Deformation mechanisms and P-wave velocity patterns from mylonitic metagabbros deformed at amphibolite and granulite facies conditions
Lenka Baratoux1, Karel Schulmann1, Pavla Štípská1, Alice Tomášková2
1
Institute of Petrology and Structural Geology, Charles University, Albertov 6, 128 43, Praha 2, Czech Republic2
Geophysical Institute, Academy of Sciences of the Czech Republic, Boční II/1401, 141 31, Prague 4, Czech Republiclka@natur.cuni.cz
We have investigated two mylonitic metagabbro belts of C-O protolith ages (Staré Město belt, Czech Republic) deformed at oblique transpressive regime. The upper gabbro sheet was un
derplated by a Variscan tonalite body, one to two kilometer thick. The second gabbroic sheet occurs in the footwall of the tonalitic intrusion. The deformation fabrics in metagabbro sheets show indentical W dipping foliation, N-S trending lineation and dextral kinematics. In the lower unit, the deformation is concentrated into anastomose shear zones surrounding low strain domains with magmatic textures. The deformation intensity in the upper unit is stronger leading to a development of penetrative foliation marked by monomineral amphibole-plagioclase bands. The metamorphic temperature estimated using Hbl-Pl thermometry is higher in the upper sheet (850°C) than in the lower one (750°C) due to differential thermal effect of tonalitic intrusion.The quantitative microstructural analysis suggests that the deformation in a microscale changes depending on temperature. The deformation of amphibole and plagioclase in low strain domains of the lower unit is characterized by "core and mantle" structures and by subgrain rotation recrystallization mechanism. Metagabbros from the mylonitic zones display a strong CPO and SPO of strongly elongated amphibole and less elongated grains of plagioclase. The dominant slip system for amphiboles is supposed to be (100) [001]. The EBSD and quantitative microstructural study show that already magmatic amphibole grains of the upper gabbro sheet exhibit strong CPO and SPO. The amphibole and plagioclase show features of grain boundary migration recrystallization mechanism accompanied with nucleation of new grains with chemistry different with respect to magmatic ones. With increasing intensity of recrystallization, amphibole grains lose the CPO and SPO and their shapes become more equigranular. On the contrary, the plagioclases attain stronger SPO than in the less deformed stages. The dominant slip system for amphiboles appears to be (100) [001] with minor contributions of (010) [001].
P-wave velocities measured at 400 MPa reveal the anisotropy of 3.5 % in the least deformed gabbros of the lower gabbro sheet (vmin = 7.31 km/s, vmax = 7.35 km/s) and the anisotropy of 8.2 % for the most deformed gabbros (vmin = 7.0km/s, vmax = 7.6 km/s). In the upper gabbro sheet, the anisotropies are 11.6 % (vmin = 6.5 km/s, vmax = 7.3 km/s) for the least deformed rock and 14.3 % (vmin = 6.5 km/s, vmax = 7.5 km/s) for the most deformed one. The P-wave velocity distribution in amphibolites depends on the proportion of amphibole and plagioclase as well as on the fabric intensity. High anisotropy of the P-wave velocities for the upper gabbro sheet contrasts with relatively weaker SPO and CPO for amphibole and plagioclase with respect to the lower gabbro sheet. This is due to high connectivity of monomineral layers and well developed compositional layering in this unit.