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Structural Criteria for Identifying Granitic Cumulates Author(s): R. H. Vernon and W. J. Collins Source: The Journal of Geology, Vol. 119, No. 2 (March 2011), pp. 127-142 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/10.1086/658198 . Accessed: 07/09/2011 21:41

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http://www.jstor.org Structural Criteria for Identifying Granitic Cumulates

R. H. Vernon1,* and W. J. Collins2

1. Department of Earth and Planetary Sciences and Australian Research Council (ARC) National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), Macquarie University, Sydney, New South Wales 2109, Australia; 2. Department of Geology, James Cook University of North Queensland, Townsville, Queensland 4811, Australia

ABSTRACT Cumulates are igneous rocks that reflect relative concentration of crystals and/or loss of melt and that therefore did not crystallize entirely from a magma of their current whole-rock composition. However, this definition does not help to identify cumulates, and so the problem is to determine whether the current whole-rock composition reflects cumulate processes, which is where structural criteria can be useful. Identification of cumulates in granitoids can be difficult, especially in the absence of major whole-rock compositional differences between cumulate and host. Structural and structural/mineralogical criteria diagnostic of, or at least consistent with, granitic cumulates include (1) locally high concentrations of particular minerals relative to concentrations in the bulk of the intrusion; (2) abundant euhedral crystals in contact, at least with regard to cores that touched before any overgrowths and consequent molding of minerals about each other occurred; (3) abundant antecrysts or mantled xenocrysts; (4) adjacent crystals of plagioclase with different zoning patterns; (5) crystal contact melting; (6) concentration of a wide variety of microgranitoid enclaves (“mafic inclusions”) in a megacryst-rich granitoid; (7) interpenetration of microgranitoid enclaves between abundant feldspar crystals; (8) intricate penetration of fine-grained mafic aggregates between feldspar crystals; (9) granitoid devoid of or extremely poor in quartz, especially with a strong magmatic foliation, occurring beneath or between major enclaves; (10) mobilized crystal aggregates with interstitial liquid, forming local, intrapluton intrusions; (11) graded layering, channel fills, and cross-lamination involving concentrations (schlieren) of mafic minerals, in some instances with K-feldspar megacrysts and/or microgranitoid enclaves; (12) continuous and partly disintegrated sheets of more mafic magma in mafic and silicic layered intrusions (MASLI plutons); (13) layered, magmatically foliated pluton margins from which leucogranitic lenses have been squeezed, forming projections into the pluton; (14) marked, patchy, or streaky modal variation; (15) “glomeroporphyritic” aggregates, implying local concentration of crystals by either heterogeneous nucleation or synneusis; and (16) compositional similarity of minerals in the most and least mafic rocks of the pluton. The proportion of “interstitial” mineral conveys no information as to the amount of “trapped liquid” in “orthocumulates.” Reliable application of these structural criteria requires ascertaining that magmatic structures have not been obliterated during slow cooling, as is often asserted. The common presence in granitoids of primary magmatic features indicates that magmatic structural evidence is not removed during cooling in the absence of external deformation or metamorphism and so that structural evidence is potentially useful for identifying granitic cumulates.

Introduction This article discusses structural and structural/ uncommon or only local. However, increasing evi- mineralogical evidence potentially useful for iden- dence suggests that loss of interstitial liquid from tifying granitic cumulates. The question of the ex- granitic plutons can leave behind cumulates. For tent to which granitoids are cumulates is one of example, geophysical evidence of large plutons be- long standing, some petrologists regarding them as neath calderas is consistent with the granitoids in being very common and others regarding them as these plutons being cumulates involved in fractionation before eruption (e.g., Lipman 2007). Loss of some interstitial “haplogranitic” liquid from a Manuscript received June 2, 2010; accepted October 30, 2010. * Author for correspondence; e-mail: ronald.vernon@mq crystal-rich magma (i.e., a “crystal mush”) could .edu.au. (1) be responsible for aplites and high-silica rhyo-

