Rapid denudation of the Himalayan orogen in the Nyalam area, southern Tibet, since the Pliocene and implications for tectonics–climate coupling

Identification of an Early-Middle Jurassic oxidized magmatic belt, south Gangdese, Tibet, and geological implications

The south Gangdese region is the site of subduction of the Neo-Tethys and subsequent continental collision. Compared with widespread Cretaceous and Cenozoic magmatism, Early-Middle Jurassic magmatic rocks and related deposits are rarely reported. Our work identified a >200km long felsic rock belt associated with Cu mineralization in the south Gangdese region. We report here zircon U-Pb ages, zircon Ce⁴⁺/Ce³⁺ values, and mineral assemblages of two Cu mineralized intrusions within the belt. A hornblende granite and a diorite porphyry were emplaced at 177.3Ma and 166.3Ma, respectively. Geological occurrence and magmatic hematite-magnetite-chalcopyrite intergrowths suggest that Cu mineralization formed coeval with Jurassic intrusions. Mineralized intrusions have high zircon Ce⁴⁺/Ce³⁺ and Eu_N/Eu_N^∗ ratios, and hematite-magnetite intergrowths, suggesting their parent magmas were highly oxidized. Hornblende is common and primary fluid inclusions are found in titanite and apatite, indicating their parent magmas were water-saturated and exsolved volatile phases at early stage of magmatic evolution. Those magma characters contribute to the formation of porphyry Cu deposits. Given that majority subduction-related porphyry Cu systems have been eroded following uplift and denudation, the well-preserved Early-Middle Jurassic Cu mineralized igneous rocks in south Gangdese are favorable prospecting targets for subduction-related porphyry Cu deposits.

Neoproterozoic magmatism in eastern Himalayan terrane

Geochronological investigation on gneisses of granitic to leucogranitic compositions in Cuona, south Tibet, reveal that their protoliths formed at 808.8±7.9-816.4±3.4Ma and 855.8±7.0Ma, respectively. Zircon rims from the granitic gneiss record a metamorphic age of 739.4±4.3Ma. Lu-Hf isotopic analyses on zircon grains with Neoproterozoic ages yield negative ε_Hf(t) values from -9.0 to -4.2, and the corresponding two-stage Hf model ages are 1965-2228Ma. Whole-rock geochemical data indicate that all granitic gneisses are K-riched calc-alkali series. These new data together with literature data show that (1) the Himalayan terrane experienced an episode of Neoproterozoic magmatism at 850-800Ma; (2) the Neoproterozoic magma of granitic compositions were derived from partial melting of ancient crusts, possibly due to the thermal perturbation related with the breakup of the Rodinia supercontinent.