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

摘要

We have investigated the petrology and geochemistry of whole rocks from two small-volume, Sn- and F-mineralized rhyolitic dome complexes of the Mexican tin rhyolite belt, Cerro el Lobo and Cerro el Pajaro, to determine volcanic degassing and mineralizing processes in felsic igneous systems. The abundance and distribution of volatiles (H2O, B, F, and 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 analyzing melt inclusions in quartz. The melt inclusions from both rhyolites are variably enriched in Li and the volatile constituents F and Cl, and some are extremely enriched in Li, although whole rocks are not correspondingly enriched. Compositional variations in the melt inclusions from both rhyolites also constrain magmatic differentiation. Melt evolution was dominated by crystal fractionation, modified by mass transport in a Cl- and H2O-rich magmatic-hydrothermal fluid, and resulted 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 contained gradients in volatiles and trace elements; comparatively less Cl, Be, 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 relative mobilities of magmatic constituents during and after eruption. Sodium, fluorine, Lithium, uranium, and yttrium (+/- H2O, Cl, Sn) were lost from both magmas and the Cerro el Pajaro magma apparently also lost Nb and Al as a result of eruptive and posteruptive degassing. These geochemical relationships and constraints on pre-eruptive abundances and distributions of volatiles in tin rhyolite magmas probably apply to other tin rhyolites and, moreover, the high levels of Cl and Li enrichment may be representative of other highly-evolved granitic magmas genetically associated with lithophile mineralization. [References: 50]