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Mechanism of formation of atoll garnet during high-pressure metamorphism

Published online by Cambridge University Press:  05 July 2018

S. W. Faryad ,
H. Klápová  and
L. Nosál
S. W. Faryad*
Affiliation:
Institute of Petrology and Structural Geology, Charles University, Albertov 6, 128 43 Praha 2, Czech Republic
H. Klápová
Affiliation:
Institute of Petrology and Structural Geology, Charles University, Albertov 6, 128 43 Praha 2, Czech Republic
L. Nosál
Affiliation:
Institute of Petrology and Structural Geology, Charles University, Albertov 6, 128 43 Praha 2, Czech Republic
*
*E-mail: faryad@natur.cuni.cz
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Abstract

Atoll garnet has been found in metabasites and quartz- and mica-rich rocks that have experienced low- to medium-temperature, high-pressure eclogite facies metamorphism in the Krušné Hory (Erzgebirge). They occur in several localities but are restricted to thin, texturally distinct zones, even on a thin-section scale. The mechanism of atoll garnet formation is documented by a series of micrographs and compositional maps and profiles of atoll garnet in combination with textural relations to other phases in the rocks. The core of full garnet or its relics in the atoll garnet have larger Ca and Fe, but smaller Mg contents, compared with the thin rim (ring). In addition to quartz, Na-Ca amphibole and phengite, the atoll cores are filled by a new garnet that has a composition similar to the outer rim. Formation of the atoll garnet is interpreted as resulting from fluid infiltration and element exchange between the garnet core and matrix, a process facilitated by a temperature increase during eclogite facies metamorphism. In addition to fluid access, the primary textures, mainly grain size, were also effective for the atoll garnet formation. Small grain fractions with thin rims were easily infiltrated by fluid, which used the short distance for element exchange between core and matrix. The core garnet was gradually dissolved and replaced by new garnet having the same crystallographic orientation as the rim or relics in the core.

Keywords

atoll garneteclogitefluid infiltrationBohemian Massif
Type
Research Article
Information
Mineralogical Magazine , Volume 74 , Issue 1 , February 2010 , pp. 111 - 126
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2010

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