Spreading ridge subduction of Bangong-Nujiang Ocean Evidence from geochemistry and Sr-Nd isotope of Middle Jurassic gabbro dikes in the Zongbai accretionary complex

The Dingqing ophiolite, outcropped in the eastern segment of the Bangong-Nujiang suture zone, is divided into eastern part covering 400 km² and western part covering 150 km² by the Zongbai accretionary complex.The Zongbai accretionary complex is composed of the allogeneic Yazong mélange and the overlying autochthonous epicontinental clastic sedimentary rocks.The Yazong mélange consists of low metamorphic rock zone, conglomerate zone, basaltic tuff interlayered with thin layers of argillaceous siliceous rock, and argillaceous shale interbedded with thin layers of greywacke.The argillaceous rock is intruded by Middle Jurassic gabbro dikes with bilateral chill margins and striking 90°~110°.Compositions of major, rare earth and trace elements of all gabbro samples are between E-MORB and OIB.Combined with Nd isotope data of ε_Nd(t) (1.88~2.41) and Sr data of (⁸⁷Sr/⁸⁶Sr) _t(0.70912~0.70919), it is suggested that the gabbroic magmas are the product of the mixture of OIB-type melts rich in volatile and incompatible elements at the top of the seismic low-velocity zone(LVZ) and N-MORB-type melts originated from the lower part of LVZ.The gabbros were formed in a slab window setting during the interacting of the forearc spreading ridge within a subduction zone.