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.