Metamorphic evolution of Early Precambrian khondalite series in North Shandong Province

Abstract

Early Precambrian khondalite series,which mainly consist of Al-rich schists and gneissic rocks with thin layers of marble,are widespread in north Shandong Province,SE Sino-Korean craton.Four stages of the metamorphic history have been recognized in mineral assemblages based on petrographic features,genetic mineralogy,metamorphic reaction integrated with garnet,biotite and cordierite zonation patterns.The P-T conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers.Garent along with fine-grained inclusion minerals,defines a prograde assemblage(M1)garnet+biotite+plagioclase+quartz.The peak of metamorphism(M2)at about 760℃ and 10.0kb is defined by kyanite-bearing granulite-facies mineral assemblage.Symplectite or coronas of cordierite surrounding relict garnet indicate garnet occurring breakdown reaction at conditions around 750~800℃ and 4.0~5.4kb.Kelyphites of fine-grained biotite ±quartz around garnet formed during the cooling process(M4)at about 550~650℃ and ~4.0kb.These results define a sequential P-T path containing near-isothermal decompression(ITD)and near-isobaric cooling(IBC)stages.The clockwise hybrid ITD and IBC P-T paths of the Early Precambrian khondalite series in North Shandong Province imply a model of thickening followed by rapid uplifting in a collisional orogen.