Cordierite in Felsic Igneous Rocks: a Synthesis

Cordierite in felsic igneous rocks: a synthesis

D. B. CLARKE Department of Earth Sciences, Dalhousie University, Halifax, N.S., Canada B3H 3J5

Abstract Cordierite is a characteristic mineral of many peraluminous felsic igneous rocks. A combination of T-P-X parameters, which overlap the stability conditions for felsic magmas, control its formation. Critical among these parameters are relatively low T, low P, and typically high (Mg+Fe2§ Mg/Fe2+, A/CNK, aAl20~, and fo2. Spatial and textural information indicate that cordierite may originate in one of three principal ways in felsic igneous rocks: Type 1 Metamorphic: (a) xenocrystic (generally anhedral, many inclusions, spatial proximity to country rocks and pelitic xenoliths); (b) restitic (generally anhedral, high-grade metamorphic inclusions); Type 2 Magmatic: (a,b) peritectic (subhedral to euhedral, associated with leucosomes in migmatites or as reaction rims on garnet); (c) cotectic (euhedral, grain size compatibility with host rock, few inclusions); (d) pegmatitic (large subhedral to euhedral grains, associated with aplite-pegmatite contacts or pegmatitic portion alone); and Type 3 Metasomatic (spatially related to structural discontinuities in host, replacement of feldspar and/or biotite, intergrowths with quartz). Of these, Type 2a (peritectic) and Type 2c (cotectic) predominate in granitic and rhyolitic rocks derived from fluid-undersaturated peraluminous magmas, and Type 2d (pegmatitic) may be the most common type in fluid-saturated systems.

KEYWORDS: cordierite, pinite, granite, aplite, pegmatite, rhyolite, migmatite, restite, xenocryst, metamorphic, magmatic, metasomatic.

Introduction overlaps considerably with that of felsic magmas. Simply stated, cordierite nucleates as a result of CORDIERITE Occurs as a characteristic, usually changes in T-P-X that drive the chemical system into accessory, phase in many types of felsic peralumi- the cordierite stability field. But what are the spatial nous extrusive and intrusive igneous rocks (rhyolites, and textural criteria that reveal the environment of granitoids, pegmatites). Such rocks commonly have formation? This paper reviews the occurrences of high A/CNK (> 1.1) [A/CNK = mol. A1203/(CaO + cordierite in felsic igneous rocks, and presents a Na20 + K20)], STSr/86Sri (> 0.7075), and glSo comprehensive classification for understanding its (> +10 %OSMOW). In these rocks, cordierite shows a origins. wide range of distribution both relative to contacts with country rocks, and to contacts between internal facies in felsic igneous bodies. It also exhibits a Cordierite composition and structure variety of coexisting mineral assemblages and textures Cordierite is an orthorhombic tectosilicate with the (grain sizes, grain shapes, and relationships with the general formula: Xo_lM2T9018 , where X = H20, coexisting phases, such as inclusions, intergrowths, CO2, Ar, Xe, Na, K occupying large channels parallel and reactions). The principal question concerning the to the c-axis created by the pseudohexagonal rings presence of cordierite in felsic igneous rocks is not (Armbruster and Bloss, 1982); M = Mg 2+ , Fe2+ (with whether this mineral is diagnostic of a particular possibly some Li§ in octahedral co-ordination; and T genetic type of granite (White, 1989), but how it = AlnSi5 (with possibly some Be 2§ in tetrahedral co- formed. In general terms, the answer is relatively ordination. Cordierite has a structure similar to that straightforward. Like any other mineral, cordierite is a of beryl, forming prismatic crystals commonly predictable thermodynamic entity in temperature (T)- showing a prominent (001) parting, and it may pressure (P)-composition (X) space, a space that develop simple, lamellar, or cyclic twinning. Indialite

