K2O prograde zoning pattern in clinopyroxene from the Kokchetav diamond-grade metamorphic rocks: Missing part of metamorphic history and location of second critical end point for calc-silicate system |
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| Authors: | Anastasiya O Mikhno Andrey V Korsakov |
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| Institution: | V.S. Sobolev Institute of Geology and Mineralogy of Siberian Branch of Russian Academy of Sciences, Koptyug Pr. 3, Novosibirsk 630090, Russia |
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| Abstract: | Despite extensive studies of calc-silicate rocks of the Kokchetav massif, there is no satisfactory explanation of the origin of potassium-bearing clinopyroxene in alkali poor metamorphic rocks. In this paper we report the finding of potassium-bearing clinopyroxene with prograde zonation (K2O increases from core to rim) from diamond-grade, but diamond-free UHP calc-silicate rocks of the Kokchetav massif. We believe that the crystallization of potassium-bearing clinopyroxene started on the prograde stage and slightly prior to the peak of UHP metamorphism. Thus, prograde metamorphic history is only traceable in diamond-free UHP calc-silicate rocks, while in diamond-bearing UHPM rocks it is completely reset. Fluid and polyphase solid inclusions, originally representing melt inclusions, occur in the core of potassium-bearing clinopyroxene and imply that melt and fluid may coexist in calc-silicate rocks even at 1000–1100 °C and 6–7 GPa. |
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