Sources of the alkaline magmatism in the Turkish-Iranian Plateau: a link with the dynamic of stagnant lithospheres within the Mantle Transition Zone

Continental alkaline lavas are volatile-rich mantle derived magmas erupted either in orogenic setting, close to subduction zones, or in intracontinental setting far away from any active subduction zone. Their genesis involves partial melting of mantle sources either at shallow depths ( 150 km) down to the top of the Mantle Transition Zone. In the Turkish-Iranian Plateau, alkaline lavas younger than 80 Ma have geochemical signatures pointing to both types, i.e. subduction-influenced and intraplate (Oceanic Island Basalt-like). Based on SiO2 content and alkalinity index, the lavas can be grouped into three main types, group I, II and II. Groups I and II are ultrapotassic-potassic and sodic alkaline lavas with subduction-influenced and Oceanic Island Basalt-like chemical patterns, respectively whereas group III lavas have a transient composition between high-K calc-alkaline to calc-alkaline series with subduction-influenced chemical patterns. Particularly the Salavat Range, located in NW Iran, is characterised by lavas (flow, pillow structures dykes) displaying an abundant population of zoned clinopyroxene macrocrysts. A combination of mineral chemistry of the zoning patterns with the bulk rock major and trace element features evidence that the different populations of zoned clinopyroxene are representative of melts of group I and/or II. Our synthesis of the most recent available seismic tomography images show the presence of a large set of elliptic, ~100 km size, low velocities bodies, we named “Compaction pockets”, scattered from ~400 km up to ~ 100 km depth. A model is developed showing that the low velocity of the compaction pockets result from the percolation and concentration into large pockets of volatile-rich melt liberated at ~ 410 km depth in the “Big Mantle Wedge”. It is shown to result from interaction of volatile-rich melts with the surrounding mantle at a temperature ~100-200 °C lower than the usual sub-continental mantle adiabat, which eventually results in the precipitation of hydrated and carbonated mineral phases at ~8-6.5 GPa, partial melting of metasomatized mantle at 6.5 down to 4 GPa and finally in the precipitation of hydrated/carbonated phases below 4 GPa. Melt extraction occurs via swarm of dykes triggered when the top of the compaction pocket successively crosses these critical depths: ~240 km (8GPa), ~165 km (5.5 GPa) and ~135 km (4.5 GPa). It is shown that the three groups of lavas (group I, II and III) may be linked to these three successive melt extraction events. Strikingly, the clinopyroxene zonings recorded in the Salavat Range lavas may be representative of group I melt transported by a dyke generated at 240 km depth which pond close to the spinel-garnet mantle transition and eventually partially crystalized there. About 0.3 Ma later, when a 135 km deep dyke transporting a group II melt crosses that last interface, it traps partially crystalized assemblage of group I melt. Thereafter, the couple of mantle-derived melts may pound in a cold horizon where they simultaneously solidify and can explain the formation of clinopyroxene zonings in equilibrium with different alkaline melts.