Mineralogical Magazine, June 1995, Vol. 59, pp. 311-325 Copyright the Mineralogical Society 312 D. B. CLARKE is the high-temperature hexagonal polymorph, with a Z(Mg+Fe2+), Mg/Fe 2§ A/CNK, aA12o3, and fo2 continuum of AI:Si ordering linking it to cordierite relative to cordierite-free granitic rocks (Clemens (Miyashiro, 1957). and Wall, 1988; Zen, 1989; Patifio Douce, 1992). (The rare occurrence of sekaninaite (Fe > Mg) as large crystals in pegmatite (Stan~k, 1954, 1991) T-P-X framework represents an exception to the condition of high Mg/ The stability region of cordierite occupies a discrete Fe). Additional tests of mineral assemblage, texture, part of T-P-X space that embraces both the normal chemical partitioning, and phase equilibrium metamorphic and igneous domains (Fig. 1). In this constraints combine to determine whether cordierite overlap lies the dilemma -- is any given cordierite is cognate or foreign to its host felsic igneous rock grain metamorphic or igneous? In practice, the (Clarke, 1981b). problem is even more complicated, because a In a related connection, Gordillo (1984) used cordierite could have both metamorphic and cordierite-garnet pairs in migmatites, and Fang and igneous growth histories, or be entirely metaso- He (1985) used the same coexisting phases in matic. Figure 1 shows a schematic T-P stability field granites and volcanic rocks, to estimate magmatic for cordierite, including many of the important T-P conditions, with varying degrees of success bounding reactions discussed subsequently in this depending on the extent of (re)equilibration between paper. In reality, the bounding reactions are zones, cordierite and garnet. rather than lines, as a consequence of extensive solid solution in most of the phases, and the boundaries Formation of cordierite also shift their positions in T-P space as a function of other chemical parameters. Favourable composi- The following sub-sections classify cordierite in tional (X) conditions include high values for felsic igneous rocks into three genetic types --

A - aluminosilicate B - biotite C - cordierite G - gaarnet K - K-feldspar L L - silicate melt G M - muscovite [K) M L O - orthopyroxene B Q - quartz V V - aqueous fluid ! Tla, b

P Tla, b B~

T2d- 'Iv CKQV

r3 T FI~. I. Schematic petrogenetic grid for cordierite in felsic magmas. The shaded region indicates the stability region of cordierite in association with felsic silicate melts. Arrows refer to types of reactions producing cordierite. Precise location of reactions depends on compositional parameters of the system, and most univariant boundaries are in reality divariant zones. Sources: Hensen and Green, 1973; Green 1976; Abbott and Clarke, 1979; Grant, 1985; Vielzeuf and Holloway, 1988; Clemens and Wall, 1988; Jones and Brown, 1990. CORDIERITE IN FELSIC IGNEOUS ROCKS 313 metamorphic, magmatic and metasomatic (Table 1). morphic rocks of semi-pelitic to pelitic composi- The organization of the types is from foreign (Types tions, and incorporation of cordierite-bearing la,b) to cognate (Type 2), and from melt-solid fragments of these country rocks into the invading reactions (Types la,b and 2a-c), to melt-solid- magmas is almost inevitable. Particularly in those vapour reactions (Type 2d), to solid-vapour cases where cordierite in the pluton concentrates near reactions (Type 3). The selected examples attempt the contact with the country rocks, or in regions with only to provide clarity of illustration; Zen (1988) high contents of pelitic xenoliths, the spatial evidence describes further examples. for a xenocrystic origin is strong. Ideally, such cordierite grains have poikiloblastic textures, and show some textural evidence of their disequilibrium Metamorphic --formation as the result of solid-solid with the melt phase (corrosion, euhedral epitaxial reactions overgrowth) (Vernon and Collins, 1988). In reality, Type 1 Metamorphic: (a) Xenocrystic. The former, however, the original habit, types of inclusions, and and mistaken, notion that cordierite was an chemical composition of the xenocrystic cordierite exclusively metamorphic mineral led to the vener- may be highly variable. able interpretation that all cordierite in felsic igneous Brammall and Harwood (1932) used a xenocrystic rocks was xenocrystic. Indeed, granitic magmas explanation for some of the cordierites in the commonly intrude low- to medium-grade meta- Dartmoor granite, and D'Amico et al., (1981)

