garnet or cordierite, (c) cotectic (water-undersaturated, T#) direct We examine 108 samples of andalusite-bearing felsic rocks from crystallization from a silicate melt, and (d) pegmatitic (water- more than 40 localities world-wide. Our purpose is to determine the saturated, T#), associated with aplite–pegmatite contacts or peg- origin of andalusite, including the T–P–X controls on andalusite matitic portion alone; Type 3 Metasomatic—(water-saturated, formation, using eight textural and chemical criteria: size— magma-absent), spatially related to structural discontinuities in compatibility with grain sizes of igneous minerals in the same rock; host, replacement of feldspar and/or biotite, intergrowths with shape—ranging from euhedral to anhedral, with no simple correla- quartz. The great majority of our andalusite samples show one or tion with origin; state of aggregation—single grains or clusters of more textural or chemical criteria suggesting a magmatic origin. Of grains; association with muscovite—with or without rims of mono- the many possible controls on the formation of andalusite (excess crystalline or polycrystalline muscovite; inclusions—rare mineral Al2O3, water concentration and fluid evolution, high Be–B–Li–P, inclusions and melt inclusions; chemical composition—andalusite high F, high Fe–Mn–Ti, and kinetic considerations), the two most with little significant chemical variation, except in iron content important factors appear to be excess Al2O3 and the effect of (008–171 wt % FeO); compositional zoning—concentric, sec- releasing water (either to strip alkalis from the melt or to reduce tor, patchy, oscillatory zoning cryptically reflect growth conditions; alumina solubility in the melt). Of particular importance is the compositions of coexisting phases—biotites with high siderophy- evidence for magmatic andalusite in granites showing no significant llite–eastonite contents (Aliv  268  007 atoms per formula depression of the solidus, suggesting that the And ¼ Sil equilibrium unit), muscovites with 057–401 wt % FeO and 002– must cross the granite solidus rather than lie below it. Magmatic 285 wt % TiO2, and apatites with 353  018 wt % F.
andalusite, however formed, is susceptible to supra- or sub-solidus Coexisting muscovite–biotite pairs have a wide range of F contents, reaction to produce muscovite. In many cases, textural evidence and FBt ¼ 1612FMs þ 0015. Most coexisting minerals have of this reaction remains, but in other cases muscovite may compositions consistent with equilibration at magmatic conditions.
completely replace andalusite leaving little or no evidence of its The three principal genetic types of andalusite in felsic igneous rocks former existence.
are: Type 1 Metamorphic—(a) prograde metamorphic (in ther-mally metamorphosed peraluminous granites), (b) retrogrademetamorphic (inversion from sillimanite of unspecified origin), KEY WORDS: andalusite; granite; magmatic; origin; xenocrystic CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS INTR OD UCT IONPurposeAndalusite occurs as an accessory mineral in a wide rangeof felsic peraluminous {A/CNK ¼ molar [(Al2O3)/(CaO þ Na2O þ K2O)] > 1} extrusive and intrusiveigneous rocks. The purposes of this contribution are: (1) to present textural observations and chemical data from a wide range of andalusite-bearing felsic igneousrocks, including fine-grained glassy volcanics, anatecticleucosomes, fine-grained aplites, medium- to coarse-grained granitoids, and very coarse-grained granitepegmatites; (2) to discover the criteria (mineral assemblages, textures, chemical partitioning, and phase equilibriumconstraints) for distinguishing between magmatic, meta-morphic, and metasomatic andalusite; (3) to evaluate the conditions and controls that promote the formation of andalusite in naturallyoccurring felsic igneous rocks.
If andalusite can have a primary magmatic origin, its occurrence places important constraints on the T–P–Xconditions of magma crystallization.
Petrological framework Fig. 1. Relationship between the granite solidus and the andalusite– The positions of the water-saturated granite solidus and sillimanite stability field boundary. (a) The combination of the haplo- the andalusite–sillimanite stability field boundary in T– granite solidus (Tuttle & Bowen, 1958) and the And ¼ Sil boundary P–X space are critical to the origin of andalusite in felsic of Holdaway (1971; H71) permits no overlap of the stability fields ofandalusite and silicate melt, and precludes the stable crystallization of igneous rocks. At one extreme, simple synthetic systems primary magmatic andalusite, whereas the combination of the haplo- involving the water-saturated haplogranite (Na2O–K2O– granite solidus and the And ¼ Sil boundary of Pattison (1992; P92) permits the formation of primary magmatic andalusite. (b) The com- solidus (Tuttle & Bowen, 1958; bination of the peraluminous granite solidus ( Johannes & Holtz, 1996) Holland & Powell, 2001) and the aluminosilicate stability and the And ¼ Sil boundary of Richardson et al. (1969; R69) expands fields (Holdaway, 1971; Holdaway & Mukhopadhyay, the stability field for andalusite þ silicate melt (shaded area labelled 1993) show no overlap between the stability fields of ‘AND MAX').
silicate melt and andalusite, precluding a primary mag-matic origin for andalusite (Fig. 1a). Accordingly, anda-lusite in felsic igneous rocks must be xenocrystic, curve depending on their concentrations (Chorlton & metasomatic, or the product of growth from a strongly Martin, 1978; London & Burt, 1982; Pichavant & undercooled melt. At the other extreme, simple synthetic Manning, 1984). Natural Ca-bearing plagioclase raises systems involving the water-saturated peraluminous the haplogranite solidus curve by 10–20C, depending on granite solidus (Abbott & Clarke, 1979; Holtz et al., the amount of Ca in the system ( Johannes, 1978).
1992; Joyce & Voigt, 1994) and the aluminosilicate sta- The position of the andalusite–sillimanite field bound- bility fields of Richardson et al. (1969) show substantial ary in P–T space has been investigated many times, but overlap, thereby permitting a primary magmatic origin its precise location remains controversial (Kerrick, 1990; for andalusite (Fig. 1b).
Pattison, 1992, 2001; Holdaway & Mukhopadhyay, The position of the water-saturated granite solidus 1993; Tinkham et al., 2001; Pattison et al., 2002; Cesare curve is sensitive to the presence of other components.
et al., 2003). Uncertainties in the position of the And ¼ Sil In particular, excess Al2O3 lowers the solidus curve by field boundary arise, in part, from the strong dependence c. 30C (Fig. 1b), and creates a more favourable composi- of the thermodynamic equilibrium conditions on the tional environment in which to grow Al2SiO5 poly- structural state of the material under investigation morphs (Abbott & Clarke, 1979; Clemens & Wall, (Salje, 1986). Considerable discrepancy exists between 1981; Holtz et al., 1992; Joyce & Voigt, 1994). Fluorine, the experimental studies of Richardson et al. (1969), lithium, and boron are other components that may have who used fibrolitic sillimanite, and those of Holdaway important roles in lowering the haplogranite solidus (1971) who used prismatic sillimanite. According to JOURNAL OF PETROLOGY Salje (1986), a ‘transition field' between the polymorphs is request on the granite-research network for further more appropriate than a ‘transition line'. Grambling & contributions to expand the coverage. The result is a Williams (1985) and Kerrick (1990) suggested an effect of database of 111 felsic igneous rock samples, 108 of impurities (mainly Fe3þ and Mn3þ) on the stability rela- them containing andalusite, contributed by the authors tions of the Al2SiO5 polymorphs. Incorporation of Fe and of this paper. All authors have participated in the pro- Mn enlarges the stability field of andalusite relative to duction of this paper through an exchange of text, tables, that of sillimanite; however, Pattison (2001) argued that and figures on the Internet.
this effect is generally modest for natural Fe and Mn Most of the samples were submitted as hand specimens and prepared as thin sections by Gordon Brown at Owing to these difficulties in deciding between the Dalhousie University. Petrographic observations of all different experimental calibrations, many investigators samples were made by Barrie Clarke and Michael Dorais, turned to natural parageneses to constrain the equilib- and verified by the person submitting the samples. In this rium (e.g. Greenwood, 1976; Vernon, 1982; Holland & way, we have applied a uniform nomenclature to all Powell, 1985; Pattison, 1992; Pattison et al., 2002). Most samples. Bernardo Cesare examined all samples for of these studies placed the And ¼ Sil equilibrium in melt inclusions. Dan Kontak examined all samples for positions intermediate between the Holdaway (1971) fluid inclusions. Where applicable, mineral abbreviations and Richardson et al. (1969) curves. Of particular signifi- used in this paper are those of Kretz (1983).
cance to this investigation is that several studies of meta-pelitic And ¼ Sil phase equilibria in low-pressure settings(i.e. those most relevant to the issue of andalusite þ P ETROGR APHIC O BSE RVA TIONS silicate melt stability) rejected the Holdaway (1971)And ¼ Sil curve because it created too small an andalusite stability field to reconcile with a number of other phase In a field and petrographic study, Hills (1938) noted that equilibrium constraints (e.g. Vernon, 1982; Vernon et al., ‘it is chiefly from those uncontaminated . . granites, 1990; Pattison & Tracy, 1991; Pattison, 1992; Johnson & pegmatites, and aplites . . that what appears to be pri- Vernon, 1995). Pattison (1992) provided an evaluation of mary pyrogenetic andalusite has been recorded'. Hills' the And ¼ Sil equilibrium against a number of key phase evidence included modal abundance, uniform distribu- equilibrium constraints that supported his calculated tion, large size and euhedral habit of andalusite, lack of position about midway between the Holdaway (1971) oriented carbonaceous inclusions (chiastolite), absence of and Richardson et al. (1969) positions. This position metasedimentary xenoliths, association with topaz and allows for an andalusite þ haplogranite melt stability tourmaline in two-mica granites, and, for some, apparent field below 3 kbar, even without the need to invoke lack of opportunity for the magmas to assimilate pera- F-, B-, Li- or excess Al-bearing components in the melt luminous wall-rock. To establish the igneous origin for a (Fig. 1a), and it has found support in a number of recent particular mineral requires matching a number of these, papers (Spear et al., 1999; Tinkham et al., 2001; Cesare and other, inherently equivocal textural criteria, detailed et al., 2003; Johnson et al., 2003; Larson & Sharp, 2003).
below. If andalusite in a felsic igneous rock satisfies at In addition, the presence of melt inclusions in andalusite least some of these criteria, an igneous origin for that from volcanic rocks (Cesare et al., 2003), the presence of andalusite is tenable.
euhedral crystals of andalusite in some glassy felsic volca- Electronic Appendix Table A1 contains informa- nic rocks (Pichavant et al., 1988), and the occurrence of tion about the samples, including source, location, euhedral andalusite crystals in granitic rocks and anatec- environment of crystallization, and a literature reference tic leucosomes (Clarke et al., 1976; Clemens & Wall, (if any); electronic appendices may be downloaded from 1981; Vernon et al., 1990; Pattison, 1992) suggest an the Journal of Petrology website at overlap of the stability fields of andalusite and silicate melt and a magmatic origin for the andalusite.
Dimensional compatibility of a mineral of unknown ori- This project began as the result of an exchange of ideas gin with other magmatic rock-forming minerals in the about andalusite in granites on the Granite-Research same sample could be used to argue a co-magmatic Internet discussion group ([email protected], origin. The grain sizes of primary magmatic minerals in now [email protected]). Subsequent to that an igneous rock can, however, vary by orders of magni- discussion, Barrie Clarke and Michael Dorais tested tude; therefore, any grain-size test is not particularly some ideas with their own andalusite-bearing and discriminating. Conversely, dimensional incompatibility andalusite-free granitoid samples, and then put out a may suggest, but does not necessarily demand a different CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS aplites, e.g. WIL-01 (Fig. 3b), pass the grain-shape testas potentially primary magmatic phases. The andalusitein CLA-12 is skeletal (Fig. 3c), suggesting formation dur-ing a temperature or pressure quench. Many subhedralor anhedral andalusites in felsic igneous rocks have pinkcores that are euhedral to subhedral (VIS-01, Fig. 3d),suggesting that those cores, at least, might be igneous.
Anhedral andalusite grain shapes may reflect late-stage grain interference during primary magmatic growth, theresult of a reaction relationship of an andalusite of anyorigin with the silicate melt phase, an originally anhedralxenocrystic morphology, or an originally euhedral xeno-crystic morphology out of equilibrium with the melt.
Distinctly anhedral andalusite grains, apparently out ofequilibrium with the felsic magma, include volcanic sam-ple BAR-01 (Fig. 3e) and plutonic sample ROT-05(Fig. 3f ).
