The Antrona nappe : lithostratigraphy and metamorphic evolution of ophiolites in the Antrona valley (Pennine Alps)

The Antrona ophiolite (western Central Alps) represents a tectonic fragment of the oceanic lithosphere of the Upper Jurassic - Lower Cretaceous Ligurian- Piedmont basin, a section of the Western Alpine Tethyan Ocean. The Antrona ophiolite occurs at lower structural levels in the Alpine nappe stack and is sandwiched between the overlying continental Monte Rosa Nappe (upper Penninic) and the underlying Camughera-Moncucco continental Unit (middle Penninic). The Monte Rosa Nappe is overlain by the Zermatt Saas ophiolitic Unit. Despite the tectono-metamorphic reworking, the Antrona ophiolite exhibits all typical lithologies of oceanic lithosphere: ultramafic and mafic plutonic rocks, mafic volcanic rocks and deep-sea sediments can be recognized. Several rock types were distinguished among mafic rocks. New findings and inferences are: i) the occurrence of probable relics of magmatic structures in some amphibolites, inferred to be pillow lavas or pillow breccia; ii) the occurrence of lawsonite pseudomorphs-bearing amphibolites, not described so far in the study area as precursor of the origin of epidote-amphibolites the Antrona Valley. A qualitative P-T diagram deduced from the stability conditions of mineral parageneses is presented and compared with published P-T paths for the Antrona and Zermatt-Saas ophiolites. The metamorphic evolution of the studied rocks is characterized by blueschist prograde path followed by high pressure (eclogitic) metamorphic peak. P-T estimates for the metamorphic peak were calculated by the Na-clinopyroxene garnet equilibria and the jadeite content in omphacite. T = 372°C for a nominal pressure of P = 1 GPa and T = 386°C for a nominal pressure of P = 1.5 Gpa were obtained. Jd30 as maximum Jadeite content suggests P > 1 Gpa. Retrograde path, although not well constrained, is dominated by epidote-amphibolite/ amphibolite facies conditions, in accord with published data, differing from those inferred so far for the overlying Zermatt-Saas ophiolite.