GEOLOGIC HISTORY OF A GNEISSIC SUITE FROM SOUTHEASTERN CALIFORNIA

Gneisses collected in lower Big Wash in the Eagle Mountains, southeastern California, were characterized using field observations, thin sections, density analysis, and geochemistry of major oxides and trace elements. Prebatholithic rocks in the Eagle Mountains have been divided into the older Hexie Mtns. and Eagle Mtns. assemblages (Powell, 1993), both intruded by the 1.19 Ga San Gabriel anorthosite suite (SGA). Although originally mapped as part of the SGA, our results suggest the Big Wash gneisses are a slightly older rock unit with a distinct origin. These rocks display metamorphic textures and are L-S tectonites, with foliation that usually dips N or E at approximately 25 degrees, and a consistent NE-trending lineation. At least four different varieties of zircon are seen in cathodoluminescence images from an intermediate gneiss (VP-10); oscillatory-zoned (os) domains surround either bright or dark cores, and faintly-zoned embayments transect oscillatory-zoned domains. U-Pb ages obtained from os zircon are concordant to slightly discordant, and yield a weighted mean 207Pb*/206Pb* age of 1198.4±7.7 Ma (MSWD=0.8), relative to zircon standard R33 (419 Ma; Black et al., 2004). We interpret this to be the age of the igneous protolith of the gneiss. Embayments formed along or helped propagate paths of weakness highlighted by fractures or mineral inclusions, and thus form a younger and chemically distinct generation of zircon that may be related to deformation. The gneisses have granodioritic to granitic silica contents, and are more silica-rich and do not generally show the Fe- and HFSE-enrichment characteristic of the slightly younger SGA suite. The Big Wash gneisses display characteristics of A-type granites, including depletion in Mg (relative to Fe) and high alkali concentrations. The protolith of the Big Wash samples is similar to some granodioritic igneous rocks found in arc-continent or continent-continent collision tectonic settings.