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UHP-03 Ultra-high pressure metamorphism: Minerals, microstructures and nanoscale observations - Part 1
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Origin of diamond and graphite inclusions in garnet and kyanite porphyroblasts from the UHPM kyanite gneisses, Kokchetav Massif, Northern Kazakhstan
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Andrey Korsakov, Institute of Geology and Mineralogy of Siberian Branch of Russian Academy of Sciences (Russian Federation)
Maria Perraki, School of Mining and Metallurgical Engineering, National Technical University of Athens (Greece)
Dmitry Zedgenizov, Institute of Geology and Mineralogy of Siberian Branch of Russian Academy of Sciences (Russian Federation)
Aleksey Ragozin, Institute of Geology and Mineralogy of Siberian Branch of Russian Academy of Sciences (Russian Federation)
Hiroyuki Kagi, Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo (Japan)
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The transformation of carbonaceous material remains an open question in ultrahigh-pressure metamorphic (UHPM) terrains. Graphites exhibiting different ordered/disordered pattern from diamond-bearing metapelites of the Greek Rhodope were proposed to have formed during incomplete diamond-graphite transformation (Perraki et al., 2006). In the case of the Kokchetav UHPM rocks, Korsakov & Shatsky, (2004) reported growth of graphite in the diamond stability field not associated with retrograde partial graphitization as previously supported (Massonne et al., 1998; Dobrzhinetskaya et al., 2003). Our aim is to study the diamond/graphite formation mechanisms in UHPM rocks from the Kokchetav Massif. Weakly foliated diamondiferous kyanite gneisses consisting of garnet+kyanite+K-feldspar+ phengite+biotite+quartz were studied. Large kyanite and garnet phorphyroblasts occur in medium-grained quartzofeldspatic matrix. A zonal distribution of carbon polymorphs is frequently observed in both garnet and kyanite porphyroblasts with abundant preferred-oriented graphite inclusions dominating in the cores, making them non-transparent, and diamond occurring in the clean mantle or external zones. Quartz also frequently accompanies graphite in the graphite-bearing cores of garnet and kyanite porphyroblasts. However, it should be noted that this zonal distribution of carbon polymorphs is not indicative of prograde trend of P-T evolution. Relics of coesite were identified in kyanite porphyroblasts within graphite-rich zone, indicating that quartz coexisting with graphite possibly pseudomorphoses after coesite. In some cases, coesite formed in intergrowth with graphite. Interestingly, not all graphites show the same degree of disordering.
Coexistence of highly and poorly ordered graphite within the same growth zone might be explained by strong variation in degree of metamorphism, especially T. However, this is unlikely within 1 μm3 indicating that other factors control the graphite crystallinity in UHPM terrains and therefore Raman spectroscopy can not be used for estimating the peak metamorphic temperatures. Although abundant CO2-inclusions occur in quartz captured by kyanite no fluids were found to be associated with graphite eliminating the possibility of fluid-precipitated disordered graphite.
The P-T field in which graphite and coesite coexist is very narrow and is constrained to the high-temperature area. The presence of poorly ordered graphite in intergrowth with coesite indicates that disordered graphite can form even at high temperatures (>600°C) in ultrahigh-pressure rocks opposite to what has been claimed for regional metamorphic low- to normal pressure metamorphic rocks (Beyssac et al., 2002).
Recrystallization of carbonaceous material into either graphite or diamond by partial melting in the diamond stability field is likely. This work was supported by the Russian Foundation for Basic Research (07-05-00308-a), MK-259.2008.5 and Russian Science Support Foundation.
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