The Characteristics Of Subductional Ophiolites And Its Magmatic Arc In Dangjiang-duocai

Abstract:As one of the twenty opholite mélange zones in Qinghai-Tibet Plateau, the material composition, structure and tectonics, subduction and closure age and subduction polarity of Jinsha ophiolite mélange zone had attracted more and more attention in the long term. Based on the research of the anterior research fruit of other people, through measureing and making tectonic cross sections, collecting related samples of Duocai ophiolite and Dangjiang volcanic rocks(gabbro, basalt, greenschist, rhyolite, dacite, andesite, etc.), investigating and researching of tectonic patterns, gathering geometry and kinematics characteristics, building tectonic stage and matching relationship, identificating material composition and analyzing isotopic geochronology, etc. of Duocai ophiolite mélange in Zhiduo area , west part of Jinsha ophiolite méglange belt, the following thoughts and conclusions were obtained:1. By the field and interior research in the tectonic patterns of different sliver, the deformations of Duocai ophiolite mélange can be divided into four stages:①The first stage was characterized by the rootless hook-shaped folds in the flysch clastic which represent the early subduction stage, belonged to the deep tectonic arrangement and indicated the NNW-trending ductile shearing.②The second stage was characterized by the pint-sized similar fold and top-thickness fold developed from the penetrational schistosity which present the mainly subduction stage, indicated the NE-trending extrusion and NW-trending laevogyrate shearing.③The third stage was characterized by the trust ductile shearing from NE-SW which incised the top-thickness fold of the second stage and presented the collisional stage, belong to middle-shallow arrangement, suggested the NNW-trending ductile shearing.④The fourth stage was characterized by the large-scale steep plunging vertical fold which presented the late uplift stage, indicated NW-trending laevogyrate strike-slip ductile shearing in accordance with the NW-trending stretching lineation in the first sliver of the section.2. The Duocai ophiolites mainly consist of gabbro, cumulate gabbro and basalt(basic lavas), and the basic lavas can be divided into OIB and MORB-IAT two series: seriesⅠwere alkalescent, related with the mantle plume or hotspot, derived from the Em2 of enriched mantle, didn't belong to the member of ophiolite. seriesⅡare tholeiitic and also can be divided into MORB and IAT two transitional series. The geochemistry of gabbros were similar to the basalt of seriesⅡand derived from the transitional zone between the prime mantle and depleted mantle which provide the evidence of both have the characteristic of island arc.3. There were three basic lavas of different component in Duocai ophiolites(OIB-MORB-IAT), the combination of OIB and MORB indicated that the Jinsha Ocean may have experienced the transitional process from the continental rift to ocean, while the exit of IAT suggested that the opiolites were related to the subduction of plate.4. The metamorphic minerals of ophiolite mélange were mainly composed of chlorite, epidote and actinolite and have no mineral of high temperature and high pressure expect there existed some sillimanites in quartz veins by the later hydrothermal contact metamorphism.5. The combination of rhyolite, dacite and andesite in Dangjiang volcanic rocks were rich in SiO2 and Na2O and poor in TiO2, belonging to calc-alkaline type rocks. The chondrite-normalized REE-patterns were characterized by LREE enrichment(Rb,Ba)with variable degree of negative Eu anomalous values and the primary mantle-normalized trace elements-parterns by HFSE depleted as well as P element, indicating that it formed in the island arc environment related to active continental margin.6. The single zircon U-Pb age of cumulate gabbro was 252.5±0.58Ma (MSWD=0.95) which showed that Duocai ophiolites formed in the end of late permian, may present the age of for-arc spreading related to the subduction in Zhiduo area of the Jinsha Ocean. The single zircon U-Pb age of rhyolite porphyry of Dangjiang volcanic rocks was 237.6±1.7Ma(MSWD=2.3)which presented the crystallization age. The two group of ages suggested that the subduction was continuing in the end of Permian and lasted to the occurrence of arc volcanic rocks in middle Triassic.7. The geochemistry characteristic and the discrimination diagrams of basic lavas showed that the ophiolites were formed in the environment of island-arc. The well matching in time, space and causes which not only indicated the ophiolites belong to SSZ type, but also suggested the subduction polarity were from northeast to southwest.8. By systematical analyzing the characteristics of deformation, metamorphosis, contact relationship in ophiolite méglanges and throuth the contrastive analysis of the mainly three emplacement mechanism types related with continental movement, we concluded that the emplacement mechanism of Duocai ophiolite méglange belonged to subduction scrape adhesion type.