On the porphyroblastic eclogites of the Śnieżnik Mountains in the Polish Sudetes

Abstract

A rare textural variety of eclogite characterized by a two-fold occurrence of omphacite — as older microlites and as later porphyroblasts enclosing numerous microlites — has been found at only two localities in the Polish Sudetes: Wojtówka and Nowa Wieś. The chemical compositions of 27 mineral grains in four samples of such eclogites have been determined by electron microprobe spot analysis. All omphacite analyses of both porphyroblasts and microlites show almost identical compositions except for one microlite enclosed in an armoured relic of tremolite. This microlite displays a lower Na content and is considered to belong to an earlier stage of metamorphic evolution, preceding the stage of eclogitization. It is deduced that these porphyroblastic eclogites experienced three successive stages of metamorphic evolution: 1. (1) Pre-eclogite stage, when the primary rocks developed the mineral composition of a low-temperature metabasite (greenschist facies). 2. (2) Eclogite stage, when the rock gained typical eclogite mineral compositions as a consequence of a strong increase in total pressure unaccompanied by an increase in P H 2O (eclogite facies). 3. (3) Post-eclogite stage, when the garnet—omphacite association ceased to be stable and was transformed into a hornblende—plagioclase association as a result of the decrease of total pressure which became equal to P H 2O (amphibolite facies).