Early Cretaceous high-silica granite in the central southern part of North China: Implications for thinning and reworking of lower crust

The North China Craton (NCC) is widely regarded to have experienced considerable lithospheric thinning during the late Mesozoic. However, how the lower crust responded to the lithospheric thinning remains a topic of debate. Here we report zircon U-Pb-Hf isotopes, whole-rock geochemistry and Nd-Pb isotopic compositions for the Tianmushan pluton in Xinyang (Henan Province) from the central southern part of the craton. Zircon U-Pb dating of three texturally distinguishable phases from this pluton yield concordant ages of 122.4 ± 1.3 Ma for porphyritic monzogranite, 123.1 ± 1.1 Ma and 122.8 ± 0.9 Ma for medium-fine grained monzogranites, and 120.3 ± 1.4 Ma for fine-grained syenogranite. All the studied rocks have high SiO2 (74.56–77.80 wt%) and total alkali (K2O + Na2O = 8.11–9.10 wt%), and low Al2O3 (12.0–13.4 wt%), Fe2O3T (0.24–2.86 wt%) and MgO (0.09–0.26 wt%) contents with weakly peraluminous characteristic (A/CNK = 1.02–1.13). Meanwhile, they contain abundance in Ga, Rb, Nb, Y, Pb, Th, U and heavy rare earth elements (HREEs) with high 104Ga/Al and Nb/Y ratios. These geochemical features, combined with mineral morphology evidence, suggest that the Tianmushan pluton is intrinsically A1-type granite, formed in an intraplate extensional environment. The depletion in Ba, Sr, P, Eu and Ti and the “seagull” REE pattern might be attributed to extensive fractionation of plagioclase, K-feldspar, biotite, magnetite, apatite and monazite in the shallow crust. Furthermore, the granites show negative whole-rock εNd(t) values (−10.8 to −10.1), zircon εHf(t) values (−12.8 to −5.2), and relatively high initial Pb isotopes with (206Pb/204Pb)i ratios of 17.2833–17.6280, (207Pb/204Pb)i of 15.4774–15.5103 and (208Pb/204Pb)i of 38.1792–38.3587, respectively. They are comparable to the mafic granulitic xenoliths but different from those of the southern basement rocks in the craton (such as the Taihua and Xiong'er Groups), suggesting that the mafic granulitic lower crust could be the potential source for the Tianmushan granites. Together with the lithologic architecture and seismic profile of the deep crust beneath the Xinyang area, we proposed that the lower crust beneath the central southern part of North China had been further reworked and thinned in an extensional intracontinental environment after delamination in the Early Cretaceous. Moreover, the widespread occurrence of coeval A-type granitoids (130–110 Ma) in the whole eastern NCC indicates that the Early Cretaceous was a significant period of crustal extension and melting, which probably represent the peak of lithospheric thinning and cratonic destruction.