BASALTIC CLASTS CONTAINED IN BRECCIA OF THE EASTERN RITTER RANGE PENDANT, SIERRA NEVADA, CALIFORNIA

The eastern Ritter Range pendant in the eastern Sierra Nevada of California contains Mesozoic metavolcanic rocks that range in age from 221 Ma to 167 Ma. This study analyzes clasts from the breccia of Rosalie Lake, exposed in the southeastern part of the pendant. Lava breccia and tuff breccia comprise a unit up to 40 m thick that extends at least 1.5 km along strike. The mafic tuff breccia contains ellipsoidal basaltic clasts up to 1 meter in length and is overlain by meter- to multi-meter-scale layers of mixed clast breccia, in turn overlain by the thick and widespread, Middle Jurassic rhyolitic tuff of Rosalie Lake. The most common breccia clasts have a preserved porphyritic texture with relict subhedral feldspar phenocrysts up to ~ 2.5 mm in long dimension. Geochemical analysis of freshest whole clasts from the tuff breccia shows only 51 to 55% SiO₂ and high alumina levels ranging from 19-22%; low Ca and Na and high K₂O levels ranging from 8 to 11% are likely due to hydrothermal alteration. High-alumina basalts from Okmok, Atka, and Moffett volcanoes of the Aleutian volcanic arc were compared to clast samples from this study; results showed similarities in SiO₂, TiO₂, Al₂O₃, and MgO contents. Primitive mantle-normalized trace element abundances show high Th/Nb and La/Nb, supporting the hypothesis that these clasts were derived from an arc-volcanic source. Comparing geochemistry of the basaltic clasts to those from older east Sierran arc breccias and Aleutian volcanos allows comparison of the environment of formation of the Rosalie Lake breccia clasts. Rosalie Lake breccia clasts are overall depleted in REE and enriched in Th, Hf and Zr compared to Triassic east Sierran basalt and andesite breccia clasts. Similarities in geochemistry and rock type suggest magma chamber processes comparable to the modern Aleutian volcano arc. The nearby tuff of Rosalie Lake has also been identified as having formed in a marine environment during the Jurassic period. The position of the samples of this study in relation to the tuff of Rosalie Lake suggests similar time period and environments of formation.