Abstract

The Little Drum Mountains represent a deeply eroded Eocene-Oligocene volcano, consisting of a vent complex which erupted mafic flows and flow breccias, accompanied by lahars. Flows are dominated by members of the shoshonite suite and contain up to 3.95 percent K₂O, mainly occult in K-rich glass, with K₂O/Na₂O ratios greater than 1.0. In a few interbedded flows, apparently of the calc-alkaline series, pyroxene with varying amounts of plagioclase in a fine-grained groundmass of plagioclase, mafic minerals, and interstitial glass. An ash-flow tuff of the Oligocene Needles Range Formation unconformably overlies the volcanic sequence. Contemporaneous eruptions of calc-alkaline and shoshonitic lavas are possibly related to different depths of magna derivation corresponding to two mid-Cenozoic imbricate subduction zones beneath the western United States.

Degree

College and Department

Physical and Mathematical Sciences; Geological Sciences

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