Compositional changes in Trans‐Pecos Texas Magmatism Coincident with Cenozoic stress realignment

Abstract

The composition and inferred sources of Cenozoic magmas in Trans‐Pecos Texas changed abruptly between 32 and 28 Ma, coincident with a fundamental change in stress regime. Magmas emplaced between 48 and 32 Ma comprise extended differentiation suites beginning with relatively evolved basaltic magmas. These rocks have low Nb and Ta compared to Zr, Hf, La, Ba, and K, characteristics typical of continental volcanic arcs. However, the Texas rocks are more alkalic, have higher concentrations of incompatible elements, and have higher ratios of Nb and Ta to Zr, Hf, La, Ba, and K than is typical of rocks from arcs near trenches. These differences reflect relative position within the Cordilleran arc. Trace element models involving partial melting in the mantle wedge combined with fractional crystallization and assimilation of upper and lower crust can account for most of the observed trace element concentrations and ratios. Trans‐Pecos magmatism may have involved smaller degrees of partial melting than is typical of arcs located closer to the subduction zone. Magmas emplaced after 31 Ma were either bimodal, alkali basalt‐rhyolite, or exclusively basaltic. They have high absolute abundances of incompatible trace elements and high ratios of Nb and Ta to Zr, Hf, La, Ba, and K. The mafic rocks are typical of continental rift or ocean island basalts. Trace element models using Nb, Ta, Y, Yb, Hf, and Zr reproduce the range of concentrations and ratios observed in these rocks and require very small degrees of partial melting, variable amounts of fractional crystallization, but little or no assimilation. The change in magma compositions and sources is best illustrated by a drop in Zr/Nb, from between 18 and 6.25 in the earlier, arc rocks to between 6.25 and 2 in the later, rift‐related rocks. Stress regime appears to play an important role in the process of magma generation and evolution. The change in trace element compositions indicative of tectonic setting supports interpretations of paleostress data that activity up to 31 Ma occurred in a continental volcanic arc, whereas later activity occurred in a setting of intraplate extension. The magmatic change may have been simultaneous with the change in stress regime or may have lagged by as much as 3–4 m.y.