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
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This paper has been many years in development, and
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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