Deformation processes in a peridotite shear zone: reaction-softening by anH2O-deficient, continuous net transfer reaction

The Turon de Tecouere peridotite, in the North Pyrenean Zone, is composed o f protomylonites grading to a 20-40 m wide zone of ultramylonites within a 0.6 km diameter exposure. The progressive mylonitization is marked by incre asing volume fractions of very fine-grained matrix that comprise up to 90% of the ultramylonite. Deformation of the fine-grained matrix took place by grain size sensitive creep, as suggested by a very fine grain size (<10 mu m), lack of dislocations in matrix grains, a weak crystallographic preferre d orientation, and the alignment of grain boundaries parallel to the foliat ion. As the percentage of fine-grained matrix increased, weakening and loca lization resulted from a change in the dominant deformation mechanism front dislocation creep in the porphyroclasts to grain size sensitive creep in t he fine-grained matrix. Production of the matrix grains took place by the n ucleation of a number of different phases at the margins of porphyroclasts, indicating that the grain size reduction resulted primarily from reaction, and not from dynamic recrystallization. The nucleation of many phases alon g a single polphyroclast margin can be explained by a syntectonic continuou s net transfer reaction associated with the spinel- to plagioclase-lherzoli te transition. This continuous net transfer reaction produced new matrix gr ains with the same mineralogy as the original assemblage (olivine, orthopyr oxene, clinopyroxene, spinel), with new compositions, plus plagioclase. Pre liminary geothermobarometry indicates that the reaction took place over a r ange of temperatures and pressures (750-850 degrees C, and possibly as high as 950 degrees C and 0.5-1.1 GPa). The presence of only small amounts of a mphibole, the lack of primary fluid inclusions, and no relation between the presence of amphibole and the intensity of mylonitic deformation led Visse rs et al. [Tectonophysics 279 (1997) 303-325] to conclude that the deformat ion took place in an H2O-deficient environment. Reaction-enhanced softening may occur in the upper mantle wherever rocks move in pressure-temperature space and cross-reaction boundaries. Reaction boundaries are often modeled as univariant (lines in pressure-temperature space), yet mantle minerals ar e solid solutions so that reactions are continuous (multivariant) and take place over a broader region of pressure-temperature space than end-member r eactions. It is therefore likely that shear zone deformation in polymineral ic rocks will involve reaction-enhanced ductility over much of pressure-tem perature space in the lithospheric mantle. (C) 1999 Elsevier Science B.V. A ll rights reserved.