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<br />

 of + 3.9 to + 7.0, unradiogenic 87Sr/86Sri 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 EiNd

 of + 6.4 to + 9.2, variable 87Sr/86Sri 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.