1 Drivers of Cordilleran Magmatic Processes and Geochemistry

Wednesday, 17 May 2023
8:00 AM - 12:00 PM
Whitney Peak Ballroom 2

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

Paper number: 1-10
Program: Theme Sessions
Time: 10:40 AM – 11:00 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 55, No. 4

doi: 10.1130/abs/2023CD-387225

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