Nd-Sr isotopic, geochemical, and petrographic stratigraphy and paleotectonic analysis: Mesozoic Great Valley forearc sedimentary rocks of California
- Lawrence Berkeley Lab., CA (United States)
- Univ. of California, Los Angeles, CA (United States)
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 {epsilon}{sub Nd} decreases, Th, U, La, Nb, Zr, Hf, Pb, Rb, SiO{sub 2}, and K{sub 2}O concentrations increase, and FeO, MgO, TiO{sub 2}, 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 {epsilon}{sub Nd}-{epsilon}{sub Sr} relation of San Joaquin Valley sandstone is the same as the plutonic rocks, but Sacramento Valley sandstone is elevated in {epsilon}{sub Sr} because of seawater exchange, weathering, and diagenesis. Whole-rock sandstone decreases in {epsilon}{sub Nd} from +7 to -5 and increases in {sup 87}Sr/{sup 86}Sr 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 {epsilon}{sub Nd} to the sandstone, indicating that sandstone and shale were derived from the source and that the Nd isotopic composition is independent of grain size. The shale is higher in {sup 87}Sr/{sup 86}Sr than the sandstone, possibly due to concentration of biotite in the fine fraction during transport and subsequent Rb loss during diagenesis. 75 refs., 13 figs., 4 tabs.
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 543101
- Journal Information:
- Geological Society of America, Bulletin, Journal Name: Geological Society of America, Bulletin Journal Issue: 10 Vol. 104; ISSN BUGMAF; ISSN 0016-7606
- Country of Publication:
- United States
- Language:
- English
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