Journal of Petrology Advance Access originally published online on November 24, 2004
Journal of Petrology 2005 46(3):441-472; doi:10.1093/petrology/egh083
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Occurrence and Origin of Andalusite in Peraluminous Felsic Igneous Rocks
1 DEPARTMENT OF EARTH SCIENCES, DALHOUSIE UNIVERSITY, HALIFAX, NS, CANADA B3H 3J5
2 DEPARTMENT OF GEOLOGY, BRIGHAM YOUNG UNIVERSITY, PROVO, UT 84602, USA
3 LABORATOIRE MAGMAS ET VOLCANS, UNIVERSITÉ BLAISE PASCAL, 5 RUE KESSLER, F63038 CLERMONT-FERRAND CEDEX, FRANCE
4 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF TEXAS, AUSTIN, TX 78712, USA
5 DIPARTIMENTO DI MINERALOGIA E PETROLOGIA, UNIVERSITÁ DI PADOVA, I-35137 PADOVA, ITALY
6 SCHOOL OF GEOSCIENCES, UNIVERSITY OF SYDNEY, SYDNEY, N.S.W. 2006, AUSTRALIA
7 LABORATOIRE D'EXPLORATION ET GESTION DES RESSOURCES NATURELLES, DÉPARTEMENT DES SCIENCES DE LA TERRE, FACULTÉ DES SCIENCES ET TECHNIQUES, BENI MELLAL, MOROCCO
8 GEOFORSCHUNGSZENTRUM POTSDAM, D-14473 POTSDAM, GERMANY
9 UNIVERSITÀ DEGLI STUDI DI ROMA LA SAPIENZA, DIPARTIMENTO DI SCIENZE DELLA TERRA, PIAZZALE ALDO MORO 5, 00185 ROME, ITALY
10 NOVA SCOTIA DEPARTMENT OF NATURAL RESOURCES, PO BOX 698, HALIFAX, NS, CANADA B3J 2T9
11 DEPARTMENT OF GEOLOGICAL SCIENCE, UNIVERSITY OF VIENNA, A-1090 VIENNA, AUSTRIA
12 DEPARTAMENTO DE CIÊNCIAS DA TERRA, UNIVERSIDADE DO MINHO, 4710-057 BRAGA, PORTUGAL
13 SCHOOL OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF OKLAHOMA, NORMAN, OK 73019-0628, USA
14 DEPARTAMENTO DE CIENCIAS DA TERRA, UNIVERSIDADE DE COIMBRA, 3000-272 COIMBRA, PORTUGAL
15 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF CALGARY, CALGARY, AB, CANADA T2N 1N4
16 INSTITUT DES SCIENCES DE LA TERRE D'ORLÉANS (ISTO, UMR 6113), 45071 ORLÉANS CEDEX 2, FRANCE
17 CENTRO DE INVESTIGACIONES GEOLÓGICAS, 644 CALLE NO. 1, 1900 LA PLATA, ARGENTINA
18 INSTITUTE OF MINERALOGY, FREIBERG UNIVERSITY, D-09596 FREIBERG, GERMANY
19 SCHOOL OF GEOSCIENCES, UNIVERSITY OF NEWCASTLE, NEWCASTLE, N.S.W., AUSTRALIA
20 THE COUNCIL FOR GEOSCIENCE, PO BOX 5347, PORT ELIZABETH 6065, SOUTH AFRICA
21 DIPARTIMENTO DI SCIENZE DELLA TERRA E GEOLOGICO-AMBIENTALI, UNIVERSITÁ DI BOLOGNA, 40126 BOLOGNA, ITALY
22 INSTITUTO DE RECURSOS NATURALES Y AGROBIOLOGIA, CSIC, 37071 SALAMANCA, SPAIN
23 NEG-LABISE, DEPARTMENT OF GEOLOGY, FEDERAL UNIVERSITY OF PERNAMBUCO, RECIFE, PE 50670-000, BRAZIL
24 UNIVERSIDAD NACIONAL DE TUCUMAN, FACULTAD CIENCIAS NATURALES, INSTITUTO SUPERIOR CORRELACIÓN GEOLÓGICA, 4000 SAN MIGUEL DE TUCUMAN, ARGENTINA
25 DEPARTAMENTO DE GEOLOGIA, FACULTAD DE CIENCIAS, 37008 SALAMANCA, SPAIN
26 DEPARTMENT OF MINERAL DEPOSITS, FACULTY OF NATURAL SCIENCES, THE COMENIUS UNIVERSITY, MLYSKA DOLINA G, 842 15 BRATISLAVA, SLOVAKIA
27 DEPARTAMENTO DE PETROLOGIA Y GEOQUIMICA, FACULTAD DE CC. GEOLOGICAS, UNIVERSIDAD COMPLUTENSE, 28040 MADRID, SPAIN
28 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF MINNESOTA, MINNEAPOLIS, MN 55455, USA
29 DEPARTMENT OF MINERALOGY, THE NATURAL HISTORY MUSEUM, LONDON SW7 5BD, UK
30 DEPARTMENT OF GEOLOGY, BELOIT COLLEGE, BELOIT, WI 53511, USA
RECEIVED AUGUST 4, 2003; ACCEPTED SEPTEMBER 22, 2004
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONAndalusite occurs as an accessory mineral in many types of peraluminous felsic igneous rocks, including rhyolites, aplites, granites, pegmatites, and anatectic migmatites. Some published stability curves for And = Sil and the water-saturated granite solidus permit a small stability field for andalusite in equilibrium with felsic melts. We examine 108 samples of andalusite-bearing felsic rocks from more than 40 localities world-wide. Our purpose is to determine the origin of andalusite, including the T–P–X controls on andalusite formation, using eight textural and chemical criteria: size—compatibility with grain sizes of igneous minerals in the same rock; shape—ranging from euhedral to anhedral, with no simple correlation with origin; state of aggregation—single grains or clusters of grains; association with muscovite—with or without rims of monocrystalline or polycrystalline muscovite; inclusions—rare mineral inclusions and melt inclusions; chemical composition—andalusite with little significant chemical variation, except in iron content (0·08–1·71 wt % FeO); compositional zoning—concentric, sector, patchy, oscillatory zoning cryptically reflect growth conditions; compositions of coexisting phases—biotites with high siderophyllite–eastonite contents (Aliv
2·68 ± 0·07 atoms per formula unit), muscovites with 0·57–4·01 wt % FeO and 0·02–2·85 wt % TiO2, and apatites with 3·53 ± 0·18 wt % F. Coexisting muscovite–biotite pairs have a wide range of F contents, and FBt = 1·612FMs + 0·015. Most coexisting minerals have compositions consistent with equilibration at magmatic conditions. The three principal genetic types of andalusite in felsic igneous rocks are: Type 1 Metamorphic—(a) prograde metamorphic (in thermally metamorphosed peraluminous granites), (b) retrograde metamorphic (inversion from sillimanite of unspecified origin), (c) xenocrystic (derivation from local country rocks), and (d) restitic (derivation from source regions); Type 2 Magmatic—(a) peritectic (water-undersaturated, T
