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Matej Machek, Czech Academy of Sciences (Czech Republic)
Stanislav Ulrich, Czech Academy of Sciences (Czech Republic)
Vojtech Janousek, Czech Geologycal Survay (Czech Republic)
Shah Wali Faryad, Charles University (Czech Republic)
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In the Bohemian Massif, a suite of mantle rocks occur as isolated lenses mostly within the felsic granulites, orthogneisses and migmatites of the internal orogenic domain. Two bodies of garnet peridotites enclosed in granulites of the Kutna Hora Crystaline Complex have been studied. They enclose small eclogite bodies and contain closely-spaced layers of garnet clinopyroxenite. The granulites and granulite gneisses show two planar fabrics.The former is steep NE-SW trending high-temperature foliation, which is parallel to layers of the clinopyroxenite. Such fabric is preserved in the low-strain domains surrounded by the later mylonitic penetrative fabric.
The main microstructural feature is sub-vertical stretching lineation revealed from the EBSD measurements and it is developed in all rock types within the low-strain domains. In peridotites, olivine LPO shows either [100](010) or [001](010) pattern indicating steeply dipping and folded foliation.
Clinopyroxene LPO measured in eclogites and clinopyroxenites exhibits foliation planes parallel to peridotites and layering measured in the field, respectively. The peak PT conditions estimated for granulites and granulite gneisses are about 1.8-2.2. GPa and 800-920 °C (Vrána et al. 2005). Garnet from garnet peridotite contains spinel and show an increase of pressure up to 4 GPa /900 °C. Pressures and temperatures, calculated for the eclogite in peridotite range between 850-900 °C at 3.0-3.4 GPa.
Peridotites are geochemicaly rather primitive, originally depleted upper mantle rocks that were re-enriched by metasomatic fluids or small-scale melts. It supports a model that clinopyroxenite layers are products of interaction between basaltic melts and lithospheric mantle peridotite with significant role for high-pressure clinopyroxenite, spinel and ilmenite accumulation. The eclogite sample corresponds to a subalkaline basaltic melt.
Peridotites are supposed to be relics of mantle wedge hydrated to various extents (above the Saxothuringian Ocean subduction zone) that triggered mantle wedge flow along the subduction zone and a subvertical rheological boundary within the upper mantle. During orogenic thickening hydrated mantle has been deformed and the basaltic melt flow from the subduction zone became strain-controlled. Presented data suggest two possible scenarios. The former works with shearing along NE-SW trending transpressional foliations well developed from clinopyroxenites in the mantle through coarse-grained granulite in the lower crust to the hanging-wall middle to upper crust (Zak et al., 2005).
It could lead to the emplacement of mantle bodies to the granulitized orogenic root in the hanging-wall. The latter suggest deep subduction of at least part of granulites within mélange at slab-mantle interface and than coalescence of mantle and mélange rocks during vertical movement along the previously developed (dry vs. wet peridotite) rheological boundary towards the granulitized orogenic root.
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