Abstract

New U-Pb isotopic data for detrital zircon populations from the Upper Devonian(?) and lower Carboniferous Bragdon Formation add constraints to the nature of source areas for epiclastic rocks in the ensimatic eastern Klamath arc terrane. Bragdon clastic units include lithic-rich sandstone containing quartz, chert, and sedimentary-lithic clasts, volcanic-lithic-rich sandstone, and crystal-rich tuffaceous sand- stone. These three compositions reflect a composite source that contained sedimentary and low-grade metasedimentary rocks, volcanic rocks, and penecontemporaneous volcanic debris. U-Pb isotopic data from detrital zircon indicate an ultimate Precambrian continental source for components of the Bragdon Formation and corroborate the previous suggestion that lower Paleozoic rocks of the Shoo Fly Complex, Yreka terrane, or related rocks may have been the immediate (last-cycle) sources. In modern convergent margins, a complex and ongoing relationship between ensimatic arc volcanism and continental sediment sources may be expected from the pattern of oceanic subduction zones that generally dip toward continents and intervening marginal basin systems, providing a context for understanding the presence of continent-derived detritus in an ancient ensimatic arc.

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