Low-initial-Sr felsic plutons of the northwestern Peninsular Ranges batholith, southern California, and the role of mafic-felsic magma mixing in continental crust formation

We studied the formation of low-initial-Sr felsic plutons by using data from the Early Cretaceous western Peninsular Ranges batholith near Escondido, California. The systematically sampled Escondido plutons have a uniformly low initial 87Sr/86Sr isotope ratio of Sri < 0.704, but a wide range of SiO2 compositions, from 46 to 78 wt%, which fall in three distinct groups: 20% gabbros, 35% tonalites, and 45% granodiorites. These low-Sri plutons are unique in having undergone one cycle of mantle melting to give basalt composition rocks, and a second cycle of arc basalt melting to give a range...