A westward propagating slab tear model for Late Triassic Qinling Orogenic Belt geodynamic evolution: Insights from the petrogenesis of the Caoping and Shahewan intrusions, central China

摘要

Late Triassic granitoid intrusions are widespread in the South Qinling Belt (SQB), providing excellent subjects to understand the geodynamic evolution of the Qinling Orogenic Belt and the collision between the North China Craton (NCC) and Yangtze Craton (YZC). This study shows newly obtained geological, geochemical and zircon U-Pb-Hf isotopic data of the Caoping and Shahewan intrusions, revealing that the Caoping intrusion consists of similar to 215 Ma fined-grained granites, and similar to 221-215 Ma porphyritic and coarse to medium-grained tonalites, granodiorites and monzogranites, which assemble with coeval mafic magmatic enclaves (MMEs). The Shahewan intrusion is composed of similar to 215-210 Ma porphyritic granodiorites and monzogranites, which also assemble with coeval MMEs. The fine-grained granites from the Caoping intrusion are characterized by high SiO2, Rb and (La/Yb)(N) ratio, but low MgO, CaO and Sc contents, with epsilon(Hf)(t) values of -8.6 to +43 and T-DM2(Hf) ages of 883-1596 Ma, suggesting that they are mainly derived from partial melting of the Meso- to Neoproterozoic metagreywackes. The porphyritic and coarse to medium-grained granitoid rocks from both Caoping and Shahewan intrusions are characterized by higher MgO, CaO, Sc, Mg# values, but low SiO2, Rb contents and (La/Yb)(N) ratio, with epsilon(Hf)(t) values of -0.7 to +2.8 and T-DM2(Hf) values of 961-1158 Ma, suggesting that they are mainly formed by magma mixing between melts that were derived from Meso- to Neoproterozoic basement rocks of the SQB and metasomatized lithospheric mantle. The MMEs from Caoping and Shahewan intrusions are characterized by low SiO2, Sr/Y ratio, high MgO, K2O, Rb, Sc, total REE contents, with epsilon(Hf)(t) values of +0.5 to +6.1 and T-DM(Hf) values of 661-846 Ma, suggesting that they are produced by partial melting of metasomatized lithospheric mantle. The rapakivi-like textures of the rocks from Shahewan intrusion may be caused by continued underplating and injection of mafic magina resulting in higher temperature (T-Zr = 770-817 degrees C, comparing with Caoping intrusion of T-Zr = 727-773 degrees C), together with the magma mixing and isothermal decompression processes. Integrated with previous regional data, the SQB shows that mantle-derived magmatic activations initiated at similar to 234 Ma near Wudang in the east, and westward systematically decreased to similar to 222-219 Ma near Caoping and to 212-208 Ma near Yangba. We attribute this temporal-spatial distribution of mantle-derived magmatism within the SQB to the progressive tear of the subducted oceanic slab. Combined with sedimentary studies on the foreland basins, paleomagnetic studies as well as numerical and seismic tomography models of slab break-off, we propose that the similar to 225-205 Ma magmatism in the SQB resulted from a westward slab tear following the collision between the NCC and YZC. (C) 2016 Elsevier B.V. All rights reserved.