Geologic Implications of a Geo-Chemical Study of Three Two-Mica Granites in Southern Arizona

Abstract

The biotite + muscovite ± garnet-bearing Texas Canyon (TC), Presumido Peak (PP), and Gunnery Range (GR) granites are members of an enigmatic suite of Eocene age granites in southern Arizona. The late orogenic granites intrude Precambrian through Jurassic metasediments and metavolcanics. The major and minor element geochemistry of the high silica, weakly peraluminous granites is rather uniform. However, trace element concentrations, REE patterns, and isotopic compositions imply gross similarities between the TC and GR granites when both are compared to the PP granite. The TC and GR granites were derived from a depleted Precambrian lower crustal source area with low Rb/Sr, while the PP granite was the result of anatectic melting of an enriched Precambrian mid-to-upper crustal source. The PP granite is an integral part of a metamorphic core complex, and this tectonic setting accounts for the geochemical differences between it and both the TC and GR granites.