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Shuguang Song - One of the best experts on this subject based on the ideXlab platform.
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Variation of mineral composition, fabric and oxygen fugacity from massive to foliated eclogites during exhumation of subducted ocean crust in the North Qilian suture zone, NW China
Journal of Metamorphic Geology, 2011Co-Authors: Shuguang Song, Haemyeong JungAbstract:Eclogites from the North Qilian suture zone are high-pressure low-temperature metamorphic rocks of ocean crust protolith, and occur in both massive and foliated varieties as individual blocks of tens to hundreds of metres in size. The massive type is weakly deformed and shows granoblastic texture characterized by a coarse-grained peak mineral assemblage of Grt1 + Omp1 + Ph + Rt ± Lws (or retrograde Cz). In contrast, the foliated type is strongly deformed and shows a fine-grained retrograde mineral assemblage of Grt2 + Omp2 + Cz + Gln + Ph. Both total FeO and aegirine contents in Omphacite, as well as XFe[=Fe3+/(Fe3+ + AlVI)] in clinozoisite/epidote, increase significantly from massive to foliated eclogites. Lattice preferred orientation (LPO) of Omphacite, determined by electron back-scatter diffraction analysis, is characterized by weak and strong SL-type fabrics for massive and foliated eclogites, respectively. Clinozoisite/epidote also developed SL-type fabric, but different from the LPOs of Omphacite in and axes, owing to their opposite crystallographic long and short axis definitions. The transition of deformation mechanism from dislocation creep to diffusive mass transfer (DMT) creep in Omphacite and the concomitant retrograde metamorphism both are efficiently facilitated when the original coarse-grained Omp1 + Grt1 + Lws assemblage is dynamically recrystallized and retrogressed into the fine-grained Fe3+-rich assemblage of Omp2 + Grt2 + Cz + Gln. The DMT process with concomitant anisotropic growth assisted by fluids is considered to be an important deformation mechanism for most minerals in the foliated eclogite. P–T estimates yielded 2.3–2.6 GPa and 485−510 °C for the massive eclogite and 1.8–2.2 GPa and 450−480 °C for the foliated eclogite. The significant increase in total Fe and Fe3+ contents in Omphacite and clinozoisite/epidote from massive to foliated eclogite suggests changes in mineral compositions accompanied by an increase in oxygen fugacity during ductile deformation associated with exhumation. The LPO transition of Omphacite, clinozoisite and rutile from weak SL-type in massive eclogites to strong SL-type in foliated eclogites is interpreted to represent the increment of shear strain during exhumation along the ‘subduction channel’.
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Variation of mineral composition, fabric and oxygen fugacity from massive to foliated eclogites during exhumation of subducted ocean crust in the North Qilian suture zone, NW China
Journal of Metamorphic Geology, 2011Co-Authors: Shuguang Song, Haemyeong JungAbstract:Eclogites from the North Qilian suture zone are high-pressure low-temperature metamorphic rocks of ocean crust protolith, and occur in both massive and foliated varieties as individual blocks of tens to hundreds of metres in size. The massive type is weakly deformed and shows granoblastic texture characterized by a coarse-grained peak mineral assemblage of Grt1 + Omp1 + Ph + Rt ± Lws (or retrograde Cz). In contrast, the foliated type is strongly deformed and shows a fine-grained retrograde mineral assemblage of Grt2 + Omp2 + Cz + Gln + Ph. Both total FeO and aegirine contents in Omphacite, as well as XFe[=Fe3+/(Fe3+ + AlVI)] in clinozoisite/epidote, increase significantly from massive to foliated eclogites. Lattice preferred orientation (LPO) of Omphacite, determined by electron back-scatter diffraction analysis, is characterized by weak and strong SL-type fabrics for massive and foliated eclogites, respectively. Clinozoisite/epidote also developed SL-type fabric, but different from the LPOs of Omphacite in and axes, owing to their opposite crystallographic long and short axis definitions. The transition of deformation mechanism from dislocation creep to diffusive mass transfer (DMT) creep in Omphacite and the concomitant retrograde metamorphism both are efficiently facilitated when the original coarse-grained Omp1 + Grt1 + Lws assemblage is dynamically recrystallized and retrogressed into the fine-grained Fe3+-rich assemblage of Omp2 + Grt2 + Cz + Gln. The DMT process with concomitant anisotropic growth assisted by fluids is considered to be an important deformation mechanism for most minerals in the foliated eclogite. P–T estimates yielded 2.3–2.6 GPa and 485−510 °C for the massive eclogite and 1.8–2.2 GPa and 450−480 °C for the foliated eclogite. The significant increase in total Fe and Fe3+ contents in Omphacite and clinozoisite/epidote from massive to foliated eclogite suggests changes in mineral compositions accompanied by an increase in oxygen fugacity during ductile deformation associated with exhumation. The LPO transition of Omphacite, clinozoisite and rutile from weak SL-type in massive eclogites to strong SL-type in foliated eclogites is interpreted to represent the increment of shear strain during exhumation along the ‘subduction channel’.
