MINERALOGIC AND 40AR/39AR ANALYSIS OF LAYERED MAFIC CUMULATES, WESTERN SIERRA NEVADA BATHOLITH, CA

Layered hornblende troctolite cumulates crop out as m- to km-scale xenoliths along the westernmost margin of the Early Cretaceous Sierra Nevada batholith at approximately 36.5ºN latitude. Near identical 40Ar/39Ar dates of 112.94±0.67 and 112.31±1.13 Ma on hornblendes from cumulate localities separated by 24 km demonstrate that regional magmatism ceased with emplacement of the 123 to 115 Mya ring dikes that engulf the cumulate xenoliths.

Twenty core samples were analyzed to investigate the stratigraphic variation of mineral compositions within two cumulate sections: a 10-m-thick section located south of Tucker Mountain, and a 100-m-thick section on Stokes Mountain. Electron microprobe analyses of virtually unaltered samples yield the following compositional data: cumulate olivine (ol) (Fo68.7-77.7; n=50); rimming orthopyroxene (opx) (En71.2-78.6; n=18); cumulate opx (En65.0-78.0; n=21); rimming clinopyroxene (cpx) (En51.9-57.3; n=4); cumulate cpx (En41.2-53.8; n=9); plagioclase (pl) (An88.5-95.3; n=27); intercumulus hornblende is dominantly edenite (n=27). The general upward decrease of Fo and En contents suggests that the Tucker Mountain section (Fo68.7-72.2) crystallized at lower temperatures than the Stokes Mountain section (Fo70.5-77.7). The composite Stokes-Tucker section indicates the following crystallization order: ol + pl; ol + opx + pl; ol + opx + cpx + pl; ol + opx + pl.

Nickel contents of olivines throughout both stratigraphic sections form sawtooth patterns suggesting repeated injection of relatively primitive, Ni-rich magma(s) followed by continued olivine fractionation. Stratigraphic variations of plagioclase composition are not coupled with those of the ferromagnesian minerals. Instead, repeated increases in An content may reflect variations in water fugacity, which might be generated by venting of the magma chamber(s). Together with abundant soft-sediment deformation structures, the chemical data support formation of cumulates in shallow, dynamic magma chamber(s) undergoing fractional crystallization, repeated recharge and venting.