TABt.Z 1. Characteristics of cordierite types

Type Origin Ideal spatial and textural characteristics

I a Xenocrystic Spatial correlation between cordierite in granite and cordierite in (Fig. 2) L-, Crd- (at least initially) country rocks and/or enclaves; anhedral to euhedral grain shapes; small (mm) grain size lb Restitic Textural features consistent with high-grade metamorphic origin, (Fig. 3) L § Crd or but ranging to cordieritites; in clots with, or with inclusions of, L +, Crd + fibrous sillimanite, spinel, foliated biotite; other textural characteristics transitional between Type la above and Type 2c below 2a Peritectic Magmatie (F) Most readily identified spatially in leucosomes (and/or melano- (Fig. 4) L-, Crd§ S- somes) of migmatites; early cordierites may have rounded quartz inclusions in core; larger (cm) grain size; as degree of partial melting increases, indistinguishable from Type 2c below 2b Peritectic Magmatic (T) Characteristic of rapidly ascending melts (aplites, rhyolites); (Fig. 4) L-, Crd§ S- occurring as reaction rims on garnet, with or without inclusions of biotite and/or orthopyroxene -- if reaction complete, then possibly indistinguishable from Type 2c below 2c Cotectic Magmatic Ubiquitous; euhedral grain shapes, few inclusions, grain size (Fig. 5) L-, Crd § S § compatibility with host rock (whether fine-grained rhyolite or aplite, or coarse-grained granite) 2d Fluido-Magmatic Large (cm-m) grain size; anhedral to euhedral grains associated (Fig. 6) L-, V § Crd§ with aplite-pegmatite contacts, miarolitic cavities, and/or pegmatitic cores 3 Subsolidus Metasomatic Euhedral grain to anhedral clots; distribution along structural (Fig. 7) V § S-, Crd§ weaknesses in granite; commonly within leucocratic biotite-free haloes; possible pseudographic intergrowths with quartz

Notes 1. Abbreviations: Crd -- cordierite; L -- silicate melt; S -- other solid phases; V -- aqueous fluid phase; + phase increasing in modal abundance as reaction proceeds; - phase decreasing in modal abundance as reaction proceeds. 2. Exceptions to practically every textural criterion, and late-stage reactions of cordierite with the magma and/or subsolidus reactions with fluids, combine to diminish the usefulness of textural evidence as a reliable criterion to infer the origin of cordierite. 314 D.B. CLARKE

FIG. 2. Type la xenocrystic cordierites and Type lb restitic cordierites. (a,b) Sample LIP-2. Type la xenocrystic cordierite, 2 mm across, in plane-polarized light (ppl) and crossed nicols (xn), respectively, from rhyolite lava flow, Lipari, Aeolian Islands. Note anhedral shape, relict sector twinning, and vermicular inclusions of iron-rich glass (arrow, medium gray in ppl and black in xn) produced as the cordierite melted. (c,d) Type lb restitic cordierite (in ppl and xn, respectively) with inclusions of sillimanite in microgranite from the Petite Kaby|ie Massif, northern Algeria (photographs courtesy of S. Fourcade). determined that much of the cordierite in the pelitic xenoliths (Fig. 2a, b), although some cordier- peraluminous granites of the Calabrian arc is ites at the margins of xenoliths have grown in a xenocrystic. Maillet and Clarke (1985) recognized a reaction relationship between the melt and the textural and compositional gradation from meta- xenolith. Ugidos and Recio (1993) cite the strong morphic to xenolithic to magmatic cordierite and correlation between the modal abundance of made a case for metamorphic seed crystals for cordierite in granites of the Central Iberian Massif, cordierites of the South Mountain Batholith. Barker and the proximity of cordierite-rich country rocks (1987) interpreted anhedral cordierites in a rhyolite and enclaves, as evidence for a xenocrystic origin for from Lipari as xenocrysts from the disaggregation of the cordierite present in the granites. CORDIERITE IN FELSIC IGNEOUS ROCKS 315

