Séminaire Alberto Saal

Abstract

The continental lithosphere is created and modified at convergent plate boundaries through complex tectonic and magmatic processes. There is an interdependent coevolution of the subducted slab and overriding continental lithosphere, which modifies the conditions within the mantle wedge, and therefore, the generation and composition of magmas at subduction zones. One approach to unraveling interactions between the continental lithosphere and asthenosphere in subduction zones is through the evolution of pre-, syn- and post-collisional mafic volcanism (i.e., MgO ≥ 6 wt%).  Based on a combined analysis of new and published compositional, age and spatial patterns of mafic lavas (MgO ≥ 6 wt%), we delineate two distinct periods of Tibetan mafic magmatism, with a major compositional change occurring at roughly 46 Ma. Early (pre-46 Ma) mafic lavas erupted only in central and southern Tibet, and were generated by melting within the mantle wedge during the northward subduction of the Neo-Tethyan oceanic slab. Late (post-46 Ma) mafic lavas were produced by the partial melting of metasomatized subcontinental lithospheric mantle. The latter group is spatially restricted to central Tibet between 46-30 Ma and after 30 Ma, l expanded both northward and southward, encompassing the entire Tibetan Plateau by 20 Ma. These observations suggest that melting of the lithospheric mantle beneath Tibet initiated earlier, and was more widespread, than previously thought. Our results also suggest that widespread melting of the mantle lithosphere across Tibet post-dates the cessation of upper crustal shortening in northern Tibet at ∼30 Ma, and precedes the initiation of crustal extension at ∼15-10 Ma. Hence, we suggest that plateau-wide melting of the metasomatized subcontinental mantle lithosphere initiated following the collision of India with Eurasia, at ∼46 Ma. This melting was induced by detachment of the Neo-Tethyan slab concomitant with thinning of the continental mantle lithosphere. Such melting has continued until the present, with the exception of volcanic quiescence in south-central Tibet since ∼10 Ma. This quiescence likely arose from the underthrusting of Indian lithosphere beneath Eurasia.