Abstract

Measurements of Nd-Sr isotopes, major and trace elements, and model mineralogy were made on Upper Jurassic and Cretaceous Great Valley forearc sedimentary rocks to test models for the temporal and spatial evolution of Sierra Nevada arc sources. Isotopes and major and trace elements are sensitive provenance indicators because of the large west-east isotopic, geochemical, and age gradients in the plutonic rocks of the Sierra Nevada batholith, and because petrographic models indicate that source areas moved east during the Cretaceous.

Isotopic and chemical variations are correlated in the forearc sandstone; as ϵNd decreases, Th, U, La, Nb, Zr, Hf, Pb, Rb, SiO2, and K2O concentrations increase, and FeO, MgO, TiO2, Ni, and Cr concentrations decrease. This relation is the same as that observed in the plutonic rocks and indicates that the arc was the primary source of sediment and that the sandstone chemistry was not disturbed by sedimentary processes. The ϵNdSr relation of San Joaquin Valley sandstone is the same as the plutonic rocks, but Sacramento Valley sandstone is elevated in ϵSr because of seawater exchange, weathering, and diagenesis. Whole-rock sandstone decreases in ϵNd from +7 to -5 and increases in 87Sr/86Sr from 0.7045 to 0.7073 with decreasing stratigraphic age. The Nd-Sr isotopic composition is correlative with the plagioclase to feldspar ratio and indicates that source areas moved inland during the Cretaceous. Upper Cretaceous San Joaquin Valley shale is similar in ϵNd to the sandstone, indicating that sandstone and shale were derived from the same source and that the Nd isotopic composition is independent of grain size. The shale is higher in 87Sr/86Sr than the sandstone, possibly due to concentration of biotite in the fine fraction during transport and subsequent Rb loss during diagenesis.

Nd-Sr isotopes were used to construct models to locate source areas. Parameters include lithology, drainage basin geometry, and erosion rate. The age and isotopic compositions of the calculated igneous component of the sandstone correspond to the age and isotopic compositions of the plutonic rocks of the batholith; this correspondence indicates that (1) the isotopic composition of the plutonic rocks and the coeval volcanic cover were similar, (2) the volcanic front was denuded within a few million years, and (3) the sediment was derived from the head of the drainage basin, located at the migrating volcanic front.

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