Middle Devonian–Carboniferous crustal differentiation promoted by hydrous plume-related magmatism along the Paleo-Pacific active margin: A case study of the Gashun Nuur Complex in the Mongolian Altai

Petrology, whole-rock geochemistry, Sr–Nd isotopic data, and zircon U–Pb geochronology of magmatic rocks from Gashun Nuur Complex (Tseel Sum, Mongolian Altai) confirm catastrophic mid–late Devonian–Tournaisian (c. 385–350 Ma) mantle melting. Early (c. 385 Ma), scattered, large (∼ 100 m across) metagabbroic bodies carry a weak subduction-like signature (elevated contents of Large Ion Lithophile Elements – LILE, depletion in High Field Strength Elements – HFSE, high ${\epsilon }_{\mathrm{Nd}}^i$ of + 3.9 to + 7.0, unradiogenic ⁸⁷Sr/⁸⁶Sr_i of 0.7036–0.7044). Younger (c. 375–350 Ma) Fe-poor and Fe-rich dismembered amphibolite sheets show an EMORB character typical of melts resulting from the interaction of a subduction-modified depleted lithospheric mantle with a mantle plume (moderate LILE and HFSE, highly positive ${\epsilon }_{\mathrm{Nd}}^i$ of + 6.4 to + 9.2, variable ⁸⁷Sr/⁸⁶Sr_i of 0.7037–0.7068). Their chemistry marks the arrival of a mantle plume in an active margin setting far behind the magmatic arc. The Gashun Nuur Complex belongs to a vast (> 300,000 km²) mosaic of Devonian magmatic provinces which encompass the Altai suprasubduction, Altai–Sayan intracontinental and Mongol–Okhotsk oceanic domains. The studied middle Devonian–Tournaisian magmatic event is interpreted as the surface expression of hydrous plume(s) developed above a dehydrating flat-slab resting upon the lower mantle, at the mantle transition zone. The flat slab favoured the retreat of the trench causing extension in the overriding lithosphere. In the Gashun Nuur Complex, heat and fluids from mantle-derived intrusions induced extensive partial melting of a fertile Cambrian–Ordovician volcanic–sedimentary accretionary wedge, promoting a wide-rift mode of extension. Ascent and emplacement of granitic magmas to the middle crust left a still partially hydrated lower crust with an intermediate composition. This process of crustal differentiation may have played an important role in the stratification and stabilisation of the continental crust through the Earth’s history.