Contributions to mineralogy and petrology

High-silica topaz-bearing rhyolites of Cenozoic emplaced as domes, lava flows or shallow intrusions. They age are widely distributed across the western USA and may also contain Fe-Mn garnet, beryl, bixbyite, pseudoMexico. They are characteristically enriched in fluorine brookite, hematite and fluorite in miarolitic cavities or with(>0.2 wt.%) and incompatible lithophile elements (e.g. Li, in the groundmass of devitrified samples. They are geoRb, Cs, U, Th, Be). In addition to topaz, the rhyolites chemically distinct as well, due to their dramatic enrichment contain garnet, bixbyite, pseudobrookite, hematite and in lithophile elements such as U, Be, Li, Rb, Th and Nb. fluorite in cavities or in their devitrified groundmasses. Based on a review of topaz rhyolites from the western Magmatic phases include sanidine, quartz, oligoclase and United States (Christiansen et al. 1982), this paper summarFe-rich biotite. Allanite, fluorite, zircon, apatite and magneizes the petrogenetic implications of their geochemistry and tite occur in most; pyroxene, hornblende, ilmenite and tidistribution. We suggest that topaz rhyolites are the fluortanite occur in some. The rhyolites crystallized over a wide ine-rich equivalents of anorogeneic granites as such they temperature interval (850" to 600" C) at,fo2 that ranges from provide some insights regarding the origin of anorogenic QFM to NNO. The REE patterns of most topaz rhyolites granites and about the possible role of fluorine in their are almost flat (La/Yb, = 1 to 3) and have deep Eu anomaevolution. lies (Eu/Eu* = 0.01 to 0.02). Both parameters decrease with Practical interest in topaz rhyolites stems from their gedifferentiation. Titanite-bearing rhyolites have prominent netic association with volcanogenic deposits of Be, Sn, U, middle REE depletions. Topaz rhyolites appear to have and F (Burt and Sheridan 1981). The similarity of these evolved from partial melts of a residual granulitic source rocks to those associated with porphyry molybdenum dein the Precambrian lower crust. According to the proposed posits suggests that topaz rhyolites may be indicators of model, the passage of hot mafic magmas through the crust subsurface porphyry, greisen or pegmatite mineralization produced partial melts as a result of the decomposition (Burt et al. 1982a). The petrogenetic model outlined here of F-rich biotite or amphibole. An extensional tectonic carries important implications about the ultimate source setting allowed these small batches of magma to rise withof metals and fluids in these systems. out substantial mixing with contemporaneous mafic magmas. Some of the compositional differences between topaz Distribution and Ages rhyolites and peralkaline rhyolites may be attributed to the accumulation of fluorine and fluorphile elements (Al, Be, Topaz rhyolites are widespread in western North America Li, Rb, U, Th, HREE) in melts which give rise to topaz and their occurrence closely coincides with the limit of late rhyolites and chlorine and chlorophile elements (Ti, Fe, Cenozoic extensional faulting (Fig. 1). In the United States, Mn, Zn, Zr, Nb and LREE) in melts which yield peralkaline the emplacement of these rhyolites appears to have spanned rhyolites. Hence the F/CI ratio of the melt or its source most of the Cenozoic Era; their isotopic ages range from may determine the alumina saturation of the magma series. 50 m.y. (Little Belt Mountains, Montana) to 0.5 m.y. (MinTopaz rhyolites are distinguishable from calc-alkaline rhyoeral Mountains, Utah), although all but two are younger lites by lower Sr, Ba, Eu and higher F, Rb, U and Th. than 30 m.y. old (Table 1). Most topaz rhyolites lie within The usually low La/Yb ratios of topaz rhyolites distinguish the eastern and southern Basin and Range province (Idaho, them from both peralkaline and calc-alkaline rhyolite Nevada, Utah and Arizona) and along the Rio Grande suites. rift (New Mexico and Colorado) and thus appear to surround the Colorado Plateau. Similar middle to late Tertiary topaz rhyolites appear to be widespread in central Mexico Introduction (Sinkankas 1959,1976; Smith et al. 1950; Foshag and Fries 1942). The Mexican occurrences are associated with small A distinctive suite of fluorine-rich rhyolites is widely distribvolcanogenic tin deposits (Huspeni et al. 1982) and may uted across the western United States. These rhyolites charform a continuous belt with the topaz rhyolites in the westacteristically contain topaz (AI,SiO,F,) and were generally ern United States. However, little geochemical data is available about the Mexican occurrences and they are not conOffprint requests to: E.H. Christiansen sidered further here.