MID-MIOCENE VOLCANIC FIELD DEVELOPMENT IN NORTHERN NEVADA: NEW AGE CONSTRAINTS ON THE TIMING OF SANTA ROSA-CALICO VOLCANISM

Miocene volcanism in the Pacific Northwest commenced at ~17-16 Ma with the inception of Steens-Columbia River flood basalt volcanism and dike emplacement along the northern Nevada rift. Concurrent with the initial phase of regional mafic magma input was the development of numerous volcanic systems characterized by large, caldera-forming eruptions (e.g., McDermitt). In contrast to the younger Yellowstone-Snake River Plain volcanic systems, these Oregon Plateau volcanic fields were active contemporaneously with regional lithospheric extension and flood basalt volcanism. Unlike other mid-Miocene Oregon Plateau volcanic fields, the Santa Rosa-Calico volcanic field (SC) of northern Nevada is characterized by a widespread component of intermediate composition eruptive products and seems to lack a history of large caldera-forming explosive volcanism. Additionally, the timing of SC volcanism is less well understood. New Ar-Ar ages (all relative to Fish Canyon Tuff at 27.55 Ma) from SC eruptive products suggest that SC volcanism commenced at ~16.4 Ma and was active for at least ~2 Ma. Basal mafic lavas from the western SC (Hinkey Summit area) and eastern SC (Calico Mountains) yield new ages of 16.45 ± 0.20 and 16.23 ± 0.20 Ma respectively, thus constraining the onset of SC volcanic field development. Furthermore, mafic intrusive bodies and vents from the western SC yield younger ages of 15.70 ± 0.10 and 14.05 ± 0.40 Ma, confirming that SC mafic input was contemporaneous with the entire >2 Ma duration of regional Steens flood basalt volcanism (Brueseke and Hart, 2001). Silicic lavas and porphyritic dikes from the western SC yield ages between ~15.9 and 16.1 Ma, which thus far represent the youngest documented phase of SC silicic activity in this part of the volcanic field. These new ages demonstrate that SC volcanism was occurring concurrently with the formation of other mid-Miocene Oregon Plateau volcanic fields and also confirm that local mafic magma input into the crust was an important catalyst for SC volcanic system development throughout the entire duration of SC volcanism. As a result, interactions between mantle- and crustal-derived materials occurred throughout the duration of SC volcanism, facilitating the generation and eruption of intermediate and silicic products.