Magmatic Clues on the Triassic evolution of the Western Southern Alps before the opening of the Jurassic Tethys.

Three-points internal isochrons based on Sm-Nd isotopic data of whole rock, plagioclase and clinopyroxene fractions have been carried out for diorites belonging to a dyke swarm cutting the mantle ultramafic massif of Baldissero (Ivrea-Verbano Zone, IVZ: Western Southern Alps, Italy).The isochron slopes correspond to (i) 19829 Ma, with 143Nd/144Ndi = 0.512669 and ?Ndi = +5.6 (MSWD = 0.63) and of (ii) 204?31 Ma, with 143Nd/144Ndi = 0.512680 and Ndi = +6.0 (MSWD = 1.5), straddling the upper Triassic-lower Jurassic boundary. They are consistent with those obtained by Obermiller (1994) with two-points Sm-Nd internal isochrons. Petrographic and geochemical features of the diorites were not perturbed after the intrusion, and thus such geochronologic data are considered emplacement ages.The dioritic dykes are cumulates and have high Mg# values in bulk-rock and mafic phases that positively correlate with the incompatible trace element concentrations. The geochemical characteristics of the diorites are explained by flow of a hydrous, silica-saturated melt accompanied by reaction with the ambient peridotite and fractional crystallisation, at deep crust condition. The hypothetical parent melts calculated in equilibrium with the diorite minerals show tholeiitic to mild-alkaline affinity. Isotopic composition, characterised by high radiogenic Nd (This Study, Obermiller, 1994) and low radiogenic Sr (Obermiller, 1994), further support a tholeiitic affinity.The strongly LREE-depleted and fractionated compositions of bulk rock and minerals, associated with fertile major element and modal bulk-rock composition, suggest that the mantle lherzolites are refractory residues after low degrees of near-fractional melting. In particular, REE composition of the clinopyroxenes is consistent with that expected after ca. 4% near-fractional melting under garnet-facies. After the melting event, the peridotites experienced subsolidus re-equilibration at spinel-facies conditions and very modest, if any, metasomatism that produced accessory amount of titanian pargasite. Re-Os model age are 376 Ma, straddling Devonian-Carboniferous boundary. It is well in agreement with the age of 378?48 Ma obtained by Obermiller (1994) from the best fit of the Sm-Nd isotopic compositions of clinopyroxenes separated from fourteen Baldissero peridotite samples (Ndi = 0.51247, ?Ndi = +6.2 and MSWD = 26). This age is considered to record a partial melting event related to extensional phases prior the Variscan orogeny.Triassic igneous events are unknown in the southern IVZ, but the intrusion of hydrous melts, mostly silica-saturated and of tholeiitic affinity, have been largely documented in the Finero region, i.e. the northernmost part of IVZ (Gebauer, 1993; Lu et al., 1997; Vavra, 1999; Oppizzi and Schaltegger, 1999; Peressini et al., 2004). Besides, the Finero mantle sequence experienced pervasive, channelled and intrusive migration in Triassic time (Hunziker 1974; Stahle et al., 1990; 2001; Von Quadt et al., 1993; Hartmann and Wedepohl, 1993; Grieco et al., 2001; Morishita et al., 2008) of melts often showing large crustal component (Hartmann and Wedepohl, 1993; Zanetti et al., 1999; Raffone et al., 2006).Triassic magmatism is also abundant throughout the Central-Eastern Southern Alps, where it shows calc-alkaline to shoshonitic affinityThe Middle Triassic tectonic evolution has been interpreted in a number of ways, spanning from aborted rift, possibly related to the development of the Central Atlantic Magmatic Province (Furin et al., 2006), to W-directed subduction with slab-retreating towards E (Doglioni, 2007).Our data represent a further evidence that in the late Trias the westernmost part of the Adria plate experienced the intrusion of melts with tholeiitic affinity s.l.: the eastwards progressive change of the melt affinity to calk-alkaline and shoshonitic could be consistent with a different crustal emplacement level in the westernmost sector, or with different slab contributions to the mantle wedge during a W-directed subduction, as well as with a rifting process. It is, however, matter of fact that the occurrence of the former scenario could also account for the puzzling suprasubduction affinity of the Triassic metasomatic melts that migrated through the Finero mantle sequence.