How can metamorphic cordierites become xeno- dacite from Cerro del Hoyazo in SE Spain (Zeck crysts in a granitic melt? The combination of heating 1970, 1992); subhedral cordierite with inclusions of of an enclave leading to melting along its grain sillimanite from the Petite Kabylie Massif, northern boundaries, and differential thermal expansion of the Algeria (Fig. 2c, d) (S. Fourcade, pers. comm., 1993); minerals of the xenolith or country rock, may result zoned cordierites in rhyolites of the Cerberean in disaggregation of the xenolith (Ugidos, 1988, Cauldron, SE Australia (Birch and Gleadow, 1974); 1990; Ugidos and Recio, 1993). Thus, cordierites of clots of Crd-Bt-Sil in a granite (Morin and Turnock, metamorphic origin may be released into a silicate 1975); anhedral Type B cordierites with inclusions of melt and, in general, they would be out of chemical biotite and/or sillimanite and spinel in the Strathbogie equilibrium with that melt. Their subsequent history batholith (Phillips et al., 1981); cordierite-bearing then depends on the degree to which they are out of leucosomes in highly aluminous Qtz-P1-Kfs-Crd-Grt- equilibrium with the magma, and on the chemical Sil-Ilm-Gr gneisses from central Massachusetts kinetics of the new environment. Xenocrystic (Tracy and Robinson, 1983); anhedral cordierite cordierite may disappear rapidly in a high- from the subsolidus protolith (coexisting with temperature, well-mixed, relatively fluid metalumi- newly nucleated magmatic cordierite grains) in the nous melt, survive largely unmodified in a near- leucosomes of the Kelly's Mountain migmatites of solidus, static, viscous peraluminous melt, or grow in Nova Scotia (Jamieson, 1984); and Type B cordierite a highly peraluminous melt. Expressed as a simple in the Violet Town volcanics of SE Australia equation, (Clemens and Wall, 1984). Cordieritites (rocks predominantly consisting of Crdl + L ~ Crd2 (1) cordierite) constitute an important subset of the where Crdl is the original metamorphic cordierite, L monomineralic rock problem in igneous petrology. is the silicate melt phase, and Crd2 is the re- Such unusual bulk rock compositions arise either by equilibrated cordierite in the melt. Crd2 is almost concentrating the chemistry to make the minerals, or certainly different in size, shape, and chemical by concentrating the minerals to make the chemistry. composition compared with Crdl. Ugidos (1988, p. 131) described the cordieritites of Type 1 Metamorphic: (b) Restitic. Wall et al. the Central Iberian Massif as the residues of partial (1987) described some general principles for melting and re-equilibration, and concluded that the recognizing restitic material. Chappell et al. (1987) cordierite composition is the product of removal of a presented the specific case for the occurrence of granitic component from a pelitic source rock in restite in granitic rocks and discussed its implica- which "Leucogranites and cordieritites...represent tions. In simple terms, restite minerals are those that melt and residue, respectively". In this scenario, it comprise the refractory residua of partial melting. is not surprising that such cordierites may also occur Given that partial melting is the rule in magma in the complementary granitic magma. generation, restite always exists, somewhere. Whether that refractory restite remains largely in Magmatic --formation as the result of solid-melt situ after a partial melting event, or ascends with the reactions magma, is still an open question. In this paper I make the theoretically important (but practically difficult) Type 2 Magmatic: (a) Peritectic (TT). This distinction between restitic cordierite that was category includes cordierite that appears only as the present in the source rocks before partial melting result of a melt-producing reaction in response to began (Type lb), and magmatic peritectic cordierite rising temperature. In other words, cordierite is not that appears only as a result of the melt-producing part of the subsolidus mineral assemblage at the onset reaction (Type 2a below). In Type lb, modal of partial melting, but it appears as melting begins in cordierite in the anatectic system may increase or a peritectic reaction such as decrease as melting proceeds, whereas in Type 2a, Als + Bt + Qtz ~ L + Kfs + Crd (2) modal cordierite first nucleates as melting takes place and increases during the initial stages of partial Spatially, this cordierite may form along the contact melting. Any magma that was in equilibrium with between pelitic xenoliths and melt (Fig. 3a), or cordierite as a restite phase in the region of partial associate with the melt phase (initially as leuco- melting is itself saturated in cordierite, and is likely somes) rather than the refractory residuum (restite) to remain saturated in cordierite during its ascent, (Fig. 3b). These cordierites have no subsolidus whether or not it is accompanied by entrained restite, metamorphic history, and thus could possibly be and whether or not the melt becomes contaminated euhedral and free of inclusions. Other reactions, most with typical upper crustal country rocks. of which are dehydration melting reactions (Hensen Examples of Type lb restitic cordierite include: and Green, 1973; Green 1976; Abbott and Clarke, anhedral cordierite with fibrolite inclusions in a 1979; Hoffer and Grant, 1980; Grant 1985; Vielzeuf 316 D.B. CLARKE

