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Chemical zonation and relative timing of growth sections in garnets from eclogites of the Leaota Massif, South Carpathians

Published online by Cambridge University Press:  05 July 2018

G. Săbău ,
E. Negulescu  and
H.-J. Massonne
G. Săbău*
Affiliation:
Geological Institute of Romania, 1 Caransebeş st., RO-012271 Bucharest 32, Romania
E. Negulescu
Affiliation:
Geological Institute of Romania, 1 Caransebeş st., RO-012271 Bucharest 32, Romania
H.-J. Massonne
Affiliation:
Institut für Mineralogie und Kristallchemie, Azenbergstr. 18, D-70174 Stuttgart, Germany
*
*E-mail: g_sabau@yahoo.co.uk
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Abstract

We have investigated the chemical zoning of garnet in three texturally different eclogite types from the Leaota Massif, Romania, to test the various zonation patterns of Ca, Fe, Mg and Mn as indices of time-lines in porphyroblastic garnets. Mn zonation was found to mark various growth stages very well. It indicates in each of our three case studies a complex growth history dominated by multiple nucleation and coalescence, but also interrupted by partial resorption, as shown by morphological characteristics. Chemical variations of the main elements in garnet during prograde high-pressure metamorphism define time-lines other than those of Mn which decrease, highlighting the stages of porphyroblastic growth. In particular, the Ca/Mn, Mg/Mn and Mg/Fe ratios are useful in late growth stages, when low Mn contents and radial gradients do not allow a satisfactory resolution of Mn time-lines. These ratio-based time-lines further substantiate the potential of Mn time-lines in revealing non-concentric porphyroblastic growth. This is important in order to correctly relate garnet growth stages to stable mineral assemblages, a prerequisite for a reliable derivation of the metamorphic history of the host rock.

Keywords

garnetcrystal chemistrygrowth sectionseclogiteRomania
Type
Research Article
Information
Mineralogical Magazine , Volume 70 , Issue 6 , December 2006 , pp. 655 - 667
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2006

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