PETROLOGY OF LATE MIOCENE VOLCANIC ROCKS OF THE CHICAGO VALLEY IN THE SOUTHERN RESTING SPRING RANGE, SOUTHERN DEATH VALLEY, CALIFORNIA

Extension-related Miocene volcanic rocks of the Chicago Valley Formation (southern Death Valley region) are well exposed in the southern Resting Spring Range and show clear evidence for open system petrogenesis in a shallow-level volcanic system. The Chicago Valley Formation sits unconformably on Cambrian dolomites of the Bonanza King Formation. The entire section has been uplifted and tilted east during Miocene normal faulting. Dips are steeper in the Paleozoic rocks and shallower near the top in the overlying Tertiary strata, indicating volcanism was synextensional. The base of the Chicago Valley Formation consists of 0 to 30 m of volcaniclastic sandstones, breccias, and thin silicic tuffs. These rocks are overlain by approximately 200 m of interlayered dacite flows, basaltic flows and breccias. The section is intruded and capped by a dacite dome up to 600 m thick, which has fed thick dacite flows. Dikes feeding flows and a partially eroded cinder cone indicate a local source for the volcanic rocks. Sparse age determinations in the southern Resting Spring Range bracket volcanism between 9-11 Ma. Partial magma mixing and mingling between dacite and basalt is spectacularly preserved in the thick upper dacite. Conspicuous mafic enclaves vary in outcrop volume from relatively few to abundant (locally >5% of outcrop). Some basaltic dikes are seen feeding enclave swarms, whereas in other areas similar dikes solidified and were not dispersed into enclave swarms. Enclave shapes range from rectangular and blocky to elliptical and wispy, and enclave size ranges from microscopic (sub-cm) to 1 m. Petrographic examination indicates that physical mingling was remarkably thorough with small rounded and wispy blobs of mafic material found throughout most of the dacite, even where conspicuous mafic enclaves were not observed in outcrop. The scale of mingling suggests a strong rheologic contrast between the dacite and basalt magmas, but interaction occurred in an environment that promoted very effective dispersal and disaggregation of partially quenched dikes and larger blobs.