U-PB ZIRCON GEOCHRONOLOGY OF THE EARLY TRIASSIC KOIPATO FORMATION: IMPLICATIONS FOR THE PERMO-TRIASSIC SONOMA OROGENY

The Early Triassic Koipato Formation of central Nevada is a volcanic and volcaniclastic unit that was deposited on the highly deformed Golconda Allochthon and has traditionally been interpreted to reflect initial continental magmatism following the end of the Sonoma Orogeny. The Koipato is best observed in the Humboldt Range of central Nevada where three distinct units are described: the lower unit (Limerick Greenstone) is a complex of intermediate hypabyssal intrusives and volcaniclastics, while the upper two units (Rochester and Weaver Rhyolites) consist of felsic flows, tuffs, and sedimentary units. Currently, most of the age assignments for the Koipato are based on sparse fossil evidence from sedimentary units in the upper Weaver Rhyolite and old Pb-α radiometric data from the upper sections of the Rochester Rhyolite and its coeval intrusives, which collectively give an age of Early Triassic for the deposition of these upper sections. The lowermost unit of the Koipato has yet to be dated using either technique and could be significantly older than the currently accepted age of Early Triassic for the felsic units. New U-Pb zircon geochronological data from volcanic rocks of all three units of the Koipato suggest that this formation was deposited over a million year time span in the Early Triassic and that the arc system evolved from intermediate to felsic in less than half a million years. The preliminary ages are: Limerick Greenstone (249.594 ± 0.081 to 249.371 ± 0.096 ma), Rochester Rhyolite (249.13 ± 0.27 to 248.912 ± 0.086 ma), and Weaver Rhyolite (248.649 ± 0.086 to 248.358 ± 0.098 ma). Also, U-Pb zircon geochronological data from Hoffman Canyon, the type locality of the Sonoma Orogeny in the Tobin Range, yields a preliminary age of 248.92 ± 0.24 ma, which shows that Koipato deposition was occurring in this locality coevally with Rochester Rhyolite deposition in the Humboldt Range and also in the East Range. Furthermore, to date only felsic Koipato volcanism has been documented outside of the Humboldt Range and this fact combined with the U-Pb zircon geochronological data may be evidence of the development of a foredeep basin, to the east of the approaching tectonic wedge, which served as the primary deposition center for the early Koipato volcanics that may have been erupting syntectonically with the Sonoma Orogeny.