Applications of lead and strontium isotopic relationships to the petrogenesis of granitoid rocks, central Sierra Nevada batholith, California

Abstract

Pb isotope data are given on 80 whole-rock and/or feldspar samples from 51 dated samples covering a traverse across the central Sierra Nevada batholith, largely between latitudes 36° and 37°N. In addition, Sr isotope data were measured on 30 whole-rock samples. No correlation is observed between age and isotope ratios. In agreement with previously published data, 87 Sr/ 86 Sr ratios are low (∼0.704) along the western margin and increase to high values (>0.707) at the eastern margin. 206 Pb/ 204 Pb ratios increase from ∼18.6 at the western margin to ∼19.5 in the middle of the batholith, then decrease regularly to ∼18.5 toward the eastern margin. 2O8 Pb/ 204 Pb ratios mirror the 206 Pb/ 204 Pb data, but do not exhibit the decrease in values toward the eastern margin. Pb and Sr isotopic data from the western-margin plutons plot within or close to the field of primitive island-arc magmas, indicating predominantly mantle sources. The high 206 Pb/ 204 Pb ratios in the central batholith suggest assimilation of sediments derived from Precambrian basement rocks. The decrease in 206 Pb/ 204 Pb ratios in the eastern segment of the batholith without corresponding reduction in 208 Pb/ 204 Pb ratios is attributed to assimilation of lower crustal granulitic/amphibolitic rocks by mafic magmas. Initial Pb ratios for samples with 87 Sr/ 86 Sr > 0.706 from the central and eastern segments of the batholith indicate components from recycled crustal rocks with a mean 207 Pb/ 206 Pb age of ca. 1.8 Ga. Nd and O isotopic data from the literature are consistent with the Pb-Sr model obtained in this study. Very similar isotope patterns to the present model are reported across the Salinian block.