THE RECORD OF EOCENE TO OLIGOCENE CONTINENTAL ARC VOLCANISM AND PLUTONISM IN THE FISH CREEK MOUNTAINS REGION, NORTHERN NEVADA

Northern Nevada and northeastern California include a phenomenal record of continental arc magmatism from ca. 40 Ma to 3 Ma that progressed southwesterly across Nevada into eastern California, presumably due to rollback of the subducting Farallon Plate. Andesite and dacite were the major components of this activity in northeastern Nevada between about 40 and 36 Ma. By 37 Ma, a northwest-trending belt of rhyolitic ash-flow calderas began to develop through central Nevada, termed the “ignimbrite flare-up”. The flare-up was accompanied by smaller-volume intermediate to felsic lava flows, with rare basalts. The Fish Creek Mountains, located south of Battle Mountain in north-central Nevada, includes a wide range of volcanic rock compositions erupted between ca. 35 Ma and 24.9 Ma that record the evolution of igneous systems during continental arc magmatism. The oldest lavas are basaltic andesites through dacites that are exposed in the western Shoshone Range, the eastern Tobin Range, and the northern and eastern Fish Creek Mountains. Plagioclase-rich andesites, dacite intrusions, and volcanic breccias occur in a belt along the western side of the Fish Creek Mountains. Basalts are concentrated in the central Fish Creek Mountains. Lavas from the northern Fish Creek Mountains are HFSE-depleted and LILE-enriched, typical of subduction-related volcanic rocks. The mafic volcanic rocks exhibit highly variable radiogenic isotopic compositions (εNd = -2 to -10) whereas intermediate to felsic rocks are more homogeneous (εNd = -4 to -6). The mafic rock isotopic compositions suggest a variably enriched, ancient lithospheric mantle source, and the evolved lavas indicate a crustal contribution. 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 centers such as the Caetano caldera (34.0 Ma) and Shoshone Range (40-35 Ma) and relative to the southwest magmatic migration, placing it in a back-arc setting. εNd values for the tuff range from 0 to -2, significantly more positive than the Eocene lavas and ignimbrites. Since crustal compositions do not change, this implies that the Oligocene magmatic system was driven by magmas from a more depleted mantle source.