Unraveling the link between mantle upwelling and formation of Sn-bearing granitic rocks in the world-class Dachang tin district, South China

Increasing evidence shows that the man-tle contributes (directly or indirectly) to Sn-bearing granites worldwide. However, the specific role of mantle in the formation of tin granites and related mineralization remains poorly understood. In the world-class Dachang district, South China, tin mineralization is related to the Longxiang-gai equigranular/porphyritic biotite granites and tin orebodies are cut by granite por-phyry dykes hosting mafic microgranular enclaves (MMEs). A combination of zircon U-Pb dating and Hf-O isotopes, mineral chemistry, and whole-rock elemental and Sr-Nd isotopic compositions-for granitic rocks and MMEs, is employed to constrain the petrogenesis and to unravel the link between tin fertility and mantle upwell-ing. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating indicates that the biotite granites were emplaced at ca. 93 Ma, and the granite porphyry dykes and MMEs were formed at ca. 86 Ma. The biotite granites are silica-and alkali-enriched with A/CNK ratios of 1.04-1.36, and exhibit elevated con-centrations of Li, F, P, Rb, Cs, Ta, Sn, W, and U, showing affinities with highly fraction-ated S-type granites. Whole-rock geochemi-cal and Nd isotopic (epsilon(Nd)(t) = -10.0 to -7.8) data, and in situ zircon Hf-O (epsilon(Hf)(t) = -9.9 to -3.9, delta O-18 = 6.2-8.9%0) isotopes indicate that the biotite granites were formed by partial melting of metasedimentary rocks at relatively high temperatures (>= 782 degrees C), possibly with minor input of mantle mate-rial. Likewise, the post-ore granite porphyry dykes have similar chemical and mineral-ogical characteristics as fractionated S-type granites. Zircon Hf-O isotopes (epsilon(Hf)(t) = -9.0 to -4.9, delta O-18 = 6.5-8.2 parts per thousand) and whole-rock geochemical data suggest they were derived from a similar source as the biotite granites, whereas elevated epsilon(Nd)(t) values of -5.0 to -3.3 for granite porphyry dykes relative to biotite granites reveal an increasing mantle input. Distinct epsilon(Nd)(t) (-0.4 and -0.3) and zircon Hf-O (epsilon Hf(t) = 1.5-5.0, delta O-18 = 6.5-7.2 parts per thousand) iso-topes of the MMEs, suggest that the mafic melt could be sourced from the astheno-spheric mantle, contaminated by subconti-nental lithospheric mantle/continental crust during magma ascent, and hybridized by fel -sic melt at emplacement-level. The magmatic sequence in the Dachang district is indica-tive of an extensional tectonic setting where mantle-derived magmas are predicted to mi-grate to shallower crustal levels as the crust progressively becomes thinner and hotter. High-temperature partial melting of mature metasedimentary crust triggered by heat in-put from the upwelled mantle, may contrib-ute to biotite breakdown, which is important for concentrating tin in melts. Fractional crystallization of initially Sn-rich felsic melts under reduced conditions makes further tin enrichment and produces Sn-bearing gran-ites (the Longxianggai pluton). Prolonged mantle upwelling results in distinct magma mixing and the formation of granite por-phyry dykes and MMEs. These dykes are highly fractionated with elevated Sn and W contents, which show great potential to form hydrothermal Sn-W mineralization.