Igneous rocks in the Fish Creek Mountains and environs, Battle Mountain area, north-central Nevada: A microcosm of Cenozoic igneous activity in the northern Great Basin, Basin and Range Province, USA

Abstract

The Great Basin of the western United States, the northern component of the Basin and Range Province, is a region of Cenozoic lithospheric extension with multiple periods and types of igneous activity. The composition and volume of Cenozoic magmas reflect a complex interaction between mantle-derived magmas and highly diverse crust, where both mantle sources and magmatic processes were modulated by tectonic environment. The Fish Creek Mountains in north-central Nevada underwent multiple igneous events ranging from ca. 40 Ma to 1 Ma that span all of the complex Cenozoic tectono-magmatic episodes of the Great Basin. The Fish Creek Mountains, therefore, is an ideal location to evaluate the different sources and processes involved in magma generation. Many plutons were emplaced in the region between about 40 and 38 Ma, several of which host base and precious metal deposits. Between 36 and 33 Ma, lava fields and calderas of the 37–19 Ma Ignimbrite Flare-up were emplaced. Both these and the preceding plutons resulted from southwestward rollback of the Farallon plate beneath North America during by far the most voluminous phase of Cenozoic magmatism. The lavas range from rare basalt and basaltic andesite to andesite, dacite, and rhyolite, have continental arc-like incompatible element patterns, and high initial 87 Sr/ 86 Sr and low εNd that require a metasomatized lithospheric mantle source combined with minor crustal component. Ignimbrites of the 34.4 Ma Cove Mine (trachydacite to rhyolite) and 34.0 Ma Caetano calderas (rhyolite to high-silica rhyolite) are abundantly porphyritic, include hydrous phases, were largely derived from partial melts of crustal rocks, but likely include 20–30% of a mantle-derived component. Igneous activity ceased in the region as the rollback-arc migrated to the southwest, but at 24.9 Ma a new caldera formed in the southern Fish Creek Mountains that was filled by ignimbrites of the Fish Creek Mountains Tuff. Intracaldera rhyolite ignimbrites range from aphyric, pumice-rich deposits at the base to progressively more quartz-feldspar phyric ignimbrites at higher levels; all flow units lack hydrous phases. No contemporaneous mafic or intermediate igneous activity accompanied caldera formation, but initial 87 Sr/ 86 Sr values in the Fish Creek Mountains tuffs are lower than in the Caetano Tuff, suggesting a greater mantle contribution to the 24.9 Ma ignimbrites. After another hiatus in igneous activity, the region was intruded and overlain by basalt to rhyolite dykes and lavas of the northern Nevada rift between 16.8 and 15.1 Ma. The primarily tholeiitic igneous suite is of the same age, chemistry, and isotopic composition as the Grande Ronde Formation of the Columbia River flood basalts, and evolved members (trachydacite and rhyolite) are crustally contaminated. The youngest northern Nevada rift lava is an alkali olivine basalt with isotopic affinity to basalts of the eastern Snake River Plain. After 10 Ma of quiescence, the region was locally covered by mafic lava flows with high-alumina olivine tholeiite compositions, represented by the 5.4 Ma Pumpernickel Valley flows. Their mid-ocean ridge-like incompatible element compositions indicate a depleted mantle source for the lavas, but radiogenic isotopic compositions indicate that the lavas of this region include a significant contribution from a mafic to ultramafic, high- 87 Sr/ 86 Sr source. The final igneous event in the Fish Creek Mountains region, the 4.0 to 1.0 Ma Buffalo Valley volcanic field, includes flows and spatter cones of transitional to alkalic basalt that are divided into two geochemical groups with identical isotopic compositions. They represent variable, low percent partial melts of the asthenosphere at different depths, yielding different rare earth element characteristics. Similar to the Lunar Crater volcanic field, the Buffalo Valley rocks represent a rare case where the lithosphere in the central Great Basin is now thin enough to allow melting of the underlying asthenosphere. Cenozoic magmatism in the northern Great Basin exhibits several transitions in magma sources and tectonic setting with time. Magmatism began as pre-extension, subduction-related, primarily lithospherically-derived magmas emplaced on/in tectonically-thickened crust. The onset of extension was partially driven by impingement of the Yellowstone plume that resulted in emplacement of rift-related volcanic and intrusive rocks in the northern Nevada rift, followed by the eruption of extension-related HAOT lavas along the northwest margin of the Great Basin. Finally, lithospheric thinning allowed for partial melting of the asthenosphere and eruption of alkaline basaltic lavas.