The development and refinement of continental arcs by primary basaltic magmatism, garnet pyroxenite accumulation, basaltic recharge and delamination: insights from the Sierra Nevada, California

The lower crust of the Mesozoic Sierra Nevada batholith was made up of high MgO, garnet-poor and low MgO, garnet-rich pyroxenites. Both groups are genetically linked and are collectively complementary to the mafic to intermediate Sierran plutons. High MgO pyroxenites represent high pressure cumulates from a mantle-derived hydrous basalt or basaltic andesite, resulting in derivative magmas having unusually low MgO for a given SiO₂ as represented by the numerous mafic enclaves found in many Sierran plutons. The low MgO pyroxenites are either (1) shallow pressure cumulates from these derivative magmas or (2) partial melting residues (restites) of these derivative magmas after they were emplaced and solidified at lower crustal levels. In both cases, the complementary melt to the low MgO pyroxenites is driven to higher SiO₂ contents, generating diorites and granodiorites. However, this simple two-stage scenario for the origin of Sierran granitoids cannot explain the observation that the Mg# of Sierran intermediate magmas remains roughly constant at ∼0.45–0.50 with increasing SiO₂. Basaltic recharge/mixing with the lower crust is suggested as one means of buffering Mg#s and re-melting the lower crust to generate granitic melts, the latter of which mix with more juvenile magmas to complete the Sierran differentiation series.