Geochemistry Of Sifangtai Mafic-ultramafic Complex,and Its Tectonic Implications

Sifangtai mafic-ultramafic complex, a Caledonia intrusion in the North Qinling tectonic belt, ~40km2 in outcrop area, is situated in the Zhouzhi county of SW Xi'an. It was emplaced into the Danfeng group within the Shangdan suture. This thesis mainly focus on geology, petrology, geochemistry and geochronoiogy, by which some new points are achieved:1. The Sifangtai mafic-ultramafic complex is proven to be an intrusion from the evidence of the intrusion contact between the Danfeng group and Sifangtai mafic-ultramafic complex. Sifangtai complex intruded into Danfeng group lava nX 10 Ma after the Danfeng group emplacement.2. Four different lithofacies are recognized as peridotite+pyroxenite, norite+gabbronorite, gabbro+diabase, and diorite including ultramafic rocks to intermediate rocks from core to margin of the intrusive body.3. Give a denial to the "ophiolite viewpoint of the Sifangtai complex" which purposed by other geologists. The intrusion-related structure, the distinct geochemistry difference to the typical ophiolite, the asynchronism to Danfeng ophiolite, all the above evidences support that the Siangtai mafic-ultramafic complex must not be a part of the Danfeng ophiolite.4. Evidence against arc-related magmatism for the Sifangtai mafic-ultramafic complex is achieved in this work. Geochemical approach indicates the primary magma is free of arc-related components, especially the distinct geochemistry difference to the world typical gabbros formed in the arc setting.5. Based on the detailed research on field geology, petrology, isotope geochronoiogy and geochemistry, the author suggest the Sifangtai complex is related to the delamination of subducted slabs and associated crust-mantle interaction.6. Reconstruct the tectonic evolution of Shangdan suture from Jinning to Caledonia. The formation and evolution process can be divided five stages. They are, 1) initial oceanic basins (~670Ma); 2) subducting of the oceanic slabs (~490Ma); 3) initial plate collisions (>440Ma); 4) delamination of subducted slab and the eruption of the magma (400Ma).