Provenance and paleogeography of the Black Rock Terrane, northwestern Nevada: Implications of U-Pb detrital zircon geochronology

Abstract

U-Pb ages have been determined for 50 detrital zircon grains from Mississippian and Triassic strata of the Black Rock terrane, northwestern Nevada. The Devonian(?) to Mississippian Pass Creek unit has three broad age groups: 976-1132 Ma (n = 8), 1595-1927 Ma (n = 10), and 2504-2660 Ma (n = 3). The Triassic Bishop Canyon formation contains a dominant group of grains between 268 and 441 Ma (n = 11), a cluster of ages between 1868 and 1925 Ma (n = 5), and scattered ages between 1184 and 1813 Ma (n = 10) and between 2183 and 3183 Ma (n = 3). Most of the ages in these samples match well with the ages of grains present in basement provinces and off-shelf assemblages in the western United States. Grains in the Pass Creek unit were most likely recycled from lower Paleozoic strata of the Roberts Mountains allochthon and from strata exposed in the Salmon River arch region of Idaho, western Montana, and eastern Washington. These provenance links suggest that the Black Rock terrane was located along the northern Nevada-Idaho segment of the Cordilleran margin, near its current location, during late Paleozoic time. Because the upper Paleozoic stratigraphy of the Black Rock terrane is similar to that found in more outboard arc assemblages, including those of the Klamath Mountains and Sierra Nevada, this relation provides an indirect but important link between the U.S. continental margin and the more outboard arc assemblages. Zircon grains in the Upper Triassic Bishop Canyon formation were derived from a source containing both upper Paleozoic igneous rocks and clastic strata bearing 1.1-3.2 Ga detrital zircons. The most likely source for this combination of grains is Paleozoic basement rocks of Mesozoic arc assemblages in the eastern Klamath Mountains and Black Rock terrane. These provenance links provide evidence of uplift and erosion of arc basement in the western Cordillera during early Mesozoic time, and support interpretations that lower Mesozoic arc assemblages in this region of the Cordillera were isolated from the continental margin by a Triassic backarc basin.