Formation of the Late Cretaceous-Paleogene Sn-rich Granites in the Tengchong Block:The Magma Origin and Differentiation Conditions

Tin metallogenesis in Tengchong block is associated with the Late Cretaceous and Paleocene magmatism. However, only the ore-forming granitoids are characterized by high Sn content (25 x 10(-6) on average). The regional contemporary unmineralized granitoids hardly show Sn enrichment. By collecting and analyzing available research material, the differences between the Sn-rich ore-forming rocks and regional unmineralized rocks are summarized in terms of their magma origins, evolution conditions and differentiation degrees. Zircon epsilon(Hf)(t) Hf values (- 15. 1 similar to + 3. 39) of the regional unmineralized rocks indicate that the magmas in this area are added with increasing mantle material from northeast to southwest, while the addition of the mantel material is disadvantageous to tin enrichment in the mixing magma according to the Sn content (1. 61 x 10(-6)) of the regional basalt. Part of the regional unmineralized granitoids formed in Late Cretaceous have the same epsilon(Hf)(t) values of -9. 7 with the Sn-rich ore-forming rocks, suggesting both of them were derived from partial melting of ancient crustal material. Considering these rocks are all metaluminous or weak peraluminous, and the crustal model age (t(DM2)) calculated from zircon Hf isotopes (1724Ma) are nearly identical to that from the whole-rock Nd isotopes (1836Ma) , the magma may not be derived from the Sn-rich Gaoligongshan Group consisting of metasedimentary rocks, but probably the inner metamorphic granitoids. The metamorphic granitoids are at least 30km underground (corresponding to 8. 4kbar) based on the regional strata thickness (28km) and the Moho depth (47 similar to 35km). The initial melting temperature (1066 degrees C) is unable to be reached according to geothermal gradient (25 degrees C/km) , therefore, the partial melting process is likely influenced by mantle heat. According to Fe2O3/FeO values, both the unmineralized granitoids (0. 59) and the tin mineralized granitoids (0. 48) have low oxygen fugacity, and belong to ilmenite series. While the fractionation of Sn-rich granitoids is much higher than that of unmineralized granitoids, moreover, the Sn-rich granitoids contain abundant volatiles, which decrease the magma consolidation temperature (650 similar to 550 degrees C) , prolong magma differentiation process, and promote Sn gathering in the late magma. In conclusion, the Sn-rich granitoids in the Tengchong block is mainly resulted from the high fractionation of normal magma under a low oxygen fugacity.