Different rift-drift tectonics associated with the formation of the Paleo-Tethys and Neo-Tethys in Iran, as deduced from Paleozoic and Mesozoic continental margin ophiolites

In Iran, two incomplete ophiolitic sequences, which are classifiable as “Continental Margin Ophiolite” (CMO), record the tectono-magmatic processes that may occur during the rift-drift tectonic phase preceding oceanic basin formation. They are represented by: 1) the Early Carboniferous Misho Mafic Complex (NW Iran), which is associated with the formation of the Paleo-Tethys and 2) the Triassic sequences in the Kermanshah ophiolite (Zagros Belt), which are associated with the formation of the southern Neo-Tethys. Both CMO sequences consist of gabbros, sheeted dykes and basaltic lavas. The Kermanshah CMO also includes exhumed sub-continental mantle lherzolites associated with metagabbros and metadykes. Both sequences include rocks showing variable incompatible element enrichments, ranging from normal (N) MORB to enriched (E) MORB, plume-type (P) MORB and alkaline basalt compositions. Nonetheless, geological and petrological evidence suggest that the continental break-up of the Paleo-Tethys and Neo-Tethys occurred in two quite different ways. The Misho CMO. Petrogenetic modeling shows that all rock-types have originated from polybaric partial melting starting in the garnet-facies mantle and continuing to much larger degree in the spinel-facies mantle. N-MORBs originated from partial melting of a depleted MORB mantle source (DMM), whereas P-MORBs originated from a DMM source metasomatized by variable proportions of plume-type (OIB-type) geochemical components. The tectono-magmatic evolution of the Misho Mafic Complex indicates that the initial rift-drift tectonics of the Paleo-Tethys was triggered by a mantle plume activity and was strongly affected by plume-related magmatism and associated lithospheric weakening at a regional scale. This conclusion is consistent with the models proposed for the Paleo-Tethys margins in central-eastern Asia. Moreover, regional geological evidence (e.g., regional doming, basaltic plateaux in adjacent areas) further supports the hypothesis of existence of mantle plume activity in this area. The Kermanshah CMO. Metagabbros and metadykes show N-MORB chemistry with either low or high Sm/Yb ratios. Pillow basalts display geochemical signatures ranging from E-MORB to P-MORB and alkaline basalts. REE petrogenetic modeling show that alkaline basalts were formed by partial melting of an enriched mantle source, whereas E-MORBs and P-MORBs were generated from partial melting of a DMM source metasomatized by variable proportions of OIB-type geochemical components. Partial melting of all rock-types occurred at shallow level (spinel-facies) mantle. The initial rift-drift tectonics of the Neo-Tethys was characterized by a type of rifted margin, which is intermediate between the amagmatic type (Ligurian Tethys type) and the magmatic, plume-influenced type. Indeed, likewise the Ligurian Tethys, rifting occurred through passive (possibly, asymmetric) extension, which led to the exhumation of the sub-continental mantle. Meanwhile, high Sm/Yb rocks formed at the continent-ocean transition zone by partial melting of a DMM source locally bearing sub-continental garnet-pyroxenite relics. However, in contrast with the Ligurian Tethys model, the Neo-Tethys rift stage was also associated with volcanism featuring a marked influence of plume-type components. Nonetheless, no geological evidence supporting the existence of a Triassic mantle plume activity in this area has been documented. Therefore, the plume-type geochemical signature observed in the Kermanshah CMO sequences can likely be explained with the re-activation of portions of enriched mantle (mantle heterogeneities) that were inherited from the Paleozoic mantle plume associated with the opening of Paleo-Tethys.