Petrogenesis Of Two-Pyroxene Basic Granulite In Gushan,Datong,Shanxi Province

The petrogenesis and metamorphism of basic granulite in Gushan Datong have been controversial.Some scholars believe that it is gabbro formed direct by crystallization of magma,while other scholars believe that it is a metamorphic rock that has experienced high pressure granulite facies.This paper mainly selects the two-pyroxene granulite in Gushan Datong as the research object,and the purpose is to explore the petrogenesis of Gushan Datong basic granulite.The main research methods of this paper are field geological survey,petrographic observation,mineral chemistry,phase equilibrium modelling.It is observed in the field that the two-pyroxene granulite occur as lenses in pelitic gneiss and TTG gneiss.Petrographic observation shows that the clinopyroxene-orthopyroxene-plagioclase is equilibrium structure,and hornblende and biotite are retrogressive edg structure.Mineral chemistry and mineral genesis analysis show that clinopyroxene,orthopyroxene,hornblende,magnetite and ilmenite are metamorphic minerals.Plagioclase can be divided into three types according to their An:(1)An=38?40,its An is lower than that of plagioclase in gabbro,similar to most of the basic granulites in North China,so it is of metamorphic origin;(2)An=18?22,which may be formed by late alteration;(3)An=76?81,only occurs in sample 1910-13,its An is much higher than sample 1910-17 and 17D2,the reason is that local fluid distribution is uneven,so the plagioclase of sample 1910-13 may retain the original rock composition or belong to peak metamorphic minerals.Therefore,according to petrographic observation and mineral chemical study,it is shown that the two-pyroxene granulite is a rock of metamorphic origin,which has experienced three metamorphic stages,namely peak medium pressure granulite facies,retrogressive amphibolite facies and retrograde greenschist facies.The mineral assemblages of the three metamorphic stages are: clinopyroxene+orthopyroxene + plagioclase ± magnetite/ilmenite,hornblende + plagioclase ± biotite ± magnetite/ilmenite,chlorite +plagioclase ± magnetite/ilmenite.Phase equilibrium modelling shows that the maximum pressure range of biotite two-pyroxene granulite(1910-13)peak assemblage is 6.6-10.5kbar,and that of biotite amphibole granulite(1910-17)peak assemblage is 6.3?10.65 kbar.The peak temperatures of biotite two-pyroxene granulite(1910-13)and biotite amphibole twopyroxene granulite(1910-17)are 873?906? and 821?853? by using two-pyroxene thermometer,the temperature and pressure of hornblende facies is 4.2?6.4kbar/771?809?calculated by amphibole-plagioclase thermobarometer.The temperature and pressure conditions of each metamorphic stage are obtained by phase equilibrium modelling combined with traditional geothermobarometry,the peak is 6.6?7.2kbar/862?906?,the retrogressive amphibolite facies is4.2?6.4kbar/771?809?,and the greenschist facies temperature is less than 450?,indicating that the two-pyroxene granulite experienced a clockwise P-T-t trajectory.The calculation results show that the sample of this studied recorded a clockwise P-T-t trajectory,indicating that the geological body experienced the dynamic process of crustal uplift and exhumation to the surface.