西藏丁青弧前蛇绿岩的地球化学特征

The geochemical charac teristics of fore-arc ophiolite from Dingqing area, Tibet.

丁青蛇绿岩位于班公湖 -丁青 -怒江蛇绿岩带的东段 ,其地幔岩出露规模是该带中最大的。本文报道的丁青蛇绿岩主要由地幔橄榄岩、堆晶岩、辉长岩和斜长花岗岩组成。蛇绿岩剖面上覆硅岩中的放射虫化石是早侏罗世和晚三叠世诺利克期的 ,中侏罗统砂岩和砾岩不整合覆盖在蛇绿岩之上 ,由此确定丁青蛇绿岩是晚三叠 -早侏罗世的 ,在中侏罗世之前侵位。丁青蛇绿岩属于玻安岩系 ,玻安岩的特点是富 Si、Mg和大离子亲石元素 (L IL E) ,贫高场强元素 (Ti、P、Zr、Y、Yb和 Nb)。丁青蛇绿岩的堆晶岩、辉长岩和辉绿岩均具“U”型 REE分布 ,暗示丁青玻安岩是由亏损的地幔源岩和来自消减带的水和流体两组分的混合形成的。丁青玻安岩的地球化学特征类似西太平洋第三纪玻安岩 ,而明显不同于 MORB的地球化学性质 ,表明丁青玻安岩应当形成于洋内岛弧的弧前环境 。

Dingqing ophiolite lies in the eastern part of Bagonghu-Dingqing-Nujiang ophiolite belt. Dingqing ophiolite is mainly composed of peridotite, accumulate, gabbro, diabase and plagioclase, and the peridotite in Dingqing ophiolite is the largest one in the scale in the whole belt. The radiolarian fossils in the silicalite overlying the ophiolite sequence is recognized as Norian of the early Jurassic and late Triassic age. The middle Jurassic sandstone and conglomerate unconformably cover above the ophiolite. The Dingqing ophiolite therefore formed in the late Triassic-early Jurassic age and emplaced before middle Jurassic age. The Dingqing ophiolite belongs to boninite series, which is characteristic of enrichment in Si, Mg and LILE and depletion of HFSE (Ti, P, Zr, Y, Yb and Nb). The REE patterns of the accumulate, gabbro and diabase are all U-shape, implying that the Dingqing boninite is derived from depleted mantle source with the mixture of slab-derived fluid. The geochemical characteristics of the Dingqing boninite are similar to those of Tertiary boninite in the western Pacific, but totally different from those of MORB, indicating that Dingqing boninite is formed in the fore-are setting, and belongs to fore-arc ophiolite.