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

Abstract

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 crust. The large-scale dyke swarm with bimodal signature suggests an extensional setting in the Carboniferous continental arc. Combined with the age data (305–299 Ma) of the Late Carboniferous bimodal magmatic rocks in the study area which denote the maximum age of the slab break-off, the emplacement time of the dyke swarm (289–274 Ma) provides robust constraints on the Early Permian extensional tectonic regime (305–274 Ma) in the Hegenshan accretionary belt. The Early Permian extension in this belt occurred prior to the closure of the Paleo-Asian Ocean and formation of the Central Asian Orogenic Belt, and the Hegenshan belt likely represents a back-arc formed through the northward subduction of the Paleo-Asian Ocean during Permian.