PETROLOGY AND GEOCHEMISTRY OF ECLOGITE AND ASSOCIATED GARNET PERIDOTITE IN BIXILING, YUEXI, ANHUI PROVINCE
Han Zongzhu(Department of Marine Geology, Ocean University of Qingdao, Qingdao 266003) Yuan Qilin(Institute of Geochemistry, Academia Sinica, Guiyang, 550002) Sheng Xingtu(Hunan Institute of Test and Utilization of Mineral Resources, Changsha, 410007)
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.