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A representative sampling of the oceanic lithosphere that floored the Jurassic Ligurian Tethys is shown by the Northern Apennine ophiolites (Fig. 4). Ophiolites consist of a peridotite-gabbro basement stratigraphically covered by ophiolitic breccias, pillowed basaltic lava flows, and oceanic sediments (Abbate et al., 1994). Field, structural and petrological evidence indicates that MORB(Mid-Ocean-Ridge-Basalt)-type melts from the asthenosphere were intruded into the overlying lithospheric mantle peridotites forming gabbroic intrusive bodies. Subsequently, the peridotite-gabbro association experienced a tectonic-metamorphic retrogression evolution, testified by deformation and recrystallization along shear zones. The uppermost portion of the serpentinites underwent intensive fracturing with development of tectonic breccias (ophicalcites), which were partially covered by sedimentary breccias. Ophicalcites and sedimentary breccias were discontinuously covered by MORB-type pillow lava flows and by Middle to Late Jurassic radiolarian cherts. This indicates progressive uplift and final exposure at the seafloor, where the peridotites were extensively serpentinized. Discrete MORB-type basaltic dikes, related to the basaltic extrusions, commonly crosscut serpentinized peridotites and foliated gabbros, as well as the overlying tectonic breccias.
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Figure 3 – Hypothetical cross section showing the main stratigraphic and structural features of a typical Internal Liguride ophiolite section. It shows the seafloor exposure of mantle peridotites, with an uppermost level of ophicalcites, which have been intruded by gabbroic bodies. The gabbro-peridotite basament is discontinuously covered by ophiolitic breccias, pillowed basalt, and pelagic sediments (i.e. radiolarian cherts, Calpionella limestones and Palombini shales).
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