VOLATILE AND LITHOPHILE TRACE-ELEMENT GEOCHEMISTRY OF MEXICAN TIN RHYOLITE MAGMAS DEDUCED FROM MELT INCLUSIONS

We have investigated the petrology and geochemistry of whole rocks fro m two small-volume, Sn- and F-mineralized rhyolitic dome complexes of the Mexican tin rhyolite belt, Cerro el Lobo and Cerro el Pajaro, to d etermine volcanic degassing and mineralizing processes in felsic igneo us systems. The abundance and distribution of volatiles (H2O, B, F, an d Cl) and lithophile trace and ore elements (Li, Rb, Cs, Be, Sr, Y, Ce , Th, U, Nb, Sn, and Mo) in the parental liquids were established by a nalyzing melt inclusions in quartz. The melt inclusions from both rhyo lites are variably enriched in Li and the volatile constituents F and Cl, and some are extremely enriched in Li, although whole rocks are no t correspondingly enriched. Compositional variations in the melt inclu sions from both rhyolites also constrain magmatic differentiation. Mel t evolution was dominated by crystal fractionation, modified by mass t ransport in a Cl- and H2O-rich magmatic-hydrothermal fluid, and result ed in increasing abundances of U, Nb, and Cs (+/- Li, F, Cl, B, Y, Ce, Be, Rb, Mo, and Sn) in both liquids. The rhyolite liquids apparently were heterogeneous prior to eruption. The Cerro el Lobo liquid contain ed gradients in volatiles and trace elements; comparatively less Cl, B e, B, Al2O3, and CaO (+/- Li, F, U, and Th) were present in the early- erupted, H2O-rich fractions of liquid. Comparing compositions of whole rocks with the mean compositions of melt inclusions constrains relati ve mobilities of magmatic constituents during and after eruption. Sodi um, fluorine, Lithium, uranium, and yttrium (+/- H2O, Cl, Sn) were los t from both magmas and the Cerro el Pajaro magma apparently also lost Nb and Al as a result of eruptive and posteruptive degassing. These ge ochemical relationships and constraints on pre-eruptive abundances and distributions of volatiles in tin rhyolite magmas probably apply to o ther tin rhyolites and, moreover, the high levels of Cl and Li enrichm ent may be representative of other highly-evolved granitic magmas gene tically associated with lithophile mineralization.