Thermometry of Quartz Mylonites: Importance of dynamic recrystallization on Ti-in-quartz re-equilibration.

Djordje, G., Stipp, Michael and Wooden, J. (2011) Thermometry of Quartz Mylonites: Importance of dynamic recrystallization on Ti-in-quartz re-equilibration. [Talk] In: 18. Conference on Deformation Mechanisms, Rheology and Tectonics, DRT 2011 Meeting . , 31.08.-02.09.2011, Oviedo, Spain .

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Quartz mylonites from the Tonale Fault Zone in the Alps (northern Italy) have been investigated by the Ti-in-quartz geothermometer (TitaniQ) in order to test its applicability to measure deformation temperatures. The eastern part of the Tonale Fault Zone was contact metamorphosed by the synkinematic intrusion of the Adamello pluton, forming a ~800 m-wide mylonitic shear zone, with a synkinematic temperature gradient from ~280 °C at the frictional-viscous transition to ~700 °C at the pluton contact as derived from metamorphic mineral assemblages.
Deformation microstructures from quartz mylonite samples, systematically collected across the mylonitic shear zone, display the entire range of dynamic recrystallization in quartz, which comprise bulging recrystallization (BLG), subgrain rotation recrystallization (SGR), and grain boundary migration recrystallization (GBM).
With regard to the trace element composition of quartz there are samples which are unaffected by contact metamorphism and Tonale shearing and those which show a resetting in trace element concentration. Ti and Al data in particular, show a sharp transition from samples with a decreasing concentration with decreasing peak synkinematic temperature to samples with a fairly constant concentration caused by pre-Alpine regional metamorphism. This shift corresponds exactly to the GBM to SGR transition at 540 °C. For all GBM samples, it was observed that TitaniQ registers the temperature at which quartz “locks in” its microstructure; i.e., when deformation and dynamic
recrystallization ceases. Based on new SHRIMP zircon U/Pb dating of the Presanella tonalite (30.5 ± 0.5 Ma), we further ascertain that the time of contact metamorphism was very short, on the order of 1 Ma. The time of deformation at the particular peak synkinematic metamorphic conditions was actually much shorter than that. Therefore, the available time was insufficient to re-equilibrate the
trace element composition of quartz by volume diffusion. The best explanation for the reequilibration in the zone of GBM is an increased diffusivity along the migrating grain boundaries of the investigated quartz mylonites.
Hence, dynamic recrystallization by fast grain boundary migration in the zone of GBM was a very effective re-equilibration mechanism for Ti and other trace elements, while within the zones of BLG and SGR no re-equilibration of the Ti concentrations could be detected. Grain boundary migration recrystallization is likely one of the most important processes for the enhancement of diffusive equilibration during deformation and it is much faster than equilibration under static conditions by volume and grain boundary diffusion.

Document Type: Conference or Workshop Item (Talk)
Keywords: Geodynamics; Geochemistry
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Date Deposited: 19 Dec 2011 08:16
Last Modified: 23 Feb 2012 05:09

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