GEOLOGIC MAPPING OF THE SAHWAVE INTRUSIVE SUITE

The Sahwave and Nightingale Ranges, covering parts of eight 7.5’ quadrangles about 80 km northeast of Reno, Nevada, have been mapped at 1:24,000 scale, revealing a large (~1000 km²), compositionally zoned intrusion, informally named the Sahwave Intrusive Suite. This intrudes a combination of pre-existing Cretaceous granitoids and Triassic-Jurassic deep-water metasedimentary rocks of the Auld Lang Syne group. The intrusive suite varies in composition from the K-feldspar megacrystic and relatively felsic Sahwave Granodiorite and Granodiorite of Bob Spring in the central Sahwave Range to the more mafic Granodiorite of Juniper Pass, an equigranular biotite hornblende granodiorite that surrounds the more felsic central units on the north, south and west. Zircon U-Pb SHRIMP-determined ages span ca. 93-88 Ma, contemporaneous with the last, voluminous pulse of Mesozoic arc magmatism in the Sierra Nevada, as represented by the similarly large and compositionally zoned intrusive suites of the Cathedral Range intrusive event along the Sierra Nevada crest. Subsequent to this magmatic burst, erosion dominated in this part of the arc, stripping away the upper 3-7 kilometers of crust until mostly volcanic strata began to accumulate in the mid-Tertiary. Miocene to present normal faulting associated with Basin and Range extension raised the Sahwave and Nightingale ranges as a warped and split horst block surrounded by sedimentary basins while leaving the Sierra Nevada intact. In concert with geochronology, petrology, and geochemistry, this new mapping demonstrates the continuity of the Mesozoic Cordilleran batholith across NW Nevada despite the obscuring effects of Cenozoic cover and extension.