Field, Chemical, and Physical Constraints on Mafic-Felsic Magma Interaction in the Lamarck Granodiorite, Sierra Nevada, California

Home , Felsic, Granodiorite, Mafic

Field, chemical, and physical constraints on mafic-felsic magma interaction in the Lamarck Granodiorite, Sierra Nevada, California

THOMAS P. FROST U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 and Department of Geology, Stanford University, Stanford, California 94305 GAIL A. MAHOOD Department of Geology, Stanford University, Stanford, California 94305

ABSTRACT that account for compositions, heats of fu- and Suragawa, 1985); however, sub- or near- sion, heat capacities of liquids and crystals, liquidus magmas may be effectively "immisci- Evidence of msigma interactions resulting and a range of initial temperatures, crystallin- ble" due to large initial differences in viscosity or in both hybridisation and mingling are ities, crystal sizes, and magma water contents to viscosity differences caused by cooling and preserved in a diverse suite of gabbroic to indicate that in most circumstances the basalt crystallisation of the more mafic magma on at- intermediate rocks associated with the com- end member quenches; the resulting large vis- tainment of thermal equilibrium (Sparks and positionally zoned hornblende-biotite La- cosity contrast between the end members Marshall, 1986). Mixing of physically interact- marck Granodiorite of the eastern Sierra Ne- prevents hybridization. Homogenization is ing magmas may form a homogeneous hybrid vada, California. Ellipsoidal mafic enclaves likely only if the compositional difference be- when viscosities of the end members are similar were formed by quenching of small amounts tween host and injected mafic magma is less and low (Huppert and others, 1984; Campbell of high-alumina basaltic magma upon injec- than 10% SiOz or if the mass fraction of mafic and Turner, 1986). Mixing between magmas of tion into and dispersal through granodiorite magma is greater than 0.5. Resulting mixtures high viscosity or between magmas having large magma early in its crystallization. Synplu- have the composition of tonalite or mafic viscosity differences is slower and is likely only if tonic intrusions of hornblende gabbro granodiorite; thus, the more silicic rocks of cooling rates are low and if turbulent convec- through hybridized mafic granodiorite repre- the granodiorite pluton must represent differ- tion, chemical diffusion, or other effective diffu- sent injection of mafic magma at a later stage entiation products rather than direct hybrids sive processes are operative. Mingling, in which of crystallization of the granodiorite, as they of mafic or intermediate magma and felsic interacting magmas form heterogeneous banded crosscut regional trends in foliation and com- magma. or enclave-bearing rocks, occurs when one positional zoning in the host pluton. Where magma is effectively quenched by another and is compositional contrasts between intrusion likely when viscosity contrasts are large or when INTRODUCTION and host granodiorite are large, contacts are the time available for homogenization is small sharp and abundant enclaves derived from (Blake and others, 1965; Walker and Skelhorn, Inclusions more mafic than their host are the mafic intrusion are present in the grano- 1966; Yoder, 1973; Eichelberger, 1975; Hup- common in intermediate to silicic plutonic and diorite. Where the host is relatively mafic or pert and others, 1984; Grove and others, 1982; volcanic rocks. Inclusions not derived from where the local-scale proportion of mafic Bacon, 1986). country rock in metaluminous hornblende- magma is large, contacts are zones of exten- bearing granodiorite plutons most commonly Variables that control the style of magma in- sive hybridization that contain both enclaves are fine-grained, igneous-textured ellipsoids of teraction are the temperature, latent heat of fu- and hybrid schlieren. Uncontaminated mafic hornblende diorite to mafic granodiorite (Phil- sion, composition, and water content of each intrusions have hi^h-alumina basaltic compo- lips, 1880; Pabst, 1928; Bateman and others, magma, as well as relative amounts and total sitions, whereas hybridized intrusions have 1963; Didier, 1973; White and Chappell, 1977; volume of the end members, and the amount of silica contents as high as 63.5%. Mafic intru- Vernon, 1983). Many different names have been time available for mixing to occur. Secondary sions locally contain coarse-grained cumulus proposed for such inclusions; we follow Vernon variables include crystallinity and viscosity of gabbro inclusions. Mafic schlieren in grano- (1983) in using the term "enclave." In this re- the end members at the equilibrium temperature diorite far from mafic intrusions represent lo- port, the term "enclave" is restricted to fine- of the mixture (Sparks and Marshall, 1986). calized accumulatiions of hornblende, Fe-Ti grained, ellipsoidal types; all others are called Field, geochemical, and experimental evidence oxides, and biotite from the granodiorite. In- "inclusions." Enclaves of undisputed magmatic suggest that most mafic enclaves result from trusion of late malic dikes mobilized and en- origin and heterogeneous banded rocks of hy- magma mingling (Lipman, 1963; Walker and trained granitic residue from the granodiorite brid origin are common in intermediate to silicic Skelhorn, 1966; Yoder, 1973; Vogel and Wil- and formed composite dikes of aplite and pil- volcanic rocks (Wilcox, 1944; Eichelberger, band, 1978; Eichelberger, 1975,1981; Reid and lowed diorite. 1978, 1980; Bacon and Metz, 1984; Bacon, others, 1983; Vernon, 1983; Bacon and Metz, Whether interacting magmas mix or mingle 1986). 1984; Vogel and others, 1984; Huppert and is a function of the heat contents and mass Superliquidus silicate magmas of mafic others, 1984; Kouchi and Sunagawa, 1985). fractions of the end members. Calculations through silicic composition are miscible (Kouchi Mechanisms proposed for magmatic enclave

Geological Society of America Bulletin, v. 99, p. 272-291, 20 figs., 2 tables, August 1987.

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formation include forcible injection of mafic are synplutonic with it; that is, the basalt in- 1979, 1982) and Paleozoic metasedimentary magma (Sparks and others, 1977; Blake, 1984), truded after emplacement of the felsic magma rocks (Fig. 1). Wall rocks include micaceous disruption of mafic dikes (Lipman, 1963; Fur- but before complete crystallization of it. Interac- quartzite, pelitic hornfels, biotite schist, marble, man and Spera, 1985), and convective dispersal tion between mafic intrusions and partially crys- and calcareous hornfels (Moore, 1963; Bate- of stratified magma chambers (Huppert and tallized granodiorite ranged from formation of man, 1965; Moore and Foster, 1980). Meta- others, 1984). Sparks and others (1984), Hup- enclaves by quenching of mafic magma globules morphic grade is hornblende-hornfels facies as pert and others (1982, 1984), and Kouchi and in felsic magma to localized mechanical mixing shown by the presence of hornblende and plagi- Sunagawa (1985) have shown experimentally and partial homogenization of interacting end oclase in siliceous rocks and diopside and gros- that convective mixing may incorporate blobs of members around margins of mafic intrusions. sularite in calcareous rocks. Coarse sillimanite, initially low viscosity fluid into higher viscosity We explain the range of relations through a andalusite, and cordierite coexist in some meta- fluid along an interface between them. quantitative model that predicts viscosities of in- sedimentary rocks (Moore, 1963), suggesting The Lamarck Granodiorite of the Sierra Ne- teracting magmas after thermal equilibrium is pressures of 200-300 MPa (2-3 kbar), corre- vada of California hosts a diverse suite of mafic reached. sponding to depths of 8-10 km. enclaves and mafic intrusions (Fig. 1) that we South of Echo Pass, the Lamarck Granodio- interpret as resulting from physical interaction of GEOLOGIC SETTING rite and the mafic intrusions are cut by the alas- mafic and felsic magma over a range of viscosity kite of Evolution Basin, which locally forms contrasts. Mayo (1941), Bateman and others The Lamarck Granodiorite, named by Bate- near-horizontal dikes as thick as 200 m that in- (1963), and Bateman (1965) referred to widely man (1961), is a large Late Cretaceous (89.6 trude the older rocks (Fig. 1). South of Mather scattered mafic plutonic rocks of the Sierra m.y., U-Pb zircon age, Stern and others, 1981) Pass, all other rocks are cut by the Cartridge Nevada batholith as "mafic forerunners" based pluton exposed over about 400 km2 in the east- Pass pluton (Moore, 1963). Biotite K-Ar ages on the interpretation that mafic intrusions pre- central Sierra Nevada (Moore, 1963; Bateman, for the alaskite and the Cartridge Pass pluton are ceded the felsic plutons of the batholith. We 1965; Bateman and Moore, 1965; Lockwood 80 and 81 m.y., respectively (Dodge and Moore, find, however, on the basis of additional field and Lydon, 1975). The eastern margin of the 1968; Evernden and Kistler, 1970). The mafic and geochemical evidence, that high-alumina pluton dips steeply under Late Jurassic grani- bodies shown in the younger granitoids in Fig- basaltic intrusions in the Lamarck Granodiorite toids (Stern and others, 1981; Chen and Moore, ure 1 are like those in the Lamarck but are

Younger granitoids jjli Mafic intrusions [\ Q Lamarck Granodiorite Older granitoids H§| Metamorphic rocks

Figure 1. Regional geologic and location map of the Lamarck Granodiorite, east-central Sierra Nevada, California. Adapted from Moore (1963), Bateman (1965), and Bateman and Moore (1965).

