THE STONE CABIN FORMATION, NEVADA, USA: GEOCHEMICAL AND ISOTOPIC INDICATORS OF MAGMA SOURCES AND CRUSTAL INFLUENCE IN AN EOCENE CALDERA SYSTEM

Stone Cabin Formation (SCF) ignimbrites and related Eocene volcanic rocks are located in eastern Nevada within the Central Nevada Volcanic Field. The SCF is a part of the 37-19 Ma, intermediate to silicic Ignimbrite Flareup that resulted from rollback of the subducting Farallon slab. This project aims to determine the magma source(s) of the SCF and determine the existence/extent of crustal contamination during petrogenesis using whole-rock, major-trace element and Pb-Sr-Nd-Hf isotopic analysis. On a broader scale, these Eocene igneous rocks are important in a larger study of the isotopic transition zone that runs through Nevada between eastern Precambrian basement terranes and western Phanerozoic accreted mafic terranes. The source of SCF ignimbrites has recently been determined to be the White River caldera southwest of Ely, NV. Seventy-one samples of Eocene volcanic and plutonic rocks were collected from intracaldera and extracaldera exposures. ⁴⁰Ar/³⁹Ar dates from SCF tuffs and post-collapse rhyolite to dacite lavas are indistinguishable at ~35.8 Ma. The SCF area igneous rocks range from subalkaline basaltic andesite to high-Si rhyolite. The samples show normalized incompatible element patterns similar to rhyolite ignimbrites from other caldera complexes of the Ignimbrite Flareup. Initial isotope ratios in SCF and nearby Eocene igneous rocks are: ⁸⁷Sr/⁸⁶Sr = 0.708863 to 0.714040, ¹⁴³Nd/¹⁴⁴Nd = 0.511698 to 0.512133 (εNd = -8.96 to -17.44), ¹⁷⁶Hf/¹⁷⁷Hf = 0.282057 to 0.282449 (εHf = -11.18 to -24.94), and ²⁰⁸Pb/²⁰⁴Pb = 40.05 to 38.84. The basaltic andesite lavas have the highest Nd and Hf and the lowest Sr and Pb isotope ratios. The primary magma source appears to be Precambrian mantle as mafic lava ⁸⁷Sr/⁸⁶Sr values are well over 0.708 with a minimum value of 0.708863. The evolved magmas show strong correlations between both Th/La and isotopic ratios with increasing SiO₂ content, indicating that crustal contamination is a major process in magma evolution. The crustal component in the felsic lavas has highly negative εNd and εHf values that are consistent with a Precambrian crustal source. Other Eocene volcanic rocks in the region underlain by Precambrian basement (e.g., Butte, N. Schell Ck. Ranges) have more negative εNd values indicating that a more ancient/enriched source exists north and east of the SCT caldera.