Accretion of the oceanic lithosphere in an ultra-slow spreading oceanic basin: inference from the Alpine-Apennine ophiolites

Group GSI.IR
Location International Geological Congress,oslo 2008
Author Piccardo, Giovanni Battista
Holding Date 17 September 2008

Studies on ophiolites from the Alpine—Apennine orogenic belt have evidenced that the oceanic basin of provenance, the Jurassic Ligurian Tethys, was characterized by: i) mantle peridotite exposure at the sea-floor; ii) discontinuous basaltic cover, and iii) lack of sheeted dyke complex and gabbroic Layer 3. These characteristics, coupled with the presence of both MORB and alkaline melts and the extreme compositional heterogeneity of peridotites, support the interpretation that the Ligurian Tethys was a Jurassic analog of modern ultra-slow spreading ridges (Piccardo, 2008).
Lithosphere extension by the action of far-field tectonic forces led to continental break-up, formation of non-volcanic passive margins and sea-floor exposure of subcontinental mantle peridotites. Lithosphere extension was already active at 220 Ma, asthenosphere decompressional melting and MORB melt formation occurred not later than 180 Ma, continental break-up and oceanic opening (i.e. basalt extrusion and chert deposition) occurred at 164-155 Ma.
Mantle peridotites deriving from different palaeogeographic settings of the basin show significantly different structural and compositional features. Peridotites from OCT (Ocean-Continent Transition) settings record extension-related tectonic-metamorphic evolution along km-scale shear zones, which indicates that subcontinental mantle was progressively exhumed to the sea-floor during passive lithospheric extension. Peridotites from MIO (More Internal Oceanic) settings show extreme compositional heterogeneity due to melt-peridotite interaction processes which were induced by percolation of MORB-type basaltic melts from the asthenosphere through the extending mantle lithosphere. Melt percolation followed inception of asthenosphere decompressional melting, which occurred prior to continental break-up and ocean formation (Piccardo, 2008, and references therein).
Accordingly, lithospheric peridotites were significantly modified by melt reactive porous flow to depleted/enriched rocks types prior to the accretionary stages of the oceanic crustal rocks of the basin, i.e., formation of MORB gabbroic intrusives and basaltic volcanites.
In summary, present knowledge indicates that important events of melt percolation and storage in the extending subcontinental mantle characterized the rifting stages of the Ligurian Tethys following inception of asthenosphere decompressional melting. During ongoing lithosphere extension and subsequent continental break-up the strongly heterogeneous peridotites formed by melt-peridotite interaction were exhumed and exposed at the sea-floor of the basin. A discontinuous basaltic cover was, later on, extruded above the peridotite basement to form the peculiar oceanic lithosphere (i.e., the direct association of mantle peridotites and basaltic volcanites) which characterized the ultra-slow spreading basin of the Jurassic Ligurian Tethys.
Piccardo G.B., 2008. Geol. Soc. London, Spec. Pubbl., 293, 11-33.