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Migmatization of the Gföhl gneiss in Waldviertel, Lower Austria


Radmila Nahodilová

Geoscience Research Reports 48, 2015 (GRR for 2014), pages 129–134

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Published online: 12 October 2015

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The Gföhl Unit (Moldanubian Zone) in Waldviertel, Lower Austria consists predominantly of orthogneisses with intercalated bodies of metasediments - paragneisses. The orthogneisses exhibit different degrees of anatexis from banded orthogneisses, via stromatitic to nebulitic migmatites. Anatexis occured during exhumation of these rock from mid-crustal (˜ 6 kbar) to upper-crustal (˜ 4 kbar) levels during the varsican metamorphism around 340 Ma. The degree of migmatization increases from west to east. I argue that partial melting occurred in the presence of fluids. Orthogneisses are generally quite refractory and can form only a few % of melt under common crustal conditions (700 °C a 4-6 kbar). A larger amount of melt creating nebulitic migmatites may be achieved only if either the pressure/temperature conditions of granulite facies are attained or if a water-rich melt or H2O were introduced. In contrast, pelitic metasediments have lower solidus temperature and can easily produce a large amount of melt (ca. 20-40 %) under similar conditions. I suggest that the paragneiss lenses provided the required aqueous fluids or melt. These fluids then infiltrated the surrounding orthogneisses and caused their extensive migmatization. Extensive melting of the Gföhl orthogneiss probably resulted from metamorphic reaction: Bt + Kfs + Qtz + Pl + H2O → melt + Grt. Consequently, as the fluid or melt infiltration was heterogeneous, the individual orthogneiss/migmatite types represent different degrees of interaction between the migrating melt and the orthogneiss.


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