西藏丁青蛇绿岩早侏罗世堆晶斜方辉石岩的弧前特征及其地质意义

Forearc characteristics and geological significance of the Early Jurassic cumulate orthopyroxenite in the Dingqing ophiolite,Tibet

丁青蛇绿岩位于班公湖-怒江缝合带东段,其形成时代和构造背景还存在争议,而洋壳底部的堆晶岩能够较好限定古洋盆的构造环境。本文在对丁青东蛇绿岩扎西觉测区开展1/5万地质填图过程中,识别出堆晶杂岩底部的伟晶斜方辉石岩与方辉橄榄岩呈构造接触。伟晶辉石岩的锆石U-Pb年龄为189.56±0.46Ma。斜方辉石平衡温度为919.23~964.93℃(平均939.51℃),与角闪石的结晶温度(915.29~939.5℃,平均930.27℃)相似。伟晶辉石岩的形成压力大致在0.31~0.47GPa,结晶深度至少为13.53~18.44km,明显大于现代海域扩张脊的洋壳厚度。辉石岩的矿物和全岩地球化学成分以及计算的平衡熔体均与玻安质岩浆相似,结合辉石岩下部方辉橄榄岩具有弧前地幔橄榄岩和大陆岩石圈地幔这一特征,本文认为丁青东早侏罗世伟晶斜方辉石岩形成于洋-陆俯冲带的弧前环境。

The Dingqing ophiolite is located in the eastern part of the Bangong Lake-Nujiang suture zone,and so far,its formation age and tectonic setting remain controversial.Fortunately,the cumulate rocks at the bottom of ophiolite can provide a better understanding of the early tectonic environment of the ancient oceanic basin.During a recent 1/50,000 scale geological mapping of the Zhaxijue area of the East Dingqing,the tectonical contact between the pegmatitic orthopyroxenite and the harzburgite of the ophiolite was identified.In-situ U-Pb zircon dating of the pegmatitic orthopyroxenite has yielded a concordia age of 189.56±0.46Ma.Furthermore,the calculated equilibrium temperature of the orthopyroxenes is 919.23~964.93℃(averaged at 939.51℃),which is similar to the crystallization temperature of amphiboles(915.29~939.5℃,averaged at 930.27℃);while the formation pressure roughly ranges within 0.31~0.47GPa,corresponding to a minimum crystallization depth at about 13.53~18.44km.The mineral compositions and whole-rock geochemical data of the orthopyroxenites and their calculated equilibrium melts are similar to those of the boninitic rocks.Combined with studies on the harzburgite which show features of the forearc peridotite and subcontinental lithosphere mantle at the bottom of orthopyroxenite,we propose that the Early Jurassic pegmatitic orthopyroxenite in the East Dingqing ophiolite was formed in a forearc setting over the ocean-continental subduction zone.