Petrology And Dingqing Ophiolite In The Eastern Segment Of Bangong-Nujiang Suture Zone,Tibet

The Bangong-Nujiang suture zone and the Yarlung Zangbo suture zone are known as the two most famous suture zones on the Qinghai-Tibet Plateau,representing the oceanic lithosphere fragments of the Middle Tethys Ocean and the Neo-Tethys Ocean located in the continental orogenic belt.The Dingqing ophiolite is located in the eastern section of the Bangong-Nujiang suture zone,which is transformed from east to west,and produces the largest peridotite in the suture zone.Among them,83 chromite bodies were reported by the predecessors.There are 27 ore bodies in the east ophiolite and 56 ore bodies in the west ophiolite.I had the privilege of participating in the mapping and chromite survey of The Second Geo-Exploration Institute of CMGB in the area,and I got a lot of help to carry out research on the peridotite and chromite in the Dingqing ophiolite mantle,and completed this thesis..This research has carried out a large number of field investigations and detailed indoor research,and has made the following progress and understanding:(1)According to the output,structure and tectonic characteristics of the mantle peridotite,harzburgite is divided into four types: blocky,porphyry,spherulitic and directional-oriented peridotite.Dunite is divided into lenticular or strip-shaped,thin-shell and agglomerate dunite.(2)The results of mineral chemistry analysis of the Dingqing peridotite show that the chemical composition of the Dingqing peridotite has the characteristics of deep-sea mantle peridotite and fore-arc mantle peridotite and their transitional type,which may be formed in the mid-ocean ridge environment and fore-arc environment in subduction zone or a superposition of both.(3)On the basis of examining the 83 chromite bodies and mineralization points discovered by the predecessors in the peridotite of the Dingqing ophiolite,according to the output,structure and mineral chemical characteristics of the chromite,the chromitite is classified into high chromium type,medium high chromium type,medium chromium type and low chromium type chromitite.These different types of chromitites,mantle peridotites and their field output have a certain correspondence.Among them,low chromium type chromitite Cr# is extremely low,ranging from 9.23 to 14.01,with an average of 11.89;TiO2 content is 0.00% to 0.04%,and the average value is 0.01%,which may be a new chromitite output.(4)According to a large number of field investigations and special mappings,it is found that the spatial distribution of chromite bodies and dunites are related to the unit density of pyroxene veins and serpentine veins on the surface.That is,the larger the density of the veins when the chromite body or dunite is exposed,the serpentine veins are generally developed in the dunite,and the pyroxene veins are rare.Among them,the changes of spherulitic peridotite,porphyry peridotite and their transitional type are influenced by pyroxene rock mass/vein invasion,and the closer to the pyroxene rock mass,the more obvious the granular structure,otherwise Patchy structure transition.The mineral chemistry characteristics of the spherulitic and porphyry harzburgite show distinct differences.The spherulitic peridotite shows the structural characteristics of the fore-arc peridotite,while the porphyritic peridotite shows the characteristics of the deep-sea peridotite.(5)The zircon U-Pb age of the pyroxene mass is 185.7 ± 2.8Ma,which is basically the same as the age of the plutonic intrusion of boninitic series from in Dingqing ophiolite;according to this,the basicity of the plate subduction process is inferred.The magma upwelling reacts with the mantle peridotite to form pyroxene vein/body.Due to the structural change of mantle peridotite caused by hot baking or plastic rheology,spherulitic peridotite is formed near the magma chamber,and porphyritic peridotite is formed instead.