The Mineralogical Study On Pyroxene From Dashihe,Jilin Province

Jiaohe City,Jilin Province is rich in cenozoic alkaline basalts.These alkaline basalts often carry a large number of peridotite.In this paper,the pyroxene in the peridotite is taken as the research object.Microscope observation,electron probe test,infrared spectrum test,raman spectrum test and microscopic infrared test are taken to the research of the mineralogical characteristics of pyroxene in Dashihe area.Microscope observation shows that the rocks collected in Dashihe are mainly two types of peridotite:harzburgite and lherzolite.These two types of rocks are consist of peridot?cpx?opx,containing a small amount of spinel inclusions.The width of reaction zone between opx and basalt is normally 0.1-0.5mm.Electronic probe result shows that the peridotite in this area is consist of peridot,cpx,opx.Cpx is consist of diopside?augite while opx is consist of enstatite.Through the analysis of major elements and the ratios of Al ^VI/Al^?and Ca/Al,it is found that the peridotite in the Dashihe area has experienced partial melting and metasomatism.The analysis of Ca/Al along with Mg#shows that most of the samples present the characteristics of metasomatism by silicate melts.Infrared spectroscopy shows that the composition of the brown pyroxene in peridotite is enstatite,and the composition of the brilliant green pyroxene is diopside.The Raman test shows that the pyroxenes in the peridotite are enstatite?diopside?augite,which is consistent with the result of the electronic probe.And the peaks are mainly caused by the vibration of Si-O^-,Si-O-Si and M-O.Through the calculation of cpx thermobarometry,it is found that the formation pressure of cpx in the Dashihe area is between 8-40kbar,the formation temperature is between 889-1107?C,The formation depth is between 26km to 132km.The calculation result of the cpx-opx thermometer shows that the formation temperature of peridotite in the Dashihe area is between 955-1112?C.By comparison,the formation pressure of cpx in Dashihe area has a negative correlation with Al^VI.When the pressure exceeds 20 kbar,the Al^VIcontent in cpx decreases sharply with increasing pressure.