i~IG. 3. Type 2a and Type 2b peritectic magmatic cordierites. (a) Sample L83-L51(A). Concentration of Type 2a cordierite (arrows) along the contact between semi-pelitic xenolith (lower part of photograph) and host granite, South Mountain batholith (field of view 8 mm wide). (b) Sample KM82. Large (6 mm) reaction cluster of Type 2a cordierite, biotite, and opaque oxides in leucosome of Kelly's Mountain gneiss. Normal grain size of mesosome evident on right side of photograph. (c) Garnet with reaction rim of Type 2b cordierite in Velay granite (after Weber et al., 1985). (d) Garnet with reaction rim of Type 2b cordierite in Violet Town volcanics (after Clemens and Wall, 1984). and Holloway, 1988; Clemens and Wall, 1988; Jones the presence of a silicate melt phase. The close and Brown, 1990), which may account for the association between peraluminous source rocks and appearance of cordierite during melting of pelitic the appearance of cordierite during melting explains and semipelitic rocks, are: the intimate association between cordierite and peraluminous felsic igneous rocks. This statement Als+Bt+Kfs+Qtz+V ~ L+Crd (3) does not diminish the importance of other mechan- Als + Bt + Pll + Qtz ~ L + Crd + Kfs + P12 (4) isms for producing peraluminous granites Als+Bt+PI+Qtz+V ~L+Crd+Grt (5) (fractionation of hornblende from metaluminous Als + Bt + Qtz ~ L + Kfs + Crd + Grt (6) melts, contamination, vapour-phase transfer; Clarke Als + Grt + Qtz ~ L + Crd (7) 1981a; Halliday et al., 1981), nor other ways in Bt + Grt + Qtz ~ L + Kfs + Crd + Opx (8) which cordierite may form (this paper). And perhaps Bt+Pl+Grt+Qtz+V ~ L+Crd+Opx (9) among all of these cordierite-producing peritectic Bt + Qtz ~ L + Kfs + Crd + Opx (10) reactions, Equation 2 involves the most common For example, Ellis and Obata (1992) used reaction phases as a starting assemblage; it takes place at the (3) to account for the formation of melt in the lowest temperature, and is thus probably one of the migmatite at Cooma, SE Australia, under conditions most important reactions producing cordierite in of T = 670-730~ and PH20 = 3.5--4.0 kbar. felsic igneous rocks. Cordierite produced by any of the above reactions Type 2 Magmatic: (b) Peritectic (T ~ and~or P],). A is magmatic because it owes its existence entirely to second type of peritectic magmatic reaction occurs in CORDIERITE IN FELSIC IGNEOUS ROCKS 317 which cordierite appears in response to falling batholith (Phillips et at., 1981); and Bavarian Forest temperature and/or pressure. Figure 1 .shows that (Propach and Gillessen, 1984). That many of these drops of pressure across several reaction boundaries reactions (11-14) may go to completion, especially can produce cordierite, possibly as reaction rims on in the case of isobaric cooling, could explain the other phases (particularly garnet and aluminosilicate) relative scarcity of this type of cordierite. The in rapidly ascending magmas, or even in isobafically cordierite so produced, may be indistinguishable cooling granites. Some reactions are: from Type 2c cordierites below. Type 2 Magmatic: (c) cotectic --fluid under- L + Kfs + Grt --* Crd + Bt + Qtz (11) saturated. In theory, at least, the overlap between the L+Grt ~ Crd+Bt+Qtz+V (12) stability fields of cordierite and granitic magmas L + Grt + Qtz + Als ~ Crd + Bt (13) suggests that cordierite can nucleate directly from a L + Als --* Crd + Kfs + Qtz + V (14) silicate melt. The problem is to distinguish this type Examples of this type, in which cordierite occurs as a from all the others (xenocrysts, restites, peritectic reaction rim on garnet, include: Velay granite (Weber magmatic). Whereas a non-descript, anhedral to et al., 1985) (Fig. 3c); Violet Town volcanics euhedral cordierite megacryst in a granitic rock (Clemens and Wall, 1984) (Fig. 3d); Strathbogie may have a number of possible origins, an inclusion-

FIG. 4. Type 2c cotectic magmatic cordierites. (a) Sample KM79-043e. Euhedral sixling, 2 mm in diameter, in leucosome of Kelly's Mountain migmatite. (b) Sample 103-A5. Euhedral sixling, 0.5 mm in diameter, in South Mountain batholith aplite. (c) Sample D15-0056. Large (8 mm) subhedral prismatic cordierite with good (001) parting in Musquodoboit batholith, Nova Scotia. (d) Euhedral microphenocryst in Violet Town volcanics (after Clemens and Wall, 1984). 318 D.B. CLARKE

