Eocene break-off of the Neo-Tethyan slab as inferred from intraplate-type mafic dykes in the Gaoligong orogenic belt, eastern Tibet

Eocene (40–42 Ma) basaltic dykes in the Gaoligong–Tengliang belt, eastern Tibet, are characterized by high Na2O (2–4%), in contrast with the widespread post-collisional potassic and ultrapotassic rocks in the Tibetan plateau. Despite the ubiquitous negative Nb anomalies, these dykes have relatively high Nb and Zr contents, making them distinct significantly from the Gangdese arc magmas. All these, together with the positive Nb anomaly in some samples, indicate an intraplate affinity for the Gaoligong–Tengliang dykes. Specifically, the Gaoligong dykes represent the asthenosphere-derived melts which has been contaminated to various degrees by the lithosphere mantle-derived melts, whereas the Tengliang samples were directly derived from an enriched lithosphere mantle. The thin lithosphere (< 80 km) inferred from basalt geochemistry is unusual in Tibet, thus demanding a peculiar mechanism to thin lithosphere. A slab break-off model is preferred given the geochemical contrast between pre-40 Ma and post-40 Ma magmas in Tibet, and the temporal correlation among this intraplate magmatism, the termination of Gangdese arc magmatism and regional thermally-driven metamorphism. Therefore, the occurrence of intraplate-type magmas in the Gaoligong orogenic belt likely represents magmatic expression of the detachment of subducting Neo-Tethyan slab from the Indian continental plate during the Eocene. In the light of the slab break-off concept and thermo-mechanical modeling, the Eocene slab break-off furthermore suggests the onset of the India–Asia collision between 52 and 57 Ma.