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sharply cut by the younger plutons. This relation suggests that the mafic bodies in the young plutons are roof pendants derived from the Lamarck and its associated mafic intrusions. The Lamarck is intruded and brecciated along its northeastern margin by the Late Cretaceous granodiorite of Lake Edison (Stern and others, 1981).

sopleth LAMARCK GRANODIORITE Specific gravity Data point Younger granitoids Compositional variation from hornblende- Mafic intrusions biotite granodiorite: to granite is present in the Lamarck Granodiorite Lamarck Granodiorite; the variation is reflected Older granitoids in concentric specific gravity lows that are sepa- Met a sedimentary rocks rated by crosscutting mafic intrusions (Fig. 2). Specific gravities in the Lamarck range from as high as 2.76 near the pluton margin and in hybrid rocks associated with mafic intrusions to as low as 2.62 in the cores of the concentric zones. There is also a regional trend of decreasing specific gravity toward the south. Primary foliation in the Lamarck Granodio- rite is defined by flattened mafic enclaves and planar orientation of biotite and hornblende crystals. Foliation is best developed at, and is parallel to, the eastern intrusive margin of the pluton (Fig. 3). Foliation is also prominent and subparallel to the western margin of the pluton, although in detail it is truncated by younger plutons. Toward the cores of specific gravity lows, foliation becomes less well developed; enclave abundance and degree of flattening decrease, and mafic silicates are more randomly A J. A oriented. In Palisade Basin, a synformal zone of foliation plunges gently southeast; south of there, Figure 2. Specific gravity map of the southern part of the Lamarck Granodiorite. Concentric foliation defines ar antiform that is crosscut by compositional zonation is crosscut by and locally reversed near mafic intrusions. Adapted from mafic intrusions. Moore (1963), Bateman (1965), and Bateman and Moore (1965) with additional data. The pluton is predominantly coarse grained and equigranular or seriate northwest of the Bishop Pass area. Quartz and perthitic micro- mineral modal percent (color index) is as high as commonly, hornblende are phenocrysts in most cline are subequal in most rocks. Seriate 25 in the northern part of the study area; in the enclaves. Texture varies prominently; in a single plagioclase as large as 2 cm make up 30% to southern part of the pluton, color index is 4 to outcrop, some enclaves are porphyritic or varia- 50% of the rock; most crystals are subhedral and 15, with an average of 7 (Moore, 1963; Bate- bly porphyritic, whereas others are equigranular. randomly oriented. Anorthite content is highest man, 1965). Accessory minerals include euhed- Enclaves are present throughout the Lamarck, north of Mather Piss (An45-An2o); some crys- ral apatite having axial ratios of less than 5:1 and although their abundance varies within short tals have euhedral patchy-zoned cores between zircon. distances from less than 1 m'2 to localized An45 and An52. South of Mather Pass, plagio- swarms in which more than 50% of the outcrop clase ranges from An32 to An¡2, although MAFIC ENCLAVES IN THE is composed of enclaves in a matrix of coarse patchy-zoned calcic cores are present in some LAMARCK GRANODIORITE granodiorite. Enclave abundance near external crystals. South of Bishop Pass, at the pluton's contacts varies around the perimeter of the eastern margin, peithitic microcline phenocrysts Mafic enclaves in the Lamarck Granodiorite granodiorite; only locally do enclaves decrease as large as 3 cm compose as much as 30% of the are well-rounded ellipsoids, typically less than in abundance with distance from the pluton rock; south of Palisade Basin, similar pheno- 50 cm in maximum exposed dimension, al- margin, as has been observed elsewhere in the crysts are abundant throughout the pluton. Mi- though some enclaves are as large as 10 by 15 Sierra Nevada (Bateman and others, 1963; crocline is present in all rocks as interstitial m. Enclaves have sharp, cuspate margins convex Dodge and Moore, 1968; Moore, 1963; Bate- crystals. Quartz is anhedral and interstitial. Bio- toward the host and are fine to medium grained man, 1965). Enclave abundance at the eastern tite and hornblende are evenly distributed and allotriomorphic or hypidiomorphic granu- margin of the pluton south of Dusy Basin is less 2 through the rock as discrete subhedral to eu- lar. Many enclaves larger than 15 cm have very than 1 m~ and increases irregularly inward hedral crystals as large as 4 mm and as smaller fine grained margins that grade to fine- to across the pluton toward a mafic intrusion. In clots of several intergrown crystals. Mafic medium-grained interiors. Plagioclase and, less the Lake Sabrina area, enclaves are abundant

accumulations of enclaves brought together by settling or by convection in the granodiorite magma. Large zones as long as 4 km marked by abundant enclaves in extensively hybridized granodiorite are intrusive in origin, as are enclaves in granodiorite along contacts of mafic intrusions; both are discussed in following sec- tions. Some small discontinuous swarms also may be intrusive, but post-emplacement flow of the granodiorite magma has obscured their origin.

Foliation, showir>g ' c SYNPLUTONIC MAFIC INTRUSIONS dip, vertical, variable < Mafic intrusions ranging from hornblende gabbro through mafic granodiorite are present at many localities in the Lamarck Granodiorite (Fig. 1). Similar intrusions are not present in older plutons east of the Lamarck Granodiorite, and mafic bodies in younger plutons to the west are older than those plutons. Mafic intrusions range from less than 200 m2 to 10 km2 in area. Most are elliptical in plan; those that have partially hybridized with the granodiorite host tend to have lens-shaped outcrop patterns. Contacts with the host granodiorite range from gradational zones where extensive hybridization has occurred (Fig. 4A) to sharp contacts with abundant enclaves where mingling has domi- nated (Fig. 4B). At sharp contacts, the mafic rock commonly has a fine-grained, chilled margin. Enclaves are abundant in the granodiorite at sharp contacts, and their abundance and size decrease irregularly with distance from the contact over a zone as wide as 200 m (Fig. 5). Figure 3. Foliation map of the southern part of the Lamarck Granodiorite. Foliation in granodiorite parallels contacts of mafic intrusions locally but is crosscut by mafic intrusions on Foliations in mafic intrusions and host grano- a regional scale. Ornament as in Figure 2. Adapted from Moore (1963), Bateman (1965), and diorite are parallel along contacts; however, the Bateman and Moore (1965) with additional data. outcrop patterns of the intrusions transect regional trends in both foliation and composition in the host (Figs. 2, 3; see also Figs. 7, 8, and 9 (up to 5 m~2) near the pluton's margin; how- (Reid and others, 1983; Vernon, 1983). In some below). This suggests that the intrusions were ever, this area also hosts many mafic intrusions enclaves, quartz xenocrysts are rimmed by fine- emplaced in partially crystallized felsic magma from which many nearby enclaves were derived. grained hornblende; in other enclaves, quartz is and that they remelted and mobilized it. Dis- Enclaves are identical in mineralogy to the interstitial and makes up less than 5% of the rupted mafic dikes that can be traced to mafic host granodiorite; only the proportions of min- rock. No miarolytic cavities have been observed intrusions are present in the granodiorite. Indi- erals are different. Plagioclase phenocrysts as in any enclaves, although it is possible that some vidual dike remnants are as long as 5 m and large as 7 mm are present in variable amounts to of the interstitial quartz is secondary in origin have randomly oriented, sharply defined ellip- as much as 20% of the rock; many have patchy- and fills original void space. soidal forms. Dikes of partially hybridized gran-

zoned cores ranging from An60-An50. Horn- Horizontally and vertically discontinuous en- odiorite as wide as 3 m cut some mafic in- blende is also present locally as subhedral clave swarms range from patches of enclaves less trusions. The dikes have matching walls and phenocrysts. Hornblende and biotite amounts, than 1 m2 in area to 7 by 40 by 15 m high. Most rotated inclusions of the mafic host, and many typically in the proportion 2:1, vary between swarms are elongate parallel to the longest ex- have complexly folded internal mafic schlieren 25% and 65%. Both minerals range from small posed dimension of the entrained enclaves. In and pods of hornblendite. Smaller dikes are less subhedral to anhedral crystals less than 0.25 mm small swarms, the matrix between enclaves is regular and may have gradational contacts and to larger subhedral forms. Apatite in enclaves is similar in composition to that of the granodiorite nonmatching walls, although internal mafic acicular with axial ratios as high as 50:1; some that hosts the swarm; as swarm dimensions in- schlieren and rotated host-rock inclusions are crystals are skeletal. Acicular and skeletal apatite crease, the matrix becomes more mafic than the evidence of their intrusive origin. morphologies have been produced experimen- granodiorite that hosts the swarm. Contacts be- Euhedral plagioclase phenocrysts as large as tally on thermal quenching (Wyllie and others, tween swarm and host granodiorite are gra- 1 cm range between 0 and 30% of the rock. 1962) and are present in other plutonic rocks dational zones less than 2 m wide. Small, Patchy-zoned euhedral cores (An60- An4j) in that are interpreted to have crystallized rapidly discontinuous swarms may represent localized some phenocrysts are similar to those common

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in enclaves and locally present in the granodiorite. Rimming the core is normal and oscillatory

zoned plagioclase (An45-An25). All rocks having plagioclase phenocrysts also contain 1- to 3-mm subhedral lath-shaped plagioclase. Nor- mal zoning (Afys-A^s) is similar to that in the rims of the phenocrysts. Hibbard (1981) suggested that similar patchy-zoned plagioclase is a result of rapid skeletal growth of calcic plagioclase on undercooling of mafic magma during interaction with cooler felsic magma. The presence of patchy-zoned cores in plagioclase in some intrusions and host granodiorite, however, suggests hybridization. The absence of plagioclase phenocrysts in some intrusions suggests that at least two basaltic magmas interacted with the magma of the Lamarck pluton during its crystallization.

