LOW-GRADE METAMORPHIC ROCKS IN THE DABIE-SULU OROGENIC BELT: AN ACCRETIONARY-WEDGE MODEL FOR CONTINENTAL SUBDUCTION

During subduction of the Yangtze plate, the sedimentary cover and its underlying basement of the subducting plate were partly scraped off by the overthrusted North China plate, causing accumulation of malange materials in front of the overriding plate with dynamic metamorphism mainly at greenschist-facies conditions. Such low-grade metamorphic rocks correspond to an accretionary wedge of a continental plate and have been identified in the Dabie-Sulu orogenic belt of east-central China. The proposed accretionary wedge consists of two parts: (1) large masses, along the northern margin of the belt, composed of metasedimentary rocks (including slates, schists, phyllites, metasandstones and marble) and deformed intrusions of mid-Neoproterozoic age; (2) sporadic outcrops, in the interior of the belt, composed of metavolcanics, metaclastics, phyllite, and marbles. An integrated study of available geochronological, petrological and paleontological observations demonstrates that the metasedimentary rocks were deposited along the northern passive continental margin of the Yangtze plate as a flysch facies prior to Triassic, and that the intrusions are product of bimodal magmatism during the mid-Neoproterozoic along the northern margin of the Yangtze plate. Except for the striking contrast in metamorphic grade, the low-grade rocks generally can be correlated in protolith origin and age with HP to UHP metamorphic rocks in the orogenic belt to the south. Relationships in time and space between these rocks are reasonably interpreted by assuming a model linking their evolution during the Triassic subduction of the Yangtze plate. Our results provide a constraint on the location of the Triassic suture zone between the North China plate and the Yangtze plate. It must be located north of the accretionary wedge, i.e. north of the Beihuaiyang zone in the Dabie region and of the Wulian and Penglai zones in the Sulu region.