GEOCHRONOLOGY AND PETROLOGY OF UPPER CRUSTAL MAFIC PLUTONS SUPPORT A MANTLE CONNECTION TO ARC MAGMATIC PRODUCTIVITY: EASTERN SIERRA NEVADA BATHOLITH, CA

It is widely recognized that magma flux in continental arcs is cyclic in nature, resulting in “flare-ups” and lulls, but the cause of these arc tempos is uncertain. Variable subduction angle and convergence rate, lithospheric underthrusting, and fluctuating mantle input are invoked to explain these cycles. Though the upper crust of continental arcs is predominantly silicic, mantle-derived basalts are critical components in arc productivity, as they provide heat and mass to generate more silicic melts. Therefore, variation in the flux of mantle-derived basalts has been proposed as a driver of magmatic tempos and arc batholith construction. To interrogate the role of mantle-sourced melts in the arc crustal column, we focus on a series of upper crustal mafic plutonic complexes in the predominantly granodioritic Sierra Nevada batholith (SNB) paleo-continental arc.

We present field observations, bulk-rock and mineral chemistry, and geochronology of 17 mafic plutons (<25 km²) in the eastern SNB. Calculated melt compositions in equilibrium with the most primitive minerals in these mafic complexes (melt Mg# 60-45) are significantly less evolved than the bulk of the SNB, but non-primitive. This suggests 40-60% differentiation of mantle-derived basalt to produce the parental melts of the plutons. Bulk-rock compositions from the mafic plutons define continuous trends in geochemical variation diagrams (SiO₂ 39-64 wt%, Mg# 73-36), consistent with upper crustal fractionation within each complex. New U-Pb zircon geochronology reveals two main pulses of mafic magmatism: (1) 89-99 Ma (n = 9 plutons) and (2) 145-153 Ma (n = 7 plutons), with two older bodies intruded at 170.5 and 190 Ma. These pulses are coeval with the late Cretaceous (100-85 Ma) and late Jurassic (160-145 Ma) flare-ups in the SNB, when immense volumes of felsic plutonic material were added to the arc (DeCelles et al., 2009). Additionally, the mafic pulses are contemporaneous with regional diking events at 90-94 Ma and 148-149 Ma (Coleman et al., 2000), suggesting that an extensional regime may favor upper crustal mafic intrusion and voluminous magma production. The temporal association between flare-ups and mafic plutonism suggests a link between these processes, and supports the idea that arc magmatic flux is modulated by mantle melt production and intrusion.