127 128 R. H. VERNON AND W. J. COLLINS lites, which are chemically similar (Bachmann and forming processes in plutons, present detailed cri- Bergantz 2008), and (2) tend to drive the composi- teria for identifying granitoid cumulates. tion of the residual material toward more mafic compositions. If the magma contained 50%–60% crystals, it would form a semirigid skeleton inhib- Definition of Felsic Cumulate iting chamberwide convection currents but per- meable enough for melt to be extracted effectively Irvine (1982, p. 127) defined cumulate structure as (e.g., Bachmann and Bergantz 2008). The resulting being “characterized by a framework of touching pluton need not show much evidence of local crys- mineral crystals and grains that evidently were con- tal accumulation if the crystal network remains centrated through fractional crystallization of their largely intact during the liquid loss (e.g., Collins et parental magmatic liquids.” He also wrote (p. 131), al. 2006) and the mass settles uniformly. Some rel- A cumulate is defined as an igneous rock char- atively homogeneous granites could be partial re- acterized by a cumulus framework of touching sidual cumulates formed in this way. Plutons that mineral crystals or grains that were evidently lose melt through recharge can consist partly, if not formed and concentrated primarily through frac- entirely, of cumulate, according to Brophy et al. tional crystallization. (Note that only a frame- (1996), Bachmann and Bergantz (2004, 2008), Hil- work of touching crystals is required. Not all dreth (2004), Eichelberger et al. (2006), and Miller crystals have to touch.) The fractionated crystals and Wark (2008). are called cumulus crystals. They typically are However, clear evidence of granitic cumulates subhedral to euhedral, and generally they are ce- depends on detailed examination of pluton struc- mented together by a texturally later generation tures. If crystal-free or crystal-poor synplutonic of postcumulus material that appears to have crystallized from intercumulus liquid in the in- feeder dikes (discussed below) are present, their terstices or pores of the cumulus framework. composition can be compared with that of the host granitoid, enabling estimation of the amount of cu- However, it cannot be assumed that an oikocrystic mulate present. However, in the absence of feeder “interstitial” mineral crystallized later than the cu- dikes, recognition of granitic cumulates can be dif- mulus minerals, as discussed below. ficult, especially where compositional differences Collins et al. (2006, p. 2090) defined cumulate as between the possible cumulate and the remainder “a touching framework of subhedral cumulus min- of the pluton are relatively minor, for example, (1) erals with overgrowths and smaller crystals of the if most of the rock is cumulate, through having lost same minerals filling the interstices.” This defi- interstitial liquid consistently during growth of the nition has the advantage of eliminating the prob- pluton (e.g., Collins et al. 2006), (2) if a granitoid lem of the relative growth timing of oikocrystic loses relatively small proportions of residual liquid minerals by emphasizing interstitial aggregates after all minerals have partly crystallized, or (3) if rather than single mineral grains. However, a prob- the magma is compositionally near a low-variance, lem with both definitions is that although touching multiply saturated cotectic. In these instances, crystals with overgrowths and interstitial mineral trace-element chemical composition (e.g., McCar- aggregates are consistent with a cumulate origin, thy and Hasty 1976; McCarthy and Robb 1978) can the description could also apply to granitoids that be an indicator of fractional crystallization and crystallize without evidence of crystal accumula- hence provide some indirect evidence of cumulate tion. formation. A technically correct and simple definition of cu- McCarthy and Groves (1979) and Miller and Mil- mulates is “igneous rocks that reflect relative con- ler (2002) have suggested that granitoids of a centration of crystals and/or loss of melt and that cumulate-like character may be more common therefore did not crystallize entirely from a magma than is generally recognized. However, the amount of their current whole-rock composition.” The of physical separation of crystals and liquid is prob- compositional difference can be achieved by addi- ably highly variable, ranging from crystal-rich cu- tion of crystals and/or loss of residual liquid (ac- mulates (either throughout the pluton or locally) companied by compaction and/or sintering of the to granitoids with a small proportion of cumulate, residual crystals, unless they are replaced by an which McCarthy and Hasty (1976, p. 1354) referred equivalent volume of fresh liquid). However, this to as forming from “cumulus enriched melts.” definition does not help to identify cumulates, and We begin with definitions and then discuss cri- so the problem is to determine whether the current teria for identifying pluton melt compositions from whole-rock composition reflects cumulate pro- feeder dikes, after which we outline cumulate- cesses; here, structural criteria can be useful. Journal of Geology IDENTIFYING GRANITIC CUMULATES 129

