Areal geology and petrology of eclogites and associated metabasites of the Piemonte ophiolite nappe, breuil—st. Jacques area, Italian Western Alps

Abstract

The Breuil—St. Jacques area is located in the upper Valtournanche and Ayas valleys on the north side of the middle Aosta Valley. The principal unit exposed here consists of calc-schists + greenstones of the Piemonte ophiolite nappe. This complex is interposed between the underlying Pennine Monte Rosa nappe and the overlying Austroalpine Dent Blanche + Sesia—Lanzo nappe. The juxtaposed representatives of two structural units occur within the Piemonte section; they differ in terms of lithologic associations, metamorphic assemblages and paleogeographic significance. 1. (1) The structurally lower Zermatt—Saas unit consists of an important basal sequence of largely serpentinized peridotite tectonites, an overlying group of discontinuous metagabbros locally containing magmatic clinopyroxene relics, and a capping series of various types of tholeiitic to slightly alkaline metabasalt. Syn- and post-volcanic sedimentary strata, now predominantly garnetiferous, ankeritic mica schists with associated marbles and minor calcareous, manganiferous metaradiolarites (spessartine-, piemontite- and braunite-bearing metacherts) form a superjacent cover series. The Zermatt—Saas unit exhibits a composite metamorphic character. Eclogites and early glaucophane schists of the eoalpine stage of recrystallization have been incipiently to pervasively replaced by a greenschistic assemblage produced during the Lepontine metamorphic event. This latter recrystallization involved the renewed growth or recrystallization of sodic amphibole. Serpentinite of the Zermatt—Saas unit contains numerous gabbroic dikes, some of which have been partly transformed to fine-grained rodingitic material; other rodingites represent metasomatic reaction rims between ultramafic material and lithologically diverse surrounding rocks. 2. (2) The structurally overlying Combin unit possesses a distinctive stratigraphy which contrasts with that of typical ophiolites and correlative oceanic crust. It consists of locally preserved Upper Permian (?), Triassic and Liassic strata of continental affinities, overlain by a section made up chiefly of regular intercalations of calc-schists, marbles and metavolcanic layers derived from submarine basaltic flows, hyaloclastites, tuffs and/or tuffites. The mafic rocks have been pervasively recrystallized to porphyroblastic albitebearing greenschists (prasinites). This volcanoclastic sequence contains interbeds of manganiferous metaradiolarite and quartz + albite-bearing chlorite schists, in part with associated stratiform Cu-Fe sulfides; it also includes some lenses — tectonic slivers and/or olistostromes — of metagabbro and serpentinite. The Combin unit does not exhibit the characteristic relict eclogitic association of the Zermatt—Saas unit; instead it displays only the effects of greenschist facies recrystallization (including very rare relics of sodic amphibole) attributable to the Lepontine metamorphic event, and corresponding to the greenschist overprinting of both the Zermatt—Saas unit and the lower tectonic member of the Austroalpine nappe. Bulk XRF analyses of seventeen mafic rock samples demonstrate that, although postigneous metasomatism has produced sodium enrichment, original lithologies possessed affinities with oceanic tholeiite. The Zermatt—Saas and Combin metabasites do not exhibit distinctive compositional differences. The relatively high-pressure metamorphic prograde path displayed by Zermatt—Saas mineral assemblages of eoalpine age is characteristic of subduction zone metamorphism, whereas the retrograde P—T trajectory represents nearly adiabatic decompression —hypothesized to have accompanied buoyant return of the imbricated subducted complex towards the surface after its detachment from the downgoing lithospheric slab. The greenschist facies overprinting, which according to isotopic age measurements is connected at least in part with the Lepontine metamorphic event, seems to be related to a post-collisional thermal reequilibration of the pile of nappes, including both Zermatt—Saas Combin units.