Petrogenesis of Early Cretaceous sodic lamprophyres from Central Tibet: Slab break-off triggers metasomatized lithosphere and asthenosphere interaction

Abstract

Lamprophyres are rare but important rocks that can provide crucial information about deep mantle source regions and geodynamic settings. The Bangong–Nujiang collision zone was formed by the collision of the Qiangtang and Lhasa terranes and experienced intense post-collisional magmatism. However, the deep mantle processes and mechanisms involved in this magmatism remain controversial. Here, we present the first complete petrological, geochemical, and geochronological characterization of sodic lamprophyres in the northern Lhasa terrane, central Tibet, with the aim of deciphering their relationship with late Early Cretaceous magmatic flare-up events. These lamprophyres were emplaced at ∼114 Ma. They exhibit sodic alkaline series affinities with high Na₂O contents (2.17–3.52 wt%) and Na₂O/K₂O ratios (1.8–3.8) and are also characterized by variable MgO contents (5.0–7.9 wt%) and Mg# values (52–68); slight enrichment in LREEs ([La/Yb]_N = 4.9–5.9); relatively flat HREE patterns ([Gd/Yb]_N = 1.46–1.53); no Eu anomalies (Eu/Eu* = 0.94–1.04); and Rb, Ba, Th, U, and Pb enrichments as well as Nb and Ta depletions. The samples have relatively low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70848–0.70886) and ε_Nd(t) values (−3.2 − −2.4) and variable zircon ε_Hf(t) values (−4.4 − +10.4). These geochemical characteristics indicate that these rocks were derived from an enriched lithospheric mantle source. However, relatively depleted Hf isotopic compositions and Ti/V (20.2–32.5) ratios are comparable to those of MORB. The calculated melting temperature ranges from 1159 to 1244 °C, with an average of 1183 °C, further supporting the involvement of a high-temperature asthenospheric component. The rapid emplacement and relatively low melting pressure (8.0–10.4 kbar) imply that they formed in an extensional setting. Therefore, we propose that the parental magma of the lamprophyres was generated by the interaction of asthenospheric and metasomatized lithospheric mantle due to slab break-off beneath the Bangong–Nujiang collision zone.