RETROGRADED ECLOGITES IN THE OLEKMA GRANITE GREENSTONE REGION, ALDAN SHIELD, SIBERIA

Relict eclogite facies assemblages occur in basic material which forms boudins and isoclinally folded bands in tonalite-trondhjemite gneisse s in the northeastern part of the Archaean Olekma granite-greenstone r egion, 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 co ntain relict garnet and clinopyroxene with symplectic intergrowths of sodic plagioclase. Garnets are surrounded by kelyphitic rims of plagio clase and hornblende, and clinopyroxenes are partially replaced by hor nblende. In the margins of the boudins (which have similar bulk compos ition to the central parts) garnet and clinopyroxene are absent, and t he mineral assemblage is hornblende and plagioclase, i.e. an amphiboli te facies paragenesis. The petrographic observations combined with min eral chemistry suggest that during the early stages of metamorphism th e (anhydrous) basic rocks had garnet+jadeite-rich clinopyroxene assemb lages. Subsequently, during decompression and continuing deformation, sodic plagioclase exsolved from the clinopyroxene, and garnet and clin opyroxene 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 faci es assemblages suggests conditions of 650-700-degrees-C and 5.5-7 kbar . During decompression and retrogression of the eclogites, local parti al melting of the host gneisses took place, particularly between boudi ns of mafic material.