Influence of deformation partitioning and metamorphic re-equilibration on P–T path reconstruction in the pre-Alpine basement of central Southern Alps (Northern Italy)

The dominant foliation (S2) in the metapelites of the Southalpine basement, near the western side of the Tertiary Adamello intrusive stock, is a Variscan greenschist facies planar fabric, slightly reworked during thick-skin Alpine tectonics. S2 is defined by muscovite and chlorite and was formed by decrenulation of pre-existing foliations, which are confined to metre-size, less-deformed domains and defined by biotite and white mica. The pre-S2 fabric is composite (D1a & D1b) and defined by contrasting amphibolite facies metamorphic assemblages in different residual sites. Cld+Bt_I+Grt+Ms_I+Pl+Qtz and St+Bt_II+Grt+Ms_II+Pl+Qtz assemblages mark D1a and D1b fabrics respectively; these developed during successive steps of a single, temperature-prograde polyphase event, rather than during separate tectonometamorphic imprints affecting different tectonic units, later coupled during a D2 greenschist facies stage. Thermobarometric estimates of assemblages formed during D1a, D1b and D2 show a transition from T  =480–540  °C (during D1a) to T  =570–660  °C (during D1b), corresponding to a slight pressure-increase from 0.75–0.95  GPa to 0.85–1.15  GPa. D2 greenschist retrogression corresponds to a pressure and temperature decrease ( T  <400–550  °C and P <0.3–0.4  GPa). This P–T– deformation–time path is inferred to be the result of uplift from a depth of c. 35  km, after Palaeozoic subduction and continental collision; it is consistent with models postulated for other metamorphic units of the Variscan Belt in Europe. This is the first documented example in the Southern Alps of temperature-prograde metamorphism before Palaeozoic collision.