Abstract

Although compositional variation in zoned calc-alkalic plutons is often ascribed to crystal fractionation, diagnostic large-scale field evidence of crystal accumulation in these slowly cooled bodies is generally missing. In many plutons, however, small-scale crystal cumulates have been preserved as layered schlieren and in microcosm may allow an assessment of the importance of crystal fractionation in their host pluton's development. Small, widely separated patches of schlieren in the Tuolumne Intrusive Series, Yosemite National Park, California, formed as cumulates. Their darkest layers show high concentrations of magnetite, sphene, biotite, horn-blende, and zircon, and have unusually fractionated major and trace element compositions (FeO >33%; Al2O3 <7%; La/Lain chondrites ∼750; Zr ∼2000 ppm). The layers define smooth trends on major and trace element Al2O3 variation diagrams that diverge strongly from patterns for the main-sequence rocks of the Tuolumne Series and granitoids throughout the Sierra Nevada. Removal of such cumulates from any main-sequence magma would produce Al-rich evolved rocks, not the Al- poor felsic rocks of the pluton. The findings suggest that fractional crystallization did not produce the dominant chemical patterns seen in the Tuolumne and similar Sierra Nevada granites.

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