Anatomy of two Permian greenschist- to blueschist-facies tectonic mélanges in the Solonker Suture Zone (Inner Mongolia, NE China): Evidences for divergent double subduction and soft collision

The timing and processes of the Paleo–Asian Ocean (PAO) closure are still controversial issues due to ambiguous interpretations of the regional geological records. We describe the structural and kinematic features of two Permian greenschist-/blueschist-facies tectonic mélanges at Zhurihe and Erdaojing in Inner Mongolia (northeastern China), separated by the Solonker Suture. The tectonic mélanges have block-in-matrix fabrics. The Zhurihe tectonic mélange in the south is dominated by NE-trending folds, and E–W to NE-trending reverse/strike-slip faults, indicating NW–SE compressive direction. Blocks in the Zhurihe mélange include ophiolitic fragments (~292 Ma) and ocean island sequences (~274 Ma), which show E–MORB and OIB geochemical affinities, respectively. The matrix includes blueschist, greenschist, quartz schist and clastics, of which the blueschist yielded a zircon U–Pb age of ~255 Ma, and is geochemically E–MORB-like. The Erdaojing mélange north of the Solonker Suture is dominated by E–W trending folds, and NE- and NW-trending reverse faults, indicating a N–S compressive orientation. The Erdaojing mélange is composed of early Permian (~281 Ma) ophiolite blocks, and middle Permian (~266 Ma) actinolite schist and clastics as the matrix. The Erdaojing ophiolitic rocks are geochemically N–MORB tholeiitic. Our results define two parallel Permian MORB-type ophiolitic belts separated by the central Solonker Suture. These observations evidenced the Permian divergent double subduction and Early Triassic soft-collision model for the PAO along the Solonker Suture.