BASEMENT INFLUENCES ON CENOZOIC ANCESTRAL CASCADES ARC VOLCANIC ROCK ISOTOPIC COMPOSITIONS IN NORTHEASTERN NEVADA, USA

Cenozoic arc magmatism migrated southwestward across northern Nevada towards eastern California from ca. 45 Ma to 3Ma as a result of rollback of the subducting Farallon slab, forming the ancestral Cascades Volcanic Arc. Whereas the modern Cascades Arc includes mafic volcanic rocks that originated primarily in the mantle wedge (87Sr/86Sr<0.7045), mafic to intermediate volcanic rocks of the Ancestral Arc exhibit a pronounced gradient in isotopic compositions from west to east (87Sr/86Sr=0.704-0.713). We infer that the composition of mafic (and evolved) magmas are a function of basement age, from Phanerozoic in the west to Precambrian in the east, and that mafic magmas are derived primarily from metasomatized lithospheric mantle rather than the asthenosphere/mantle wedge. Geological and Nd isotopic studies indicate that the northeastern corner of Nevada is underlain by Archean Wyoming Province basement. eNd (-16 to -21) and eHf (-21 to -32) from Eocene intermediate and felsic volcanic rocks in northeast Nevada (Pequop Mountains, East Humboldt Range, Tuscarora, Jerritt Canyon) are lower at a given 87Sr/86Sr, and have dramatically different Pb isotopic compositions (6/4 < 18.3, 7/4 < 15.7), compared to similar-age volcanic rocks emplaced south of the proposed Archean/Proterozoic basement boundary (eNd= -10 to -19; eHf= -13 to -25; 6/4 > 18.5, 7/4 > 15.7) . Cenozoic plutons from the northeast corner of Nevada also have anomalously low Pb isotope ratios (“lower array”, Wright & Wooden 1991). The low Pb isotope ratios require that northeast Nevada magma sources are Early Proterozoic or older; the best-fit line through our volcanic rock data yields a 207Pb/206Pb isochron age of ~2.3 Ga. Alternatively, this line could indicate mixed contributions from both Archean and Proterozoic sources. Young lamproites from the Leucite Hills, proposed to be derived from the lithospheric mantle of the Wyoming Province, have similarly low Pb isotopic ratios, as do entrained crustal xenoliths (Mirnejad & Bell, 2006, 2008). The isotopic data from Cenozoic arc volcanic rocks in northeast Nevada are consistent with previous results to the west: all volcanic rocks have trace element characteristics of subduction-related magmas, but radiogenic isotopes indicate a primary contribution from the underlying lithospheric mantle and crust.