Topaz Rhyolites-Distribution, Origin, and Significance for Exploration
Economic Geology Vol. 77, 1982, pp. 1818-1836 Topaz Rhyolites-Distribution, Origin, and Significance for Exploration DONALD M. BURT, MICHAEL F. SHERIDAN, JAMES V. BIKUN~, AND ERIC H. CHRISTIANSEN~~ Department of Geology, Arizona State University, Tempe, Arizona 85287 Abstract Topaz rhyolites are fluorine.,.rich alkaline silicic lavas and shallow intrusives that are char acterized by the presence of topaz (AI2Si04F 2) in gas cavities, commonly associated with Mn Fe garnet, bixbyite, pseudobrookite, specularite, quartz, and other minerals. In the western United States, Cenozoic (O.5--50m.y. .old for dated examples) topaz rhyolites occur on both sides of the Colorado Plateau (in Colorado, New Mexico, Arizona, Utah~ and Nevada) and in Idaho and Montana. They also occur in a single linear belt in Mexico. Their enrichment in lithqphile (flu~rophile) elements (Li, Rb, Cs, U, Th, Nb, Ta, Sn, W,'Be, etc.) leads to the term rare metal rhyolites. SimilarF-rich rocks from Mongolia and the Soviet Union have been called ongonites. Topaz rhyolites appear to represent a special class of the bimodal or high silica rhyolites of the western United States. Their extensional tectonic setting and geochemical characteristics suggest that topaz rhyolites are the extrusive equivalents of anorogenic or residual (A- or R-type) granites. Their petro genesis presumably involves partial melting of Precalnbrian continental crust (they appear to be restricted to areas of such crust) in the presence of a high heat flow (which tends to enrich F in solids at the expense of H 20). Mafic magmas may provide the heat for melting. Further differentiation may depend on (1) zone refining during ascent, (2) extreme fractional crys tallization, (3) dehydration due to early pyroclastic volcanism, and (4) apical enrichment of near-surface magma chambers due to liquid state processes.
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