青海可可西里地区蛇绿岩的时代及形成环境

青海可可西里地区发现两条蛇绿混杂岩带,它们沿逆冲带分布。古生物、地层和同位素定年资料表明其时代为早石炭世-早二叠世。其岩石组合代表大洋岩石圈残迹。镁铁质岩主要形成于洋岛环境。硅质岩形成于深海、半深海环境,部分生成于洋岛或洋脊附近热水活动区。这是一套弧前蛇绿混杂岩,是古特提斯洋壳俯冲过程中形成的增生楔。这些资料表明,可可西里地区至少在早石炭世就已存在古特提斯洋,洋盆中有一些洋岛,洋盆在早二叠世末期基本闭合。

AGE AND TECTONIC SETTING OF OPHIOLITE IN THE HOH XIL REGION, QINGHAI PROVINCE

The Xijir Ulan-Shexinggou and Gangqiqu ophiolitic melange belts were found in the Hoh Xil region, Qinghai Province, during the comprehensive scientific expendition in 1990. These ophiolitic melange belts and the Bayingchawuma ophiolitic melange belt are distributed along thrusts. The rocks discovered might be slices of a large ophiolitic melange belt buried deeply underneath very thick Triassic flysch which were thrust and exhumed. The melanges consist of blocks of harzburgite, gabbro, pillow basalt, massive basalt, picritic basalt, diabase, limestone, marble and chert as well as phyllite matrix. The age of the ophiolites has been determined to be Early Carboniferous-Early Permian by paleontological dating, stratigraphic unconformity and isotopic dating. The rock association in the ophiolites represents relict oceanic crust. The petro- chemical study suggests that mafic rocks, including basalts, gabbro and picritic basalt mainly formed in an ocean island environment, whereas cherts were mainly formed in abyssal or bathyal regions and partly in hydrothermally active areas near ocean islands or ocean ridge. The association of abyssal radiolarian cherts, abyssal red clay blocks and collapsed ocean island rocks as well as tectonic features show that the ophiolitic melanges belong to forearc ones and are an accretional wedge formed during subduction of the Paleotethyan oceanic crust. These data indicate that the Paleotethyan ocean had existed at least in the Early Carboniferous in the study region and that some ocean islands had existed in the ocean which largely closed at the end of the Early Permian.