The Round Top intrusion, one of five shallow rhyolite laccoliths in the Tertiary Trans-Pecos Texas magmatic province near Sierra Blanca, Texas, was chemically modified by pegmatitic vapor-phase crystallization. Evidence of pegmatitic crystallization includes the presence of cryolite (Na3AlF6), Li-rich trioctahedral micas with variable Fe contents, ru-tilated quartz, and vapor-rich fluid inclusions defining quartz overgrowths on magmatic grains. The laccoliths are enriched in Li, Be, F, Zn, Rb, Y, Zr, Nb, Sn, REEs, and Th and are depleted in Mg, Ca, and Ti relative to typical calc-alkaline rhyolites. They are peralumi-nous, making them unusual among Trans-Pecos silicic igneous rocks, which are typically peralkaline or metaluminous. The presence of cryolite and the strong HREE enrichment, which may also be the result of vapor-phase crystallization, sets the Sierra Blanca rhyolites apart from otherwise chemically similar topaz-bearing rhyolites. Accessory biotite and Li-rich micas are enriched in F, Mn, and Zn and are depleted in Mg and Ti. Rare metals are contained in part by bastnaesite(?), cassiterite, columbite, priorite(?), Nb-rich rutile, tan-talite, thorite, HREE-rich xenotime, yttrocerite and yttrofluorite, and zircon. The rhyolites are the sources of F and Be in beryllium deposits in fluoritized limestones along the contacts with the laccoliths.

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