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

摘要

At the southern border of the city of Schwaz (Tyrol, Austria) fine-grained metarodingite bodies are tectonically embedded within quartzphyllites (Innsbruck Quartzphyllite and/or Wildschonau Schists). The mineral assemblage of the metarodingites is Cr-Al-Zn-Fe-rich spinel_1, + Cr-Fe spinel_2 + magnetite_2, + grossular-rich garnet, + Cr-rich garnet_2+ Ti-bearing garnet_3 + andraditerich garnet_4, + clinopyroxene_1, + clinopyroxene_2 + titanite + calcite ± hyalophane ± K-feldspar ± albite ± chlorite ± Ba-biotite ± amphibole ± clinozoisite ± barite. Cr-bearing spinels_(1,2) 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 garnet, is represented by cores of grossular-rich garnet. Adjacent to spinel, Cr-rich garnet, replaces spinel. The subsequently crystallized Ti-bearing garnet_3 is characterized by oscillatory zoning and the last generation garnet, is andradite-rich, which occurs at the outermost rims of garnet_3. 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 TiO_2 and BaO contents where TiO_2 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.