RADIOGENIC ISOTOPIC DATA FROM CENOZOIC VOLCANIC ROCKS IN THE EUREKA, ELY AND WELLS AREAS DOCUMENT SPATIALLY DISTINCT ARCHEAN AND PROTEROZOIC CRATONIC DOMAINS ACROSS NORTHERN NEVADA, USA

The Great Basin of northern Nevada hosts abundant Cenozoic igneous activity that has been the topic of research since the 1960s. A Mesozoic continental arc (Sierra Nevada) formed as a result of the subduction of the Farallon plate along the North American plate margin, which was terminated by flat slab subduction in the latest Cretaceous, but subsequent Cenozoic slab rollback caused renewed widespread magmatism. Much of this magmatism can be observed in Nevada and Utah where the southwest-trending arc rollback has produced calderas, plutons, lava complexes and abundant ignimbrites. Arc magmatism was emplaced on two drastically different cratons; a Precambrian craton to the east and Proterozoic accreted basement to the west. Geochemists have attempted to map this terrane boundary isotopically, exemplified by the contentious 0.706 ⁸⁷Sr/⁸⁶Sr line. To help evaluate isotopic boundaries, this study focuses on three localities in north -central Nevada, east of the 0.706 line. Using a multi-isotopic approach (Sr, Hf, Nd and Pb), we studied 35-45 ma volcanic flows in and around the Eureka, Ely and Wells areas. Major, trace element and petrographic data were obtained for a total of seventy-three samples while isotope ratios were collected from thirty-five of those samples. Volcanic rocks from the three sample areas have distinct isotopic compositions. Using Nd as an example, initial εNd for lavas from Eureka ranges from -9 to -7, in Ely from –19 to -13, and in Wells from –22 to -16. Ely and Eureka lavas overlap in ²⁰⁶Pb/²⁰⁴Pb (19 to 20), while samples from Wells have lower ²⁰⁶Pb/²⁰⁴Pb (17 to 18) but high ²⁰⁷Pb/²⁰⁴Pb, similar to other lavas proposed to be sourced from an Archean craton. A west to east gradient in isotopic composition is recorded in the volcanic rocks from Eureka to Ely, consistent with changing mantle and crustal sources. Samples from Eureka and the southern region of Ely show isotopic signatures consistent with Precambrian mantle and crustal sources, while the northern Ely and Wells samples show isotopic signatures consistent with an even older source, the Archean Wyoming Craton. Based on a parallel study of Mesozoic and Cenozoic igneous rocks of this region (Cousens et al., this meeting), we conclude that isotopic studies to define tectonic boundaries are best conducted on rocks of Late Cretaceous to mid-Cenozoic age.