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Description
The Picacho area of SE California is underlain by an extensional duplex consisting of four major plates bounded by faults. From oldest to youngest the duplex consists of (1) the Late Cretaceous to Early Tertiary Orocopia Schist, (2) probable Jurassic granodioritic gneiss, (3) probably Jurassic Winterhaven Formation, and (4) Cenozoic volcanic and sedimentary rocks. The oldest unit in structural unit (4) is the Oligocene Quechan Volcanics. The Quechan Volcanics were named by Crowe (1978) who described them as being ~300 meters thick. According to Crowe (1978) the lower part consists of ~30 to ~50 meters oftrachybasalt, overlain by a thick section of pyroxene rhyodacite which was in tum overlain by glassy pyroxene dacite. As outlined below, field and laboratory work completed during spring 2003 suggests that this stratigraphic hierarchy is in need of modification. Approximately 14 square miles (23 km2) were mapped at a scale of 1: 12000, and 8 samples were collected for thin section and chemical study. Results of this work indicate that the Quechan is ~300 meters thick, consisting of a lower part, ~50 m thick, of pyroxene basaltic porphyry and an upper part composed of pyroxene-plagioclase porphyry. Phenocrysts in the lower part vary from 0.5 mm to 2.0 mm in size while phenocrysts in the upper part vary from 0.25 mm to 6.5 mm. Intergranular and trachyitic textures are common throughout the Quechan. Chemical analyses confirmed that the lower part of the Quechan is basaltic as SiO2 is ~51 wt. percent while SiO2 values for samples from the upper part vary from ~57 to ~60 wt. percent. On the conventional SiO2 versus alkali diagram, the one specimen analyzed from the lower part plots on the boundary separating trachybasalt from basaltic trachyandesite, while specimens from the upper part plot mostly in the trachyandesite field. On the TiO2/Zr vs Y /Nb classification diagram the one specimen from the lower Quechan plots in the alkali basalt field, while all other specimens cluster tightly within the trachyandesite field. Hence, I conclude that the Quechan Volcanics consist of a twopart stratigraphic hierarchy consisting of alkali basalt in the lower ~50 meters and trachyandesite in the upper ~250 meters.