State of aggregationAndalusite in felsic igneous rocks may occur as singlegrains (Figs 3a, b, d–f; 4a–d), isolated from other andalu-site grains by more common rock-forming minerals. Itmay also occur as clusters of small grains. In some clus-ters, the individual andalusite grains have random orien-tations relative to one another (Fig. 5a–d). Why should amodally scarce mineral cluster? Either the individualandalusites crystallized elsewhere and were brought tothat location by some physical process such as synneusisor settling, or they represent the sites of advanced diges-tion of pelitic xenoliths, or they nucleated and grew atthat position in the sample. These common clusters of Fig. 2. Andalusite grain sizes. Photomicrographs illustrating andalusite randomly oriented grains of andalusite may have genetic grains that are significantly larger than the average grain size of the rock, suggesting that they may not have the same origin as the other In other clusters, the individual andalusite grains are in minerals in these felsic igneous rocks. (a) Sample BBR-01 (granite;Oulad Ouslam Pluton, Morocco). (b) Sample CES-01 (dacite, optical and crystallographic continuity (e.g. Figs 3c and on, Spain). Scale bars represent 1 mm. A, andalusite.
4b). If in crystallographic alignment, the andalusite grainseither grew as a spray of quench crystals (Figs 3c and origin. Any andalusite grains that are significantly smal- 5b), or the clustering may only be apparent, as in the ler, or significantly larger, than the main rock-forming cases of many optically continuous andalusite grains silicate minerals are potentially non-igneous. Figure 2 embedded in muscovite (Fig. 4b). In cases such as the illustrates two samples (BBR-01 and CES-01) in which latter, a single grain of andalusite was irregularly replaced andalusite fails the grain-size test because the crystals are by muscovite, yielding an apparent ‘cluster' of anhedral, much larger than the other minerals in the rock. Many but crystallographically aligned, andalusite in muscovite.
other samples contain andalusite grains that are consid-erably smaller than the main rock-forming minerals;although they also fail the grain-size test, they may still Textural relationship with muscovite have an igneous origin.
Many andalusite grains in felsic igneous rocks have man-tles of muscovite, and these muscovite rims may consist of a single crystal or a polycrystalline aggregate. Figure 6 Euhedral andalusite in a felsic igneous rock may indicate combines the state of aggregation of andalusite grains a former cotectic or peritectic relationship with a silicate (above), and the common association of andalusite with melt phase; however, euhedral andalusite occurs both in muscovite, to establish a six-fold textural classification igneous and metamorphic rocks, and thus idiomorphic of andalusite. In some cases, more than one class of grain shapes alone are not diagnostic. Some of the anda- andalusite can occur in the same rock (e.g. sample GOT- lusites in volcanic samples, e.g. LON-01 (Fig. 3a), or 02 contains andalusite textural types S1, C1, and C2).
JOURNAL OF PETROLOGY Fig. 3. Andalusite grain shapes. (a) Sample LON-01 (rhyolite obsidian clast, Macusani, Peru; US National Museum catalog no. 2143) shows twosmall euhedral to subhedral andalusite crystals in a predominantly glassy matrix. (b) Sample WIL-01 (aplitic granite; Velay Massif, France)contains euhedral andalusite. (c) Sample CLA-12 (aplite–pegmatite; South Mountain Batholith, Nova Scotia, Canada; section is slightly too thick)has elongate–skeletal andalusite grain shapes suggesting crystallization by quenching. (d) Sample VIS-01 (granite; Makalu north side, Tibet) hasandalusite with an overall anhedral grain shape, but with a more euhedral pink core. (e) Sample BAR-01 (rhyolite; Lipari, Italy) is a volcanic rockwith anhedral andalusite. (f) Sample ROT-05 (granite; Telve, Cima d'Asta pluton, southern Alps, Italy) contains anhedral andalusite that exhibitsdeformation twinning in crossed polars (not shown). Scale bars represent 1 mm. A, andalusite.
In the Macusani rhyolites, muscovite and andalusite andalusite and muscovite are negatively correlated, sug- coexist throughout the entire volcanic field (Pichavant gesting that, during the main crystallization stage of the et al., 1988). No textural evidence exists for replacement Macusani magmas, the reaction Ms þ Qtz ¼ And þ San of one phase by the other, but the modal proportions of (in presence of melt) controls the modal proportions of CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS Fig. 4. Single grains of andalusite. (a) Sample CLR-01 (migmatite; Mt. Stafford, Arunta Block, Australia) with subhedral andalusite in a migmatiteleucosome (textural type S1). (b) Sample TOS-05 (pegmatite; Velasco Batholith, Argentina) shows a single optically continuous andalusite grainenclosed in single grain of muscovite (textural type S2). (c) Sample ROT-02 (granite; Cotronei, Sila Batholith, Calabria, southern Italy) showing asubhedral andalusite enclosed in a single crystal of muscovite (textural type S2). (d) Sample UGI-04 (granite; Plasencia granite, west CentralIberian Massif, Spain) showing an anhedral andalusite with a polycrystalline rim of muscovite (textural type S3). Classification of textural types S1,S2, and S3 is given in Fig. 6. Scale bars represent 1 mm. A, andalusite.
andalusite and muscovite. This reaction depends on P, T, (ROT-02, Fig. 4c), and thereby complicate any determi- and aH O, implying that the mineral assemblage charac- nation of the origin of the andalusite. Because muscovite teristic of the main crystallization stage of the Macusani can have primary magmatic or secondary hydrothermal magmas (Qtz, San, Plag, Ms, And,  Bt) could have origins, with much the same texture (Miller et al., 1981; crystallized over a range of P, T and fH O conditions.
Zen, 1988), interpretation of this textural relationship However, the F content of muscovite is also an important between andalusite and muscovite is difficult. One reason controlling factor in this reaction. For a given aH O, for little or no bulk chemical compositional difference elevated fHF would drive the reaction to the left (consum- between some andalusite-bearing two-mica granitoids ing andalusite, producing muscovite). Muscovite crystal- and andalusite-free two-mica granitoids is just a question lization at the expense of andalusite does not necessarily of how completely the andalusite is replaced (effectively imply high aH O (it could be lower T, higher P, or higher under magmatic conditions by primary muscovite, less fHF). The inverse correlation between the modal propor- effectively under subsolidus conditions by secondary mus- tions of Ms and And in the Macusani volcanics also covite). Whether andalusite is preserved in plutonic rocks occurs in peraluminous granites from the Bohemian depends on its survival under conditions of slow cooling, Massif (samples ROT-03,04; D'Amico et al., 1982– allowing magmatic peritectic relations of the type Muscovite overgrowths on andalusite in plutonic L þ And þ Other Phases rocks may obscure a possible original euhedral shape ! L þ Ms þ Other Phases JOURNAL OF PETROLOGY Fig. 5. Clusters of andalusite grains. (a) Sample ROT-04 (granite; Rasna quarry, Telc, southwestern Moravia, Czech Republic) showing a smallcluster of anhedral andalusite crystals in quartz (textural type C1). (b) Sample ELB-01 (aplite; Beni Bousera, Morocco) showing a sub-parallelcluster of andalusite grains in an aplite (textural type C1). (c) Sample ROB-02 (granite; South Bohemian Pluton, Austria) shows a cluster ofrandomly oriented andalusites in a single crystal of muscovite (textural type C2). (d) Sample VIL-02 (granite; Pe na-Hombre Pluton, Spain) shows a cluster of anhedral andalusite grains in a polycrystalline aggregate of muscovite (textural type C3). Classification of textural types C1, C2, and C3is given in Fig. 6. Scale bars represent 1 mm.
(where L is melt), or subsolidus reactions such as And þ Kfs þ ðH2OÞ ! Ms þ Qtz to eliminate the early formed andalusite. Addition ofwater to the left sides of these equations converts ‘dry'andalusite-bearing granitoids to ‘wet' muscovite-bearing,and normally two-mica, granitoids; in other words, theyare compositional equivalents except for the amount ofwater (Zen, 1989). Kinetically, a high-temperature, Fig. 6. Textural classification of andalusite in felsic igneous rocks.
melt þ fluid, condition may favour the formation of Three textural parameters (the occurrence of andalusite either as single coarse-grained single muscovite crystals, whereas a grains or as clusters of grains, the occurrence of andalusite with or subsolidus low-temperature, fluid-only, condition may without muscovite, and if with muscovite, whether that muscoviteconsists of a single grain or an aggregate of grains) produce the favour the formation of some fine-grained polycrystalline following six textural categories: S1, single andalusite grains, no mus- covite; S2, single andalusite grains, monocrystalline muscovite over- Figure 7 illustrates four of the many types of growth or reaction rim; S3, single andalusite grains, polycrystallinemuscovite overgrowth or reaction rim; C1, clustered andalusite grains, textural relations between andalusite and muscovite.
no muscovite; C2, clustered andalusite grains, monocrystalline musco- The original andalusite may be a single grain or a cluster, vite overgrowth or reaction rim; C3, clustered andalusite grains, poly- the muscovite rim may be magmatic or subsolidus hydro- crystalline muscovite overgrowth or reaction rim. Textural types S1 thermal, and the And ! Ms reaction may be incomplete and C1 can occur as discrete grains, or as inclusions in other grainssuch as plagioclase or quartz.
or complete. In the last case, the andalusite is completely

CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS Fig. 7. Development of several possible textural relationships between andalusite and muscovite (arrows represent the crystallographic c-axis ofandalusite). Different processes can have similar end-points. (a) Single grain of magmatic muscovite overgrows a single grain of magmaticandalusite. Suprasolidus or subsolidus muscovite continues to grow to the ultimate elimination of andalusite. No textural evidence for the formerexistence of andalusite remains. (b) Subsolidus replacement of a single grain of andalusite to produce a polycrystalline muscovite pseudomorph.
(c) Quenched skeletal andalusite overgrown by magmatic muscovite resulting in an apparent cluster, but the ‘grains' are in optical continuity.
(d) Optically discontinuous cluster overgrown by magmatic muscovite.
consumed in the reaction, leaving little or no evidence of those inclusions may help to determine the origin of the its former existence.
host andalusite. If an andalusite contains carbonaceousmaterial defining the chiastolite cross (e.g. BBR-01,Fig. 2a), a metamorphic origin is probable. Some chias- Inclusion relationships tolite-like andalusite may also form by peritectic melting Mineral inclusions reactions in graphitic schists where inclusion of graphite If andalusite occurs as inclusions in igneous minerals such particles may take place behind advancing crystal faces, as feldspar and quartz (e.g. REN-03, UGI-06), little can but at the same time the andalusite should also trap melt be deduced about its origin; however, andalusite rarely inclusions (Cesare & G omez-Pugnaire, 2001). Few of the occurs as inclusions in any phase other than muscovite. If andalusites that we believe are igneous on other grounds andalusite itself contains inclusions of magmatic miner- contain any mineral inclusions, and thus the mineral als, the sizes, shapes, abundances, and compositions of inclusion criterion is not particularly useful.
JOURNAL OF PETROLOGY in a volcanic rock is almost certainly magmatic, whereas alarge anhedral andalusite with a chiastolite cross and areaction rim is probably xenocrystic. Also, we note thatthere is no a priori textural reason why a felsic igneous rockcannot contain more than one genetic type of andalusite(e.g. magmatic and xenocrystic).
CHEMICAL COMPOSITION OF ANDALUSITE IN FELSIC IG NEOUS ROC KSIn this section, we examine the chemical composition ofandalusite, the nature of any chemical zoning, and thechemical compositions of coexisting micas and apatite tosearch for criteria that might provide information about Fig. 8. Melt inclusions in andalusite. Volcanic sample PIC-01 (rhyolite; the origin of andalusite in felsic igneous rocks. Electronic Macusani, Peru) showing conspicuous melt inclusions. Also to be notedis the sharp straight contact between the pleochroic core and the Appendix Tables A2–A5 contain compositional data colourless rim of the andalusite. Scale bar represents 01 mm.
for average biotite, muscovite, andalusite, and apatite,respectively, in the samples we have studied. Not allsamples contain all four minerals, and even if they do, we do not necessarily have analyses for all four phases in Melt inclusions in andalusite attest to its growth in the presence of melt (Cesare et al., 2003). Glass inclusions areeasy to recognize in andalusite from felsic volcanic rocks, Chemical composition such as those from Lipari (BAR-01), Mazarr If a mineral exhibits a wide range of chemical substitu- 01,02) (Cesare & G omez-Pugnaire, 2001; Cesare et al., tions that reflect its conditions of formation [e.g. Ti in 2002, 2003), and Macusani (Pichavant et al., 1988; Fig. 8).
muscovite (Miller et al., 1981)], then the origin of that In slowly cooled plutonic rocks or migmatites, any melt mineral may be determined from its chemical composi- inclusions trapped in andalusite will have crystallized as tion alone. In stoichiometric andalusite (Al2SiO5), half polyphase aggregates of quartz, feldspars, and micas, a the Al cations reside in octahedral sites, and the other useful criterion to infer an igneous origin for andalusite.
half reside in five-coordinated polyhedra, whereas all the Polyphase inclusions in andalusite crystals of samples Si cations occupy tetrahedral sites. Such simple chemistry CLA-01,05,11,12,13, CLR-02, GOM-03, RIC-03, and and relatively simple structure provide limited opportu- TOS-06 provide additional support for their coexistence nity for chemical substitution (Deer et al., 1982). Electro- with a felsic silicate melt.
nic Appendix Table A4 shows that the studiedandalusites from felsic igneous rocks have transition-element compositions with the following ranges: FeO (measured as Fe, reported as FeO) 003–170%, MnO Examination of all our andalusite samples for fluid inclu- 000–009%, and TiO2 000–036%. Without a compar- sions yielded negative results. Either there was no fluid in able database of andalusite compositions from meta- equilibrium with the andalusite as it grew (unlikely in the morphic rocks, little can be said about the existence of cases of pegmatites), or the surface properties of andalu- chemical discriminants to determine the origin of the site are such that it is not readily ‘wetted' by fluids.
andalusite. Trace elements might prove to be more usefulthan major elements.
