Microstructural, modal and geochemical changes as a result of granodiorite mylonitisation – a case study from the Rolovská shear zone (Čierna hora Mts, Western Carpathians, Slovakia)
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Date
2016-12
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Publisher
Instytut Geologii UAM
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Abstract
Strong tectonic remobilisation and shear zone development are typical features of the easternmost part of the Vepori-
cum tectonic unit in the Western Carpathians. The granodiorite mylonites in the area of the Rolovská shear zone (Čierna
hora Mts) underwent a complex polystage evolution during the Hercynian and Alpine orogenies. Deformation during
the latter reached greenschist facies under metamorphic conditions. Mylonites are macroscopically foliated rocks with
a stretching lineation and shear bands. Structurally different mylonite types, ranging from protomylonites to ulramylo-
nites with typical grainsize reduction from the margins towards the shear zone centre, have been assessed. The modal
mineralogy of the different mylonite types changes considerably. Typical is a progressive decrease in feldspar content
and simultaneously the quartz and white mica content increases from protomylonites towards the most strongly defor-
med ultramylonites. The deformation had a brittle character in less deformed rocks and a ductile one in more deformed
tectonites. Obvious chemical changes occur in mesomylonites and ultramylonites. During mylonitisation, the original
biotite granodiorite was depleted of Mg, Fe, Na, Ca and Ba, while K, Rb and mainly Si increased considerably. Other
(major and trace) elements reflect erratic behaviour due to lateral mobility. Chemical changes indicate the breakdown
and subsequent recrystallisation of biotite and feldspars and, in turn, the crystallisation of albite and sericite. REE de-
crease in ultramylonites due to the breakup of accessory minerals during deformation and alteration.
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Keywords
shear zone, protomylonite, mesomylonite, ultramylonite, Veporic unit
Citation
Geologos, 2016, 22, 3, pp. 171-190
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ISBN
ISBN 978-83-232187-4-6
ISSN
ISSN 1426-8981