鲁东南莒南县一带榴辉岩的特征

CHARACTERISTICS OF ECLOGITES IN JUNAN COUNTY, SOUTHEASTERN SHANDONG

莒南榴辉岩原生矿物共生组合是石榴石+绿辉石±石英。石榴石的AIm58.3～62.6％,Pyr11.0～22.9％;绿辉石含Jd 40.7～46.3。它们形成在P为14.7～21.78×10~8Pa、T为635～994℃的条件,形成时的地热梯度大约16.2～16.9℃/km,埋藏深度约39～60km。

The eclogites in Junan County are a part of the eastern section of the eclogite belt in middle China. They occur among the metamorphic rocks of amphibolite facies. The eclogite bodies have no roots and occur mainly in the form of lenses, and possess the characteristics of cold emplacement. Their stretch is consistent with the mylonite zones and the tectonic lines of thefolds. Their paragenetic association of the primary minerals is Gt+Omp±Q (accessory minerals are omitted). In general, the eclogites correspond to gabbros in their total chemical compositions, and are similar to basic magmatic rocks in their geochemical characteristics of rare earth elements. The composition of garnets in the eclogites is poor in Mg and rich in Fe. The garnets are composed of 58.3～62.6％ almandine, 11.0～22.9％ pyrope, 2.2～13.3％ andradite, 8.4～19.5％ grossular and 0.4～22.9％ spessartine. The composition of omphacites in the eclogites is rich in Al and Na, but poor in Mg. The omphacites are composed of 40.7～46.3％ jadeite, 11.4～28.3％ hedenbergite, 20.8～27.3％ diopside, 0.2～2.9％ johannsenite and 1.1～6.5％ tschermark. The mineralogical characteristics of the omphacites show that the pressure for their formation was higher. The P-T conditions for formation of the eclogites in this area were estimated by using Gt-Cpx geothermometry and single variation line in X_(Jd)^(Cpx) to be 14.7～21.78×10~8 Pa and 635～994℃. It shows a geothermal gradient of approximately 16.2～16.9℃/km and a depth of roughly 39～60 km during formation of the eclogites. Four diagrams constructed on the basis of chemical compositions of the garnets and omphacites indicate that all of the four samples are of the crustal genesis and not of A group eclogite. The eclogites among the Jiaonan Group in the studied area may be classified into B group eclogite. A part of the eclogites in the ultramafic rocks should be also classified into B group eclogites.