ORIGINAL STUDY OF GARNET FROM THE TAIPINGZHAI-JINCHANGYU AREA, IN EASTERN HEBEI PROVINCE
Garnet are commonly found in the metamorphic rocks in the Taipingzhai-Jin-changyu area.They can be divided into two types based upon their field occurence, relationship with other minerals of rocks and formed conditions.One is paragenetic relation with minerals in garnet-bearing felsic gneisses. The other is maily around the pyroxene and oxide of iron. The hornblende and biotite are occasionally rimmed by garnet,showing coronitic texture in the metamafic rocks, BIF and charnockites. Chemical compositions and geological time are different between the two garnet types.The former had been formed before the main tectonics in this area. It is composed mainly of almandine, and pyrope is subordinate component. The later was formed after main tectonics.Its major component is almandine,pyrope and calc-gar-net are subordinate.Garnet composition is controlled by Cao and FeO in the bulk rock composition. Within the same section, the composition of garnet in coronas cored by clinopyro-xene differs only slightly from that in coronas cored by oxide of iron.FeO/FeO + Mg ratio of garnet-hypens theae and Fe2+/Fe2+ + Mg ratio of garnet-colinopyroxene and garnet-hornblende all show a tendency with positive correlation, indicating that they are subjected to the similar P-T conditions.PH2o is one of significant factors which controls formation of garnet. Lower PH2O is favourable for the formantion of garnet. On the contrary, higher PH2O is favourable for formation of hornblende. The inhomogeneous ultrami-cro-cracks brought about different PH2O and yielded aggregate parts of garnet and hornblende in the metamafic dykes. It is indicated that garnet and hornblende are formed in the same temperature conditions in the same time.By methods of geothermometer and geobarometer,the calculated temperature is 760-770℃ at 1 GPa for garnet in coronitic rims,and 820-830℃ at 0.88-1 GPa for coexisting garnet.The former is equal to amphibolite granulite subfacies and the later, to pyroxene granulite subfacies.