Variscan and Alpine metamorphic events in the Northern Apennines (Italy): a review.

The Northern Apennines consist of oceanic and continental Mesozoic-Tertiary units thrust over the deepest Tuscan metamorphic units, including Paleozoic successions. Tuscan metamorphic units underwent two Alpine tectonometamorphic deformation phases, and only pre-Carboniferous ones were subjected to polyphase Variscan metamorphism. For the first Variscan phase with a Sudetian age of 328 +/- 5.3 Ma, the estimated P-T values are similar to 0.8 GPa and similar to 670 degrees C; for the second Variscan phase, P similar to 0.4-0.5 GPa, T similar to 530 degrees C. These P-T conditions were attained only in the Gneiss and Micaschist Complexes. In Larderello Micaschist C. and Gneiss C., pre-Alpine andalusite and muscovite (285 +/- 11Ma) might be related to a Late Variscan thermal event (third Variscan phase) with P=0.15-0.25 GPa and T >550 degrees C. During the first Alpine compressional phase (27-20 Ma), Mesozoic-Tertiary sediments were buried to a maximum depth of 40-55 km. Peak pressure values ranged from 0.8 to 1.2 GPa for Verrucano metasediments (Tuscan metamorphic units), with the exception of those on Giglio Island (1.0-1.5 GPa). The Schistes Lustres of Gorgona show peak pressure values of 1.3-1.6 GPa, while the same rocks on the Argentario Promontory and Giglio Island are in the range 0.6-0.85 GPa. For both units, temperatures varied from 300 degrees to 420 degrees C (450 degrees - 480 degrees C in the Massa Unit). Extensional tectonics began in the early to late Miocene according to different models, caused very rapid exhumation (rough estimates: 2.0-2.5 mm/yr for the Apuan units and 3.9-4.0 mm/yr for the Schistes Lustres of Gorgona) up to 10-15 km depth and produced a second main folding metamorphic phase (14-12 Ma) with P=0.3-0.5 GPa and T=300 degrees-370 degrees C. The distribution pattern of Al-silicates reveals that Tuscany is divided into NW-SE trending narrow parallel zones. Metamorphic grade increases from an eastern kaolinite-bearing zone to a central kyanite-bearing zone corresponding to the Massa Unit-Larderello alignment and then shows a westward decrease in the Punta Bianca-Elba areas. Similarly, non-metamorphic rocks show a rough decrease in illite crystallinity (degrees Delta 2 Theta increase) from west to east and from the lower to upper tectonic units. In Southern Tuscany and in the Tuscan Archipelago, Tortonian-Quaternary magmatism produced contact metamorphism and hydrothermal alterations.