Figure 4. Schematic diagrams drawn from photographs showing types of contacts between Hornblende in mafic intrusions ranges from granodiorite host and synplutonic mafic intrusions. Ranges of silica content are indicated. 25% to 45% of the rock and has a considerable (A) Gradational contact with hybrid enclaves and hybridized mafic rock. Near Piute Pass. variety in habit. Most commonly it has subhed- (B) Quenched mafic enclaves derived from synplutonic intrusion in granodiorite host. Dikes of ral to anhedral prismatic forms 1-7 mm long. remobilized felsic material that intrude the mafic rock are partially hybridized. Mount Gilbert Both single crystals and clots of anhedral horn- intrusion, near traverse A of Figure 7. blende intergrown with biotite, Fe-Ti oxides, and sphene are common. Subhedral to anhedral biotite is subordinate to hornblende and makes up 5%-25% of the intrusions. Biotite is less than 1 mm in size in most rocks. Secondary biotite Figure 5. Mafic enclave abundance in replacing hornblende is common. Interstitial granodiorite as a function of distance from quartz and minor microperthite constitute contact with Mount Gilbert intrusion along 0-15% of the intrusions. Accessory minerals in- three traverses shown in Figure 7. Each de- clude Fe-Ti oxides, sphene, zircon, and apatite. termination made by averaging percentage of Near chilled external contacts with granodiorite horizontal outcrop composed of enclaves in and in some rocks with hybrid compositions, five 1-m2 plots. fine-grained acicular apatite crystals having axial ratios as high as 30:1 have quench morphologies like those in the mafic enclaves (Fig. 6A). Rocks from interiors of the mafic bodies have coarse- 50 40 30 20 10 METERS FFIOM CONTACT

Figure 6. Apatite morphologies in mafic intrusion of Palisade Creek. Width of view, 0.5 nun. (A) Needles having quench morphologies within 5 cm of contact with granodiorite. (B) Stubby equilibrium crystallization morphologies 3.5 m from same contact.

grained apatites with axial ratios of less than 5:1 (Fig. 6B), suggestive of relatively slow growth (Wyllie and others, 1962). Irregular pods of hornblende quartz diorite pegmatite having both sharp and gradational contacts with the enclosing rock are present locally. Euhedral skeletal hornblende prisms as long as 7 cm occur in radiating masses and are cored by plagioclase. Zoned plagioclase is also present as euhedral crystals as large as 5 mm. Anhedral quartz and minor microcline are interstitial. The intrusion of Mt. Gilbert (Fig. 7) has sharp contacts with the granodiorite around its entire exposed perimeter. Enclaves and disrupted mafic dike remnants as large as 5 by 15 m (Fig. 4B) are abundant in the granodiorite at the contact but decline in abundance away from it (Fig. 5), suggesting that at least some enclaves formed from the mafic intrusion. Remobilized granodiorite locally reinjected the Mt. Gilbert mafic intrusion and the associated disrupted mafic dikes.

HYBRID ROCKS 500 11S°35' Contacts between mafic intrusions and gran- Meters odiorite along which hybridization occurred Quaternary Coniaci, showing dip are heterogeneous gradational zones defined by — dashed wtiere approximate Younger plutons interdigitation of weakly to unfoliated grano- ——w Gradaliooal contaci diorite with hybrid rock which contains abun- Mafic Intrusions Foliation Strik« and dip dant enclaves (Fig. 5A). The heterogeneous Hybrids if Vertical foliation zones average 5 m but range up to 50 m in O Foliation absent width and grade inward to more homogeneous Lamarck Granodiorite Silica content mafic rock. In some localities, no discrete intru- Older piutons Granodiorite sion can be recognized; only heterogeneous and 50* Enclaves extensively hybridized mafic granodiorite to Calc hornfels. biotite schist 55A Mafic intrusionsand hybrids quartz diorite having abundant enclaves crops out. The local reversal in concentric composi- Figure 7. Mafic intrusion of Mount Gilbert. Contacts are sharp and marked by abundant tional zonation of the host granodiorite in Pali- enclaves around entire perimeter of the intrusion. A, B, and C are traverses near contact shown sade Basin (Fig. 2) appears to result from in Figure 5. Geology by T. Frost; adapted and with emendations from Bateman (1965). extensive hybridization over a wide zone. Bimodal grain-size distributions of hornblende are common in hybrid rocks and rock. At its narrow eastern end, the intrusion is granodiorite far removed from mafic intrusions schlieren around margins of intrusions. Hybrid composed almost entirely of ellipsoidal enclaves or hybrid rock. These schlieren also have diffuse rocks have as much as 15% quartz and microper- enclosed in granodiorite that show little evi- margins but have cumulus textures characterized thite and are texturally similar to the felsic La- dence of hybridization; the wide western part of by high proportions of euhedral hornblende, Fe- marck Granodiorite; they are gradational to it as the intrusion has hybridized extensively with the Ti oxides, and sphene; plagioclase and quartz are color index drops below 25. granodiorite. anhedral and interstitial. Compositional evi- Most mafic intrusions of the Lake Sabrina dence for their cumulus origin is discussed in a area (Fig. 8) are small bodies composed of en- MAFIC SCHLIEREN following section. claves in hybridized rock. Foliation in the granodiorite around the intrusions is parallel to the Mafic schlieren in hybrid zones between HORNBLENDE GABBRO contact over wide zones (Fig. 3). The single mafic intrusions and granodiorite are character- INCLUSIONS large intrusion of Lake Sabrina has hybridized ized by diffuse margins and are associated with extensively with the host granodiorite around its abundant mafic enclaves. Schlieren in hybrid Angular inclusions as large as 1.5 m of coarse- margin. zones result from incomplete hybridization be- grained, cumulate-textured hornblende gabbro The Upper Basin intrusion (Fig. 9) crosscuts tween interacting magmas as suggested by bi- are present locally in the large mafic intrusions regional foliation in the granodiorite; however, modal hornblende size distributions. A second at Mount Gilbert, near Mather Pass, and in the foliation is parallel at the contact due to remobi- schlieren type, mesoscopically similar to hybrid northern part of Upper Basin. Contacts between lization of the granodiorite by the intruding mafic schlieren, is found in otherwise homogeneous inclusions and host are sharp, despite their sim-

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118°37'30"

Figure 8. Mafic intrusions of Lake Sabrina area. Hybridization between intrusion and host is common over wide zones around margin of intrusion. Ornament as in Figure 7. Geology by T. Frost; adapted and with emendations from Bateman (1965).

ilar color indices. Gabhro inclusions have not mally zoned plagioclase (An65-An45) is anhed- Walker and Skelhorn, 1966; Vogel and Wil- been found in granodiorite or in hybrid rocks. ral and interstitial to hornblende. Apatite has band, 1978; Taylor and others, 1980) locally cut The gabbro inclusions are composed of horn- stubby prismatic forms 0.2 mm long, indicative the Lamarck Granodiorite. The dikes are partic- blende, plagioclase, Fe-Ti oxides, biotite, and of slow cooling (Wyllie and others, 1962). ularly abundant near the core of the pluton in sphene. Euhedral-zoned green-hornblende Dusy Basin, where they dip steeply and strike prisms as large as 10 mm constitute 50% of the COMPOSITE DIKES IN THE northeast, perpendicular to the structural trend rock and have a ciude orthocumulate texture LAMARCK GRANODIORITE of the pluton; elsewhere they are more variable (Fig. 10). Hornblende is partially replaced by in attitude. Dike lengths of 80 m are not un- euhedral brown biotite books that do not inter- Composite diorite-aplite dikes similar to those common, although few are more than 1 m thick. sect hornblende crystal margins (Fig. 10). Nor- described elsewhere (Blake and others, 1965; The dikes are in sharp contact with the enclosing

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Km N

Figure 9. Mafic intrusion of Upper Basin. Intrusion crosscuts regional foliation in granodiorite although foliation at contact is parallel. Partial hybridization is dominant at wide western end; mingling is dominant at narrow eastern end. Ornament as in Figure 7. Geology by T. Frost; adapted and with emendations from Moore (1963).

Figure 10. Orthocumulate texture in angular hornblende gabbro inclusion in mafic intrusion of Upper Basin. Biotite replaces cumulus hornblende and is responsible for high alkali content of the gabbro inclusions (Table 1). Width of view, 12 mm.

