Petrogenesis of mafic collision zone magmatism: The Armenian sector of the Turkish-Iranian Plateau

- Mafic magmatism is a common feature of continental collision zones.
- Elemental and Sr-Nd-Pb-Hf isotope results from < 2.5 Ma Armenian alkali basalts
- Low-degree melting of metasomatised mantle with limited basement interaction
- Melting controlled by slab break-off and/or small-scale asthenospheric convection
- Collision magmatism is a mantle-derived, low Th/La addition to continental crust.

The Turkish-Iranian Plateau grew after the Middle Miocene following the initial Paleogene Arabia-Eurasia collision. Authors attribute uplift to break-off of the southern Neo-Tethys slab beneath the Bitlis-Zagros Suture at ~ 15-10 Ma, coupled with continued plate convergence and regional crustal shortening. Since this time there has been an upsurge in mantle-derived collision magmatism over large parts of NW Iran, Eastern Anatolia and the Lesser Caucasus, potentially hundreds of kilometres from the site of southern Neo-Tethys slab break-off, > 10 Myr after the proposed break-off event. Whole rock elemental and Sr-Nd-Pb-Hf isotope data are presented for < 3 Ma trachy-basalt to trachy-basaltic andesite lavas erupted in Armenia in the South Caucasus. Samples formed by < 5% melting of fertile subduction-modified spinel-facies lithospheric mantle, and few display elemental or isotopic evidence for contamination by the 45-km thick Mesozoic-Paleogene arc crust or South Armenian Block continental crust. Recent magmatic activity in Armenia may not be a direct consequence of southern Neo-Tethys slab break-off 300-450 km away, beneath the Bitlis Suture. Late Miocene break-off of a second (northern Neo-Tethys) slab beneath the Pontide Arc may have allowed asthenospheric upwelling over a wider area than was affected by southern Neo-Tethyan break-off. However, whole-scale delamination of mantle lithosphere is ruled out due to the modest degrees of partial melting, a lack of asthenospheric components and limited crustal involvement in magmatism. Small-scale sub-lithospheric convection may be complementary to break-off, causing localised removal of lithospheric mantle and aiding the occurrence of melting for a significant time interval after the break-off event(s). Collision magmas such as those in Armenia represent mantle-derived additions to continental crust, enriched in incompatible elements but with Th/La ratios ≤ 0.2, much lower than those calculated for continental crust (0.25-0.3). Collision magmatism in Turkic-style orogens must be balanced by infra-crustal recycling and delamination to produce bulk continental crust.