EXPLORING CAUSES OF MESOZOIC ARC FLARE-UPS IN THE PENINSULAR RANGES BATHOLITH, SOUTHERN CALIFORNIA

Both bedrock and detrital U/Pb zircon ages define magmatic flare-ups at ~250, ~170, ~100 Ma in a broad corridor across the PRB to Mojave in southern California and additional smaller peaks only farther east in the Transverse Ranges and Mojave at 83 and 75 Ma. Models to explain such flare-ups involve forearc underthrusting of western arcs and sediments, retroarc underthrusting of North American crust and increased mantle melt input. Scarce Triassic geochemical data indicate lower time averaged SiO₂, positive eNd_i values, elevated Sr_i and variable Sr/Y values both before and during this flare-up. Jurassic data indicate variable time averaged SiO₂, fairly constant slightly negative eNd_i, variable Sr_i, flat Sr/Y of ~20 before and during this flare-up. A much larger and more detailed Cretaceous dataset indicates that the flare-up built an arc across an oceanic-continental margin transition, and that volumes of magmatism are similar in the western oceanic and eastern continental columns (Morton et al., 2014) although the flare-up culminates in large La Posta style plutons in eastern continental crust. Published positive eNd_i, low Sr_i, low d18O isotopic values and low Sr/Y ratios indicate that western magmas formed in thinner crust and remained primitive with little to no involvement of continental crust. Averaged SiO₂ show only a small west-east increase while decreasing eNd_i, increasing Sr_i and d18O isotopic values and increasing Sr/Y ratios indicate an eastern increase of up to 30% continental crust assimilated into the magmas in regions where the arc column thickens. In the younger 83 and 75 Ma eastern flare-ups there is widespread evidence of midcrustal melting and migmatite formation in the eastern continental part of the arc. And although these eastern resulting magmas are more evolved than most PRB magmas the only known source to drive melting at this time is an increase in asthenospheric mantle magma input into the lower crust. Thus these data indicate that variable mantle input is the likely main driving mechanism for all five of the above magmatic flare-ups, with crustal column thickness and amount of assimilated continental crust not reflecting the main flare-up triggering mechanisms but only their importance in controlling final magma chemistries.