Geologic and petrographic study of the veins in the metabasites of the ultra-high pressure Lago di Cignana Unit (upper Valtournenche, western Alps).

The ultra-high pressure Lago di Cignana Unit (LCU), which consists of coesite-eclogite facies metabasics and metasediments, is among the best examples of deeply subducted oceanic crust worldwide. This contribution is a petrographic study of veins, and of their relative chronology, occurring in metabasites, in order to distinguish the events of fluid circulation occurred during the metamorphic evolution of the LCU. A detailed petrographic characterization of the different generations of veins occurring in the LCU metabasites is, in fact, still missing, except for some information on few quartz-bearing vein types studied by van der Klauw et al. (1997), most of them associated to the late greenschist facies retrogression. The hosting metabasites are mainly represented by eclogite (Omp + Grt + ex-Lws ± Gln ± Phe ± Cb ± Qtz + Rt), variably retrogressed eclogite (characterized by the pervasive growth of green-blue Amp + Czo/Ep ± Ab) and minor prasinite. Layers and pods of omphacitite and garnetite, locally occurring in fresh eclogites, have been interpreted as primary compositional layering in the protolith (i.e. not related to fluid circulation). The observed mineral assemblages allowed to constrain the tectono-metamorphic evolution, which is characterized by a HP prograde stage, an UHP peak stage (coesite-bearing eclogite facies) often coinciding to the development of the main foliation (Sp ecl ), followed by several retrograde stages under Lws-blueschists, Ep-blueschist, blueschist/greenschist and greenschist –facies conditions. Concerning the evidence of fluid circulation, the following vein types have been recognised:  syn-Sp ecl veins (UHP eclogite facies): Omp+Gln veins; Qtz+ex-Lws veins;  post-Sp ecl veins (UHP eclogite facies): Qtz (ex-Coe) -bearing veins with selvages enriched in Omp, Gln and Grt; Qtz+Omp+Gln+ex-Lws veins;  post-Sp ecl veins (Lws-eclogite/blueschist facies): Grt+Gln+Whm+Qtz veins; Gln+Grt veins; Gln+Grt+ ex-Lws veins; Qtz+Gln+Whm veins; Whm veins; Qtz+Cb+ex-Lws veins; Qtz+Cb+Gln±ex-Lws veins; Qtz veins with selvages enriched in Whm and Gln;  post-Sp ecl veins (Ep-eclogite facies): Qtz+Ep+Gln veins; Zo+Qtz veins; Qtz+Zo/Czo veins; Ep+Whm+Grt veins;  post-Sp ecl veins (Ep-blueschist facies): Qtz+Czo+Ttn veins;  post-Sp gs veins (greenschist facies): Ep+Ab; Ep+blue-green Amp+Ab+Chl veins. The obtained results allowed to reconstruct in detail the relative chronology of fluid circulation in the oceanic crust during subduction and the following exhumation, providing a quite more complex picture than previously suggested in the literature. This study has therefore important implications for the understanding of the nature of the fluids circulating in the subducting slab, and of the fluid mobility in subduction zones. van der Klauw S.N., Reinecke T. & Stöckhert B. 1997. Exhumation of ultrahigh-pressure metamorphic oceanic crust from Lago di Cignana, Piemontese zone, western Alps: the structural record in metabasites. Lithos, 41, 79-102.