Summary of textural observationsOf the several possible textural tests for the origin of andalusite in felsic igneous rocks, no single criterion Optically zoned andalusite is common in metamorphic, (grain size, grain shape, clustering, textural relations hydrothermal, and magmatic environments [e.g. review with muscovite, inclusion relations) is necessarily diagnos- by Kerrick (1990)]. Andalusites from the studied felsic tic of the origin of andalusite. The agreement of two or igneous rocks show four types of zoning, as follows.
more of these textural and chemical criteria constitutes a (1) Concentric zoning. Concentric zoning consists of a stronger collective case. For example, a euhedral, grain- sharp to gradational variation in the mole fraction of size compatible, andalusite with melt inclusions occurring CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS Fig. 9. Chemical zoning in andalusite. (a) Sample CLA-05 [migmatite(?); South Mountain Batholith, Nova Scotia, Canada] shows normal–concentric zoning, with one sharp zone boundary and one gradational zone boundary, mimicking the external morphology of the crystal.
(b) Sample VIL-05 (granite; Berrocoto Pluton, Spain) shows irregular normal–concentric zoning with straight sharp and curved gradational zoneboundaries. (c) Sample ERD-01 (aplite–pegmatite; South Mountain Batholith, Nova Scotia, Canada) shows well-developed sector zoning.
(d) Sample GOM-12 (pegmatite; Pacos de Ferreira, Portugal) shows irregular, sharp to gradational, oscillatory zoning. (e) Line scan for ironalong profile in (d). The pink zones correspond to high iron contents. (f ) Sample WHI-01 (granite; Nigde Massif, Turkey) shows preservation oforiginal zoning in andalusite subjected to sillimanite-grade metamorphism (arrow). Scale bars represent 1 mm.
simply as TE content) from core to rim, with some convolute–lobate and/or irregularly stepped boundaries boundaries subparallel to the external morphology of the crystal (Fig. 9a). However, the cores of such (2) Sector zoning. Sector zoning is characterized by grains may be highly irregular in shape, showing higher TE contents parallel to {001}, {100}, and {010} JOURNAL OF PETROLOGY (Hollister & Bence, 1967). Regular steps in some of the other and magmatic in origin, and if they are also in concentric zone boundaries may be sector zone chemical equilibrium with andalusite, then the andalusite boundaries. The most striking example is from sample should also be magmatic.
ERD-01, which shows sharp subhedral sector zoning(Fig. 9c); the steps in the zoning of andalusite VIL-05 (Fig. 9b) may also represent preferential sector growth.
(3) Oscillatory zoning. Oscillatory zoning is character- Figure 10a is a trioctahedral mica plot showing the aver- ized by alternating high-TE and low-TE, continuous age biotite compositions in all the studied samples. Given to discontinuous, growth shells (Fig. 9d and e). Most the global distribution of the samples, the consistency of boundaries between the growth zones are either rounded the Aliv [mean 268  007 atoms per formula unit or irregularly stepped.
(a.p.f.u.)] in the biotites is remarkable, suggesting that (4) Patchy zoning. In contrast to sector zoning above, the biotites have had their alumina contents fixed by patchy zoning shows neither sharp nor obviously equilibrating with some Al-rich phase (e.g. andalusite), crystallographically controlled boundaries (Fig. 4a).
probably under conditions of restricted temperature and Once formed, such andalusite zoning patterns appear pressure. Although a magma containing abundant anda- to be robust, as indicated by samples WHI-01 (Fig. 9f ) lusite and biotite xenocrysts might also attain this equili- and NEV-04 (not shown) in which the pink TE-rich brium, the simplest interpretation is that the biotite and andalusite cores have survived high-temperature sillima- andalusite are both primary magmatic in origin.
nite-grade metamorphism, but the outer parts of thecolourless rims have inverted to sillimanite.
Although the different types of zoning in andalusite are Figure 10b and c shows the TiO2 and Na/(Na þ K) well known, little is understood about their origins and distributions in all analyzed muscovite grains. According their potential for revealing diagnostic information about to the chemical criteria of Miller et al. (1981), very few of these muscovites have TiO2 >1%, consistent with a andalusite commonly shows concentric zoning (high-TE primary magmatic origin; however, if a highly evolved core, low-TE rim) or sector zoning (Cesare, 1994; magma has a very low TiO2 content, so presumably, will Whitney & Dilek, 2000), whereas metamorphic andalu- its primary magmatic muscovite. According to their Na/ site commonly shows gradational patchy zoning, and (Na þ K) ratios, however, these muscovites are predomi- may also exhibit concentric zoning (Yokoi, 1983; Shiba, nantly magmatic (Monier et al., 1984). Figure 10d shows 1988; Cesare, 1994), or sector zoning (Grambling & the variable, but non-diagnostic, range of FeO concen- Williams, 1985). If distinctions between environments of trations in the muscovites coexisting with andalusite.
crystallization exist, they are not yet well defined. Never- For the composition of muscovite to be more useful, we theless, zoning patterns may help to exclude a certain need a detailed study of muscovite associated with anda- origin for a grain in question (e.g. oscillatory zoning is lusite versus the rest of the muscovite in the rock. Further- unlikely for metamorphic andalusite, but likely for hydro- more, we need to determine if there is any chemical thermal or magmatic andalusite). Several features of difference between the monocrystalline muscovite rims zoned andalusites are, at least, consistent with a mag- on andalusite (magmatic?) and the polycrystalline musco- matic origin (e.g. sharp compositional zone boundaries, vite rims on andalusite (hydrothermal? quenched?).
oscillatory zones, possible quench phenomena with pre-ferential sector growth). Unfortunately, we do not yethave sufficient textural and chemical information about Biotite þ muscovite zoned andalusites in veins and metamorphic rocks to be In general, andalusite-bearing plutonic rocks contain bio- able to distinguish clearly between one environment of tite and muscovite with high alumina contents; however, crystallization and another, and what, if any, character- this criterion alone does not necessarily separate igneous istics of zoning are unique to magmatic andalusites.
from metamorphic micas. Figure 11a shows TiO2 con-tents for averages of all analyzed mica pairs. The similarslopes of the tie lines between coexisting micas suggest Chemical equilibrium with other minerals attainment of chemical equilibrium between the mica For minerals showing extensive mutual solid solution, pairs, namely D Bt/Ms range is 266–2517, mean systematic disposition of tie lines between coexisting 468  150 (excluding all values greater than 800), n ¼ phases is an indication of an equilibrium relationship.
20 [compare the values obtained by Brigatti et al. (2000), In this section, we consider whether the compositions of range 194–333, mean 274, n ¼ 7). Given biotite, muscovite, and apatite coexisting with andalusite the texture and the unaltered state of these biotites, we are consistent with their being an equilibrium assem- conclude that the equilibrium is more likely to be blage. If they are in chemical equilibrium with each magmatic than subsolidus–hydrothermal.
CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS Figure 11b shows mean fluorine concentrations for the same coexisting micas. Of note is the wide range of Fcontents in the micas in these andalusite-bearing rocks,and the generally regular disposition of tie lines suggest-ing equilibrium compositions. Tie lines with distinctivelysteeper or shallower slopes suggest that the compositionof at least one mica in the assemblage has changed, andthat some degree of subsolidus re-equilibration of Fbetween coexisting micas may have taken place. Insuch cases, D Bt/Ms increases with subsolidus cooling because muscovite re-equilibrates more readily than bio-tite (Ferrow et al., 1990). Such disequilibrium between themicas may also raise questions about the origin of thecoexisting andalusite.
Samples with FBt/FMs <15 are VIL-14 and WOO-01, which appear to be otherwise unremarkable. Sampleswith FBt/FMs >38 are JAM-01, JAM-02, RIC-01,RIC-02, RIC-03, RIC-05, RIC-06, and NEV-02. Signif-icantly, seven of these samples are migmatites with lowFBt contents, and the other sample is a pegmatite (NEV-02). Sample CLA-05 has the lowest F contents in itscoexisting micas; in this respect, its similarity to the mig-matites suggests that it may also have an early anatecticorigin. The mean of all samples with average FBt/FMs>14 and <38 is 227  059 (n ¼ 31). In two sampleswith crossing tie lines (NEV-03, NEV-05), andalusiteoccurs in clusters with biotite-rich xenolithic materialand texturally (but not chemically) secondary muscovite.
Because all of these samples contain andalusite, high F is,apparently, not a precondition for the occurrence ofandalusite in felsic igneous rocks.
If all the analyzed samples had come from one differ- entiating pluton, such a regular disposition of tie linesmight be expected; however, given that the samples comefrom more than 40 localities of different types, the reg-ularity of the tie lines in Fig. 11a and b suggests animportant repetition of T–P–X conditions in andalusite-bearing peraluminous felsic igneous rocks through spaceand time. As a first-order approximation, we consider thebundles of roughly parallel tie lines (Fig. 11a and b) andthe samples with Ti and F partitioning between coexist-ing micas similar to those determined experimentally(Icenhower & London, 1995), as magmatic micas.
Figure 11c is similar to Fig. 11b, except that the verticalaxis is expanded and most of the tie lines have beenremoved. Additional plotted samples are from thetopaz-bearing two-mica Lake Lewis leucogranite in theSouth Mountain Batholith (Clarke & Bogutyn, 2003).
Fig. 10. Compositions of coexisting micas. (a) Biotite compositions in Sample GOT-02 is the most fluorine-rich, andalusite- the system phlogopite–annite–eastonite–siderophyllite. Despite the bearing, topaz-absent, sample from our database, and it genetically unrelated nature of the sample set, biotite compositionsfrom the studied suite of andalusite-bearing rocks have tightly con- helps to constrain the position of the andalusite–topaz strained Aliv  268  007 a.p.f.u. (b) TiO2 contents of all analyzed boundary in this system.
muscovites in the sample set. The distribution is strongly skewed to low Figure 11d shows the systematic partitioning of F TiO2 contents (<1%). (c) Na/(Na þ K) values showing the boundarybetween non-igneous and igneous muscovites (Monier et al., 1984). (d) between biotite and muscovite expressed by the equation FeO contents of all analyzed muscovites.
FBt ¼ 131 FMs þ 002. This empirical relationship is JOURNAL OF PETROLOGY Fig. 11. Average muscovite and biotite compositions. Tie lines join coexisting pairs. (a) Molecular (FeO þ MnO þ MgO) vs TiO2. (b) Molecular(FeO þ MnO þ MgO) vs F. In both plots, most mica compositions appear to represent equilibrium pairs, and the simplest interpretation is thatthey are magmatic phases in equilibrium with (magmatic) andalusite. (c) The andalusite-bearing sample with the highest F content in coexistingmicas is GOT-02 (high fluorine, high phosphorus, granite; Satzung, Erzgebirge, Germany). Also plotted are micas coexisting with primarymagmatic topaz in the Lake Lewis leucogranite in the South Mountain Batholith (LLL; Clarke & Bogutyn, 2003). The dashed line is the inferredupper limit for fluorine in micas of andalusite-bearing granites. (d) Partitioning of fluorine between coexisting micas in the sample set. Thecalculated partition coefficients are generally consistent with experimentally determined values (Icenhower & London, 1995).
reasonably consistent with other data from coexisting and 277 for muscovite. Furthermore, samples with low micas in granites (FBt/FMs ¼ 18  05; Neves, bulk-rock fluorine contents, as proxied by the FBt values, 1997), and on coexisting micas in peraluminous ex- have the fluorine strongly partitioned into the apatites (as perimental systems (FBt/FMs ¼ 122–155; Icenhower & before, many of these samples are migmatitic leuco- London, 1995).
somes). If the bulk-rock fluorine contents are high, Fstrongly partitions into the micas. The systematic parti-tioning of F between apatite and the micas suggests Biotite þ muscovite þ apatite equilibrium conditions. If the apatite is magmatic, then The magmatic origin of apatite is normally not in ques- probably so should be the micas.
tion. Apatite should, therefore, exhibit systematic parti-tion relationships for fluorine with the two othermagmatic F-bearing phases, i.e. biotite and muscovite, if they are all in equilibrium. Figure 12a and b shows the The broad overlap of stability fields for sillimanite and complex relationship FAp/FMs vs FAp/FBt, contoured for felsic melt means that, in contrast to andalusite, an FBt and FMs, respectively. In general, the array of points igneous origin for sillimanite in felsic igneous rocks is defines a curved trend, and in both plots the fluorine not a petrogenetic problem. Sillimanite can occur as the concentrations are highest in those micas with the lowest only aluminosilicate phase (D'Amico et al., 1982–83a, FAp/FMica values. This relationship appears to be the 1982–83b; Pichavant et al., 1988), or it can occur with result of relatively constant fluorine concentrations in andalusite (Barker, 1987; Pichavant et al., 1988; Messina the apatites. The ratio of Fmax/Fmin in each of the phases et al., 1991; Rottura et al., 1993; Cesare et al., 2002; in our entire sample set is 14 for apatite, 88 for biotite, Visona & Lombardo, 2002). Our sample set was CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS a priori chemical reason why a felsic igneous rock may notcontain more than one genetic type of andalusite (e.g.
magmatic and xenocrystic).
