Characteristics And Tectonic Significance Of Nawucuo’s Magmatite In The Western Segment Of Bangonghu-nu Jiang Suture

Based on the geological survey in the field, intermediate-acid intrusive and intermediate-basic volcanic rock of the Early Cretaceous Qushenla group samples from Nawucuo in the western segment of Bangonghu-Nujiang suture zone were selected for detailed studies, containing petrologic, geochemical and zircon U-Pb geochronology studies, find out the main types, combination of rocks and its rule of spatial and temporal distribution; to analysis and discuss its magmatic activity and tectonic environment; also discuss its subduction direction and the tethys ocean’s time of subduction.In junction, diorite porphyrite on the dominate of intermediate-acid intrusive, quartz diorite porphyrite, quartz monzodiorite, quartz plagiophyre, plagioclase granite porphyry, granodiorite porphyry occur sporadic, belonging to calc-alkaline and high-K calc alkaline series; displaying a steeply right declined REE pattern with weakly negative Eu anomalies, show that the degree of magmatic differentiation is lower, and plagioclase fractional crystallization is not obvious. Rb、K、Ba are enriched and Nb、Ta、Ti、P are depleted, as a whole, the curve is flat, similar to the characteristics and evolution trend of arc-type calc-alkaline. In the tectonic environment discrimination diagram shows that the have the volcanic arc and syn-collision granite characteristics, explained its formed in the subduction phase of the Bangonghu-Nujiang tethyan oceanic crust.Intermediate-acid intrusive of yanhu belong to high-K calc alkaline and calc-alkaline series. REE pattern diagram shows that they are richer in LREE with not obvious Eu anomaly, displayed the source from crust-mantle. The right shelving of primitive mantle-normalized indicating that its have the feature of island arc-type with subduction magmatic. And all the samples fall into the VAG area, illustrate its formed in the active continental margin with Bangonghu-Nujiang’s Tethys oceanic of south subduction stage. The belt and yanhu rocks have similar characteristics on the petrologic and geochemical, its petrogenesis from the partial melting of mantle wedge of the subduction zone, occurred metasomatism with the sediment melt from subduction plate. The method of Zircon U-Pb dating reports the age of quartz diorite porphyrite is 104.5Ma, K feldspar granite is 100.7Ma, quartz monzodiorite is 108.3Ma. Considering field survey data, we known its mainly diagenetic era from as early as the Early Cretaceous to the Late Cretaceous. Suggest its magmatic arc granite which formed in the environment of subduction.Volcanic rocks of the Early Cretaceous Qushenla are series of basic-intermediate volcanic rocks, their REE elements show a right pattern with LREE weak rich, and Eu takes on weak anomaly; trace elements enriched in LILE, such as Rb、Ba、Th、U,depleted in HFSE, such as Nb、Ta, clearly displaying compositional characteristics of island-arc volcanic rock. Explain its formed in the southward subduction of Bangonghu-Nujiang tethyan ocean.