Abstract

Two belts within the Trans-Pecos Magmatic Province have been recognized: an eastern alkalic and a western metaluminous belt. Representative rocks from these two belts have been collected and analyzed for their major and trace element compositions.

Although mafic rocks of the two belts are chemically indistinguishable, as rocks evolve to higher silica contents compositions in the two belts diverge with the eastern belt becoming increasingly alkaline, while the western belt compositions become metaluminous to peraluminous in character.

Within the eastern belt, four stages of magma evolution have been identified and modeled through processes of extensive crystal fractionation, magma replenishment, and crustal assimilation. These processes were operative in varying proportions throughout the four stages of evolution. The geochemical modeling is supported through physical evidence including glomerocryst-matrix disequilibrium (indicating magma replenishment) and partially resorbed inclusions of both sedimentary and igneous clasts (indicating assimilation) in host igneous rocks.

Although similar processes were operative early on in the evolution of the western belt rocks, the markedly low levels of incompatible elements (e.g. Zr, Nb, and Th), minimum melt compositions, and elevated oxygen isotopic compositions argue for more direct crustal involvement in the western belt. Two trends of rocks exist within the western belt. Based on combined geochemical evidence, one trend appears to have evolved through direct crustal melting followed by fractionation while the second trend represents hybrid magmas produced through fractionation from trachytic compositions and episodic mixing with a crustal melt component.