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Department of Earth Sciences, University of Lowell, Lowell, MA 01854, U.S.A.
Alkaline magmatism in far W Texas extended from 48 to 16 Ma ago, peaking in volume during the interval 37-26 Ma when all the felsic rocks were emplaced. Primitive magmas are rare, if not entirely lacking, but ne- and {ol+hy}-normative hawaiite and mugearite liquids (Ni <120 ppm; 100Mg/(Mg+Fe)<66) erupted through the entire time span. These evolved through ne-normative and q-normative trachytes to phonolite and rhyolite, including peralkaline varieties of both. Mafic q-normative rocks are distinctly subordinate.
Transition from a contractional to an extensional regime occurred between 32 and 30 Ma ago, after which Trans-Pecos magmatism was almost exclusively mafic. Major- and trace-element compositions show gradation south-westward to the calc-alkaline rocks of the Sierra Madre Occidental in western Mexico, which were erupted from 35 to 28 Ma ago. This compositional gradient suggests that the alkaline and calc-alkaline rocks were all related to subduction of the Farallon plate. The Trans-Pecos province is not a continental rift, and probably is not a product of back-arc spreading. Peralkaline phonolites and rhyolites in Texas and calc-alkaline rhyolites in Mexico were erupted at the same time and probably in the same stress regime, and their compositional differences must reflect differences in source rocks and conditions of melting.
