THE EASTERN PENINSULAR RANGES BATHOLITH MAGMATIC FLARE-UP: INSIGHT FROM ZIRCON U-PB AGES AND OXYGEN ISOTOPE RATIOS

Mesozoic-Cenozoic circum-Pacific batholiths are a prime example of convergent continental margin magmatic processes thought to drive growth and maturation of continental crust. Although it's now clear that non-steady-state magmatic flare-ups of ~5-15 Ma duration account for the bulk of magmatic addition in long-lived continental margin arcs, understanding of deep crust & mantle processes related to the build-up and triggering of flare-up events remains elusive. “La Posta-type” intrusions that dominate the eastern Peninsular Ranges batholith of southern and Baja California provide an important perspective on this issue. These rocks are chemically similar to high-Al tonalite-trondhjemite-granodiorite gneiss terrains of Archean crust produced by high pressure melting of mafic source regions. New zircon U-Pb ages (n=43) from most of the major La Posta intrusive centers document synchronous and surprisingly brief emplacement at 96±3 Ma throughout this 1600-km long batholith segment. High del18O values of La Posta-type zircon (~7-11 per mil) indicate large components of supracrustal contaminants into deep crustal magma source regions. Because the volume La Posta-type crust in the Peninsular Ranges may easily exceed a million cubic kilometers, simple mass balance considerations require supracrustal recycling on a massive scale. The fact that high del18O La Posta-type zircon are from rocks with Sri values mostly <0.706 constrains the nature of the supracrustal contaminant. Assimilation of high Sri Julian Schist-type metasedimentary wallrock to account for La Posta zircon oxygen isotope compositions is unrealistic on several counts. We speculate that large-scale sediment underplating of isotopically primitive accretionary prism material may have played an important role leading up to La Posta-type melt generation. This view is supported by a sediment deficit in adjacent forearc basin & accretionary prism belts of the California borderland.