Microstructural, compositional and petrophysical properties of mylonitic granodiorites from an extensional shear zone (Rhodope Core complex, Greece)

At the southern boundary of the Rhodope Massif, NE Greece, the Kavala Shear Zone (KSZ) represents an example of the Eastern Mediterranean deep-seated extensional tectonic setting. During Miocene time, extensional deformation favoured syntectonic emplacement and subsequent exhumation of plutonic bodies. This paper deals with the strain-related changes in macroscopic, geochemical and microstructural properties of the lithotypes collected along the KSZ, comprising granitoids from the pluton, aplitic dykes and host rock gneisses. Moreover, we investigated the evolution of seismic anisotropy on a suite of granitoid mylonites as a result of progressive strain. Isotropic compressional and shear wave velocities (Vp,Vs) and densities calculated from modal proportions and single-crystal elastic properties at given pressure–temperature (P–T) conditions are compared to respective experimental data including the directional dependence (anisotropy) of wave velocities. Compared to the calculated isotropic velocities, which are similar for all of the investigated mylonites (average values:Vp~ 5.87 km s−1,Vs~ 3.4 km s−1,Vp/Vs= 1.73 and density = 2.65 g cm−3), the seismic measurements give evidence for marked P-wave velocity anisotropy up to 6.92% (at 400 MPa) in the most deformed rock due to marked microstructural changes with progressive strain, as highlighted by the alignment of mica, chlorite minerals and quartz ribbons. The highest P- and S-wave velocities are parallel to the foliation plane and lowest normal to the foliation plane. Importantly,Vpremains constant within the foliation with progressive strain, but decreases normal to foliation. The potential of the observed seismic anisotropy of the KSZ mylonites with respect to detectable seismic reflections is briefly discussed.

Strain Scan and Texture of an Anhydrite-Dolomite Rock from the Piora-Syncline (Switzerland)

An anhydrite-dolomite rock was investigated in order to find correlations between intracrystalline strain and texture. The sample represents a composite rock, called Zuckerdolomit, occurring for instance in the central Alps. Zuckerdolomit may occasionally respond to mechanical impulses with a complete break down of its structure. Residual strain was scanned at the strain/stress diffractometer EPSILON-MDS and the crystallographic texture was obtained by neutron time-of-flight diffraction at the texture diffractometer SKAT at the pulsed neutron source IBR-2 in Dubna, Russia. Texture was additionally determined by the U-stage microscopy. The texture of the rock could be derived from the main constituents: anhydrite and dolomite. The anhydrite texture appears to be characterised by an orthorhombic symmetry. One plane of the symmetry trends parallel to the rock’s foliation plane, with basal planes subparallel to and a small circle distribution within the foliation plane. There is a subordinated small circle orientation pattern relating in 45° to the rock’s foliation plane. On the other hand, the subordinated regulation pattern of anhydrite is fixed to the orientation of the foliation plane. The determined residual strain data for anhydrite and dolomite reflect a generally opposed directed state, regarding both, compression and dilatation. Moreover, taking into account the elastic constants for both minerals, which differ approximately by the factor two, the special geomechanical behaviour of the studied Zuckerdolomit appears to be considerably determined by interactions between its residual strain and textural properties. This may be of particular importance for underground activities (mining, tunnelling etc.). The dangerous geomechanical behaviour of the rock seems to be determined by its residual straintexture relations more than so far believed (anhydrite hydration).

Pseudomigmatites in the Abitau Lake Area, District of Mackenzie, Northwest Territories, Canada

The Abitau Lake area, N.W.T., chiefly consists of medium-to-coarse-grained quartzo-feldspathic gneisses. Many of these are granoblastic, presenting little or no microscopic evidence of deformation. Others are highly tectonized and foliated varieties that sometimes form pseudomigmatites easily mistaken in the field for true migmatites formed by lit-par-lit injection of pink granite into more basic grey gneiss. The last, however, are found after detailed examination to be granitic rocks that owe their misleading appearance and distinctive fabric to deformation during isochemical amphibolite fades metamorphism. Surprisingly they are not mylonites. Their fabric, like that of the other foliated gneisses is dominated by the parallelism of dark minerals and elongated quartz crystals. It resembles that of typical European granulites and often seems largely due to compression acting in a direction normal to the foliation plane with little or no transport in that plane. Tectoniza-tion did not promote the formation of myrmekite or microperthite but may have assisted to some extent the production of epidote from hornblende in certain specimens.

Petrofabric Analyses of the Loch Tay Limestone from Strachur, Argyll

The paper records the results of petrofabric analyses of four specimens of marble from the South-West Highlands of Scotland. The marble fabric has a high degree of homogeneity. Calcite grains are flattened in the foliation plane and are elongate in a direction normal to the fold axis. The c crystal axes lie in a distinct girdle about b, with a maximum near the pole of the foliation; the a crystal axes tend to occupy three maxima, one of which is close to b. Late twinning has occurred on {011¯12} apparently in response to a simple stress system, operating in the ac plane. The symmetry of the general calcite fabric, the twinning and the megascopic structures coincide.