Lawsonite, blueschist and eclogite formation in the southern Sesia zone (western Alps, Italy)

Abstract

The sou thern Sesia zone in the western Italian Alps consists of fou r tectonic su b-units separated by maj or thru sts. These su b-units inclu de : "gneiss minu ti" (GM) , blu eschist-facies "polymetamorphie paraschists" (PP) similar to the "11 zona diorito-k inzigitica" , "eclogitic micaschists" (E M) and blu eschist-facies "Rocca Canavese thru st sheets" (RCT). Du ring Alpine metamorphism these su b-units underw ent a polyphase early Alpine history at high-P, follow ed by a late-Alpine low-P event. Evidence for an early-Alpine prograde path has been only fou nd in the EM and GM. The early-Alpine metamorphic peak s occu rred at T = 500-550 °C and P = 13-20 kbar in the EM, T = 400-450°C and P � 11-12 kbar in the GM and at T = 300-400°C and P � 8-10 kbar in the PP and RCT. Decompression du ring the early-Alpine event occu rred at decreasing T in the EM (trom T = 500-550 °C to T < 450-500 0C) and at more or less constant T in the other su b-u nits. In the sou thern EM, the early-Alpine post-eclogitic recrystallization produ ced law sonite porphyroblasts which grew in eq uilibriu m with quartz ± jadeite. Formation of law sonite occu rred at low XC0 2 ; it was favou red by a temperatu re decrease after the HP climax and probably the syn-metamorphic infiltration of H20-rich flu ids along preferential channels. Law sonite also occu rs in the other Sesia su b-u nits, where it crystallized du ring the metamorphic climax. The post-eclogitic T decrease inferred here for the sou thern EM is less evident in more northerly parts of this su b-u nit ; it may be related to a process of condu ctive-cooling du ring early-Alpine ex hu mation and the stack ing at deep cru stal levels with the relatively cooler blu eschist-facies su b-u nits. After stack ing and recrystallization under fairly comparable blu eschist-facies conditions, the different Sesia su b-units underw ent a low-P event of probable late-Alpine age. Su ch a low-P event cau sed a metamorphic zonation, with low er T in the stru ctu rally high er and eastern RCT.

The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite from the southern Sesia Zone, Western Alps, Italy

“…These estimated temperatures close to those of a previous study by Pognante (1989), who reported T = 520 45 C at P = 14 kbar, where the temperature was determined using Powell (1985) garnet clinopyroxene geothermometer.…”

“…During the later stage of the exhumation, the EMC was overprinted by a greenschist facies metamorphism (T 400 C and P 4 5 kbar). On the other hand, Pognante (1989) reported that the southern part of the EMC underwent early Alpine eclogite facies metamorphism with peak P T conditions of T = 500 550 C and P = 13 20 kbar, and then underwent a blueschist facies metamorphism during an early stage of the exhumation, which was characterized by a significant temperature drop at high pressures (T < 450 500 C, at pressures in the jadeite lawsonite stability field). During a later stage of the exhumation, pumpellyite and albite grew under P T conditions around of T < 350 C and P < 5 kbar.…”

“…The Eclogitic Micaschist Complex (EMC) of the Sesia Zone is one of the best studied quartz eclogite facies terranes in the world (e.g., Compagnoni and Maffeo, 1973;Compagnoni, 1977;Compagnoni et al, 1977;Rubie, 1984;Oberha nsli et al, 1985;Pognante, 1989;Droop et al, 1990;Castelli, 1991, Fig. 1).…”

Analysis of fabric evolution and metamorphic reaction progress at Lago della Vecchia-Valle d’Irogna, Sesia-Lanzo Zone, Western Alps

“…The exhumation was accomplished during subduction of the Alpine Tethys lithosphere Polino, Dal Piaz, & Gosso, 1990;Roda, Spalla, & Marotta, 2012;Spalla et al, 1996). Some authors (Babist et al, 2006;Giuntoli & Engi, 2016;Pognante, 1989;Ramsbotham, Inger, Cliff, Rex, & Barnicoat, 1994;Regis et al, 2014;Venturini, 1995) found evidence of a strong blueschist-to greenschistfacies retrogression within EMC, thus testifying to a heterogeneous record of the tectono-metamorphic stages. On this ground, Regis et al (2014) interpreted the SLZ as represented by a composite unit that can be divided into several slices with different pre-Mesozoic lithologies and Alpine tectono-metamorphic evolutions: (1) the Druer slice represents the southeastern portion of the SLZ and mainly consists of micaschist and minor metabasite and metagranitoid with a pervasive Alpine eclogite-facies metamorphic imprint; (2) the Intermediate Unit and GMC, in the northwestern portion of SLZ, mainly consist of fine-grained gneiss with dominant retrograde blueschist-to greenschist-facies assemblages; (3) the Fondo slice is a thin and discontinuous layer of Mesozoic metasediment with subordinate metabasite interposed between the Druer slice and GMC-Intermediate Unit.…”

“…The SLZ is classically divided into four main complexes ( Figure 1(B); Compagnoni et al, 1977;Gosso, 1977): (1) the II Dioritic-Kinzigitic Zone (IIDK) preserves a pervasive pre-Alpine HT metamorphic imprint; (2) the Gneiss Minuti Complex (GMC) and (3) the EMC display a dominant Alpine metamorphic imprint and locally preserve pre-Alpine HT relicts (Compagnoni et al, 1977;Gosso, Messiga, Rebay, & Spalla, 2010;Lardeaux, Gosso, Kiénast, & Lombardo, 1982;Manzotti et al, 2014;Zanoni, 2016); (4) the Rocca Canavese Thrust Sheets are characterized by a blueschist Alpine imprint with no eclogite-facies relicts (Cantù, Spaggiari, Zucali, Zanoni, & Spalla, 2016;Pognante, 1989;Spalla & Zulbati, 2003). The EMC and GMC consist of metapelite, paragneiss, metagranitoid, metabasite, and impure marble recording Alpine polyphasic deformation and eclogite-facies peak conditions.…”