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TABLE 1. SELECTED MAJOR-ELEMENT DETERMINATIONS, LAMARCK GRANODIORITE AND RELATED ROCKS, CALIFORNIA

Granodiorite-enclave pairs Hybridized intrusions Locality Lake Sabrina Dusy Basin Dusy Basin Piute Pass Mt. Lamarck Sample 20a 20b 96a 96b 115c 115d 115a 87d 87a 87g 87e 16d 16e 4a type grd enc grd enc enc hyb grd enc int hyb grd hyb grd grd

Si02 65.0 50.3 62.0 49.9 52.5 59.9 64.1 50.8 52.6 57.7 67.5 56.0 61.5 70.6

TO2 0.53 1.11 0.66 1.25 1.35 0.91 0.64 1.02 0.95 0.83 0.39 0.82 0.68 0.30 AljOj 16.2 19.7 16.9 18.8 18.0 16.4 15.9 18.5 17.6 17.6 15.7 16.3 16.5 14.4

Fe203* 4.60 10.0 5.40 9.89 8.90 6.35 4.91 9.95 9.17 6.94 3.56 7.64 5.40 2.55 MnO 0.08 0.19 0.09 0.19 0.14 0.11 0.09 0.19 0.15 0.11 0.07 0.19 0.10 0.05 MgO 2.00 4.60 2.30 4.40 4.50 3.10 2.00 5.30 5.30 3.60 1.50 5.60 2.50 1.10 CaO 4.23 7.00 4.86 7.27 7.33 5.40 4.30 6.90 8.22 6.33 3.15 7.20 4.93 2.51 Na20 3.60 4.10 3.90 4.20 3.90 3.70 3.70 4.10 3.30 3.70 3.30 3.80 4.20 3.10

K20 3.45 2.33 2.71 2.01 2.07 2.70 3.20 2.33 1.90 1.85 4.09 1.81 1.99 4.40

P205 0.18 0.35 0.22 0.36 0.36 0.28 0.21 0.31 0.25 0.27 0.14 0.23 0.23 0.11 LOI 0.30 0.60 0.59 0.70 0.55 0.50 0.22 0.45 0.64 0.40 0.25 0.54 0.57 0.34

Total 100.17 100.28 99.63 98.97 99.60 99.35 99.27 99.85 100.08 99.33 99.65 100.13 98.60 99.46

Type codes: grd, mesoscopiaJly unhybridized Lamarck Granodiorite; enc, mafic enclave; int. mesoscopically unhybridized mafic intrusion; hyb, mesoscopically hybridized rock; sch, cumulus schlieren; inc, hornblende gabbro inclusions in mafic

intrusions; pillow, diorite pillow; in composite dikes. Determinations by X-ray fluorescence on replicate fused discs. Total iron reported as Fe203. LOI is the loss on ignition at 920 °C. Mass of crushed sample exceeded 5 kg for all but mafic enclaves. Branch of Geochemistry, U.S. Geological Survey, Menlo Park, California. T. Frost, analyst.

vidual crystals of biotite and hornblende are lo- Figure 11. Schematic cally present within a few millimetres of the drawing of zoning in contact with the diorite. The diorite is fine composite dikes in La- grained and consists of subhedral hornblende, marck Granodiorite. Fine- anhedral biotite, subhedral- to anhedral-zoned grained diorite pillows intermediate plagioclase, and low but variable (black) are in sharp, con- proportions of quartz and potassium feldspar. vex contact with aplite Apatite in the diorite pillows has acicular and (clear). Late fractures in skeletal quench morphologies. Rare interme- pillows are filled with diate rocks appear to result from mechanical aplite, indicating that fel- incorporation of individual crystals of horn- sic liquid was last mobile blende and biotite in aplite. fluid. COMPOSITIONAL VARIATION

Major-element XRF determinations on more than 150 samples of the Lamarck Granodiorite and associated enclaves and mafic intrusions de- granodiorite and contain rotated, angular inclu- composed of pillow-like forms of fine-grained fine a calc-alkaline trend (Fig. 12) similar to sions of granodioriie. Composite dikes have not biotite-hornblende diorite which have very those from other Sierran metaluminous plutons been found in the younger alaskite of Evolution sharp, fine-grained, and cuspate margins against (Bateman and Nokleberg, 1978; Bateman and Basin, bracketing the dike age between 98 and aplite (Fig. 11). During emplacement, some Chappell, 1979; Noyes and others, 1983a, 81 m.y. (Evernden and Kistler, 1970; Stern and diorite pillows developed brittle fractures that 1983b; Reid and others, 1983; Furman and others, 1981). were filled by aplite. Spera, 1985), although the trend reaches higher The composite dikes are symmetrically The aplite is composed of subequal amounts MgO and FeO* contents due to the associated zoned. An irregular selvage of aplite is in contact of fine to very fine grained anhedral quartz, diorite and gabbro. Selected major-element de- with the wall rock. Interiors of the dikes are microcline, and sodic plagioclase. Small indi- terminations for the Lamarck, mafic rocks, and aplite are presented in Table 1; additional ana- lytic data are in Frost (1986). Silica contents for mesoscopically noncumu- late, nonhybridized rocks of the Lamarck range Figure 12. AFM diagram for between 59.5% and 73.2% (normalized to 100% Lamarck Granodiorite and as- anhydrous). Mesoscopically unhybridized mafic

sociated mafic rocks. A, Na20 + intrusions have silica contents as low as 50.0%;

KzO; F, FeO + 0.9*Fe203; M, hybrids have between 54.3% and 63.5% silica. MgO. The widely scattered enclaves not spatially associated with synplutonic intrusions have between

50.1% and 53.5% Si02 and, on average, are similar in composition to the least silicic rocks of the mafic intrusions. Although alkali migration appears to have enriched them in Na and K, en-

TABLE 1. (Continued)

Cumulus schlieren Unhybridized intrus. Cumulus gabbro incl. Composite dikes Lake Sabrina Ml. Gilbert Mt. Gilbert Mather Pass Dtisy Basin 44c 45 76a 76b 76c 81b 80fM 80fL GB-5c PC-5a UB-3d 104a 104b 121a 121d enc hyb grd sch sch int int int ine ine ine aplite pillow aplite pillow

50.4 57.0 64.6 60.6 51.4 53.0 49.0 60.7 47.9 48.2 46.8 75.8 52.6 76.0 55.4 1.10 0.96 0.60 0.80 1.42 1.03 1.30 0.86 1.87 1.12 1.53 0.06 1.60 0.06 1.23 18.7 17.7 15.5 16.5 12.4 17.8 18.3 16.5 13.6 12.2 14.8 12.6 17.6 13.0 17.1 9.78 7.22 5.03 6.70 13.1 9.15 10.5 6.75 11.5 10.1 11.2 0.47 9.10 0.45 8.00 0.17 0.11 0.08 0.11 0.28 0.12 0.18 0.10 0.18 0.20 0.14 0.04 0.11 0.01 0.13 4.86 3.20 2.20 3.00 7.00 4.80 5.50 2.80 10.4 12.5 10.3 0.05 4.40 0.09 4.40 7.56 6.15 4.22 5.59 7.97 8.25 7.13 5.25 8.10 7.83 8.36 1.00 7.50 1.12 6.45 3.90 3.90 3.30 3.90 2.40 3.20 3.80 3.80 2.00 1.68 1.91 3.40 4.30 2.60 3.90 2.05 2.46 3.66 1.87 2.29 1.34 1.69 1.55 2.57 3.36 2.65 5.12 1.70 5.84 2.16 0.30 0.32 0.18 0.22 0.43 0.33 0.36 0.29 0.09 0.28 0.18 0.04 0.41 0.04 0.40 0.60 0.52 0.50 0.50 0.50 0.82 2.27 1.03 0.94 1.15 1.51 0.10 0.60 0.11 0.75

99.42 99.54 99.87 99.79 99.19 99.84 100.03 99.63 99.15 98.62 99.38 98.68 99.92 99.32 99.92

claves and mafic intrusions are otherwise similar the data for A1203, FeO*, MgO, CaO, Ti02, high-alumina basaltic magma, involving both in major-element composition to high-Al basalts and P2O5 can be resolved into two line segments quenching of enclaves and local hybridization. common from volcanic arcs (Gill, 1981; Grove with a subtle change in slope at about 63% silica We show in a following section on rheologic and others, 1982). Enclaves present in areas (Fig. 13). Correlation coefficients are lower for modeling that thermal and crystallinity barrier characterized by hybridized mafic granodiorite rocks with less than 63% silica than for more to mixing between basaltic and granitic magmas are themselves hybrids as shown by silica con- silicic rocks (Frost, 1986). The slope change cor- limit the maximum silica content of a hybrid tents as high as 57.2% (Table 1 and Fig. 13). responds to the maximum silica content deter- magma at crustal pressures to -63%. Granodio- Although major-element data with respect to mined for rocks classed as hybrids based on field rite lacking mesoscopic or microscopic evidence silica from enclaves, intrusions, hybrids, and evidence. These lines of evidence suggest that of hybridization has more than 59% silica; granodiorite define nearly linear trends with most rocks below 63% silica result from magma major-element trends are consistent with frac- high linear least-squares correlation coefficients, interaction between granodiorite magma and tional crystallization of hornblende, plagioclase,

: ^MmL CaO s- ; * -0.92 18-

16 : y ** e :

S 9 " i T , , , , , , , , , e 12 • i 50 60 70 (fe - ; ^ • Fe0* Na20 e- ; ^^ 10: •ÂêMK'^t^'^^^S^^ e I*4' e ^«^V 6' -0.38 J _ , I ' ' ' ' ' 'e 'e '6. e ge . S'è 2-

5 6 9 9 I |° I , , |P 70 ( «B ' * MgO 12 • m 0.85 : *** -0.96 io- : * ** 8:

• •vyk.^jti...^ P2O5

.3.9. . •n . ^ r?v<>8.9.9.^,1 so 60 sjc>2 70 80 50 60 SÌ02 70 80

Figure 13. Anhydrous Harker diagrams for Lamarck Granodiorite and associated mafic rocks. Linear least-squares correlation coefficients are shown for mafic enclaves, intrusions, hybrids, and granodiorite, excluding data from cumulus hornblende gabbro inclusions, cumulus schlieren, and aplite dikes. Symbols as in Figure 12.

