Metamorphic evolution of calcareous schists of the Margna Nappe at Valmalenco, Central Alps

Metamorphic rocks cropping out north of the Periadriatic Line are objects of our study to better understand the Cenozoic subduction process during the collision of microcontinent Adria with the European plate forming the Central and Eastern Alps. For this purpose, we collected rocks along the Malenco valley that are suitable to allow us to decipher their pressure-temperature (P-T) evolution. About 1.5 km northwest of the village of San Giuseppe, calcareous schists rich in phengite were sampled. Their bulk-rock compositions were determined. The minerals in three samples were carefully chemically characterized with the electron microprobe. Thermodynamic modelling followed for two garnet-bearing samples using the program package PERPLE_X.

The protoliths of the samples were probably carbonate-bearing psammopelites. The two modelled rocks are characterized by alternating, a few mm thick layers either enriched in phengite or quartz representing the main foliation. The phengite-rich layers host most of the mafic minerals, whereas quartz-rich layers also contain plagioclase, which is nearly pure albite. The mafic minerals are hornblende, idiomorphic garnet with diameters between 50 to 200 µm, and minor biotite and chlorite. One sample also contains some epidote, the other one some titanite. Accessories are zircon, apatite, and opaque phases. Carbonate is lacking.

Modelling of the peak metamorphism is based on phengite with Si contents between 3.25 (rim) and 3.35 per formula unit (pfu) and zoned garnet. The zonation is characterized by significantly decreasing Mn, slightly decreasing Mg (0.05 to 0.04 Xpyrope), and clearly increasing Ca contents (e.g., 0.25 to 0.36 Xgrossular in one sample) from core to rim. The modelling yielded, consistently for both samples, a pressure decrease from 13.5 kbar at 570°C to 11.5 kbar at 550°C. In spite of the temperature decrease, a growth of garnet occurred because the modelling predicts 2-2.5 vol% garnet coexisting with about 20-25 vol% phengite and 30-35 vol% as well as significant quantities of omphacite (20-25 vol%), biotite (7-10 vol%), and paragonite (7-10 vol%) at the pressure peak, but 4.5 vol% garnet at 11.5 kbar due to breakdown of omphacite, biotite, and paragonite. The decomposition of these minerals also led to increasing contents of phengite with Si contents of 3.25 to 3.30 pfu as well as significant quantities of hornblende and albite during further pressure release. The observed chlorite seems to be a late retrogression product.

We suggest that the studied samples are monocyclic metamorphic rocks. They were located at (or near) the surface of the downgoing European plate and subducted to Earth’s depths of about 50 km in the Cenozoic. Hydrous fluids were present during the subduction process and early exhumation evidenced by the aforementioned mineral reaction (paragonite breakdown). Major deformation occurred at the pressure peak and during early exhumation. The corresponding tectonic movements led to nappe stacking, so that the contact to the Malenco Unit, which represents Permian lowermost crust and underlying mantle, in the south was established.