Abstract
As for most upper mantle shear zones, mylonitic and ultramylonitic peridotites exposed in the northern Lanzo shear zone display thoroughly mixed, fine-grained (< 25 mu m) bands. The weak crystallographic preferred orientations (CPO) of all their phases are indicative for deformation by diffusion creep. Although interpreted as significantly decreasing the strength of the upper mantle if interconnected, the formation process of fine-grained phase mixtures is still debated. Microstructural analysis of gradually higher strained peridotites of the northern Lanzo shear zone revealed the formation of fine-grained (< 25 mu m) polymineralic tails adjacent to clino- and orthopyroxene porphyroclasts in mylonitic samples. Phase mixtures (av. 65% phase boundaries) are dominated by neoblasts derived from the porphyroclast (Opx/Cpx) and olivine, with minor abundances (< 10 area-%) of plagioclase, spinel +/- amphibole. Neoblast microstructures and mixing of phases directly at the border of the pyroxenes, indicate a reaction of pymxene porphyroclasts to neoblast phase mixtures. Neoblast temperature estimations (similar to 860 degrees C), their systematic change in composition, and plagioclase/spinel abundances suggest that continuous net-transfer reactions, enhanced by the spinel lherzolite to plagioclase lherzolite transition are likely the main driving forces for the formation of ultramylonitic phase mixtures in the Lanzo shear zone. These reactions were potentially enhanced by the presence of fluid as evidenced by pargasitic amphibole crystallization. Comparing these results to other upper mantle shear zones and deformed xenoliths, the importance of reactions (melt-rock, fluid-rock and/or metamorphic) for the formation of polymineralic, ultramylonitic bands is confirmed.
NSTL
Elsevier