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 Varis can greenschist facies planar fabric, slightly reworked during thick-skin A lpine 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 fa bric is composite (D1a & D1b) and defined by contrasting amphibolite facies metamorphic assemblages in different residual sites. Cld + Bt(I) + Grt + M s-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 t ectonometamorphic imprints affecting different tectonic units, later couple d during a D2 greenschist facies stage. Thermobarometric estimates of assem blages formed during D1a, D1b and D2 show a transition from T = 480-540 deg rees C (during Dla) to T = 570-660 degrees C (during D1b), corresponding to a slight pressure-increase from 0.75-0.95 GPa to 0.85-1.15 GPa. D2 greensc hist retrogression corresponds to a pressure and temperature decrease (T < 400-550 degrees C and P < 0.3-0.4 GPa). This P-T-deformation-time path is i nferred to be the result of uplift from a depth of c. 35 km, after Palaeozo ic subduction and continental collision; it is consistent with models postu lated for other metamorphic units of the Variscan Belt in Europe. This is t he first documented example in the Southern Alps of temperature-prograde me tamorphism before Palaeozoic collision.