) associated with leucosomes in migmatites, (b) peritectic (water-undersaturated, T
), as reaction rims on garnet or cordierite, (c) cotectic (water-undersaturated, T) direct crystallization from a silicate melt, and (d) pegmatitic (water-saturated, T), associated with aplite–pegmatite contacts or pegmatitic portion alone; Type 3 Metasomatic—(water-saturated, magma-absent), spatially related to structural discontinuities in host, replacement of feldspar and/or biotite, intergrowths with quartz. The great majority of our andalusite samples show one or more textural or chemical criteria suggesting a magmatic origin. Of the many possible controls on the formation of andalusite (excess Al2O3, water concentration and fluid evolution, high Be–B–Li–P, high F, high Fe–Mn–Ti, and kinetic considerations), the two most important factors appear to be excess Al2O3 and the effect of releasing water (either to strip alkalis from the melt or to reduce alumina solubility in the melt). Of particular importance is the evidence for magmatic andalusite in granites showing no significant depression of the solidus, suggesting that the And = Sil equilibrium must cross the granite solidus rather than lie below it. Magmatic andalusite, however formed, is susceptible to supra- or sub-solidus reaction to produce muscovite. In many cases, textural evidence of this reaction remains, but in other cases muscovite may completely replace andalusite leaving little or no evidence of its former existence. KEY WORDS: andalusite; granite; magmatic; origin; xenocrystic
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INTRODUCTION |
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Purpose
Andalusite occurs as an accessory mineral in a wide range of felsic peraluminous {A/CNK = molar [(Al2O3)/(CaO + Na2O + K2O)] > 1} extrusive and intrusive igneous rocks. The purposes of this contribution are:
- to present textural observations and chemical data from a wide range of andalusite-bearing felsic igneous rocks, including fine-grained glassy volcanics, anatectic leucosomes, fine-grained aplites, medium- to coarse-grained granitoids, and very coarse-grained granite pegmatites;
- to discover the criteria (mineral assemblages, textures, chemical partitioning, and phase equilibrium constraints) for distinguishing between magmatic, metamorphic, and metasomatic andalusite;
- to evaluate the conditions and controls that promote the formation of andalusite in naturally occurring felsic igneous rocks.
If andalusite can have a primary magmatic origin, its occurrence places important constraints on the T–P–X conditions of magma crystallization.
Petrological framework
The positions of the water-saturated granite solidus and the andalusite–sillimanite stability field boundary in T–P–X space are critical to the origin of andalusite in felsic igneous rocks. At one extreme, simple synthetic systems involving the water-saturated haplogranite (Na2O–K2O–Al2O3–SiO2–H2O) solidus (Tuttle & Bowen, 1958
; Holland & Powell, 2001) and the aluminosilicate stability fields (Holdaway, 1971; Holdaway & Mukhopadhyay, 1993) show no overlap between the stability fields of silicate melt and andalusite, precluding a primary magmatic origin for andalusite (Fig. 1a). Accordingly, andalusite in felsic igneous rocks must be xenocrystic, metasomatic, or the product of growth from a strongly undercooled melt. At the other extreme, simple synthetic systems involving the water-saturated peraluminous granite solidus (Abbott & Clarke, 1979; Holtz et al., 1992; Joyce & Voigt, 1994) and the aluminosilicate stability fields of Richardson et al. (1969) show substantial overlap, thereby permitting a primary magmatic origin for andalusite (Fig. 1b).
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The position of the water-saturated granite solidus curve is sensitive to the presence of other components. In particular, excess Al2O3 lowers the solidus curve by c. 30°C (Fig. 1b), and creates a more favourable compositional environment in which to grow Al2SiO5 polymorphs (Abbott & Clarke, 1979
; Clemens & Wall, 1981; Holtz et al., 1992; Joyce & Voigt, 1994). Fluorine, lithium, and boron are other components that may have important roles in lowering the haplogranite solidus curve depending on their concentrations (Chorlton & Martin, 1978; London & Burt, 1982; Pichavant & Manning, 1984). Natural Ca-bearing plagioclase raises the haplogranite solidus curve by 10–20°C, depending on the amount of Ca in the system (Johannes, 1978).
