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Kosmochlor and chromian jadeite aggregates from the Myanmar jadeitite area

Published online by Cambridge University Press:  05 July 2018

G. H. Shi*
Affiliation:
China University of Geosciences, Beijing 100083, China Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
B. Stöckhert
Affiliation:
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
W. Y. Cui
Affiliation:
School of Earth and Space Sciences, Peking University, Beijing 100871, China
*

Abstract

Four distinct textures and related compositions of kosmochlor (Ko) and chromian jadeite in rocks from the Myanmar jadeitite area are described: (1) spheroidal or ellipsoidal aggregates with a corona texture surrounding relict chromite; (2) spheroidal or ellipsoidal aggregates with a core of jadeite; (3) granoblastic textures in undeformed coarse-grained clinopyroxene rocks; and (4) recrystallized fine-grained aggregates in deformed jadeitite. Nearly pure kosmochlor (97 mol.% NaCrSi2O6) was found in type 2 textures, closest to the end-member reported so far from a terrestrial rock. Sharp compositional boundaries between kosmochlor and chromian jadeite of variable composition are interpreted to be related to progressive crystallization or replacement at differing conditions. The compositions analysed plot along the kosmochlor-jadeite join. In contrast to conclusions of previous studies on the Myanmar clinopyroxenes there is no unequivocal evidence for miscibility gaps. The preservation of relict chromite in the centre of coronitic spheroidal or ellipsoidal aggregates of kosmochlor and jadeite indicates a metasomatic origin from a peridotite protolith at an inferred minimum pressure of 1.0 GPa and temperatures of 250—370°C. Recrystallization during later ductile deformation of the clinopyroxene rocks in the dislocation creep regime leads to fine-grained aggregates of chromian jadeite, which are of particular gemmological interest.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2005

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