CONTRIBUTIONS OF MAFIC MAGMATISM TO THE UPPER CRUSTAL SIERRA NEVADA BATHOLITH, CALIFORNIA (Invited Presentation)

Although volumetrically minor compared to high-silica granodiorites to granites that dominate batholiths, mafic plutons (non-primitive gabbros and diorites) are widely present in the upper crust of preserved arc sections. However, the role that high-Al, low-Mg basalts (thought to be parental melts of these mafic plutons) play in the formation of large volume, high-silica batholiths that make up the upper crust of continental arcs has been relatively understudied. Here we present whole-rock & mineral chemistry, as well as, U-Pb zircon geochronology of 17 mafic plutons from the central-eastern Sierra Nevada batholith (SNB). We show that the majority (88%) of the studied mafic plutons intruded in two pulses (155-147 Ma and 99-90 Ma) during the late-Cretaceous and late-Jurassic flare-ups that define magmatism in the high-silica batholith. Further, whole-rock and mineral chemistry support the hypothesis that the mafic plutons represent the complementary melt to lower crustal cumulates as documented in garnet-hornblende-pyroxenite xenoliths and deeper crustal (0.7-0.9 GPa) mafic rocks of the southern SNB. We show that these high-Al, low-Mg basalts that intruded to upper crustal levels have a relatively uniform and restricted composition in terms of Mg# (40-55) likely due to variations in magmatic rheology during hydrous melt evolution in the lower crust. In particular, basalts differentiating in the lower crust obtain low densities and viscosities over a narrow window of Mg# enhancing the ascent of a relatively narrow range of melt compositions. Further, we suggest that these same properties allowing for ascent are also conducive to mixing with more evolved melts to produce the compositional spectrum observed in the high-silica batholith. As such, the mafic plutons of the SNB could represent an important end-member in the generation of the compositional diversity observed in continental arc batholiths.