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

• Published in: Lithos 2016-10, Vol.262, p.486-506
• Main Authors: Hu, Fangyang, Liu, Shuwen, Zhang, Wanyi, Deng, Zhengbin, Chen, Xu
• Format: Article
• Language: English
• Subjects: Caoping and Shahewan intrusions; Petrogenesis; South Qinling Belt; Westward slab tear; Zircon geochronology and Hf isotopes
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2016.07.034

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 ~215Ma fined-grained granites, and ~221–215Ma porphyritic and coarse to medium-grained tonalites, granodiorites and monzogranites, which assemble with coeval mafic magmatic enclaves (MMEs). The Shahewan intrusion is composed of ~215–210Ma 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 εHf(t) values of −8.6 to +4.3 and TDM2(Hf) ages of 883–1596Ma, 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 εHf(t) values of −0.7 to +2.8 and TDM2(Hf) values of 961–1158Ma, 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 εHf(t) values of +0.5 to +6.1 and TDM(Hf) values of 661–846Ma, 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 magma resulting in higher temperature (TZr=770–817°C, comparing with Caoping intrusion of TZr=727–773°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 ~234Ma near Wudang in the east, and westward systematically decreased to ~222–219Ma near Caoping and to 212–208Ma near Yangba. We attribute this temporal–spatial distribution of mantle-derived magmatism within the SQB to the progressive tear of the subducted ocean