Transport of Volatile-rich Melt from the Mantle Transition Zone via Compaction Pockets: Implications for Mantle Metasomatism and the Origin of Alkaline Lavas in the Turkish–Iranian Plateau

A Soltanmohammadi (1) , M. Gregoire (1) , M. Rabinowicz (1) , Muriel Gerbault (1) , G. Ceuleneer (1) , M. Rahgoshay (2) , M. Bystricky (3) , M. Benoit (1)

1 GET - Géosciences Environnement Toulouse
2 Faculty of Earth Sciences, Shahid Beheshti University
3 IRAP - Institut de recherche en astrophysique et planétologie

This study is based on an integrated approach combining results from petrology, geophysics and modeling to explain the origin of continental alkaline magmatism in the Turkish–Iranian plateau (TIP). Synthesis of the petrological and chemical characteristics of the alkaline magmatism of the TIP, extending from 80 Ma to the present, shows that the alkaline lavas can be classified as ultrapotassic (UK), transitional potassic to sodic (TK) and sodic-high-potassium (HK) and sodic-low-potassium (LK) lavas, all derived from a heterogeneous mantle source. Synthesis of the most recent seismic tomography images for the region shows the presence of a large set of low-velocity elliptical bodies, ∼100 km in size, referred to as ‘Compaction Pockets’ (CP), scattered from the top of the mantle transition zone (MTZ) to the base of the lithosphere beneath the TIP. A model is developed, which shows that the low velocities in these Compaction Pockets result from the percolation and concentration of volatile-rich melts liberated from the MTZ. These volatile-rich melts interact with their surrounding mantle, which has a temperature ∼100°C lower than the usual subcontinental mantle adiabat. It is argued that this results in the precipitation of hydrated and carbonated mineral phases (at ∼8–6·5 GPa and <4 GPa, respectively), and partial melting of the resulting heterogeneous mantle (6·5–4 GPa) at critical depths. Melt extraction via dykes occurs when the top of the Compaction Pocket successively crosses these critical depths. The three groups of UK, sodic-LK and sodic-HK lavas may be linked to distinct melt extraction events. The chemical composition of TK melts may result from the mixing of UK and sodic melts, or partial melting of metasomatized mantle at lithospheric depths. The ‘Compaction Pocket’ model offers a robust new concept to explain alkaline magmatism in the context of continental orogeny.

Mots clés: Compaction Pocket; orogenic belt; volatile-rich melt; alkaline lava; mantle Transition Zone

Domaines: Tectonique; Interfaces continentales, environnement; Volcanologie; Géochimie

A Soltanmohammadi, M. Gregoire, M. Rabinowicz, Muriel Gerbault, G. Ceuleneer, et al.. Transport of Volatile-rich Melt from the Mantle Transition Zone via Compaction Pockets: Implications for Mantle Metasomatism and the Origin of Alkaline Lavas in the Turkish–Iranian Plateau. Journal of Petrology, 2018, 59 (12), pp.2273-2310. ⟨10.1093/petrology/egy097⟩. ⟨hal-01979646⟩