Serpentinization Process And Magnetite Formation Of Ultramafic Rocks From Angwu District,north Tibe

Various serpentinised ultramafic rocks are widely distributed in the Bangong-Nujiang suture zone?BNSZ?and magnetite is an important product during the process of serpentinization.Here we research the process of serpentinization in ultramafic rocks and its influencing factors,which contribute to the study on physico-chemical properties of rocks,the discussion of life origin and provide a new direction for magnetite prospecting.The Angwu district is located in the central segment of BNSZ.By means of petrology,petrography,mineralogy,mineral chemistry,as well as geochemistry,this paper conduct the exploration of petrogenesis and research on the process of serpetinization and its influencing factors in ultramafic rocks from Angwu,which provides a referrence for magnetite prospecting in ultramafic rocks of suture in future.To compared with the typical mantle peridotites,the collecting samples of this paper are clinopyroxene peridotites and belong to cumulative ultramafic rocks,where the mineral assembly is olivine+clinopyroxene+lizardite+chlorite+chromite+magnetite.It was investigated that ultramafic rocks in Angwu form at the initial extension environment of Bangong-Nujiang ocean.The expansion of the ocean caused the upwelling of asthenosphere material,which was replaced with lithospheric mantle to produce E-MORB type magma with the signatures of OIB.Subsequently the magma formed the protolith in research area before serpentinization through cumulating,where the main crystal phases were olivine and clinopyroxene.According to microscopic observation and mineral compositon characteristics,the serpentinization process of the ultramafic rocks can be divided into three stages:?I?formation of relatively rich-Fe lizardite?Mg#<0.85?;?II?formation of relatively rich-Mg lizardite?Mg#>0.90?,with precipitation of magnetite in serpentine veins;?III?alteration of lizardite into chlorite.A comprehesive analysis indicated that the serpentinization of ultramafic rocks was mainly affected by the composition of primitive rocks,temperature and the activity of SiO₂.Based on comparesive analyses with the former achievement,computation of the reactions and thermodynamic modelling associated with alteration process for the above stages,we found the serpentinization degree of dunites composed of olivine was higher than that of clinopyroxenite peridotites,and its magnetite content was relatively large;in the reaction system,high SiO₂ activity made serpentine transform into chlorite,and suppress the precipitation of magnetite;within this temperature interval?100³00??,the lizardite we observed is stable,but high temperature caused the conversion of serpentine to chlorite,and precipitation of magnetite was not favoured.The principal reason that magnetite content in Angwu ultramafic rocks was relatively low was that the clinopyroxenes in protolith reduced the total content of magnetite.Meanwhile,the desilication of pyroxene caused the enhancement of SiO₂activity and restricted the formation of magnetite.Furthermore,the phenomenon that there were no brucites in the reaction system suggested a relatively high temperature.Therefore,the advantages of magnetite precipitation in serpentinized ultramafic rocks in the suture zone are:at constant temperature and pressure,the low SiO₂ activity in the reaction system and the less composition of easy desilication contribute to the formation of magnetite;within the stable temperature interval of lizardite,the lower temperature is preferable.