Classification of eclogites in terms of physical conditions of their origin

Abstract

The apparent FeMg distribution coefficient between garnet and clinopyroxene K′ is defined by K″= X Fe X Mg ga X Fe X Mg cpx · The effects of pressure, temperature, and chemistry of the rocks on K′ were examined, using formula volumes of end members of these minerals, and the K′ values of natural eclogites. It is shown that K′ increases with increasing pressure, and decreases with increasing temperature, and that the effect of chemistry on K′ is usually not large. It is shown that the eclogite types defined by the geological mode of occurrences are also characterized by particular range of K′ values, and that the relative temperatures of eclogite crystallization as estimated by K′ and by ordinary petrological considerations are usually in harmony with each other. Further, K′ can be applied to distinguish the difference in temperatures among the eclogites of some types. The crystallization temperature of eclogites increases in the following order, which is shown by localities: Colombia, Ural and Guatemala, California, Alps and Japan, Bavaria and Spain, Norway, East Sudetes, and granulite facies eclogites. Eclogite inclusions in basalt and in kimberlite represents highest temperature but the former represents higher pressure than the later. The crystallization temperature of some eclogites which are not included above are also discussed.