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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 of SiO2 plutons, but no third cycle of melting and contamination by old continental crust to yield high-Sri rocks. After doing two-cycle partial melting and fractional crystallization calculations, it was recognized that mixing of gabbro and granodiorite magmas was necessary to yield the tonalites. The linear data pattern on Harker diagrams is interpreted as resulting from mixing of mafic magma from partial melting of the mantle and felsic magma from partial melting of the lower crust to form intermediate magma. These plutons provide a simplified two-cycle Phanerozoic example of the petrogenetic process for forming continental crust.

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