山东大疃榴辉岩和石榴透辉石岩的成因

The Genesis of Eclogite and Garnet Diopsidite in Datan,Shandong Province

山东大疃迟家店石榴二辉橄榄岩中榴辉岩与石榴透辉石岩呈过渡关系，前者位于中心，后者为边缘带。在边缘带的透辉石中出现不规则状和条纹状绿辉石。通过对两类岩石的岩石学、矿物学特征及岩石平衡温压条件的研究，并与国内外不同地区、不同产状同类岩石进行了对比后认为，二者的关系虽然基本符合常见的蚀变规律，即中心的榴辉岩中绿辉石蚀变成透辉石，从而形成石榴透辉石岩边缘带，但是根据榴辉岩、石榴透辉石岩的ＲＥＥ特征和岩石平衡温压条件的分析，结合近年来国外一些学者的实验研究证明，榴辉岩可以由玄武质岩石（如辉长岩）转变而来，石榴透辉石岩是转变过程中的产物，即在高温下Ｊｄ和Ｄｉ呈混合的二元系，来自上地幔的玄武质岩石由于压力增高Ｊｄ成分增加，经固溶体分离，在透辉石中形成绿辉石。由于压力再增高．Ｊｄ成分继续增加，逐渐形成绿辉石，从而形成榴辉岩。无论哪种解释，两种岩石的原岩是相同的，应来自上地幔，属地幔成因。

Most researchers consider that Chijiadian garnet lherzolite in Datan of Shandong Provinceincludes mainly eclogite composed chiefly of omphacite + garnet, with small amounts of intercalated garnet diopsidite. There exists controversy on relationship and material sources of eclogite and garnet diopsidite. Some geologists hold that the two sorts of rocks were derived fromdifferent sources, and some believe that garnet diopsidite was formed through alteration of eclogite. The author observed the existence of transitional relationship between eclogite and garnet diopsidite in garnet lherxolite of this area, with the former at the center, and the lattermaking up the fringe; in diopside of the fringe there appears irregular and lamellar omphacite.Based on investigations into petrological and mineralogical features as well as equilibrium T-Pconditions of rocks together with a comparison with the same sorts of rocks in different areas,the author holds that the two sorts of rocks are both basaltic rocks (e. g. gabbro) derived fromthe upper mantle. The elevation of pressure led to the appearance of garnet and the reduction ofplagioclase, forming the garnet, pyroxene and plagioclase assemblage. At that time, pyroxenewas dominated by Di, and at high temperature, Jd and Di made up a mixed binary system.Further elevation of pressure resulted in urmixing of solid solution, forming omphacite in diopside. At the same time, the ratio of garnet to pyroxene increased, and plagioclase became moreenriched in sodic materials, leading to the formation of garnet diopsidite assemblage. Under thehighest pressure, sodic plagioclase was decomPOsed into jadeite and entered solid solution toform omphacite. Moreover, Jd in diopside also increased and was gradually converted into omphacite, thus forming the mineral assemblage of eclogite.