Permian dyke swarm with bimodal affinity from the Hegenshan ophiolite-arc-accretionary belt, Central Inner Mongolia: Implications on lithospheric extension in a Carboniferous continental arc

The Hegenshan ophiolite-arc-accretionary belt adjacent to the Solonker suture in central Inner Mongolia marks the closure of a back-arc ocean basin and an Early Permian extensional setting is a critical region to understand the final closure of the Paleo-Asian Ocean in relation to the construction of the Central Asian Orogenic Belt (CAOB). Since the Paleo-Asian Ocean still existed in Solonker during Permian, the Early Permian extensional regime as indicated by the widely distributed Permian A-type granites in this accretionary belt remains enigmatic. In order to address this issue, we investigate the large-scale Early Permian EW-trending dyke swarm in the Hegenshan belt through an integrated study on petrology, zircon U-Pb isotopic ages, whole rock major-trace elements, and Sr-Nd-Pb isotopes. These dykes were emplaced in the volcanic-plutonic complex of the Carboniferous continental arc. Our zircon U-Pb results show an age range of 289.9 ± 3.6 Ma to 274.6 ± 4.6 Ma for the dyke emplacement, which is slightly younger than the post-subduction bimodal volcanic-plutonic suite (305–299 Ma) which is closely associated with the dyke swarm. Based on field characteristics and geochemical data on the dykes, we recognize two magma types: calc-alkaline dykes and A-type granitic dykes. The calc-alkaline dykes include diorite and granodiorite, whereas the A-type granitic dykes are composed of granite porphyry and rhyolite, corresponding to a major phase of Early Permian bimodal magmatism in the Hegenshan belt. The diorite-granodiorite dykes exhibit a continuous evolutionary trend of calc-alkaline magma from gabbro through diorite to granodiorite. They also display enrichment in LILEs and LREEs, and depletion of HFSEs. These characteristics are consistent with the Late Carboniferous calc-alkaline plutons which are spatially and temporally associated with the diorite-granodiorite dykes. We therefore infer a common magma system, with the parent magma sourced from enriched lithosphere mantle. The granite porphyry-rhyolite dykes show marked consistency in geochemical and Sr-Nd-Pb isotopic compositions with the Late Carboniferous A-type granite and rhyolite in this area. Compared with the calc-alkaline diorite-granodiorite dykes, the granite porphyry-rhyolite dykes have significantly higher Rb, Th, HFSEs (Nb, Ta), HREEs (Yb) and Y, and lower Sr and Ba. These rocks exhibit the features of A2-type granites, possibly inherited from pre-existing subduction-related juvenile lower c