The Cenozoic Igneous History of the Great Basin, southwestern USA, As Recorded in the Fish Creek Mountains of northern Nevada
The Fish Creek Mountains, located in north-central Nevada, is a site of multiple igneous events ranging from ca. 35 Ma to 1 Ma, covering most of the igneous history of the Great Basin. The oldest lavas are ca. 35-33 Ma basaltic andesites through dacites that are exposed in the western Shoshone Range, the eastern Tobin Range, and the northern and eastern Fish Creek Mountains. Eocene mafic rocks of the FCM and Tobin Range are typical of lavas erupted during the southwesterward magmatic sweep across Nevada during the Cenozoic. The Fish Creek Mountains are located close to the 0.706 and Pb lines that separate Precambrian from transitional to oceanic lithosphere in central Nevada, and have a range of isotopic compositions that reflect variations in lithospheric mantle sources and crustal assimilants in evolved lavas. The southern Fish Creek Mountains is composed of the 24.9 Ma Fish Creek Mountains rhyolitic tuff that is largely confined to an undeformed caldera structure. The caldera and tuff are anomalously young compared to nearby felsic caldera centers such as the Cove Mine (34.4 Ma) and Caetano (34.0 Ma) calderas and relative to the southwestward magmatic migration. With time, the sources for ignimbrite/caldera complexes have lower 87Sr/86Sr that may reflect melting of younger or less evolved crustal rocks or different mantle input, or both. The isotopic trend toward more upper mantle-like compositions in volcanic rocks of this region continues into the Miocene. Adjacent to the Fish Creek Mountains are exposures of ca. 15 Ma Northern Nevada Rift flood basalts that are associated with the Yellowstone plume of the Columbia River flood basalts. Northern Nevada Rift lavas have incompatible element patterns and isotope ratios similar to Grande Ronde basalts. North and west of the Fish Creek Mountains are exposures of Miocene to Quaternary mafic lava flows termed high-alumina olivine tholeiites (HAOT). One 5.4 Ma HAOT is found only 50km NW of the Fish Creek Mountains, and this lava sequence is MORB-like in character but with elevated Sr isotope ratios. Along the northwestern margin of the Fish Creek Mountains and in the center of the caldera complex are exposed late Pliocene to Quaternary alkalic lava flows and cinder cones of the Buffalo Valley volcanic field. Buffalo Valley lavas are melts of the asthenosphere and are intraplate volcanic rocks, like lavas of the Lunar Crater field to the south. The exceptional exposure of volcanic rocks in this area allows for a reconstruction of magma sources through time, which reflect the interplay of lithospheric extension and magma generation in the mantle (asthenosphere and lithosphere) and the crust.