Deformation And Metamorphism Characteristics Of Rocks In South Qinling Acctionary Complex Belt

The Qinling orogen was a composite orogen which experienced subduction during Paleozoic and collision during Mesozoic. As a main part of Qinling, the traditional geodynamic model for South Qinling acctionary complex belt involved two stages: an initial rifting and the formation of the juvenile oceanic basin(Devonian to Carboniferous), and the development and closing of the mature ocean(Permian to Early Triassic). Accordingly, the South Qinling had been traditionally inferred as a part of the passive margin of northern Yangtze plate. However, recent researches view the northern Yangtze plate as active margin in Paleozoic. Deformation and metamorphism as well as related geochronology in the South Qinling indicated that the Qinling Ocean closed because of the bidirectional subduction(southward subduction in South Qinling and northward subduction in North Qinling), which resultes in the formation of the South Qinling accretionary complex belt during Triassic.In this paper, the investigation of six cross-sections provides new significant data on the deformation and metamorphism of the South Qinling accretionary complex belt: characteristic formations in this belt are block-in-matrix structures with undeformed rigid blocks(limestone, basic-ultrabasic blocks and so on) floating in an apparently highly strained meta-sedimentary matrix. Tectonic deformation stages in matrix are divided into four stages. The first stage(D1) is characterized by intensively N-S directed contraction. In the second stage(D2), northward thrusting is dominated. The deformation of the third stage(D3) is characterized by sinistral strike-slip. The fourth tectonic movement(D4) is typical of a thrust process from north to south. Brittle deformation developed later and locally transformed the ductile structures. During the process of deformation, ductile shear zones develop in the accretionary complex and divide it into several thrust sheets. P-T path of the intermediate metamorphic rocks in matrix shows that it experienced an early prograde metamorphic stage, the peak high amphibolite facies metamorphic stage and the late-stage low amphibolite facies retrogression. The last two stages are related to the first and second stage of deformation, respectively. In contrast, the deformation and metamorphism of blocks are weaker.LA-ICP MS U-Pb dating of zircons from syn-tectonic migmatite veins yields age of 198.5 ± 2.0 Ma, which constraints the time of northward thrust shearing(D2). Also, we interpret the biotite plateau age of 183.0 ± 2.0 Ma as representing cooling age of deformation in South Qinling. What’s more, the geochemical characteristics of Tangjiagou granite which intrudes into the accretionary complex show the nature of island-arc. In combination with the formation age of hydrothermal zircon rim(205 ± 2.6 Ma), they suggest the existence of magmatic hydrothermal activity during the subduction-accretion process. In view of these dating results and the tectonic setting of South Qinling accretionary complex belt, we propose that they represent the time of closing of Qinling Ocean and the end of accretion.Based on the description above, the deformation and metamorphism newly recognized in the South Qinling accretionary complex belt allow a detailed reconstruction of processes in a fossil accretionary wedge: this belt resulted from the southward subduction of Qinling Ocean in South Qinling and it recorded the contraction(D1), thrusting(D2), strike-slip process(D3) during subdcution – collision process, and the collision after the close of the ocean(D4). In the accretionary complex, mélanges formed due to the mixing of different blocks and matrix under the control of several kinds of tectonism. Among them, the development of underplating facilitated the exposure of intermediate metamorphic rocks.