Abstract

The Sleeping Beauty area of the southeastern Cady Mountains, central Mojave Desert, contains a sequence of volcanic rocks >3 km thick which was erupted approximately 20 m.y. ago. Plate reconstructions indicate that the change from subduction to transform-fault tectonics off the coast of California occurred about 20 m.y. ago as well, and so these rocks contain a record of volcanism, extensional faulting, and potassic metasomatism during this time of tectonic transition.

The volcanic sequence comprises basalt to rhyolite flows and tuffs which are overlain by the widespread Peach Springs Tuff and by terrestrial sediments. Bedding below the Peach Springs Tuff generally dips 10°-50° to the southwest and is cut by numerous steep northwest- to north-trending faults. Tilting predated deposition of the Peach Springs Tuff. Bedding-fault relationships indicate that deformation was not by "tilted-book" geometry in its simplest form. No significant low-angle faults are exposed, but such faults occur to the west and may underlie the area. A major open, southeast-trending anticline in the area may be a drag fold related to the post- late Miocene, right-lateral, north-trending Ludlow fault.

All units below the Peach Springs Tuff were locally affected by severe potassic metasomatism, which raised measured K2O contents to as high as 13.3 wt%. Metasomatized rocks occur in irregular zones that follow northwest-trending faults and are best developed around northwest-trending breccia zones which have jasper matrices. Ba and Mn prospects are invariably found in metasomatized rocks. Geologic and geochemical constraints indicate that metsomatism occurred at shallow depth (<1 or 2 km) and low temperature (<150 °C). Metasomatism occurred in at least two distinct pulses and apparently predated deposition of the Peach Springs Tuff. The K may have been derived from percolating closed-basin brines or through hydrogen metasomatism of rocks deeper in the complex.

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