Recognition of Feeder Dikes ing behind cumulates commonly characterized by mafic layers or schlieren. Nonporphyritic granitic feeder dikes to plutonic systems are magmas that should closely approxi- mate melt compositions, but they are difficult to recognize. Rare, unequivocal field criteria include Structural Criteria of Granitic Cumulates exposures of pluton bases with dikes cutting the Many petrologists have inferred the presence of cu- contact and diffusing into the host granitoid in the mulates on the basis of so-called cumulate mi- pluton interior (e.g., Collins et al. 2006). The geo- crostructure (texture), without specifying what this chemical test is that the dikes lie on the same actually is. For many, this structure implies eu- chemical variation trends as the plutonic rocks. hedral “cumulus” crystals with interstitial liquid, Feeder dikes can be distinguished from late-frac- from which an “intercumulus” mineral later crys- tionated (aplitic) dikes by mineralogical and mi- tallizes. However, simultaneous crystallization of crostructural relationships. They contain minerals both minerals produces the same structure (dis- with the same chemical composition as minerals cussed below), and so more reliable criteria are in the pluton, whereas aplites typically lie at the needed. high-silica end of the plutonic system and have Many of the structural and structural/mineral- higher contents of incompatible elements. Also, ogical criteria discussed in this article refer to local the mineral compositions of aplites are typically occurrences and so should not be used to infer that more evolved, reflecting the fractionated nature of a whole pluton is of cumulate origin. Some of the the magma. Aplites also typically have fine- structural features are stronger indicators than oth- grained, granular (“saccharoidal”) microstructures, ers, but all are at least consistent with a cumulate whereas feeder dikes tend to be more variable, usu- origin. Application of as many of them as possible ally containing a range of euhedral minerals, none to the rock under consideration should strengthen of which are abundant (e.g., Collins et al. 2000, a cumulate interpretation. 2006). 1. Local mineral concentrations well in excess of the usual concentrations elsewhere in the pluton— Formation of Cumulates in Granitoid Plutons such as K-feldspar megacryst aggregates (e.g., Gil- bert 1906; Emeleus 1963; Smith 1975; Vernon 1986; Fractional crystallization can involve the forma- Abbott 1989; Elburg and Nicholls 1995; Clarke and tion of granitic cumulates in two ways. Clarke 1998; Weinberg et al. 2001; Paterson et al. 1. Crystals or crystal aggregates can be dislodged 2005; Vernon and Paterson 2008b, 2008c and ref- from elsewhere in the chamber and transported to erences therein), schlieren rich in euhedral horn- the deposition site (e.g., Paterson et al. 2005; Col- blende or biotite (e.g., Barrie`re 1981; Frost and Ma- lins et al. 2006; Vernon and Paterson 2008c;Za´k et hood 1987; Elburg and Nicholls 1995; Paterson et al. 2008). This process applies most clearly to al. 2005; Vernon and Paterson 2008c), and concen- schlieren rich in K-feldspar and/or hornblende and trations of quartz xenocrysts rimmed with fine- to tubes and pipes rich in K-feldspar megacrysts, grained mafic minerals (“ocelli”) extracted from a especially where sedimentary structures, such as hybrid zone—are all consistent with a cumulate cross-stratification and grading, are evident (e.g., origin (figs. 1–7). For example, Backlund (1938, p. Barrie`re 1981; Vernon and Paterson 2008c). 362) described rapakivi feldspar crystals in linear 2. For many other granitic cumulates, especially concentrations suggestive of flow patterns, reflect- those consisting of all the main minerals of the ing mechanical aggregation and sorting. rock, accumulation may be passive, occurring by 2. Euhedral crystals in contact, without molding upward release of lighter residual liquid produced of one mineral about the other or with a small pro- as a result of crystallization (“convective fraction- portion of molding, are consistent with accumu- ation”), as discussed by Sparks and Huppert (1984) lation and compaction rather than in situ growth and Sparks et al. (1985). This appears to be basically (figs. 3, 6). Moreover, because of the likelihood of the same as the process suggested by McBirney et postaccumulation overgrowth, the euhedral stipu- al. (1985) and Spera et al. (1995), involving marginal lation should apply only to the first point of contact (e.g., sidewall) crystallization and loss of the re- (neglecting possible overgrowths), which can be de- sulting less dense felsic liquid upward or laterally, termined in concentrically zoned crystals. This cri- in response to gravity (McBirney et al. 1985; Sawka terion is strengthened if the crystals show euhedral et al. 1990) or magmatic deformation (McCarthy growth-zoning patterns (e.g., Brigham 1984, figs. 1– and Groves 1979; Vernon and Paterson 2008c), leav- 8C), as shown in figure 3, because such patterns 130 R. H. VERNON AND W. J. COLLINS

Figure 1. K-feldspar megacrysts concentrated in some layers and aligned parallel to biotite-rich schlieren in shallow troughs or channels. South Mountain batholith, Chebucto Head, Nova Scotia, Canada. Base of photo 50 cm long. imply independent growth in liquid before accu- tained (Davidson et al. 2007)—or mantled xeno- mulation. crysts, such as rimmed alkali feldspar (rapakivi 3. Abundant antecrysts—crystals that originate structure) or mantled quartz grains (ocelli), inter- in the magma system but are not true phenocrysts spersed with unmantled mineral equivalents, re- because they largely grow in precursor magma(s) flect derivation from elsewhere and so are consis- rather than the liquid in which they are finally content with felsic cumulate (fig. 8). This situation is

Figure 2. Local concentration of K-feldspar megacrysts, many of which are in contact. Cathedral Peak Granodiorite, Tuolumne Batholith, Sierra Nevada, California. Scale (ruler) in centimeters. Journal of Geology IDENTIFYING GRANITIC CUMULATES 131

