GEOCHEMICAL CHARACTERISTICS AND THE ORIGIN OF ALKALI BASALTS OF THE TRANS-PECOS MAGMATIC PROVINCE, WEST TEXAS

Extension-related alkali basalts of the Basin and Range age (24-17 Ma) constitute the member nine unit of the Rawls Formation in the Bofecillos Mountains area of west Texas. These basalts were emplaced after a period of quiescence (27-24 Ma) of the Bofecillos volcano. They occur as lava flows capping early members (units 1-8, age 28-27 Ma) of the Rawls Formation, as interbedded flows in the graben fill (Bolson fill) sediments and as dikes and small stocks throughout the area.

Phenocrysts of olivine, plagioclase, and clinopyroxene are set in a fine-grained matrix of plagioclase laths. Olivine shows alteration to a red colored mineral (iddingsite/ aenigmatite), and plagioclase exhibits disequilibrium textures such as reaction rims and compositional zoning. Xenoliths (gneisses and metasedimentary rocks) and xenocrysts of probable crustal origin are also present in the basalts.

Most of the basalts are nepheline-normative and a few are hypersthene-normative. The narrow compositional range (SiO2 44-49 %), high Mg numbers (sixties), and high concentrations of Ni, and Cr (as high as 1363 ppm) suggest that these basalts are close to being primitive. The spidergram pattern shows characteristic highs in Nb, Ta, Zr, and Y, which are generally attributed to the involvement of asthenospheric mantle sources.