ERUPTIVE HISTORY OF THE UNION PASS VOLCANIC CENTER, COLORADO RIVER EXTENSIONAL CORRIDOR, NORTHWEST ARIZONA

The Union Pass area lies on the flank of the Black Mountains accommodation zone and is dominated by east-tilted fault blocks. A thick early-to-mid-Miocene volcanic sequence suggests that it represents a volcanic center. The sequence is especially well exposed in a relatively coherent block bounded on the west by the east-dipping Arabian Mine Fault and on the east by the west-dipping Frisco Mine Fault. The section here is a >1 km thick (neither base nor top is exposed) and dips uniformly 15-25° ENE with no systematic variability. Exposed strata include, from oldest to youngest, (1) ~500 m of massive trachyandesites and trachydacites, some of which possibly represent domes; (2) a heterogeneous, ~150 m sequence that includes trachydacite and rhyodacite flows and relatively sparse felsic tuffs, with volcaniclastic sandstones near the top; and (3) at least 600 m of very thick, massive, high-silica rhyolite breccia and lava. A trachydacite plug intrudes the rhyolite. All units are characterized by biotite and plagioclase phenocrysts; other mafic phenocrysts are absent, and sanidine is limited to the most evolved rhyolites. SiO2 ranges from 57-78 wt%, and all units are potassic, with K2O >~4 wt % even in andesites. REE patterns are extremely uniform (enriched LREE ~250 x chondrite, flat HREE ~10 x chondrite, minimal Eu anomaly) except that the high-silica rhyolite is poorer in LREE and has a large negative Eu anomaly. The abrupt transition from monotonous intermediate volcanism to more variable volcanism with abundant felsic eruptives is typical of the region. The absence of apparent hiatuses within the section suggests rapid transition. The only direct evidence of mafic input is the presence of sparse mafic enclaves in trachyandesite. Geochemical similarities throughout the section are consistent with all units being petrogenetically related. The thick outpourings of viscous, highly evolved, high-silica rhyolite may indicate tapping of late, fractionated liquid. The trachydacite plug may represent reestablishment of the fundamental magmatic system.