The position of the andalusite–sillimanite field boundary in P–T space has been investigated many times, but its precise location remains controversial (Kerrick, 1990; Pattison, 1992, 2001; Holdaway & Mukhopadhyay, 1993; Tinkham et al., 2001; Pattison et al., 2002; Cesare et al., 2003). Uncertainties in the position of the And = Sil field boundary arise, in part, from the strong dependence of the thermodynamic equilibrium conditions on the structural state of the material under investigation (Salje, 1986). Considerable discrepancy exists between the experimental studies of Richardson et al. (1969), who used fibrolitic sillimanite, and those of Holdaway (1971) who used prismatic sillimanite. According to Salje (1986), a ‘transition field’ between the polymorphs is more appropriate than a ‘transition line’. Grambling & Williams (1985) and Kerrick (1990) suggested an effect of impurities (mainly Fe3+ and Mn3+) on the stability relations of the Al2SiO5 polymorphs. Incorporation of Fe and Mn enlarges the stability field of andalusite relative to that of sillimanite; however, Pattison (2001) argued that this effect is generally modest for natural Fe and Mn contents.
Owing to these difficulties in deciding between the different experimental calibrations, many investigators turned to natural parageneses to constrain the equilibrium (e.g. Greenwood, 1976; Vernon, 1982; Holland & Powell, 1985; Pattison, 1992; Pattison et al., 2002). Most of these studies placed the And = Sil equilibrium in positions intermediate between the Holdaway (1971) and Richardson et al. (1969) curves. Of particular significance to this investigation is that several studies of metapelitic And = Sil phase equilibria in low-pressure settings (i.e. those most relevant to the issue of andalusite + silicate melt stability) rejected the Holdaway (1971) And = Sil curve because it created too small an andalusite stability field to reconcile with a number of other phase equilibrium constraints (e.g. Vernon, 1982; Vernon et al., 1990; Pattison & Tracy, 1991; Pattison, 1992; Johnson & Vernon, 1995). Pattison (1992) provided an evaluation of the And = Sil equilibrium against a number of key phase equilibrium constraints that supported his calculated position about midway between the Holdaway (1971) and Richardson et al. (1969) positions. This position allows for an andalusite + haplogranite melt stability field below 
3 kbar, even without the need to invoke F-, B-, Li- or excess Al-bearing components in the melt (Fig. 1a), and it has found support in a number of recent papers (Spear et al., 1999; Tinkham et al., 2001; Cesare et al., 2003; Johnson et al., 2003; Larson & Sharp, 2003). In addition, the presence of melt inclusions in andalusite from volcanic rocks (Cesare et al., 2003), the presence of euhedral crystals of andalusite in some glassy felsic volcanic rocks (Pichavant et al., 1988), and the occurrence of euhedral andalusite crystals in granitic rocks and anatectic leucosomes (Clarke et al., 1976; Clemens & Wall, 1981; Vernon et al., 1990; Pattison, 1992) suggest an overlap of the stability fields of andalusite and silicate melt and a magmatic origin for the andalusite.
Methods
This project began as the result of an exchange of ideas about andalusite in granites on the Granite-Research Internet discussion group (granite-research{at}ac.dal.ca, now granite-research{at}lists.dal.ca). Subsequent to that discussion, Barrie Clarke and Michael Dorais tested some ideas with their own andalusite-bearing and andalusite-free granitoid samples, and then put out a request on the granite-research network for further contributions to expand the coverage. The result is a database of 111 felsic igneous rock samples, 108 of them containing andalusite, contributed by the authors of this paper. All authors have participated in the production of this paper through an exchange of text, tables, and figures on the Internet.
Most of the samples were submitted as hand specimens and prepared as thin sections by Gordon Brown at Dalhousie University. Petrographic observations of all samples were made by Barrie Clarke and Michael Dorais, and verified by the person submitting the samples. In this way, we have applied a uniform nomenclature to all samples. Bernardo Cesare examined all samples for melt inclusions. Dan Kontak examined all samples for fluid inclusions. Where applicable, mineral abbreviations used in this paper are those of Kretz (1983).
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PETROGRAPHIC OBSERVATIONS AND DISCUSSION |
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In a field and petrographic study, Hills (1938)
noted that ‘it is chiefly from those uncontaminated ... granites, pegmatites, and aplites...that what appears to be primary pyrogenetic andalusite has been recorded’. Hills' evidence included modal abundance, uniform distribution, large size and euhedral habit of andalusite, lack of oriented carbonaceous inclusions (chiastolite), absence of metasedimentary xenoliths, association with topaz and tourmaline in two-mica granites, and, for some, apparent lack of opportunity for the magmas to assimilate peraluminous wall-rock. To establish the igneous origin for a particular mineral requires matching a number of these, and other, inherently equivocal textural criteria, detailed below. If andalusite in a felsic igneous rock satisfies at least some of these criteria, an igneous origin for that andalusite is tenable. Electronic Appendix Table A1 contains information about the samples, including source, location, environment of crystallization, and a literature reference (if any); electronic appendices may be downloaded from the Journal of Petrology website at http://petrology.oupjournals.org/.