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and magnetite (Frost, 1986). Wilcox (1944), Vogel and Wilband (1978), Vogel and others Figure 14. AFM diagram, (1984), and Reid and others (1983) have also composite pillowed dikes. Line observed high linear correlations in oxide varia- shows trend for the granodiorite tions in suites of mixed and mingled rocks. and mafic rocks.

Schlieren Compositions

Schlieren in hybridized zones between mafic intrusions and granodiorite have compositions that lie on mixing lines between intrusion and host (Table 1 and Fig. 13), suggesting that they are simple hybrid«; between magmas of mafic cumulus origin. Plagioclase is not a cumulus Composite Dike Compositions intrusions and granodiorite host. Mafic schlieren phase, nor is there pétrographie evidence for having cumulus textures isolated in the grano- pyroxene or olivine in the inclusions (Fig. 10). The bimodal nature of the composite aplite- diorite are low in silica (52.1%); other major Hornblende on the liquidus and suppression of pillowed diorite dikes is shown in Figure 14. oxides lie well off the linear trends defined by plagioclase crystallization require water con- The diorites have between 50.9% and 59.5% sil- intrusions, hybrid!!, and granodiorite (sample tents that approach saturation for granodiorite ica; most are above 57%. The aplites have

76c in Table 1 and Fig. 13). The textures and through gabbroic compositions (Stern and oth- greater than 72.5% Si02 and are close to the compositions suggest the schlieren are localized ers, 1975; Cawthorn and O'Hara, 1976; Wyllie, granite minimum composition at 200-400 MPa accumulations of hornblende and Fe-Ti oxides 1977; Gill, 1981; Naney, 1983). Because the in- (Tuttle and Bowen, 1958). The lack of hybridi- from the granodiorite host. clusions do not contain plagioclase phenocrysts zation between mafic and felsic magmas in the like those of their hosts or the enclaves in the dikes is evident in Figure 14; only a few analyses Hornblende Gabbro Inclusion Compositions granodiorite, it is likely that the gabbro inclu- plot between the two fields. sions are not directly related to their hosts or to The hornblende gabbro inclusions are low in the mafic enclaves; instead they crystallized at ASPECTS OF MAGMA silica (46.8%-52.8%) and lie off the linear trends high water pressures at deeper levels of the crust INTERACTION

defined by the other lithologie units for A1203, and were entrained with later batches of basalt

MgO, CaO, Na2(>, and K20 (Fig. 13). High magma. High K20 and Rb as high as 200 ppm Field, pétrographie, and chemical evidence MgO and low AI 2O3 contents with respect to (Frost, 1986) in the inclusions may be due to suggest that all styles of interaction from nearly the host rocks, as well as Ni to 210 ppm and Cr late-stage alkali enrichment as shown by partial complete local homogenization of largely fluid to 610 ppm (Frost, 1986), are consistent with a replacement of hornblende by magmatic biotite. magmas to mingling of quenched mafic magma

TABLE 2. SUMMARY OF COMPOSITIONAL VARIATION AND FIELD RELATIONS. LAMARCK GRANODIORITE AND ASSOCIATED MAFIC ROCKS, SIERRA NEVADA, CALIFORNIA

Locality Mafic Roc c Type Silica content range (%) Width of Enclaves Hybrid Foliation Mixing or Gabbro Mafic At In contact zone at contact schlieren In mafc In host mingling inclusions body contact host at contact body dominant

Throughout Enclaves 49-53 53-67 Sharp Minor Variable Variable Mingling Lamarck

Piute Pass Hybrid int "usions 52-57 55-58 54-67 0-50 m Swarms Abundant Strong Moderate Mixing No Enclaves 49-52 54-67 Sharp Variable Mingling

Lake Sabrina Hybrid infusion 51-63 55-63 58-68 0-30 m Locally Abundant Strong Strong Mixing No

Mt. Gilben Intrusion 51-55 51-55 63-68 Sharp Abundant Locally Stroig Weak Mingling Yes

Echo Pass Hybrid int -usions 51-55 (53-55) 57-63 0-5 m Swarms Abundant Strong Variable Mixing No Enclaves 49-52 57-63 Sharp Locally Variable Variable Mingling

Giraud Pk. Hybrid infusions 54-64 54-64 64-67 0-25 m Locally Abundant Moderate Variable Mixing No Enclaves 49-52 57-67 Sharp Abundant Moderate Variable Mingling

Palisade Hybrid intrusion 50-60 55-62 60-65 I-I00m Swarms Exclusively Variable Variable Mixing No Basin Enclaves 49-52 60-65 Sharp Abundant

Mather Pass Intrusions 51-60 52-60 67-69 0-20 m Swarms Abundant Strong Variable Variable Yes Enclaves 49-53 51-69 Sharp Locally Variable Weak Mingling

Upper Basin Intrusion 52-60 52-(60) 65-69 0-20 m Swarms Abundant Weak Weak Variable Yes Enclaves (49-52) (55)-69 Sharp Locally Weak Weak Mingling

Lake Intrusion 52-60 52-(60) 65-67 0-10 m Swarms Abundant Variable Weak Mixing No Maijorie Enclaves (49-52) 52-69 Sharp Locally Weak Weak Mingling

Dusy Basin Composite dikes 52-57 72-77 Sharp No No Mingling No (pillows) (aplite)

Range of silica in enclaves isted for each locality is for enclaves present in granodiorite or hybrid rock near the contact with associated mafic intrusion at that locality. Parentheses indicate best estimates from specific-gravity measurements correlated to other analyzed samples; • • , not applicable.

with felsic magma occurred at different times magnitude larger than chemical diffusivities in The heat generated by a unit mass of mafic and at different scales during crystallization of silicate melts (Shaw, 1974; Hofmann, 1980; magma in cooling and crystallizing from its in-

the Lamarck pluton. Variation in style of inter- Watson, 1981). Sparks and Marshall (1986) itial temperature (Tm) to any equilibrium action around margins of individual mafic intru- have presented a heat-balance model in which temperature (0) is: sions is prominent, especially where these the equilibrium temperature of a mixture of >) S01 transect compositional gradients in the pluton magmas depends upon the initial temperature, HM = Jt (XlsCpm + (1 - Xls) CpJ'O + (for example, Mount Gilbert and Upper Basin heat capacity, heat of fusion, change in crystal- <9Xls fusion intrusions, Figs. 7, 9). Mixing styles along con- linity, and mass fraction of each magma. It is A Hm ) dT tacts also depend on relative amounts of inter- possible to model viscosities of each magma by IT acting magmas. Where the local proportion of the methods of Shaw (1972), modified for sub- where Xls is a function that describes the change mafic magma is high, hybridization and forma- liquidus magmas (McBirney and Murase, 1984), in crystallinity in the magma with changing so1 liq tion of schlieren dominates; where small if the equilibrium temperature and crystallinities temperature, Cpm and Cpm are heat capaci- amounts of mafic magma interact with large can be estimated. Sparks and Marshall (1986) ties of the aggregate crystals and liquid as func- fusion amounts of granodiorite magma, mingling dom- have shown that (1) hot mafic magma tends to tions of temperature, and A Hm is the heat inates. Where the compositional contrast be- quench in cooler felsic magma at low mass frac- of fusion of the crystals changing phase at any tween host and intrusion is small, interacting tions of mafic magma (XM), and, (2) as XM temperature. A similar function (Hp) can be de- magmas tend toward homogenization, although increases, the equilibrium temperature (0) in- fined for the heat absorbed by heating and melt- enclaves are present in hybrid rocks where the creases, and the equilibrium crystallinity and ing of the felsic magma in going from its initial

contrast is only a few weight percent silica. Min- viscosity of both magmas decrease. At some crit- temperature Tf to 0. At thermal equilibrium, the

gling becomes common as compositional con- ical value of XM, XM, the viscosity of the mafic mass fraction of mafic magma (Xm) is: trasts increase. Incorporation of enclaves in magma (??m) equals that of the initially cooler

