Genesis Of Podiform Chromitite In The Luobusa Ophiolite(Southern Tibet):Insights From Mineralogical And Geochemical Studies

Podiform chromitite of the Luobusa ophiolite in the eastern part of Yarlung Zangbo Suture zone(YZSZ)is the biggest podiform chromitite deposites in China.Recently,diamonds and a wide range of unusual minerals were sucessively discovered from many podiform chromitite deposites in different suture zone,which challenge the typical accepted model that podiform chromitite form in shallow mantle.Luobusa chromitite deposite is the most classic podiform chromitite deposits in China.What's more,it is the most studied unusual minerals from ophiolite.To further constrain the origin environment of podiform chromitite,it is necessary to systematically study the petrology,mineralogy,geochemistry and so on of Luobusa chromitite and its host rock peridotite.pMELTS is a software package designed to facilitate thermodynamic modeling of phase equilibria in magmatic system.It provides the ability to compute equilibrium phase relations for a given bulk composition especially "mantle-like" bulk compositions at near solidus conditions.This study tried to use pMELTS thermodynamic calculations to explain the observed modal and geochemistry composition variations in Luobusa peridotite,Tibet and to stimulate the mantle processes such as partial melting,melt-rock reaction and so on,hoped to obtain the best environment for podiform chromitite generation.Mineral compositon,major and trace element of bulk rock suggest that Luobusa peridotite have been refertilized by late enriched melts or fluids after moderate to high degrees of partial melting.The oxygen fugacity calculation results of Luobusa peridotite show that Luobusa peridotite maybe form in a more oxidized environment relative to mid-ocean ridge(MOR),which indicate that Luobusa peridotite may once suffer metasomatism of enriched mid-ocean ridge basalt(MORB)or subduction fluid.Simulations of partial melting and melt-rock reaction processes show that the formation of large scale podiform chromitite especially the massive chromitite is hard to explained by all these mantle processes.The olivine inclusions in Luobusa chromitite always display high Cr2O3 content,and sometimes there are exsolutions of clinopyroxene and spinel symplectite in matrix olivine.The compositions of symplectites in Luobusa chromitite matrix olivine are similar to those in olivine from lunar basalt and troctolite,but different from those in olivine found from terrestrial so far.All of these imply that these olivine grains in Luobusa ophiolite maybe form in reduced environment in deep mantle.The chromite probably protect the olivine inclusions from later oxidation and retain the high Cr content,while,as without the protection of chromite,the matrix olivine contain exsolutions of clinopyroxene and spinel symplectite when the olivine upwell to the shallow mantle and oxidated.