Figure 3. Local concentration of zoned K-feldspar megacrysts, most of which are in contact, with interstitial aggregates of quartz, feldspar, and biotite, at least some of which could be cumulative. Cathedral Peak Granodiorite, Tuolumne Batholith, Sierra Nevada, California. The curved boundary between the two megacrysts at the top-left of center (X) could be the result of contact melting. Scale (ruler) in centimeters and inches. common in rapakivi granites (e.g., Volborth 1962; itoid enclaves (“mafic inclusions”) in a megacryst- Elders 1967; Anderson 1980). rich granitoid (figs. 11–13) is consistent with me- 4. Coexistence of abundant crystals of K-feldspar chanical accumulation of both enclaves and or plagioclase with different zoning patterns, im- megacrysts (e.g., Collins et al. 2006; Vernon and plying different crystallization/corrosion histories, Paterson 2008c). Xenoliths can also be concen- is also consistent with a cumulate origin (fig. 9). trated with the igneous enclaves (e.g., Flood and 5. Compaction of accumulated crystals in con- Shaw 1979; Vernon and Paterson 2008c), as shown tact can cause contact melting (Park and Means in figure 11. The variety of enclave types distin- 1996). For example, local zoning truncations, where guishes mechanical accumulation from incipient one zoned megacryst impinges on another in K- disintegration of synplutonic dikes (e.g., Cobbing feldspar aggregates (Paterson et al. 2005), can be due and Pitcher 1972; Pitcher 1979, 1991; Whalen and to contact melting (figs. 3, 10). The problem of iden- Currie 1984; Furman and Spera 1985; Foster and tifying contact melting has been discussed by Ver- Hyndman 1990), which produces collections of non et al. (2004), and care should be taken when identical or very similar enclaves. evaluating these microstructures. Nevertheless, if 7. Interpenetration of microgranitoid enclaves the two impinging crystals, both with truncated between abundant feldspar crystals (figs. 5, 11) sug- zoning, have separated cores and each core is com- gests that the crystals were present when the partly pletely surrounded by some zones (figs. 3, 10), in- crystallized magmatic enclaves were compacted dependent nucleation and growth of the crystals, against them (e.g., Collins et al. 2006; Vernon and followed by impingement and contact melting, is Paterson 2008c, their figs. 14, 15). This is consistent a reasonable interpretation (Vernon et al. 2004). with, though not necessarily diagnostic of, a cu- However, although contact melting is consistent mulate origin for the feldspar. with cumulate, it is not a sufficient criterion on its 8. Intricate penetration of fine-grained mafic- own. intermediate aggregates between feldspar crystals 6. Concentration of a wide variety of microgran- (fig. 5) has been interpreted as small remnants of 132 R. H. VERNON AND W. J. COLLINS

crogranitoid enclaves represent cumulate plus interstitial liquid mobilized by mechanically or ther- mally induced instabilities (e.g., Reid et al. 1983; Elburg and Nicholls 1995; Tobisch et al. 1997; Weinberg et al. 2001; Paterson et al. 2005; Vernon and Paterson 2008c). Interstitial liquid can be squeezed out during the injection. 11. Graded layering, channel fills, and cross-lamination involving concentrations (schlieren layering) of mafic minerals, in some instances with K- feldspar megacrysts and/or microgranitoid enclaves (e.g., McCarthy and Groves 1979, fig. 3; Barrie`re 1981; Barbey et al. 2008; Vernon and Paterson 2008c; Barbey 2009), are consistent with mechanical accumulation and sedimentation of crystals or crystal aggregates (figs. 1, 13). The mineral grains in these concentrations are of about the same general size as those of the equivalent minerals in the rest of the pluton, suggesting that they are physical accumulations. Modal and grain-size grading occurs in some schlieren, consistent with deposition Figure 4. Concentration of mantled quartz xenocrysts of crystals at a rheological boundary, such as a (“ocelli”) formed in a zone of magma mixing and later chamber floor (Gilbert 1906; Wilshire 1969; Barri- physically accumulated, together with some K-feldspar e`re 1981). megacrysts and small remnants of microgranitoid en- 12. Continuous and partly disintegrated (“pil- claves. Kameruka Granodiorite, Bega Batholith, Wog Wog River, southeastern New South Wales, Australia. Base of photo 10 cm long. still-magmatic enclave material that were physically extracted from larger enclaves and flowed between solid (i.e., cumulate) crystals, displacing former interstitial liquid (Collins et al. 2006). 9. A granitoid locally devoid of or extremely poor in quartz, especially with a strong magmatic foliation, occurring beneath or between major enclaves suggests that the dense enclaves settled and were compacted against a crystal mush, from which interstitial liquid was squeezed (e.g., Vernon et al. 1988; Wiebe and Collins 1998; Collins et al. 2006; Vernon and Paterson 2008c, their figs. 16–18). Such compaction can produce changes in local whole- rock compositions in the cumulate; for example, syenitic patches (figs. 3, 9, 11, 12) and dioritic patches (figs. 6, 14, 15) can be formed in granodiorite by removal of Si-rich liquid from K-feldspar- Figure 5. Concentration of microgranitoid enclaves, rich and plagioclase-rich cumulate, respectively. In with variable composition and structure, and K-feldspar some magmas, depending on pressure, melt com- megacrysts, together with abundant, coarse-grained, feldspar-rich (syenitic) aggregates, inferred to be cumulate. position, and water content, K-feldspar compo- Kameruka Granodiorite, Bega Batholith, Wog Wog River, nents could be preferentially removed in the inter- southeastern New South Wales, Australia. The margins stitial liquid, leaving enrichments of quartz of many of the enclaves interpenetrate, some intricately, phenocrysts. with feldspar grains, which suggests that the enclaves 10. Local, minor intrapluton intrusions rich in were pressed against accumulated crystals before fully K-feldspar megacrysts, hornblende, biotite, or mi- solidifying. Base of photo 30 cm long. Journal of Geology IDENTIFYING GRANITIC CUMULATES 133

