P-T-deformation-Fe3+/Fe2+ mapping at the thin section scale and comparison with XANES mapping: application to a garnet-bearing metapelite from the Sambagawa metamorphic belt (Japan)

摘要

Quantitative X-ray maps of composition from a chlorite, K-white mica, albite, quartz and garnet bearing thin section from a Sambagawa blueschist facies metapelite were combined with a multi-equilibrium calculation method to calculate a P-T-Fe3+/Fe2+-deformation map at the millimetre scale. The studied sample was chosen because elongated chlorite crystallization tails (pressure shadows) rimmed by phengite are present, which is an appropriate assemblage for the quantification of the P-T evolution. Chlorite temperature and Fe3+ content maps were calculated by successive iterations for each pixel analysis of Fe3+ until convergence of the four chlorite-quartz-H2O equilibria that can be written using the Fe- and Mg-amesite, clinchlore, daphnite and sudoite chlorite end-members. The calculated map of Fe2+/Fe3+ in chlorite is in good qualitative agreement with the in situ mapping of this ratio using XANES (X-ray absorption near edge structure) techniques. The temperature map indicates that high temperature chlorite zones with low Fe3+ contents alternate with lower temperature zones and higher Fe3+ contents in the crystallization tail. Late fractures perpendicular to the elongation axis of the tail are filled by very low temperature chlorite (< 250 degrees C) showing Fe3+/Fe-total up to 0.4. Groups of chlorite and mica pixels were then identified based on compositional and structural criteria, and a P-T-deformation map was calculated using representative analyses of these groups. The calculated P-T-deformation map suggests that in contrast to chlorite, the composition of most mica did not change significantly during exhumation. Mica reequilibrated in late EW shear bands only. EW shearing was already active at 0.1 GPa, 500 degrees C, which corresponds to the peak temperature (and probably pressure) conditions, at reduced redox conditions. The intensity of deformation probably decreased with decrease in temperature to similar to 350-400 degrees C. At this temperature, a second main deformation event corresponding to a further EW stretching occurred and was still active below 250 degrees C and more oxidizing conditions. These results indicate that the scale at which P-T data can be obtained is now close to the scale of observation of structural geologists. A close link between deformation and mineral reaction is therefore possible at the microscopic scale, which provides information about the relationship between deformation and mineral reactivity, the modalities of deformation with time and the P-T conditions at which it occurred.