free, anhedral to euhedral cordierite, in textural associated with peraluminous granites (Fig. 5a-c). equilibrium with quartz and feldspars in an aplite Individual cordierite crystals may range in size from or rhyolite beating no evidence of contamination, is millimetres to metres, and they commonly occur near much less problematic. If any cordierite is magmatic the quartz-rich cores of the pegmatites (Heinrich cotectic, this type should be. 1950; Goad and (~ern2~, 1981; Povondra et al., 1984). Examples include euhedral cordierite sixlings from Their widespread occurrence in pegmatites, the Kelly's Mountain migmatite (R. A. Jamieson, commonly in association with other characteristic pers. comm., 1993) (Fig. 4a) and from an aplite in the peraluminous minerals such as aluminosilicates, South Mountain batholith (M. A. MacDonald, pers. spinel, corundum, tourmaline, dumortierite, topaz, comm., 1993) (Fig. 4b); the Musquodoboit batholith, beryl, pyrophyllite and diaspore, suggests conditions Nova Scotia (Fig. 4c); euhedral cordierite without highly favourable for nucleation in the fluid-saturated any spatial relationship either to the contacts of the magmatic system. But is the role of the super-critical Halifax Pluton or the abundance of xenoliths aqueous fluid phase as a medium for precipitation, or (MacDonald and Horne, 1988); the Dartmoor is it as an agent to modify the silicate melt granite (Brammall and Harwood, 1932); euhedral composition? cordierite in peraluminous granite plutons of the Deduced origins for cordierite in pegmatites southern Appalachians (Speer, 1981); Type A include xenocrystic (Schumacher, 1990), direct cordierite in the Strathbogie batholith (Phillips et crystallization from the pegmatitic fluid locally al., 1981); and Type A euhedral cordierite contaminated by A1 and Mg from biotite gneisses phenocrysts in the Violet Town volcanics (Clemens (Gordillo et al., 1985), and direct crystallization from and Wall, 1984) (Fig. 4d). the pegmatitic melt phase as a result of alkali loss Nucleation of cordierite in the magmatic cotectic (Goad and ~ern~, 1981). Excluding cordierite of environment is simply a case of the silicate melt demonstrably foreign origin, the common thread reaching saturation in cordierite through changes in linking other occurrences appears to be a change in T, P, or X (reaching an appropriate chemical the bulk chemical composition of the fluido- composition, favoured by high A/CNK, high magmatic system, resulting from extensive Y,(Mg+Fe2+), and generally low fH2o, by some Rayleigh distillation of alkalis into the highly process such as fractional crystallization). With a mobile and fugitive fluid phase. In this case, the large field of overlap between the stability fields of residual silicate melt may experience a considerable cordierite and granitic melts in T-P space, and with increase in A/CNK with alkali loss resulting in its cordierite stable down to the solidus temperature in somewhat unexpected saturation in cordierite. A most cases, the reaction must approximate general equation is: L --* Crd + Other Solid Phases (15) L ~ Na-K Aqueous Fluid + Crd (17) Zen (1989) identified the critical reaction Although aH20 ~ 1, the formation of cordierite involving several AFM minerals as occurs because loss of K from the silicate melt makes the formation of biotite unlikely. The role of the Kfs + Grt + Crd + H20 ~ Ms + Bt + Qtz (16) aqueous fluid phase is significant in two respects: it The addition of water to the left side of the abruptly raises the A/CNK of the melt, and it equation converts 'dry' Crd __+ Grt-bearing granites facilitates chemical migration of elements to to 'wet' two-mica granites; in other words they are produce the large cordierite crystals characteristic compositional equivalents except for the amount of of some pegmatites (Heinrich, 1950). This fluid-loss water. This is a first-order approximation to the process for the formation of cordierite probably differences between exotic Crd-Grt granites and explains the zones of cordierite in pegmatites better common Bt-Ms (two-mica) granites. than a magmatic cotectic (Type 2c) process, as might Type 2 Magmatic: (d) cotectic --fluid saturated. occur in the type of pegmatitic system envisaged by Zen (1989) noted the association of cordierite with London (1992). water-undersaturated granitic melts. Indeed, in many cordierite-bearing plutons, cordierite belongs to the early stages of crystallization prior to saturation in an Metasomatic --formation as the result of solid-fluid reactions aqueous vapour phase. The occurrence of cordierite in pegmatites, however, limits the usefulness of the Type 3: metasornatic -- subsolidus. Only two types generalization that cordierite necessarily signifies of cordierite can occur in association with a felsic 'early and dry'; it may also mean 'late and wet'. silicate melt (Type 1 metamorphic and Type 2 Cordierite can occur in small pegmatitic 'sweats' magmatic, described above). However, another in cordierite-bearing granulite facies rocks variety occurs so intimately with granites and (Thomson, 1989) or in large zoned pegmatites pegmatites that it requires discussion here. t~ern2~ et CORDIERITE IN FELSIC IGNEOUS ROCKS 319

