冀东太平寨—金厂峪地区石榴子石成因研究

ORIGINAL STUDY OF GARNET FROM THE TAIPINGZHAI-JINCHANGYU AREA, IN EASTERN HEBEI PROVINCE

本文通过对冀东太平寨—金厂峪地区石榴子石的地质产状、成分特征和形成条件进行详细研究,认为P_(H2O)是控制石榴子石和角闪石产出的重要因素之一。该区石榴子石可分为与岩石中其它矿物呈平衡共生关系和呈反应边产出的两种类型,前者形成于820℃左右,压力在0.88～1GPa,相当于辉石麻粒岩亚相的变质条件;后者形成于770℃、1GPa,相当于角闪麻粒岩亚相的变质条件。

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.