Abstract:We studied clastics of high-alumina garnet-kyanite-mica schists and garnet-kyanite-quartz granofelses, including diamond-bearing ones, found in the eluvial sediments near Lake Barchi. In contents of major elements the studied rocks correspond to argillaceous shales. The garnet-kyanite-quartz granofelses are poorer in K (0.49-1.35 wt.% K 2 O) than the garnet-kyanite-mica schists (4.9-2.2 wt.% K 2 O) but have the same contents of other major components. The REE patterns of most of the garnet-kyanite-phengite sch… Show more
“…It is commonly manifested in the presence of (a) a graphite-rich core with graphite and quartz/coesite inclusions ( Figure 1b) and (b) a clean overgrowth zone enclosing cuboctahedral diamond crystals (up to 20 μm in size; Figure 1c). [28,56] A similar pattern of inclusion arrangement is observed in garnet porphyroblasts. As a rule, matrix phengite is partly replaced by biotite and chlorite.…”
Section: Geological Setting and Samples Descriptionsupporting
confidence: 63%
“…The zonal distribution of C and SiO 2 polymorphs observed in kyanite from the diamond-bearing Barchi-Kol rocks has been interpreted previously to originate on the prograde metamorphic stage during the increase of PT parameters. [56] However, the graphite-rich cores of the kyanite porphyroblast studied here contain diamond crystals, which indicates the kyanite crystallization under UHP conditions. [28] High Si (3.34 apfu) content in phengite further proves the UHP formation (>4 GPa) of the kyanite porphyroblasts in the Barchi-Kol diamondiferous gneisses.…”
Section: Insights From Mineral Inclusions In the Kyanite Porphyroblmentioning
A series of precise nondestructive analytical methods (Raman spectroscopy, cathodoluminescence, and EBSD—electron backscatter diffraction) has been employed to investigate the internal textures of kyanite porphyroblasts from diamondiferous and diamond‐free ultrahigh‐pressure metamorphic rocks (Kokchetav massif, Northern Kazakhstan). Such internal kyanite characteristics as twinning, radial fibrous pattern, and spotty zoning were identified by means of Raman and cathodoluminescence imaging, whereas an intergrowth of two kyanite crystals was distinguished only by Raman imaging. The EBSD analysis recorded an ~10–25° changing of orientations along the elongation in the investigated kyanite porphyroblasts. The absence of a radial fibrous pattern and a spotty zoning on the EBSD maps indicates that these textures are not related to variations in crystallographic orientation. The absence of clear zoning patterns (cores, mantles, and rims) on the Raman, cathodoluminescence, or EBSD maps of the kyanite porphyroblasts indicates the rapid single‐stage formation of these porphyroblasts near the peak metamorphic conditions and the lack of recrystallization processes. The obtained results provide important implications for deciphering of mineral internal textures, showing that the data obtained by cathodoluminescence mapping can be clearly reproduced by Raman imaging, with the latter method occasionally being even more informative. This observation is of significant importance for the study of minerals that are unexposed on a thin section surface or Fe‐ and Ni‐rich minerals that do not show luminescence emission. The combination of the Raman spectroscopic, cathodoluminescence, and EBSD techniques may provide better spatial resolution for distinguishing different domains and textural peculiarities of mineral than the selective application of individual approaches.
