SILVER CREEK CALDERA, PROBABLE SOURCE OF THE MIOCENE PEACH SPRING TUFF, OATMAN MINING DISTRICT, ARIZONA

The eastern half of a major silicic caldera is preserved at the western foot of the north-trending Black Mountains ~10km northwest of Oatman, Arizona. The western half is either faulted away and/or buried by Colorado River valley fill. The caldera is filled with feldspar phenocryst-rich ash-flow tuff that has no known cooling unit divisions, preserved top, or confirmed base. Lithic swarms of mesobreccia, and megabreccia blocks up to several hundred meters abound. Some are so large and apparently so coherent that they were previously interpreted as part of a coherent stratigraphic sequence. Overturned beds in the largest block, a complex mass of dacitic lava, breccia, and coarse-grained volcaniclastic turbidite, strongly implicate it as a megaclast. Breccia clasts within the caldera of Proterozoic granite, coarse-grained turbidite, and dacitic lava correlate with the three main regional units that were exposed in the west-facing caldera wall.

No moat-fill or caldera rim features are preserved. Instead, the caldera margin is intruded by epizonal plutonic rocks dated at 18.5 + 0.5Ma (U-Pb zircon, DeWitt et al., 1986: USGS Bull 1857). The nearest exposures of outflow sheet are ~10km to the north, east, and south of the caldera. In each of these areas, the sheet overlies rocks that correlate with the same volcanic sequence that is found in the caldera wall.

Correlation of the caldera fill with the Peach Spring Tuff outflow sheet is based on nearly identical phenocryst assemblages and ratios. The intracaldera tuff contains 20-35% feldspar phenocrysts with the sanidine plagioclase ratio ranging between 5:1 to 3:1, 1-2% biotite, <1% quartz, and traces of hornblende, clinopyroxene, sphene, zircon, and apatite. A positive correlation is also indicated by lithic blocks within the caldera which match only units that are known to underlie the outflow sheet. Clasts of younger units are conspicuously absent. The synchronous age of the caldera margin plutonic suite with the reported age of 18.5 + 0.02Ma (⁴⁰Ar/³⁹Ar sanidine and biotite, Nielson et al., 1990: JGR, v. 95) for the outflow sheet also provides a strong correlation.