Grain size
Dimensional compatibility of a mineral of unknown origin with other magmatic rock-forming minerals in the same sample could be used to argue a co-magmatic origin. The grain sizes of primary magmatic minerals in an igneous rock can, however, vary by orders of magnitude; therefore, any grain-size test is not particularly discriminating. Conversely, dimensional incompatibility may suggest, but does not necessarily demand a different origin. Any andalusite grains that are significantly smaller, or significantly larger, than the main rock-forming silicate minerals are potentially non-igneous. Figure 2 illustrates two samples (BBR-01 and CES-01) in which andalusite fails the grain-size test because the crystals are much larger than the other minerals in the rock. Many other samples contain andalusite grains that are considerably smaller than the main rock-forming minerals; although they also fail the grain-size test, they may still have an igneous origin.
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Grain shape
Euhedral andalusite in a felsic igneous rock may indicate a former cotectic or peritectic relationship with a silicate melt phase; however, euhedral andalusite occurs both in igneous and metamorphic rocks, and thus idiomorphic grain shapes alone are not diagnostic. Some of the andalusites in volcanic samples, e.g. LON-01 (Fig. 3a), or aplites, e.g. WIL-01 (Fig. 3b), pass the grain-shape test as potentially primary magmatic phases. The andalusite in CLA-12 is skeletal (Fig. 3c), suggesting formation during a temperature or pressure quench. Many subhedral or anhedral andalusites in felsic igneous rocks have pink cores that are euhedral to subhedral (VIS-01, Fig. 3d), suggesting that those cores, at least, might be igneous.
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Anhedral andalusite grain shapes may reflect late-stage grain interference during primary magmatic growth, the result of a reaction relationship of an andalusite of any origin with the silicate melt phase, an originally anhedral xenocrystic morphology, or an originally euhedral xenocrystic morphology out of equilibrium with the melt. Distinctly anhedral andalusite grains, apparently out of equilibrium with the felsic magma, include volcanic sample BAR-01 (Fig. 3e) and plutonic sample ROT-05 (Fig. 3f).
State of aggregation
Andalusite in felsic igneous rocks may occur as single grains (Figs 3a, b, d–f; 4a–d), isolated from other andalusite grains by more common rock-forming minerals. It may also occur as clusters of small grains. In some clusters, the individual andalusite grains have random orientations relative to one another (Fig. 5a–d). Why should a modally scarce mineral cluster? Either the individual andalusites crystallized elsewhere and were brought to that location by some physical process such as synneusis or settling, or they represent the sites of advanced digestion of pelitic xenoliths, or they nucleated and grew at that position in the sample. These common clusters of randomly oriented grains of andalusite may have genetic significance.
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In other clusters, the individual andalusite grains are in optical and crystallographic continuity (e.g. Figs 3c and 4b). If in crystallographic alignment, the andalusite grains either grew as a spray of quench crystals (Figs 3c and 5b), or the clustering may only be apparent, as in the cases of many optically continuous andalusite grains embedded in muscovite (Fig. 4b). In cases such as the latter, a single grain of andalusite was irregularly replaced by muscovite, yielding an apparent ‘cluster’ of anhedral, but crystallographically aligned, andalusite in muscovite.
Textural relationship with muscovite
Many andalusite grains in felsic igneous rocks have mantles of muscovite, and these muscovite rims may consist of a single crystal or a polycrystalline aggregate. Figure 6 combines the state of aggregation of andalusite grains (above), and the common association of andalusite with muscovite, to establish a six-fold textural classification of andalusite. In some cases, more than one class of andalusite can occur in the same rock (e.g. sample GOT-02 contains andalusite textural types S1, C1, and C2).
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In the Macusani rhyolites, muscovite and andalusite coexist throughout the entire volcanic field (Pichavant et al., 1988
). No textural evidence exists for replacement of one phase by the other, but the modal proportions of andalusite and muscovite are negatively correlated, suggesting that, during the main crystallization stage of the Macusani magmas, the reaction Ms + Qtz = And + San (in presence of melt) controls the modal proportions of andalusite and muscovite. This reaction depends on P, T, and aH2O, implying that the mineral assemblage characteristic of the main crystallization stage of the Macusani magmas (Qtz, San, Plag, Ms, And, ± Bt) could have crystallized over a range of P, T and fH2O conditions. However, the F content of muscovite is also an important controlling factor in this reaction. For a given aH2O, elevated fHF would drive the reaction to the left (consuming andalusite, producing muscovite). Muscovite crystallization at the expense of andalusite does not necessarily imply high aH2O (it could be lower T, higher P, or higher fHF). The inverse correlation between the modal proportions of Ms and And in the Macusani volcanics also occurs in peraluminous granites from the Bohemian Massif (samples ROT-03,04; D'Amico et al., 1982–1983a, 1982–1983b).
Muscovite overgrowths on andalusite in plutonic rocks may obscure a possible original euhedral shape (ROT-02, Fig. 4c), and thereby complicate any determination of the origin of the andalusite. Because muscovite can have primary magmatic or secondary hydrothermal origins, with much the same texture (Miller et al., 1981; Zen, 1988), interpretation of this textural relationship between andalusite and muscovite is difficult. One reason for little or no bulk chemical compositional difference between some andalusite-bearing two-mica granitoids and andalusite-free two-mica granitoids is just a question of how completely the andalusite is replaced (effectively under magmatic conditions by primary muscovite, less effectively under subsolidus conditions by secondary muscovite). Whether andalusite is preserved in plutonic rocks depends on its survival under conditions of slow cooling, allowing magmatic peritectic relations of the type
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). Kinetically, a high-temperature, melt + fluid, condition may favour the formation of coarse-grained single muscovite crystals, whereas a subsolidus low-temperature, fluid-only, condition may favour the formation of some fine-grained polycrystalline muscovite aggregates.
