Hydrous and oxidised deep lithospheric mantle revealed by amphibole megacrysts from the northwest North China Craton

Abstract

The destruction of the North China Craton (NCC) is generally attributed to the complex thermal-mechanical-chemical effects of the (Paleo-) Pacific plate subduction on the lithospheric mantle. However, the effects of other tectonic regimes, especially the subduction of the Paleo-Asian oceanic (PAO) plate, remain poorly constrained. Here, this is explored using major- and trace-element, and Sr-isotope data on newly discovered amphibole megacrysts in the ∼89 Ma basalts from Langshan area, northwest NCC. The megacrysts are chemically homogeneous with a few Fe–Ti oxide inclusions and classified as Pargasite & Magnesio-Hastingsite with high Mg# (71.6–78.3) and contents of Al₂O₃ (14.55–15.39 wt%), TiO₂ (2.65–3.02 wt%) and K₂O (1.88–2.12 wt%). They are characterized by low rare earth element (REEs) concentrations (21.4 to 40.6 ppm), notable enrichments in fluid-loving elements (e.g., Ra, Ba, Sr) and variable Sr-isotopic ratios (0.7018–0.7049). The megacrysts are inferred to crystallize from oxidised (mean ΔFMQ = +0.8) and hydrous (3.6–4.7 wt%) parental magmas in the crust-mantle transition zone (1.1–1.2 GPa). Such parental melts are likely derived from amphibole-bearing lithospheric mantle due to the impregnation of hydrous fluids released from the shallowly subducted PAO slab. Together with studies on the host basalt-xenolith association and the general knowledge on the eastern NCC lithosphere, our study reveals hydration and oxidization effects related to the fossil subduction from the north and emphasizes the modification of the craton lithospheric mantle by accumulated effects from circum-craton subduction episodes.