G ENET IC TYPE S OF A NDA LUS IT E IN FELSIC IGNEOUS ROCKSTheoretically, andalusites in felsic igneous rocks can fallinto three main genetic categories detailed below.
Type 1 Metamorphic (melt phase notinvolved in the formation of andalusite)Type 1a Metamorphic–in situ progradeBarrera et al. (1985) and Zaleski (1985) described theeffects of contact metamorphism in granites where anda-lusite formed as euhedral to subhedral prisms replacingoriginal biotite. None of the andalusite in our samplesappears to have formed in situ by thermal metamorphism Fig. 12. Fluorine in coexisting apatite, biotite, and muscovite. (a) F of a felsic igneous rock. In the sample most obviously FMs vs FAp/FBt with FBt contours. The systematic distribution of FBt affected by thermal metamorphism (WHI-01; Fig. 9g), contours suggests equilibrium among the three phases. High FAp/FMs pre-existing andalusite has been partially converted to and FAp/FBt ratios correlate with low bulk fluorine contents as mon- itored by F in biotite. (b) FAp/FMs–FAp/FBt with FMs contours. Again,high FAp/FMs and FAp/FBt ratios correlate with low bulk fluorinecontents, and the systematic distribution of contours suggests equili-brium among these three phases.
Type 1b Metamorphic–retrograde inversion ofsillimanite of various origins assembled solely on the basis of the presence of andalu- If sillimanite of any origin (magmatic, metamorphic) were site; the additional occurrence of sillimanite in any present in a granite magma, it could undergo inversion to sample was incidental. Our database is not sufficiently andalusite above or below the granite solidus, possibly comprehensive to draw any general conclusions about resulting in andalusite pseudomorphs after the sillima- the coexistence of andalusite and sillimanite in felsic nite. Barker (1987) has argued that, on the basis of size igneous rocks.
and shape of the andalusites in sample BAR-01 fromLipari, they have inverted from xenocrystic sillimanite.
Otherwise, none of our andalusite appears to haveformed by inversion from sillimanite.
Summary of chemical criteriaWe have considered three chemical tests for the origin ofandalusite in felsic igneous rocks. The chemical composi- Type 1c Metamorphic–xenocrystic derived from local tion of andalusite itself provides little information about peraluminous country rocks its origin. The nature of chemical zoning may have Andalusite crystals may be released from disaggregating, greater potential, but it first requires a more detailed contact-metamorphosed, metapelites into a silicate melt examination of chemical zoning patterns in andalusites and, in general, such xenocrystic grains would be out of from metamorphic rocks and hydrothermal veins. The chemical equilibrium with that melt. These xenocrysts chemical-equilibrium-with-other-phases test is the most may be anhedral and contain many mineral inclusions, quantitative and most objective. Systematic partitioning including carbonaceous material. Their subsequent his- of Ti and F between coexisting biotite, muscovite, and tory in the magma then depends on the degree to which apatite in our sample set suggests that they are in equilib- they are out of equilibrium with the silicate melt, and on rium and are almost certainly magmatic phases. That the the kinetics of the new environment. Xenocrystic magmatic biotite also has its Aliv controlled by equilib- andalusite may disappear rapidly in a high-temperature, rium with andalusite is, we believe, the most compelling well-mixed, relatively fluid metaluminous melt, or in a chemical argument in favour of a magmatic origin for the peraluminous melt undersaturated in Al2SiO5, survive andalusite; however, this view does not entirely preclude largely unmodified in a near-solidus, static, viscous the equilibration of xenocrystic biotite and andalusite at peraluminous melt, or even develop magmatic over- magmatic temperatures. We note again that there is no growths in a highly peraluminous melt. Xenocrysts in JOURNAL OF PETROLOGY an advanced state of dissolution, especially if mantled by Table 1: Examples of melting reactions involving late muscovite, would be difficult to distinguish from aluminosilicate (Als) as an original phase of the protolith anhedral magmatic grains.
Bouloton et al. (1991) and Bouloton (1992) described xenocrystic andalusite in Hercynian granites from Bt þ Als þ Qtz þ Kfs þ H2Ov ! L Morocco where chiastolite-type crystals, up to 5 cm long, Als þ Bt þ H2Ov þ Kfs þ Pl þ Qtz ! L occur. Samples BBR-01 (Fig. 2a) and BBR-02 are from Als þ Bt þ H2Ov þ Kfs þ Ms þ Qtz ! L the same pluton. These large andalusites fail the grain- Bt þ Als þ Qtz þ Pl þ H2Ov ! L þ Grt/Crd size test as magmatic, and they have significant reaction Bt þ Pl þ Ksp þ Qtz þ Als þ H2Ov ! L rims indicating disequilibrium with the melt. Also, sam- Bt þ Als þ Qtz þ H2Ov ! L þ Crd ples NEV-03 to NEV-05 contain ovoid polymineralic Bt þ Als þ Qtz þ H2Ov ! L þ Grt aggregates of biotite, andalusite, and muscovite, with or Grt þ Als þ Kfs þ Pl þ Qtz þ H2Ov ! L þ Bt without sillimanite, showing a symplectitic relationship.
Als þ Kfs þ Qtz þ H2Ov ! L These aggregates only occur close ( 300 m) to the con- Bt þ Qtz þ Als ! L þ Grt þ Kfs tact with younger porphyritic biotite granites, and they Bt þ Pl þ Qtz þ Als ! L þ Grt/Crd þ Kfs appear to be foreign to their granite host.
Als þ Qtz þ Kfs þ Pl ! L opez Ruiz & Rodrı´guez Badiola (1980) interpreted the origin of andalusite in some high-K dacites as xeno- L, silicate melt.
crystic because typical anhedral andalusite grains aresurrounded by plagioclase and spinel reaction rims. Suchandalusite grains may also contain inclusions, includingthe chiastolite cross, as well as textural evidence of dis- Table 2: Examples of reactions that produce aluminosilicate equilibrium (e.g. corrosion). Alternatively, because some (Als) only as a result of peritectic melting relations of these andalusites also contain melt inclusions, Cesareet al. (2003) regarded them as Type 1d or 2a (below).
Grt þ Crd þ Kfs þ H2Ov ! L þ Als þ Bt Type 1d Metamorphic–original constituent of source Crd þ Pl þ Kfs þ Qtz þ H2Ov ! L þ Als Ms þ Qtz  H2Ov ! L þ Als þ Kfs We define restite minerals as those minerals, present in Bt þ Crd ! L þ Als þ Grt þ Qtz the protolith prior to partial melting, that survive as the Qtz þ Ksp þ Bt ! Als þ L refractory residua of partial melting. Table 1 lists several melting reactions in which aluminosilicate (Als) is part of the original subsolidus mineral assemblage of the (meta-pelitic) protolith. Given the low T–low P stability region L, silicate melt.
of andalusite, and its limited region of overlap with thefield of granite magmas, andalusite is an unlikely phase to surmicaceous enclaves may be restites from the source occur as part of a truly restitic assemblage in many area (Didier, 1991; Montel et al., 1991; Gaspar & granitoid magmas, especially if extensive partial melting Inverno, 1998), but in the absence of minerals or textures has taken place at high temperatures. Fluid-present melt- typical of high temperatures and pressures (Wall et al., ing reactions with (H2O)v are likely to be lower T, and 1987), such enclaves are more likely to be partially ¼ andalusite. Fluid-absent melting reactions, digested xenoliths of country rocks. Unless some of our especially those involving biotite dehydration, are likely andalusites represent disaggregated relicts from such to be high T, and Als ¼ sillimanite. Depending on the enclaves, restitic andalusite must be rare.
bulk composition of the protolith, and the degree ofpartial melting, Als can remain as part of the restiticrefractory residuum. If any magma had been in equili- Type 2 Magmatic (melt phase an integral brium with andalusite as a restite phase in the region part of the formation of andalusite) of partial melting, that magma would be saturated in Type 2a Magmatic–peritectic ( T") andalusite, and would probably remain saturated during Table 2 lists several reactions in which andalusite appears its ascent to lower pressures. Such magmas are strong solely as the result of melt-producing reactions in origin- candidates for crystallizing magmatic andalusite (below).
ally andalusite-free rocks. In none of these reactions is Distinguishing between former restitic andalusite and andalusite also present in the subsolidus mineral assem- new magmatic andalusite is extremely difficult, especially blage, but it appears peritectically in an incongruent in the absence of melt inclusions. Andalusite-bearing melting reaction. We regard such andalusite as being CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS magmatic because, in phase equilibrium terms, it demon- et al., 1991), which is more like a heterogeneous diatexite strates a stability field overlap with a silicate melt.
than a granodiorite, andalusite occurs in biotite reaction High-temperature fluid-absent melting reactions will rims around cordierite macrocrysts. Ellis & Obata (1992) favour Als ¼ sillimanite, but low-temperature, low- described the origin of that andalusite as a typical back- pressure, water-saturated melting reactions will favour reaction when the melting reaction was reversed. Samples Als ¼ andalusite. Spatially, andalusite of this type may form CLA-07 (Fig. 13a) to CLA-10, inclusive, from the along the contact between pelitic xenoliths and melt, or Musquodoboit Batholith (Abbott & Clarke, 1979), also associate with the melt phase (initially as leucosomes) represent this down-temperature peritectic reaction.
rather than the refractory residuum (restite) in anatecticmigmatites. Such andalusites have no subsolidus meta- Type 2c Magmatic–cotectic ( T# and/or P#), morphic history, and thus may be euhedral and free of the mineral inclusions metamorphic andalusites com- Any overlap between the stability fields of andalusite monly contain. Kawakami (2002) has described andalu- and granitic magmas means that andalusite can become site of magmatic origin from migmatites in Japan. Small saturated in a silicate melt of appropriate composition.
crystals of euhedral andalusite in some Himalayan leuco- Aluminosilicate saturation in the melt phase is favoured granites are surrounded by thin rims of sillimanite by excess alumina (high A/CNK), and must be achieved (Castelli & Lombardo, 1988; Visona & Lombardo, by some closed- or open-system process, or combination 2002), and may be the products of a T " (rising tempera- of processes, such as source inheritance, fractional ture) peritectic melt-producing reaction. In such a reac- crystallization, contamination, or possibly water satura- tion, andalusite initially grows in the metapelites by tion and escape of a fluid phase. Figure 1b shows that peritectic melting reactions and replacement by topotac- isobaric cooling might produce andalusite in upper- tic sillimanite is the result of rising temperature (e.g.
crustal magmas (coarse-grained granitoids), and that Cesare et al., 2002).
adiabatic pressure decreases are capable of producing Prime candidates for T " peritectic andalusite occur in andalusite in rapidly ascending magmas (aplites and the migmatites from our sample set. Sample RIC-06 shows abundant large andalusite crystals growing along With a significant field of overlap between the stability the leucosome–melanosome contact, and samples CLR- fields of andalusite and granitic melts in T–P space, and 01 (Fig. 4a) and CLR-02 also have high modal abun- with andalusite stable down to the solidus temperature dances of andalusite in the leucosomes. Such high in most cases, the general andalusite-forming reaction concentrations of andalusite in early melts are important in considering the origin of andalusite in all felsic igneousrocks. Furthermore, the coarse grain size of some of these L ! And þ Kfs þ Plag þ Qtz andalusites is normal for leucosomes (Kriegsman, 2001).
