Evolution of a shear zone in calcite marble on Thassos Island, Northern Greece: results from microfabrics and stable isotopes.

Journal article


Publication Details

Author(s): Bestmann M
Journal: Erlanger Geologische Abhandlungen
Publication year: 2000
Volume: 131
Pages range: 1-127
ISSN: 0071-1160
Language: English


Abstract


The metamorphic succession of Thassos Island, Northern Greece, is dominated by massive marble complexes in alternation with paragneisses, orthogneisses and amphibolites that show fabric development under retrograde metamorphic conditions. During extensional tectonics, linked to the exhumation process of the metamorphic core complex strain localization occurred in response to decreasing pressure-temperature conditions. In pure calcite marble complexes, this stage of deformation is recorded by several shear zones of up to 3 m thickness which are oriented parallel to the foliation plane in the host rocks. The mylonitic shear plane of the shear zone interior is also oriented parallel to the shear zone boundary (SZB) which is considered to be the basic reference plane. The history of a monophase shear zone complex in calcite marble is reconstructed by detailed studies of the crystallographic preferred orientation (CPO, texture) and microstructural patterns. Combined cathodoluminescence (CL), stable isotope (d18O, d13C), and geochemical analyses are used to evaluate possible metamorphic fluids and/or mass transfer during the deformation history of the shear zone complex.



Outside the investigated shear zone, the calcite fabric of the protolith marble is equigranular and coarse-grained (grain size 1-3 mm) with a high density of twin lamellae. Twins are oriented both



clockwise and anticlockwise to the long axes of the grains, which are oriented more or less parallel to the macroscopic foliation. The texture is characterized by a single c-axis maximum oriented perpendicular to the reference plane (SZB). Both a-axis pole figures (+a and –a) show very similar girdle distributions within the reference plane. The textures of the protolith are comparable with patterns found in experimentally deformed calcite marble and simulated textures under pure shear conditions within the twinning regime. The overall microfabric (texture & microstructure) of the protolith exhibits orthorhombic symmetry indicating a dominant coaxial deformation (pure shear strain path).



The investigated foreland-directed shallow-dipping shear zone cuts through the ‘already‘ deformed coarse-grained calcite marble complex. The boundaries of the shear zone are marked by sharp transitions in the grain size, with a narrow zone of protomylonitic fabric. Orientation Imaging Microscopy (OIM) demonstrates that lattice rotation represents the starting point in the process of dynamic recrystallization accompanied by grain size reduction. Preferred orientation of grain shape and one dominant twin orientation in the porphyroclasts of the protomylonite are oblique to the SZB and reveal a monoclinic symmetry, due to a non-coaxial strain component (simple shear strain path). Related to the protolith the texture of the protomylonite is slightly rotated with the sense of shear, but still preserves the orthorhombic internal texture symmetry of the protolith. The protomylonite shows inhomogeneous microstructural fabrics that are related to the degree of dynamic recrystallization, which is strain and time dependent. In contrast, the fine-grained ultramylonite in the interior of the shear zone is completely dynamically recrystallized with a homogeneous microstructure, which is more likely to represent a ‘steady state’ of flow. Dynamic recrystallization is accompanied by a strong grain size reduction (grain size 10-30 µm). The interior of the dynamically recrystallized grains is generally free from twins. Therefore, twinning does not play any role during deformation of ultramylonite, as in the case of protolith and protomylonite. Mylonitization under dominant non-coaxial deformation conditions in the shear zone is indicated by a grain shape preferred orientation in oblique angle to the SZB.