“…It is commonly manifested in the presence of (a) a graphite-rich core with graphite and quartz/coesite inclusions ( Figure 1b) and (b) a clean overgrowth zone enclosing cuboctahedral diamond crystals (up to 20 μm in size; Figure 1c). [28,56] A similar pattern of inclusion arrangement is observed in garnet porphyroblasts. As a rule, matrix phengite is partly replaced by biotite and chlorite.…”
Section: Geological Setting and Samples Descriptionsupporting
confidence: 63%
“…The zonal distribution of C and SiO 2 polymorphs observed in kyanite from the diamond-bearing Barchi-Kol rocks has been interpreted previously to originate on the prograde metamorphic stage during the increase of PT parameters. [56] However, the graphite-rich cores of the kyanite porphyroblast studied here contain diamond crystals, which indicates the kyanite crystallization under UHP conditions. [28] High Si (3.34 apfu) content in phengite further proves the UHP formation (>4 GPa) of the kyanite porphyroblasts in the Barchi-Kol diamondiferous gneisses.…”
Section: Insights From Mineral Inclusions In the Kyanite Porphyroblmentioning
A series of precise nondestructive analytical methods (Raman spectroscopy, cathodoluminescence, and EBSD—electron backscatter diffraction) has been employed to investigate the internal textures of kyanite porphyroblasts from diamondiferous and diamond‐free ultrahigh‐pressure metamorphic rocks (Kokchetav massif, Northern Kazakhstan). Such internal kyanite characteristics as twinning, radial fibrous pattern, and spotty zoning were identified by means of Raman and cathodoluminescence imaging, whereas an intergrowth of two kyanite crystals was distinguished only by Raman imaging. The EBSD analysis recorded an ~10–25° changing of orientations along the elongation in the investigated kyanite porphyroblasts. The absence of a radial fibrous pattern and a spotty zoning on the EBSD maps indicates that these textures are not related to variations in crystallographic orientation. The absence of clear zoning patterns (cores, mantles, and rims) on the Raman, cathodoluminescence, or EBSD maps of the kyanite porphyroblasts indicates the rapid single‐stage formation of these porphyroblasts near the peak metamorphic conditions and the lack of recrystallization processes. The obtained results provide important implications for deciphering of mineral internal textures, showing that the data obtained by cathodoluminescence mapping can be clearly reproduced by Raman imaging, with the latter method occasionally being even more informative. This observation is of significant importance for the study of minerals that are unexposed on a thin section surface or Fe‐ and Ni‐rich minerals that do not show luminescence emission. The combination of the Raman spectroscopic, cathodoluminescence, and EBSD techniques may provide better spatial resolution for distinguishing different domains and textural peculiarities of mineral than the selective application of individual approaches.
“…Multiple reports exist on the UHP nature of metapelitic schists and gneisses in the Barchi-Kol area (e.g. Korsakov, Shatsky, Sobolev, & Zayachokovsky, 2002;Shatsky, Skuzovatov, Ragozin, & Sobolev, 2015;Stepanov, Rubatto, Hermann, & Korsakov, 2016), based on thermobarometry and findings of microdiamonds. The eastern domain is represented mainly by pelitic schists and paragneisses (e.g.…”
Section: Brief Geological and Geochronological Outlinementioning
U-Pb geochronological, trace-element and Lu-Hf isotopic studies have been made on zircons from ultrahigh-pressure (UHP) mafic eclogite from the Kumdy-Kol area, one of the diamond-facies domains of the Kokchetav Massif (northern Kazakhstan). The peak eclogitic assemblage equilibrated at > 900 C, whereas the bulk sample composition displays light rare-earth element (LREE) and Th depletion evident of partial melting. Zircons from the eclogite are represented by exclusively newly formed metamorphic grains and have U-Pb age spread over 533-459 Ma, thus ranging from the time of peak subduction burial to that of the late post-orogenic collapse. The major zircon group with concordant age estimates have a concordia age of 508.1 ±4.4 Ma, which corresponds to exhumation of the eclogite-bearing UHP crustal slice to granulite-or amphibolite-facies depths. This may indicate potentially incoherent exhumation of different crustal blocks within a single Kumdy-Kol UHP domain. Model Hf isotopic characteristics of zircons (ε Hf (t) +1.5 to +7.8, Neoproterozoic model Hf ages of 1.02-0.79 Ga) closely resemble the whole-rock values of the Kumdy-Kol eclogites and likely reflect in situ derivation of HFSE source for newly formed grains. The ages coupled with geochemical systematics of zircons confirm that predominantly late zircon growth occurred in Th-LREE-depleted eclogitic assemblage, that experienced incipient melting and monazite dissolution in melt at granulite-facies depths, followed by amphibolite-facies rehydration during late-stage exhumation-related retrogression.
“…The diamond-bearing kyanite gneisses investigated (samples B-11-14 and B-16-14) were collected on the south-west extension of the Barchi-Kol terrane (Fig. 1b), described previously by Shatsky et al (2015). The samples have a medium- to coarse-grained texture without obvious foliation.…”
Section: Geological Outline and Sample Descriptionmentioning
confidence: 99%
“…2c,d,f). A more detailed petrographic description of the diamondiferous kyanite gneisses from the Barchi-Kol terrane is provided elsewhere (Shatsky et al , 2015; Shchepetova et al , 2017). Fig.…”
Section: Geological Outline and Sample Descriptionmentioning
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