南迦巴瓦峰地区雅鲁藏布构造带中石英(片)岩的岩石化学特征及变质条件探讨

PETROCHEMICAL AND METAMORPHIC STUDY OF QUARTZITE(SCHIST) IN INDUS-YARLUNG TSANGPO TECTONIC ZONE,NAMCHE BARWA,TIBET

雅鲁藏布构造带在喜马拉雅东构造结地区大致呈弧形展布,宽2km～10km的蛇绿混杂岩带,带内岩石类型为云母石英(片)岩、绿片岩和变超镁铁岩块体等。其中云母石英(片)岩类广泛分布,宏观上呈巨大而狭长且岩性较单一的岩片,或与绿片岩(变玄武岩)混杂在一起。在化学成份上,MnO/TiO2比值低,Al/(Al+Fe+Mn)比值高,与洋脊附近热水成因深海硅质岩有很大的区别。在Al Fe Mn图解上,落在"生物成因硅质岩"范围内,原岩应为不纯的硅质岩类和砂、泥质岩,形成于弧后盆地小洋盆。矿物温压计算及多硅白云母的特点说明变质作用曾经历了角闪岩相,变质温压条件可能达到T=500℃～650℃,P=0.75GPa～0.8GPa,之后,构造带边界阿尼桥断层和米林 鲁朗断层发生强烈走滑,产生低温高压动力变质作用,形成高压、超高压型多硅白云母。

Indus-Yarlung Tsangpo ophiolitic zone (IYS) extends in the eastern Himalayan syntaxis as a big bend roughly along the great canyon. It is a zone of 2 to 10 km wide mainly composed of blocks of quartzite, quartz schist, green schist, meta-ultramafic and mafic rocks. Quartzite and quartz schist occur as big blocks and matrix interbeded with green schist (meta-basalt). Petrochemical study reveals that quartzite in IYS has lower MnO/TiO_2 ratio and higher Al/(Al+Fe+Mn) ratio compared with those of deep sea hydrothermal genetic chert. It shows similarity to the ″biogenetic chert″ on Al-Fe-Mn diagram. It also shows difference from the radiolarian chert in Xigatze in the same IYS belt petrochemically. The original rock was infered to be impure chert and sandstone deposited in back-arc basin or marginal basin. Similar conclusion has been made by previous trace element and REE studies on paragenesis dismembered ophiolite. Geobarometer and geothermometer calculation suggested that IYS in this region experienced metamorphism of high-pressure amphibole phase. Metamorphic condition in this belt caused by the Himalayan orogenesis after the Neo-Tethys closure might reach T=500 ℃～650 ℃,P=0.75 GPa～0.8 GPa. Strong strike-slip faulting of both boundary faults,namely the Ani and Milin-Lulang faults, initiated low-temperature and high-pressure dynamic metamorphism which produced high-pressure high-silica muscovite.This region was dominated by detachment and rapid up-lift since 3 Ma ago.