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Relict coesite exsolution in Omphacite from Western Tianshan eclogites, China
American Mineralogist, 2005Co-Authors: Lifei Zhang, Shuguang Song, J G LiouAbstract:Exsolution rods of relict coesite together with quartz were identified in Omphacite in eclogites from western Tianshan, China. They are oriented along the c-axis of the host clinopyroxenes and have grain size up to 30 μm long and 2-3 μm wide. Raman spectra of exsolved lamellae yield consistent but weak bands at 521, 270, 181, 151, and 118 cm - 1 , typical for coesite, in addition to those of quartz and the host Omphacite. Such occurrences together with textured observations suggest a two-stage evolution of SiO 2 exsolution rods in Omphacite. Lamellae of coesite were apparently exsolved from supersilicic Omphacite at P 5.0 GPa and the transformation from coesite to quartz occurred during retrograde metamorphism.
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relict coesite exsolution in Omphacite from western tianshan eclogites china
American Mineralogist, 2005Co-Authors: Lifei Zhang, Shuguang Song, J G Liou, Yongliang Ai, Xuping LiAbstract:Exsolution rods of relict coesite together with quartz were identified in Omphacite in eclogites from western Tianshan, China. They are oriented along the c-axis of the host clinopyroxenes and have grain size up to 30 μm long and 2-3 μm wide. Raman spectra of exsolved lamellae yield consistent but weak bands at 521, 270, 181, 151, and 118 cm - 1 , typical for coesite, in addition to those of quartz and the host Omphacite. Such occurrences together with textured observations suggest a two-stage evolution of SiO 2 exsolution rods in Omphacite. Lamellae of coesite were apparently exsolved from supersilicic Omphacite at P 5.0 GPa and the transformation from coesite to quartz occurred during retrograde metamorphism.
Haemyeong Jung - One of the best experts on this subject based on the ideXlab platform.
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Variation of mineral composition, fabric and oxygen fugacity from massive to foliated eclogites during exhumation of subducted ocean crust in the North Qilian suture zone, NW China
Journal of Metamorphic Geology, 2011Co-Authors: Shuguang Song, Haemyeong JungAbstract:Eclogites from the North Qilian suture zone are high-pressure low-temperature metamorphic rocks of ocean crust protolith, and occur in both massive and foliated varieties as individual blocks of tens to hundreds of metres in size. The massive type is weakly deformed and shows granoblastic texture characterized by a coarse-grained peak mineral assemblage of Grt1 + Omp1 + Ph + Rt ± Lws (or retrograde Cz). In contrast, the foliated type is strongly deformed and shows a fine-grained retrograde mineral assemblage of Grt2 + Omp2 + Cz + Gln + Ph. Both total FeO and aegirine contents in Omphacite, as well as XFe[=Fe3+/(Fe3+ + AlVI)] in clinozoisite/epidote, increase significantly from massive to foliated eclogites. Lattice preferred orientation (LPO) of Omphacite, determined by electron back-scatter diffraction analysis, is characterized by weak and strong SL-type fabrics for massive and foliated eclogites, respectively. Clinozoisite/epidote also developed SL-type fabric, but different from the LPOs of Omphacite in and axes, owing to their opposite crystallographic long and short axis definitions. The transition of deformation mechanism from dislocation creep to diffusive mass transfer (DMT) creep in Omphacite and the concomitant retrograde metamorphism both are efficiently facilitated when the original coarse-grained Omp1 + Grt1 + Lws assemblage is dynamically recrystallized and retrogressed into the fine-grained Fe3+-rich assemblage of Omp2 + Grt2 + Cz + Gln. The DMT process with concomitant anisotropic growth assisted by fluids is considered to be an important deformation mechanism for most minerals in the foliated eclogite. P–T estimates yielded 2.3–2.6 GPa and 485−510 °C for the massive eclogite and 1.8–2.2 GPa and 450−480 °C for the foliated eclogite. The significant increase in total Fe and Fe3+ contents in Omphacite and clinozoisite/epidote from massive to foliated eclogite suggests changes in mineral compositions accompanied by an increase in oxygen fugacity during ductile deformation associated with exhumation. The LPO transition of Omphacite, clinozoisite and rutile from weak SL-type in massive eclogites to strong SL-type in foliated eclogites is interpreted to represent the increment of shear strain during exhumation along the ‘subduction channel’.