pering leucogranite or aplite dykes emplaced normal to the foliation but flattened within the foliation (Gee and Groves 1971) are consistent with squeezing of a crystal mush against its walls, with filter-pressing and release of interstitial melt into zones of tension, leaving behind cumulate (Mc- Carthy and Groves 1979). Evidence of the squeezing of evolved interstitial liquid from this general type of cumulate during noncoaxial deformation, forming felsic veins or local intrapluton felsic intrusions (see criterion 10), has been described by Barbey et al. (2008), Vernon and Paterson (2008c, figs. 22, 27, 29, 32), and Barbey (2009). 14. Marked, lenticular, or streaky modal variation (figs. 11, 13) is consistent with local-flow sorting of crystals and aggregates and hence with cumulate. 15. Coarse-grained aggregates of plagioclase crystals in “glomeroporphyritic aggregates” or “chain structures” (figs. 14–17) and coarse-grained aggregates of mafic minerals in “mafic clots” (fig. 7) imply local concentration of crystals, either by het- Figure 6. Plagioclase-rich granodiorite or tonalite, con- erogeneous nucleation of one crystal on another or sisting of abundant, mainly euhedral crystals of zoned by drifting of crystals together in the liquid and plagioclase, many of which are in contact in two dimen- adhesion along low-energy planes, this process be- sions, which implies that all could be in contact in three ing known as “synneusis” (Vance 1969). Close dimensions, although this is not necessary for a plagio- proximity of several such aggregates (fig. 14) is con- clase cumulate framework to be present (Irvine 1982). sistent with physical accumulation of the aggre- Also shown are interstitial quartz (Qtz), minor horn- gates. blende (Hbl), and minor biotite (Bt). Ravenswood Grano- 16. Compositional similarity of minerals in the diorite, north Queensland, Australia. Crossed polars; most and least mafic parts of a pluton is consistent base of photo 7 mm long. lowed”) sheets of more mafic magma in mafic and silicic layered intrusions, or MASLI plutons (Pat- rick and Miller 1997; Wiebe and Collins 1998; Col- lins et al. 2002; Miller and Miller 2002; Harper et al. 2005), indicate the presence of a crystal-rich base that is strong enough to support the injected, more mafic magma. This situation is consistent with granitic cumulate, as suggested by Wiebe (1974, 1993, 1994, 1996), Wiebe and Collins (1998), Wiebe et al. (2001, 2002), and Collins et al. (2006), although it is not a diagnostic criterion on its own, because advanced, noncumulate crystallization could produce the same strong base. 13. Alternating layers or schlieren rich or poor in mafic minerals, parallel to a pluton margin (e.g., McCarthy and Groves 1979; Paterson and Vernon 2008a; fig. 11) are consistent with a cumulate origin involving “sidewall crystallization” with upward loss of less dense interstitial felsic liquid (“liquid Figure 7. Coarse-grained, biotite-rich aggregate in the fractionation”), as discussed by McBirney (1980), Kameruka Granodiorite, Bega Batholith, southeastern McBirney et al. (1985), and Sawka et al. (1990). Ta- Australia. Plane-polarized light; base of photo 7 mm long. Figure 8. Polished slab of Finnish rapakivi granite showing unmantled alkali feldspar megacrysts interspersed with less numerous mantled or rimmed megacrysts (R) with rapakivi structure. Base of photo about 30 cm long.

Figure 9. Accumulated quartzofeldspathic aggregate rich in K-feldspar megacrysts (left) and a patch of very heterogeneous, hybrid, more maﬁc cumulate (right) containing microgranitoid enclaves and variably zoned K-feldspar megacrysts. Kameruka Granodiorite, Bega Batholith, Bega River, southeastern New South Wales, Australia. Some megacrysts (gray) have plagioclase rims (white); others have no rims; and still others have been completely replaced by plagioclase (although a few may be mantled grains in which only the rim was intersected), which is most clearly seen in the more maﬁc part of the outcrop. Some of the K-feldspar has oscillatory zoning, as in the large crystal to the right of center. The different crystallization histories suggest that the crystals were formed and mantled in different parts of the magma chamber or by mixing in separate magma batches and were accumulated mechanically. Journal of Geology IDENTIFYING GRANITIC CUMULATES 135

interconnected framework of plagioclase, augite, olivine, orthopyroxene, hornblende, and Fe oxide, with vesicular, interstitial felsic glass (Grove and Donnelly-Nolan 1986, their ﬁg. 2A,2B; Brophy et al. 1996).