granodiorite at the contact, regardless of the size felsic magma (RJF). At XM less than XM, rjm is = HP / (H - HM). of the mafic intrusion, dominates when compo- larger than rft due to high crystallinities; at F sitional contrasts are greater than 15% SiC>2. higher X , RJ is less than 17F. In the latter case, M M The crystallinity of the lavas and hypabyssal Table 2 summarizes field relations of some mixing to yield a hybrid is then possible if vis- intrusions considered by Sparks and Marshall mafic-felsic magma interactions as a function cosities are low enough, turbulent convection or (1986) could be measured. Because initial of the compositions of the interacting end other effective dispersive forces are active, and magmatic crystallinity and crystal size of sub- members. enough time is available for homogenization to liquidus end members are not preserved in plu- occur. The actual minimum value of XM at Enclaves not spatially associated with mafic tonic complexes, we have generalized the model which mixing between magmas is likely to occur intrusions must record mafic magma injection at by estimating the crystallinity of a given magma is impossible to predict precisely because the an early stage in the crystallization of the grano- composition as a function of temperature and amount of time spent below the liquidus is criti- diorite, as they are distributed throughout the water content. Crystallinity is modeled as the cal in determining both the crystallinity and the pluton and their long axes define regional folia- error function of T', where T' is a nondimen- viscosity of a magma (McBirney and Murase, tion in the host. The enclaves did not mix with sionalized temperature between the liquidus and 1984), and little is known about the mixing of the magmas in which they were incorporated; solidus (Marsh, 1981). Given this assumption for fluids of large and contrasting viscosities. Slow petrographic evidence suggests rapid crystalliza- magma kinetics, the crystallinity of a magma plutonic cooling rates allow more time for both tion on interaction with cooler felsic magma. changes most rapidly around the midpoint be- mechanical and diffusive homogenization of Enclaves in unhybridized granodiorite have a tween liquidus and solidus. Heat balance is cal- mixtures with lower XM and larger viscosities limited compositional range equivalent to high- culated based on normative mineralogies, as and viscosity contrasts. Although homogeniza- alumina basalt compositions (Table 1), suggest- heats of fusion of hydrous phases and solid solu- tion is likely to occur at XM somewhat lower ing that they represent the most primitive tions are poorly known. Residual or newly than XM, it is convenient in the following discus- magmas for which evidence of interaction is formed liquid compositions are calculated based sion to track the value of XM in order to deter- preserved. upon modal mineralogies. Crystal size is as- mine how end-member compositions, water sumed to vary as a linear function of the percen- The composite aplite-pillowed diorite dikes in contents, temperatures, and crystallinities affect tage crystallized. Liquidi and solidi at a total the Lamarck Granodiorite did not interact with whether or not magmas mix or mingle. pressure of 200 MPa (equal to 8 km depth, the the felsic granodiorite at the present level of ex- pressure suggested by wall-rock metamorphic posure except to fill fractures. The pillowed We have applied the heat-balance model of assemblages) are estimated as functions of bulk forms of the mafic rock and acicular apatite Sparks and Marshall to the magmas represented composition and water content based on exper- morphologies suggest that the mafic magma was in the Lamarck with several modifications. We imental data (Frost, 1986). Initial magma quenched in cooler aplitic magma during dike have extended the model to include heat capaci- temperatures above the calculated liquidus at intrusion. Although both facies of the dikes were ties of both the normative minerals and the cal- the initial water content are not allowed in the intruded simultaneously at the present level of culated residual or accumulating liquids of the model, although superliquidus temperatures are exposure, large differences in viscosity prevented magmas, as well as heats of fusion of the miner- possible in the felsic end member after thermal mixing. als crystallizing or melting as each magma equilibrium is reached. reaches 0. A complete description of the model, RHEOLOGIC MODELING a program listing, and the sources of data on Magmas interact in an isolated system in the heat capacities, heats of fusion, and the composi- model; that is, the interacting magmas remain as Interacting magmas reach thermal equilib- tional dependence of liquidi and solidi as func- discrete physical entities until all heat has ex- rium before significant chemical interaction can tions of pressure and water content are in Frost changed between them, and heat flow to wall occur because thermal diffusivities are orders of (1986). A brief description of the model follows. rocks or to magmas distant from the interacting

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domain is not considered. Thermal equilibration requires considerable time, and magmas may surrounding rocks. Finally, the model does not between small volumes of mafic magma in an cool slowly enough for hybridization to occur at allow for vapor exsolution as magmas cool and

infinite reservoir (for example, mafic enclaves in Xm lower than Xm. Additionally, although the crystallize, a process that may be important in granitic magma) is most accurately modeled. As model allows superheated felsic magmas after reducing specific gravity differences between en-

both Xm and total volume of interacting mag- thermal equilibration, such a state cannot be claves; arid host magmas (Eichelberger, 1980; mas increase, however, thermal equilibration maintained long due to dissipation of heat to Turner and others, 1983). Keeping these limita-

A 55% Si02 B 60%SiO2

-I I L. o 0.5 1.0 °o 0.5

Figure 15. Mass-fraction viscosity diagrams for differing initial silica contents and initial temperatures of interacting anhydrous magmas. Plots show calculated viscosities of end members (bold line for basalt and light for silicic end member) at thermal equilibrium as a function of mass

fraction of mafic magma (Xm) in the mixture. Dry liquidus basalt (50% Si02 at 1245 °C) interacts with dry felsic magmas (SiOz from 55% on A to

70% on D; at initial temperatures, Tf, ranging from solidus to liquidus). Silica content (in percent) of hyb rid magma at point of equal viscosity

(rj, where each set of curves cross), as well as Tf (°C) and initial crystalllnity of the felsic magma are indicated (for example, 1100/0.1). See text for discussion.

tions in mind, we now consider the interdepen- Effects of Composition and Temperature (Nicholls and Stout, 1982) and the viscosities of dence of factors influencing the manner in which the end members after thermal equilibrium is magmas interact, and we estimate the ranges of Latent heat released by crystallization of reached. Because heats of fusion of crystals in composition, initial temperature, mass fraction mafic magma and absorbed by melting of felsic mafic magmas are larger than those of crystals in of mafic magma, and water content over which magma or rock is critical in determining the felsic magmas, the equilibrium temperature is

magma mingling or mixing may occur. equilibrium temperature of a given mixture not a linear function of Xm, the mass fraction of

Figure 16. Mass-fraction viscosity diagrams for liquidus felsic magmas (60% and 70% Si02) with variable H20 contents (l%-5%), and liquidus

basalt (50% Si02) with 0% and 1% H20. Numbers by each rj( curve indicate Tf and water content (in percent) of the felsic magma. Water content

of felsic magma with which basalt interacts is shown beside each r\m curve. Increasing H20 in liquidus felsic magmas raises Xm. Increasing water

content of liquidus mafic magma (compare A-C and B-D) at any water content of felsic magma lowers Xm, although it raises r\, due to lowering of liquidus temperature with increasing water content.

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mafic magma in the mixture. At small Xm, most ity, more heat is required from crystallization of crystallinity and lowering the viscosity of the of the neat released by crystallization of mafic the mafic end member to melt the crystals in the liquid phase (McBirney and Murase, 1984). magma is absorbed by the melting crystals in the felsic magma, resulting in lower 0, greater crys- This change does not necessarily lead to greater felsic magma; at large Xm, all crystals in the tallinity of the mafic end member, and larger ease in mixing with basalt, however, because the felsic magma may be melted, and any heat re- viscosities for both end members at thermal concomitant decrease in liquidus temperature

leased by further crystallization of the mafic equilibrium for any Xm (Fig. 15). As Tf of any results in lower 0 and higher crystallinities for

magma simply raises the temperature of the felsic host decreases, Xm increases dramatically, mafic end members. Progressively larger mixture. but viscosities of the end members at Xm, rj, amount!! of mafic magma are required for hy- Plots similar to those used by Sparks and increase only slightly. As the silica content of the bridization as the water content of the felsic end

Marshall (1986) show viscosities of interacting felsic magma increases for a given Tf, Xm de- member increases (Fig. 16), but the lower vis-

magmas as a function of Xm for different end- creases slightly due to smaller latent heat of crys- cosities of water-rich felsic magmas make hy-

member compositions and initial temperatures tals in more silicic magmas, but rj increases bridization at Xm more efficient by lowering rj. after thermal equilibration. Dry felsic magmas dramatically due to the intrinsically higher vis- For example, a hybrid of initially liquidus

having between 55% and 70% Si02 at a range of cosities of more silicic magmas. The value of rj is granite (Si02 = 70%, H20 = 5%) and basalt at 4 3 Tf equilibrate with dry liquidus basalt (50% about 10 poise for a silicic end-member magma Xm = 0.85 will have a viscosity of about 10 Si02,1250 °C (Fig. 15). Because hybridization having 55% Si02, and it increases about an poise, and a silica content of only about 53% is most likely to occur when viscosities of the order of magnitude for each 5% increase in silica (Fig. 16B). In order to minimize the number of end members are similar, we note the silicacon- content. For this reason, hybridization of basalt changing variables, the plots in Figure 16 are for tent of the hypothetical hybrid magma at Xm in with the most silicic magmas probably would be initially liquidus felsic magmas; lower Tf only Figure 15. Again we emphasize that hybridiza- slow and would go to completion only if the decreases the probability of hybridization. This tion may occur at Xm lower than Xm, and that mixture were stirred mechanically by processes decrease suggests that basalts should hybridize rates of cooling, chemical diffusion, and turbu- such as forcible injection of basalt, turbulent readily with small proportions of hydrous partial lent convection rates determine how much mixing during flow to a vent, or collapse of the melts of crustal rocks, leaving little mesoscopic lower. The slopes of the viscosity curves where magma-chamber roof during caldera formation. evidence of interaction. Resultant hybrids would they cross are also important, as hybridization The importance of the latent heat of crystalli- not be more silicic than basaltic andesite or an- may occur at X much lower than X if viscos- m m zation of mafic magma is highlighted in the case desite, however. ity changes are small with changing X . m of partial melting of dry plutonic wall rocks, Increasing the water content of the mafic Liquidus temperatures of dry magmas having illustrated by curves for the solidi in Figure 15. magma (Figs. 16C, 16D) lowers its liquidus, silica contents as high as 72% (between 1150 Granitic through dioritic wall rocks initially at thereby making it less likely to quench by crys- and 1225 °C; Eggler, 1972a; Wyllie, 1977; their solidi are predicted to melt completely if tallizing upon injection into cooler silicic

