班公湖-怒江洋的扩张脊俯冲: 宗白增生杂岩中侏罗世辉长岩脉地球化学和Sr-Nd同位素特征

丁青蛇绿岩位于班公湖-怒江缝合带东段，被宗白增生杂岩分为东、西2个蛇绿岩体，面积分别约为400 km2和150 km2。宗白增生杂岩由异地体亚宗混杂岩和上部原地体陆缘碎屑沉积岩组成。亚宗混杂岩由低变质岩带、砾岩带、玄武质凝灰岩夹薄层泥硅质岩带和作为主要基质的泥页岩夹薄层杂砂岩组成，其中泥页岩基质被中侏罗世辉长岩脉侵入。辉长岩脉发育双侧冷凝边，走向90°~110°不等，主量、稀土和微量元素成分均介于富集型洋中脊玄武岩和洋岛玄武岩之间，结合εNd（t）=1.88~2.41和（87Sr/86Sr）t=0.70912 ~ 0.70919，指示宗白辉长岩岩浆为大洋岩石圈地幔底部地震波低速带（LVZ）顶部富集挥发分和不相容元素的洋岛玄武岩型熔体，与源自LVZ下部亏损软流圈地幔的正常洋中脊玄武岩型熔体混合的产物，形成于弧前扩张脊与俯冲带相互作用的板片窗环境。

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 εNd(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.