Petrogenesis of Late Cretaceous Muscovite-Bearing Peraluminous Granites in the Youjiang Basin, South China Block: Implications for Tin Mineralization

Abstract

Most primary Sn deposits worldwide are associated with muscovite-bearing peraluminous granites, commonly believed to originate from the partial melting of metasedimentary rocks. We studied the whole-rock geochemistry and Sm–Nd isotopes of Late Cretaceous (~90 Ma) Laojunshan muscovite-bearing peraluminous granites in the Youjiang Basin, South China Block. The globally significant Dulong tin mineralization was co-genetic with the Laojunshan muscovite-bearing monzogranites. The Laojunshan granites exhibit slightly higher εNd(t) values than the Precambrian basement, indicating a hybrid crustal source comprising both Precambrian rock and juvenile components. Characterized by weakly peraluminous compositions, these granites display highly evolved geochemical features: notably low levels of Ca, P, Mg, Fe, and Ti contents, elevated Si content, a high FeOT/MgO ratio, and a low Zr/Hf ratio. These distinctive geochemical features can be attributed to the differentiation of plagioclase, biotite, and zircons, with the remarkably low Nb/Ta and K/Rb ratios further suggesting a fluid exsolution process. The geochemical data propose that tin-enriched Laojunshan granites originate from mineral differentiation and fluid exsolution of crust-derived melts during magmatic evolution. By integrating these novel findings with existing data on coeval muscovite-bearing granites co-genetic with tin mineralization in the Youjiang Basin, it is deduced that these granites share a unified origin. Their genesis can be attributed to mineral differentiation and fluid exsolution of crust-derived melts rather than a direct melting of metasedimentary rocks.