The origin of Alpine-Himalayan K-rich orogenic lavas: an integrated experimental and geochemical approach

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Mantle-derived magmatism that occurs within Alpine-Himalayan orogenic belts is dominantly K-rich and its origin is controversial, particularly the significance of the widespread geochemical signal typical for recycled continental crust. Three major components are recognized: i) The crustal component indicated by incompatible-element enrichment, elevated 87Sr/86Sr, 207Pb/204Pb, 187Os/188Os and low 143Nd/144Nd and 176Hf/177Hf ratios of the lavas; ii) An ultra-depleted component identified by usual presence of refractory Cr-spinel, high Fo olivine and low FeO abundances; iii) Extremely high Th/La coupled with high Sm/La points to a genetic relationship with the melange. The above observations suggest that the source of orogenic lavas cannot be realistically modelled as homogeneous peridotite. It is envisaged that the metasomatic assemblages are situated in the veins within peridotitic wall-rock of different fertility, originated through melt-mantle reaction, during three major orogenic phases: subduction, collision and postcollision. In order to simulate the melting processes within mantle source of Alpine-Himalayan orogenic magmas, which may involve a mixture of peridotite and nonperidotite-hydrous mineral assemblages, we will perform two types of experiments: i) The first one mimic recycling and re-melting of different continental crustal material that may have happened during subduction; it involves sandwich experiments up to 3 GPa including terrigenous siliciclastic sediments, marly sediments, carbonated pelites and blueshists, combined with peridotite of different fertility. ii) The second series of experiments will simulate melting events related to postcollisional stage with ultimate goal to produce melts that compositionally resemble Alpine-Himalayan orogenic lavas. It will combine i) hydrous assemblages produced in the first series of experiments, and ii) hydrous assemblages envisioned to comprise source of the orogenic lavas like phlogopite-clinopyroxenites and glimerites (MARID), again with peridotite of different fertility at pressures up to 3 GPa. By performing these experiments, we simulate major stages involved in orogenic magmatism, where first hydrous sediment melt from the slab infiltrates the overlying mantle wedge and undergoes wholesale thermal and chemical equilibration (partial to complete reactive freezing), and the second when the activation of the resulting metasomes undergoes postcollisional partial melting in the orogenic mantle. The novelty of the proposed research lies largely in the paradigm shift from melting of peridotite to the melting of combined peridotitic and non-peridotitic assemblages. This reveals immense gaps in our understanding of the melting of non-peridotitic ultramafic rocks that this project will make the first steps in filling.