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Kenny Wong, The University of Hong Kong (China)
Min Sun, The University of Hong Kong (China)
Guochun Zhao, The University of Hong Kong (China)
Chao Yuan, Guangzhou Institution of Geochemistry, Chinese Academy of Science (China)
Wenjiao Xiao, Institution of Geology and Geophysics (China)
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The Central Asian Orogenic Belt (CAOB) is a well-known Phanerozoic accretionary orogen which recorded the evolution of the Paleo-Asian Oceans. Although many subduction-related processes were reported for this belt, ridge-trench interaction, which is a common phenomenon in present-day subduction systems, has not been recognized yet. In this account, we present several lines of evidence which possibly indicate ridge subduction along an island arc on the southern margin of Siberia in the Devonian. In northwestern Altai, Xinjiang, China, we discovered two dismembered ophiolitic belts comprising of pyroxenite, diabase and pillow basalts interbedded with flysch and greywacke and andesite. Majority of these mafic rocks resemble transitional mid-ocean ridge basalt (T-MORB) in their rare earth element (REE) pattern and trace element abundances. Two other components, a supra-subduction zone (SSZ) suite and an ocean island basalt type (OIB-type) magma, are also found and our geochemical data suggest mixing among these three components.
The two ophiolitic belts may represent an ocean floor spreading near the fore-arc region at the southern margin of the Chinese Altai during ridge subduction. Although arc magmatism virtually ceased in such a setting, pre-existing sub-alkaline melts would be able to give rise to the conspicuous supra- subduction zone (SSZ) signatures. On the other hand, formation of slab window allowed influx of asthenospheric mantle of the Ob-Zaisan Ocean. And, the alkaline OIB which originated from enriched part of this mantle, served as a source of enrichment. Further south near the Erqis shear zone, adakites, high-Mg rocks and picrites also occur and can be explained by this tectonic model. While the adakite represents melt derived from the edge of subducted slab, the high-Mg rocks and picrites were probably products of high-degree partial melting of the refactory peridotite in mantle wedge due to the raised geotherm. Finally, waning stage of the spreading was marked by the eruption of boninitic lava discovered in the Ashele area.
Overall, we suggest that the late Paleozoic evolution of the Chinese Altai was mainly controlled by ridge subduction. Such a process provided not only heat for the ∼380Ma metamorphism but also mantle input for the granitoids and mineralization.
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