Trace element and Nd-Sr isotope constraints on origin of the Chifeng floodbasalts, North China

The Chifeng flood basalt field is a component part of the Neogene basalt be lt in North China. The basalts erupted on the northern edge of the North Ch ina craton during the Miocene and are subalkaline tholeiites in composition , with normative olivine and quartz. They show a limited variation in SiO2 contents, ranging from 48.47 to 51.61 wt.%, and evolved nature as indicated by both Mg# = 47.4-65.2 and large variations in Ni and Cr contents. They s hare the geochemical characteristics of the High-Ti group of the Gondwana c ontinental flood basalts, with TiO2 = 2.10-2.48 wt.%, Ti/Y = 399-584, Zr/Y = 4.35-7.21, and Sr > 299 ppm, being very similar to the Paranapanema High- Ti basalts, Brazil. In contrast to the Paranapanema lavas which generally h ave large negative Nb, Sr, and P anomalies and negative epsilon Nd values, the Chifeng basalts have no striking Sr and P anomalies, with variable Nb a nomalies and positive epsilon Nd values. The Chifeng basalts are characteri sed by low Rb/Ba and Rb/Sr, high Ti/Y, Ti/Yb, and Ba/Y, and variable but hi gh Nb/La ratios, and show progressive enrichment of K2O, P2O5, Nb, Ba, Sr, and Zr with decreasing epsilon Nd values, which preclude the possibility th at the magmas were contaminated by crustal materials. Their Sr-87/Sr-86 rat ios and epsilon Nd values are in the ranges of 0.7039 to 0.7052 and of +1.6 to +4.4, respectively, showing more restricted variations compared with th ose of the mantle xenoliths from the Hannuoba basalt field. Nd-Sr isotopic compositions and the trace element ratios such as La/Nb, K/Nb, Rb/Nb, Ba/Nb , and Zr/Nb show the GIB-like geochemical signatures. The Chifeng basalts w ere derived from a heterogeneous source region in the asthenosphere, with l ittle interaction with lithospheric mantle. It is likely that a combination of varied degrees of partial melting with the chemical heterogeneity of a source region could result in negative correlation arrays of epsilon Nd vs. K2O, P2O5, Rb, Sr, Ba, Nb, Zr, Ti/Yb, Ba/Y, and La/Sm, in which relatively low LIL and HFS element contents and ratios and high epsilon Nd values wou ld be produced by the relatively high degrees of partial melting of the het erogeneous source region. (C) 1999 Elsevier Science B.V. All rights reserve d.