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

摘要

The Chifeng flood basalt field is a component part of the Neogene basalt belt in North China. The basalts erupted on the northern edge of the North China craton during the Miocene and are subalkaline tholeiites in composition, with normative olivine and quartz. They show a limited variation in SiO_2 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 share the geochemical characteristics of the High-Ti group of the Gondwana continental flood basalts, with TiO_2 = 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 have 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 anomalies and positive #epsilon#Nd values. The Chifeng basalts are characterised by low Rb/Ba and Rb/Sr, high Ti/Y, Ti/Yb, and Ba/Y, and variable but high Nb/La ratios, and show progressive enrichment of K_2O, P_2O_5, Nb, Ba, Sr, and Zr with decreasing #epsilon#Nd values, which preclude the possibility that the magmas were contaminated by crustal materials. Their ~(87)Sr/~(86)Sr ratios 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 those 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 OIB-like geochemical signatures. The Chifeng basalts were derived from a heterogeneous source region in the asthenosphere, with little 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. K_2O, P_2O_5, 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 would be produced by the relatively high degrees of partial melting of the heterogeneous source region.