Granulite-facies Metamorphism In Sand River Area From The Central Zone Of The Limpopo Belt

Limpopo Belt is located between Kaapvaal Craton and Zimbabwe Craton in South Africa.It is a world-famous Archaean to early Proterozoic tectonic belt experiencing multiple tectonic events and high-grade metamorphism.The Limpopo belt can be further divided into the North Marginal Zone,the Central Zone and the South Marginal Zone according to its structural and lithologic characteristics.The study of central granulite facies,especially high-pressure granulite facies metamorphic rocks is widely regarded by scholars at home and abroad.At the same time,there is a lot of debate about the P-T trajectory and the structural evolution of the Limpopo Belt.We will explore the origin of the mafic granulites in Sand River area and the tectonic evolution of the Central Zone in Limpopo Belt in this paper based on the research of the granulite in Sand River area.Sand River area is located in the northeast of the Central Zone in Limpopo Belt.Based on the study of granulite facies rocks on petrography,conventional geothermobarometers,phase equilibrium,geochemistry and metamorphism of the granulite,we can know the protolith type is island arc tholeiite.And its metamorphism evolution can be divided into three different metamorphic stages: peak high-pressure metamorphic stage,early retrograde metamorphic stage and late retrograde metamorphic stage.The mineral assemblage of high-pressure metamorphic stage(M1)is Grt+Cpx+Qtz+Ilm+Mag;the mineral assemblage of early retrograde metamorphic stage(M2)is Opx+Pl±Grt±Cpx+Qtz;the mineral assemblage of late retrograde metamorphic stage(M3)is Amph+Pl ± Opx ± Grt ± Cpx + Qtz,.Retrograde metamorphism P-T trajectory is clockwise,near isothermal decompression to cooling and depressurization.The peak temperature and pressure conditions(M1)are calculated at T=820-900?and P=13.5-15.4kbar by the calculation of conventional geothermobarometers.The temperature and pressure conditions of early retrograde metamorphism(M2)is 800-890?and 8.8-11.2kbar;the late metamorphic conditions(M3)are 540-650? and 2.7-4.4kbar.The phase equilibrium simulation obtains the peak pressure metamorphic temperature and pressure conditions(M1)for T=810-890 ?,P=12.5-14.5kbar;early retrograde metamorphic temperature and pressure conditions(M2)for T=830-880?,P=6.5-7.5kbar;late retrograde metamorphic temperature and pressure conditions(M3)for T=580-670?,P=4.5-5.5kbar.The results of the two methods are basically consistent.We consider the granulites in Sand River area from the Central Zone of Limpopo Belt probably formed in about 2 billion years,and have connection with the the thickening of the crust throngh continent-continent collision process.After crustal thickening and granulite facies metamorphism,the tectonic movement,rapid uplift to shallow crust,developed the PTt path of near isothermal decompression(ITD).In the shallow crust level,rock mass may be affected by fluid effects,experienced late amphibolite facies metamorphism.