FEATURES AND EVOLUTION OF METAMORPHISM IN THE ALTAI PALEOZOIC OROGENIC BELT OF CHINA

The Altai Paleozoic orogenic belt is located in the northwest of China and has undergone three stages of metamorphism and deformation.The tectonic-thermal events in the Altai orogenic belt were taken place in Hercynian period.The isotopic ages is frome 300 to 390Ma. During the first stage,the whole area underwent intensive deformation(D1) and simple low-temperature greenschist phase metamorphism(M1).The second stage of metamorphism(M2) was happened in the places of heat axis and heat spots,and formed mid-temperature progressive metamorphic belts with main deformation(D2).The third stage of metamorphism(M3) is low-temperature retrogressive metamorphism.The Hercynian progressive metamorphic belts are distributed in two types of occurrences.The planar type is concordant with the stratigraphic sequences and with rather wide iso-metamorphic belt.The linear type is developed in small scale Paleozoic volcano-sedimentary basins and with narrow iso-metamorphic belts. The geothermal gradients were very different in the two type of progressive metamorphic belts.The different type of progressive metamorphic belts were formed in different geological environments and divided into both of staurolite-kyanite type and staurolite-andalusite type.They were formed under different condition of pressure and occurred different mineral associations.The staurolite-kyanite association represent midderate pressure metamorphic condition.The staurolite-andalusite association illustrate low-pressure metamorphic condition.The both have similar clockwise PTt path and metamorphism process.