Die Entwicklung von polymineralischen Scherzonen im oberen Mantel
Zusammenfassung der Projektergebnisse
The proposal focused on the feedback processes between deformation, reactions, grain size reduction and phase mixing during shear zone evolution. In order to look at different phase mixing processes, three shear zones were analysed in detail. Each of these shear zones represent a phase mixing process: meltrock reactions (Lanzo, Italy), metamorphic reactions (Erro-Tobbio, Italy) and grain boundary sliding with neighbour switching (Ronda, Spain). The idea behind the proposal was to analyse and compare the different microstructures (i.e, grain size, area percentage of phases, CPO), in order to see how grain size reduction and phase mixing occurs during these processes. Furthermore, the objective of the proposal was to determine when the phase mixing occurred during the shear zone evolution and what the effect on strain localization was. Grain size reduction can result in strain localization due to the switch in deformation mechanism from dislocation creep (grain size insensitive) to diffusion creep (grain size sensitive). The detailed study of the Lanzo peridotite showed that pyroxene porphyroclasts formed polymineralic tails dominated by the host pyroxene (opx or cpx) and olivine. In addition, plagioclase was always present in these tails. After comparing the chemical and microstructural data with previous studies it was deduced that these polymineralic tails formed by metamorphic reactions due to the exhumation from the spinel peridotite to plagioclase peridotite stability field. These metamorphic reactions were very effective in decreasing the grain size and mixing the different phases. The ultramylonite bands found within the mylonites had a similar grain size and phase composition, suggesting that these bands were formed by the metamorphic reactions and ongoing deformation. In the ultramylonites diffusion creep was dominant, and the strain could localize in these bands. The Erro-Tobbio showed slightly different microstructures. Both pyroxene porphyroclasts again show tails of neoblasts. However, the cpx porphyroclasts partly show close to monomineralic cpx tails. These tails were formed by dynamic recrystallization of the cpx porphyroclast, something not observed in the Lanzo peridotite. Further away from the porphyroclast border, some olivine was included within the tail. In addition, some smaller cpx porphyroclasts formed olivine and cpx tails. Clusters of cpx grains with a similar orientation and irregular grain shapes suggest that the olivine was formed due to a reaction of a SiO2 undersaturated melt with cpx. The opx porphyroclasts show little evidence for dynamic recrystallization and the polymineralic tails are dominated by opx, olivine and some cpx and spinel. This also suggest that these tails formed from the reaction of opx with a SiO2 undersaturated melt. This can be correlated with the evolution of the Erro-Tobbio peridotite derived in previous research. Spinel shear zones accommodated a first stage of thinning without melt. Then with ongoing thinning the adiabatic uprising of the asthenosphere led to the infiltration of SiO2 undersaturated melts preferentially along the shear zones. The dynamic recrystallization of the cpx therefore occurred in these first stages of thinning, whereas the fine-grained layers and ultramylonites formed in a later stage during melt infiltration. Using olivine flow laws, the grain size reduction led to a strain rate increase of one order of magnitude in the ultramylonites. This calculation uses olivine deformation flow laws and does not consider the pyroxenes within the phase mixtures. The shear zones are likely also weakened further by the infiltration of the melt. The preliminary results of Ronda shear zone (garnet/spinel mylonite and spinel tectonite), indicate two metasomatic events leading to phase mixing. The first re-fertilized the olivine matrix and interstitial pyroxenes recrystallized in between the olivine grains. Polyphase tails adjacent to cpx porphyroclasts and fine-grained patches of neoblasts bordering irregular, lobate opx porphyroclasts, with indentations of all neoblast phases, were formed by metasomatic reactions. Indentations of amphibole into pyroxene porphyroclasts underline that amphibole is part of the primary neoblast assemblage. There is no grain size difference between the olivine-rich matrix and these tails, and the grain sizes remain similar over the entire mylonitic shear zone. The strain however did increase towards the shear zone front as indicated by the increasing elongation of the opx porphyroclasts. Strain localization therefore did occur, but the cause for that was not the grain size reduction and phase mixing process during the metasomatic reactions. No ultramylonites were formed and dislocation creep was dominant in the whole shear zone. The results of Ronda show that reactions alone do not necessary lead to shear localization. However, comparing the results of the studies three shear zones with other shear zones, it does show the importance of reactions for reducing grain sizes and the mixing of phases. Especially reducing the opx grain sizes. Grain size reduction of opx porphyroclasts by dynamic recrystallization has only been observed in deformed xenoliths and not in upper mantle shear zones. Therefore, it is crucial for the formation of polymineralic (olivine + opx + cpx) ultramylonites, that reactions take place. These reactions can occur during melt/fluid infiltration or due to changes in PT conditions.
Projektbezogene Publikationen (Auswahl)
- (2021). Formation of Ultramylonites in an Upper Mantle Shear Zone, Erro- Tobbio, Italy. Minerals, 11(10), 1036
Linckens, J., and Tholen, S.
(Siehe online unter https://doi.org/10.3390/min11101036) - (2022) Reaction-induced phase mixing and the formation of ultramylonitic bands. Tectonophysics 827
Tholen, S., Linckens, J., Heckel, C. and Kemperle, M.
(Siehe online unter https://doi.org/10.1016/j.tecto.2022.229230)
