Sahwave Batholith, NW Nevada: Cretaceous arc flare-up in a basinal terrane

Abstract

Detailed mapping and sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology centered around the Nightingale and Sahwave Ranges, ∼100 km northeast of Reno, Nevada, reveal that most of the Mesozoic basement in this area is composed of predominantly granodiorite-composition plutonic rocks intruded ca. 110–88.5 Ma. These rocks are similar in age, petrology, and composition to the mid-Cretaceous eastern part of the Sierra Nevada Batholith, and are likely related. The youngest plutonic rocks, ca. 93–88.5 Ma, form a large, compositionally zoned intrusive suite, referred to as the Sahwave intrusive suite. This suite is composed of a set of nested, inward-younging intrusions, varying between mafic, equigranular granodiorite around the periphery to more felsic, K-feldspar–megacrystic granodiorite in the center. The Sahwave intrusive suite is coeval with the Cathedral Range intrusive event along the crest of the Sierra Nevada, including the Tuolumne intrusive suite. The geochemistry and petrology of this intrusion also support similar magma genesis and emplacement. Intrusions of the Cathedral Range intrusive event in the Sierra Nevada were emplaced along the margin of North American continental crust, whereas the Sahwave intrusive suite was intruded into a thick package of basinal metasedimentary rocks that were likely underlain by transitional crust. More primitive initial 87Sr/86Sr and εNd values (ca. 0.7047 and –0.2, respectively) reflect this difference. In light of this likely fundamental difference in lower-crustal character, other factors, possibly related to subducted, water-rich material, must be responsible for creating similar melting conditions among the series of large intrusions that represent the last magmatic flare-up of the Cretaceous arc.