CORRELATED O-SR-PB ISOTOPE TRENDS IN GRANITOIDS FROM THE WEST-CENTRAL SIERRA NEVADA, CA

Zircon (Zrc) d¹⁸O values from Early Cretaceous granitoids west of Sequoia National Park (~ 36.5 N) display a clear trend of increasing d¹⁸O from west to east (5.8 to 8.0 per mil) over the western 40 km of the batholith (projected onto a SW to NE transect). Whole rock (WR) d¹⁸O also increases from west to east (6.7 to 10.8 per mil) and is more variable. Oxygen isotope data are well correlated with Sr _i and Pb isotope variations (Chen & Tilton, GSA Bulletin, 1991). A positive correlation of d¹⁸O with Sr _i mimics the linear trend observed in the Peninsular Ranges Batholith. It is interpreted to be due to contamination by the Kings sequence (continental metasediments), in agreement with the conclusions of Kistler (GSA Memoir 174, 1990). O-Sr data also indicate that the rocks studied have the same depleted-mantle source as ring dike complexes in the Stokes Mountain area (Clemens-Knott, unpub. Ph.D., 1992), at the western end of the transect. We note different O-Pb correlations in the Weakly Contaminated (WC) and Strongly Contaminated-Reduced (SCR) zones of Ague and Brimhall (GSA Bulletin, 1988): d¹⁸O(Zrc) increases significantly (5.9 to 7.0 per mil) in the WC zone (westernmost 20 km of transect) with little variation in ²⁰⁶Pb/²⁰⁴Pb (18.658-18.741); SCR zone plutons (remaining 20 km of transect) have a smaller range of d¹⁸O(Zrc) (6.8 to 7.4 per mil) and a significant increase and range in ²⁰⁶Pb/²⁰⁴Pb (18.929-19.254). The WC trend is best explained by assimilation of material with mantle-like Pb isotope compositions and elevated d¹⁸O, such as rocks of the Kings-Kaweah ophiolite belt. Thin crust in the WC region may also explain limited variance in Pb data. The O-Pb trend in SCR rocks is attributed to assimilation of the Kings sequence. The decrease in d¹⁸O 40 km from the western margin of the batholith occurs at the Giant Forest Pluton of the Sequoia Intrusive Suite (SIS), a zoned plutonic suite immediately west of the proposed North American-Panthalassan lithospheric break of Kistler (ibid.). The decrease in d¹⁸O at the SIS contrasts with Pb and Sr trends, which continue east to the lithospheric break (Chen and Tilton, ibid.). Values of d¹⁸O identify the SIS as chemically transitional between granitoids west and east of the lithospheric break beneath the central Sierra Nevada.