Figure 7 illustrates four of the many types of textural relations between andalusite and muscovite. The original andalusite may be a single grain or a cluster, the muscovite rim may be magmatic or subsolidus hydrothermal, and the And 
Ms reaction may be incomplete or complete. In the last case, the andalusite is completely consumed in the reaction, leaving little or no evidence of its former existence.
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Inclusion relationships
Mineral inclusions
If andalusite occurs as inclusions in igneous minerals such as feldspar and quartz (e.g. REN-03, UGI-06), little can be deduced about its origin; however, andalusite rarely occurs as inclusions in any phase other than muscovite. If andalusite itself contains inclusions of magmatic minerals, the sizes, shapes, abundances, and compositions of those inclusions may help to determine the origin of the host andalusite. If an andalusite contains carbonaceous material defining the chiastolite cross (e.g. BBR-01, Fig. 2a), a metamorphic origin is probable. Some chiastolite-like andalusite may also form by peritectic melting reactions in graphitic schists where inclusion of graphite particles may take place behind advancing crystal faces, but at the same time the andalusite should also trap melt inclusions (Cesare & Gómez-Pugnaire, 2001
). Few of the andalusites that we believe are igneous on other grounds contain any mineral inclusions, and thus the mineral inclusion criterion is not particularly useful.
Melt inclusions
Melt inclusions in andalusite attest to its growth in the presence of melt (Cesare et al., 2003). Glass inclusions are easy to recognize in andalusite from felsic volcanic rocks, such as those from Lipari (BAR-01), Mazarrón (CES-01,02) (Cesare & Gómez-Pugnaire, 2001; Cesare et al., 2002, 2003), and Macusani (Pichavant et al., 1988; Fig. 8). In slowly cooled plutonic rocks or migmatites, any melt inclusions trapped in andalusite will have crystallized as polyphase aggregates of quartz, feldspars, and micas, a useful criterion to infer an igneous origin for andalusite. Polyphase inclusions in andalusite crystals of samples CLA-01,05,11,12,13, CLR-02, GOM-03, RIC-03, and TOS-06 provide additional support for their coexistence with a felsic silicate melt.
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Fluid inclusions
Examination of all our andalusite samples for fluid inclusions yielded negative results. Either there was no fluid in equilibrium with the andalusite as it grew (unlikely in the cases of pegmatites), or the surface properties of andalusite are such that it is not readily ‘wetted’ by fluids.
Summary of textural observations
Of the several possible textural tests for the origin of andalusite in felsic igneous rocks, no single criterion (grain size, grain shape, clustering, textural relations with muscovite, inclusion relations) is necessarily diagnostic of the origin of andalusite. The agreement of two or more of these textural and chemical criteria constitutes a stronger collective case. For example, a euhedral, grain-size compatible, andalusite with melt inclusions occurring in a volcanic rock is almost certainly magmatic, whereas a large anhedral andalusite with a chiastolite cross and a reaction rim is probably xenocrystic. Also, we note that there is no a priori textural reason why a felsic igneous rock cannot contain more than one genetic type of andalusite (e.g. magmatic and xenocrystic).
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CHEMICAL COMPOSITION OF ANDALUSITE IN FELSIC IGNEOUS ROCKS |
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In this section, we examine the chemical composition of andalusite, the nature of any chemical zoning, and the chemical compositions of coexisting micas and apatite to search for criteria that might provide information about the origin of andalusite in felsic igneous rocks. Electronic Appendix Tables A2–A5 contain compositional data for average biotite, muscovite, andalusite, and apatite, respectively, in the samples we have studied. Not all samples contain all four minerals, and even if they do, we do not necessarily have analyses for all four phases in each rock.
Chemical composition
If a mineral exhibits a wide range of chemical substitutions that reflect its conditions of formation [e.g. Ti in muscovite (Miller et al., 1981)], then the origin of that mineral may be determined from its chemical composition alone. In stoichiometric andalusite (Al2SiO5), half the Al cations reside in octahedral sites, and the other half reside in five-coordinated polyhedra, whereas all the Si cations occupy tetrahedral sites. Such simple chemistry and relatively simple structure provide limited opportunity for chemical substitution (Deer et al., 1982). Electronic Appendix Table A4 shows that the studied andalusites from felsic igneous rocks have transition-element compositions with the following ranges: FeOT (measured as Fe, reported as FeO) 0·03–1·70%, MnO 0·00–0·09%, and TiO2 0·00–0·36%. Without a comparable database of andalusite compositions from metamorphic rocks, little can be said about the existence of chemical discriminants to determine the origin of the andalusite. Trace elements might prove to be more useful than major elements.
Chemical zoning
Optically zoned andalusite is common in metamorphic, hydrothermal, and magmatic environments [e.g. review by Kerrick (1990)]. Andalusites from the studied felsic igneous rocks show four types of zoning, as follows.
- Concentric zoning. Concentric zoning consists of a sharp to gradational variation in the mole fraction of transition-element content (hereafter referred to simply as TE content) from core to rim, with some boundaries subparallel to the external morphology of the crystal (Fig. 9a). However, the cores of such grains may be highly irregular in shape, showing convolute–lobate and/or irregularly stepped boundaries (Fig. 9b).
- Sector zoning. Sector zoning is characterized by higher TE contents parallel to {001}, {100}, and {010} (Hollister & Bence, 1967). Regular steps in some of the concentric zone boundaries may be sector zone boundaries. The most striking example is from sample 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.
- Oscillatory zoning. Oscillatory zoning is characterized by alternating high-TE and low-TE, continuous to discontinuous, growth shells (Fig. 9d and e). Most boundaries between the growth zones are either rounded or irregularly stepped.
