Geochemistry and petrology of the Kermanshah ophiolites (Iran): Implication for the interaction between passive rifting, oceanic accretion, and OIB-type components in the Southern Neo-Tethys Ocean

摘要

The Mesozoic southern branch of the Neo-Tethys Ocean was located between the Arabian shield and the Sanandaj-Sirjan continental block of Iran. The Kermanshah ophiolitic complex crops out in the Main Zagros Thrust Zone in Iran and consists of several rock associations, which record the geodynamic evolution of this ocean. In particular, the ophiolitic rocks that record the early stage of this oceanic evolution are represented by mantle metalherzolites, metagabbros and crosscutting metabasaltic dykes, as well as by basaltic pillow lavas and dykes. Mineral chemistry and whole-rock geochemistry of the metalherzolites show that they represent portions of exhumed sub-continental mantle. Four groups of rock can be recognized within the metagabbro and volcanic/subvolcanic sequences, based on many geochemical indicators and Sr-Nd isotopic data. They are: (1) metagabbros and metadykes with a N-MORB signature; (2) alkaline basalts and trachybasalts with ocean-island basalt (OIB) affinity; (3) rocks showing plume-type MORB (P-MORB) affinity; and (4) rocks showing enriched-type MORB (E-MORB) affinity. Metagabbros and metadykes show either low or high Sm/Yb ratios, which point to a genesis from partial melting of a pure depleted MORB mantle (DMM) source or a DMM source bearing garnet-pyroxenite relics, respectively.All volcanic and subvolcanic rock-types have variable, but generally high La/Yb, Sm/Yb, Th/Yb, Ta/Yb and Zr/Y ratios, and low Zr/Nb ratios. These features coupled with relatively low ~(143)Nd/~(144)Nd ratios and high ~(87)Sr/~(86)Sr indicate that the alkaline rocks were generated from an enriched, OIB-type (plume-type?) mantle source, whereas P- and E-MORB rocks can be interpreted as the product of variable interaction between depleted MORB-type asthenosphere and OIB-type material.We propose a tectono-magmatic model that can explain the different rock associations cropping out in the Kermanshah ophiolites involving a type of rifted margin that is intermediate between the amagmatic Iberia/Ligurian Tethys type and the magmatic, plume-influenced East Greenland type. This model presupposes that the Sanandaj-Sirjan block was rifted from the northern margin of Arabia through passive extension, which led to the exhumation of the sub-continental mantle now represented by the metalherzolites. Meanwhile, the asthenosphere upwelling associated with the partial melting of a heterogeneous mantle source locally bearing garnet-pyroxenite relics allowed the formation of both low and high Sm/Yb N-MORB-type metagabbros and metadykes at the continent-ocean transition zone. This model is similar to the model proposed for the Ligurian Tethys. But, in contrast with the Ligurian Tethys model, extension was also associated with volcanism characterized by a marked influence of OIB-type components linked to the uprising of MORB-type, depleted asthenospheric mantle. If so, the Kermanshah ophiolites represent a particular type of continental margin ophiolite, comprising a combination of continental margin- and plume-type ophiolites.