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 P-MORB 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 G-MORB 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.
The Vermion Massif (northern Greece) is located across the boundary between the Pelagonian and Vardar Zones and includes several tectonic units bearing ophiolitic rocks, which represent remnants of the oceanic lithosphere formed in the Neotethyan Vardar Ocean, between the Pelagonian and the Serbo- Macedonian continental realms. This massif consists of tectonic units belonging to the Pelagonian Domain, which are tectonically overlain by units associated with the Almopias sub-Zone (Vardar Zone). Ophiolitic rocks consist of mantle harzburgites and ophiolitic melanges. The ophiolitic melanges incorporate rocks exhibiting a wide range of composition, including various intrusive rocks and volcanic rocks ranging from basalts, basaltic andesites, andesites, dacites, to rhyolites.
Incompatible elements and rare earth elements analyses indicate that a number of different rock-types formed in distinct tectonic settings can be identified. Mantle harzburgites have a very depleted nature and represent portions of the supra-subduction (SSZ) mantle developed in an intra-oceanic arc setting. The melanges units include six rock types variably distributed in the Pelagonian and Almopias Units. They are: (1) calc-alkaline rocks with marked depletion in Nb, Ta Ti and enrichment in LREE, Th; (2) LREE-depleted N-MORB; (3) LREE-enriched E-MORB; (4) alkaline within-plate basalts showing marked enrichments in Th, Ta, Nb, LREE; (5) low-Ti island arc tholeiites featuring depletion in HFSE; and (6) very-low-Ti boninites characterized by strong depletion in HSFE and REE. Previous interpretations have referred the Vardar ophiolites to a MORB-type oceanic setting and to a MORB-type backarc setting; however, the widespread occurrence of SSZ ophiolites has never been documented in the Vardar Zone before and is particularly important as it testifies for the existence of an intra-oceanic arc basin in the Vardar oceanic domain.
The results presented in this paper compared with literature data on other magmatic rocks within the Vardar Zone suggest that the opening and closure of the Vardar Ocean record several distinct accretion events in this basin, that is oceanic crust generation at mid-ocean ridge, alkaline seamounts in the oceanic domain and SSZ setting, as well as two accretion events in the western realm of the Serbo-Macedonian continent, that is volcanic arc and backarc settings.
Based on the comparison between the modern west Pacific subduction system and the results from this study a new model for a multistage tectono-magmatic evolution of the Vardar Ocean is proposed.