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Variation of mineral composition, fabric and oxygen fugacity from massive to foliated eclogites during exhumation of subducted ocean crust in the North Qilian suture zone, NW China
Journal of Metamorphic Geology, 2011Co-Authors: Shuguang Song, Haemyeong JungAbstract:Eclogites from the North Qilian suture zone are high-pressure low-temperature metamorphic rocks of ocean crust protolith, and occur in both massive and foliated varieties as individual blocks of tens to hundreds of metres in size. The massive type is weakly deformed and shows granoblastic texture characterized by a coarse-grained peak mineral assemblage of Grt1 + Omp1 + Ph + Rt ± Lws (or retrograde Cz). In contrast, the foliated type is strongly deformed and shows a fine-grained retrograde mineral assemblage of Grt2 + Omp2 + Cz + Gln + Ph. Both total FeO and aegirine contents in Omphacite, as well as XFe[=Fe3+/(Fe3+ + AlVI)] in clinozoisite/epidote, increase significantly from massive to foliated eclogites. Lattice preferred orientation (LPO) of Omphacite, determined by electron back-scatter diffraction analysis, is characterized by weak and strong SL-type fabrics for massive and foliated eclogites, respectively. Clinozoisite/epidote also developed SL-type fabric, but different from the LPOs of Omphacite in and axes, owing to their opposite crystallographic long and short axis definitions. The transition of deformation mechanism from dislocation creep to diffusive mass transfer (DMT) creep in Omphacite and the concomitant retrograde metamorphism both are efficiently facilitated when the original coarse-grained Omp1 + Grt1 + Lws assemblage is dynamically recrystallized and retrogressed into the fine-grained Fe3+-rich assemblage of Omp2 + Grt2 + Cz + Gln. The DMT process with concomitant anisotropic growth assisted by fluids is considered to be an important deformation mechanism for most minerals in the foliated eclogite. P–T estimates yielded 2.3–2.6 GPa and 485−510 °C for the massive eclogite and 1.8–2.2 GPa and 450−480 °C for the foliated eclogite. The significant increase in total Fe and Fe3+ contents in Omphacite and clinozoisite/epidote from massive to foliated eclogite suggests changes in mineral compositions accompanied by an increase in oxygen fugacity during ductile deformation associated with exhumation. The LPO transition of Omphacite, clinozoisite and rutile from weak SL-type in massive eclogites to strong SL-type in foliated eclogites is interpreted to represent the increment of shear strain during exhumation along the ‘subduction channel’.
Jürgen Konzett - One of the best experts on this subject based on the ideXlab platform.
