The Queyras ophiolite West of Monte Viso (Western Alps): Indicator of a pecular ocean floor in the mesozoic tethys

Abstract

The Queyras ophiolite and its pelagic sedimentary cover originate from the Ligurian Tethys ocean, which opened in the Late Middle Jurassic and spread during Late Jurassic and Cretaceous times. Three superimposed sequences are distinguished in the obducted units. (A) Ultramafic sequence: its lower part (A1) is mainly made up of mantle-derived tectonite lherzolites which are intruded by a few gabbro bodies (dykes and small magma chambers); the partial melting and the intrusion of the gabbro magma seem to have occurred inside the subcontinental mantle during the continental, preoceanic rifting (− Lissic). Both the lherzolites and the gabbros are sealed up by a major discontinuity (d1) that marks the base of the sedimentary upper part (A2) of the sequence, which is made up of ultramafic breccias, namely the sedimentary ophicalcites OC2 (serpentinite pebbles in a carcareous matrix). (B) Basaltic sequence: thin (a few metres to a few tens of metres, exceptionally a few hundreds) and above all discontinuous theoleiitic pillow lavas. (C) Pelagic sedimentary sequence (Late Middle Jurassic to Late Cretaceous): radiolarian cherts, shales, limestones, calcschists, with numerous intercalations of ophiolitic arenites, breccias and olistoliths; these detrital intercalations which occur especially in the Upper Jurassic and Lower Cretaceous beds witness a repeated tectonic activity of the ocean floor during its spreading. Similar peculiarities are known from other ophiolites that derive from the Ligurian ocean, especially those of Corsica and of the Northern Apennines. Therefore, a significant part of this ocean was characterized by a thin and very discontinuous basaltic layer and by the lack both of a continuous gabbroic layer and of a sheeted dyke complex. Sediments (serpentinite breccias OC2 and pelagic deposits) and/or local, thin basaltic submarine lava-flows were emplaced above an already uncovered mantle-derived ocean floor. If the radiometric dating of the gabbros is accepted without any discussion, the two magmatic events (gabbros in sequence A and Basalts B) were separated by a rather long time span, possibly 30–40 my, which sets a problem. During this interval, however, a complex suite of tectonic, metamorphic (e.g. foliation and metamorphism of the gabbros) and sedimentary events occurred. Nevertheless, other ophiolite massifs of the Alps display different features; for instance, the Viso ophiolite comprises a continuous (40 km long) gabbroic layer overlain by a continuous basaltic layer; but without a sheeted dyke complex. A comparison with the axial part of the Central Atlantic ocean shows both similar features and variations: Queyras-type successions are known close to the mid-Atlantic ridge, and, conversely, the outcrops surveyed along the Vema escarpment exhibit a 3 km thick ocean crust with a gabbroic layer overlain by a 1–1.5 km-thick sheeted dyke complex. Such variations between two parts of the same ocean recall the differences known in the Ligurian vanished ocean, e.g. between the Queyras-Northern Appennines type ophiolite and the Viso-type ophiolite. Genetic models might account for such “patchwork” features.