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Orozbaev R.,Kyoto University | Orozbaev R.,Kyrgyz National Academy of science | Hirajima .,Kyoto University | Bakirov A.,Kyrgyz National Academy of science | And 7 more authors.
Lithos | Year: 2015

Polyphase mineral aggregates (PMAs) composed of clinozoisite. +. kyanite. +. quartz. ± chlorite. ± paragonite. ± phengite have been found within garnet and in the matrix of talc-garnet-chloritoid schists from the Makbal ultrahigh-pressure complex in the northern Kyrgyz Tian-Shan. These mineral textures are interpreted as pseudomorphs after lawsonite, and we reconstructed the compositions of PMAs of clinozoisite. +. kyanite. +. quartz, consistent with lawsonite. Petrological study demonstrated that lawsonite was stable during the prograde to the UHP peak stage (P = 28-33. kbar and T = 530-580. °C) and decomposed to the PMAs during isothermal decompression around P = 16-20. kbar and T = 510-580. °C. Trace element characteristics of the clinozoisite grains in the PMAs (former lawsonite) show a flat rare earth element (REE) chondrite-normalized pattern, comparable with the typical reported REE pattern of lawsonite, although the abundance of REE varied from sample to sample. Thus, the REE content of clinozoisite in the PMAs included in garnet was likely inherited from the former lawsonite as the decomposition reaction took place isolated from the matrix. Discrete clinozoisite grains in the matrix have high light REE enrichment over heavy REE in the chondrite-normalized pattern, consistent with the typical epidote pattern. Our results indicate that the talc-garnet-chloritoid schists in the Makbal complex were buried to great depth (>. 100. km) with a low geothermal gradient (<. 6. °C/km) during the Early Paleozoic (480-509. Ma). Lawsonite decomposition and clinozoisite-forming reactions accompany fluid release during the isothermal decompression stage, implying that the fluids can be generated not only during subduction, but also during exhumation of ultrahigh-pressure rocks in cold subduction settings. © 2014 Elsevier B.V..

Orozbaev R.T.,Kyoto University | Orozbaev R.T.,Kyrgyz National Academy of science | Yoshida K.,Kyoto University | Bakirov A.B.,Kyrgyz National Academy of science | And 4 more authors.
Journal of Mineralogical and Petrological Sciences | Year: 2011

We report the occurrence of preiswerkite and högbomite as inclusion phases within the garnets of eclogite from the Aktyuz area of Northern Tien Shan, Kyrgyzstan. Preiswerkite and högbomite occur both as a constituent of multiphase solid inclusions (MSI) and as single discrete grains in the mantle and rim of the garnets. However, they do not occur in the core of the garnet and in the matrix of the eclogite. Preiswerkite is associated with the minerals paragonite ± staurolite ± Mg-taramite ± Na-biotite ± hematite ± högbomite ± chlorite ± titanite ± phengite ± magnetite, and högbomite is associated with paragonite ± preiswerkite ± staurolite ± hematite ± chlorite ± Na-biotite ± magnetite in MSI. The average compositions of preiswerkite and högbomite are (Na 0.96K 0.02Ca 0.01) 0.99(Mg 1.52Fe 2+ 0.54 VIAl 0.93) 2.99( IVAl 1.93Si 2.07) 4.00O 10(OH) 2 and (Mg 1.47Fe 2+ 3.02Zn 0.04Fe 3+ 1.45) 5.98(Fe 3+ 0.31Al 15.13Ti 0.56) 16O 30(OH) 2, respectively. Na-biotite, with an average composition of (Na 0.89K 0.07Ca 0.01) 0.97(Mg 1.66Fe 2+ 0.69 VIAl 0.63) 2.98( IVAl 1.57Si 2.43) 4.00O 10(OH) 2, corresponding to the intermediate composition between preiswerkite and aspidolite (i.e., Naphlogopite), is also observed. The compositions of the newly found preiswerkite and Na-biotite with similar X Mg values (0.66-0.78) are arrayed along preiswerkite-aspidolite solid solution series. The mode of occurrence of inclusion phases in garnets may suggest that the activity of Na-Al-rich and Si-undersaturated aqueous fluids played a major role in the formation of preiswerkite during the prograde stage of high-pressure eclogitic metamorphism.

Togonbaeva A.,The University of Shimane | Takasu A.,The University of Shimane | Tagiri M.,Ibaraki University | Bakirov A.B.,Kyrgyz National Academy of science | And 2 more authors.
Journal of Mineralogical and Petrological Sciences | Year: 2010

Eclogites occurring in the Neldy Formation of the Makbal district in the Kyrgyz Northern Tien-Shan are newly described. The metamorphism of the eclogites is divided into two events, a precursor metamorphic event and a subsequent eclogitic metamorphic event. The metamorphic conditions of the precursor event are represented by the amphibolite facies. The peak metamorphic conditions of the eclogitic event are estimated to be T = 550-610 C and P = 22-25 kbars. The prograde, peak and retrograde metamorphic conditions define a hairpin-type clockwise P-T path for the Neldy eclogites.

Orozbaev R.T.,The University of Shimane | Orozbaev R.T.,Kyrgyz National Academy of science | Takasu A.,The University of Shimane | Bakirov A.B.,Kyrgyz National Academy of science | And 2 more authors.
Journal of Metamorphic Geology | Year: 2010

Eclogites and related high-P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien-Shan (Tianshan) Mountains, which are located in the south-western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1-M3). In the eclogites, the first MP-HT metamorphic event (M1) of amphibolite/epidote-amphibolite facies conditions (560-650 °C, 4-10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg-taramite + Fe-staurolite + paragonite ± oligoclase (An<16) ± hematite. The eclogites also record the second HP-LT metamorphism (M2) with a prograde stage passing through epidote-blueschist facies conditions (330-570 °C, 8-16 kbar) to peak metamorphism in the eclogite facies (550-660 °C, 21-23 kbar) and subsequent retrograde metamorphism to epidote-amphibolite facies conditions (545-565 °C and 10-11 kbar) that defines a clockwise P-T path. thermocalc (average P-T mode) calculations and other geothermobarometers have been applied for the estimation of P-T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP-HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600-640 °C; 11-12 kbar and 675-735 °C and 14-15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote-amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An11-16), phengite, biotite, quartz and rutile indicate 635-745 °C and 13-15 kbar. The P-T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote-amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent-continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo-ocean subduction to its completion by continent-continent collision. © 2010 Blackwell Publishing Ltd.

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