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high pressure synthetic na0 97mg0 03 mg0 43fe0 173 si0 40 si2o6 with six coordinated silicon isostructural with p2 n Omphacite
Acta Crystallographica Section E-structure Reports Online, 2012Co-Authors: Esther S. Posner, Jürgen Konzett, Daniel J. Frost, Robert T. Downs, Hexiong YangAbstract:The title compound, (sodium magnesium) [magnesium iron(III) silicon] disilicate, (Na0.97Mg0.03)(Mg0.43Fe0.173+Si0.40)Si2O6, is isotypic with ordered P2/n Omphacite. Its structure is characterized by single chains of corner-sharing SiO4 tetrahedra, extending along the c axis, which are crosslinked by bands of edge-sharing octahedra (site symmetry 2), statistically occupied by (Mg2+ + Fe3+ + Si4+). Between the bands built up of the octahedra are two non-equivalent highly distorted six-coordinated sites (site symmetry 2), statistically occupied by (Na + Mg). In contrast to Omphacites, the great differences in size and charge between Mg2+ and Si4+ result in complete, rather than partial, ordering of Mg and Si into two distinct octahedral sites, whereas Fe3+ is disordered between the two sites. The octahedron filled by (Mg + Fe) is larger and markedly more distorted than that occupied by (Si + Fe). The average (Mg + Fe)—O and (VISi + Fe)—O bond lengths are 2.075 and 1.850 A, respectively.
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High-pressure synthetic (Na0.97Mg0.03)(Mg0.43Fe0.173+Si0.40)Si2O6, with six-coordinated silicon, isostructural with P2/n Omphacite
Acta Crystallographica Section E Structure Reports Online, 2012Co-Authors: Esther S. Posner, Jürgen Konzett, Daniel J. Frost, Robert T. Downs, Hexiong YangAbstract:The title compound, (sodium magnesium) [magnesium iron(III) silicon] disilicate, (Na0.97Mg0.03)(Mg0.43Fe0.173+Si0.40)Si2O6, is isotypic with ordered P2/n Omphacite. Its structure is characterized by single chains of corner-sharing SiO4 tetrahedra, extending along the c axis, which are crosslinked by bands of edge-sharing octahedra (site symmetry 2), statistically occupied by (Mg2+ + Fe3+ + Si4+). Between the bands built up of the octahedra are two non-equivalent highly distorted six-coordinated sites (site symmetry 2), statistically occupied by (Na + Mg). In contrast to Omphacites, the great differences in size and charge between Mg2+ and Si4+ result in complete, rather than partial, ordering of Mg and Si into two distinct octahedral sites, whereas Fe3+ is disordered between the two sites. The octahedron filled by (Mg + Fe) is larger and markedly more distorted than that occupied by (Si + Fe). The average (Mg + Fe)—O and (VISi + Fe)—O bond lengths are 2.075 and 1.850 A, respectively.
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oriented quartz calcic amphibole inclusions in Omphacite from the saualpe and pohorje mountain eclogites eastern alps an assessment of possible formation mechanisms based on ir and mineral chemical data and water storage in eastern alpine eclogites
Lithos, 2008Co-Authors: Jürgen Konzett, Christine Miller, Eugen Libowitzky, Clivia Hejny, Alberto ZanettiAbstract:Abstract The composition of mineral phases and their modal proportions have been determined for three representative Eoalpine eclogites from the Saualpe type locality/Eastern Austria (sample SKP31) and the Pohorje Massif/Slovenia (CM31/03 and CM15/01) using electron microprobe, laser ICP-MS, IR spectroscopy and modal analysis to evaluate possible mechanisms for the formation of composite oriented calcic amphibole + quartz inclusions (COIs) in Omphacite and to assess the relative importance of hydrous and nominally anhydrous phases as H 2 O carriers in these eclogites. For Omphacites in CM31/03 with a zonal distribution of COIs, a comparison of water and trace element concentrations of areas containing COIs and those free of COIs and a comparison with the trace element concentration of calcic amphibole indicate that COIs have formed through an open-system alteration of clinopyroxene and not through a closed system exsolution process. In sample SKP31, both textural and mineral chemical evidence suggests that COIs did not form by exsolution involving a Ca-Eskola component in clinopyroxene but formed by progressive growth under eclogite-facies P-T conditions and prior to the onset of retrogressive symplectite formation analogous to the formation of poikiloblastic quartz–calcic amphibole grains in the matrix. Bulk H 2 O contents of the eclogites are between ca. 750 and 2150 ppm with 6–25% of the total water contributed by nominally anhydrous minerals (NAMs). Because of high modal amounts of 37–65%, Omphacite is the major nominally anhydrous water carrier, containing 145–580 ppm H 2 O with significant concentration variations on a thin section scale. Due to their very low H 2 O concentrations of 2 O content. Calcic amphibole forming part of COIs may be a major carrier of H 2 O as evidenced by CM31/03 containing both COIs and texturally primary calcic amphibole. In this sample calcic amphibole of the COIs contributes 63% whereas primary calcic amphibole only accounts for 13% of the bulk water. The relative order of H 2 O concentrations in NAMs is H 2 O rutile > H 2 O Omphacite ≫ H 2 O garnet for CM31/03 and CM15/01 and H 2 O rutile ≈ H 2 O Omphacite ≫ H 2 O garnet in SKP31.