Possible Grain Shape Changes in Cooling Cumulates In view of the emphasis on structural criteria in this article, it is necessary to consider the possi- bility of structure modiﬁcation during cooling, to ensure that primary magmatic evidence is not ob-

Figure 10. Possible example of contact melting between two zoned plagioclase grains (numbered), each of which has a complete core. Wards Mistake Adamellite, New England Batholith, New South Wales, Australia. Crossed polars; base of photo 5.5 mm long. with a cumulate origin for the most maﬁc rocks (Flood and Shaw 1979).

An Unreliable Criterion Some have suggested that the abundance of the host mineral in poikilitic intergrowths reﬂects the amount of trapped liquid in an inferred cumulate (e.g., Kamiyama et al. 2007, pp. 300–301). However, as explained by Vernon (1977, 2004, pp. 102–108), Flood and Vernon (1988), and McBirney and Hunter (1995, p. 116), poikilitic structures do not necessarily imply later growth of the enclosing mineral but instead can reﬂect differences in the ratios of nucleation rates to growth rates of two simultaneously crystallizing minerals. Because the “interstitial” or enclosing mineral in “orthocumulates” can form simultaneously with the included mineral, the volume occupied by an “interstitial” mineral conveys no necessary information as to the amount of “trapped liquid.” However, if the interstitial material is a mineral aggregate occurring between crystals in contact, it is more reasonable to infer a former trapped liquid (e.g., Collins et al. Figure 11. Concentration of microgranitoid enclaves, K- feldspar megacrysts, and xenoliths in a coarse-grained, 2006). Moreover, the presence of glass between compositionally and structurally heterogeneous cumu- touching euhedral crystals is clear evidence of for- late aggregate that varies from dioritic to syenitic. Ka- mer interstitial liquid, although this occurrence is meruka Granodiorite, Bega River, southeastern New restricted to volcanic rocks. For example, gabbro South Wales, Australia. The enclaves have very variable cumulates in rhyolite at Little Glass Mountain, structure and composition, many being hybrids and some Medicine Lake volcano, California, consist of an being double enclaves. Scale in centimeters. Figure 12. Concentration of microgranitoid enclaves of variable composition, structure, and size in a heterogeneous aggregate showing very variable mineral proportions, being locally quartzofeldspathic, feldspathic (syenitic), and biotite rich; this heterogeneity is consistent with a cumulate origin. The margins of many of the enclaves interpenetrate with feldspar grains, suggesting that the enclaves were pressed against accumulated crystals before fully solidifying. Sierra Tablelands, Sierra Nevada, California. Base of photo about 40 cm long.

Figure 13. Concentrations of very variable microgranitoid enclaves, many of which are porphyritic, as well as K- feldspar megacrysts (resulting in local syenitic compositions) and abundant maﬁc schlieren in truncated troughs resembling cross-bedding. Dinkey Creek pluton, Sierra Nevada, California. Knife 9 cm long. Journal of Geology IDENTIFYING GRANITIC CUMULATES 137

crystal faces of both crystals, or a higher-energy interface, for random impingements. A polygonal aggregate results if all the melt is locally expelled, after which further adjustment of grain boundaries can occur in the solid state (e.g., Voll 1960; Weedon 1965; Vernon 1970, 1976, 2004, pp. 224–229; Hul- bert and von Gruenewaldt 1985; Reynolds 1985; Hunter 1987, 1996; Mathison 1987; Higgins 1991, 1998). Because this process can occur only by solid state diffusion along grain boundaries and through grains, it is slower than “liquid-phase sintering” (Hunter 1987). 2. Stress concentrated at points of contact of compacting crystals can be released by contact melting, a term introduced by Park and Means (1996) to describe a solution process in deforming melt-present systems that is broadly analogous to pressure solution or strain solution in melt-absent systems. Park and Means (1996) observed, in syn- kinematic analogue experiments, that contact melting generally occurs at boundaries oriented at high angles to the direction of shortening, suggest- Figure 14. Aggregates of coarse-grained plagioclase in- ing that the melting could be related to high normal terspersed with aggregates of coarse-grained biotite (Bt) stress at these boundaries. Possible evidence of con- and small patches of interstitial quartz (Qtz) in a plagio- tact melting in rocks is provided by truncation of clase-biotite (dioritic or tonalitic) cumulate phase of the Kameruka Granodiorite, Bega Batholith, southeastern Australia. Crossed polars; base of photo 7 mm long. scured or obliterated. Grain shapes in cumulates theoretically could change during slow cooling in two ways. 1. Grain interfaces can move in order to reduce the total crystal/liquid surface area, which in- creases the proportion of crystal/crystal interfaces, as is common in cooling mafic and ultramafic cumulates. The process has been related to “liquid- phase sintering” (Hulbert and von Gruenewaldt 1985; Jurewicz and Watson 1985; Reynolds 1985), which is an industrial process in ceramics and met- als involving the growing together of powders to form solid aggregates. A similar process occurs in the conversion of snow to firn and then to solid ice in glacial ice sheets (e.g., Vernon 2004, fig. 4.43). The process involves minimization of the total interfacial free energy by increasing the area of solid/ solid interfaces, which have less interfacial free energy than solid-fluid interfaces (Kingery 1960, p. 370; Coble and Burke 1963), and expelling melt Figure 15. Aggregate of many plagioclase crystals form- from the crystal aggregate. The process involves ing “chain structure” in the Kameruka Granodiorite, diffusion and especially solution precipitation (e.g., Bega Batholith, southeastern Australia. This structure Cooper and Kohlstedt 1984). Once the crystals can be interpreted as resulting from either heterogeneous come into contact, a solid-solid grain boundary is nucleation or adhering separate crystals, followed by set- formed between them: either a fortuitously low- tling of the aggregate. Crossed polars; base of photo 7 energy interface, if the crystals happen to abut on mm long. 138 R. H. VERNON AND W. J. COLLINS

