Petrology and biotite geochemistry of Mengku granitoids in the Changning-Menglian suture zone, southwest China: Insights into magma evolution and Sn mineralization

Abstract

The Changning-Menglian suture zone (CMSZ) is one of the major Sn metallogenic zones related to Triassic granitoids. However, the genetic relationship between magmatic petrogenesis and metallogeny is not well-constrained. Highly evolved granitoids are an important type of granitic rocks, given their close relationship with Sn mineralization. Here we provide monazite U-Pb data and comprehensive major-trace element (including halogen) data for biotites from the Mengku granitoids, to constrain their magma evolution and link to regional Sn metallogeny. Our U-Pb dating on monazite from the highly evolved granitoids yielded weighted mean age of 223.6 ± 1.4 Ma (n = 20, MSWD = 1.3). The new geochronologic results, coupled with recently reported ages from the Lincang batholith, suggest that most of the Lincang granitoids formed within a relatively consistent period at ca. 230–215 Ma. Geochemically, these Triassic granitoids can be divided into three groups. Group 1 granitoids display low Al2O3, P2O5, FeOT + MgO, Eu, Sr and Zr contents with high differentiation index (DI = 88–94) and A/CNK (1.07–1.17), consistent with the features of highly evolved granites. In contrast, Group 2 monzogranites and Group 3 dioritic enclaves have higher TiO2, P2O5, FeOT + MgO, Eu, Sr and Zr contents, comparable to I-type granites. All magmatic biotite grains are Mg-rich in composition (XMg = 0.44–0.60). A positive correlation between F and MnO contents in biotites from Mengku granitoids indicate that more evolved Group 1 magmas underwent fractional crystallization. The most evolved magmas (Group 1) have relatively low log(fH2O/fHF) values (4.34–5.05) and high log(fHF/fHCl) values (4.85–5.39), suggesting lower Cl contents and higher F contents than other magmas. Oxygen fugacities (logfO2) calculated from biotite equilibria for the Mengku granitoids are –22.2 to −17.7, straddling on the NNO and QFM buffers. Low log(fO2) values and positive correlation between Sn and log(fHF/fHCl) suggest that relatively reduced condition, highly evolved and F-rich granitic magmas are conducive to Sn mineralization in the CMSZ.