Petrogenesis of the Dengzhazi A-type pluton from the Taihang–Yanshan Mesozoic orogenic belts, North China Craton

The voluminous Mesozoic monzonitic to monzogranitic rocks in the north China craton (NCC) mostly show high-K calc-alkaline and I-type granitoids features. The Dengzhazi granitic pluton, however, shows features typical of A-type granites. The A-type pluton was emplaced in the Taihang–Yanshan orogenic belts of the northern margin of the NCC, with zircon U–Pb ages of around 140 Ma. The Dengzhazi A-type granites are characterized by high SiO2 (70.2–77.7 wt.%), K2O + Na2O, Zr, Nb, Ga, Zn, and Y contents as well as high Ga/Al ratios, and extremely low CaO, Ba, Sr. In addition, they show high zircon saturation temperatures (870–950 °C), low water and low oxygen fugacity. All these features are consistent with the A-type affinity of the pluton. In situ Hf isotopic analyses for the dated zircons show relatively small range of eHf(t) (−13 to −17). They also have homogeneous initial Nd isotopic compositions with eNd(t) ranging from −15.1 to −16.3. The Hf and Nd isotopic data suggest that the Dengzhazi A-type granites originated from a homogeneous crustal source, probably the Archean mafic-intermediate granulites. Taking into account the high temperatures, the low H2O and fO2 of the magma system, we believe that partial melting of the granulites should have been triggered by underplating of mantle-derived magmas at the base of the mafic lower crust in an extensional regime. The Dengzhazi A-type granite is the oldest pluton of the Taihang–Yanshan Mesozoic magma belts, signifying the commencement of extensive underplating of mafic magmas, and thus of lithospheric thinning in the northern NCC.