Original paper

Ba-Ti-bearing metarodingites from the Austroalpine units (Innsbruck Quartzphyllite, Wildschonau Schists) north of the Tauern Window (Tyrol, Eastern Alps, Austria)

Piber, Andreas; Tropper, Peter

Abstract

At the southern border of the city of Schwaz (Tyrol, Austria) fine-grained metarodingite bodies are tectonically embedded within quartzphyllites (Innsbruck Quartzphyllite and/or Wildschönau Schists). The mineral assemblage of the metarodingites is Cr-Al-Zn-Fe-rich spinel1 + Cr-Fe spinel2 + magnetite2 + grossular-rich garnet1 + Cr-rich garnet2 + Ti-bearing garnet3 + andradite-rich garnet4 + clinopyroxene1 + clinopyroxene2 + titanite + calcite ± hyalophane ± K-feldspar ± albite ± chlorite ± Ba-biotite ± amphibole ± clinozoisite ± barite. Cr-bearing spinels1,2 are thought to represent remnants of the protolith, a pre-Alpine magmatic, ultramafic assemblage. Although garnet grew continuously, four types of garnet can be distinguished texturally and chemically: the oldest generation garnet1 is represented by cores of grossular-rich garnet. Adjacent to spinel, Cr-rich garnet2 replaces spinel. The subsequently crystallized Ti-bearing garnet3 is characterized by oscillatory zoning and the last generation garnet4 is andradite-rich, which occurs at the outermost rims of garnet3. The presence of Ti-rich garnets with intensive oscillatory zoning, and strongly zoned Ba-bearing phases such as barite, Ba-rich feldspar and Ba-bearing biotite, is indicative for a localized fluid flux during a metamorphic overprint. BSE images and X-ray distribution maps indicate that Ba-rich feldspar (hyalophane) represents the first feldspar generation followed by Ba-bearing K-feldspar, which shows oscillatory zoning. The last generation is albite. Biotites also reveal a complex chemical zoning pattern with up to three different growth zones based on different TiO2 and BaO contents where TiO2 contents range from 0.14 to 0.67 wt.%, and Ba contents range from 0.07 to 6.49 wt.%. Mineral chemistry and textural relations indicate a growth sequence of Ca-Al-, Ca-Fe-Ti-, Ba-, and Ca-Fe-bearing minerals, indicating changes in the fluid composition during mineral growth. Although the absolute timing of rodingitzation cannot be precisely established, the presence of Ba mineralizations in adjacent Triassic sediments and the correlation with available geochronological data from this area, indicates that localized, episodic minor element (Ti, Ba) transport most likely occurred during the greenschist-facies Eo-Alpine metamorphic overprint.

Keywords

metarodingitesinnsbruck quartzphyllitewildschÖnau schistseo-alpineba-ti zoning