GEOLOGIC MAPPING, GEOCHEMISTRY, AND DETRITAL ZIRCON GEOCHRONOLOGY IN HUNTINGTON VALLEY AND THE EASTERN PIÑON RANGE, NORTHEAST NEVADA: IMPLICATIONS FOR THE PALEOGEOGRAPHIC EVOLUTION OF THE ELKO BASIN AND SURROUNDINGS

The Ruby Mountains–East Humboldt Range (RMEH) is a classic example of a metamorphic core complex, which exposes deep crustal levels providing insight into the complex Tertiary and Mesozoic magmatic and deformational history of the Cordillera. The extensional history of the RMEH has been controversial for decades but comparatively little study has focused on the surrounding supracrustal rocks and sedimentary basins, which record an impressively complete history of Cenozoic sedimentation and volcanism, thus revealing insight into the paleogeographic and paleotopographic evolution of what was once the cover sequence for the RMEH.

Depositional rates in the Elko Basin were relatively minor from Cretaceous to Oligocene time, and changed to rapid sedimentation in the Middle Miocene. The Eocene Elko Formation is only ~125 m thick (maximum ~850 m thick regionally; Haynes, 2003) and sediments within the Eocene–Oligocene Indian Well Formation are significantly less voluminous than previously reported. Detrital zircon signatures recording erosion of the metamorphic and igneous rocks of the RMEH arrive in the Middle Miocene, suggesting that the RMEH was not exposed to erosion prior to that time. A small number of Jurassic zircons in the Elko Formation are likely derived from plutons in the nearby Cortez Range to the west, and a population of ~46 Ma zircons may represent air fall from the Challis volcanic field in Idaho.

The ~40–31 Ma calcic to calc-alkalic Robinson volcanic field records early peraluminous to weakly metaluminous “ignimbrite flare-up” volcanism of basaltic andesite to trachydacite and rhyolite composition. Minor ~E–W-striking faults are observed during or immediately following eruption of 38–37 Ma volcanic rocks. A gentle (10–15º) angular unconformity is mapped between ~37 Ma volcanics and a ~31.5 Ma ignimbrite, the Tuff of Hackwood. Between ~31.5–16 Ma another unconformity is developed at the base of the Miocene section that is not well constrained from the mapping but could be as little as 5º or as much as 45º. An east-dipping, range-bounding normal fault system between Huntington Valley and the Piñon Range to the west is inferred to have slipped after 31 Ma and may be synchronous with deposition of the Miocene Humboldt Formation, as is the major W-dipping fault that bounds the RMEH (Colgan et al., 2010).