Tertiary alkaline magmatism in Trans-Pecos Texas, in Alkaline Igneous Rocks, edited by

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 toan 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 tothe 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 n Mexico were erupted at the same time and probably in the same stress regime, and their compositional differences must reflect differences in ource rocks and conditions of melting. Location and extent Early work The Trans-Pecos province contains all the Ceno-zoic igneous rocks within an area bounded on the N by an E-W line 12 km N of the New Mexico-Texas border, on the E by the Pecos River, and on the W and S by the international border defined by the Rio Grande (Fig. 1). The south-western and south-eastern limits of the province are arbitrary; compositions of igneous rocks grade to the SW into less alkaline but coeval rocks, and to the SE alkaline rocks like those of the Trans-Pecos province xtend for an unknown distance into Mexico (Bloomfield & Cepeda-Davila 1973; Robin 1974). The northern bound-ary is also arbitrary, because similar rocks of the same ages do occur farther N in New Mexico (Foord et al. 1983). The north-eastern limit of the province, coinciding with the inland edge of the Basin and Range structural province in Texas, is a natural boundary, because no Cenozoic igneous rocks are exposed within 50 km W of the Peco