Geochemistry And Petrogenesis Of Cenozoic Basalts In The Trans-north China Orogen

Basalt is the partial melting product of the mantle.A wide range of major and trace element and radiogenic isotopic composition of basalt have been demenstrated.Understanding the mantle source compositon is a key goal in the long-term history of mantle geochemistry.Continental basalts can provide important imformation of the nature of the mantle source,and their heterogeneity is linked to chemical geodynamics such as plate subduction process.It has been advocated that the stagnant Pacific slab within the mantle transition zone played a critical role in the genesis of the Cenozoic basalts in the eastern part of North China Craton?NCC?,however,it is not clear whether this recycled oceanic crust contributed to the chemical makeup of the Cenozoic basalts in the Trans-North China Orogen?TNCO,the central zone of the NCC?.Here,whole rock major and trace elements and Sr-Nd isotopic compositions as well as precise measurements of Fe/Mn and Nb/Ta ratios and olivine compositions for Cenozoic basalts from the TNCO were analyzed to investigate the nature and lithology of the mantle sources and to assess the evolution of the lithosphere beneath the TNCO.These basalts are predominantly alkali basalts with small amounts of tholeiites.Compared to partial melts of mantle peridotite,the TNCO basalts have higher TiO₂,total Fe₂O₃,FC3MS?FeO/CaO-3*MgO/SiO₂,all in wt%?,Fe/Mn and Zn/Fe ratios and lower CaO contents.The olivine phenocrysts from the TNCO basalts have lower Ca and Mn and higher Ni and Fe/Mn than olivines crystallized from peridotite partial melts.These features indicate that the TNCO basalts may predominantly be derived from a mantle source of pyroxenite lithology.Combining Nb/Ta ratios with the CS-MS-A pseudo-ternary projections suggests that the residue mineralogy in the mantle source of alkali basalts with MgO>7.5 wt%are mainly composed of clinopyroxene+garnet±olivine±rutile,whereas those of the tholeiites with MgO>6 wt%are composed of clinopyroxene+garnet±orthopyroxene.The ocean island basalt?OIB?-like trace element distribution patterns?mostly Ba,Sr,Nb and Ta positive anomalies and Th and U negative anomalies?and enriched Sr-Nd isotopic compositions of the TNCO basalts suggest that subducted oceanic crust?mixing of altered oceanic crust plus sediment?was involved in the depleted mantle source.We speculate that reaction between the oceanic crust derived-melts and the ambient mantle peridotite could have formed the pyroxenite source,with high and low melt/peridotite ratios producing silica-rich and silica-deficient pyroxenite,respectively.Due to the lower solidus of the second-stage silica-excess pyroxenite than the silica-deficient pyroxenite,the former would preferentially melt to a higher degree to generate tholeiites and the latter to generate alkali basalts,which is consistent with the lower incompatible trace element contents in the tholeiites compared to the alkali basalts.The Cenozoic basalts in the TNCO evolved from alkali basalts of Late Eocene–Oligocene age to coexisting alkali and tholeiitic basalts of Late Miocene–Quaternary age.This temporal variation in basalt geochemistry was previously interpreted as reflecting progressive lithospheric thinning in the TNCO during the Cenozoic.However,this model of lithospheric thinning was based on the conventional assumption of peridotite melts.The estimated average melting pressure of TNCO tholeiitic basalts was³ GPa?¹00 km?,significantly deeper than those?<60km?based on a convectional assumption of a peridotite mantle source but in agreement with the present lithosphere thickness beneath the north region of the TNCO?⁹0-120km?.Therefore,our work emphasizes that the previous model of lithospheric thinning in the TNCO during the Cenozoic may need to be reconsidered.The geochemistry of these basalts varies symmetrically from the center to both the north and the south sides,suggesting that the recycled oceanic crust in the mantle of the TNCO is likely related to the southward subduction of the Paleo-Asian oceanic plate and/or the northward subduction of the Tethyan ocean plate.