The petrogenesis of Cenozoic basalts from Daihai, western North China Craton: Constraints from 40Ar-39Ar chronology, major and trace elements, and Sr-Nd-Pb-Hf isotopes

The Daihai Cenozoic intraplate basalts are distributed in the western North China Craton (NCC), which is a part of the Cenozoic volcanic province in eastern China, and they are all alkaline basalts. The fine-scale determination of their mantle source region properties, partial melting mechanisms, and petrogenesis can provide crucial information for exploring lithospheric destruction and thinning in the western NCC. ⁴⁰Ar-³⁹Ar dating of potassium feldspar grains from the Daihai alkaline basalts yielded plateau ages of 18.22 ± 1.84 Ma and 26.86 ± 0.72 Ma, indicating that the Daihai basalts underwent multiple eruptive cycles during the Late Oligocene-Middle Miocene. These basalts exhibit ocean island basalt (OIB)-like geochemical features and were subjected to negligible crustal contamination. Moreover, basaltic magmas underwent intense fractional crystallization of olivine and clinopyroxene. The geochemical differences in the Daihai basalts were controlled by partial melting. The Daihai basaltic magmas were composed of at least two types (type I-enriched mantle and prevalent mantle) of mantle end-members that partially melted and then mixed, and the lithology of the mantle source region was predominantly peridotite. Under enriched mantle conditions, the mixing of garnet lherzolite partial melts (<1%) and spinel lherzolite partial melts (2-5%) can reasonably explain the elemental variations characteristic of the Daihai basalts. Most importantly, the melting depth and lithospheric thickness of the Daihai basalts were < 70 km or even close to 50 km, implying that the western NCC underwent lithospheric destruction and thinning during the Cenozoic. However, these effects were spatially heterogeneous. The most plausible genetic mechanism for the generation of the Daihai basalts was the coupled effects of subduction of the Pacific slab and subduction–collision of the Indo–Eurasian Plate since the Oligocene. The mantle flows generated by these two events convected, blocked and triggered upwelling mantle flows at the eastern margin of the Ordos Block. The upwelling mantle flows resulted in frequent magmatism in the region.