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GEOCHRONOLOGIC AND ISOTOPIC INVESTIGATION OF THE KOIPATO FORMATION, NORTHWESTERN GREAT BASIN, NEVADA: IMPLICATIONS FOR LATE PERMIAN-EARLY TRIASSIC TECTONICS ALONG THE WESTERN U.S. CORDILLERA

The Koipato Formation unconformably overlies the Golconda Allochthon, and this relationship has been used to define the timing of the Sonoma Orogeny. New lithologic and high-precision CA-TIMS U-Pb zircon geochronology from the Koipato Formation in the Humboldt, East, and Tobin Ranges allows for a more detailed understanding of the Koipato Formation’s stratigraphic architecture and its importance for the Early Mesozoic tectonic setting of the U.S. Cordillera. New U-Pb geochronology reveals that Koipato Formation units were deposited predominately in the Early Triassic (Olenekian) and that the majority of Koipato-type volcanism lasted only ~1.2 Ma. The existence of ~254 Ma inherited zircons within the leucogranite of the Humboldt Range has been inferred to represent the earliest stages of Limerick Greenstone-type Koipato volcanism, which extends the age of Koipato Formation volcanism to the latest Permian. The Koipato Formation also records the transition from intermediate to felsic volcanism and a short-term hiatus in volcanism. Volcanism within the Koipato Formation most likely lasted until just after deposition of the youngest Weaver Rhyolite sample from this study (248.32 Ma). U-Pb geochronology was performed on the felsic units (volcanic and intrusive) and shows that the volcanic Rochester and lower Weaver Rhyolites are coeval with the intrusive units. Two phases of