Petrology And Geochemistry Of Pyroxenites In The Yarlung Zangbo Ophiolites

The pyroxenite veins are widely developed in the mantle peridotites of the Yarlung Zangbo ophiolite belt.We conducted a detailed study on the pyroxenite veins exposed in ophiolite belt.Through filed investigations and research of petrography indoor and geochemistry characteristics,the following proceedings and recognitions have been achieved:The pyroxenite veins studied in the thesis exposed in the Dongbo,Pulan,Luqu,Dazhuqu,Renbu and Zedang ophiolites of the Yarlung Zangbo ophiolite belt.The pyroxenite veins mainly intrude into the mantle peridotite,and the surrounding rocks are mainly harzburgite,we can hardly find pyroxenite veins intrude into dunite in the Dazhu ophiolite.According to the lithology of pyroxenite veins,they can be divided into clinopyroxenites?websterites?orthopyroxenites.According to the contact relationship between pyroxenite veins with surrounding rocks,they can be divided into continual contact and transitional contact.Accroding to the microscopic identification,The main minerals of three kinds of pyroxenites veins are clinopyroxenes and orthopyroxenes,and a small amount of olivine and spinel,which are distributed in the boundarie or gap of pyroxene.clinopyroxene/orthopyroxene is classified into early coarse and later fine granules with similar compositions characteristics.The Mg^#of clinopyroxenites?websterites?orthopyroxenites are 87.92-91.48?90.1-90.8?89.10-91.69 separately,the amount of SiO₂ are 52-54.26 wt%?51.3-54.73wt%?51.71-56.31wt%separately,the amount of Al₂O₃ are 0.93-1.95 wt%?1.37-2.66wt%?0.76-1.58wt%separately.The REE distribution of clinopyroxenites is nearly“spoon”type,the?REE is 1.45-4.36ppm;The REE distribution of websterites is“spoon”type,the?REE is 0.79-1.96ppm;The REE distribution of orthopyroxenites is“V”type the?REE is 0.24-1.43ppm.These characteristics indicate that the melt sourace of pyroxenites had been suffered metasomatism of fluid.The major element characterist of spinel and pyroxene of pyroxeneite veins reveal the transitional trend from the deep-sea mantle peridotite to the pre-arc mantle peridotite.According to the partition coefficient of the clinopyroxene in the basaltic melt,the equilibrium melt formed with the three kinds of pyroxenites is calculated to be similar to the boninite.In summary,The pyroxenites veins that widely exposed in Yarlung Zangbo ophiolite belt can be divided into clinopyroxenites?websterites?orthopyroxenites,the geochemical characteristics of pyroxenites indicate the sourace of them had been suffered metasomatism of fluid;The simulation calculations show that the melt equilibrium with pyroxenites has a compositional characteristics similar to Boninite.The above characteristics indicate that the melt sourace of pyroxenites may be fromed in the subduction zone environment rather than the product of the mid-ocean ridge.