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An (in-)coherent metamorphic evolution of high-P eclogites and their host rocks in the Chinese southwest Tianshan?

Meyer, M. ; Klemd, R. ; John, T. ; Gao, J. ; Menneken, M.

Journal of metamorphic geology, 2016-02, Vol.34 (2), p.121-146 [Periódico revisado por pares]

Oxford: Blackwell Publishing Ltd

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  • Título:
    An (in-)coherent metamorphic evolution of high-P eclogites and their host rocks in the Chinese southwest Tianshan?
  • Autor: Meyer, M. ; Klemd, R. ; John, T. ; Gao, J. ; Menneken, M.
  • Assuntos: (U)HP metamorphism ; eclogite ; Geochemistry ; Metamorphism ; Mineralogy ; P-T path ; thermobarometry ; Tianshan
  • É parte de: Journal of metamorphic geology, 2016-02, Vol.34 (2), p.121-146
  • Notas: ArticleID:JMG12175
    Deutsche Forschungsgemeinschaft - No. KL692/17-3
    Figure S1. Sample locations, structural measurements (foliation) and field relations of (metasedimentary) host rocks and mafic blocks, boudins and layers. All samples are derived from the same valley (indicated by the star in Fig. ). The small grey circles indicate other occurrences of mafic rocks embedded in voluminous metasedimentary host rocks. Figure S2. Temperature distribution maps (RSCM thermometry) of metasedimentary samples 11-06-4 (a), 11-11-8 (b) and 11-12-8 (c) mapped with Raman spectroscopy. For temperature calculation the calibration of Beyssac et al. () was used. Table S1. Representative mineral chemical composition of eclogites, metasedimentary rocks and a metavolcanic schist. Elements are given in wt%, structural formulae in atoms per formula unit. Table S2. Mineral assemblages of host rocks and mafic rocks investigated in this study. Table S3. Comparison of maximum temperatures calculated with different calibrations of RSCM thermometry.
    ark:/67375/WNG-P0QZ2QK7-V
    istex:F4B2231B7B4AA8165BFF0EFB84D8831A648D3231
    National Natural Science Foundation of China - No. 41025008
  • Descrição: In the (ultra‐)high‐P–low‐T metamorphic terrane of the Chinese South Tianshan, discontinuous mafic blocks and boudins (former upper oceanic crust) are now embedded in voluminous (mainly metasedimentary) host rocks. Two different models were proposed and relate the occurrence of both high‐P and ultra‐high‐P mafic and metasedimentary rocks to either (i) a tectonic mélange style exhumation, with no exhumation of coherent units, but different lithologies derived from different depths juxtaposed and intermingled during exhumation in the subduction channel, or (ii) the evolution of two coherent metamorphic belts: one with high‐P and the other with ultra‐high‐P conditions. In contrast to most previous studies in the Chinese South Tianshan which focused either on single eclogites or metasedimentary rocks (assumed as representative), this study concentrates on the systematic investigation of both mafic boudins and their immediate sedimentary host rocks, because the investigation of both lithologies and the comparison of their metamorphic evolution is crucial to reconstruct the geodynamical context of the whole (ultra‐)high‐P–low‐T metamorphic complex. Several sample pairs consisting of both lithologies were geochemically investigated and their respective metamorphic evolution was reconstructed using geothermobarometry and thermodynamic modelling. The latter approach considers changes in the mineral assemblage during the metamorphic evolution, as well as changes in mineral composition, which may help to determine the metamorphic history of a rock despite the preservation of critical mineral assemblages. All samples experienced a clockwise P–T path with overall maximum P–T conditions of 540–550 °C and 1.9–2.25 GPa for the host rocks, and 555–575 °C and 2.2–2.5 GPa for the eclogites. Peak‐metamorphic temperatures of ~525–540 °C of the metasedimentary host rocks were also confirmed by Raman spectroscopy of carbonaceous material. Results from thermobarometry and thermodynamic modelling are consistent with the observation that none of the samples contains mineral relicts indicating UHP conditions (like coesite in garnet) and neither conventional thermobarometry, nor thermodynamical modelling resulted in P–T conditions in the stability field of coesite. Thus, no evidence of ultra‐high‐P conditions was found. Given that the whole sampled river valley lies within the proposed ‘ultra‐high‐P sub‐belt’ and considering former studies, which showed that at other places within this ‘unit’ both ultra‐high‐P and high‐P rocks are now juxtaposed on a small scale, the formation of the whole (ultra‐)high‐P–low‐T metamorphic belt in the Chinese South Tianshan as a tectonic mélange style exhumation is more convincing than the formation and juxtaposition of two coherent metamorphic units with high‐P and ultra‐high‐P conditions respectively.
  • Editor: Oxford: Blackwell Publishing Ltd
  • Idioma: Inglês
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  • Realização: ABCD-USP USP   Logos de Redes Sociais: Freepik

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