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Post-collisional (ultra)potassic lamprophyre dykes are the key probes for understanding mantle metasomatism and reconstructing tectonic evolution. In this study, we present new petrological, geochronological, geochemical and zircon Lu-Hf isotopic data for lamprophyre dykes in the northern Qiangtang terrane (central Tibet), aiming to constrain their petrogenesis and geodynamic setting. The studied lamprophyres are minettes with phenocrysts of siderophyllite and phlogopite, which intrude into Triassic granite of 236.9 Ma. These lamprophyres yield zircon U-Pb ages of 39.7–40.9 Ma. They exhibit high contents of K2O (7.61–8.59 wt.%) and ultrapotassic features with high K2O/Na2O (11.43–14.38) ratios. They are characterized by increased values of Mg# (69.1 to 72.1) and high concentrations of compatible elements (e.g., Cr = 277–529 ppm, Ni = 232–322 ppm), which are diagnostic of mantle-derived primitive magma. The studied lamprophyres have a high abundance of rare earth elements (∑REE = 902–1061 ppm) with significantly fractionated REE patterns ((La/Yb)N = 66.3–100.6), and they are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in high field strength elements (HFSE) (e.g., Nb, Ta and Ti) and heavy rare earth elements (HREE) with enriched zircon Hf isotopes (εHf(t) from −6.40 to 3.80). This indicates their derivation from an enriched mantle source which was metasomatized by subduction-related fluids and sediment-derived melts. A petrogenetic study suggests that the lamprophyres were generated by the partial melting of a phlogopite-bearing lherzolite within the garnet stability field. We propose that the Cenozoic ultrapotassic mafic rocks in the central Tibetan Plateau originated in the lithospheric mantle metasomatized by the subduction-related components, and are the magmatic response to the detachment of the subducted Tethyan slab. [ABSTRACT FROM AUTHOR]

Late Mesozoic igneous rocks from the north Lhasa block record the Neo-Tethyan orogeny in the southern Tibet Plateau. This study presents geochronological and geochemical data of Bieruozecuo quartz diorite pluton in the northern margin of the Lhasa block to constrain its petrogenesis and tectonic implications. The LA–ICP–MS zircon U–Pb geochronology of quartz diorites shows that the emplacement occurs at ca. 114–116 Ma, belonging to the products of Early Cretaceous magmatic activities. The high concentrations of TiO2, MgO, and MnO, together with SiO2 vs. K2O and A/NK vs. A/CNK diagrams, all suggest that the Bieruozecuo quartz diorites are meta-aluminous and high-potassium calc-alkaline rocks. Their high Sr and low Y and Yb contents, as well as high Sr/Y and La/Yb ratios, are consistent with the typical adakitic rocks. The REE patterns show a large distribution of compositions, which have LREE, while HREE are buffered, along with large ratios of (La/Yb)N, as well as high values of K2O/Na2O, Mg#, Cr, and Ni, all of which imply the partial melting of a delaminated lower crust, without obvious fractional crystallization during the magma ascending and emplacement. This study suggests that, with the closure of the Bangonghu–Nujiang Tethys Ocean Basin, the post-collisional extension of the north Lhasa block will have started no later than ca. 114–116 Ma. Combined with the previous studies, our new data demonstrates that the partial melting of the delaminated lower crust, in a post-collisional setting, may be the main mechanism responsible for the ca. 116–82 Ma adakitic magmatism in the north Lhasa block. [ABSTRACT FROM AUTHOR]