Petrochemical and isotopic evidences for multiple sources involving in the generation of post-collisional granitoid complex: A case study of the Mangling complex from the eastern Qinling orogen, China

Abstract

Granitoids widely occur in collisional orogens, which are a critical target for probing the nature of deep crust and mantle-crust interaction. In this study, we present an integrated study of in situ zircon U–Pb dating and Hf–O isotopes, in combination with whole-rock geochemistry, for the Mangling complex in the eastern Qinling orogen. The Mangling complex is composed mainly of monzogranites with some mafic microgranular enclaves (MMEs) and subordinate diorites. Zircon U–Pb dating yields identical crystallization ages of 141–143 Ma for the diorites and monzogranites, suggesting that they are formed simultaneously and belong to post-collisional magmatism. The Mangling diorites have relatively low SiO2 but high MgO, Cr and Ni contents and Mg# values. They are characterized by enrichment of large ion lithophile elements (LILEs) but depletion of high field-strength elements (HFSEs). They show initial Sr isotope (ISr) of 0.706491 to0.706622, εNd(t) of −10.82 to −10.38, and zircon εHf(t) of −10.8 to −7.9 and δ18O values of 6.48–7.18‰. These geochemical features indicate that the diorites were derived from an enriched lithospheric mantle source. In contrast, the Mangling monzogranites have high silica and low Mg# values. They exhibit more variable isotopic compositions: Isr = 0.707191–0.708310, εNd(t) = −13.74 to −8.10, and zircon εHf(t) = −17.2 to −7.0 and δ18O = 5.83–7.08‰. Some inherited zircon cores have been observed in the monzogranite samples with U–Pb ages of 1773–1791 Ma. These inherited cores have δ18O values of 5.42–5.69‰ and εHf(t) values of −7.8 to −5.9. Their U–Pb ages and Hf–O isotopic compositions are similar to those of the Xiong'er Group in the southern margin of the North China Craton (NCC), which have underthrusted the juvenile (in terms of isotopes) Kuanping unit where the Mangling complex has been emplaced. Isotopic modeling shows that the monzogranites were likely produced by fusion of the Kuanping unit and Xiong'er Group, with the involvement of enriched mantle-derived materials. The crustal rocks as well as the enriched sub-lithospheric mantle were both contributed to the Mangling complex, and some of the crustal components show more juvenile isotopic fingerprints. Therefore, it is not a mandatory to involve a depleted juvenile mantle-derived component to recognize mantle-crust interaction during granitoid generation, especially in which the preceded juvenile arcs were superimposed on ancient crust and/or enriched mantle. The ‘inverse’ isotopic compositions in crust and mantle components imply that a detailed petrochemical and isotopic study should be conducted to address the issue about crust-mantle interaction during granitoid generation.