BASEMENT STRUCTURES AND RIO GRANDE RIFT MAGMATISM IN WEST TEXAS

Magmatism in the Trans-Pecos Magmatic Province began earlier (~46 Ma) than that of the northern Rio Grande Rift (~36 Ma), differed in rock compositions (more alkalic) and eruptive styles (extensive silicic lavas and fewer calderas), and ended earlier (~17 Ma). Proterozoic basement rocks underlying the northern rift form southwest-northeast trending belts, younger to the south, parallel to the southern margin of the Archean Wyoming Craton. These belts were largely formed by accretion of volcanic arcs and their associated sedimentary rock and intrusions in a succession of orogenies to the ancient craton. Major shear zones separate these belts, and these shear zones appear to have influenced later magmatism and segmentation of the Rio Grande Rift. In Trans-Pecos Texas, two additional major lithospheric boundaries occur across the axis of the Rio Grande Rift: the Grenville and Ouachita fronts. The Grenville Front is exposed in the Van Horn Mountains where the metamorphosed Proterozoic Carrizo Mountain Group is thrust over craton-margin sedimentary and volcanic rocks. The Grenville Front roughly separates the Davis Mountains volcanic field and the older Laramide Tornillo Basin from the Diablo Plateau to the north. The Ouachita front roughly separates the Davis Mountains portion of the field from the Christmas Mountains - Big Bend portion of the rift. The thicker, older arc crust in the northern rift favored production of magmatic suites with subduction geochemical signatures (e.g. high Ba, low Nb,Ta) whereas the thinner, younger non-arc crust in West Texas more strongly favored production of peralkaline silicic and silica-undersaturated series.