Don’t always use metapelites: what do amphibolites and pegmatites reveal about the prograde Eoalpine metamorphic evolution of the Austroalpine nappe stack in the Vinschgau valley (S-Tyrol, Italy)?

The geological map sheet Schlanders (Project CARG F012) offers the chance to carefully investigate the metamorphic evolution of the Austroalpine units in the Vinschgau and their tectonic contacts and to implement them into a tectonic model based on new petrological, geochronological and structural data. The Austroalpine nappe stack in the investigated area, located in the Vinschgau area (South Tyrol), comprises from bottom to top the Campo-, Texel-, Ötztal- and Matsch Units. The Matsch unit in the northern flank of the Vinschgau valley shows a clear polymetamorphic history (Variscan, Permian, Eoalpine) which can be well reconstructed with metapelites using the spatial distribution of alumosilicates (kyanite, andalusite, sillimanite), the chloritoid-isograd and the observation of chemical zoning patterns in garnets, which, depending on the geographical position and the geological setting, exhibit single-phase, two-phase or even three-phase compositions. The Ötztal and Texel Units (without the Lodner Unit) also show a polymetamorphic history (Variscan, Eoalpine) but without the Permian overprint. In contrast to the Ötztal Unit, the Texel Unit contains rare Eoalpine eclogites (e.g. Ulvas, Saltaus). Geothermobarometry from all three units yielded a strong increase in Eoalpine P-T conditions from ca. 450°C and 0.6 GPa in the west (Matsch valley) to 650°C and 1-1.2 GPa in the east (Naturns).

 

The study of amphibole composition is central to the understanding of metamorphic processes of metabasic rocks, especially when analyzing pressure and temperature conditions. This study analyzes the chemical composition of amphiboles along a W-E traverse along the Vinschgau Valley (South Tyrol). The composition of amphiboles changes from actinolite to hornblende along the prograde E-W-trending metamorphic gradient, and shows increasing chemical substitutions such as the edenite-, glaucophane- and tschermak vectors. This is also accompanied by an increase in Ti content (0.004 to 0.36 wt.% TiO₂) in the amphiboles, as well as the X_An in the coexisting plagioclase from 0.1 to 0.2. Temperatures based on the Ti-in-hornblende- and the amphibole-plagioclase geothermometers yielded a T increase from 490°C to 600°C.

 

Tourmaline from Permian pegmatites in the Matsch unit show chemical evidence for the Eoalpine metamorphic overprint in the rim zoning along fractures and growth zones in tourmaline associated with muscovite (also showing Eoalpine growth rims), K-feldspar growing along veins, An-bearing plagioclase, quartz, and a second generation of garnet. The Permian tourmaline cores can be classified as schorl according to the [Y]-position and have the same composition in the entire area. The Eoalpine rims show compositionally a transition from schorl to dravite and show increasing contents of Ca[X] from 0.06 to 0.2, Mg/Fe[Y] from 0.02 to 2, and a significant decrease in Al[Y] from 0.4 to 0.1 from W to E. This confirms the from NW to SE increasing Eoalpine P-T conditions as reconstructed based on analysis of metapelitic rocks mentioned above.

 

The data show that lithologies such as amphibolites and pegmatites also show great potential to contribute significantly to our knowledge of prograde metamorphic evolution.