Mantle thermal state controls the post-110 Ma basalt-borne megacryst species across central-eastern Asian continent

Volcanic-borne magmatic megacrysts (e.g., clinopyroxene and amphibole) are commonly deep cumulate phases bearing critical insights into magmatic thermochemical conditions (e.g., P-T-fO₂-H₂O). The post-110 Ma diffuse basaltic province covering the central-eastern Asian continent share similar geochemical characteristics but the entrained clinopyroxene megacrysts exclusively occur in the east with unknown controlling factors. This study focuses on the Langshan-Chaoge basaltic field far west from the eastern coast, where megacrysts of clinopyroxene (Mg# = 60–70) and amphibole (Mg# = 70–80) exist in ~95 Ma and ~89 Ma basalts, respectively. They display comparable trace-element patterns, Sr-isotope compositions and crystallization P-T conditions (clinopyroxene: 0.8–1.2 GPa, 1061–1143°C; amphibole: 1.1–1.2 GPa, 1068–1129°C) and likely represent fragmented lower crustal cumulates from compositionally similar parental magmas with different degrees of fractionation. Given the limited water storage capacity of asthenosphere, the estimated magmatic water contents (H₂O = 2.1–2.8 wt.% for clinopyroxene vs. H₂O = 3.6–4.7 wt.% for amphibole) should reflect the contamination by low-extent melts from variably hydrated lithospheric lower boundary. Together with the established cooling trend of regional lithosphere during 110–80 Ma, the basalt-borne megacryst transition from clinopyroxene to amphibole likely mirrors the stability of amphibole in the cooling deep lithospheric mantle. This recognition has general relevance to the exclusive occurrences of clinopyroxene megacrysts in eastern Asian continent with hot lithosphere and emphasizes the potential of volcanic-borne megacryst species as an independent reflection of province-scale lithospheric mantle thermal states.