Abstract: The Hannuoba basalts, erupted in the Neogene in northern North China, contain not only complete rock outcrops but also widespread ultramafic xenoliths and a variety of high-pressure megacrysts. The Daihuanggou profile, one of the better exposed profiles of the Hannuoba basalts generation, can be divided into the upper three layers and base from top to bottom. Petrographic observations showed that these basalts contained 1%5% of xenocrysts and trace of phenocrysts. The peridotite xenoliths were spinel-iherzolite. They were slightly depleted in LREE and had no fractionation of LREE and HREE, which is consistent with primary mantle xenoliths after some partial melting. Through mineralogical analysis of phenocrysts and xenocrysts, such as olivine, pyroxene and plagioclase in the Daihuanggou basalt, we concluded that the compositional changes in phenocrysts or xenocrysts are caused by the rapid rising basalt after mixing with the decomposition product of mantle peridotite. Compared to the native melt in equilibrium with lherzolite, the Mg# (52.062.7), CaO (7.3%8.5%), Ni ((82192)×10-6) and Cr ((65192)×10-6) contents were lower. The REE patterns for basalts showed right-wing type enrichment; HFSE (Nb, Ta and Zr) and LILE (Ba and Sr) were also enriched, with δEu=1.011.05 and δCe=0.951.02. Trace elemental ratios Ba/Rb and Rb/Sr showed that magmatic source might have been fluid metasomatism. According to trace elemental compositions, this series of rocks is mainly controlled by partial melting, where large scale fractional crystallization does not occur, and the residual source contains garnet. The analysis suggests that the basalt magma should be a primary rather than an evolved magma. The process that affects the formation of the entire magma may be partial melting rather than fractional crystallization. Based on current published experimental results and theoretical petrology analysis, the Hannuoba basalts are in equilibrium with pyroxenite, and the magma source contains mainly clinopyroxene and garnet and possibility a small amount of olivine.

Key words: Hannuoba, basalt, peridotite, primary magma, partial melting, magmatic source