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Textures of diffusion-controlled reaction in contact-metamorphosed Mg-rich granulite, Kokchetav area, Kazakhstan

Published online by Cambridge University Press:  05 July 2018

J. R. Ashworth ,
V. V. Reverdatto ,
V. YU. kolobov ,
V. V. Lepetyukha ,
V. S. Sheplev  and
N. A. Bryxina
J. R. Ashworth
Affiliation:
School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
V. V. Reverdatto
Affiliation:
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of Russian Academy of Science, Universitetsky pr. 3, Novosibirsk 90, 630090, Russia
V. YU. kolobov
Affiliation:
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of Russian Academy of Science, Universitetsky pr. 3, Novosibirsk 90, 630090, Russia
V. V. Lepetyukha
Affiliation:
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of Russian Academy of Science, Universitetsky pr. 3, Novosibirsk 90, 630090, Russia
V. S. Sheplev
Affiliation:
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of Russian Academy of Science, Universitetsky pr. 3, Novosibirsk 90, 630090, Russia
N. A. Bryxina
Affiliation:
United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of Russian Academy of Science, Universitetsky pr. 3, Novosibirsk 90, 630090, Russia
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Abstrsct

In a granulite from the Kokchetav massif, a complex mineral assemblage and intricate textures have resulted from a combination of unusual rock composition and two–stage metamorphic history. The second, contact metamorphism produced mainly cordierite and anthophyllite, reflecting a bulk composition attributed to pre–metamorphic alteration of basic igneous rock. From the first, highpressure metamorphism, garnet relics persist while another mineral has been completely pseudomorphed. The garnet is partly replaced by a symplectite of three minerals: orthopyroxene vermicules in a coarser intergrowth of cordierite and calcic plagioclase. Despite variable proportions of cordierite and plagioclase, the Al:Si ratio of the symplectite is almost constant, because the proportion of orthopyroxene is smaller where the dominant aluminous mineral is cordierite (Al:Si ≈ 0.8) than where the even more aluminous plagioclase (Al:Si ≈ 0.89) is prominent. The bulk Al:Si ratio of this symplectite, approximately 0.69, is very close to that of reactant garnet (0.66), indicating that Al and Si have been retained almost completely during the local reaction, while other elements were more mobile. In the pseudomorphs, aluminous cores (with Al:Si ratios 1.61–1.93) indicate that the mineral which has been completely replaced was probably kyanite. These cores comprise plagioclase, zoisite, corundum and spinel, and are surrounded by layers of plagioclase and cordierite. Fe, Mg, and Ca have diffused to the core, through layers with low bulk concentrations of these elements, probably by grainboundary diffusion in the solid state.

Keywords

symplectitepseudomorphdiffusioncordierite-orthoamphibole rocksKokchetav massif
Type
Research Article
Information
Mineralogical Magazine , Volume 62 , Issue 2 , April 1998 , pp. 213 - 224
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1998

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