þ Other Magmatic Solid Phases: Cesare et al. (2003) regarded andalusites with melt inclu- The problem is to distinguish this primary magmatic sions as having formed in xenoliths during peritectic cotectic andalusite from all other possible origins.
melting reactions, and subsequently having been released The euhedral, or mechanically fragmented, andalusite into the main magma by disaggregation of the xenolith.
crystals in volcanic rocks [Macusani sample LON-01 Sample BAR-01 (Fig. 3e) may also have formed in this (Fig. 3a), Macusani sample PIC-01 (Fig. 8), and way, and is now in a melt with which it is not in equili- Morococala sample MOR-01 (Fig. 13b)] are strong brium, hence its irregular grain shape and reaction rim of candidates for a primary cotectic magmatic origin.
cordierite. This reaction is the up-temperature reverse of According to Pichavant et al. (1988), the Macusani the andalusite-forming reaction (Type 2b) below.
magma crystallized at T  650C, P ¼ 15–20 kbar,and aH O 1 and the andalusite at 15–175 kbar. For Type 2b Magmatic–peritectic, ( T#), the andalusite-bearing rhyolites of Morococala, Morgan et al. (1998) estimated andalusite formation at 740–750C A second, much more restricted, type of peritectic mag- and a pressure 4–5 kbar. Some Morococala rocks con- matic reaction occurs in which andalusite appears as a tain muscovite rather than andalusite, suggesting a pre- result of T# (falling temperature) in a water-undersatu- rated melt-solid reaction such as And þ Kfs þ (H2O)v reaction to Qtz þ Ms.
L þ Crd ! And þ Bt þ Plag þ Qtz½þðH2OÞ : Type 2d Magmatic–cotectic ( T# and/or P#), The andalusite so produced may be chemically, although perhaps not texturally, indistinguishable from Type 2c Almost every felsic magma reaches water saturation andalusites below. In the Cooma ‘granodiorite' (Chappell JOURNAL OF PETROLOGY Fig. 13. Photomicrographs illustrating andalusite textures with genetic significance. (a) Sample CLA-07 (granite; Musquodoboit Batholith, NovaScotia) shows reaction rims of andalusite and biotite on cordierite. (b) Sample MOR-01 (rhyolite; Morococala, Bolivia) shows a single fragmentedandalusite in a crystal tuff. (c) Sample GOM-13 (aplite–pegmatite; Penafiel, Portugal) shows an apparent cluster of pink andalusite coresovergrown by colourless andalusite. (d) Sample JAM-03 (aplite–pegmatite; South Mountain Batholith, Nova Scotia, Canada) shows a contactbetween textural type S1 andalusite-bearing pegmatite (upper left) and textural type C1 andalusite-bearing aplite (lower right). Scale barsrepresent 1 mm.
low-temperature, water-saturated, conditions favour General crystallization sequences for andalusite forma- maximum overlap with the andalusite stability field.
Water saturation could occur with, or without, andalusitealready present in the system, and the occurrences of L  Solids ! L  Solids þ Andalusite andalusite in pegmatites, and even quartz veins (Whitney ! L  Solids þ Andalusite þ Aqueous Fluid & Dilek, 2000) demonstrate its ability to nucleate andgrow in a water-saturated environment. Andalusite can L  Solids ! L  Solids þ Aqueous Fluid occur in small pegmatitic melt pods in andalusite-bearing ! L  Solids þ Aqueous Fluid þ Andalusite granulite-facies rocks (Vernon & Collins, 1988), or inlarge zoned pegmatites associated with peraluminous L  Solids ! L  Solids granitoids (Voloshin & Davidenko, 1973; Leal Gomes, þ Aqueous Fluid þ Andalusite: 1984). Individual andalusite crystals may range in sizefrom micrometres to metres, and they commonly occur The first case is just a continuation of Type 2c above, near the quartz-rich and alkali-deficient (least likely namely the magma became fluid-saturated after anda- magmatic) cores of the pegmatites in association with lusite crystallization had begun. Mantling of these other characteristic peraluminous minerals (e.g. spinel, andalusites with magmatic muscovite may follow. In the second case, the creation of a separate aqueous fluid phase precedes the formation of andalusite, and that CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS fluid phase may, or may not, be instrumental in the and Mt. Stafford migmatites representing early, low-F, formation of andalusite in that particular system. The andalusite-rich partial melts, and we propose that sample aqueous fluid phase is also significant in that it facilitates CLA-05 is an extensively melted metapelite.
chemical migration of elements to produce the large Several types of andalusite can cross physical and, andalusite crystals characteristic of some pegmatites.
therefore, classificatory boundaries. For example, a Large crystals of andalusite in coarse-grained pegmatites Type 2a magmatic peritectic andalusite, formed in an include sample GOM-13 with euhedral pink cores incongruent melting reaction, may become a Type 1c (Fig. 13c), and sample TOS-05 (Fig. 4b) with an xenocrystic andalusite in another magma (e.g. BAR-01); apparent reaction relationship between early andalusite a Type 2a magmatic peritectic andalusite, formed during and later, but still primary, muscovite. Andalusite in a melting reaction, may become a Type 2c magmatic sample CLA-12 (Fig. 3c) occurs as apparent clusters of cotectic andalusite after more extensive melting; or a optically continuous grains that are probably large single Type 1c metamorphic xenocrystic or Type 1d meta- skeletal crystals (Clarke et al., 1998).
morphic restitic andalusite may also become the core of Sample JAM-03 (Fig. 13d) shows andalusite occurring a later overgrowth of Type 2c magmatic cotectic andalu- on both sides of an aplite–pegmatite contact: single large site. Restitic (and xenocrystic) andalusite could become textural type S1 crystals on the pegmatitic side, and magmatic Type 2c instead of metamorphic as it equili- clusters of small textural type C1 crystals on the aplitic brates with a granitic melt (but not by direct crystalliza- side. The environment of crystallization seems to be tion from it). In short, a single andalusite grain may have similar to that produced by periodic build-up and release had a lengthy history, and some or all of the evidence of of fluids described by Lowenstern & Sinclair (1996). The its former incarnations is overwritten.
large S1 single andalusite crystals in the pegmatite appearto have grown from the water-saturated melt under staticconditions. The small C1 clusters in the aplite may have Classification and origin of andalusite grown as the result of periodic buildup and release of in the sample set water pressure resulting in saturation of Al2SiO5 in the Electronic Appendix Table A6 shows the key textural silicate melt, either by removal of alkalis from the melt and chemical characteristics of the 108 andalusite-bear- (Clarke, 1981), or by decreasing the water and alumina ing felsic igneous rocks in our sample set and the most solubility in the melt (Acosta-Vigil et al., 2003). Thus, the probable origin of the andalusite in each sample.
andalusite in the aplite may be the result of combinedpressure quenching and compositional oversaturation.
The magmatic case Significantly, sample JAM-03 may be a macrocosm for In general, the evidence in favour of magmatic andalusite what happens in the final interstitial melt of crystallizing granites to produce fine-grained C1, C2, and C3 clusters (1) presence of melt inclusions in andalusite; of andalusite.
(2) euhedral grain shapes of andalusite, especially in volcanic rocks and aplites; (3) grain-size compatibility with minerals acknowl- Type 3 Metasomatic (melt phase absent, edged to be magmatic; fluid phase present) (4) coexisting biotite with Aliv  268  007; Type 3: Metasomatic (5) coexisting biotite–muscovite–apatite in chemical Metasomatic andalusite in felsic igneous rocks is rare, (and probably magmatic) equilibrium; requiring the removal of Ca–Na–K from the solid rock (6) zoning in andalusite that resembles zoning in by aqueous fluids. Such andalusite should be associated other magmatic phases.
with a subsolidus hydrothermal alteration process. Few On the strength of this evidence, we conclude that 99 examples of metasomatic andalusite exist in the litera- andalusite-bearing samples in Electronic Appendix ture; however, Corey (1988) described a high-alumina Table A6 and in Table 3 have one or more textural or hydrothermal alteration zone in the South Mountain chemical criteria to suggest that they are Type 2a–2d Batholith containing And þ Sil þ Spl þ Ms þ Crd þ magmatic in origin, i.e. the andalusite grew from, or at Ap þ Pyr. Sample CLA-05 (Fig. 9a) comes from the same least in the presence of, a silicate melt phase. We recog- locality. What is remarkable about this sample is its high nize a fundamental petrogenetic difference between the modal abundance of andalusite, otherwise the grain sizes, size-compatible single andalusite grains (textural type S; grain shapes, and oscillatory chemical zoning satisfy the Fig. 6) and the size-incompatible (small) clustered conditions to be primary magmatic. This sample is also andalusite grains (textural type C; Fig. 6). Textural type unusual because its coexisting biotite–muscovite tie line S single grains of andalusite may represent Al2SiO5 (Fig. 11b) lies at the lowest F content of our entire spec- saturation in the melt before water saturation, resulting trum of samples. In this regard, it is similar to the Cooma in normal primary magmatic crystallization, whereas JOURNAL OF PETROLOGY Table 3: Numbers of samples with magmatic textural and/or (3) the range of T–P–X conditions for xenocrystic andalusite in granites is greater than the range of T–P–X chemical characteristics conditions for magmatic andalusite in granites; (4) the whole-rock A/CNK parameter is irrelevant if the andalusite in the rock is xenocrystic, and it carrieslittle or no weight when the origin of the andalusite is Andalusite compatible with To illustrate the complexity of the problem of deter- accepted magmatic minerals mining the origin of andalusite, we selected three specific Andalusite smaller than accepted examples (BAR-01, GOT-02, UGI-02 to 07) for a more magmatic minerals detailed examination (Electronic Appendix Table A7). In each case, foreign material is clearly present in the rock, and the textural–chemical evidence for magmatic anda- lusite is equivocal; therefore, a xenocrystic origin for the Textural type S only all volcanic rocks, and andalusite is possible. The result of this type of detailedreconsideration of the observations for the entire sample 8 of 12 migmatites set would be that the frequency of Type 1c may increase Textural type C only at the expense of Types 2a, 2c, and 2d.
Most of our samples appear to contain magmatic anda- Textural types S and C in lusite (Type 2a–d). The greatest problem is to distinguish between Type 2c cotectic magmatic, Type 2a peritectic, and Type 1c xenocrystic (if the assimilation of the foreign Melt inclusions or polycrystalline andalusite is in an advanced stage). Logically, however, if andalusite in volcanics, aplites, pegmatites, and migma- tites is magmatic, why should andalusite in composition- Sharp concentric, sector, or ally equivalent medium- to coarse-grained granites be oscillatory zoning xenocrystic? Also, why should xenocrysts be of uniform Mineral compositions size, and why should they cluster? And what special Samples with biotite conditions must obtain to permit a euhedral andalusite xenocryst from the country rock to remain euhedral in a Coexisting equilibria magma? Furthermore, Type 2a peritectic andalusite can Biotitemuscovite in only be positively identified in its migmatitic spatial con- text; some Type 2a andalusites, removed from their mig- Biotitemuscovite in F equilibrium 46 matitic origins, may be misclassified as Type 2c.
Although these Type 2a peritectic melt reactions mayproduce abundant andalusite (e.g. CLR-01 and CLR- textural type C clusters of andalusite grains may repre- 02), if these reactions were the principal method of gen- sent water saturation before Al2SiO5 saturation, resulting erating andalusite, we might expect to find andalusite in fine-grained ‘quench' clusters of andalusite.
more commonly in peraluminous batholiths, unless itremained in the refractory residuum of partial melting The xenocrystic case or was incorporated into the melt at higher temperatures.
The textural parameters of andalusites that we have used Also, the high temperatures required to generate large (size, shape, inclusions, state of aggregation, relation to quantities of granitic magma are inconsistent with anda- muscovite) are open to alternative explanations. Grains lusite being more abundant than sillimanite in felsic with anomalous sizes, shapes, inclusions, and/or textural igneous rocks. Many felsic magmas must have been relations with magmatic rock-forming minerals (or other Al2SiO5-undersaturated while in the sillimanite stability metamorphic minerals) are likely to be Type 1c xeno- field, and reached critical saturation in Al2SiO5 only at crystic (e.g. BBR-01,02 based on anomalously large size, low temperatures in the andalusite stability field.
irregular shape, chiastolite inclusions, and obvious reac-tion rims). General arguments in favour of a xenocrystic C ONT ROLS ON THE FORMATION origin for andalusite include: (1) euhedralism is not an exclusively magmatic OF MAGMATIC ANDALUSITE (2) many peraluminous granite magmas originate at Most of the andalusites in our sample set satisfy one or depths incompatible with andalusite stability; more of the textural and chemical criteria for a magmatic CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS origin. On this basis, the stability fields of andalusite and Table 4: Amount of fractional crystallization of feldspars naturally occurring felsic magmas must overlap; how- required to reach A/CNK ¼ 120 and 130 from magmas ever, many peraluminous felsic igneous rocks do not with a range of (A/CNK) contain andalusite, and many plutons do not contain andalusite throughout, but rather only in restricted facies.
