Retrograded eclogites in the Olekma granite-greenstone region, Aldan Shield, Siberia

Abstract

Relict eclogite facies assemblages occur in basic material which forms boudins and isoclinally folded bands in tonalitetrondhjemite gneisses in the northeastern part of the Archaean Olekma granite-greenstone region, Aldan Shield, Siberia. The basic material is interpreted as the remains of dykes which were deformed and metamorphosed during a major orogenic event. The central parts of thicker (3–7 m) basic boudins contain relict garnet and clinopyroxene with symplectic intergrowths of sodic plagioclase. Garnets are surrounded by kelyphitic rims of plagioclase and hornblende, and clinopyroxenes are partially replaced by hornblende. In the margins of the boudins (which have similar bulk composition to the central parts) garnet and clinopyroxene are absent, and the mineral assemblage is hornblende and plagioclase, i.e. an amphibolite facies paragenesis. The petrographic observations combined with mineral chemistry suggest that during the early stages of metamorphism the (anhydrous) basic rocks had garnet +jadeite-rich clinopyroxene assemblages. Subsequently, during decompression and continuing deformation, sodic plagioclase exsolved from the clinopyroxene, and garnet and clinopyroxene were variably recrystallised to give rise to the hornblende + plagioclase amphibolite facies assemblages. The eclogites must have equilibrated at 10–15 kbar, and geothermometry of the amphibolite facies assemblages suggests conditions of 650–700°C and 5.5–7 kbar. During decompression and retrogression of the eclogites, local partial melting of the host gneisses took place, particularly between boudins of mafic material.