al. (1967), and 12ern3~ and Povondra (1967), (poikiloblastic?) cordierite containing inclusions of described fracture-controlled 'chains' of cordierite quartz in the Velay granite of the Massif Central . crystals cutting across the primary internal zoning of The nodules concentrate along structural disconti- pegmatites of west Moravia (Fig. 6a). ~ern~ and co- nuities in the host granite and are surrounded by workers suggested that this cordierite is the product leucocratic (biotite-free) zones (Fig. 6b). Didier and of the reaction Dupraz speculated that CO2-rich fluids at near- solidus temperatures exploit structural weaknesses 4Ab + 2MgO ~ Crd + 7Qtz + 2Na20 (18) in the granite and promote formation of cordierite. in which the source of the MgO could be closed Although they did not attempt to write the reaction, it system within the pegmatite (autometasomatic), or must be similar to open system (originating from the MgO-rich Fsp + Bt + Fll ~ Crd + (Na-K) F12 (19) serpentinite wall rocks). Note that this reaction potentially yields cordierite-quartz intergrowths. In The leucocratic haloes around the cordierite nodules another example, Didier and Dupraz (1985) attest to the consumption of biotite from a volume of described nodules (1-10 cm) of poikilitic rock larger than currently occupied by the nodule.

FIG. 5. Type 2d pegmatitic cordierites. (a,b) Samples D15-0640 and 103-P7. Anhedral cordierites (arrows) growing from an aplite-pegmatite contact into the coarse-grained pegmatite, South Mountain batholith (widths of photos 8 mm and 18 mm, respectively). (c) Large euhedral cordierite from pegmatite, South Mountain batholith. 320 D.B. CLARKE

gra--hic ,'e-'matite ~ ~ ~ ~ ,'.'.-:.~f71='~

50 Cm

FIG. 6. Type 3 metasomatic cordierites~-(a) Euhedral and ovoid cordierites associated with cross-cutting fracture in the Vezna pegmatite (after ~erny and Povondra, 1967). (b) Cordierite pods associated with microshears in the Intres granite, Massif Central (after Didier and Dupraz, 1985). Note the leucocratic, biotite-free haloes around each cordierite. (c) Sample M79-085. Metasomatic cordierite, with (001) parting and rich in anhedral quartz inclusions, in aplite, Musquodoboit batholith, Nova Scotia (width of photo 12 mm).

Type 3 metasomatic cordierite may not be and Clarke, 1979), in the Bavarian Forest (Propach particularly common, but because it is secondary, it and Gillessen, 1984), and at Cooma (Ellis and Obata, may occur anywhere in the felsic igneous system; all 1992). The occurrence of cordierite with a corona of that is required for its formation is fluid, feldspar, and andalusite and biotite is a useful general geobarom- a source of Fe-Mg (e.g. in an aplite, Fig. 6c). eter because such a reaction must occur at pressures greater than X (Fig. 1), however the position of X is not independent of compositional parameters in P--T Breakdown of cordierite space, although it probably should be less than 4 Cordierite crystals in felsic igneous rocks are rarely kbar. pristine or euhedral, making assignment of their Other reactions to eliminate cordierite may occur origin into Types 1-3 difficult. Again, in simple under subsolidus conditions with reactions such as: terms, elimination of cordierite begins whenever it Crd + K-Aqueous Fluid ~ Ms + Green Bt (21) leaves its own T-P-X stability field. This departure Crd + K-Aqueous Fluid ~ Ms + Chl (22) may occur above the appropriate 'granite' solidus in (pinite) a reaction such as The irony of these breakdown processes is that L + (Bt +) Crd ~ Als + (Bt +) Kfs + Qtz + V (20) they are complementary to those that produce Type e.g. as occur in the Musquodoboit batholith (Abbott 2d cordierites; in other words, the same fluids that are CORDIERITE IN FELSIC IGNEOUS ROCKS 321 indirectly responsible for the formation of cordierite Problematic cases in aplite-pegmatite systems may destroy Types la- 2c cordierite elsewhere in the same pluton. Several different types of problem arise with reliance Unfortunately, these processes not only degrade the on this spatial-textural classification of cordierite in textural and chemical features that permit determina- felsic igneous rocks: (1) various types of alteration tion of the origin, but also they may proceed to the (say, to 'pinite') may obscure genetic relationships; point where even the prior existence of cordierite (2) absence of chemical parameters, particularly trace cannot be easily recognized. Figure 7 illustrates element profiles, and knowledge of the chemical several types of cordierite breakdown reactions. compositions of cordierite and their coexisting

