Abstract

The provenance of fore-arc sandstones of the Upper Cretaceous Great Valley Group was investigated by using measurements of Sm-Nd and Rb-Sr isotope ratios and major and trace element concentrations. The objective was to elucidate the erosional history of the adjacent Sierra Nevada arc and its relation to magmatism and fore-arc sedimentation. Relatively detailed provenance information is obtainable because the currently exposed plutonic rocks of the arc (and, by inference, the Cretaceous volcanic rocks) have large geographic gradients in isotopic composition, mainly from west to east across the arc. The relation of ϵNd to ϵSr and to major and trace elements in Great Valley sandstones is the same as in the plutonic rocks of the Sierra Nevada; this observation confirms that the arc was the primary source of sediment and that the chemistry of the detritus was not significantly disturbed by sedimentary processes. With decreasing ϵNd, there is an increase in K2O, Rb, Pb, Ba, La, and U, and MgO, Ni, and Cr decrease. The ϵNd of sandstones decreases with decreasing depositional age from -0.7 to -5.0 in 97 to 73 Ma strata. The ϵNd and age of the sandstones closely match those of the plutonic rocks of the Sierra Nevada. This observation indicates that the location of the mean sandstone source and the active volcanic front were nearly coincident in time and space; thus, the isotopic compositions of the sandstones directly record the temporal and geographic variations in arc magmatism. The volcanic "roof" of the Sierra Nevada arc was evidently chemically and isotopically equivalent to its plutonic roots. Prearc metasedimentary rocks did not significantly contribute to the isotopic or geochemical composition of the fore-arc sandstones.

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