Bowen 1958; Augustithis 1973; Luth 1976, p. 336; McCarthy and Hasty 1976, p. 1358; Pitcher 1979, 1997, p. 69; Hughes 1982, p. 162; Hibbard 1995, p. 228; Eklund and Shebanov 1999, p. 130; Bartley et al. 2004; Coleman et al. 2005; Glazner and Bartley 2006; Bachmann et al. 2007, p. 7; Kemp et al. 2008, p. 337; McBirney 2009, p. 4), as discussed by Vernon and Paterson (2008a). The putative grain shape changes during cooling would be most likely to occur by high-temperature recrystallization, involving the formation of smaller new grains or the modification and rearrangement of existing grains by grain-boundary “sweeping” during grain-boundary migration recrystallization, which is most effective at high temperatures, especially if water is present (e.g., Vernon 2004, pp. 338–342). Both processes would tend to obliterate internal magmatic fea- Figure 16. Six zoned plagioclase phenocrysts (num- tures such as growth zoning (especially oscillatory bered) in parallel, apparent “synneusis” orientation, set zoning) in plagioclase, K-feldspar, quartz, and other in a much finer-grained groundmass of anhedral quartz, minerals; elongate, euhedral to subhedral feldspar K-feldspar, and plagioclase. Xu et al. (2009, their fig. 3a) or quartz grains; graphic intergrowths; poikilitic referred to this as a “tower-shape structure.” Crossed microstructures; and euhedral plagioclase inclu- polars; base of photo 7 mm long. sions in quartz and K-feldspar. Such features are common in granitoids but uncommon in or absent mineral zoning, for example, in plagioclase (Nicolas from metamorphic rocks of equivalent composi- and Ildefonse 1996; Rosenberg 2001) or K-feldspar. However, apparent truncated zoning can be produced by (a) sections through concentrically zoned, simply twinned, single crystals (Dowty 1980, fig. 1), (b) sections through concentrically zoned crystals, one of which nucleates epitaxially on the other (Dowty 1980, fig. 2), or (c) crystals that nucleate obliquely on other crystals. Two alternative expla- nations for genuinely truncated zoning are (1) for- tuitous impingement and consequent cessation of growth at the contact and (2) contact melting. A third possible interpretation is that large fragments of plagioclase, with zoning truncated by fracture surfaces, impinge and grow together in a kind of sintering process (Vernon et al. 2004). As noted above, if the two impinging crystals, both with truncated compositional zoning, have separated cores and if each core is completely surrounded by some zones (fig. 10), separate nucleation and growth of the crystals, followed by impingement and contact melting, can be inferred (Vernon et al. 2004). However, although minor intergranular processes (such as local contact melting), as well as Figure 17. “Glomeroporphyritic” aggregate of zoned intragranular processes (such as exsolution), can oc- plagioclase crystals in a much finer-grained groundmass cur in granitoids, these rocks generally show little of anhedral quartz, K-feldspar, and plagioclase. Some of or no evidence of major grain shape changes during the crystals could be in a potential “synneusis” relation- slow cooling (in the absence of deformation), de- ship, but most appear to be random and so could have spite many assertions to the contrary (e.g., nucleated on each other. Crossed polars; base of photo 7 Drescher-Kaden 1948; Tuttle 1952; Tuttle and mm long. Journal of Geology IDENTIFYING GRANITIC CUMULATES 139 tion, in which solid state recrystallization/neo- origin. Reliable application of these structural cri- crystallization processes dominate (Vernon and teria requires ascertaining that magmatic struc- Paterson 2008a). For example, quartz commonly tures have not been obliterated during slow cooling. shows spectacular oscillatory zoning (revealed by The common presence in granitoids of primary cathodoluminescence SEM imaging) in high-level magmatic features such as concentric growth zon- granitoid plutons (e.g., D’Lemos et al. 1997; Seye- ing (especially oscillatory zoning), euhedral to sub- dolali et al. 1997; Mu¨ ller et al. 2000, 2002a,2002b, hedral feldspar and quartz, graphic intergrowths, 2005; Wiebe et al. 2007; Larsen et al. 2009) but has and euhedral plagioclase inclusions in quartz and uniform composition or shows patchy, mottled, or K-feldspar indicates that magmatic structural evi- indistinct zoning in metamorphic rocks (Seyedolali dence is not removed during slow cooling, in the et al. 1997). Also, graphic intergrowths tend to re- absence of external deformation or metamorphism, crystallize to granular aggregates when metamor- and so is potentially useful for identifying granitic phosed (e.g., Vernon 2004, pp. 256–257, fig. 4.65). cumulates. A reasonable conclusion is that magmatic struc- Implications of identifying cumulate in granitoid tural evidence in granitoids is not removed during plutons are that residual liquids are fluid enough slow cooling, in the absence of external deforma- to be mobile and for crystals to move in them tion or metamorphism. It is therefore potentially (partly because the liquids become more hydrous of use in identifying felsic cumulates. as anhydrous minerals crystallize) and that many granitoids are likely to be composites of materials of contrasting intrachamber origin. Microanalysis Conclusions of those materials is a way of unraveling the struc- Although identification of felsic cumulates can be tural history of the chamber. difficult, especially in the absence of major com- ACKNOWLEDGMENTS positional differences between the possible cumulate and the host granitoid, some structural and We acknowledge the financial assistance of grants structural/mineralogical criteria appear to be di- from the Australian Research Council. We thank agnostic of, or at least consistent with, a cumulate C. Miller and S. Seaman for helpful critical reviews.

