A Geochemical Study of Cenozoic Magmatism Along an East-West Transect from Central Great Basin, Nevada to the Ancestral Cascade Arc, California – A Compositional Journey Over Space and Time

Abstract

Processes linked to shallow subduction, slab rollback and extension are recorded in the whole-rock major, trace element and Sr, Nd, Pb and O isotopic compositions of magmatic rocks in both time and space. Over 250 Eocene to Mio-Pliocene volcanic rocks were sampled along a transect across Central Great Basin (GB) in Nevada to the Ancestral Cascade Arc (ACA) in northern Sierra Nevada, California (ca. 39 to 40 o latitude), which are interpreted to represent the southwestward migration of magmatism that occurred as a result of the shallow subduction and eventual rollback of the Farallon plate. Volcanic rocks from the study area are chemically and chronologically zoned into four regions: central GB (5 to 35 Ma), western GB (5 to 14 Ma), eastern and western ACA (2.5 to 16 Ma). The volcanic products are dominantly calc-alkalic but transitional to alkalic in the eastern part of the study area. Intermediate lavas are the dominant lava-type, which evolved from the mafic lavas by fractional crystallization and variable degrees of assimilation with lower crustal melts. Central GB lavas have shoshonitic affinities with higher K, P, Ti, La/Sm as well as lower (Sr/P)pmn and Ba/Nb. Western GB lavas are chemically uniform with low K, La/Sm and Rb/K. Compared to GB lavas, ACA lavas were largely modified by subduction processes with higher La/Sm, Ba/Nb, and Th/Rb values. Pb-Pb isotope plots for ACA mafic to intermediate lavas show linear trends to higher Pb-isotopic values, which combined with lower Ce/Ce* and high Th/Nb ratios are interpreted to come from slab sediment. Pb-isotope ratios for GB lavas increase with increasing silica, indicating assimilation of continental crust. Mafic and intermediate lavas from both the GB and ACA have overlapping 87Sr/86Sr and 143Nd/144Nd values that are consistent with primary melts mixing with a subduction-modified lithospheric mantle source. Percent melting of the mantle is enhanced by fluid flux from the dehydrating subducting slab, which is lowest for GB lavas due to low P-T conditions from a shallow slab dip, and highest for western ACA lavas, generated from higher slab dehydration and P-T conditions due to greater slab dip.