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Migmatization of the Gföhl gneiss in Waldviertel, Lower Austria
Published online: 12 October 2015 Bailey, E. H. - Stevens, R. H. (1960): Selecting straining of K-feldspar and plagioclase on rock lobes and thin sections. - Amer. Mineralogist 45, 1020-1025. Franke, W. (2000): The mid-European segment of the Variscides: tectonostratigraphic units, terrane boundaries and kinematic evolution. In: Franke, W. - Haak, V. - Oncken, O. - Tanner, D., ed.: Orogenic Processes: Quantification and Modelling in the Variscan Belt. - Spec. Publ. Geol. Soc. London 179, 35-63. Hasalová P. - Štípská P. - Powell R. - Schulmann K. - Janoušek V. - Lexa O. (2008a): Transforming mylonitic metagranite by open-system interactions during melt flow. - J. Metamorph. Geol. 26, 55-80. Hasalová P. - Schulmann K. - Lexa O. - Štípská P. - Hrouda F. - Ulrich S. - Haloda J. - Týcová P. (2008b): Origin of migmatites by deformation-enhanced melt infiltration of orthogneiss: a new model based on quantitative microstructural analysis. - J. Metamorph. Geol. 26, 29-53. Jenček, V. - Matějovská, O. - Thiele, O. (1987): Groß-Siegharts. - Geologische Karte der Republik Österreich 1 : 50.000, Bl. 7, Geol. B.-A., Wien. Matte, P. - Maluski, H. - Rajlich, P. - Franke, W. (1990): Terrane boundaries in the Bohemian Massif: Result of large-scale Variscan shearing. - Tectonophysics 177, 151-170. Mehnert, K. R. (1971): Migmatites and the Origin of Granitic Rocks. - Elsevier, Amsterdam. Sawyer, E. W. (2001): Melt segregation in the continental crust: distribution and movement of melt in anatectic rocks. - J. Metamorph. Geol. 19, 291-309. Weinberg, R. F. - Hasalová, P. (2015): Water-fluxed melting of the continental crust: A review. - Lithos 212-215, 158-188. White, R. W. - Pomroy, N. E. - Powell, R. (2005): An in situ metatexite-diatexite transition in upper amphibolite facies rocks from Broken Hill, Australia. - J. Metamorph. Geol. 23, 579-602. White, R. W. - Powell, R. - Clarke, G. L. (2003): Prograde metamorphic assemblage evolution during partial melting of metasedimentary rocks at low pressures: migmatites from Mt Stafford, Central Australia. - J. Petrology 44, 1937-1960.Abstract
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.References
