A Devonian to Carboniferous intra-oceanic subduction system in Western Junggar,NW China
【摘要】：正The Devonian and Carboniferous rock assemblages in Western Junggar,southern Altaids,include ophiolitic melanges and coherent units.At Baijiantan,Yeyagou and Dagun Devonian gabbros and one group of volcanic rocks(typeⅠlava) generally crop out as blocks in a matrix of ultramafic rocks and/or mudstone.They have a depleted light rare earth element(LREE) and mid-oceanic ridge basalt(MORB)-like signature with a small negative Nb anomaly,suggesting formation in a back-arc basin.Another group of volcanic rocks(typeⅡlava) from Baijiantan,Yeyagou,Western Karamay and Maliya includes alkaline basalt,basanite,andesite,dacite and trachyte that contain microphenocrysts of clinopyroxene-plagioclase,amygdales filled with chalcedony and calcite,and accessory pyrite and ilmenite.The typeⅡlavas are further subdivided into five sub-types:(1) TypeⅡ_1 lava is strongly enriched in LREE with incompatible trace element ratios similar to oceanic island basalt (OIB),suggesting derivation from enriched mantle.(2) TypeⅡ_2 lava and tuff are slightly LREE-enriched,with a marked negative Nb anomaly and Th/Yb-enrichment indicating that they were generated in a supra-subduction zone(SSZ).(3) TypeⅡ_3 lava is also moderately enriched in LREE suggesting derivation from a more-enriched MORB mantle.(4) TypeⅡ_4 lava is enriched in LREE and has high Nb/Yb values close to those of OIB.It is extensively depleted in Nb suggesting a source containing a mixture of enriched and SSZ mantle.(5) TypeⅡ_5 lava has MORB-like REE patterns,and Nb/Yb and Th/Yb ratios.Based on field structural data,and the contemporaneity of the Baogutu adakite,a high-Mg diorite dike and the Miaoergou charnockite,these enriched and depleted magmatic features suggest generation in an intra-oceanic subduction zone influenced by ridge-trench interaction.The Altaids in Western Junggar are characterized by multiple intra-oceanic subduction systems,which may have continued to operate to the late Carboniferous.