Abstract:

The Gangdese magmatic belt is a huge tectonic-magmatic belt, which is nearly E-W directed along the Yarlung-Zangbo suture zone. Mesozoic magmatism in the southern Gangdese experienced four stages: 205-152 Ma, 109-80 Ma, 65-41 Ma and 33-13 Ma, all of which were the products of a compression environment. However, the triggering mechanism has been controversial. Focus of controversy lies in the subduction mode of the Neotethys and the Bangong-Nujiang Ocean. In order to explain the genesis and tectonic environment of Mesozoic intermediate-acid magmatic rocks in the southern margin of Gangdese belt, we analyzed the quartz monzonite in Sangye area (from Zhanang County, Tibet) and discussed the magmatic origin, petrogenesis and tectonic environment through geochronology, whole-rock geochemistry and Lu-Hf isotopes. LA-ICP-MS zircon U-Pb dating shows that the intrusion was formed in the Late Cretaceous (91-88 Ma). The rocks are metaluminous and high-K calc-alkaline. They are enriched in LILEs and LREEs, depleted in HFSEs and HREEs, with no obvious negative Eu anomaly. ε_Hf(t) value is +10.6 to +14.2. Quartz monzonite magma was likely formed in the transition zone between the plagioclase and garnet stability fields. The magma may have experienced the fractionation of amphibole, apatite and biotite. The typical trace element ratios indicate that the magma was originated from partial melting of the young subducted oceanic crust and the addition of minor juvenile mantle-derived materials, which is I-type granite. We inferred that it was formed in the subduction-related island arc setting, indicating that the Late Cretaceous southern margin of Gangdese belt was influenced by the north-dipping subduction of the Neotethys.

Key words: geochemistry, zircon U-Pb dating, Lu-Hf isotope, Late Cretaceous, southern Gangdese margin