TESTING POSSIBLE CORRELATION OF THE LOWER SIDEWINDER SERIES VOLCANICS AS A SOURCE OF THE COWHOLE VOLCANIC SERIES USING PETROGRAPHIC TECHNIQUES AND MAJOR AND TRACE ELEMENT GEOCHEMISTRY

Periods of volcanic activity and depositional events produced a well-preserved Cordilleran Jurassic arc that extends from southeast California to western Nevada and into Arizona and northern Mexico. The Cowhole Mountains are located within this arc, within the central-eastern Mojave Desert, California.

The Cowhole Volcanics series (Jcv), extruded in the Middle Jurassic and dated at approx.170 Ma, consist of rhyolite ignimbrites, a dacite flow, and volcaniclastic units (Wadsworth et al, 1995). A possible correlation between the Cowhole Volcanics and the Lower Sidewinder Series volcanics of the central Mojave Desert has been suggested. The Lower Sidewinder Series, currently dated as170 Ma age, consists of andesitic lava flows and dacitic to rhyolitic ignimbrites. The last eruptive phases resulted in a nested caldera complex with a proposed eastern margin at Stoddard Ridge, approximately 120 km southwest of the Cowhole Mountains (Schermer and Busby, 1994).

Samples were collected from each member of the Cowhole Volcanics, granodioritic Cowhole Pluton (Jcp), a rhyolite dome (Jr), and lava flows intercalated with the Aztec Sandstone (Javu) with the goal of testing this possible correlation using complete major and trace element geochemistry and standard petrographic techniques.

Data from samples Jr, Javu, Jcv, and Jcp suggests a genetic relationship between the Jr and Javu, and indicate a relationship of the Jcp to the Jr, Javu, or Jcv is not likely. The Jcv exhibits lower SiO₂ (66.30wt%) than the Jr (75.3wt%), Javu (74.9wt%), and Jcv (70.0wt%), while exhibiting higher Fe₂O₃ (4.06wt%vs 1.38, 1.27, and 2.9wt% respectively). Both the Jcv and Jcp show increased values of K₂O (5.54wt% and 4.52wt%) relative to the Jr and Javu (4.26wt% and 4.24wt%). The Jcp shows a substantially higher value of Sr (308 ppm) vs the Jr (79ppm), Javu (115ppm), and Jcv (137ppm). The Jcv has a greater Ba value (1093ppm) than the Jr (480ppm), Javu (192ppm), or Jcp (737ppm).

Elemental signatures for the samples suggest no relationship of the Jcp to the Jr, Javu, or Jcp, implying an alternative source exists. The proposed margin for the Sidewinder caldera complex is located at such a distance from the Cowhole volcanics that this possible source lacks credibility and a source more proximal to the Cowhole volcanics is more likely.