Special conditions must obtain for the formation of that andalusite (Clemens & Wall, 1981, 1988; Pati 1992). In this section, we focus on the variables that may contribute to the formation of magmatic andalusite in felsic igneous rocks.
Conditions favourable for the formation of magmatic andalusiteAttaining the critical A/CNK ratio in the Values are calculated using the Rayleigh fractionation equation CL ¼ Co(FD1), where CL is the final excessalumina in the melt, Co is the initial excess alumina in the Metaluminous felsic igneous rocks do not contain pri- melt, F is the melt fraction remaining, and D ¼ 0.
mary magmatic andalusite, but peraluminous felsicigneous rocks can. (If the andalusite is magmatic,its crystallization should be the consequence of high would be Type 2a peritectic and Type 2c cotectic (e.g.
A/CNK in the bulk magma composition; however, if the the Macusani volcanics).
andalusite is xenocrystic, the high A/CNK in the bulk Increase of (A/CNK)i by feldspar fractionation. Large volumes rock may be merely an artifact caused by the addition of of partial melt derived from mixed metapelite–metagrey- andalusite and other peraluminous phases.) The effect of wacke sources will have A/CNK >1, but may also be excess alumina in a magma is two-fold: (1) it creates a more favourable T–P range for andalusite crystallization A/CNK does a magma reach saturation in Al2SiO5? by depressing the granite solidus (Abbott & Clarke, 1979; From experimental studies, melts approaching equili- Holtz et al., 1992; Joyce & Voigt, 1994), enlarging the brium with Al2SiO5, or other strongly peraluminous region of overlap with the andalusite stability field; (2) it phases, have A/CNK values of c. 130–135 ( Joyce & creates a more favourable compositional condition Voigt, 1994; Scaillet et al., 1995). Similarly high A/CNK because, the more peraluminous a magma is, the greater values also occur in melt inclusions in quartz in the is the probability that it will become saturated in Al2SiO5 Morococala volcanics. For those of our samples for and crystallize andalusite. Halliday et al. (1981) described which we have whole-rock chemical analyses, the mean a number of processes by which a magma can increase its A/CNK value is 119  008, indicating that many peraluminosity. In this section, we focus on three com- samples have values lower than those suggested by positional conditions to enhance the crystallization of experimental work as being necessary to saturate a magma in aluminosilicate.
Inheritance of A/CNK from the source region. A magma with a The question is: can a low A/CNK melt evolve to high initial A/CNK [(A/CNK)i] inherited from pelitic A/CNK levels high enough to nucleate Type 2c cotectic source rocks will favour crystallization of andalusite.
andalusite? For any melt with A/CNK >1 that is fractio- Assimilation of highly peraluminous country rocks may nating feldspars only, the mole fraction of alumina in the also produce the same effect on alumina saturation, but melt that is not charge balanced by alkalis doubles for may result in xenocrystic andalusite rather than mag- every 50% of fractional crystallization. Table 4 shows the matic andalusite. In such cases, the source or contamin- percent of such feldspar crystallization required to reach ant controls the peraluminous character of the melt.
two levels of A/CNK (120 and 130). If the (A/CNK)i in Progressively more peraluminous melts will be produced the magma is sufficiently high (last row), little or no in equilibrium with the following residual assemblages: fractional crystallization is needed to reach Al2SiO5 (a) biotite; (b) biotite þ garnet; (c) biotite þ cordierite; saturation. Monomineralic andalusite zones in the D5 (d) biotite þ andalusite/sillimanite/kyanite. In the last pegmatite at Arreigada, Pacos de Ferreira, Portugal case, the melt is already saturated in Al2SiO5 at the (Electronic Appendix Table A1) appear to have formed time of segregation from the source rocks, and as long as the result of feldspar fractionation (Leal Gomes, 1984).
as that saturation is maintained, the appearance of anda- The mean A/CNK values in the South Mountain lusite on the liquidus is inevitable (provided the T–P path Batholith, Nova Scotia, range from 116 in early grano- of the magma passes through the andalusite stability diorites to 123 in late leucogranites (Clarke et al., 2004), field). Andalusites produced in magmas such as these and many of its moderately evolved rocks contain JOURNAL OF PETROLOGY andalusite (samples CLA-01 to 14, ERD-01, JAM-03).
Is vapour saturation and consequent alkali partitioning, The discrepancy between the A/CNK required to nucle- or stripping, an adequate mechanism for achieving ate andalusite in natural and synthetic systems can be aluminosilicate saturation in felsic melts? In a worst-case reconciled if the naturally occurring andalusite grows as a scenario, calculations for the haplogranite minimum late product from a more highly evolved interstitial melt.
composition Ab39Or26Qz35 (wt %), A/CNK ¼ 1, and Although, theoretically, feldspar fractionation can 55 wt % H2O (near saturation at 2 kbar), show that if all drive all (A/CNK)i >1 melts to Al2SiO5 saturation, in of the water in the haplogranite minimum melt were practice saturation with, and fractionation of, other to exsolve as a single batch, it would require phases such as cordierite, biotite, and muscovite will  3–4 to reach A/CNK retard, or even prevent, the magma from reaching  130. These D values appear to be too high, given that saturation in andalusite. Conversely, contamination work with macusanite indicates D vapour/melt with pelitic material may assist the increase in A/CNK.
to be of the order of 01 (London et al., If fractional crystallization is responsible, andalusite will 1988), approximately an order of magnitude lower than be restricted to those parts of the intrusion that are the value estimated for the Spoor Mountain rhyolite chemically highly evolved, or to the last interstitial melt.
(Webster, 1997). Even assuming D values of unity, com- This prediction is in general agreement with other indices plete vapour exsolution would change the A/CNK from of fractional crystallization such as low concentrations of 100 to c. 106, clearly insufficient if (A/CNK)i were so compatible elements (Ti, Sr, Zr, Ba) and high concentra- low, but much more effective if A/CNK of the melt were tions of incompatible elements (Rb, Cs, Li) in andalusite- 120 when water saturation occurred.
bearing facies of peraluminous plutons.
An alternative is to consider the role of chlorine in complexing Na and K in the vapour phase. When Increase of (A/CNK)i by evolution of an aqueous fluid phase. The Dvapour/melt ¼ 1, a fluid with 14 wt % NaCl þ KCl common occurrence of andalusite in aplites (Figs 3b, c, requires 0475 g of Cl per 55 g of H2O. This amount of 5b and 13d), which are normally associated with fluid- Cl exceeds the known solubility of Cl in H2O-saturated saturated pegmatites, and in fluid-saturated pegmatites (in fluid approaching unity) melt by a factor of 2, and themselves (Figs 4b, 9d, and 13c and d), suggests a role for also greatly exceeds the known solubility of Cl in anhy- fluids in the production of andalusite. The large euhedral drous granitic melts. As shown by Webster (1997), the crystals of andalusite in some pegmatites (TOS-05, solubility of H2O in haplogranitic melt at 2 kbar Fig. 4b; GOM13, Fig. 13c; JAM-03, Fig. 13d) show that decreases from about 55 wt % at 02 wt % Cl in the andalusite is stable with hydrothermal fluids (Cesare, melt to essentially 0 wt % H2O at 03 wt % Cl in melt.
1994; Whitney & Dilek, 2000), and that andalusite does Rayleigh fractionation of Cl-complexed alkalis into a not react under all conditions to become secondary mus- highly mobile fluid phase can, at least to some extent, covite. Any andalusite formed under such conditions enrich the residual silicate melt in A/CNK and, depend- would be Type 2d water-saturated magmatic.
ing on the A/CNK of the melt at the time of water We now consider whether some fluid-related process saturation, take the silicate melt over a critical threshold could be effective in attaining the A/CNK levels neces- to Al2O5 saturation, or possibly even to A/CNK compo- sary to crystallize andalusite.
sitions unreachable by melt–solid equilibria alone.
(1) Effect of water on shifting primary phase volumes. In Most felsic magmas that evolve a separate aqueous general terms, the appearance of a stability field for a fluid fluid phase never nucleate andalusite, so water saturation phase will have an effect on the sizes, shapes, and posi- alone is not a sufficient condition to form primary mag- tions of the primary phase volumes for all other phases, matic aluminosilicates. If the fluid saturation occurs while including andalusite. The effect of such reconfiguration the magma is inside the andalusite T–P stability field, of the liquidus topologies on the probability of crystal- andalusite may nucleate if the A/CNK of the melt lizing andalusite in complex natural systems is unknown.
reaches saturation in Al2SiO5.
(2) Effect of water on stripping alkali elements from the melt.
(3) Effect of water on Al solubility in silicate melts. The Evolution of a separate water-rich fluid phase (aH O1) experimental work of Acosta-Vigil et al. (2002, 2003), has the potential to raise the A/CNK of the silicate melt based on saturating haplogranite starting materials from which it evolves by preferential partitioning of Na individually with a variety of aluminous phases, shows and K into the fugitive fluid phase. This process could that alumina solubility in the melt depends on the potentially take the melt composition to ‘hyperalumi- coexisting phase, the temperature, and the water content of the melt. In particular, the positive correlation equilibria (Clarke, 1981). Evidence of high degrees of between XH O and A/CNK suggests that alumina alkali-element mobility in fluid phases in plutons includes solubility is a direct function of water content of the saline fluid inclusions (Na, K, Cl removal), albitization melt phase. Thus, alumina solubility in the melt is (Na addition), and greisenization (K addition).
maximized at high water contents, and decreases when CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS that water is released (and when the temperature falls).
Although the effect of beryllium is to lower the haplo- The loss of aqueous fluid from the melt, and reduction of granite solidus slightly (Evensen et al., 1999) into the water content of the melt, reduces the solubility of alumina andalusite stability field, the effect of beryl crystallization and results in the crystallization of a phase with A/CNK would be to reduce A/CNK in the melt and diminish the >1. The particular peraluminous phase that crystallizes probability of nucleating andalusite. None of our samples is determined by the precise phase relations for that shows any obvious mineralogical evidence of high Be T–P–X condition. Under suitable conditions, that phase contents, thus we judge that this element is not a factor could be andalusite, or some other peraluminous phase in promoting the presence or absence of andalusite in our such as garnet or cordierite (Rapela et al., 2002).
In conclusion, if a spatial–temporal–genetic correlation The effect of high levels of boron is to depress the of fluids and andalusite exists in plutons as a whole, then haplogranite solidus by several tens of degrees Celsius high A/CNK levels in the granitic rocks, and the appear- (Chorlton & Martin, 1978; Manning & Pichavant, ance of primary magmatic andalusite, may lie close to the 1983; Acosta-Vigil et al., 2001; Kawakami, 2001), thereby region of initiation of water saturation. This andalusite-in expanding the region of overlap with the andalusite sta- ‘isopleth' in plutons represents the place where vapour bility field. However, the precipitation of tourmaline saturation has driven the composition of the melt phase might be expected to lower the A/CNK of the silicate against the andalusite primary phase volume. The melt, and diminish the probability of nucleating andalu- appearance of (water-saturated) magmatic andalusite in site. If high levels of boron were responsible for the one place in a pluton, and the occurrence of sub-solidus formation of andalusite in most of our samples, little alkali metasomatism in another place (albitization, K- mineralogical evidence of its presence exists in our sam- feldspathization, and greisenization) could be comple- ple set; nevertheless, seven of our andalusite-bearing mentary processes in the late stages of evolution of rocks (GOM-04, JAM-03, RIC-03, RIC-04, UGI-03, peraluminous granites. As a consequence, the same fluids VIL-03, and VIS-03) do contain texturally compatible that are indirectly responsible for the formation of anda- tourmaline, suggesting that, in these rocks at least, the lusite in aplite–pegmatite systems in one part of a pluton positive effect of boron on lowering the granite solidus might destroy Types 1a–2c andalusite elsewhere in the was greater than the negative effect of Al-diversion to same pluton.
