Geochemical characteristics of the Late Proterozoic Spitz granodiorite gneiss in the Drosendorf Unit (Southern Bohemian Massif, Austria) and implications for regional tectonic interpretations

Abstract

The Spitz Gneiss, located near the Danube in the southern sector of the Variscan Bohemian Massif, represents a ~13 km² large Late Proterozoic Bt ± Hbl bearing orthogneiss body in the Lower Austrian Drosendorf Unit (Moldanubian Zone). Its formation age (U-Pb zircon) has been determined previously as 614 ± 10 Ma. Based on 21 new geochemical analyses, the Spitz Gneiss can be described as a granodioritic I-type rock (64-71 wt. % SiO 2) with medium-K composition (1.1-3.2 wt. % K 2 O) and elevated Na 2 O (4.1-5.6 wt. %). Compared to average granodiorite, the Spitz Gneiss is slightly depleted in Large-Ion Lithophile (LIL) elements (Rb 46-97 ppm, Cs 0.95-1.5 ppm), Sr (248-492 ppm), Nb (6-10 ppm), Th (3-10 ppm), the LREE (e.g. La 10-30 ppm), Y (6-19 ppm) and first row transitional metals (e.g. Cr 10-37 ppm). The Zr content (102-175 ppm) is close to average granodiorite. The major-and trace-element signature of the Spitz Gneiss is similar to some Late Proterozoic granodiorite suites in the Moravo-Silesian Unit (e.g. the Passendorf-Neudegg suite in the Thaya Batholith). However, granodiorites of such type and age do not occur elsewhere in the Moldanubian Zone of the Bohemian Massif. This observation fits existing tectonic models in which the Austrian Drosendorf Unit is considered allochthonous and part of the Moravo-Silesian Unit and the Avalonian Superterrane. Mineral chemistry data for amphibole, plagioclase and biotite allow an estimation of the Variscan peak regional metamorphic conditions for the Spitz Gneiss at ~700 °C and 7 kbar. Amphibole and plagioclase show hardly any signs of retrograde reequilibration, implying a fast late-Variscan exhumation. Partial chloritization of biotite indicates late fluid activity at T ~ 250 °C.

Topaz-bearing Lower Ordovician orthogneiss within the Ostrong Nappe System – The Laimbach Orthogneiss (Bohemian Massif, Lower Austria)

“…The Gföhl N.S. consists of various migmatic orthogneiss, paragneiss, granulite, amphibolite, eclogite and peridotite (Petrakakis, 1997;Fuchs et al, 1990;Lindner & Finger, 2018). This unit underwent eclogite-facies and partly ultrahigh-pressure peak metamorphic conditions (Kotková, 2007;Perraki and Faryad, 2014;Schantl et al, 2019) followed by high-pressure granulite-facies conditions, interpreted as continental subduction during the Variscan orogeny, followed by rapid exhumation (Carswell, 1991;Schulmann et al, 2005).…”

“…This unit underwent eclogite-facies and partly ultrahigh-pressure peak metamorphic conditions (Kotková, 2007;Perraki and Faryad, 2014;Schantl et al, 2019) followed by high-pressure granulite-facies conditions, interpreted as continental subduction during the Variscan orogeny, followed by rapid exhumation (Carswell, 1991;Schulmann et al, 2005). Granulite-facies peak metamorphic conditions followed by a low-pressure overprint (Högelsberger, 1989;Petrakakis, 1997) are characteristic for the Drosendorf N.S., which consists of paragneiss, quartzite, marble, graphitic schist, amphibolite, calc-silicate rocks and orthogneiss (Fuchs et al, 1990;Racek et al, 2006;Lindner and Finger, 2018). The provenance of this unit shows strong affinities to Avalonia (Finger and Schubert, 2015;Lindner et al, 2021).…”

“…Calc-silicate rocks, quartzite, eclogite, amphibolite and orthogneiss are also present. This unit underwent granulite-facies low-pressure metamorphic conditions (Petrakakis, 1997;Lindner and Finger, 2018). Linner (1996) divides the evolution of the Ostrong N.S.…”

“…Furthermore, a clear high degree of magmatic differentiation is shown by the high fluorine content, which is largely in the topaz. (René & Finger, 2016), Moldanubian Granulite (Janoušek et al, 2004), Dobra Gneiss (Lindner et al, 2020), Spitz Gneiss (Lindner and Finger, 2018), Blaník Gneiss (René and Finger, 2016), the Plagioclase-Orthogneiss Group IV from Teipel (2003) and Leptinitic Gneiss (Richter and Stettner, 1983). To answer the question if the investigated rock is whether a meta-granite or -rhyolite the following aspect is important: The short prismatic habit of the investigated zircons indicates a plutonic and not volcanic origin of the orthogneiss.…”

Building and Collapse of the Cadomian Orogen: A Plate‐Scale Model Based on Structural Data From the SW Iberian Massif

“…References in red, pale purple, and black refer to D C1 , D C2 , and D C3 , respectively. The numbers (1–7) indicate the reference from the following list: 1—Lindner and Finger (2018); 2—Sláma et al. (2008); 3—Graviou et al.…”