MAFIC-FELSIC MAGMA MIXING IN CONTINENTAL CRUST FORMATION: AN EXAMPLE FROM LOW-INITIAL SR GRANITOIDS OF THE NORTHWESTERN PENINSULAR RANGES BATHOLITH, SOUTHERN CALIFORNIA

A 1000 km² area within the western Peninsular Ranges Batholith (PRB) near Escondido, California is made up of granitoids having a uniformly low initial ⁸⁷Sr/⁸⁶Sr isotope ratio of Sr_i<0.704, but a wide range of SiO₂ compositions ranging from 46 to 78 wt.%. The systematically sampled Escondido plutons are made up of three distinct groups consisting of 20% gabbros, 35% tonalites, and 45% granodiorites. The linear data pattern on Harker diagrams is interpreted as resulting from the mixing of mafic magma from partial melting of the mantle and felsic magma from partial melting of the lower crust to produce magma of intermediate SiO₂ composition. These three magma types subsequently fractionated and were emplaced at shallow levels. The early Cretaceous western PRB granitoids formed in association with island arc basalts accreted to the North American craton, and this especially low Sr_i portion displays negligible contamination from old continental crust. These Escondido granitoids are unique in having undergone a cycle of mantle melting to give arc basalts, a cycle of arc basalt (or gabbroic underplate) melting to give a range of SiO₂ granitoids, but no third cycle of continental crust melting and assimilation to yield high Sr_i. Therefore, these granitoids provide a simplified Phanerozoic example of the petrogenetic process for forming continental crust. Simple partial melting, mixing, and fractional crystallization calculations were performed to quantitatively understand the relative importance of each process during differentiation. Mass balance calculations indicate that the volume of ultramafic restite left after differentiation of the lower crust is about twice the volume of the fractionated Escondido granitoids.