Eclogites occur in three districts of the northern and southern parts of Tien-Shan. Three eclogites collected from the Aktyuz, Makbal and Atbashy districts were analyzed; the P-T paths of three eclogites were estimated by analyzing compositional growth zoning and retrograde reaction of garnet and omphacite. Aktyuz and Makbal eclogites have not preserved the prograde path. An Aktyuz eclogite that underwent a quartz eclogite facies metamorphism (about T = 600"C, P = 12 kbar) has recorded three stages of retrograde metamorphism. Four stages of retrograde metamorphism were recognized in a Makbal eclogite; the garnet-omphacite geothermometer gave about T = 560°C at 20 kbar as the highest metamorphic condition. Garnet from a garnetchloritoid-talc schist of the Makbal district includes quartz pseudomorphs after coesite; some units evidently underwent a low-temperature part of coesite eclogite facies metamorphism. Prograde and retrograde paths were recognized in an Atbashy eclogite; five stages of metamorphic reaction were observed in the Atbashy sample. The prograde path from stage I to stage I11 has been recorded in garnet and omphacite in which quartz pseudomorphs after coesite are included. The peak metamorphism of stage I11 took place at about 660°C at 25 kbar. The stages IV and V are retrograde. UHP eclogite facies metamorphism took place twice in Kyrghyzstan. The Aktyuz and Atbashy eclogites gave Rb-Sr mineral-isochron ages of about 750 Ma and 270 Ma, respectively. The K-Ar age of paragonite from the Makbal eclogite is about 480 Ma.
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 pre eclogitic amphibolite or epidote amphibolite facies metamorphism (T = 560 650 °C, 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 subduction. 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 °C, 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 °C 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.
Quartz pseudomorphs after coesite were discovered in garnet from a pelitic garnet-chloritoid-talc schist. The pseudomorphs accompany radial cracks developed from them. A metamorphic condition of low temperature (<600°C) and very high pressure (>2.4 GPa) is estimated from the mineral assemblage and mineral compositions.Sedimentary rocks were brought to the depth of about 80 km or more by subduction of oceanic lithosphere or by collision of two continents.
Coesite occurs in garnets from quartz schists and pelitic schists in the Makbal Complex of Kyrgyzian TienShan. The thick quartz schists interbedded with pelitic schists are the transitional facies after coesite schists. Quartz -pseudomorph after coesite also appears widely in pelitic schists and quartz schists, but is rare in eclogitic rocks. The growth zoning of garnets and Na -Ca amphiboles in eclogites and the garnet -omphacite geothermometry indicate that the metamorphic P -T paths of eclogites are different from each. Ultra -high (UHP) pressure metamorphic rocks of the Makbal Complex are mainly composed of pelitic and quartz schists. All eclogite lenses are included in the host UHPM rocks, although most of them do not contain UHP metamorphic (UHPM) evidences. The K -Ar ages of phengite in a host pelitic schist and of paragonite in an eclogite are approximately 500 Ma, while those of biotite and phengite in a biotite -bearing mica schist and of winchite in a winchite schist are approximately 769 -717 Ma and 881 Ma, respectively. The evidence indicates that this complex is a tectonic mélange. These geological relationships suggest that the ascending substances must be pelitic and quartz schists. The UHPM rocks in the ascending material capture many exotic blocks as xenoliths during intrusion into the upper crust.
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