Crustal recycling is an important cause of mantle heterogeneity and can have significant control on basalt compositions. Recycled components from the subducted (Paleo-) Pacific slab have frequently been recognized in Cenozoic basalts from the eastern North China Craton (NCC). However, it still remains unclear if the subducted Paleo-Asian oceanic slab contributed to intraplate basalts in this Craton.
In a search of evidence for the recycled components from this slab, we have studied the Ar–Ar age and elemental and Sr-Nd-Pb isotope compositions of newly-discovered basalts from the Langshan area and compiled a regional synthesis of Cenozoic alkali basalts from the northwest NCC. This region is far from the Pacific domain but near the suture zone of the Paleo-Asian Ocean. With a Late Cretaceous eruption age (~89 Ma), Langshan basalts have low silica and high FeO, MgO and alkali contents, high incompatible elemental concentrations, positive Sr, Eu, Ba, Nb, Ta and negative Pb and Ti anomalies with high Ce/Pb, Nb/U, Ba/Rb and low Rb/Sr ratios, superchondritic Zr/Hf ratios, and uniform radiogenic isotopes (ɛNd(t) = 2.20–2.51, 87Sr/86Sri = 0.703807–0.704348, 206Pb/204Pbi = 18.200–18.384, 207Pb/204Pbi = 15.470–15.515, 208Pb/204Pbi = 38.007–39.430). These compositional characteristics are shared by the compiled Cenozoic alkali basalts from northwest NCC.
The low silica and high MgO, FeO and alkali contents together with the positive Nb, Ta and negative Ti anomalies were probably controlled by silica-deficient garnet pyroxenite in the mantle source. The partial melting is estimated to have occurred in the asthenosphere under an average mantle potential temperature of 1300–1450 °C and a pressure of ~2.5 GPa. The positive Sr, Eu, Ba, Nb, Ta anomalies and the canonical indices (high Ce/Pb, Nb/U, Ba/Rb and low Rb/Sr ratios) indicate the involvement of a subducted oceanic igneous slab. Considering 1) the particular tectonic locality of the study region, 2) the inferred northward increase of silica-deficient pyroxenite in the mantle source of Cenozoic alkali basalts, 3) the evidence for strong lithospheric modification beneath the northwest NCC induced by slab-derived components, and 4) the longevity of subducted slabs in convecting mantle, the subducted Paleo-Asian oceanic materials could have introduced ubiquitous mantle heterogeneity beneath the northwest NCC and played a significant role in the generation of the Late Cretaceous to Cenozoic intraplate alkali basalts there.
- Alkali basalts
- Silica-deficient pyroxenite sources
- Recycled components
- Subducted Paleo-Asian slab
- North China Craton