ANOTHER LOOK AT THE GEOCHEMISTRY OF THE RANCHERIAS LAVAS IN THE BOFECILLOS MOUNTAINS OF TEXAS

The Rancherias Lavas Member crops out in the Bofecillos Mountains centered in the heart of Big Bend Ranch State Park in Trans-Pecos Texas. Trachytes, trachyandesites, and basaltic trachyandesites were sampled from remote outcrops, distant from most previously sampled.

Rancherias lavas range from aphyric to sparsely porphyritic with a sugary (devitrified?) groundmass, to glomeroporphyritic. Some samples are vesicular and many have a flow foliation. The dominant phenocrysts are plagioclase and sanidine, but pyroxene, amphibole, apatite, olivine (mostly iddingsitized), and iron-titanium oxides are also present. Three phases are notable in their appearance: iddingsite is outstanding in its apparent crystallinity and beauty, apatite has an unusual appearance and color, likely due to voluminous fluid inclusions, and the feldspars are commonly “complexly” zoned and intricately resorbed.

Harker variation and rare-earth element diagrams support a co-magmatic origin for the lavas, and likely crystal fractionation. However, consistent variation in relative depletions in Nb, Ta, Sr, P, and Ti requires more than one source for the magmas, and indicates that differentiation was achieved by fractionation of plagioclase, Sr-rich apatite, and a Ti-rich phase. Three distinct co-genetic groups within the Rancherias Lavas have been identified: 1) basaltic trachyandesites to trachytes that show a typical, though moderate, continental arc signature; 2) trachybasalts to trachytes that are not depleted in Nb-Ta-Ti; and 3) a single trachyte that is not depleted in Nb-Ta-Ti, which is acmite normative (no others are), lacks apatite in the norm (the rest contain apatite), and is peralkaline while all others are metaluminous.