The geochemical evolution of the granitoid rocks in the South Qinling Belt: Insights from the Dongjiangkou and Zhashui intrusions, central China
Widespread Late Triassic granitoid rocks in the South Qinling Belt (SQB) represent excellent subjects to study the geochemical and geodynamic evolution of magmatic rocks during the collision between the North China Craton and South China Block. In this study, we report new geological, geochemical, zircon U–Pb geochronology and zircon Hf isotopic data for the Dongjiangkou and Zhashui intrusions, two large igneous complexes typical of the SQB. We also summarize published geochemical data of similar granitoid rocks. Our results show that the Dongjiangkou intrusion is composed of ca. 223–214 Ma quartz diorites, tonalites and granodiorites with abundant coeval mafic magmatic enclaves (MME), and the Zhashui intrusion consists of ca. 203–197 Ma monzogranites and K-feldspar granites with rare MME. The Dongjiangkou granitoid rocks are characterized by high K2O, MgO, Rb, and Mg# values, but low TiO2, Sc, and A/CNK (molar Al2O3/(CaO + Na2O + K2O)) values, with εHf(t) values of −18.1 to −0.4 and TDM2(Hf) values of 1141–2117 Ma.We suggest that the Dongjiangkou granitoid rocks are formed by magma mixing between 70%–90% melts derived from partial melting of the Neoproterozoic basement rocks of the SQB and 10%–30% mantle-derived mafic melts with minor involvement of Archean to Paleoproterozoic old basement materials of the SQB. The Zhashui granitoid rocks exhibit high K2O, Rb, A/CNK, AMF(Al2O3/(MgO + FeOT)), Al2O3/TiO2 and Rb/Sr values, but low MgO, TiO2, Sc, CaO/Na2O values,with εHf(t) values of−3.5 to+2.3 and TDM2(Hf) values of 979–1300 Ma, suggesting they are mainly produced by partial melting of Neoproterozoic metamorphic greywackes mixed with minor amounts of mantlederived melts. The MME are characterized by low SiO2, Sr/Y ratio, but high MgO, K2O, and Rb contents, with εHf(t) values of −0.8 to +4.3 and TDM(Hf) values of 732–905 Ma, suggesting they are produced by partial melting of metasomatized continental lithospheric mantle. A flare-up event is recognized during 225–205 Ma and the lower SiO2 and Sr/Y of the granitoid rocks during this period indicate prominent involvement of mantle-derived melts mixed with crustal melts. The granitoid rocks with higher SiO2 and Sr/Y during 225–205 Ma, however, suggest they are formed under a greater pressure than those of previous period (235–225 Ma) granitoid rocks. Few granitoid intrusions are formed after 205 Ma, and they display high SiO2 with low Sr/Y reflecting that they may be produced by low pressure partial melting of crustal materials or may represent highly fractionated magmas in the upper crust. These geochemical variations show a regular change with their formation ages, reflecting the possible variation in petrogenesis and crustal thickness. We propose that the granitoid rocks may have been formed in a transitional setting from subduction to collision during ca. 235–225 Ma, and a slab break-off event followed during ca. 225–205 Ma. Slab detachment was completed during ca. 205–190 Ma. We propose that no intensive delamination/convective removal of lower crust occurred in the SQB.