Tectono-metamorphic history recorded in garnet porphyroblasts: insights from thermodynamic modelling and electron backscatter diffraction analysis of inclusion trails

Abstract

International audienceIn a Barrovian metamorphic sequence, garnetiferous mica schists document a heterogeneously developed superposition of sub-orthogonal fabrics and multiple garnet growth episodes. In the variably deformed domains, four types of garnet porphyroblasts have been defined based on inclusion trail patterns. Modelled garnet zoning in the MnNCKFMASHTO system indicates a prograde evolution from 4-4.5 kbar and 490-510 C to 5-6 kbar and 520-550 C in the earliest subhorizontal fabric progressing towards 6.5-7.5 kbar and 560-590 C in the subsequent subvertical foliation. This fabric is heterogeneously deformed into a shallow-dipping retrograde foliation associated with garnet resorption. In situ electron backscatter diffraction measurements of ilmenite inclusions in individual garnet grains yield precise data on included planar and linear elements. Consistent orientations of internal foliations, lineations and foliation intersection axis sets indicate a superposition of three sub-orthogonal foliation systems. Weak variations of internal records with increasing intensity of deformation suggest that a moderate buckling stage occurred, but apparent lack of porphyroblast rotation is interpreted as a result of dominant passive flow. Coupling the orientation of internal fabric sets with P-T estimates is used to complement the tectono-metamorphic evolution of the thickened crust. We demonstrate that garnet porphyroblasts preserve features which reflect large-scale tectonic processes in orogens