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

Abstract

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 Betic Ophiolites and the Mesozoic Evolution of the Western Tethys

“…This hypothesis conforms to the interpretations proposed for ophiolite sequences of the Alps and Apennines as shown in [24][25][26][27][28][29][30][31]. Worth of note, recent oceanographic studies emphasize that the petrological characteristics of the current day oceanic rocks are more heterogeneous to that assumed in the past [32][33][34][35][36] and, hence, the "ophiolite concept" interpreted as "fossil oceanic lithosphere with a unique and complete lithologic sequence" has also to be revisited and updated [33].…”

“…These associations rarely fit the "Penrose three layer" model constituted by the regular coexistence of ultramafic rocks, basic magmatic rocks and sediments, showing a particular paucity of sheeted dyke complexes. On the contrary, Ligurian Ophiolites are often associated with remnants of the continental crust, and analogously the Betic Ophiolites locally present some indices of crustal assimilation, probably deriving from basement rocks of their continental margin, indicating in both cases (1) vicinity of the continental margin at least during the first stages of the oceanic-floor magmatism, and (2) intermittent spreading generally attributed to ultra-slow spreading ridges [19] or to OCT setting [30].…”

“…The enrichment in Th/Yb presented by some samples in Figure 5D, especially in rocks from Cóbdar and Algarrobo outcrops, can be related to assimilation of the thinned and rifted crustal rocks preexisting to the opening of the Betic Ocean, or to interaction with crustal rocks located at the continental margin. Tholeiitic magmas similar to those that formed the Betic Ophiolites are typical of slowly expanding oceanic ridges, especially in the initial stages of oceanization, when embryonic ridges evolve after continental breakup, i.e., during a transition between rifting and drifting before the attainment of a steady-state spreading regime, and therefore may be classified as "Continental margin ophiolites" according to the classification of [30,33] and Pearce [54] classification.…”

Tectonic significance of Triassic mafic rocks in the June Complex, Sanandaj–Sirjan zone, Iran

“…The thick sedimentary sequence was deposited either in a passive continental margin during a late stage of rifting or rather during the drift stage of the Neotethys Ocean. Recent models assume that the opening of Neotethys started not later than during Middle Triassic times (e.g., Saccani et al 2014Saccani et al , 2015. The composition of the oceanic crust is influenced by plume-type material (Saccani et al 2013a, b;Fig.…”

Variscan ophiolites in NW Iberia: Tracking lost Paleozoic oceans and the assembly of Pangea

“…The evolution proposed for the Upper Units implies that the Variscan suture ophiolites were generated within the Rheic Ocean realm. The Cambrian ophiolites, excepting the Bazar Ophiolite, would be linked to the initial stages of the opening of this ocean reminiscent of the Western Alps Tethys (Dilek and Furnes, 2014;Balestro et al 2015;Saccani et al 2015), whereas the Devonian ophiolites would have originated during the final stages of its closure analogous to many supra-subduction zone ophiolites (Dilek, 2003;Dilek and Furnes, 2011;Yang et al 2015). The Devonian ophiolites, the commonest in the Variscan suture, have been interpreted as suprasubduction zone ophiolites generated at the intraRheic subduction zone located near the Southern margin of Laurussia Sánchez Martínez et al, 2007a).…”