Closure mechanism of the Nenjiang Ocean: Constraint from the deformation pattern and age of the Yinder Complex in Jalaid Banner area, SE Inner Mongolia, China

The closure mechanism of Nenjiang Ocean (NJO) is a controversial scientific issue in the Great Xing'an Range area. The newly discovered Yinder Complex, located in the basin and mountain conjunction belt of the Great Xing'an Range and Songliao Basin, is a typical subduction-accretionary related m lange. This complex is composed of blocks of diorite, gabbro, basaltic andesite and granite within the matrix of sericite schist, chlorite schist and actinite schist, showing typical block -in -matrix texture. Field observation and microscopic fabric analysis disclosed that two deformation stages occurred in the complex: the early -stage deformation was featured by NW -dipping schistosity, gneissic schistosity and mylonite foliation, and with NNW -dipping lineation with high angle, formed by NW -SE intense compression; while the late one by NE -dipping subhorizontal stretching lineation with low angle. The quartz, K feldspar, plagioclase, mica and other minerals were clearly elongated by intracrystalline dislocation slip and bulk recrystallization, indicating a NE -SW left strike -slip shear movement. The two deformation stages are corresponding to collisional compression and postcollisional stretching strike -slip of the Xing'an Accretionary Terrane ( XAT) and Songnen Block ( SNB), respectively, and the early stage lineation were commonly replaced by the late one. The contact relationships and geochronological data of the components of the Yinder Complex limit its deformations occurred before the late Early Carboniferous ( ca. 323Ma). During the Early Carboniferous, the NJO initiated a "bi-directional subduction" model beneath SNB to the southeast and XAT to the northwest, and formed plenty of accretionary prisms at each side of these units. In late Early Carboniferous, the NJO was finally closured. Meanwhile, the strongly deformed collisional and orogenic belt between the SNB and the XAT forced a NW -SE compression that leads to significant obduction and uplifting of accretionary prisms at wedges of units at both sides, forming a typical upward oriented flower shape. The early -stage high angle foliation and lineation were formed during this process. In the late orogenic stage, a clearly left strike -slip activity was occurred between SNB and XAT, forming the late -stage NE -SW low angle lineation. The structural analysis related to closure of NJO suggests bidirectional subduction model of the ocean.