Fluorite [formula omitted] and REE constraints on fluid–melt relations, crystallization time span and bulk DSr of evolved high-silica granites. Tabuleiro granites, Santa Catarina, Brazil

Abstract

The evolved high-silica Tabuleiro granites within the Early Paleozoic Santa Catarina Composite Massif, Pelotas Batholith, southern Brazil are characterized by the presence of euhedral to subeuhedral accessory fluorite and geochemical features typical of topaz–rhyolites and related A-type granites. Sr isotopes and REE data of the Tabuleiro granites and their accessory fluorite are used to constrain fluid–melt relations, crystallization time span and bulk crystal–melt Sr partition coefficient D Sr.Correlations involving REE, Eu/Eu*, Rb/Sr, Sr and 87 Sr/ 86 Sr in fluorite and fluorite-host granites show that fluorite records the differentiation trend of the host Tabuleiro granites. REE-normalized patterns and Eu/Eu* relations in fluorite-bearing granites indicate that fluorite forms after the crystallization of the quartzo-feldspathic framework in residual melts. The Tabuleiro accessory fluorites yield high and variable 87 Sr/ 86 Sr ratios between 0.72334 and 0.8192. These ratios are neither the result of fluorite precipitation from a fluid nor Sr isotopic resetting. They result from 87 Rb decay in fluorine-rich high-Rb/Sr melts evolved by fractional crystallization in a magmatic system with a long crystallization time-span. Melt residence times of 300 to 700 ka and D Sr of 4.7 to 6.0 are necessary to yield the high fluorite 87 Sr/ 86 Sr ratios. These results are compatible with those deduced elsewhere from high-silica rhyolitic volcanic equivalents.