Influence of deformation partitioning and metamorphic re‐equilibration onP–Tpath reconstruction in the pre‐Alpine basement of central Southern Alps (Northern Italy)

Abstract

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+BtI+Grt+MsI+Pl+Qtz and St+BtII+Grt+MsII+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.

Interplay between deformation and metamorphism during eclogitization of amphibolites in the Sesia–Lanzo Zone of the Western Alps

“…The catalysing effect of strain rate on metamorphic recrystallization has been considered, so far, by different authors on a single bulk composition (Mørk 1985;Pognante 1985;Koons 1986;Austrheim 1990;Rubie 1990;Messiga et al 1995;Spalla et al 1999;Hobbs et al in press). Our observations suggest that strain localization, manifest by the mesoscopic fabric gradient, is the additional variable triggering the recrystallization from coronitic eclogitized amphibolites to amphibole-bearing eclogite (Amp-in) and finally to eclogite (Amp-out).…”

Cretaceous-Eocene compression in the central Southern Alps (N Italy) inferred from 40Ar/39Ar dating of pseudotachylytes along regional thrust faults

“…The structure of the Southern Alps west of the Giudicarie Line is typical of a thick-skinned fold-and-thrust belt (Carminati et al, 1997), which involves both the basement and a Permian to Cenozoic cover succession. Alpine metamorphism was weaker than that suffered by the Austroalpine units to the north of the Tonale Line (Carminati et al, 1997;Crespi et al, 1982;Spalla et al, 1999;Colombo and Tunesi, 1999), attaining lower greenschist facies conditions (Carminati et al, 1997;Crespi et al, 1982;Albini et al, 1994).…”

“…Relics of the S 1 foliation, marked by the SPO (Shape Preferred Orientation) of biotite and minor white mica, are best preserved within granitoid gneisses. Phase assemblages suggest amphibolite facies conditions for the sin-D 1 metamorphic event (Carminati and Siletto, 2005;Spalla et al, 1999;Milano et al, 1988;Cadel et al, 1996). The meso-and microfabric of the basement are characterised by a pervasive foliation, namely S 2 , marked by the SPO of chlorite, white mica and minor biotite.…”

“…The meso-and microfabric of the basement are characterised by a pervasive foliation, namely S 2 , marked by the SPO of chlorite, white mica and minor biotite. The development of S 2 , generally assumed to be Variscan in age (Spalla et al, 1999;Cadel et al, 1996), corresponds to a diffused retrogression under greenschist facies conditions. The Variscan age of both D 1 and D 2 deformation phases and associated metamorphism is testified by the transposition of the igneous contacts of the metaintrusives bodies of Ordovician age occurring in the area (Siletto, 1990;Bergomi, 2004).…”

Geological-structural map of the Orobic and Porcile thrust junction, central Southern Alps (N Italy)

“…The metamorphic basement exposed in the Main Map area displays a polyphase history (Albini et al, 1994;Carminati & Siletto, 2005;Carminati, Siletto, & Battaglia, 1997;Spalla, Carminati, Ceriani, Oliva, & Battaglia, 1999;Spalla & Gosso, 1999). The oldest structural relics (D 1 ) are chiefly preserved within quartz-rich lithologies and granitoid gneisses, whereas they have been seldom observed in mica schists and paragneiss.…”