The chronology of early Mesozoic arc magmatism in the Yerington district of western Nevada and its regional implications

摘要

The Yerington district, west-central Nevada, is underlain by arc volcanic, sedimentary, and plutonic rocks ranging in age from Middle(?) Triassic to Middle Jurassic. Twenty-three U-Pb radiometric dates from eight igneous units have been determined for zircon from both plutonic and volcanic rocks in the district, which establish two distinct periods of voluminous intermediate to silicic, high-K calc-alkaline magmatism. The earlier period is constrained by zircon dates of 232 and 233 Ma on a quartz porphyry and a metadiorite, respectively, which intrude and are possibly comagmatic with andesitic to rhyolitic volcanic rocks. The second period of arc magmatism is constrained by 5 concordant zircon dates that define an approximately 4-m.y. period (169-165 Ma) in the Middle Jurassic during which as much as 4 km of volcanic and volcaniclastic strata accumulated and two major batholiths were emplaced. Middle Jurassic magmatism began with eruption of the volcanics of Artesia Lake. They were intruded by the probably comagmatic 250-km2 Yerington batholith, which was differentiated, emplaced, and crystallized within approximately 1 m.y. (169.4-168.5 Ma). Granite porphyry dikes (168.5 Ma) are the latest phase of the Yerington batholith and are synchronous with porphyry and skarn copper mineralization. Disconformably overlying the Artesia volcanics is a second volcanic suite consisting of as much as 2 km of latitic volcanics of Fulstone Spring (166.5 Ma). The volcanics of Fulstone Spring were subsequently intruded by quartz monzodiorite porphyry dikes (165 Ma) and the Shamrock batholith (165.8 Ma). The Triassic through Lower Jurassic stratigraphic column was contact metamorphosed and folded adjacent to, and at least in part contemporaneously with emplacement of, the Yerington and Shamrock batholiths (169.4-165.8 Ma). We interpret the early, 232-233 Ma period of magmatism to represent a minimum age for the establishment of a west-facing Triassic volcanic arc within west- facing Triassic volcanic arc within west-central Nevada following the Permian Triassic Sonoma orogeny. The timing of Middle Jurassic arc magmatism and deformation in the Yerington district and surrounding regions is strikingly similar to the timing of magmatic and tectonic events documented in the Klamath Mountains region of northwestern California. We suggest that the similar record of arc magmatism and deformation in these two regions is not fortuitous but reflects a common tectonic evolution within a single, west-facing arc constructed along the western edge of North America.