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Oriented quartz + calcic amphibole inclusions in Omphacite from the Saualpe and Pohorje Mountain eclogites, Eastern Alps—An assessment of possible formation mechanisms based on IR- and mineral chemical data and water storage in Eastern Alpine eclogit
Lithos, 2008Co-Authors: Jürgen Konzett, Christine Miller, Eugen Libowitzky, Clivia Hejny, Anna ZanettiAbstract:Abstract The composition of mineral phases and their modal proportions have been determined for three representative Eoalpine eclogites from the Saualpe type locality/Eastern Austria (sample SKP31) and the Pohorje Massif/Slovenia (CM31/03 and CM15/01) using electron microprobe, laser ICP-MS, IR spectroscopy and modal analysis to evaluate possible mechanisms for the formation of composite oriented calcic amphibole + quartz inclusions (COIs) in Omphacite and to assess the relative importance of hydrous and nominally anhydrous phases as H 2 O carriers in these eclogites. For Omphacites in CM31/03 with a zonal distribution of COIs, a comparison of water and trace element concentrations of areas containing COIs and those free of COIs and a comparison with the trace element concentration of calcic amphibole indicate that COIs have formed through an open-system alteration of clinopyroxene and not through a closed system exsolution process. In sample SKP31, both textural and mineral chemical evidence suggests that COIs did not form by exsolution involving a Ca-Eskola component in clinopyroxene but formed by progressive growth under eclogite-facies P-T conditions and prior to the onset of retrogressive symplectite formation analogous to the formation of poikiloblastic quartz–calcic amphibole grains in the matrix. Bulk H 2 O contents of the eclogites are between ca. 750 and 2150 ppm with 6–25% of the total water contributed by nominally anhydrous minerals (NAMs). Because of high modal amounts of 37–65%, Omphacite is the major nominally anhydrous water carrier, containing 145–580 ppm H 2 O with significant concentration variations on a thin section scale. Due to their very low H 2 O concentrations of 2 O content. Calcic amphibole forming part of COIs may be a major carrier of H 2 O as evidenced by CM31/03 containing both COIs and texturally primary calcic amphibole. In this sample calcic amphibole of the COIs contributes 63% whereas primary calcic amphibole only accounts for 13% of the bulk water. The relative order of H 2 O concentrations in NAMs is H 2 O rutile > H 2 O Omphacite ≫ H 2 O garnet for CM31/03 and CM15/01 and H 2 O rutile ≈ H 2 O Omphacite ≫ H 2 O garnet in SKP31.
Lifei Zhang - One of the best experts on this subject based on the ideXlab platform.
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Ultra-deep subduction of Yematan eclogite in the North Qaidam UHP belt, NW China: Evidence from phengite exsolution in Omphacite
American Mineralogist, 2015Co-Authors: Lei Han, Lifei Zhang, Guibin ZhangAbstract:Phengite exsolution in Omphacite from the Yematan eclogite, North Qaidam UHP belt, NW China, is described. Mineralogical investigations show that the precursor Omphacite in the Yematan eclogite contains up to 1.16 wt% K2O and ~10 000 ppm H2O. Experimental studies document this Omphacite to be formed at pressures higher than 6 GPa (at 900 °C). The pressure-temperature conditions of 3.68 GPa and 892 °C for phengite exsolution in Omphacite associated with garnet during the exhumation were obtained by using Grt-Omp-Ph geothermobarometer. We conclude that omphacitic-clinopyroxene in subducted eclogites may act as a robust medium to transport H2O and potassium deep into the interior of the Earth. This study suggests that the Yematan eclogite in North Qaidam UHP metamorphic belt, NW China, may have been subducted and exhumed from depths of more than 200 km.
