Continental Margin Ophiolites of Neotethys: Remnants of Ancient Ocean-Continent Transition Zone (OCTZ) Lithosphere and Their Geochemistry, Mantle Sources and Melt Evolution Patterns

We present an overview of the geology, geochemistry and petrogenesis of continental margin ophiolites (CMO), which represent the lithospheric remnants of riftgenerated paleo ocean – continent transition zones (OCTZ) in orogenic belts. The igneous stratigraphy and geochemical signatures of Neotethyan CMOs reflect the extent of geochemical heterogeneity, partial melting degrees, and melt evolution patterns in the continental lithospheric mantle prior to the onset of seafloor spreading in rifted margins. Basaltic rocks of the Jurassic CMOs in the External Ligurian units of the Northern Apennines have N-MORB and G-MORB affinities with strong HREE/MREE depletion, and represent the products of partial melting of a heterogeneous subcontinental lithospheric mantle containing small volumes of garnet pyroxenite layers. These extrusive rocks were erupted directly on the exhumed fertile spinel lherzolites of Adria during its OCTZ evolution. Volcanic rocks of the Triassic CMOs in the Albanide-Hellenide orogenic belt are represented by calc-alkaline suites; alkaline basalts and subordinate trachybasalts, trachyandesites, and trachytes; transitional to sub-alkaline plume-type PMORB basalts; sub-alkaline enriched, E-MORB basalts; and, sub-alkaline N-MORB basalts. Upper mantle peridotites are not exposed. Magmas of these extrusive rock associations were derived from compositionally distinct mantle sources, which were affected by previous subduction and plume events in the geological history of the region. The CMOs in the Zagros orogenic belt include metamorphosed lherzolites with gabbro and mafic dike intrusions, which show N-MORB and GMORB affinities. Basalts and basaltic andesites making up the majority of the Zagros volcanic sequences have E-MORB and P-MORB affinities, whereas minor alkaline rocks that are composed of basalts, trachybasalts and trachytes display OIB signatures. The mantle sources of the Zagros CMOs were progressively enriched in Th and Nb. The OIB component of the mantle beneath the Zagros OCTZ was derived from previous plume events during the early Carboniferous, when Paleotethys was undergoing its rift-drift tectonics. The observed differences in the igneous stratigraphy and geochemical affinities of these Neotethyan CMOs are a result of extreme mantle heterogeneity caused by previous subduction and plume events during the Wilson Cycle evolution of the older Paleotethys.