GEOCHEMISTRY AND FACIES OF THE EASTERN SIERRA NEVADA AS ANALOGUES FOR MODERN CONTINENTAL ARC SETTINGS

The east-central Sierra Nevada in California have a complex tectonic history with regard to Mesozoic magmatic events. Subduction initiation in the early Mesozoic led to magmatic flare-ups in the Sierran arc, with varying degrees of mafic involvement. The Triassic flare-up was particularly impacted by mafic magma, and is thought to be exposed in roof pendants in the east-central Sierra Nevada. The Koip sequence in the Saddlebag Lake Pendant records evidence of early Mesozoic magmatism, in the form of silicic tuffs and andesitic breccias. The breccias of Frog Lakes are an assemblage of volcanic domes containing clasts ranging from basalt to andesite. The breccias are stratigraphically separated by the 220 Ma tuff of Greenstone Lake. However, facies observations, textural analysis, whole-rock, and trace element geochemistry of clasts taken from andesite domes indicate that they are coeval, cogenetic, and formed subaqueously. Facies were categorized in terms of matrix differences, clast shape, size, lithology, and whether they were mono or polylithologic. The upper and lower breccias are both polylithologic, with similar mineral assemblages, clast sizes, and clast shapes. The breccias are also geochemically similar to modern continental magmatic arc systems. Whole-rock and trace element geochemistry of breccia clasts show distinct characteristics of subduction initiation and indicate that domes in Frog Lakes were emplaced in a continental magmatic arc setting. The data are also analogous to modern continental arcs settings around the world.