TOURMALINE-RICH PARAGONITE SCHISTS IN THE CENTRAL ALPS AS PALEOGEOGRAPHIC INDICATORS?

Metapelites containing an unusual abundance of euhedral tourmaline occur near Alpe Sponda, Switzerland, and are a part of the basement of the Simano nappe in the Lepontine Alps. In addition to tourmaline, these rocks contain garnet, kyanite, plagioclase, and large porphyroblasts of cordierite; as well as four sheet silicates: muscovite, biotite, paragonite, and chlorite. There is an ample amount of rutile occurring as inclusions within other minerals, but so far no quartz has been observed.

Tourmaline displays characteristic color zoning, indicating two stages of crystal growth at different periods of mountain building. The tourmaline prisms have cross sections of up to 0.6 cm and may be as long as 2.0 cm. Quantitative chemical profiles along cross sections of tourmaline obtained with an electron microbe exhibit a pronounced compositional discontinuity at the core/rim boundary: the concentration of Al2O3 decreases from 33.7 wt% in the core to 31.4 wt% in the rim. On the other hand, there is an increase in the contents of Na2O (2.0 - 2.5 wt%), MgO (7.8 - 8.5 wt%) and total Fe (4.2 – 5.1 wt% Fe2O3).

Based on the mineral assemblage, it can be concluded these rocks were metamorphosed under amphibolite-facies conditions, which prevailed during the main Alpine metamorphism in the Tertiary. Further electron microprobe and oxygen isotopic data will be collected in order to derive temperature and pressure of metamorphism for these rocks. Previous studies have estimated the conditions of peak metamorphism in this area to be about 625 °C and 600 MPa (Irouschek, 1983; PhD thesis Univ. Basel). To find a possible explanation for the unusual abundance of tourmaline and paragonite, the whole-rock chemical composition is being analyzed. These data will contribute in evaluating possible paleogeographic environments during deposition of the protolith.