The geochemistry and geochronology of Permian granitoids from Central Inner Mongolia, NE China: Petrogenesis and tectonic implications

Abstract Permian granitoids have been identified in the Zhalageamu district, Inner Mongolia, NE China, which provide an excellent opportunity to constrain the tectonic evolution of the Central Asian Orogenic Belt (CAOB). In this study, we determined the zircon U Pb ages of the quartz diorite, granodiorite porphyry, diorite porphyry and granite porphyry at Zhalageamu. These ages are 269.5 ± 1.8, 261.4 ± 1.9, 256.4 ± 2.0 and 255.2 ± 2.0 Ma, respectively. The granitoids have high contents of SiO2 (61.95–75.41 wt%) and Al2O3 (11.87–15.88 wt%), and their A/CNK ratios range from 0.91 to 1.44. Their Na2O + K2O contents range from 6.04 to 9.28 wt% and help classify them as having calc-alkaline to high-K calc-alkaline affinity. The low P2O5 contents of the quartz diorite, diorite porphyry and granite porphyry and their decrease with increasing SiO2, in combination with low 10,000*Ga/Al ratios (1.51–2.58) and Zr + Nb + Ce + Y contents (128–264 ppm) identify them as I-type granitoids, whereas the strongly peraluminous character of the granodiorite porphyry classifies it as an S-type granite. These I-type granitoids are characterized by an enrichment in large ion lithophile elements (LILE; e.g., Rb, Ba and Th) and a depletion in high field strength elements (HFSE; e.g., Nb, Ta and Ti). This, and their weakly negative Eu anomalies on chondrite-normalized REE diagrams, are features of typical arc magmatic rocks. The ɛNd(t) values and initial 87Sr/86Sr ratios range from 2.9 to 4.2 and 0.7010 to 0.7039, respectively, and both sets of values point to a mantle source. Hafnium isotopic analysis of quartz diorite yielded positive eHf(t) values of 8.5–12.3 and young two-stage Hf model ages (TDM2) of 0.51–0.72 Ga, suggesting a juvenile crustal source. These features are similar to those of locally occurring coeval volcanic arc rocks, indicating that the Zhalageamu granitoids mostly formed by partial melting of juvenile mantle-derived crust. Our geochemical and Sr-Nd-Hf isotopic data show that the Zhalageamu granitoids were generated in a subduction setting related to the northward subduction of the Paleo-Asian Ocean.