Alkalic rocks of contrasting tectonic settings in Trans-Pecos Texas

Alkalic rocks in Trans-Pecos Texas were emplaced in two distinctly different tectonic environments: one compressional (contractional) and one extensional. Rocks (Eocene and early Oligocene) of the older compressional environment can be divided into a western alkali-calcic belt and an eastern alkalic belt. The boundary between the two belts is parallel to the paleotrench that used to lie off the west coast of Mexico. The alkalic rocks were the most inland expression of subduction-generated volcanism. The predominance of east-striking dikes and veins and the orientation of en echelon dikes indicate igneous activity during residual compression remaining from Laramide deformation. As the dip of the subducting slab became gentler with time, calc-alkaline magmatism of Laramide age in Mexico graded eastward into the alkaline magmatism in Texas. Widespread extension and normal faulting began about 24 Ma in the Texas portion of the Basin and Range province. Between 24 and 17 Ma, alkalic basalts were extruded and intruded at several localities, dominantly as north-northwest-striking dikes. Both nepheline- and hypersthene-normative basalts occur in the extensional environment. Rocks of the compressional environment follow two major lines of differentiation: hypersthene-normative basalt to rhyolite and nepheline-normative basalt to phonolite. In contrast, rocks of the extensional environment are apparently limited to basalts. During contraction, magmas rising from the mantle probably formed chambers in which differentiation could occur. During extension, less differentiation occurred, either because tectonically dilated fractures permitted more direct rise of magma to the surface or because the volume of magma was too small. Basalts of the two contrasting tectonic settings are broadly similar in alkalinity and silica saturation. The basalts of the extensional environment are, however, generally richer in magnesium than are the basalts of the compressional environment. This difference is not simply a matter of degree of differentiation but is probably related to the different pressure-temperature regimes of the mantle from which the basalts originated.