The geochemistry of Tibetan lavas: Spatial and temporal relationships, tectonic links and geodynamic implications

Abstract

The continental lithosphere is created and modified at convergent plate boundaries through complex, and interdependent, tectonic and magmatic processes. Evolution of the continental lithosphere modifies 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%). Such lavas are most likely to contain characteristic geochemical fingerprints of the composition and physical condition of the mantle source at the time of melting. Here, we combine new analyses of thirty-one samples from the Hoh Xil Basin of the northern Tibetan Plateau with a compilation of published geochemical and geochronological data for late Mesozoic and Cenozoic magmas from the Indo-Eurasian collisional orogen. Geochronology of the Hoh Xil lavas reveals unrecognized early (∼27 Ma) alkali basaltic magmatism in northern Tibet that likely triggered melting of the mafic lower and metasedimentary upper continental crust, generating trachyanadesites-trachydacites and rhyolites, respectively. 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. After 30 Ma, lavas derived from melting of the lithospheric mantle expanded both northward and southward, eventually 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 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. Although melting of the mantle lithosphere beneath Asia has previously been interpreted as an abrupt event, we propose that mantle lithosphere thinning and removal has been part of a continuum melting process during the Indo-Asian orogen that reflects an evolution from mantle wedge to mantle lithosphere melting accompanying the transition from subduction to collision.