- Patchy zoning. In contrast to sector zoning above, patchy zoning shows neither sharp nor obviously crystallographically controlled boundaries (Fig. 4a).
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Once formed, such andalusite zoning patterns appear to be robust, as indicated by samples WHI-01 (Fig. 9f) and NEV-04 (not shown) in which the pink TE-rich andalusite cores have survived high-temperature sillimanite-grade metamorphism, but the outer parts of the colourless rims have inverted to sillimanite.
Although the different types of zoning in andalusite are well known, little is understood about their origins and their potential for revealing diagnostic information about T–P–X crystallization environments. Hydrothermal andalusite commonly shows concentric zoning (high-TE core, low-TE rim) or sector zoning (Cesare, 1994; Whitney & Dilek, 2000), whereas metamorphic andalusite commonly shows gradational patchy zoning, and may also exhibit concentric zoning (Yokoi, 1983; Shiba, 1988; Cesare, 1994), or sector zoning (Grambling & Williams, 1985). If distinctions between environments of crystallization exist, they are not yet well defined. Nevertheless, zoning patterns may help to exclude a certain origin for a grain in question (e.g. oscillatory zoning is unlikely for metamorphic andalusite, but likely for hydrothermal or magmatic andalusite). Several features of zoned andalusites are, at least, consistent with a magmatic origin (e.g. sharp compositional zone boundaries, oscillatory zones, possible quench phenomena with preferential sector growth). Unfortunately, we do not yet have sufficient textural and chemical information about zoned andalusites in veins and metamorphic rocks to be able to distinguish clearly between one environment of crystallization and another, and what, if any, characteristics of zoning are unique to magmatic andalusites.
Chemical equilibrium with other minerals
For minerals showing extensive mutual solid solution, systematic disposition of tie lines between coexisting phases is an indication of an equilibrium relationship. In this section, we consider whether the compositions of biotite, muscovite, and apatite coexisting with andalusite are consistent with their being an equilibrium assemblage. If they are in chemical equilibrium with each other and magmatic in origin, and if they are also in chemical equilibrium with andalusite, then the andalusite should also be magmatic.
Biotite
Figure 10a is a trioctahedral mica plot showing the average biotite compositions in all the studied samples. Given the global distribution of the samples, the consistency of the Aliv [mean 2·68 ± 0·07 atoms per formula unit (a.p.f.u.)] in the biotites is remarkable, suggesting that the biotites have had their alumina contents fixed by equilibrating with some Al-rich phase (e.g. andalusite), probably under conditions of restricted temperature and pressure. Although a magma containing abundant andalusite and biotite xenocrysts might also attain this equilibrium, the simplest interpretation is that the biotite and andalusite are both primary magmatic in origin.
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Muscovite
Figure 10b and c shows the TiO2 and Na/(Na + K) distributions in all analyzed muscovite grains. According to the chemical criteria of Miller et al. (1981)
, very few of these muscovites have TiO2 >1%, consistent with a primary magmatic origin; however, if a highly evolved magma has a very low TiO2 content, so presumably, will its primary magmatic muscovite. According to their Na/(Na + K) ratios, however, these muscovites are predominantly magmatic (Monier et al., 1984). Figure 10d shows the variable, but non-diagnostic, range of FeO concentrations in the muscovites coexisting with andalusite. For the composition of muscovite to be more useful, we need a detailed study of muscovite associated with andalusite versus the rest of the muscovite in the rock. Furthermore, we need to determine if there is any chemical difference between the monocrystalline muscovite rims on andalusite (magmatic?) and the polycrystalline muscovite rims on andalusite (hydrothermal? quenched?).
Biotite + muscovite
In general, andalusite-bearing plutonic rocks contain biotite and muscovite with high alumina contents; however, this criterion alone does not necessarily separate igneous from metamorphic micas. Figure 11a shows TiO2 contents for averages of all analyzed mica pairs. The similar slopes of the tie lines between coexisting micas suggest attainment of chemical equilibrium between the mica pairs, namely DTiBt/Ms range is 2·66–25·17, mean 4·68 ± 1·50 (excluding all values greater than 8·00), n = 20 [compare the values obtained by Brigatti et al. (2000), i.e. DTiBt/Ms range 1·94–3·33, mean 2·74, n = 7). Given the texture and the unaltered state of these biotites, we conclude that the equilibrium is more likely to be magmatic than subsolidus–hydrothermal.
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Figure 11b shows mean fluorine concentrations for the same coexisting micas. Of note is the wide range of F contents in the micas in these andalusite-bearing rocks, and the generally regular disposition of tie lines suggesting equilibrium compositions. Tie lines with distinctively steeper or shallower slopes suggest that the composition of at least one mica in the assemblage has changed, and that some degree of subsolidus re-equilibration of F between coexisting micas may have taken place. In such cases, DFBt/Ms increases with subsolidus cooling because muscovite re-equilibrates more readily than biotite (Ferrow et al., 1990
). Such disequilibrium between the micas may also raise questions about the origin of the coexisting andalusite. Samples with FBt/FMs <1·5 are VIL-14 and WOO-01, which appear to be otherwise unremarkable. Samples with FBt/FMs >3·8 are JAM-01, JAM-02, RIC-01, RIC-02, RIC-03, RIC-05, RIC-06, and NEV-02. Significantly, seven of these samples are migmatites with low FBt contents, and the other sample is a pegmatite (NEV-02). Sample CLA-05 has the lowest F contents in its coexisting micas; in this respect, its similarity to the migmatites suggests that it may also have an early anatectic origin. The mean of all samples with average FBt/FMs >1·4 and <3·8 is 2·27 ± 0·59 (n = 31). In two samples with crossing tie lines (NEV-03, NEV-05), andalusite occurs in clusters with biotite-rich xenolithic material and texturally (but not chemically) secondary muscovite. Because all of these samples contain andalusite, high F is, apparently, not a precondition for the occurrence of andalusite in felsic igneous rocks.