As with the other light-element cations, the effect of Lowering of the granite solidus lithium is to lower the haplogranite solidus by several tensof degrees Celsius (Wyllie & Tuttle, 1964; London & Even with a suitable A/CNK ratio, andalusite will not Burt, 1982; Martin & Henderson, 1984), thereby expand- crystallize from a felsic melt unless the stability field of ing the primary phase field of andalusite. Lithium is also andalusite overlaps that of the melt. In this section, we able to sequester Al in, for example, spodumene or consider several ways in which the granite solidus may be amblygonite otherwise possibly destined to become pri- lowered (beyond the effect of high A/CNK itself) to mary aluminosilicate minerals.
increase the probability of overlap with the stability field The only sample from a pluton with measured lithium of andalusite.
contents in its micas is KOL-01 (Bt with 1100 ppm Li, High concentrations of Be–B–Li. In general, high concentra- and Ms with 500 ppm Li). Otherwise, the equations of tions of Be–B–Li in granitic rocks are normally associated Tischendorf et al. (1997, 1999) yield the following esti- with pegmatites, and those pegmatites can contain anda- mates for lithium (Li2O wt %) contents in the micas of our lusite. An example is the Alburquerque pluton, Spain sample set: biotite—range 012–115, mean 028  019; (London et al., 1999). In it, only the marginal granites muscovite—range 002–127, mean 018  019. If the contain andalusite, whereas the pegmatitic facies and between muscovite and melt is  05 (Walker et al., dykes do not. Remarkably, the pegmatites have much 1989), the melt compositions contained roughly double higher A/CNK ratios than the associated granites. The the Li2O concentrations estimated for the muscovite high Be–B–Li content appears to stabilize other Al-rich samples. Our sample set shows no obvious mineralogical phases (beryl, tourmaline, spodumene), instead of anda- evidence of high lithium contents, and the only effect of lusite. In general, the effects of high concentrations of the its presence is to have a small effect on lowering the light elements (Be, B, Li) are opposing: (1) increasing the granite solidus. High lithium contents, therefore, do not probability of stabilizing primary magmatic andalusite by seem to be essential to the appearance of andalusite in lowering the granite solidus (Fig. 1), but (2) decreasing the felsic igneous rocks.
probability of stabilizing primary magmatic andalusite bydiverting Al from potential andalusite into such minerals High concentrations of P. The presence of phosphorus has a (or chemical components) as beryl, tourmaline, spodu- complex effect on the effective value of A/CNK in the mene, and berlinite.
silicate melt phase. In general, the higher the P content of JOURNAL OF PETROLOGY the whole rock, the higher the true A/CNK must have (4) At higher concentrations of fluorine, topaz been in the melt because phosphorus decreases the Ca appears to form instead of andalusite (London et al., term in the denominator by an amount equivalent to 1999, 2001; Neves et al., 1999; Clarke & Bogutyn, 2003).
33 P. However, another effect is the amount of P in the Some peraluminous felsic igneous rocks should show feldspars (Kontak et al., 1996). Through the berlinite evidence of the peritectic reaction substitution, Al3þ P5þ $ 2Si4þ, A/CNK >1 in fractio- And þ F-richMelt ! Topaz nating feldspars, and thus the effect of P is to reduce theeffectiveness of feldspar fractionation to increase A/CNK but none is present in our sample set.
in residual melts. Also, if P and Al form discrete anionic (5) At even higher fluorine contents, F may complex complexes that do not co-polymerize with the principal with Al and Na to form cryolite-like species that would aluminosilicate framework of the melt, the net effect of decrease aAl O in the silicate melt and decrease the increased P (at constant Al) is a decrease in a probability of andalusite precipitation.
case, it will require a higher A/CNK to produce satura- To summarize, our samples are probably representa- tion in an aluminous phase such as andalusite. The one tive of high A/CNK, andalusite-bearing rocks, but not environment in which this effect may be important is in necessarily all high A/CNK rocks. In most of our rocks, highly fractionated pegmatite systems, where Al–P-rich Be–B–Li–P appear not to be important chemical compo- derivative liquids also concentrate enough other lithophile nents, and we conclude that these elements may have had components to stabilize late-stage phosphates [e.g. chil- only a small effect on lowering the granite solidus to promote the formation of andalusite. Had we specifically 4(OH)2.H2O, lithiophilite (LiMnPO4)– investigated high A/CNK, high Be–B–Li–P, granites instead, we might have concluded that andalusite is 2(PO4)(F,OH)], which may then liberate complexed Al and increase a only a rare mineralogical constituent, and that the nega- Al O . This process could, at least in part, lead to late-stage precipitation of aluminosilicates.
tive effect of Be–B–Li–P on the formation of andalusitehad prevailed because high concentrations of these High concentrations of F. The effect of F is to lower the elements lowered aAl O and prevented formation of granite solidus (Manning & Pichavant, 1983), thereby andalusite. In contrast, our samples contain a wide expanding the andalusite primary phase volume.
range of F contents (as deduced from the F concentra- Clemens & Wall (1988) suggested that high fluorine con- tions in the micas). We conclude that fluorine is not tents should stabilize And þ Bt relative to Grt/Crd þ necessary to stabilize andalusite, at least in plutonic Kfs. Our extensive chemical data permit several observa- rocks, and that high levels of fluorine serve to destabilize tions and deductions about the role of fluorine in the andalusite and form topaz, or to form complexes that formation of andalusite in felsic igneous rocks, as follows.
reduce the probability of nucleating andalusite.
(1) The extremely wide range of fluorine contents in our andalusite-bearing granitoids suggests that F con- Expanding the andalusite stability field centration, at least alone, is not the controlling factor in Small amounts of transition-element solid solution in the appearance of andalusite.
andalusite have a potentially significant effect on its P–T stability field (Grambling & Williams, 1985; Kerrick & muscovite–biotite system showing that the concentration Speer, 1988; Kerrick, 1990; Pattison, 1992, 2001; of fluorine correlates with the degree of mutual solubility andez-Catuxo et al., 1995), in particular by shifting of the two micas. In the multicomponent natural system, the And ¼ Sil boundary upward by 50–100C. Although the presence of fluorine must modify the sizes, shapes, minor elements in andalusite may be important in stabi- and positions of the primary phase volumes of biotite lizing andalusite over sillimanite in some magmatic situa- and muscovite. If they change, then so must also the tions, many andalusites contain negligible concentrations primary phase volumes of coexisting phases such as of transition elements.
andalusite, cordierite, garnet, etc. The implication of The andalusites with Fe-rich pink cores in our sample such changes in the topology of the relevant natural set (Figs 3–5, 8 and 9), including the completely pink phase diagram is to change the probability, and order, of andalusites in the Morococala volcanics, suggest that crystallization of all phases, including andalusite. High F minor elements, such as Fe3þ, may be important in contents will stabilize muscovite to higher temperatures, initially stabilizing andalusite or reducing its energy bar- thereby shrinking the andalusite primary phase volume.
rier for nucleation. Subsequent overgrowths of Fe-poor (3) The high F contents of the biotites in two of the andalusite may take place more readily on the nuclei of volcanic samples (MOR-01 and PIC-01, Electronic Fe-rich andalusite, but the sharp core–rim compositional Appendix Table A2) may indicate, however, that fluorine boundaries remain a problem. The general absence of a has a role in expanding the andalusite primary phase field compositional gradient suggests that some T–P–X para- at low-pressure, water-undersaturated conditions.
meter changes abruptly.
CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS Fig. 14. Formation of magmatic andalusite in pressure–temperature space. In this petrogenetic grid for the formation of andalusite in felsicigneous rocks, the AND MAX field is defined by the water-saturated peraluminous granite solidus (Johannes & Holtz, 1996), the And ¼ Silreaction (R69; Richardson et al. 1969), and the stoichiometric muscovite breakdown curve. (a) Isobaric cooling paths. (b) Adiabatic decompressionpaths. Heavy dashed line is a schematic water-saturation curve for granitic melts.
Kinetic considerations effects of H2O are probably the most important.
Pattison (1992) noted that, in metamorphic rocks, anda- Figure 14 summarizes the formation of magmatic anda- lusite is a common metastable relict phase, persisting well lusite under the most favourable conditions with a large upgrade of the first occurrence of sillimanite. If the And ¼ T–P stability field for andalusite (‘AND MAX') from Sil reaction is sluggish on geological time scales (105– Fig. 1b, utilizing the And ¼ Sil curve of Richardson et al.
107 years), it raises questions about the accuracy of (1969) and the most favourable melt composition (satu- laboratory determinations (e.g. 25C) on experimental rated in Al2SiO5 throughout). Whether the And ¼ Sil time scales (102–100 years), especially considering the stability curve intersects the muscovite breakdown curve difficulties of such work (Kerrick, 1990; Holdaway & above the granite solidus (as the R69 curve does in Mukhopadhyay, 1993; Pattison, 2001). Samples NEV- Fig. 14), or below the granite solidus (as the P92 curve 04 and WHI-01 (Fig. 9f ) show remarkable persistence would do in Fig. 1a), is important to the petrogenetic of andalusite in the stability field of sillimanite.
interpretations that follow. The R69 case permits some, In the case of rising temperature, magmatic andalusite or all, of the muscovite rims on andalusite (Figs 4, 5 and may continue to grow in the stability field of sillimanite 7) to be magmatic in origin; the P92 case restricts all the but, unless seeded, andalusite is unlikely to nucleate muscovite rims to be subsolidus in origin. Further work metastably in the sillimanite stability field. Interestingly, on the textures and compositions of those muscovite rims in the case of no overlap between the stability fields of is needed to determine their origins, but the variation in granite melt and andalusite (Fig. 1a), moderate (20–50C) their grain sizes and relations with the andalusite suggest to strong amounts of undercooling may result in stable, that a single subsolidus origin is unlikely. Therefore, at but rapid, growth of andalusite from metastable melt.
least in some compositional situations, the And ¼ Sil Some of the andalusites in our samples do appear to have curve lies between P92 and R69 and intersects the mus- quench textures (CLA-12, ELB-01, ERD-01, JAM-03).
covite stability curve above the granite solidus. Using the They may be the result of rapid stable growth or rapid AND MAX field in Fig. 14 allows us to examine the metastable growth, but they are almost certainly not implications of this situation.
Figure 14 subdivides the possible crystallization paths into two limiting categories: isobaric slow crystallization,and adiabatic rapid crystallization. Intermediate T–Ppaths combine features of these extremes. The isobaric PETROGENETIC MODEL crystallization paths (Fig. 14a) fall into three pressure Of the many possible controls on the formation of anda- domains, delineated by two critical invariant points.
lusite in felsic magmas, the level of A/CNK and the Isobaric cooling in the low-pressure Domain P1 results JOURNAL OF PETROLOGY in magmatic sillimanite (if T were sufficiently high), andalusite field, only those with favourable compositional followed by magmatic andalusite at temperatures conditions of fractionation-enhanced or fluid-enhanced closer to the solidus. Any muscovite in these rocks enrichment in A/CNK will attain Al2SiO5 saturation.
must be subsolidus. Isobaric cooling in the moderate- Magmas with low A/CNK may pass through the entire pressure Domain P2 results in magmatic sillimanite P–T andalusite stability field, whatever its size, and never (again, if the temperature were sufficiently high), followed nucleate andalusite. For those cases in which fluid evolu- by magmatic andalusite. That andalusite then becomes tion can achieve critical saturation of Al2SiO5 in the melt, involved in a peritectic reaction with the melt to produce the T–P conditions must be below the water saturation magmatic muscovite. At this stage of our investigation, curve (schematic heavy dashed line).
we do not know if a muscovite rim on an anhedral Figure 15 is a graphic summary of this view that the andalusite represents a slow peritectic partial resorption most important controls on the formation of magmatic of a euhedral andalusite, or a fast overgrowth on an andalusite are A/CNK and fluid saturation in the melt. It anhedral andalusite. Under conditions of perfect equili- depicts five possible crystallization paths, three of which brium crystallization, andalusite is eliminated in this reac- produce andalusite. The textural type of andalusite tion, and all that remains is magmatic muscovite (Fig. 7a).
depends on whether the magma reaches Al2SiO5 satura- Under conditions of perfect fractional crystallization, tion before water saturation (mainly textural types S1– magmatic muscovite may overgrow, but not entirely S3), or water saturation before Al2SiO5 saturation (tex- replace, the andalusite. In the high-pressure Domain tural types C1–C3).
P3, andalusite never crystallizes, thus the invariant The precise paths depend on the assemblage of phases point at about 650C and 48 kbar represents an crystallizing and the sequence in which they appear. In upper pressure limit for the formation of magmatic detail, the crystallization paths, in order of increasing (A/ CNK)i, are as follows.