F~G. 7. Cordierite breakdown reactions. (a) Sample D15-0560. Cordierite ~ Andalusite + Biotite. Anhedral grain of cordierite partially broken down to a mixture of biotite and granular andalusite, Musquodoboit batholith (width of photo 7 mm). (b) Detail of (a) showing andalusite (arrows). (c) Sample M79-032. Cordierite ~ Biotite. Anhedral grain of cordierite, 8 mm long, with wide reaction rim of biotite, Musquodoboit batholith. (d) Sample Dt5-0439. Arrested reaction Cordierite ~ Muscovite + Chlorite (Pinite), Musquodoboit batholith (original grain 9 mm long). (e) Sample D15-0545. Completed reaction Cordierite ~ Muscovite + Chlorite (Pinite), South Mountain batholith (grain with arrow 7 mm long). 322 D.B. CLARKE

Tla,b; T2b,c

.... ,~~'~-~ L70 i~!iii~i~!iii:!i:;~!i~i::~184184 "!" ,.~ )/-;(.. T2b,d; T3 + ./i~(:~ r .... T1 a,b; T2a :'~"~"~" + ~i;i;iiii!iiii~ilupper ~iii!i!]~i~ii~iiii J - ~.!~'i~// ~iiiiiiii!il;i;: crust ~:~::ii;i:~i + "J ii~i::i,!!!!i:!;!; +

FIG. 8. Cross-section showing overlapping environments of cordierite occurrences in felsic igneous rocks. CORDIERITE IN FELSIC IGNEOUS ROCKS 323 phases, both of which may yield useful information volumetrically small special cases. Probably the about attainment of equilibrium, zoning, growth single most important reason for the occurrence of history, and P-T conditions of formation, restricts cordierite in peraluminous felsic igneous rocks is the the usefulness of this classification; (3) random multitude of cordierite-producing melting reactions sections of cordierite grains may not reveal in pelitic and semi-pelitic rocks, however magmatic diagnostic features such as inclusions of sillimanite cordierites are an almost inevitable crystallization or spinel; (4) a potentially large variety of cordierite product of these peraluminous magmas as they xenocrysts, ranging from anhedral and poikiloblastic ascend into the upper crust. Figure 8 demonstrates to possibly euhedral and inclusion-free (in fact, all of the high degree of spatial overlap of the various Types 1-3), greatly complicates the simple two-fold cordierite types in a single hypothetical pluton. (xenocrystic, restitic) model presented here; and, most importantly (5) several types of cordierite can Acknowledgements cross physical and, therefore, classificatory boundaries (for example, a Type 2a magmatic peritectic I acknowledge financial support in the form of a cordierite, formed in a melting reaction, may become research grant from the Natural Sciences and a Type 2c magmatic cotectic cordierite after more Engineering Research Council (NSERC) of Canada. extensive melting, or a Type l a metamorphic Rebecca Jamieson, Daniel Kontak, and Michael xenocrystic or Type lb metamorphic restitic MacDonald offered helpful comments on a draft cordierite may also become the core of a later Type version of this manuscript. Serge Fourcade, Rebecca 2c magmatic cotectic cordierite). Jamieson, and Michael MacDonald supplied some of Nevertheless, spatial and textural relations have the essential samples and photographs. I wish to much to offer toward a first-order interpretation of thank the two official and anonymous reviewers, as the occurrence of cordierite in felsic igneous rocks. well as Jose M. Ugidos and E-an Zen, all of whom provided many comments to improve the manuscript. I also wish to thank Jose M. Ugidos for the Summary and conclusions opportunity to participate in a granite workshop in The type of analysis of cordierite types presented in Salamanca, and to test these ideas before a group of this paper (i.e. classification on spatial and textural constructively critical colleagues. criteria, definition of T-P-X space, examination of reactions, relation to genetic types) is easily References adaptable to any AFM mineral (Bt, Ms, Crd, Als, Grt, etc.) in peraluminous igneous rocks (in fact, to Abbott, R. N. and Clarke, D. B. (1979) Hypothetical any mineral in any rock). The types are probably liquid relationships in the subsystem AlzO3-FeO- similar, the T-P-X spaces are different but overlap to MgO projected from quartz, alkali feldspar and some extent, and the reactions have both theoretical plagioclase for a(H20) ~< 1. Can. Mineral., 17, and experimental validity. 549-60. Partial melting of pelitic material under high Armbruster, T. and Bloss, F. D. 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