SEMINAR - Dave Kimbrough

Growth and recycling of continental crust—new insight from the Peninsular Ranges batholith of southern and Baja California
Dr. Dave Kimbrough
Department of Geological Sciences
San Diego State University
Wednesday, October 10th CSL 422, 1pm

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.