Identification Of The Solonshan Melange Belt

Melanges are widespread in the collision-type and accretionary-type orogens and are one of the hallmark units of the accretionary complexes at convergent plate margins. They record much information about the deformation and metamorphism during the subduction and accretion processes and are one of the most important marks when tracing an ancient suture zone. They can provide much reliable evidence for us to study the evolution of the subduction and accretion processes. Solonshan is located in the border between China and Mongolia, southeastern of the Central Asian Orogenic Belt. Based on the detailed field and interior work, the author identified a melange belt in Solonshan area and named it the Solonshan melange belt. In this paper, the author makes a detailed analysis about the internal litho-structural assemblies, formation age, typical fabrics, deformation characteristics, polyphase deformation and kinematics of the melange and obtains much new research progress as follows.(1) Identification of the Solonshan melange belt. The fabrics developed in the Solonshan litho-structural units are similar with the classical definition of the melange and with the typical melange cases around the world. Various blocks with different ages and origins are embedded in an argillitic, sandy, or serpentinite matrix showing high stratal disruption and a chaotic internal structure and finally form the block-in-matrix structural characteristics. All of the blocks and matrix are suffered from a certain extent of deformation and metamorphism. So we can confirm that there is a melange belt in Solonshan area.(2) The Solonshan melange belt is divided into two units which are the late Carboniferous-early Permian melange and the middle Permian ophiolitic melange. They are in thrust fault contact with each other. In the former unit, the sandstone, limestone, biohermal limestone and basalt blocks are distributed in the graywacke, siltstone, chert, mudstone and argillite matrix. Middle Permian ophiolitic melange consists of ultramafic, metagabbro, diabase, basalt, limestone and sandstone blocks which embedded into the argillite and chert matrix.(3) Through the detailed measurement of the structural section and outcrop scale structural analyses, the deformation of the late Carboniferous to early Permian unit is divided into four stages. The D1 deformation is characterized by a pervasive foliation (S1) and intrafolial isoclinally folds (F1). D2 deformation is marked by the axial plane cleavage (S2) and tight folds (F2). D3 is characterized by thrust deformation with sinistral shearing that modified the earlier structures and produced plate cleavage (S3), intrafolial (F3) and asymmetric fabric. D4 deformation superposed in D3 and produced some open folds. The deformation in middle Permian ophiolitic melange is divided into three stages. The D1 deformation is characterized by S1 foliation, pinch-and-swell, asymmetric boudins. D2 deformation is characterized by a series of thrust deformation, tight folds, rootless folds, imbricated structures accompanied with dextral shearing. D3 superimposed on the former two stages of deformation and produced the open folds. Macroscopic and microscopic structure observation shows that the ophiolitic melange is dominated by the ductile deformation with the development of a large number of mylonite, stretching lineations, S-C fabrics and reflect the middle crustal level of deformation. Kinematic analyses indicates a top to the north thrusting during the last subduction stage of the Paleo Asian Ocean and also shows that subduction polar of the southern part of the ocean is to the south.(4) New isotopic data and paleontological evidence suggest the existence of early Permian oceanic crust in Solonshan area and the formation age of the ophiolitic melange is in the middle Permian. The main evidence including (1) zircon U-Pb age of the metagabbro blocks in the ophiolitic melange is 289 ± 4 Ma and the age of basalt blocks is 281 ± 7 Ma; (2) detrital zircon dating of the argillaceous matrix shows the peak age for the youngest group of magmatic zircon is 272 ± 6 Ma which represents the maximum sedimentary age of the argillaceous matrix; (3) the chert contain some radiolarian fossils of late early Permian; (4) zircon U-Pb age of the tonalite pluton which intrudes into the ophiolitic melange is 260 ± 3 Ma and constrain the minimum formation age of the ophiolitic melange.(5) The metagabbro and basalt from the Solonshan ophiolitic melange show the characteristics of tholeiite and their REE and trace elements patterns are similar to the mid-ocean ridge (N-MORB). They also show negative of Nb, Ti anomalies. Combined with previous studies in Mandula area, northeastern of the Solonshan area, the author suggested that the ophiolitic rocks from Solonshan ophiolitic melange may form in the subduction zone (SSZ) fore-arc setting.(6) Through our study about the formation age and deformation characteristics of the Solonshan melange, the author proposed that the early Permian residual oceanic crust in Solonshan area formed the Solonshan melange in the middle Permian (272-260 Ma) by many complex tectonic processes such as offscraping and underplating. The recognition of the middle Permian Solonshan ophiolitic melange can confirm that the Paleo Asian Ocean was still existed in middle Permian. As the product of the subduction and accretionart of the Paleo Asian Ocean, the Solonshan melange records the closure of the Paleo Asian Ocean during late Permian and the formation of Solon-Xar Moron suture zone.