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

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Field, Chemical, and Physical Constraints on Mafic-Felsic Magma Interaction in the Lamarck Granodiorite, Sierra Nevada, California 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. 272 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/2/272/3998276/i0016-7606-99-2-272.pdf by guest on 26 September 2021 CONSTRAINTS ON MAFIC-FELSIC MAGMA INTERACTION, CALIFORNIA 273 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.
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