758 MINERALOGICAL MAGAZINE, VOLUME 58A Time constraints on the metamorphic and structural evolution of the southern Sesia-Lanzo Zone, Western Italian Alps

W. Ramsbotham* University of Leeds, Leeds, LS2 9JT, UK. S. Inger B. Cliff D. Rex A. Barnicoat

Introduction number of structurally and metamorphically The Sesia-Lanzo Zone (SLZ) is important as one distinct elements: a) Eclogitic Micaschist of the most extensive segments of continental crust Complex. (EMC) The upper, eastern part of the metamorphosed under eclogite facies conditions unit, comprising metamorphosed igneous and (Compagnoni et al., 1977). Parts of the SLZ sedimentary rocks of diverse compositions, record a complex thermal and structural evolution recording in most cases an early-Alpine eclogite including metamorphism under widely varying facies event which overprinted pre-Alpine granu- conditions over a long (probably >300Ma) lite-amphibolite facies assemblages. The main timespan, whereas other elements of the SLZ high-pressure planar fabric (TE) is defined by seem to record only portions of this history alignment of minerals that appear to have formed (Lardeaux and SpaUa, 1991). Aiming primarily in a regime of general non-coaxial shear (Dl), with to understand the mechanisms by which deep crust mica fabrics accommodating almost pure-shear is exhumed, this study had a number of objectives: flattening, but with rotation of strain markers into I) Definition of a structural framework based on parallelism with TE indicating a top-east vorticity. progressive deformation phases, tracking changes A later stage of the D~ high-pressure deformation in the nature and site of the deformation flow generated crenulation cleavages (TE2), and both through time. 2) Characterisation of meta- high-pressure fabrics were kinked by a subvertical, morphism conditions under which diagnostic partly transpositional cleavage (TCC)developed mineral fabrics were formed, thereby linking the under retrograde (?blueschist-greenschist) condi- strain history with the P-T evolution. 3) Isotopic tions, b) A transition zone in which pre-Alpine dating of minerals which could be related to granulite,-amphibolite facies fabrics and early- specific segments of the P-T-deformation path, to Alpine high-pressure assemblages have been define the age of the high-pressure metamorphic partially retrogressed under greenschist facies peak and rates of subsequent exhumation which conditions. Here, TE and Tm fabrics are rotated might be related to regional tectonics. into parallelism with fabrics in the Gneiss Minuti Complex (see below), but without significant transposition. This deformation is referred to as Geological Setting D2. c) Gneiss Minuti Complex. The lowest, This study was based on the Valle dell'Orco external part of the Sesia Zone shows a section in NW Italy, which exposes a number of thorough-going recrystallisation under greens- tectonic units constituting the Alpine 'suture'. chist facies conditions, to the extent that it From west to east these are: 1) The Gran remains controversial whether eclogite facies Paradiso Massif. Continental basement of relics can truly be identified here. A penetrative European affinity, preserving Hercynian meta- deformation D3 formed a uniform and consistent morphic asemblages partially overprinted during L-S fabric (To) at greenschist facies conditions in the Alpine orogeny. 2) The Pi6mont Zone. the GMC but did not penetrate the EMC. Oceanic rocks recording an early-Alpine eclogite- The general hypothesis offered for the stuctural blueschist facies metamorphism which has been evolution in the SLZ is that eclogite facies variably overprinted by an Alpine grecnschist deformation in the eastern zone may have had a facies cycle. 3) The Sesia Lanzo Zone. top-east assymetry, whereas later, as the locus of Continental rocks of the Austroalpine realm, deformation propagated westwards during exhu- having African affinities and divided into a mation, a top-west non-coaxial flow was estab-

* This workformedpart of Will Ramsbotham's PhD research before his tragic death in June 1993. GOLDSCHMIDT CONFERENCE EDINBURGH 1994 759

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52.2+/-.5 Mal Gran Paradiso Massif Piemonte Unit Sesio-Lanzo Zone {~ Gteetschist orthogneiss ~ Ultrabasic (sopontiait~) ~ Gtet~nschists ('GneissMinuti') C,hpteenschist relictsccemmn b~sement in basic ~hists pods ~ Gnt-(omph)-glao-ep -wm basics ~ granalitesPartially retrogressed and eclogites (increasingly lessretrogressed eastwards) [~ Lowscralncarbonaleschists ~ 'EclogiticMica~qchistComplex'

FIG. I. Metamorphic sketch-map, with Rb/Sr dates, for the Valle delrOrco. lished. The basal contact of the GMC with the (cclogite) metamorphic conditions were probably Pi6mont Zone is in turn characterised by top-SE 500-600~ > 15kb. However, the large spread in directed shear (Wheeler and Butler, 1993). ages of closely spaced samples suggests that additional processes besides cooling have influ- enced closure of the Rb-Sr system; partial Geochronology rejuvenation during retrogression is ruled out by Most of the previous geochronological studies in the near-absence of greenschist facies assemblages other parts of the SLZ yielded late Cretaceous in the internal EMC. The U-Pb age of sphene at mineral K-Ar ages, augmented by a single whole- 66Ma is most likely to rccord an age during the rock and mineral Rb-Sr determination from which high pressure event, although the exact point on the widely-cited 'early-Cretaceous' age for the the P-T path is poorly defined at present. Two cclogite metamorphism was derived (Oberhnsli et 4~ determinations from the EMC yield al., 1985). However, recent laser-4~ studies good plateaux with ages exceeding the Rb-Sr date have shown that micas in the Ecogitic Micaschist of the same sample by up to llMa.A third Complex contain heterogeneously-distributed spectrum suggests minor argon loss and has a extraneous argon that must cast doubt on K-Ar plateau 8Ma lower than the Rb-Sr age.An determinations (Kelley et al., 1993) This study important point is that, even in the most utilisesthe Rb-Sr system on white micas because it internal, unretrogressed part of the EMC, no is not susceptible to excess argon problems, and ages older than 70Ma were determined. Rb-Sr has a closure temperature closer to the thermal ages in rocks with pervasive greenschist fabrics peak of the cclogite metamorphism. As a check, cluster at 39Ma, the widely-accepted age of the step-heated 40 Ar- 39 Ar analyses were made on Alpine regional metamorphism and penetrative some of the same samples, and high-pressure top-west non-coaxial deformation at these levels titanitc from calc-silicate lithologics has been (Hunziker et al., 1992). A Pi6mont marl gives an dated by the U-Pb method because it probably age of 36Ma by Rb-Sr and 40 Ar- 39 Ar; the has a closure temperature higher than the concordance of these results probably reflects the metamorphic temperature maximum and should thorough recrystallisation of assemblages under therefore record the age of crystallisation of the high fluid activities, and the younger age is assemblage in which titanitc was stable. consistent with many recent determinations from Results are plotted on the map, showing their these levels (Inger, unpublished data) which may position in the geological framework. Rb-Sr record the latest, top-east shear fabrics related to phcngite ages in the EMC vary from 46-63Ma, unroofing of the Pi6mont Zone. and could be interpreted as cooling ages since peak