If all the analyzed samples had come from one differentiating pluton, such a regular disposition of tie lines might be expected; however, given that the samples come from more than 40 localities of different types, the regularity of the tie lines in Fig. 11a and b suggests an important repetition of T–P–X conditions in andalusite-bearing peraluminous felsic igneous rocks through space and time. As a first-order approximation, we consider the bundles of roughly parallel tie lines (Fig. 11a and b) and the samples with Ti and F partitioning between coexisting micas similar to those determined experimentally (Icenhower & London, 1995), as magmatic micas. Figure 11c is similar to Fig. 11b, except that the vertical axis is expanded and most of the tie lines have been removed. Additional plotted samples are from the topaz-bearing two-mica Lake Lewis leucogranite in the South Mountain Batholith (Clarke & Bogutyn, 2003). Sample GOT-02 is the most fluorine-rich, andalusite-bearing, topaz-absent, sample from our database, and it helps to constrain the position of the andalusite–topaz boundary in this system.
Figure 11d shows the systematic partitioning of F between biotite and muscovite expressed by the equation FBt = 1·31 FMs + 0·02. This empirical relationship is reasonably consistent with other data from coexisting micas in granites (FBt/FMs = 1·8 ± 0·5; Neves, 1997), and on coexisting micas in peraluminous experimental systems (FBt/FMs = 1·22–1·55; Icenhower & London, 1995).
Biotite + muscovite + apatite
The magmatic origin of apatite is normally not in question. Apatite should, therefore, exhibit systematic partition relationships for fluorine with the two other magmatic F-bearing phases, i.e. biotite and muscovite, if they are all in equilibrium. Figure 12a and b shows the complex relationship FAp/FMs vs FAp/FBt, contoured for FBt and FMs, respectively. In general, the array of points defines a curved trend, and in both plots the fluorine concentrations are highest in those micas with the lowest FAp/FMica values. This relationship appears to be the result of relatively constant fluorine concentrations in the apatites. The ratio of Fmax/Fmin in each of the phases in our entire sample set is 1·4 for apatite, 8·8 for biotite, and 27·7 for muscovite. Furthermore, samples with low bulk-rock fluorine contents, as proxied by the FBt values, have the fluorine strongly partitioned into the apatites (as before, many of these samples are migmatitic leucosomes). If the bulk-rock fluorine contents are high, F strongly partitions into the micas. The systematic partitioning of F between apatite and the micas suggests equilibrium conditions. If the apatite is magmatic, then probably so should be the micas.
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Sillimanite
The broad overlap of stability fields for sillimanite and felsic melt means that, in contrast to andalusite, an igneous origin for sillimanite in felsic igneous rocks is not a petrogenetic problem. Sillimanite can occur as the only aluminosilicate phase (D'Amico et al., 1982–83a
, 1982–83b; Pichavant et al., 1988), or it can occur with andalusite (Barker, 1987; Pichavant et al., 1988; Messina et al., 1991; Rottura et al., 1993; Cesare et al., 2002; Visonà & Lombardo, 2002). Our sample set was assembled solely on the basis of the presence of andalusite; the additional occurrence of sillimanite in any sample was incidental. Our database is not sufficiently comprehensive to draw any general conclusions about the coexistence of andalusite and sillimanite in felsic igneous rocks.
Summary of chemical criteria
We have considered three chemical tests for the origin of andalusite in felsic igneous rocks. The chemical composition of andalusite itself provides little information about its origin. The nature of chemical zoning may have greater potential, but it first requires a more detailed examination of chemical zoning patterns in andalusites from metamorphic rocks and hydrothermal veins. The chemical-equilibrium-with-other-phases test is the most quantitative and most objective. Systematic partitioning of Ti and F between coexisting biotite, muscovite, and apatite in our sample set suggests that they are in equilibrium and are almost certainly magmatic phases. That the magmatic biotite also has its Aliv controlled by equilibrium with andalusite is, we believe, the most compelling chemical argument in favour of a magmatic origin for the andalusite; however, this view does not entirely preclude the equilibration of xenocrystic biotite and andalusite at magmatic temperatures. We note again that there is no a priori chemical reason why a felsic igneous rock may not contain more than one genetic type of andalusite (e.g. magmatic and xenocrystic).
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GENETIC TYPES OF ANDALUSITE IN FELSIC IGNEOUS ROCKS |
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Theoretically, andalusites in felsic igneous rocks can fall into three main genetic categories detailed below.
Type 1 Metamorphic (melt phase not involved in the formation of andalusite)
Type 1a Metamorphic–in situ prograde
Barrera et al. (1985) and Zaleski (1985) described the effects of contact metamorphism in granites where andalusite formed as euhedral to subhedral prisms replacing original biotite. None of the andalusite in our samples appears to have formed in situ by thermal metamorphism of a felsic igneous rock. In the sample most obviously affected by thermal metamorphism (WHI-01; Fig. 9g), pre-existing andalusite has been partially converted to sillimanite.
