GEOCHEMICAL AND TECTONIC SIGNIFICANCE OF TOURMALINE-RICH METASEDIMENTARY ROCKS IN THE CENTRAL ALPS

The Penninic Simano nappe, one of several large thrust sheets in the Central Alps, has a polymetamorphic basement core and a sedimentary cover of late Permian and Mesozoic age, and paleogeographically represents the southernmost part of the European plate at the transition from the European margin to the Tethyan ocean. After nappe stacking during the Alpine orogeny, the nappe was metamorphosed in amphibolite facies.

The sedimentary rocks consist of metaconglomerates, quartzites and dolomite marbles, with metapelites intercalated at various levels. Recent field work revealed an unusual abundance of large, euhedral tourmaline in all rock types which occur in a stratigraphic position that represents the Permian-Triassic transition. This period was characterized by an arid climate and sedimentation near the Tethyan shore and in nearby desert basins.

The abundance of tourmaline in such a variety of rocks offers the unique possibility to address several key issues: 1) because of its prismatic habit, tourmaline can provide information about the timing of crystal growth and deformation. Microstructural investigations demonstrate that tourmaline generally grew post-kinematically with respect to the main foliation (representing the nappe stacking event); 2) tourmaline displays distinct chemical zoning (continuous, discontinuous or both), and in one case oxygen isotopic zoning. These features demonstrate that tourmaline records chemical changes occurring in the rock during its metamorphic evolution. Moreover, discontinuous zoning indicates that tourmaline grew at various stages, probably as a result of episodic Bororn availability; 3) the presence of tourmaline in widely different mineral assemblages allows to infer the effect of the host-rock composition on stability and composition of tourmaline. The association of tourmaline with white mica points to a genetic relationship between the two phases; and 4) the bulk chemical composition of the host rocks can be compared to non-metamorphosed rocks, thus allowing to detect geochemical anomalies and/or to infer the sedimentary environments of the protolith.

The results suggest that tourmaline-rich metasedimentary rocks are potentially important for reconstructions of their original depositional setting and their tectono-metamorphic evolution in orogenic belts.