Petrogenesis of the Fine Gold intrusive suite, Sierra Nevada Batholith, California

Abstract

The Fine Gold Intrusive Suite FGIS, located at the western edge of the Sierra Nevada batholith north of Fresno, California, is one of a number of Late Cretaceous intrusive complexes that become progressively younger to the east. Systematic west-to-east compositional variations across the batholith include an increase in K sub 2 O and Rb abundances and initial super 87 Sr/ super 86 SrSr sub i values and a decrease in initial super 143 Nd/ super 144 Nd values. Several geochemical and tectonic boundaries have been proposed to traverse the FGIS including the Sr sub i = 0.706 isopleth, the boundary between Panthalassan and North American lithosphere, and the boundary between terranes having contrasting magnetic susceptibility intensities. The present study was developed in order to assess these proposed boundaries and characterize source magmas of this westernmost Cretaceous intrusive suite. The petrography, major-oxide and trace-element abundances, and isotopic parameters were studied for samples from each of the five major lithodemes which comprise the FGIS. These are: 1 the Tonalite of Ross Creek, a small, relatively mafic pluton; 2 the granodiorites of Arch rock; 3 the Ward Mountain Trondhjemite; and 4 the Knowles Granodiorite--all small, relatively felsic plutons; and 5 the extensive Bass Lake Tonalite, which varies from tonalite to granodiorite. All FGIS lithodemes belong to the calc-alkaline magma series and are depleted in K, Rb, Nb, and Ta. Rare-earth element REE patterns show that FGIS samples are moderately enriched in the light REE relative to chondrites La sub N = 16 to 110, exhibit moderate light REE fractionation La sub N /Lu sub N = 2 to 37, and have a Eu-anomaly which is small or negligible. Isotopic data measured on a subset of six samples show the following ranges: Sr sub i = 0.7044 to 0.7061, initial super 143 Nd/ super 144 Nd = 0.51246 to 0.51270 epsilon sub Nd = +2.0 to -3.6, delta super 18 O = 9.9 to 12.1 per mil measured on plagioclase separates, super 206 Pb/ super 204 Pb = 18.88 to 19.28, super 207 Pb/ super 204 Pb = 15.62 to 15.68, and super 208 Pb/ super 204 Pb = 38.81 to 39.34. Data from this study and from the literature on the Sherman Thomas pluton, a 126-Ma subsurface trondhjemite located just to the west of the FGIS, suggest that the Cretaceous mantle in this area had an Sr sub i of 0.7032 and an initial super 143 Nd/ super 144 Nd of 0.51297 epsilon sub Nd = +9.2, only slightly more evolved than mantle sources of mid-ocean ridge basalts. Oxygen data delta super 18 O = 9.2 per mil indicate that the Sherman Thomas magma interacted with sedimentary material. Limited data on the Ward Mountain and Knowles bodies suggest that primitive mantle magmas interacted with crustal material to a limited degree, resulting in slightly raised Sr sub i and slightly lowered super 143 Nd/ super 144 Nd values. More extensive data for the Bass Lake Tonalite available from this study and the literature suggest that this mapped body may be a composite complex consisting of four distinct members. Rocks located south and west of the Coarsegold roof pendant originated from mantle melts that interacted with crustal rocks which resulted in a little oxidized, homogeneous magma with Sr sub i of 0.7045 and initial super 143 Nd/ super 144 Nd of 0.51270 epsilon sub Nd = +1.1 that then evolved by fractional crystallization with little further crustal interaction. Rocks located just to the north and east of the pendant underwent the same sequence, but interaction with two different types of crustal rocks yielded two distinct initial magmas; one was little oxidized with Sr sub i of 0.7057 and initial super 143 Nd/ super 144 Nd of 0.51253 epsilon sub Nd = +1.3 and another one was highly oxidized with Sr sub i of 0.7061 and initial super 143 Nd/ super 144 Nd of 0.51246 epsilon sub Nd = -2.1. Both of these magmas then evolved by fractional crystallization with little further crustal interaction. A fourth variety of magma may comprise Bas

25 samples from this reference