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Tin-specialized granites, Protolith, Fractional crystallization, Metasediment melting
Abstract:
Endogenic tin (Sn) mineralization is associated with peraluminous granites that are derived from partial melting of metasedimentary rocks. These melts commonly experienced extensive fractional crystallization. There is increasing evidence that the nature of the protoliths is essential for the formation of Sn-specialized granites. Whether a Sn-rich protolith can produce a Sn-specialized granite at a minor extent of fractionation or from a small source volume, however, remains a matter of debate. We performed fluid-absent melting experiments at 750–950 ◦C, 300 and 900 MPa, and at oxygen fugacity of ~FMQ - 1.4 to ~FMQ + 2.9 on a Sn-rich, feldsparpoor metasedimentary schist. Our experimental results show that feldspar-poor schists produce larger proportions of melt than feldspar-bearing metasedimentary rocks under similar P–T conditions. Because of the Snrich nature of the studied protolith, these melts have high Sn contents (>2500 ppm), which implies that depending on Sn enrichment in the source even small intrusions may produce major mineralization. Apart from the Sn contents in the protoliths, the concentrations of Sn in melts are primarily a function of temperature, suggesting that efficient Sn mobilization requires high-temperature melting, which requires input of heat from the mantle. Our findings offer new insights into the origin of Sn-specialized granites and suggest a potential paradigm shift from “fractionation only” to “protolith is essential”. This change may also be relevant for the understanding of the genesis of other granite-related ore systems.
