TRANS-PECOS MAGMATIC PROVINCE: AN INCONVENIENT MYTH

Between the Rio Grande and the Pecos River in Texas, more than 50,000 km² of continental crust were invaded by Cretaceous and Cenozoic igneous rocks. The term “magmatic province” is defined as an area in which igneous rocks of a narrow compositional range (and, by implication, a common parentage) were emplaced in a specific geologic time span, and is not appropriate for the Trans-Pecos. A new data base incorporates trace elements and isotopic ratios for igneous rocks 1.3 Ga and younger. Although there is a crude west-east asymmetry in compositions, neighboring Cenozoic rock units (even within Big Bend National Park) differ in age, eNd_t, initial ⁸⁷Sr/⁸⁶Sr, Pb isotopes, Nb/Zr, silica saturation, and calcalkalic, alkalicalcic, and alkalic nature. Over 60 Ma, small areas were fed by independent magmas, most of which fractionated in upper- and mid-crustal reservoirs. Very few “primitive” magmas are represented. The question of why the Trans-Pecos region was so susceptible to magmatism, compared to adjacent areas, remains to be answered.