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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.

Tagiri M.,Hitachi City Museum | Dunkley D.J.,Japan National Institute of Polar Research | Adachi T.,Kyushu University | Hiroi Y.,Chiba University | Fanning C.M.,Australian National University
Island Arc | Year: 2011

Ion microprobe dating of zircon from meta-igneous samples of the Hitachi metamorphic terrane of eastern Japan yields Cambrian magmatic ages. Tuffaceous schist from the Nishidohira Formation contains ca 510Ma zircon, overlapping in age with hornblende gneiss from the Tamadare Formation (ca 507Ma), and meta-andesite (ca 507Ma) and metaporphyry (ca 505Ma) from the Akazawa Formation. The latter is unconformably overlain by the Carboniferous Daioin Formation, in which a granite boulder from metaconglomerate yields a magmatic age of ca 500Ma. This date overlaps a previous estimate for granite that intrudes the Akazawa Formation. Intrusive, volcanic, and volcaniclastic lithologies are products of a Cambrian volcanic arc associated with a continental shelf, as demonstrated by the presence of arkose and conglomerate in the lowermost Nishidohira Formation. Granitic magmatism of Cambrian age is unknown elsewhere in Japan, except for a single locality in far western Japan with a similar geological context. Such magmatism is also unknown on the adjacent Asian continental margin, with the exception of the Khanka block in far northeastern China. A 'great hiatus' in the Paleozoic stratigraphy of the Sino-Korean block also exists in the Hitachi terrane between Cambrian volcanic arc rocks and Early Carboniferous conglomerate, and may indicate a common paleogeographic provenance. © 2011 Blackwell Publishing Asia Pty Ltd.

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