LATE CARBONIFEROUS RIDGE SUBDUCTION AND CONTINENTAL GROWTH IN THE WEST JUNGGAR OF NW CHINA: INSIGHTS FROM ALASKAN-STYLE MAGMATIC ASSOCIATIONS

Ridge–trench interaction is an important geodynamic process which can be widely observed on the margins of present oceans, such as southern Alaska and western California in the Circum-Pacific subduction systems. In contrast, there are very few ridge subduction events described in the ancient geologic record. Mafic to intermediate dikes are important and pivotal in reconstructing crust-mantle geodynamic processes. The mafic to intermediate dikes commonly occur in the West Junggar, NW, China and provide an opportunity to approve the possibility of the existence of ridge subduction in the West Junggar. Our research suggests that the dikes formed in the Late Carboniferous-early Permian and consist of two special types, including Nb-enriched and magnesian dikes. The Nb-enriched dikes (305 Ma), similar to the Nb-enriched basalts, likely originate from partial melting of mantle peridotite that was metasomatized by subducted sediment-derived melts and slab fluids. The Baogutu–Karamay magnesian dikes (305 Ma) show the geochemical characteristics of Archaean sanukitoids, which were derived from the partial melting of subducting ocean slab and subsequent melt–mantle interaction. The Bieluagaxi magnesian dikes have the geochemical affinity of Cenozoic sanukitoids in the Setouchi Volcanic Belt, which may be generated by the partial melting of subducting sediments, and subsequent melt–mantle interaction. The coexistence of Nb-enriched and magnesian dikes as well as adakites, charnockites, alkali-feldspar granites, Cu-Au mineralization in the West Junggar indicates a high temperature and extensional regime during the Late Carboniferous-Early Permian. Similar magmatic associations with related slab window have also been described modern orogens, for instance the south-central Alaska Range. Thus, ridge subduction and the resultant slab window also provide an explanation for those cases in West Junggar.