AN UPPER CRETACEOUS COMPOSITIONALLY ZONED INTRUSION IN THE SHAWAVE AND NIGHTINGALE RANGES, PERSHING COUNTY, NEVADA

Preliminary geologic mapping and geochronology in two ranges 50 km northeast of Reno indicate the presence of a large, concentrically zoned intrusion comparable to the Tuolumne Intrusive Series and contemporaneous intrusions within the Sierra Nevada Batholith. Basin and Range normal faulting has warped the Shawave and Nightingale Ranges together into a broad synclinal horst, exposing plutonic rocks in both ranges and burying them in the adjacent valleys. The intrusive series varies in composition between two end-member units which are in gradational contact: a K-feldspar-phenocryst-bearing granite in the central Shawave Range and a biotite hornblende granodiorite that surrounds the granite to the south, west, and north (there is no exposure to the east). Zircons have been analyzed via SHRIMP, yielding ²⁰⁷Pb corrected ²⁰⁶Pb/²³⁸U weighted average ages with 2σ errors of 92.7 ± 1.4 Ma for the granodiorite, 88.5 ± 2.0 Ma for the central granite, and 91.2 ± 1.0 Ma for a granite in the Nightingale range that is compositionally intermediate between the two end members. This age range coincides with the first part of the Cathedral Range intrusive epoch (94 – 81 Ma) and reflects the same trend of younger, inner intrusions becoming progressively more felsic as seen in the Sierran intrusive series. Plutons to the west and northwest of the main intrusive series, dated by the same technique, yielded ages of 96.3 ± 0.8 Ma and 104.9 ± 0.8 Ma respectively, indicating that magmatism had been ongoing since mid-Cretaceous time (these two samples also contained inherited zircon populations, which were as old as 109.7 ± 0.8 Ma in the latter). The similarity of the late Cretaceous intrusive history of this eastward-bending part of the magmatic arc to the history preserved in the Sierra Nevada Batholith proper helps to establish the continuity of the arc across the northwestern Basin and Range.