Mineral parageneses and metamorphic P‐T paths of ultrahigh‐pressure eclogites from Kyrghyzstan Tien‐Shan

Abstract

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.

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

“…1). Eclogites in both the Aktyuz and Makbal areas experienced high pressure (HP) metamorphism (Bakirov, 1989;Tagiri et al, 1995), and have been regarded as exotic blocks in a pelitic matrix (Sobolev et al, 1986). Coesite inclusions in garnet have been described from talc garnet chloritoid schists forming the matrix in the Makbal area, suggesting that UHP metamorphic conditions were reached (Ishida et al, 2004).…”

Newly described eclogites from the Neldy Formation, Makbal district, Northern Tien-Shan, Kyrgyzstan

“…Dotted and solid lined areas are prograde and peak metamorphic conditions, respectively. Broken line area shows the estimated P-T conditions of the retrograde stage; grey broken line is the estimated retrograde P-T path of the Makbal eclogite (Tagiri et al, 1995). Squares with error bars: P-T estimates from THERMOCALC.…”

“…the Makbal Formation, and the metamorphic conditions were estimated as T= 498 -585 ˚C and P= 20 kbar (Tagiri et al, 1995). After peak eclogitic conditions the Makbal eclogite was retrograded through the epidote amphibolite to greenschist facies conditions (Fig.…”

“…Eclogites occur as lenticular bodies within the UHP matrix. The peak metamorphic conditions of these eclogites have been suggested to be T = 560 ºC and P = 20 kbar (Tagiri et al, 1995).…”

Preiswerkite and högbomite within garnets of Aktyuz eclogite, Northern Tien Shan, Kyrgyzstan

“…The Aktyuz Formation is located in the Zaili Range in Northern Tien Shan, Kyrgyzstan, and it consists of pelitic gneisses, gneissose -granites and migmatites, accompanied by eclogites, garnet amphibolites and amphibolites that occur as layers or lenticular bodies (Bakirov, 1978;Tagiri et al, 1995;Bakirov et al, 2003;Orozbaev et al, 2007Orozbaev et al, , 2010. A detailed description of the geology in the area and the location of studied sample (KG -426) were previously reported by Orozbaev et al (2007Orozbaev et al ( , 2010.…”