Cataclastic flow e dissolution-precipitation creep di granato: un indicatore di un alto tasso di deformazione? = Cataclastic flow and dissolution-precipitation creep in garnet: a hint for high strain rate deformation?

The rock sample studied in this work has been collected at the Sulztal shear zone, a milonitic structure of regional importance which accommodated the Variscan exhumation of high-pressure units, dated at 350-360 Ma. The shear zone has a down-dip lineation and dips 60-70° towards S and is located South of the Sulztal which connects Längenfeld and Winnebach, nearby the Ötztal, Austria. Variscan high-P metamorphic rocks, which the sample is part of, are present throughout the Austroalpine basement of the Eastern Alps and the Ötztal-Stubai complex (ÖSC) and the Silvretta crystalline complex (SC) (Miller, 1974; Maggetti & Galetti, 1988; Maggetti & Flisch, 1993; Miller & Thöni, 1995; Schweinehage & Massonne, 1999; Ladenhauf et al., 2001). In the NW zone of the Ötztal unit rocks mostly preserved the pre-Alpine metamorfic overprint, namely the Variscan, in amphibolitic to eclogite facies and the associated multiple deformation. The units show only weak evidence of the Alpine mertamorfic overprint. High-pressure events are preserved most exclusively in metabasic rocks that are present in elongated lenses and bodies, like ultramafic garnet-bearing rocks (studied sample) of kilometer-scale extension, often associated with metapelitic units. Those metabasites show affinity to gabbro-basalts of mid-ocean ridge basalt (MORB) type (Maggetti & Galetti, 1984,Miller & Thöni, 1995). Eclogitic associations with garnet + omphacite + quartz assemblages are locally preserved within these bodies as a result of incomplete re-equilibration during exhumation subsequent to the peak of Varisan metamorphism. In rare cases, relict gabbros and gabbroic textures still indicate the nature of the protoliths (Miller, 1974, Miller & Thöni, 1995). In the north-central ÖSC, high-P rocks are present in two parallel kilometre-sized elongated bodies consisting of garnet amphibolite, eclogites and olivinerich rocks enclosed in pelitic gneisses, forming the northern and southern eclogitic zone (NEZ, SEZ) (Miller, 1974; Koziol & Oberhänsli, 1995). P-T conditions for the Variscan high-P metamorphism range from 2.5-2.9 GPa at 600-730°C in the ÖSC and SC (Maggetti & Galetti, 1988; Miller & Thöni, 1995 ; Schweinehage & Massonne, 1999). The timing of the high-P event was determined for metabasic eclogites of the ÖSC and SC by Miller & Thöni (1995) using Sm-Nd datations. One thin section cut from an amphibolic-garnet bearing ultrafemic scist collected along the Sulztal shear zone has been studied in this thesis. It is defined as ultramafic because it consists predominantly of actinoltic amphibole (>80%) plus garnet, but primary clinopyroxene (diposide) is still preserved as a relict phase. Garnet is organized in millimeter-sized aggregates composed of sub-sferic, micrometer-sized grains. Single grains are in contact by triple junctions of 120°. Storey & Prior (2005) studied elongated garnets which occur within a shear zone transecting an eclogitic body located at Glenelg, NW Scotland. These authors described deformation and recrystallization of garnet under plastic conditions which has led to a similar microstructure as that observed in the rock studied during this thesis, with the significant difference that the autors observed the development of a CPO (crystallografic preferred orientation) in garnet. In contrary, the present study shows that the crystallographic orientation of garnet becomes randomized during deformation. Storey & Prior (2005) describe a variation of deformation mechanism as a function of stress, whereas in the present work it is thought to be a function of the different distribution of metamorphic fluids. In the studied thin section a prominent strain gradient is preserved, suggesting strong strain partitioning due to channelized fluid flow, the garnet microstructure in the studied rock is best explained by a high strain rate deformation process during fast exhumation. Scanning electron microscope (SEM) based techniques like EBSD (electron backscatter diffraction) and EDS (energy dispersive spectorscopy), as well as fully quantitaive electron microprobe analysis and traditional petrographic microscopy are the analytical tools chosen to study the microstruture and the mineral compositions within the selected rock sample. As alrady outlined, garnet aggregates have been of principal interest. All analysed garnet grains are caracterized by a heavy crystallographic misorientation although they are at reciprocal contact. From a chemical point of view, the aggregates and the whole mineralogy is highly homogeneous, exept some zonated garnet cores inside larger grains of the low strain domain which are grossular-rich. These relics are thought to have preserved the primary chemism because of a their lower grain size reduction . The magnitude of crystallographic misorientation does not increase with deformation intensity. The grain size reducing deformation mechanism is thought to be cataclastic flow, a mechanism working close to the brittle-ductile transition conditions (BDT) and is characterized by a continous and thourogh fracturing of previous garnet porhyroblasts and the mechanical rotation of the developing new grains. The observed “mosaic-like” structure is attributed to dissolution-precipitation creep, which is thought to heal fractures and to favour formation of straight grain boundaries with 120° triple junctions. This mechanism has also led to a progressive increase of the aspect ratio of the single grains with increasing strain. The two deformation mechanisms together explain hence the observed strong crystallographic misorientation and the “mosaic” micostructure of the garnet aggregates. Application of the garnet-clinopyroxene Fe2+ -Mg exchange geothermometer of E. Krogh Ravna (2000) has led to the assessment of an equilibration temperature of 512°C for a fixed pressure of 10 kbar. This geothermometer is not very pressure sensitive, and therefore the arbitrarely chosen pressure of 10Kbar only slightly affects the calculated temperatures. Although we do not know the exact pressure conditions, the assumed pressure is reasonable considered that the studied rock has been involved in the subduction process responsible for the eclogitic metamorphism within the Ötztal. The calculated temperature, together with the observed mineral assemblage suggest that chemical equilibration and deformation of garnet has occurred at lower amphibolite to higher greenscist facies condistions. The garnet aggregate microstructure suggests that the sample and hence the outcropping units along the Sulztal shear zone have been exhumed extremely fast during the Variscan subduction, dated at 350-360 Ma by Miller & Thöni (1995). The dissolution microstructures of garnet suggest that fluids have been abundant and channelized flow is responsible for localized strain concentration, and hence for the observed strong strain gradient within the studied rock sample