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J. 1988. Wiebe, R. A.; Wark, D. A.; and Hawkins, D. P. 2007. Shape and microstructure of microgranitoid enclaves: Insights from quartz cathodoluminescence zoning indicators of magma mingling and flow. Lithos 22:1– into crystallization of the Vinalhaven granite, coastal 11. Maine. Contrib. Mineral. Petrol. 154:439–453. Vernon, R. H.; Johnson, S. E.; and Melis, E. A. 2004. Wilshire, H. G. 1969. Mineral layering in the Twin Lakes Emplacement-related microstructures in the margin Granodiorite, Colorado. Geol. Soc. Am. Mem. 115: of a deformed tonalite pluton: the San Jose´ pluton, 235–261. Baja California, Me´xico. J. Struct. Geol. 26:1845–1865. Xu, X.-W.; Neng, J.; Yang, K.; Zhang, B.-L.; Liang, G.-H.; Vernon, R. H., and Paterson, S. R. 2008a. How extensive Mao, Q.; Li, J.-X.; et al. 2009. Accumulated pheno- are subsolidus grain-shape changes in cooling gran- crysts and origin of feldspar porphyry in the Chanho ites? Lithos 105:85–97. area, western Yunnan, China. Lithos 113:595–611. ———. 2008b. How late are K-feldspar megacrysts in Za´k, J.; Verner, K.; and Tycova´, P. 2008. Grain-scale pro- granites? Lithos 104:327–336. cesses in actively deforming magma mushes: new in- ———. 2008c. Mesoscopic structures resulting from sights from electron backscatter diffraction (EBSD) crystal accumulation and melt movement in granites. analysis of biotite schlieren in the Jizera granite, Bo- Trans. R. Soc. Edinb. Earth Sci. 97:369–381. hemian massif. Lithos 106:309–322. Ulrichsweb.com--Full Citation http://www.ulrichsweb.com/ulrichsweb/Search/fullCitation.asp?navPa...

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Click highlighted text for a new search on that item. Table of Contents: Click here to view ISSN: 0022-1376 Title: The Journal of Geology Publishing Body: University of Chicago Press Country: United States Status: Active Start Year: 1893 Frequency: Bi-monthly Volume Ends: Nov Document Type: Journal; Academic/Scholarly Refereed: Yes Abstracted/Indexed: Yes Media: Print Alternate Edition ISSN: 1537-5269 RSS Availability: Click here to view Size: Standard Language: Text in English Price: USD 197 subscription per year domestic to institutions USD 216.85 subscription per year in Canada to institutions USD 212 subscription per year elsewhere to institutions USD 216 combined subscription per year domestic to institutions (Print & Online Eds.) USD 236.80 combined subscription per year in Canada to institutions (Print & Online Eds.) USD 231 combined subscription per year elsewhere to institutions (Print & Online Eds.) (effective 2010) Subject: EARTH SCIENCES - GEOLOGY Dewey #: 551 LC#: QE1 CODEN: JGEOAZ Special Features: Includes Advertising, Illustrations, Book Reviews Article Index: Cum.index Pages per Issue: 225 Editor(s): Barbara J Sivertsen (Managing Editor) E-Mail: [email protected] URL: http://www.journals.uchicago.edu/JG Description: Covers original research across a broad range of subfields in geology, including geophysics, geochemistry, sedimentology, geomorphology, petrology, plate tectonics, volcanology, structural geology, mineralogy, and planetary sciences. Back to Top Add this item to: Request this title: Print Download E-mail I'd like to request this title. Corrections: Submit corrections to Ulrich's about this title. Publisher of this title? If yes, click GO! to contact Ulrich's about updating your title listings in the Ulrich's database. Back to Top

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