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jadeite and dolomite bearing coesite eclogite from western tianshan nw china
European Journal of Mineralogy, 2014Co-Authors: Lifei Zhang, Zhenyu ChenAbstract:Jadeite associated with Omphacite has been reported from high-pressure, low-temperature (H P -L T ) rocks worldwide. However, the upper pressure stability of this association is not clear in nature. This study describes an immiscible texture of clinopyroxenes at ultrahigh-pressure (UH P )-L T conditions from western Tianshan, NW China. The host eclogite mainly consists of quartz, garnet, carbonate, albite, Omphacite, white mica and Na-Ca amphibole. The modal contents of constituent minerals vary from place to place, and quartz-rich and carbonate-rich domains are developed. Jadeite (Jd 70–91 Aeg 2–16 ) only occurs as inclusion in garnet, mostly accompanied by patchy Omphacite. Micro-Raman analyses show that jadeite and Omphacite have different diagnostic peaks, consistent with the quantitative microprobe results. Jadeite is locally replaced by secondary sodic phases like paragonite and albite. A composite inclusion occurs in garnet and consists of lath-like paragonite, quartz, Omphacite and titanite, with or without glaucophane. Matrix porphyroblastic albite usually encloses paragonite, titanite and barroisite, with or without Omphacite, constituting a multi-phase aggregate. These two types of sodic phase-rich aggregates within and outside garnet are interpreted to be pseudomorphic replacements after jadeite at different conditions during retrogression. In addition to coesite in garnet, inclusions of coesite and its pseudomorphs without radial fractures were found in ankeritic dolomite. This first reported paragenesis of dolomite + coesite at low temperatures suggests that dolomite can be stabilized at depths of up to 90 km in a cold subduction zone. This contribution not only reports a new occurrence of Tianshan UH P rocks, but also suggests that UH P -L T metamorphism has impacted a variety of lithologies in addition to paraschist and basaltic eclogite, thus supporting the internal coherence of the UH P terrain of the Chinese western Tianshan.
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Relict coesite exsolution in Omphacite from Western Tianshan eclogites, China
American Mineralogist, 2005Co-Authors: Lifei Zhang, Shuguang Song, J G LiouAbstract:Exsolution rods of relict coesite together with quartz were identified in Omphacite in eclogites from western Tianshan, China. They are oriented along the c-axis of the host clinopyroxenes and have grain size up to 30 μm long and 2-3 μm wide. Raman spectra of exsolved lamellae yield consistent but weak bands at 521, 270, 181, 151, and 118 cm - 1 , typical for coesite, in addition to those of quartz and the host Omphacite. Such occurrences together with textured observations suggest a two-stage evolution of SiO 2 exsolution rods in Omphacite. Lamellae of coesite were apparently exsolved from supersilicic Omphacite at P 5.0 GPa and the transformation from coesite to quartz occurred during retrograde metamorphism.
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relict coesite exsolution in Omphacite from western tianshan eclogites china
American Mineralogist, 2005Co-Authors: Lifei Zhang, Shuguang Song, J G Liou, Yongliang Ai, Xuping LiAbstract:Exsolution rods of relict coesite together with quartz were identified in Omphacite in eclogites from western Tianshan, China. They are oriented along the c-axis of the host clinopyroxenes and have grain size up to 30 μm long and 2-3 μm wide. Raman spectra of exsolved lamellae yield consistent but weak bands at 521, 270, 181, 151, and 118 cm - 1 , typical for coesite, in addition to those of quartz and the host Omphacite. Such occurrences together with textured observations suggest a two-stage evolution of SiO 2 exsolution rods in Omphacite. Lamellae of coesite were apparently exsolved from supersilicic Omphacite at P 5.0 GPa and the transformation from coesite to quartz occurred during retrograde metamorphism.
Thomas Ludwig - One of the best experts on this subject based on the ideXlab platform.
