Early Alpine eclogite metamorphism in the Penninic Monte Rosa-Gran Paradiso basement nappes of the northwestern Alps

Abstract

Eclogites occur in the Monte Rosa and Gran Paradiso Penninic nappes of the Western Alps as small bodies within metasedimentary rocks and, inMonte Rosa, rarely, as boudinaged dikes within late-Variscan granitoids. Minor but characteristic constituents of these eclogites are glaucophane and paragonite. Bulk composition of the protoliths ranges from basaltic to Fe-Ti basaltic.High-pressure assemblages comprising chloritoid, phengite, garnet ± kyanite, talc, Mg-chlorite and glaucophane also occur in micaschists of both nappes. The Early Alpine high-pressure assemblages postdate the intrusion of late-Variscan granitoids and are overprinted by the greenschist, mid-Tertiary Lepontine metamorphism.Garnet-clinopyroxene geothermometry and phase compatibilities suggest highpressure metamorphic temperatures ranging from 440°C to 530°C in the Monte Rosa nappe, with pressure in excess of 8-10 Kb but lower than approximately 14 Kb. These conditions are in the same range as established for the overlying Piedmont ophiolite nappe and for parts of the Austroalpine basement nappe, implying a similar tectonometamorphic evolution during Early Alpine time. Garnet-omphacite pairs from the amphibole eclogites of the Gran Paradiso nappe yield lower equilibration temperatures (T =400°C), possibly recording a later stage in the P-T-t path of this unit.

Metamorphic evolution of the Briançonnais units along the ECORS-CROP profile (Western Alps): new data on metasedimentary rocks

“…Eclogitic rocks are known from the Piemont-Ligurian domain ("Grivola", Dal Piaz 1928;Droop et al 1990) in the southeast and as well from the Valaisan in the northwest (Schürch 1987;Oberhänsli 1994;. The most internal part of the Briançonnais domain, the Gran Paradiso massif that underlies the Schistes Lustrés, also evidences a HP history (Compagnoni and Lombardo 1974;Dal Piaz & Lombardo 1986;Ballèvre 1990). The Briançonnais domain s.str.…”

Subduction and obduction processes in the Swiss Alps

“…Most HP=LT ages measured so far in the internal massifs (Monte Rosa, Grand Paradis, Dora Maira) are older than 45 Ma (see Dal Piaz andLombardo, 1986 andHunziker et al, 1992 for a review of older ages, and Duchêne et al, 1997a for younger Michard et al (1993), Sharp et al (1993); Zermatt-Saas: Hunziker et al (1989), Reinecke (1991), Ballèvre and Merle (1993). Sesia: Hunziker et al (1989), Ballèvre and Merle (1993), Venturini (1995); Monte Rosa, Grand Paradis: Monié (1985), Hunziker et al (1989), Ballèvre and Merle (1993). ages).…”

Hydrogeological model and hydraulic behaviour of a large landslide in the Italian Western Alps

“…The schistosity is locally developed parallel to the chloritoschist layers, which can be interpreted as high strain shear zones created by metamorphic reactions along high pressure mylonitic shear bands (Dal Piaz and Lombardo, 1986). Since the shear strength of these layers is lower than the average values of the Gran Paradiso augen orthogneiss, instability problems can occur when the mean plane of the slope presents the same orientation values as the regional metamorphic schistosity.…”