Naney, 1983) are only slightly lower than those Xm exceeds 0.7; stoping of small blocks locally magma. This is reflected in a decrease in Xm for

for basalts (Holloway and Burnham, 1972; Wyl- may yield high values of Xm. Hybridization of a given silicic end member as the mafic end lie, 1977), and so 0 for mixtures of dry, liquidus basalt with melted dioritic to mafic granodioritic member becomes more hydrous (Fig. 16). Cor- magmas differs little from the initial temperatures wall rocks to form magmas of andesitic compo- responding values of rj increase slightly due to of the end members, and viscosities decrease sition can occur when rj is small. More silicic the lower initial temperatures of hydrous mafic only slightly as Xm increases (compare liquidus wall rocks would form melts having such high end members. curves in Fig. 15). Hybridization of dry, liquidus viscosities that hybridization would be inhibited. magmas is possible at all Xm because the mafic These calculations are for the dry case, and, al- Summary of Composition, Temperature, magma is never quenched and maintains its low though they are illustrative and may have some and Water-Content Effects viscosity. The likelihood of hybridization de- application to partial melting of charnockitic creases, and mechaE.ical stirring may be required lower-crustal rocks, they are not strictly applica- Projections of rj of liquidus felsic magmas to effect homogenization as silica increases in the ble to the biotite- and hornblende-bearing wall with different silica and water contents interact- felsic end member due to higher intrinsic viscosi- rocks of the Lamarck Granodiorite. ing with dry, liquidus basalt having 50% Si02 ties. Large volumes of dry, liquidus granite are are shown in Figure 17. Along the curves for not likely in the upper crust, however (Wyllie, Effects of Initial Water Content initial H;>0 in the felsic magma, viscosities of 1977; Wyllie and others, 1978; Green, 1970). interacting magmas are equal and are given by

Magma viscosities increase abruptly due to Petrographic evidence for the early crystalli- the isopleths for 77; at smaller Xm, rjm is higher interference of crystals as crystallinity rises zation of hornblende in the Lamarck Granodio- than jjf due to high crystallinity; at larger Xm, rjm above 0.15, and magmas having crystallinity rite, associated enclaves, and intrusions, as well is smaller than 77f. Increasing amounts of water higher than 0.6 behave as brittle solids due to as abundant experimental evidence (Stern and in the felsic magma increase Xm and decrease rj. partial interlocking of crystals (Marsh, 1981; others, 1975; Wyllie, 1977; Naney, 1983), sug- As compositional contrasts between magmas McBirney and Murase, 1984). For these reasons, gests that both mafic and silicic end-member decrease, rj decreases. Differences between •qf for initially subliquidus felsic magmas drops magmas contained significant water. The pre- magma viscosities away from rj cannot be plot-

rapidly as Xm increases due to decreasing crystal vious discussion of interaction of anhydrous ted on the diagram, but decrease with decreasing content, but after all crystals are resorbed, magmas provides a starting point for illustrating compositional contrast between end members. r] decreases more slowly as a function of in- the large effect of initial water content on styles Higher initial temperature, decreasing silica, or creasing 0 in accordance with Shaw's (1972) of magma interaction. decreasing water content of the felsic magma equations. Increasing water content lowers the initial promote hybridization, but resultant hybrids will With decreasing Tf and increasing crystallin- viscosity of felsic host magmas by decreasing the be basaltic or andesitic in composition.

75 —1 y1 /r 1 vT and associated mafic rocks were both hydrous and below their liquidi at the time of interaction. ^Y A We would expect, therefore, hybrids of rela- « 70 - r M / V E o V/ Y Figure 17. Summary interaction dia- tively mafic composition and a wide range of cu co 5 E gram showing X and rj for different mixing and mingling behavior dependent upon CO \ / \ / m E Ï initial Si02 in liquidus felsic magma compositional contrast, initial temperature and o 65 o Y A / E M " ® \ / / 4" having different initial water contents. crystallinity at the time of mafic magma injec- m ® Mafic magma is dry liquidus basalt tion, and the local-scale proportion of mafic 60 (50% Si02) in all cases. See text for magma. In this section, we model the conditions CM discussion. O O and ranges of mixing and mingling likely during co CM crystallization of the Lamarck Granodiorite and 55 21 • v, M its associated mafic rocks. The presence of plagioclase phenocrysts in most enclaves and in many intrusions requires 0 0.5 1 water contents below about 3% at the time of Xm their crystallization (Eggler, 1972a, 1972b; Stern and others, 1975). On the other hand, the pres-

MODEL FOR THE LAMARCK 20 wt% SiOz can hybridize in any proportions ence of early hornblende in all rock types re-

GRANODIORITE only if they are on their liquidi and if they have a quires at least 3% H20 in the melt (Eggler, combined water content of less than 2%. As 1972b; Stern and others, 1975; Wyllie, 1977; The previous discussions have served to high- either magma drops below its liquidus or be- Burnham, 1979; Naney, 1983). Thus we esti- light the effects of composition, temperature, comes more water-rich, hybridization occurs at mate an initial water content of about 2.5% for

and crystal and water content on the nature of progressively larger Xm, and resultant hybrids the high-alumina basalt magmas from which the magma interaction. We have shown that mag- are increasingly mafic in composition. Pétro- mafic enclaves and intrusions crystallized. mas having compositional differences as large as graphie evidence indicates that the granodiorite Hornblende in granodiorite magmas at 200

Figure 18. Mass-fraction viscosity diagrams of models for magmas represented by Lamarck Granodiorite and associated mafic rocks. Mafic

magma (bold lines) has 50% Si02, 2.5% H20,1050 °C Tm, and 0.05 initial crystallinity. Beside each r)m curve is shown Tf of the felsic magma with which the mafic magma interacts. Beside each rft curve is shown the initial temperature (°C) and crystallinity of the felsic magma; number

where each set of curves cross shows Si02 of hypothetical hybrid formed at rj. (A) Interaction of basalt with mafic granodiorite (light lines, 60%

Si02, 3.5% H20) at different Tf between liquidus and solidus. Xm is low (and hybrids relatively silicic) for near-liquidus felsic magma but

increases as felsic magma cools and crystallizes. (B) Interaction of basalt with granite (light lines, Si02 = 70%, H20 = 4.0%) results in

crystallization of mafic magma to large rjm at all Xm less than 0.4, even for liquidus felsic magma. See text for discussion.

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MPa requires water contents of at least 3.5% must instead result from fractional crystalliza- pluton. Simple aplite dikes that do not have (Naney, 1983). At an initial temperature of tion of mafic granodiorite or hybrid magmas. mafic pillows may also form by migration of 1050 °C, the model predicts the basalt (50% This is consistent with the general lack of corre- interstitial granite-minimum melt toward an SiC>2) will have a crystallinity of 0.05, in accord lation between enclave abundance and proxim- opening dike. with the phenocryst content of the average en- ity to the more mafic margin of the pluton and clave. Interaction of this mafic magma with fel- with the observation that intrusions crosscut CONCLUSIONS sic magmas of 60% and 70% SiC>2, having 3.5% compositional zoning in the granodiorite. Local and 4.0% H2O, respectively, for a complete reversals in zoning may be the result of hybridi- Evidence of interaction between magmas of range of initial crystallinities of the felsic magma, zation with mafic intrusions. contrasting composition throughout the crystal- is illustrated in Figure 18. Composite dikes that cut the granodiorite also lization of the Lamarck Granodiorite is present Injection of m.ific magma produces different may be due to interaction of mafic and felsic in field relations, petrographic observations, and styles of interaction, depending on the tempera- magmas. Calculations show that mafic dikes major-element compositions. Sparks and Mar- ture and crystallinity of the host at the time of having compositions like those of the mafic pil- shall (1986) have shown that the nature of

