STILL ANOTHER LOOK AT TRANS-PECOS MAGMATISM

Throughout the span (48-16 Ma) of magmatic activity in the Trans-Pecos province, tephrites, basanites, phonotephrites, tephriphonolites, alkali basalts, and trachybasalts were emplaced in small volumes, and these were the only surviving products emplaced during crustal extension (26-16 Ma). However, during the previous episode of crustal shortening (48-26 Ma), all the preceding types were accompanied by basaltic trachyandesites, trachyandesites, phonolites, trachytes, and rhyolites. The phonolites are mostly peralkaline. Trachytes and rhyolites, totalling more than 60% of 1156 analyzed Trans-Pecos samples, are dominantly metaluminous, but include many peralkaline examples. The lack of hotspot trails in this 400 by 160 km region through 32 m.y. seems to rule out an asthenospheric source. The persistence and geographic spread of mafic magmatism suggest small degrees of melting in lithospheric mantle, and depletion of rare-earth element concentrations in the younger mafic rocks points to continued derivation from the same source volume. The origin of a long-lived heat source for this melting is unknown. Compression constrained more of the mafic magmas to pool within the lithospheric mantle and crust and fractionate to more felsic, highly evolved, liquids, but later extension permitted easy ascent of mafic and less fractionated magmas. Alkalic rocks are not the exclusive hallmark of continental rifting or extension.