TOURMALINE-RICH PHENGITE-PHLOGOPITE SCHISTS FROM THE CENTRAL ALPS

The Mesozoic sedimentary cover of the Penninic Simano Nappe, Central Alps consists of shales, conglomerates, sandstones and carbonate rocks, which were subjected to amphibolite-facies metamorphism during the Tertiary orogeny.

The metacarbonates are banded dolomite marbles with thin (<1cm) mica-rich beds. The latter are composed predominantly of phengite, quartz, potassium feldspar, and tourmaline, and contain minor amounts of phlogopite, calcite, dolomite, albite, and zircon. The dolomite-rich layers consist primarily of dolomite and minor calcite, phengite and quartz. Tourmaline occurs only in the mica-rich layers, and is present as colorless, generally euhedral crystals. Most phengites are aligned parallel to the schistosity, but some large crystals are clearly post-kinematic with respect to the main deformation event (D2), which represents the nappe stacking.

Chemically, tourmaline has a nearly pure dravite end-member composition. It exhibits continuous chemical zoning, which is characterized by a decrease, from core to rim, in Na, Fe, Ti and Mg, and a concomitant increase in Ca and Al. The ratio Ca/(Ca+Na), thus, increases from core to rim, a feature that reflects increasing metamorphic grade, as documented by Sperlich et al. (1996, AmMin).

Application of the biotite-tourmaline thermometer of Colopietro and Friberg (1987, GSA Abstract) yields a temperature of 501ºC. This temperature estimate is in good agreement with other estimates from the same area. Since phengite coexists with potassium feldspar, phlogopite and quartz in these rocks, the barometer of Massone and Schreyer (1987, CMP) was applied to obtain a pressure estimate. The composition of phengite shows a rather large variation, but the Si content (3.3-3.5 per formula unit) of most syn-kinematic phengites indicates pressures between 8 and 13 kbars. Similar pressures were also reported by Irouschek (1983, Ph.D. Thesis) for metapelites in the Simano nappe. Other authors, however, estimate pressures of 5-6 kbars for the thermal peak of the Alpine metamorphism in this area. This suggests that the Simano nappe had a more complex metamorphic history than generally assumed.