mafic magma injection. Interaction of small lows (55% Si02) injected into nearly solidified magma interaction is largely a function of the amounts of near-liquidus mafic magma early in granite equivalent to the most silicic granites of viscosities of the end members after they have the crystallization of the felsic pluton may result Dusy Basin would totally melt the near-solidus thermally equilibrated; we have adapted and extended. their model to account for a wide range in hybridization (Fig. 18A); however, even granite host if Xm exceeds 0.6 (Fig. 19). Alterna- small decreases ir. Tf below the liquidus result in tively, the localized decrease in lithostatic pres- of initial compositions, temperatures, water con- quenching of the mafic magma until Xm exceeds sure as a dike opens may promote migration of tents, and crystal contents. Whether mafic 0.6. As the pluton cools and crystallizes, pro- interstitial granite-minimum composition resid- magmas mix as liquids, quench as enclaves, or gressively larger amounts of mafic magma must ual magma toward the dike. Van der Molen and crystallize as large intrusions that hybridize only be injected into a given volume of felsic magma Patterson (1979) and Hibbard and Watters locally with their hosts depends on their Theolo- for hybridization to occur. (1985) have discussed melt migration in par- gies and the manner in which rheologies change Similar relations hold for mafic magma inter- tially crystallized granitic rocks. McKenzie during interaction. Mingling is dominant in mixtures having low mass fractions of hot mafic acting with the most felsic magmas of the La- (1984), Richter and McKenzie (1984), Ribe magma at any initial temperature and crystallin- marck (Fig. 18It), except that mafic magma (1985), and Sleep (1986) have modeled similar ity of the felsic magma. Hybridization becomes quenches to high crystallinities and viscosities processes for magma migration within the man- likely for large mass fractions of mafic magma even in crystal-free felsic magma at all Xm less tle. The heat of crystallization of the mafic and (or) as thermal and compositional contrasts than 0.6. Assuming that hybridization may magma reduces the viscosity of the interstitial between interacting magmas decrease. occur between magmas having viscosity differ- granite-minimum melt (Fig. 19), increasing the ences as large as 2.0 log poise under slowly probability of melt migration to the dike. For The model utilized in this study assumes that cooled plutonic conditions, the resulting hybrid either model, incorporation of cooler granite- interacting magmas maintain a physical inter- has only 63% SiC^. Hybrids produced by minimum melt in the dikes locally decreases Xm face between them until they have melted or mixing mafic magma with felsic magma that has in the dikes and may quench the mafic magma crystallized to the extent required for thermal an initial crystallinity of 0.3 are unlikely to have as pillows. The superheated felsic magma equilibrium to be achieved. Although this sim- more than 54% Si02. This modeling suggests formed from the granite then may act as a lubri- plification enables straight-forward calculations, that mixing between end members of extreme cant to intrusion of the mafic pillows to higher, the actual situation is much more complex. Heat composition cannot produce large volumes of more completely solidified parts of the felsic exchange between magmas may be much more silicic magma, even under the most favorable thermal circumsiances under plutonic conditions. Field and petrographic observations on the Lamarck undertaken prior to the rheologic modeling are consistent with the modeling. Figure 19. Mass-fraction viscosity Rocks mapped as hybridized facies of the La- diagram of magma interaction models marck based on field observations (for example, for composite dikes. Mafic magma presence of schlieren, proximity to mafic intru- (bold lines, 55% Si02, 2.5% HzO, 1050 sions, matrix of enclave swarms) or petrographic °C Tm) interacts with near-solidus fel- criteria (bimodal hornblende populations, sic granite (light lines). Models show patchy feldspar cores) have between 53% and complete melting of nearly solidified 64% Si0 . Samples of the Lamarck that lack 2 granite near dike (upper set of curves) mesoscopic or petrographic evidence of a cumu- and mobilization of interstitial granite- late or hybrid origin have silica between 59.5% minimum melt near dike (lower and 73%, and more than 60% of the samples curves). Numbers along each rjf curve have greater than 63% SiC>2. indicate felsic magma initial Si02 and One implication of the model is that the nor- H20 percentages, crystallinity, and mal felsic-coreward zoning of the pluton cannot crystal size. See text for discussion. be produced directly by mixing processes but

7r>~ Percent uO

and compositional Si zoning

Composite Mafic dikes dikes Figure 20. Vertical section cartoons showing types of magma interaction throughout crystallization of Lamarck Granodiorite. (A) Enclave formation by mingling of small amounts of high-alumina basalt magma with granodiorite magma early in its crystallization. Mechanism shown is highly speculative. (B) Intrusion of mafic magma across compositionally zoned, partially crystallized pluton that contains early, widely distributed mafic enclaves (not shown). Crystallinity of host decreases toward core due to heat loss to margins and roof as well as decrease in liquidus of magma due to compositional zonation. (C) Differing styles of interaction around margin of intrusion depending upon each magma's initial composition, temperature, crystal and water content, and volume over which they interact. Partial hybridization dominates where initial compositional and crystallinity contrasts are small and local mass fraction of mafic magma is large. Quenching of mafic intrusion with incorporation of newly formed enclaves in the host dominates where compositional and crystallinity contrasts are initially high (although melting of felsic host occurs adjacent to intrusion). Reintrusion of mafic rock by remobilized granodiorite occurs locally. (D) Late mafic dikes intrude nearly solidified felsic pluton, remobilizing and incorporating interstitial magma of granite-minimum composition near core of pluton. See text for further discussion.

rapid than crystallization or melting may keep The earliest magma interaction events for as shown, or disruption of a compositionally pace with, resulting in supercooled or super- which evidence is preserved in the pluton are layered magma chamber. The over-all pattern of heated (although not necessarily superliquidus) mafic enclaves not spatially associated with concentric compositional zoning in the pluton magmas. Complex thermal and chemical inter- mafic intrusions. Quenching and dispersal of appears to predate incorporation of the pre- play during latent heat balance in approaching small amounts of high-Al basalt magma through served enclaves and intrusions. true equilibrium may result in double-diffusive the granodiorite magma occurred relatively The granodiorite host must have been par- exchange (Huppert and Turner, 1981; Huppert early in its crystallization history (Fig. 20A). The tially crystallized prior to incorporation of the and Sparks, 1984; Huppert and others, 1982, mechanism shown in Figure 20A is highly spec- widely distributed enclaves, because Stokes set- 1984; Morse, 1986) between interacting ulative; possible mechanisms include forcible tling velocities of nonvesicular enclaves in magmas. injection followed by convective dispersal, crystal-free granodiorite are as high as several

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kilometres per year. Wholesale, turbulent con- The composite dikes may represent small imum predicted silica content of a hybrid vection to distribute the early formed enclaves amounts of mafic magma injected as dikes very between basalt and granite-minimum melt throughout the granodiorite magma is inconsist- late in the crystallization of the pluton when under reasonable crustal conditions does not ex- ent with the high viscosities of magmas rich only small amounts of interstitial water-satur- ceed about 63%. The modeling suggests that enough in crystals to prevent enclave settling ated granite-minimum melt remain near the core most silicic magmas in arc settings are produced (McBirney and Murase, 1984), unless vesicula- of the pluton (Fig. 20D). Large thermal differ- dominantly by fractional crystallization of in- tion of the crystallizing mafic magma occurred, ences between dike magma and host quench the termediate magmas, which may themselves be as has been suggested by Eichelberger (1980). mafic magma as pillows while heating the host products of hybridization. Limited remelting of granodiorite near intru- until the interstitial granitic liquid is near or sions that serve as sources for enclaves may above its liquidus or the host is entirely melted ACKNOWLEDGMENTS permit distribution of the enclaves by convec- (Fig. 19). The opening of the dike reduces the tion in the locally remobilized host. Some en- lithostatic pressure in the dike, which causes the Discussions with Norm Sleep and Jonathan clave swarms may be accumulations that settle relatively low viscosity granitic liquid to migrate Stebbins and reviews by C. R. Bacon and J. S. after quenching in nearly crystal-free felsic toward the dike. The aplitic melt is then en- Pallister improved both content and presenta- magma (Fig. 20A); alternatively, swarms may trained with and acts as a lubricant for further tion of the manuscript. Reviews by M. J. Hib- form by emplacement and quenching of mafic intrusion of the composite dikes to higher levels bard and L. A. Marshall were also helpful. The magma into nearly solidified felsic rock which is in the pluton. U.S. National Park Service and U.S. Forest Ser- then only locally remelted in the vicinity of the A corollary of the interpretation presented vice permitted sampling in areas normally intrusion (Fig. 20 C). here for the origin of enclaves, mafic intrusions, closed to sampling. This research was supported As a mafic intrusion first invades the zoned, and composite dikes is that broadly similar high- in pan: by the Geological Society of America partially crystallized granodiorite pluton (Fig. alumina basalt magmas, or their slightly contam- (Research Grant 2624-80), the Shell and 20B), the interaction is expected to be largely by inated or evolved derivatives, were present and McGee Funds of Stanford University, and Na- mingling (Fig. 20C). As more hot mafic magma providing heat throughout the crystallization of tional Science Foundation Grant EAR 84- invades the chamber, temperatures near the in- the Lamarck Granodiorite. Additionally, there is 07822 to Mahood. trusion increase £is crystallinities and viscosities no evidence that primitive basalt reached the of both magma;, decrease. Hybridization be- present level of exposure. REFERENCES CITED

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