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Evolution of a tourmaline-bearing lawsonite eclogite from the Elekdağ area (Central Pontides, N Turkey): evidence for infiltration of slab-derived B-rich fluids during exhumation
Contributions to Mineralogy and Petrology, 2004Co-Authors: Rainer Altherr, Thomas Zack, Gültekin Topuz, Horst Marschall, Thomas LudwigAbstract:An undated high-pressure low-temperature tectonic mélange in the Elekdağ area (central Pontides, N Turkey) comprises blocks of MORB-derived lawsonite eclogite within a sheared serpentinite matrix. In their outer shells, some of the eclogite blocks contain large (up to 6 cm) tourmaline crystals. ‘Prograde’ inclusions in poikiloblastic garnet from a well-preserved eclogite block are lawsonite, epidote/clinozoisite, Omphacite, rutile, glaucophane, chlorite, Ba-bearing phengite, minor actinolite, winchite and quartz. In addition, glaucophane, lawsonite and rutile occur as inclusions in Omphacite. These inclusion assemblages document the transition from a garnet-lawsonite-epidote-bearing blueschist to a lawsonite eclogite with the peak assemblage garnet + Omphacite I + lawsonite + rutile. Peak metamorphic conditions are not well-constrained but are estimated approximately 400–430°C and >1.35 GPa, based on Fe–Mg exchange between garnet and Omphacite and the coexistence of lawsonite + Omphacite + rutile. During exhumation of the eclogite–serpentinite mélange in the hanging wall of a subduction system, infiltration of B-rich aqueous fluids into the rims of eclogite blocks caused retrogressive formation of abundant chlorite, titanite and albite, followed by growth of tourmaline at the expense of chlorite. At the same time, Omphacite I ( X _Jd=0.24–0.44) became unstable and partially replaced by Omphacite II characterized by higher X _Jd (0.35–0.48), suggesting a relatively low silica activity in the infiltrating fluid. Apart from Fe-rich rims developed at the contact to chlorite, tourmaline crystals are nearly homogeneous. Their compositions correspond to Na-rich dravite, perhaps with a small amount of excess (tetrahedral) boron (~5.90 Si and 3.10 B cations per 31 anions). δ^11 B values range from −2.2 to +1.7‰. The infiltrating fluids were most probably derived from subducting altered oceanic crust and sediments.
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evolution of a tourmaline bearing lawsonite eclogite from the elekdag area central pontides n turkey evidence for infiltration of slab derived b rich fluids during exhumation
Contributions to Mineralogy and Petrology, 2004Co-Authors: Rainer Altherr, Thomas Zack, Gültekin Topuz, Horst R Marschall, Thomas LudwigAbstract:An undated high-pressure low-temperature tectonic melange in the Elekdag area (central Pontides, N Turkey) comprises blocks of MORB-derived lawsonite eclogite within a sheared serpentinite matrix. In their outer shells, some of the eclogite blocks contain large (up to 6 cm) tourmaline crystals. ‘Prograde’ inclusions in poikiloblastic garnet from a well-preserved eclogite block are lawsonite, epidote/clinozoisite, Omphacite, rutile, glaucophane, chlorite, Ba-bearing phengite, minor actinolite, winchite and quartz. In addition, glaucophane, lawsonite and rutile occur as inclusions in Omphacite. These inclusion assemblages document the transition from a garnet-lawsonite-epidote-bearing blueschist to a lawsonite eclogite with the peak assemblage garnet + Omphacite I + lawsonite + rutile. Peak metamorphic conditions are not well-constrained but are estimated approximately 400–430°C and >1.35 GPa, based on Fe–Mg exchange between garnet and Omphacite and the coexistence of lawsonite + Omphacite + rutile. During exhumation of the eclogite–serpentinite melange in the hanging wall of a subduction system, infiltration of B-rich aqueous fluids into the rims of eclogite blocks caused retrogressive formation of abundant chlorite, titanite and albite, followed by growth of tourmaline at the expense of chlorite. At the same time, Omphacite I (XJd=0.24–0.44) became unstable and partially replaced by Omphacite II characterized by higher XJd (0.35–0.48), suggesting a relatively low silica activity in the infiltrating fluid. Apart from Fe-rich rims developed at the contact to chlorite, tourmaline crystals are nearly homogeneous. Their compositions correspond to Na-rich dravite, perhaps with a small amount of excess (tetrahedral) boron (~5.90 Si and 3.10 B cations per 31 anions). δ11 B values range from −2.2 to +1.7‰. The infiltrating fluids were most probably derived from subducting altered oceanic crust and sediments.
