Lower crustal metapelites: an ideal case for microstructural and geochronological characterization. An example from metapelites of the Serre Massif (Calabria, Southerm Italy)

In the northern Serre Massif (Calabria, southern Italy) a 8 km thick amphibolite-granulite facies Hercynian continental crust is exposed and shows a metamorphic gradient increasing from S toward N. This lower crustal section shares several similarities with well studied Palaeozoic crustal sections exposed in the southern Alps, i.e., the Ivrea- Verbano Zone. The lowermost level (the “granulite-pyriclasite unit”) consists of 1 km thick layer of mafic granulites followed by about 2 km thick layer of felsic granulites. Toward the higher structural levels, migmatitic metapelites (the “metapelite unit”) define a 5 km thick belt. So far it was believed that such a Palaeozoic lower continental crust is vertically continuous and tectonically undisturbed. However, our detailed fieldwork has revealed that the felsic granulites and the migmatitic gneisses are juxtaposed by a ca. 600 m thick high temperature shear zone. The shear zone developed at the base of the metapelite unit, where the mylonitic migmatitic gneisses are characterized by several centimetre to decimetre long ellipsoidal garnet porphyroclasts surrounded by an anastomosing matrix made up of elongated sillimanite prisms and biotite flakes. Garnet contributes to a strong strain partitioning in the mylonitic migmatites, and where garnet together with K-feldspar crystals forms a stress sustaining framework, we observe interbedded quartz layers with large recrystallized grains. Within the mylonitic matrix the grain size and abundance of quartz were strongly reduced and the crystals are primarily characterized by phase-boundaries with sillimanite prisms, biotite flakes and minor plagioclase grains. The main stretching lineation, well traced by the preferred alignment of sillimanite prisms. Locally, some decimetre-thick felsic dykes crosscut the main mylonitic foliation. Field structural data and CPOs of quartz, measured with EBSD within the different microstructural sites, testify that the shear zone developed along the flat of a normal fault, at temperatures exceeding 650°C. Preliminary microstructural investigation on accessory minerals reveals the presence of monazite, zircon and rutile within the mylonitic metapelites and zircon within the felsic dykes. LA-ICP-MS U-Th-Pb dating will be performed in order to reconstruct the T-t evolution of this lower crustal sector of the Palaeozoic continental crust. The coupling of different geochronometers will provide useful information on the timing of the evolution of the lower continental crust. Possibly, a comparison with the other well-known lower crustal section (i.e., the Ivrea-Verbano Zone) will be also presented.