Four Flavours of Orogenic Plateau Magmatism: What's Melting Beneath the Turkish-Iranian Plateau?

Orogenic plateaux are first order topographic features of the continents, occurring in collision zones such as Tibet and Andean-style continental arcs. Plateaux are sites of abundant mantle-derived magmatism, but there is little understanding of its geodynamic cause in spite of assumptions that slab break-off or lithospheric thinning are controlling factors in melt production. The Turkish-Iranian Plateau formed on the Eurasian Plate after the ~30 Ma Arabia-Eurasia collision. Neogene-recent volcanoes are found up to ~800 km from the suture, and have huge compositional variation. We define four varieties of recent mafic magmatism spatially and geochemically. (1) Close to the Bitlis Suture in Eastern Turkey, slab break-off is likely to have occurred at ~10 Myr, and there is little mantle lithosphere present. Magmatism is mostly calc-alkaline, sourced from the asthenosphere or any remaining mantle lithosphere, and is affected by crustal contamination. (2) In the Lesser Caucasus up to ~500 km from the suture, magmatism is more alkaline, less crustally-contaminated and is derived from subduction-modified lithospheric mantle. (3) Close to the Zagros Suture in Iran, the lithosphere may have thickened to >200 km during collision. Magmatism is volumetrically limited and derived almost exclusively from the lithospheric mantle, with trace element-enriched alkaline or ultrapotassic compositions. Unlike the Lesser Caucasus, there is little magmatism in the Iranian desert up to ~500 km from the suture. (4) Beyond ~500 km from the Bitlis-Zagros Suture Zone, there is sparse, compositionally variable magmatism: it is OIB-like in Eastern Iran, more akin to Zagros magmatism in the Alborz, and more felsic above the ~55 km thick crust of the Greater Caucasus. Geochemical data suggest that magmatism in (1) is caused by asthenospheric upwelling following slab breakoff, whereas in (2) it is dominated by melting of the base of the still-present lithospheric mantle during convective removal. Magmatism in (3) may be triggered by the breakdown of hydrous phases such as amphibole and phlogopite within the lithospheric mantle which release water and lower the mantle solidus. In (4) melting has diverse triggers not clearly related to the Bitlis-Zagros Suture. OIB-like magmas in (4) may form beneath lithospheric thin-spots whilst more arc-like magmas may be due to lithospheric thickening in the Alborz and Greater Caucasus. The spatial distribution of different magma types leads us to conclude that the ~500 km distance from the suture represents a cut-off at which stirring of the asthenosphere due to slab break-off ceases to be an effective propellant for decompression melting and lithospheric delamination. The relative paucity of magmatism beneath and inboard from the Zagros Mountains lends support to the idea that the mantle is not being stirred by break-off processes and that the lithosphere is particularly thick beneath this region. There is no single trigger or source of magmatism beneath the TIP, but two processes may dominate: melting due to heating and dehydration of the lithosphere as it is thickened or convectively removed, and decompression melting of upwelling asthenosphere.