Abstract

A complete alkali basalt through trachyte volcanic suite has been documented from the central part of the Mount Taylor volcanic field, New Mexico (the southeast margin of the Colorado Plateau) showing mineralogic, chemical, and field relations similar to alkalic volcanism of both continental and oceanic settings throughout the world. Thus, the Mount Taylor field is fundamentally distinct from the predominantly calc-alkalic to low-Ti alkali basaltic volcanism of the Basin and Range Provice as a whole.

Eruptions of the rocks began in Pliocene-Pleistocene time with alkali basalt (basanatoid) and continued through intermediate (hawaiite, mugearite, benmorite) and trachyte compositions. This progression, together with minor-element and isotopic data, suggests that the suite evolved by fractional crystallization of basaltic magmas.

Volcanologic diversity of the field is illustrated by the variety of land forms, including numerous maars, pit craters, silicic flow-domes, and distinct northeasterly oriented fissures. Northeast-southwest oriented faults with as much as 30-m displacements were active concurrently with the volcanism, and several faults are superposed on fissure lines, suggesting that the volcanism was a consequence of deep fracturing of the margin of the Colorado Plateau by late Cenozoic Basin and Range faulting.

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