Synextensional magmatism in the Basin and Range Province; A case study from the eastern Great Basin

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Abstract
An integrated structural, stratigraphic, geochronological, and geochemical investigation of Cenozoic volcanic and sedimentary rocks within a highly extended part of the eastern Great Basin sheds light on the interplay between magmatism and extensional tectonism. Tertiary rocks in east-central Nevada and west-central Utah can be divided into three broad groups: (1) 40 to 35 Ma, locally derived sequences of andesite and rhyolite lava flows and ash-flow tuffs; (2) the voluminous 35 Ma Kalamazoo volcanic rocks, including the compositionally zoned (rhyolite to dacite) Kalamazoo Tuff, crystal-rich hornblende dacite lavas, and the K-rich dacite tuff of North Creek and associated lavas; and (3) 35 to 20(?) Ma, predominantly sedimentary sequences. Crosscutting relations between faults and subvolcanic intrusions, decreasing tilts upward within the Tertiary sections, and sedimentologic evidence for rapid unroofing of deep structural levels demonstrate that rapid, large-magnitude extension in this region began at least 36 Ma during some of the earliest eruptions, was ongoing at 35 Ma during the culminating eruptions of Kalamazoo volcanic rocks, and continued after volcanism had largely ceased. These synextensional volcanic rocks constitute a high-K calc-alkaline andesite to rhyolite series, and closely resemble suites from the central Andes rather than the bimodal or alkalic suites commonly associated with continental rifts. Trace-element systematics and reconnaissance Sr and Nd isotopic data suggest that the suite formed by extensive contamination of mantle-derived basalt by crustal partial melts in the deep crust, followed by relatively minor wall-rock assimilation during fractionation from andesite to rhyolite, presumably at shallower levels. Modeling of the isotopic data suggests that the most voluminous rock type, hornblende dacite, consists of 30 to 50 percent mantle material. Thus, intrusions associated with Cenozoic volcanic rocks represent a significant addition of new mantle-derived material to the continental crust. A comparison of the eastern Great Basin with other highly extended parts of the Basin and Range province reveals striking similarities in eruptive and extensional histories, despite important regional variations in absolute timing. These similarities are best explained by an active rifting model that invokes a flux of basaltic magma into the crust, hybridization and mixing of these magmas with crustal melts to produce intermediate magmas that differentiate in shallower magma reservoirs, and magmatically induced thermal weakening of the crust culminating in brittle failure of the upper crust and ductile flow at depth. This model helps explain (1) the close spatial and temporal association between the onset of large-magnitude extension and voluminous volcanism throughout the province; (2) the general decrease in extensional strain rates through time; (3) the typical progression of magma compositions from early, intermediate to silicic rocks to late, relatively primitive basaltic or bimodal suites; and (4) the uniform crustal thickness and the reflective mafic lower crust of the Basin and Range province.

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