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Abstract
We report structural evidence of ductile strain localization in mantle pyroxenite from the spinel to plagioclase websterite transition in the Ronda Peridotite (southern Spain). Mapping shows that, in this domain, small-scale shear zones occurring at the base of the lithospheric section are systematically located within thin pyroxenite layers, suggesting that the pyroxenite was locally weaker than the host peridotite. Strain localization is associated with a sudden decrease of grain size and increasing volume fractions of plagioclase and amphibole as a result of a spinel to plagioclase phase transformation reaction during decompression. This reaction also fostered hydrogen extraction (‘dehydroxylation’) from clinopyroxene producing effective fluid saturation that catalyzed the synkinematic net-transfer reaction. This reaction produced fine-grained olivine and plagioclase, allowing the onset of grain-size sensitive creep and further strain localization in these pyroxenite bands. The strain localization in the pyroxenites is thus explained by their more fertile composition, which allowed earlier onset of the phase transition reactions. Geothermobarometry undertaken on compositionally zoned constituent minerals suggests that this positive feedback between reactions and deformation is associated with cooling from at least 1000°C to 700°C and decompression from 1·0 to 0·5 GPa.
Highlights
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These centimeter- to decimeter-scale pyroxenite mylonite zones occur at the tips of larger (10^15 m wide) shear zones that formed during kilometer-scale folding and shearing of the attenuated subcontinental lithospheric mantle, which accommodated the emplacement of the massif in the crust (Hidas et al, 2013)
In this paper we present a detailed microstructural and petrological study of one of these mylonitic zones associated with plagioclase^ spinel pyroxenites, aiming to unravel potential processes allowing for strain localization in the shallow lithospheric mantle
Summary
This page was generated automatically upon download from the ETH Zurich Research Collection. Structural mapping in the southwestern part of the Ronda Peridotite massif in southern Spain highlights that shear zones located in the transition between the spinel and the plagioclase lherzolite domains are systematically developed in plagioclase^spinel pyroxenites, indicating that these pyroxenites were weaker than the surrounding peridotite These centimeter- to decimeter-scale pyroxenite mylonite zones occur at the tips of larger (10^15 m wide) shear zones that formed during kilometer-scale folding and shearing of the attenuated subcontinental lithospheric mantle, which accommodated the emplacement of the massif in the crust (Hidas et al, 2013). Plagioclase tectonites formed during kilometer-scale folding of an attenuated lithospheric mantle upon decompression from spinel to plagioclase lherzolite facies prior to the emplacement into the crust in the Late Oligocene^Early Miocene, as described by Hidas et al (2013) These small-scale shear zones have foliations steeply dipping to the NNW
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