Syn- and post-collisional potassic to ultrapotassic alkaline and subalkaline volcanic rocks: Heterogeneous mantle metasomatism beneath the North Qaidam orogenic belt

Potassic to ultrapotassic and bimodal volcanic rocks related to the collisional evolution of a continental orogen can provide valuable insights into the properties of the mantle and crust-mantle interactions. Here we report two distinct suites of volcanic sequences, subalkaline and alkaline, from the Oulongbuluke block, north of the North Qaidam ultrahigh-pressure metamorphic belt. The subalkaline suite is characterized by bimodal compositions. Both mafic and felsic rocks show enrichment of LREEs and LILEs but depletion of HFSEs and EM-I type SrNd isotopic compositions (ISr = 0.70725 to 0.70811 and εNd(t) = −11.9 to −8.7). The alkaline mafic rocks are potassic to ultrapotassic in composition and characterized by enrichment in LREEs, depletion in NbTa and enriched SrNd isotopic compositions (ISr = 0.70793 to 0.71394 and εNd(t) = −7.4 to −3.5), similar to the EM-II mantle source. The subalkaline/alkaline felsic rocks were most likely fractionated from parental mantle-derived mafic rocks, according to their geochemical characteristics. UPb zircon geochronology indicates the subalkaline suite formed at 382–378 Ma, while the alkaline suite formed over a long period from 441 Ma to 372 Ma. The subalkaline and alkaline mafic magmas are derived from two distinctly enriched mantle sources: subducted fluid metasomatized mantle and the potassium-rich melt metasomatized mantle, respectively. We suggest that the mantle beneath the continental collision belt was heterogeneous and metasomatized by oceanic subduction-zone fluids and potassic-rich melts during subduction of the Proto-Tethys Ocean at ∼530–445 Ma. Melting of the metasomatized lithospheric mantle, triggered by the break-off of an oceanic slab during the continental collision at 445–430 Ma, and by delamination of the lithospheric mantle during orogenic collapse at 380–360 Ma, caused the formation of the potassic to ultrapotassic alkaline and bimodal subalkaline volcanic rocks. •Two subalkaline and alkaline volcanic suites were formed at 382–378 and 441–372 Ma.•They were derived from heterogeneous metasomatized mantle of EM-I/II affinities.•The metasomatic agents were fluid and sediment melt from subducted oceanic crust.