Peraluminous rhyolites that are chemically somewhat similar to topaz rhyolites and anorogenic granites occur in an orogenic setting near Sierra Blanca in the Tertiary Trans-Pecos magmatic province. The Sierra Blanca rhyolites are even more enriched in most incompatible trace elements than are topaz rhyolites. Some of the extreme enrichments may in part be the result of chemical modification by crystallization from an F-rich vapor phase.
The rhyolites were intruded as laccoliths at 36 Ma, during the main phase of Trans-Pecos igneous activity, which is characterized by ash-flow eruptions from numerous calderas and widespread mafic, intermediate, and silicic intrusions. A dominant east-northeast orientation of dikes and veins throughout the region indicates mild compression that was residual from Laramide deformation. This compressive tectonic setting, coupled with concurrent volcanism in Mexico and the east-northeast change in magma chemistry from calc-alkalic in western Mexico through alkali-calcic to alkalic in Texas, suggests that the rhyolites were emplaced in a continental arc. Extension did not begin in Trans-Pecos Texas until after 32 Ma; 31- to 17-Ma dikes are dominantly oriented north-northwest, perpendicular to the direction of extension during early Basin and Range deformation. Thus, the tectonic setting of the Sierra Blanca rhyolites contrasts with that of typical topaz rhyolites, most of which were emplaced during periods of crustal extension.
The Sierra Blanca rhyolites are chemical and mineralogic oddities for the region, where most rhyolites are peralkaline or metaluminous. The rhyolites are depleted in the same elements as topaz rhyolites (Mg, Ca, Ti, Sr, Ba) but are more highly evolved than topaz rhyolites. Extreme trace-element enrichments (Li, F, Zn, Rb, Y, Zr, Nb, Sn, Ta, Pb, HREE, Th, U) are accommodated in Li-rich white mica, Zn-rich biotite, Rb-rich feldspars, and numerous trace minerals, including cassiterite, changbaiite, columbite, thorite, xenotime, yttrium- and REE–rich fluorides, and zircon. The rhyolites are large-tonnage, low-grade resources of several rare metals. Also enriched in Be (as much as 180 ppm), the rhyolites are the sources of Be and F in beryllium deposits in fluoritized limestones along the contacts with the laccoliths. Interaction with the limestones probably locally elevated the Ca, Mg, and Sr contents of the rhyolites.
Vapor-phase crystallization has modified the original magmatic chemistry of the rocks. Evidence of vapor-phase crystallization includes the presence of minerals typical of pegmatites: cryolite (from 0 to 3 volume percent), alkali feldspars with nearly end-member compositions, polylithionite-zinnwaldite mica, and rutilated quartz, plus fluid inclusions defining quartz overgrowths on magmatic grains. Extreme HREE enrichments (Yb to 72 ppm; chondrite-normalized REE patterns with positive slopes) may also be the result of vapor-phase crystallization.
