Polymetamorphism of Aktyuz eclogites (northern Kyrgyz Tien-Shan) deduced from inclusions in garnets

摘要

Garnet and omphacite in eclogites from the Aktyuz area, northern Kyrgyz Tien-Shan, preserve evidence of two distinct metamorphic events. Relic polyphase inclusions of Fe-rich staurolite (X_(Mg) < 0.29), Mg-taramite, paragonite, and oligoclase (An < 16) contained in garnet cores represent a separate preeclogitic amphibolite or epidote-amphibolite facies metamorphism (T = 560-650℃, P = 4-10 kbar). This relatively low- to medium-pressure metamorphic event probably resulted from the heat of the hot hanging wall at the inception of subduc-tion. Inclusions in the garnet cores and rims and matrix omphacite (glaucophane, sodic-calcic and calcic amphiboles, paragonite, phengite, epidote, albite, rutile, hematite, and quartz) suggest that epidote-blueschist facies conditions (T= 330-570℃, P = 8 16 kbar) were experienced during the prograde stage of the eclogitic metamorphism. This event occurred within a subduction zone. The estimated peak metamorphic conditions of the eclogites are T= 600-710℃ and P = 15-25 kbar. The subsequent retrograde path through epidote-amphibolite and greenschist facies suggests a clockwise P-T path, in contrast to the anticlockwise P-T path proposed by previous work.