Type 1b Metamorphic–retrograde inversion of sillimanite of various origins
If sillimanite of any origin (magmatic, metamorphic) were present in a granite magma, it could undergo inversion to andalusite above or below the granite solidus, possibly resulting in andalusite pseudomorphs after the sillimanite. Barker (1987) has argued that, on the basis of size and shape of the andalusites in sample BAR-01 from Lipari, they have inverted from xenocrystic sillimanite. Otherwise, none of our andalusite appears to have formed by inversion from sillimanite.
Type 1c Metamorphic–xenocrystic derived from local peraluminous country rocks
Andalusite crystals may be released from disaggregating, contact-metamorphosed, metapelites into a silicate melt and, in general, such xenocrystic grains would be out of chemical equilibrium with that melt. These xenocrysts may be anhedral and contain many mineral inclusions, including carbonaceous material. Their subsequent history in the magma then depends on the degree to which they are out of equilibrium with the silicate melt, and on the kinetics of the new environment. Xenocrystic andalusite may disappear rapidly in a high-temperature, well-mixed, relatively fluid metaluminous melt, or in a peraluminous melt undersaturated in Al2SiO5, survive largely unmodified in a near-solidus, static, viscous peraluminous melt, or even develop magmatic overgrowths in a highly peraluminous melt. Xenocrysts in an advanced state of dissolution, especially if mantled by late muscovite, would be difficult to distinguish from anhedral magmatic grains.
Bouloton et al. (1991) and Bouloton (1992) described xenocrystic andalusite in Hercynian granites from Morocco where chiastolite-type crystals, up to 5 cm long, occur. Samples BBR-01 (Fig. 2a) and BBR-02 are from the same pluton. These large andalusites fail the grain-size test as magmatic, and they have significant reaction rims indicating disequilibrium with the melt. Also, samples NEV-03 to NEV-05 contain ovoid polymineralic aggregates of biotite, andalusite, and muscovite, with or without sillimanite, showing a symplectitic relationship. These aggregates only occur close (300 m) to the contact with younger porphyritic biotite granites, and they appear to be foreign to their granite host.
López Ruiz & Rodríguez Badiola (1980) interpreted the origin of andalusite in some high-K dacites as xenocrystic because typical anhedral andalusite grains are surrounded by plagioclase and spinel reaction rims. Such andalusite grains may also contain inclusions, including the chiastolite cross, as well as textural evidence of disequilibrium (e.g. corrosion). Alternatively, because some of these andalusites also contain melt inclusions, Cesare et al. (2003) regarded them as Type 1d or 2a (below).
Type 1d Metamorphic–original constituent of source rocks (restitic)
We define restite minerals as those minerals, present in the protolith prior to partial melting, that survive as the 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 (metapelitic) protolith. Given the low T–low P stability region of andalusite, and its limited region of overlap with the field of granite magmas, andalusite is an unlikely phase to occur as part of a truly restitic assemblage in many granitoid magmas, especially if extensive partial melting has taken place at high temperatures. Fluid-present melting reactions with (H2O)v are likely to be lower T, and Als = andalusite. Fluid-absent melting reactions, especially those involving biotite dehydration, are likely to be high T, and Als = sillimanite. Depending on the bulk composition of the protolith, and the degree of partial melting, Als can remain as part of the restitic refractory residuum. If any magma had been in equilibrium with andalusite as a restite phase in the region of partial melting, that magma would be saturated in andalusite, and would probably remain saturated during its ascent to lower pressures. Such magmas are strong candidates for crystallizing magmatic andalusite (below).
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Distinguishing between former restitic andalusite and new magmatic andalusite is extremely difficult, especially in the absence of melt inclusions. Andalusite-bearing surmicaceous enclaves may be restites from the source area (Didier, 1991
; Montel et al., 1991; Gaspar & Inverno, 1998), but in the absence of minerals or textures typical of high temperatures and pressures (Wall et al., 1987), such enclaves are more likely to be partially digested xenoliths of country rocks. Unless some of our andalusites represent disaggregated relicts from such enclaves, restitic andalusite must be rare.
Type 2 Magmatic (melt phase an integral part of the formation of andalusite)
Type 2a Magmatic–peritectic (T)
Table 2 lists several reactions in which andalusite appears solely as the result of melt-producing reactions in originally andalusite-free rocks. In none of these reactions is andalusite also present in the subsolidus mineral assemblage, but it appears peritectically in an incongruent melting reaction. We regard such andalusite as being magmatic because, in phase equilibrium terms, it demonstrates a stability field overlap with a silicate melt.
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High-temperature fluid-absent melting reactions will favour Als = sillimanite, but low-temperature, low-pressure, water-saturated melting reactions will favour Als = andalusite. Spatially, andalusite of this type may form along the contact between pelitic xenoliths and melt, or associate with the melt phase (initially as leucosomes) rather than the refractory residuum (restite) in anatectic migmatites. Such andalusites have no subsolidus metamorphic history, and thus may be euhedral and free of the mineral inclusions metamorphic andalusites commonly contain. Kawakami (2002)
has described andalusite of magmatic origin from migmatites in Japan. Small crystals of euhedral andalusite in some Himalayan leucogranites are surrounded by thin rims of sillimanite (Castelli & Lombardo, 1988; Visonà & Lombardo, 2002), and may be the products of a T (rising temperature) peritectic melt-producing reaction. In such a reaction, andalusite initially grows in the metapelites by peritectic melting reactions and replacement by topotactic sillimanite is the result of rising temperature (e.g. Cesare et al., 2002).
Prime candidates for T peritectic andalusite occur in the migmatites from our sample set. Sample RIC-06 shows abundant large andalusite crystals growing along the leucosome–melanosome contact, and sa