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Radmila Nahodilova, Czech Geological Survey (Czech Republic)
Shah Wali Faryad, Charles University Prague (Czech Republic)
Peter Tropper, University of Innsbruck (Austria)
Juergen Konzett, University of Innsbruck (Austria)
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The Moldanubian zone of the Bohemian Massif is characterized by the presence of high-pressure granulites, migmatites and granulite gneisses that are part of the Gföhl and Gföhl related units. The granulites are mostly felsic and their textures varies from fine-grained massive to well foliated rocks characterized by lecucocratic stripes and bands parallel to the foliation. The main mineral of granulites are quartz + mesoperthite >> garnet, rutile ± kyanite. Mafic lithologies may contain clinopyroxene and orthopyroxene.
The granulites and migmatites usually contain lenses of garnet peridotite and eclogites that show pressure conditions up to 3-4 GPa/850-1000 oC. Maximum PT condition estimated using the mineral assemblages present in granulite are about 2-2.2 GP/850-900oC. However, some textural relations and thermodynamic calculations using bulk rock and mineral stability fields as well as core composition of garnet show more higher pressures. It is not always clear, if the granulites passed also ultra-high-pressure conditions.
Felsic granulites with leucocratic stripes from the Kutna Hora Complex (Moldanubian Zone) were sampled to estimate PT conditions for partial melt formation and to analyse changing in mineral composition during granulite facies metamorphism and retrogression. The granulite consists of quartz, ternary feldspars, kyanite with garnet having relatively high Ca content (cor->rim:Grs41->29 Py8->20 Alm47->45 Sps1->0.8 xFe86->69). It shows relatively flat compositional profile in the core, but its rim composition indicates increase of Mg and decrease of XFe and Ca. Garnet from the lighter bands has relatively low Ca with rimward decrease (Grs27->15 Py22 Alm41->55 Sps0,5->1 xFe65->72). More evalvuated granulites, expeienced medium- to pressure granulite facies recrystalization, have Ca-poor ganet and high amounts of plagioclase instead of antiperthite. Beside thermodynamic calculations, the leucocratic variety was undertaken melting experiment at 1.7 GPa/850 oC and 2.1 GPa 900 oC.
The results of experiment, mainly that at higher pressure and temperature indicated formation of kyanite, ternary feldspars and relatively Ca-rich garnet, close to that in the run material. Both thermodynamic calculations and experimental investigations show that the transition to granulite facies P-T conditions and the formation of partial melt occurred by decompression and heating at ca. 2.2 GPa and 900oC during exhumation. The lack of hydrous phase coexisting with granulite facies assemblages at high-pressure conditions (while biotite represents a retrograde phase) suggests that the partial melting occurred with minimum fluid contents. Under equilibrium conditions in the absence of an H2O-rich volatile phase, melting could occur at multiphase grain junctions that include quartz and feldspar. The interpretation on partial melting in of granulites is consistent with several other models for the formation of felsic granulites in the Bohemian Massif.
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