Geochemistry of the rhyolitic magmas from the Streltsovka caldera (Transbaikalia, Russia): a melt inclusion study

The Streltsovka caldera located in Transbaikalia near the Chinese-Mongolian border represents the largest uranium field associated with volcanics in t he world. All the rocks, volcanics and underlying granites. are deeply alte red by hydrothermal fluids and their original major and trace element chemi stry was strongly modified, particularly their primary mobile element conce ntrations such as fluorine, alkalis, uranium. To determine the primary geoc hemical characteristics of the magma associated with this giant ore field, a study of melt inclusions has been undertaken on the rhyolites. Many of th e melt inclusions are well preserved in quartz phenocrysts from strongly al tered rhyolites of the caldera. Si. Al, Fe, Na, K, Ca, F, Cl, Mn, Ti, P, S and Mg were analyzed by electron microprobe. U by fission tracks and H, U, Tn, La. Ce, Pr, Nd, Sm, Eu, Gd. Dy, Er, Yb, Lu, Sc, Ti, V, Sr, Y, Nb, Pa, H f by ion microprobe. The presence in melt inclusions, of daughter minerals such as fluorite, F-rich mica and REE fluocarbonate, and the composition of the silicate melt after homogenization, indicate that the Streltsovka rhyo lites correspond to highly fractionated mildly peralkaline melts (1.04 < Na + K/Al < 1.10) whereas whole rock analyses indicate a peraluminous composi tion. The Streltsovka rhyolitic melts are rich in F (from 1.4 to 2.7 wt.%), and in U (14 to 23 ppm). They are relatively dry with a water content infe rior to 2.4% H2O. The low Cl contents (0.2% Cl) in melt inclusions of the p resent study may be related to the mildly peralkaline character of the Stre ltsovka melts (NK/Al: 1.04 to 1.10). These melts are rich in REE and presen t moderate LREE fractionation, low HREE fractionation and a strongly negati ve Eu anomaly. The similar REE contents of Tulukuevskoe melt inclusions and whole rock indicate that there was no significant Fractionation after trap ping of the magma. On the other hand, the glasses from Krasny Kamen and Str eltsovskoe rhyolites are richer in HREE than the whole rock. This agrees wi th an REE saturation of the magma at the trapping time that is confirmed by the crystallization of parisite as a daughter mineral in melt inclusions. Melt temperatures obtained from homogenization of the inclusions are relati vely high for such magma (1015 +/- 30 degreesC) and may be overestimated be cause of a possible volatile loss since the rhyolite formation. The lower T -h (1000 +/- 20 degreesC) obtained in Tulukuevskoe inclusions agrees with t heir higher F content (3.2% to 3.2%). The higher homogenization temperature of Krasny Kamen inclusions also agrees with the lower F content (0.6% to 0 .8%). (C) 2001 Elsevier Science B.V. All rights reserved.