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Neoproterozoic magmatism associated with the assembly and configuration of the Rodinia supercontinent is widely distributed in the India-Himalayan terrane. However, its petrogenesis and tectonic settings remain controversial. This study provides new geochronological and geochemical data on the Neoproterozoic bimodal magmatism from the eastern Himalayan orogen. In situ zircon U Pb dating revealed that the protoliths of amphibolites were emplaced at ca. 826 Ma and the granitic gneisses have crystallization ages of 825–820 Ma. The granitic gneisses exhibit geochemical features of A-type granites, with high initial (87Sr/86Sr)i ratio (0.7182–0.7394), low whole-rock eNd(t) (−8.4 to −6.6), and variable zircon eHf(t) (−7.4 to +1.0) values. They were probably generated by partial melting of the ancient lower crust with minor input of mantle components. The amphibolite samples are enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs), suggesting an arc affinity. They have relatively high initial (87Sr/86Sr)i ratios (0.7113–0.7136), low whole-rock eNd(t) (−1.1 to 1.4) and a wide range of zircon eHf(t) (−4.1 to 8.3) values, indicating that the protoliths of amphibolites were likely generated by partial melting of an enriched subduction-modified continental lithospheric mantle. Their geochemical signatures are similar to typical back-arc basin basalts. The presence of coeval A-type granites and arc-related mafic rocks is probably due to the existence of a back-arc system. We argue that the Neoproterozoic bimodal magmatism is a product of back-arc extension initiated at an early stage, resulting from the rollback of the Mozambique Oceanic slab. Combined with previous studies on Neoproterozoic magmas from India and the Himalayas, we suggest that an extensive Neoproterozoic back-arc system may have existed along the northwestern margin of the Rodinia supercontinent. This theory supports a scenario of an Andean-type continental margin for the India-Himalayan terrane during the middle Neoproterozoic.

The Cuonadong deposit is located in southern Tibet and represents the first large-scale tin polymetallic deposit in the Himalayan region. Sn-(W) skarn mineralization is spatially related to leucogranites, whereas Be-Nb-Ta mineralization mainly develops in pegmatites. Alteration and veining in this deposit can be divided in to five stages: prograde skarn (stage I), retrograde skarn (stage II), cassiterite-quartz vein (stage III), cassiterite-sulfide vein (stage IV) and fluorite quartz vein (stage V). 40Ar 39Ar dating of muscovite from the skarn and phlogopite from the cassiterite-sulfide vein yield isochron ages of 15.4 ± 0.3 Ma and 15.0 ± 0.3 Ma, respectively, whereas U Pb dating of cassiterite from the skarn yields a Tera-Wasserburg low-intercept precise age of 14.2 ± 0.2 Ma. Zircon and monazite U Pb ages of the causative stanniferous leucogranites are 15.3 ± 0.1 Ma and 14.9 ± 0.2 Ma, respectively. Mineralization ages are consistent with the emplaced ages of the granites within the analytical uncertainties, which indicates that the tin polymetallic mineralization is genetically related to the Miocene leucogranites. Sn-bearing leucogranites have zircon eHf(t) values that vary from −13.3 to −8.5 (-10.7 on average), with an average TDM2 value of 1.56 Ga, which reveals that the leucogranites are derived from partial melting of ancient metasedimentary rocks. According to the zircon trace elements, the Miocene leucogranites are highly differentiated reduced S-type granites that formed at relatively high temperature. The average homogenization temperatures of fluid inclusions in different minerals that formed at stages II, III, IV and V are 351, 315, 240 and 175 °C, respectively, whereas their salinities are 8.3, 4.9, 9.5 and 3.8 wt% NaCl equiv., respectively. The C-H-O-S-Pb isotopes indicate that the ore-forming fluid and materials mainly originated from Miocene leucogranite. Dehydration and partial melting of mica in the Greater Himalayan crystalline complex due to east-west extension at 18–14 Ma developed the stanniferous leucogranite and ore-controlling fault system. Because of the pervasive occurrence of gneiss domes and Miocene Sn-bearing leucogranites similar to Cuonadong, the Himalaya has strong potential to be a new globally important Sn-(W) rare metal metallogenic belt.