Late Permian intermediate and felsic intrusions in the eastern Central Asian Orogenic Belt: final-stage magmatic record of Paleo-Asian Oceanic subduction?

Abstract

The Central Asian Orogenic Belt (CAOB), which formed by closure of the Paleo-Asian Ocean, is one of the largest Phanerozoic accretionary orogens on Earth. However, the timing of final closure of the Paleo-Asian Ocean remains debated. Here we present new zircon U-Pb dating, oxygen fugacity estimates, and whole-rock geochemical and Sr-Nd isotopic data from intermediate and felsic intrusive rocks in the Chunhua area of NE China to constrain their petrogenesis and assess the tectonic implications for the evolution of the eastern CAOB. Zircon U-Pb dating indicates that the intrusive rocks formed in the late Permian (257-261 Ma). Based on their chemical compositions, two groups of rocks can be identified. Group I consists mainly of dioritic rocks (e.g., gabbroic diorites, diorites and tonalites) with relatively low SiO2 but high MgO contents and Mg# values. Petrological and geochemical characteristics show that Group I rocks were formed by mixing of mantle- and crust-derived magmas. Mafic microgranular enclaves within Group I rocks have high MgO contents (up to 9 wt%) and depleted isotopic compositions (epsilon(Nd)(t) = +4.3 to +7.2), and probably represent mafic end-member magmas derived from a metasomatized lithospheric mantle, which subsequently were injected into the felsic crustal magmas. In contrast, Group II rocks are mostly granodiorites characterized by high SiO2 and low Mg# values, with high Sr/Y and La/Yb ratios. These rocks were probably generated by partial melting of mafic tower crust with little or no addition of mantle materials. Trace element modeling suggests that intracrustal differentiation was dominated by homblende fractionation and caused the high Sr/Y and La/Yb signatures of Group II rocks. Their low (Sr-87/Sr-86)(i) (0.70345-0.70485) and positive epsilon(Nd)(t) values (+3.8 to +4.6) suggest predominantly juvenile sources, typical of newly underplated mafic lower crust. The low oxidation states (FMQ-1.1 to FMQ-7.9) for both Group I and Group II rocks, as indicated by Ce4+/Ce3+ ratios in zircon, imply reducing conditions for the lithospheric mantle and the overlying lower-crustal source rocks. The Chunhua intrusions show petrological and geochemical characteristics of arc-type igneous rocks, including abundant euhedral hydrous minerals, talc-alkaline affinities, and enrichment in large ion lithophile elements (e.g., Rb, Ba, and Pb) and depletion in high field strength elements (e.g., Nb, Ta, and Ti), indicating that they formed in a subduction-related setting. Given the occurrences of Early Triassic syn-collisional granites (235-252 Ma) in the area, the late Permian intrusions (257-261 Ma) likely represent the final stages of magmatism related to subduction of Paleo-Asian Ocean, signifying that final closure of the Paleo-Asian Ocean in the eastemmost CAOB occurred during the end-Permian. (C) 2018 Elsevier B.V. All rights reserved.