CONTRASTING METALUMINOUS AND PERALKALINE SILICIC MAGMATISM IN THE TRANS-PECOS TEXAS MAGMATIC PROVINCE

Trans-Pecos Texas has long been recognized as containing one of the largest alkalic provinces in North America, yet metaluminous igneous rocks are one of its major components. Cenozoic alkalic magmatism was concentrated along a NW-SE belt, almost 400 km long, extending from the New Mexico border into the Big Bend country, marked by a chain of silica-undersaturated intrusions. Silicic alkalic volcanic centers parallel this trend, concentrated in the Davis Mountains and Big Bend regions. A second, generally younger, metaluminous igneous center trend, located southwest of that of the alkalic, produced some of the largest silicic units of the Trans Pecos Province, including those of the Paradise Mountain caldera in the Davis Mountains, the Chinati and Infernito calderas, and the San Carlos caldera complex in adjacent Mexico. Minor peraluminous rocks are associated with the metaluminous centers.

Phenocryst assemblages of the silicic alkalic units include dominant anorthoclase, Fe-rich cpx, titanomagnetite, apatite, with some units containing fayalite, quartz and ilmenite. Strongly peralkaline units lack zircon. Metaluminous phenocrysts assemblages may include, in addition to alkali feldspar, cpx amd FeTi oxides, plagioclase, opx, biotite and/or hornblende. Neodymium isotopic data for Davis Mountains rocks show higher initial ratios for metaluminous rocks, although data for both peralkaline and metaluminous silicic rocks indicate little assimilation of Grenville continental crust believed to underlie the area. Rhyolite units for both series are highly fractionated, with high values of Rb, Nb and low values for Sr and Ba.

Trans-Pecos igneous rocks have been related to foundering of the subducted Farallon Plate and associated asthenospheric upwelling, triggering lithospheric mantle melting, with the chain of silica-undersaturated intrusions marking the axis of initial upwelling. The southwestward asymmetry of the metaluminous belt to the axis of initial upwelling may represent continued southwestward foundering of the Farallon Plate, with greater degrees of melting of lithospheric mantle, producing the metaluminous series, which migrated from Trans-Pecos Texas into the Mexican state of Chihuahua.