Deformation processes in a peridotite shear zone:: reaction-softening by an H2O-deficient, continuous net transfer reaction
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abstract
The Turon de Tecouere peridotite, in the North Pyrenean Zone, is composed of protomylonites grading to a 20-40 m wide zone of ultramylonites within a 0.6 km diameter exposure. The progressive mylonitization is marked by increasing 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 m), lack of dislocations in matrix grains, a weak crystallographic preferred orientation, and the alignment of grain boundaries parallel to the foliation. As the percentage of fine-grained matrix increased, weakening and localization resulted from a change in the dominant deformation mechanism from dislocation creep in the porphyroclasts to grain size sensitive creep in the fine-grained matrix. Production of the matrix grains took place by the nucleation 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 along a single porphyroclast margin can be explained by a syntectonic continuous net transfer reaction associated with the spinel- to plagioclase-lherzolite transition. This continuous net transfer reaction produced new matrix grains with the same mineralogy as the original assemblage (olivine, orthopyroxene, clinopyroxene, spinel), with new compositions, plus plagioclase. Preliminary geothermobarometry indicates that the reaction took place over a range of temperatures and pressures (750-850C, and possibly as high as 950C and 0.5-1.1 GPa). The presence of only small amounts of amphibole, the lack of primary fluid inclusions, and no relation between the presence of amphibole and the intensity of mylonitic deformation led Vissers et al. [Tectonophysics 279 (1997) 303-325] to conclude that the deformation 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 are solid solutions so that reactions are continuous (multivariant) and take place over a broader region of pressure-temperature space than end-member reactions. It is therefore likely that shear zone deformation in polymineralic rocks will involve reaction-enhanced ductility over much of pressure-temperature space in the lithospheric mantle.
