MAGMATISM IN THE EOCENE EMIGRANT PASS VOLCANIC FIELD, NORTHERN NEVADA: TRACE ELEMENT AND ISOTOPIC INDICATORS OF MAGMA SOURCES AND CRUSTAL INFLUENCES

The Emigrant Pass Volcanic Field is a part of the Eocene igneous activity associated with continental arc volcanism in northern Nevada, western United States. The aim of this project is to use whole-rock major element, trace element, and the isotopic compositions of lead, hafnium, strontium and neodymium to determine the magma source(s) of this volcanic suite and establish if crustal contamination is important in petrogenesis. The Emigrant Pass suite is also important in that it falls within the transition zone between Phanerozoic accreted terrane basement to the west and Precambrian basement to the west, and isotopic compositions will indicate which of the two basement terranes influences magma chemistry. The volcanic rocks sampled for this study are from four units: Primeaux Lava Sequence (38.3–38.1 Ma), Mack Creek Lava Sequence (38.4-37.1), the Bob Creek lava dome (37.7 Ma), and late porphyritic dykes (36.7-36.4 Ma). The Emigrant Pass igneous rocks range from basaltic andesite to rhyolite in composition. Major mineral phases include plagioclase, hornblende, quartz, biotite, and pyroxene. All of the lavas and intrusive rocks have normalized incompatible element patterns typical of subduction-related magmas, like other Eocene lava sequences in the northern Great Basin (e.g., Fish Creek Mountains). Initial isotope ratios are: ⁸⁷Sr/⁸⁶Sr = 0.705143 to 0.707914, ¹⁴³Nd/¹⁴⁴Nd = 0.512122 to 0.512420 (εNd = -3.3 to -9.2), ¹⁷⁶Hf/¹⁷⁷Hf = 0.282393 to 0.282610 (εHf = -5.4 to -13.1), and ²⁰⁸Pb/²⁰⁴Pb = 38.9401 to 39.2212. The basaltic andesites have the lowest Sr and Pb, and highest Nd and Hf isotope ratios, and the primary magma source appears to be Phanerozoic lithospheric mantle (⁸⁷Sr/⁸⁶Sr < 0.706). The evolved magmas show strong correlations between trace element (e.g., Th/La) and isotopic ratios with increasing SiO₂ content, and crustal contamination is a major process in magma evolution. The crustal component in the felsic lavas has highly negative εNd and εHf that may indicate a Precambrian crustal source. Based on this study and work at nearby Eocene volcanic centres, the Phanerozoic-Precambrian transition zone appears to include mantle and crustal components of varying age that are structurally interleaved.