Magmatism in Trans-Pecos Texas


The lithosphere under the Trans-Pecos region supplied granitic to mafic magmas from 1300 to 1100 Ma. From the Proterozoic to Late Cretaceous time, magmatism was absent. A few occurrences of Late Cretacous and Paleocene rocks are known. Starting in the Eocene, magmatism was voluminous and continued into Miocene time. The general pattern centered around a NW-SE axis, parallel to the strike of the subducting Farallon plate, but was compositionally and temporally asymmetric. Calcalkalic and alkalicalcic rocks tend to be older and are concentrated in the SW. To the NE, rocks become more strongly alkalic, culminating in a 350-km long belt of phonolites and nepheline-normative trachytes. Activity then shifted to the SW from 31 to 27 Ma, and mafic lavas and dikes from 26 to 17 Ma accompanied the onset of Basin-and-Range faulting over the entire width of the Trans-Pecos. Times of magmatic stasis and migration probably reflect roll-back of the subducting and detaching Farallon slab; this roll-back sometimes kept pace with SW migration of the North American plate. Rock types range from tephrite/basanite through minor alkali basalts to more abundant hawaiites, mugearites, and benmoreites, trachytes, phonolites, and rhyolites. Small discrete areas share distinctive trace-element and isotopic characteristics. Mostly small-volume magma batches stalled in the upper crust, fractionating, mingling, and assimilating crust to achieve the wide variety of compositions.