The adiabatic decompression paths similarly consist of Path 1. Initial melt is haplogranitic [(A/CNK)i ¼ 1] and three domains (Fig. 14b). In the high-temperature Domain T1, the rocks may, or may not, show magmatic unchanged in the residual melt, water saturation is sillimanite followed by magmatic andalusite (possibly never attained, and no andalusite forms.
sample GOT-02) and the texture should be saccharoi- Path 2. Initial melt is weakly peraluminous and water dal–aplitic or volcanic. Any muscovite in these rocks must undersaturated. Fractionated magma does not reach be either subsolidus (not shown on the path), or grains saturation in aluminosilicate, but it does reach water crystallized at a higher pressure from a greatly expanded saturation, and eventually becomes an ordinary two- stability field (high Fe or F?). The greater the expansion of mica granite.
the muscovite stability field, the more andalusite crystal- Path 3. Initial melt is moderately peraluminous and lization is restricted to low pressures. In the extremely water undersaturated. Cystallization of anhydrous phases narrow moderate-temperature Domain T2, primary raises the A/CNK of the melt, but the melt reaches water magmatic muscovite may be followed by primary mag- saturation before Al2SiO5 saturation. The evolution of an matic andalusite. None of our samples shows an over- alkali-bearing fluid phase may provide the essential growth of andalusite on muscovite, although such a increase to A/CNK to drive the residual melt against, textural relationship is not required. Finally, in the low- or even into, the aluminosilicate field resulting in crystal- temperature Domain T3, the rocks contain magmatic lization of andalusite. Alternatively, adiabatic decom- muscovite, and potentially late subsolidus muscovite, pression of the water-saturated melt would reduce the but no magmatic andalusite.
solubility and concentration of water in the melt, and Less favourable compositional conditions for crystal- decrease the threshold of Al2SiO5 saturation to encom- lizing magmatic andalusite include restriction of the pass the melt composition, resulting in andalusite preci- andalusite stability field (water-saturated solidus curve pitation. In the first case, the melt enters the andalusite moves to higher temperature because of lower Al2O3, stability field, and in the second case, the andalusite lower Be–B–Li–P–F, higher Ca, aH O < 1, etc.; the stability field overtakes the melt composition. In either And ¼ Sil curve moves to lower temperature because of case, the result is crystallization of andalusite, probably as lower minor constituents such as Fe; and the muscovite quench clusters.
stability field expands as a result of Fe and F solubility). If Path 4. Initial melt is highly peraluminous and water Al2O3 decreases, the A/CNK composition of the melt is undersaturated. It reaches aluminosilicate saturation by less favourable for crystallizing Al2SiO5, i.e. the AND fractionation alone to form single grains of andalusite, MAX field shrinks as A/CNK ! 1. With a much smaller and may later form andalusite from the evolved fluid stability field for andalusite, many more P–T crystalliza- tion paths for felsic magmas will not intersect it. For Path 5. Initial melt is aluminosilicate saturated from the crystallization paths that do intersect the reduced outset and water undersaturated. It can contain single CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS Fig. 15. Plot of A/CNK vs percent crystallization of haplogranitic and peraluminous melts. Double-headed arrows show the key system variables:(A/CNK)i of the melt and (A/CNK)f of the fractionating phases; (A/CNK)c, the critical value of A/CNK required to saturate the melt inaluminosilicate; and aH O ¼ 1, the point in the crystallization history at which the melt becomes saturated in water. Path 1 crystallizes only quartz and feldspar. Path 2 crystallizes micas as the mineralogical expression of the weak peraluminosity of the melt. Path 3 reaches Al2SiO5 saturationeither by a combination of feldspar fractionation and water saturation, or by decrease in pressure causing a decrease in water and aluminasolubility. Path 4 reaches Al2SiO5 saturation by feldspar fractionation alone. Path 5 is saturated in Al2SiO5 from the outset.
andalusite (or sillimanite) grains that may be magmatic, melts probably reach Al2SiO5 saturation by evolving a or xenocrystic, or both.
fluid phase. Only the last (interstitial) melt fraction attains According to this model, andalusites in peraluminous sufficiently high A/CNK to nucleate andalusite. Whether felsic igneous rocks are texturally and genetically bimo- it is the escape of alkalis from the melt, or the reduction in dal: those magmas that reach Al2SiO5 saturation before alumina solubility in the melt, the effect is the same; water saturation contain single grains of andalusite; those namely, to saturate the melt in Al2SiO5 and to crystallize magmas that reach water saturation before Al2SiO5 andalusite. This model for andalusite formation may saturation (and Al2SiO5 saturation by water saturation), explain why many andalusites occur as clusters in late contain clusters of andalusite. Some rocks may contain interstitial patches surrounded by reaction rims of mus- both single andalusite grains and clusters of andalusite covite. It can also explain why many of the andalusite- grains (e.g. when Path 4 or 5 above reaches water satura- bearing whole-rock compositions have lower A/CNK tion). As discussed previously, sample JAM-03 (Fig. 13d), ratios than experimentally determined values necessary with S1 and C1 andalusites, may be a macrocosm for for Al2SiO5 saturation. Finally, if a correlation exists what happens in the final interstitial melt of crystallizing between the appearance of andalusite and appearance granites. The interstitial melt reaches water saturation, of a fluid phase, the number of Type 2d water-saturated and the release of the water causes a small quench cluster magmatic andalusites may be even greater than believed.
of andalusite to form.
The formation of muscovite reaction rims is indepen- dent of the mode of andalusite formation. Both textural SUMMAR Y A ND CONCL USIONS types of magmatic andalusite may develop a muscovitereaction rim, depending on the conditions prevail- Occurrence of andalusite in felsic ing when the system encounters the muscovite stability field. Monocrystalline muscovite rims may be mag- Andalusite is an accessory mineral occurring in a wide matic, whereas polycrystalline muscovite rims may be range of peraluminous felsic igneous rocks, including volcanic rocks, aplites, granites (commonly only in We conclude that, because most of our whole-rock spatially restricted regions of plutons), pegmatites, and samples have A/CNK values less than the experimentally anatectic migmatites. It can occur as single grains, or as determined values needed to saturate in Al2SiO5, and clusters of grains, with or without overgrowths of because the effects of fractionation of feldspars and muscovite. It is commonly subhedral, compatible in (biotite þ cordierite) counterbalance, many late residual grain size with its host rock, has an iron-rich JOURNAL OF PETROLOGY (01–17 wt % FeO) core, and is normally not associated Controls on the formation of andalusite with sillimanite.
Of the many possible controls on the formation of anda-lusite (excess Al2O3, water concentration and fluid evolu- Textural and chemical criteria for tion, high Be–B–Li–P, high F, high Fe–Mn–Ti, and determining the origin of andalusite kinetic considerations), the two most important factors Textural and chemical evidence for a magmatic origin is appear to be excess Al2O3 and the effect of releasing strong for volcanic, aplitic, pegmatitic, and migmatitic water (either to strip alkalis from the melt or to reduce andalusites, but is equivocal for medium- to coarse- alumina solubility in the melt).
grained plutonic andalusites. We considered grain size,grain shape, textural relations with other minerals, inclu- Origin of andalusite sion relations, chemical compositions, chemical zoning, Our deductions about the origin of andalusite rest on a and chemical equilibria with other minerals as tests for sample set that encompasses a wide range of andalusite- determining the origin of andalusite in a given rock. In bearing felsic igneous rocks world-wide. The strongest general, inclusion relations (other than melt inclusions) evidence for a magmatic origin for andalusite includes and andalusite chemical compositions (except possibly grain-size compatibility (including fine-grained clusters), zoning patterns) are generally not useful criteria, but the euhedral or quench shapes, melt inclusions, and equili- other criteria are extremely useful in specific cases. The brium chemical compositions of coexisting phases. We chemical-equilibrium-with-other-minerals test shows that believe that the majority of andalusite in peraluminous many rocks have biotite–muscovite–apatite compositions felsic igneous rocks is of magmatic origin and, therefore, a that appear to be in equilibrium with each other, and significant stability region exists for magmatic andalusite, with andalusite, suggesting that they all have the same delimited by the water-saturated granite solidus, the magmatic origin.
And ¼ Sil reaction, and the stability field of muscovite.
The size of the magmatic andalusite stability field can Genetic classification of andalusite expand or contract depending on a number of composi- We have proposed the following genetic classification of tional parameters and, thus, the reactions shown as andalusite in felsic igneous rocks.
discrete lines in Fig. 14 are in practice broad zones.
The strongest evidence for a xenocrystic origin for Type 1 Metamorphic andalusite includes grain-size incompatibility, anhedral (a) in situ prograde (resulting from thermal metamorph- grain shapes, textural disequilibrium (reaction rims), ism of peraluminous granitic rocks); and general matching of textural and chemical para- (b) retrograde (resulting from inversion of sillimanite); meters with andalusite in the country rock. Xenocrysts (c) xenocrystic (generally anhedral, many inclusions, of andalusite do occur, but they are not as common as spatial proximity to country rocks and/or pelitic xeno- magmatic ones. True restitic andalusite is probably rare because the region of significant generation of granitoid (d) restitic (residua of partial melting, generally magma occurs at higher temperatures and pressures than anhedral with inclusions of high-grade metamorphic permitted by the andalusite stability field. Occurrences of metasomatic andalusite may represent only volumetri-cally small and special cases.
Partial melting of semi-pelitic material under high pres- (a) peritectic (water-undersaturated, T"), subhedral to sure in the middle to lower crust probably results in the euhedral, associated with leucosomes in migmatites; formation of peraluminous granitic melts in equilibrium (b) peritectic (water-undersaturated, T#), subhedral to with garnet ( sillimanite), not andalusite (Green, 1976; anhedral, as reaction rims on garnet or cordierite; Vielzeuf & Holloway, 1988). Ascent (T# P#) of the (c) cotectic (water-undersaturated, T#), euhedral, grain- magma may bring those melts into the stability field of size compatibility with host rock, few inclusions; andalusite. Alternatively, partial melting of pelitic ma- (d) pegmatitic (water-saturated, T#), large subhedral to terial under lower pressures in the middle to upper crust euhedral grains, associated with aplite–pegmatite con- results, in many cases, in the formation of andalusite in tacts or pegmatitic portion alone.
the same peritectic reaction that forms the peraluminousmelt phase. Andalusite may be a normal primary mag- Type 3 Metasomatic matic mineral in some water-poor peraluminous mag- Water-saturated, magma-absent conditions; andalusite mas. Fractional crystallization of feldspars may increase spatially related to structural discontinuities in the host the A/CNK ratio of the original magma to andalusite rock, coincident replacement of feldspar and/or biotite, saturation and precipitate single cotectic grains of intergrowths with quartz.
andalusite. Alternatively, the inevitable attainment of CLARKE et al.
ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS water-saturated conditions in the magma can also result in the precipitation of andalusite from the silicate melt.
Abbott, R. N. & Clarke, D. B. (1979). Hypothetical liquid relationships Escape of fluid, with consequent depletion of alkalis or in the subsystem Al2O3–FeO–MgO projected from quartz, alkali reduction of Al-solubility in the melt, creates saturation or feldspar and plagioclase for a(H2O) ¼ 1. Canadian Mineralogist 17, oversaturation in Al2SiO5. Continued release of aqueous fluid keeps driving the interstitial residual melt into the Acosta-Vigil, A., Pereira, M. D., Shaw, D. M. & London, D. (2001).
Contrasting behaviour of boron during crustal anatexis. Lithos 56, 2SiO5 (over)saturation region, removing heat from the system, and resulting in the precipitation of quench clusters of optically discontinuous andalusite grains.
Acosta-Vigil, A., London, D., Dewers, T. A. & Morgan, G. B., VI (2002). Dissolution of corundum and andalusite in H Clustered andalusites are at least as common as indivi- haplogranitic melts at 800C and 200 MPa: constraints on dual grains, even more so in plutonic rocks.
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ditions are thermodynamically or kinetically unfavour- (2003). Solubility of excess alumina in hydrous granitic melts in able. Either the bulk composition is right but it is not in equilibrium with peraluminous minerals at 700–800C and the T–P stability field of andalusite, or T–P conditions are 200 MPa, and applications of the aluminum saturation index.
Contributions to Mineralogy and Petrology 146, 100–119.
right, but the melt A/CNK is inappropriate, or both.
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HOW TO CREATE A STRUCTURAL SUMMARY Rapid Reference 4.1 Preparing the Transparency Template HOW TO CREATE A STRUCTURAL 1. Photocopy the templates from Appendixes 5a and 5b onto an 8'' x 11'' SUMMARY FOR THE RORSCHACH transparency. Do not enlarge or reduce the size of the form. (It's ofteneasiest to take the book to a full-service photocopy store where you canpurchase a single transparency and have them photocopy it for you.)


Vi ringraziamo per aver acquistato una fotocamera digitale Olympus. Prima di iniziare ad usare la vostra fotocamera, leggete attentamente queste istruzioni per ottenere i migliori risultati e per una maggior durata della macchina. Conservate il manuale in un posto sicuro per futuro riferimento. Prima di fare fotografi e importanti, vi consigliamo di scattare alcune fotografi e di prova, per acquisire