安徽岳西碧溪岭榴辉岩及伴生石榴石橄榄岩的岩石学和地球化学

PETROLOGY AND GEOCHEMISTRY OF ECLOGITE AND ASSOCIATED GARNET PERIDOTITE IN BIXILING, YUEXI, ANHUI PROVINCE

本文对安徽岳西碧溪岭榴辉岩-石榴石橄榄岩伴生岩体作了岩相学、岩石化学、矿物化学及稀土元素地球化学研究。利用湿法化学分析完成了13个榴辉岩和8个石榴石橄榄岩的成分分析,利用电子探针分析了15个石榴石,14个单斜辉石、2个斜方辉石、4个橄榄石的化学组成,利用等离子光谱完成了5个榴辉岩和2个石榴石橄榄岩的稀土元素含量测定,利用地质温压计估算了榴辉岩和石榴石橄榄岩的成岩温度和压力,最后讨论了其物质来源和形成条件。

In this article we have comprehensively studied on mineral chemistry, petroche- mistry, geochemistry of eclogite and associated garnet peridotite in Bixiling,Yuexi, Anhui province. The chief part of Bixiling rock body is eclogite and garnet peridotire in forms of belt or lenticular in eclogite. The rock body area is about 0.7km^2, taking dolomitic plagioclase gneiss of Chengjiahe group of Dabiequn as its country rock. The occurrence of boundary surface of the rock body is the same as that of schistosity of country rocks. Eclogite is chiefly composed of garnet, clinopyroxene, quartz and futile, containing little amount of orthopyroxene, kyanite, dolomite and amphibole.According to petrochemical composition,it could be divided into three groups: Ⅰ、Ⅱ、Ⅲ. The eclogite of group Ⅰ is characterized with low silicon and high magnesium content; eclogite of group Ⅱ is similar to eclogite of type B in the respect of average composition, and that of group Ⅲ is characterized with high iron and high titanium and low magnesium contents. Garnet peridotite is composed of garnet, olivene, clinopyroxene and magnetite. The composition of garnet in eclogite varies greatly. Garnet in eclogite of group Ⅰ is rich in pyrope and now its clinopyroxene is augite. Garnet in eclogite of group Ⅲ is rich in almandine, the clinopyroxene is omphacite. The distribution of rare earth elements in eclogite is different and the variation of content of REE is from 18 ppm to 124 ppm. The chondrite-normalized REE patterns of eclogite belong to the type that exhibits possitive Eu anomaly and rich in light REE. The content of REE in garnet peridotite is low and it is 10ppm or so.Its chondrite-nomalized REE patterns also belong to the type showing possitive Eu anomaly and rich in light REE. For the eclogite from group Ⅰ to group Ⅲ the contents of SiO_2, TiO_2 FeO and Na_2O increase and that of MgO decreases. The content of jadeite in clinopyroxene increases and pyrope in garnet decreases and that of almandine increases.The total amount of REE increases,the possitive Eu anomaly reduces and (La/Yb)_N decreases. The formation pressure and temperature of eclogite and garnet peridotite estimated by geothermometer and geobarometer are 18kbar,600—800℃ and 16kbar, 600—700℃ respectively. We suppose that eclogite and associated garnet peridotite are formed by differentiation of basic magma through plutonic metamorphism.