Asymmetric (‘s-shaped’) quartz grains with wedge shaped appendages are incorporated in the calcite matrix and correspond to a top-to-the SW shear direction in the ultramylonite. The texture of the ultramylonite shows a single c-axis maximum normal to the SZB. Whereas both of the separated a-axis pole figures of the protolith and protomylonite show similar girdle distributions, the ultramylonite exhibits a different distribution of distinct maxima in the respective a-axis pole figures. Thus the texture of the ultramylonite has clearly no orthorhombic sample symmetry but contains the symmetry elements of a monoclinic sample system as also indicated by the microstructures. The ‘steady state’ microstructure and the CPO favour slip systems with a stable end orientation of ‘easy slip’, with slip planes within the shear plane, and slip direction parallel to the shear direction. Here, the ‘easy slip’ system would be (0001)<10-10> (basal-m), which has never been reported for calcite. The texture of the ultramylonite with the different a-axis maxima at a specific angle relative to the shear direction can be explained by duplex slip on different but energetically equivalent <a>-directions on the basal plane (0001).



This study demonstrates that pure shear, as well as simple shear strain paths, can produce similar c-axis fabrics by different dominant deformation mechanisms. The separated a-axis pole figures show different density distributions indicating orthorhombic texture symmetry for the protolith and trigonal-monoclinic symmetry for the ultramylonite. The differently strained calcite marbles (protolith, protomylonite, and ultramylonite) show different rheological behaviour during deformation, and are respectively accompanied by characteristic strain configurations (degree of coaxial flow to non-coaxial flow). It seems probable that the entire calcite marble complex has undergone a component of early coaxial deformation during the uplift of the metamorphic core complex. Finally, the shallow dipping shear zone cuts through the 'already' deformed coarse-grained calcite marble complex related to foreland-directed extensional tectonics under retrograde conditions. High strain is accommodated together with non-coaxial flow.



There are no significant isotopic differences in d18O and d13C within and outside the calcite marble shear zone. The oxygen (d18O 28.8-30.7‰) and carbon (d13C 2.6-3.1‰) values of the entire shear zone complex are in the range of unmetamorphosed Mesozoic marine carbonates. However, in the shear zone, areas of small-scale depletions of 18O (D18O up to 1.5‰) are observed, while d13C values remain constant. This oxygen depletion coincides systematically with increasing cathodoluminescence intensity related to lamination pattern (luminescence streaks) oriented parallel to the SZB. The isotopic gradients and the CL variation are probably due to some kind of diffusive mass transfer during dynamic recrystallization produced. No significant geochemical variation exists between the coarse-grained marble protolith and the dynamically recrystallized ultramylonite in the interior of the shear zone. The concentration of trace elements, like Mn (61-188 ppm), Fe (53-369 ppm), and Sr (103-146 ppm) are relatively constant throughout the shear zone complex. No systematic variation of the concentration of Mn (main luminescence activator for calcite) related to the CL intensity can be detected. The geochemical and stable isotope data indicate that the development of the calcite shear zone took place under ‘closed system’ conditions, i.e. without influence of externally derived fluids.



 



Dolomite is absent in the protolith as well as in the protomylonite and only appears within the pervasive dynamically recrystallized calcite ultramylonite; it is presumably related to exsolution reactions occurring during dynamic recrystallization. The angular shape of the dolomite grains points to a brittle rheological behaviour of dolomite during continuous deformation of the calcite matrix. Solvus thermometry indicates that exsolution of dolomite from Mg-calcite with a composition of the protolith (XMgCO3 ~0.01) will take place at a temperature of about 300-350°C, which seems to be geologically reasonable for the penetrative dynamic recrystallization of calcite in the shear zone.



FAU Authors / FAU Editors

Bestmann, Michel Dr.
Professur für Tektonik


How to cite

APA:
Bestmann, M. (2000). Evolution of a shear zone in calcite marble on Thassos Island, Northern Greece: results from microfabrics and stable isotopes. Erlanger Geologische Abhandlungen, 131, 1-127.

MLA:
Bestmann, Michel. "Evolution of a shear zone in calcite marble on Thassos Island, Northern Greece: results from microfabrics and stable isotopes." Erlanger Geologische Abhandlungen 131 (2000): 1-127.

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Last updated on 2018-09-08 at 05:39

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