Petrogenesis of the Cenozoic Alkaline Basalts From Daihai, North China Craton: Geochronology, Geochemistry and Sr‐Nd‐Pb‐Hf Isotopic Evidence

ABSTRACT

The mechanism of Cenozoic lithospheric thinning in the western North China Craton (NCC) remains debated. To address this issue, we conducted an integrated geochronological, geochemical and isotopic study of the Daihai alkali basalts. ⁴⁰ Ar/ ³⁹ Ar dating yields eruptive ages ranging from ~24 to ~16 Ma, which record multi‐episodic magmatism. These eruptions constitute part of a broader, eastward‐migrating magmatic phase that initiated in the Late Eocene and comprised four distinct eruptive episodes in the western NCC. Geochemically, these basalts have SiO ₂ and alkali (K ₂ O + Na ₂ O) contents ranging from 42.54 to 51.90 wt% and from 3.90 to 7.25 wt%, respectively. They display oceanic‐island‐basalt (OIB)‐like trace element and isotopic patterns, characterized by enrichment in incompatible elements (e.g., Nb/U = 37–90, Ce/Pb = 10–21), relative enrichment in light rare earth elements (LREEs; (La/Yb) _N  = 4.8–12.9) and the absence of negative Sr and Eu anomalies. They have low ⁸⁷ Sr/ ⁸⁶ Sr ratios (0.70417–0.705581), high ¹⁴³ Nd/ ¹⁴⁴ Nd (0.512387–0.512818) and ¹⁷⁶ Hf/ ¹⁷⁷ Hf (0.282717–0.282988) ratios and Pb isotopic compositions of ²⁰⁶ Pb/ ²⁰⁴ Pb = 16.8998–17.8061, ²⁰⁷ Pb/ ²⁰⁴ Pb = 15.3665–15.5483 and ²⁰⁸ Pb/ ²⁰⁴ Pb = 37.1192–37.9121. The Daihai basalts show negligible alteration or crustal contamination. They primarily underwent fractional crystallization of olivine and clinopyroxene. Geochemical compositional variations are primarily controlled by 2%–7% partial melting of a peridotitic asthenospheric mantle. Isotopic data further indicate that the mantle source represents a mixture of prevalent mantle (PREMA) and enriched mantle I (EMI) components. The lithospheric thickness beneath the Daihai region is estimated to be < 80 km, locally approaching ~50 km, confirming significant Cenozoic thinning with notable spatial heterogeneity across the western NCC. We suggest that both the petrogenesis of these basalts and the associated lithospheric thinning were jointly controlled by mantle flow stagnation, induced by the combined effects of Pacific Plate subduction and the Indo‐Eurasian collision. The observed spatial heterogeneity in lithospheric thickness is interpreted to reflect variations in the degree of this mantle flow stagnation and the magnitude of associated upwelling. This study thus provides direct evidence linking intraplate volcanism in the western NCC to deep geodynamic processes and associated lithospheric thinning.