Abstract:  The Petrogenesis of the Awengcuo-Yanhu magmatic arc in Lhasa terrane is the key to solve the subduction polarity and time of the Bangong-Nujiang Tethy Ocean. In this paper, the monzonitic granite in the north Awengcuo was selected for petrography, geochemistry, zircon U-Pb geochronology and Hf isotopic studies. The zircon ²⁰⁶Pb/²³⁸U weighted mean age of the monzonitic granite is (107.0±0.5) Ma, MSWD＝2.6, belonging to the Late Early Cretaceous. The samples are a series of high-silicon and potassium-rich high-potassium calc-alkaline rocks with A/CNK values ranging from 1.006 to 1.019, belonging to weak peraluminum. The trace elements are enriched in large ion lithophile elements Rb, K, U, Th and LREE, depleted in high field strength elements such as Nb, Ta, P, Ti, with moderate to weak negative Eu anomalies (δEu = 0.55-0.78), belonging to a weakly peralumious undifferentiated I-type granite. The monzonitic granite yields the zircons ε_Hf(t) values of 3.7-6.3 with a mean of 5.0, except for one characteristic value of 11.1, and the model ages (t_DM2) range from 928 Ma to 765 Ma. Based on integrated isotopic and geochemical data, the north Awengcuo pluton is probably formed by the heterogeneous mixing of felsic magma and mafic magma generated by the melting of the juvenile crust, with the participation of a small amount of mantle-derived materials. Combined with Hf isotope analysis of magmatic rocks in the north-central Lhasa terrane, the Awengcuo-Yanhu magmatic arc was formed under the tectonic regime of the backward subduction of the Bangong-Nujiang Tethy Ocean, and the formation age of the pluton from the north Awengcuo (107-104 Ma) represents the time for the transformation from the end of slab break-off to the collision environment.

Key words: Bangong-Nujiang suture zone')">

Bangong-Nujiang suture zone, Lhasa terrane, Awengcuo, petrogenesis, tectonic setting