Geochemical characteristics of Cenozoic Jining basalts of the Western North China Craton: Evidence for the role of the lower crust, lithosphere and asthenosphere in petrogenesis

Abstract

The Jining volcanic field located in the southern margin of the Mongolian plateau and the western North China Block consists of four rock types: quartz tholeiite, alkaloolivine tholeiite, alkali olivine basalt and basanite. These rocks have a wide range of K-Ar ages from ∼36 to < 0.2 Ma. The early volcanism was voluminous and dominated by flood-type fissure eruptions of tholeiites, whereas the later phase was represented by sparse eruptions of basanitic lavas. Thirty-six samples analyzed in this study show a wide range in SiO2 contents from 44% - 54%. They all are sodium-rich and high-Ti basalts that, however, show marked isotopic variations between two end-members: (1) tholeiites that have higher 87Sr/86Sr of 0.7048 - 0.7052, and lower eNd of-0.8 to -2.4 and Pb isotope ratios (206Pb/ 204Pb of 16.9 - 17.2, 207Pb/204Pb of 15.3 - 15.4 and 208Pb/204Pb of 37.1 - 37.7); and (2) basanites that have lower 87Sr/86Sr of 0.7035 - 0.7044, and higher €Nd of+ 1.3 to +4.9 and Pb isotope ratios (206Pb/ 204Pb of 17.7 - 18.0, 207Pb/204Pb of 15.4 - 15.5 and 208Pb/204Pb of 37.8 - 38.2). Alkali olivine basalt that occurs as a subordinate rock type is geochemically similar to the basanites, but isotopically similar to the tholeiites, characterized by the highest 87Sr/86Sr ratio among the three basaltic suites, coupled with a low Nb/U value (∼33). In Sr-Nd-Pb isotopic plots, the tholeiites extend toward the EM1 (i.e., enriched mantle type 1) component, whereas the basanites trend toward the Indian Ocean mid-ocean ridge basalt (MORB) field. Adopting the 'plum-pudding' model by Morris and Hart (1983), we suggest that the Oligocene tholeiites were generated by high degree melting of an ascended asthenospheric mantle that was contaminated with a large amount of EM1-type continental lithospheric material during the early Cenozoic. On the other hand, the late Tertiary and Quaternary basanites may have originated predominantly from a depleted asthenosphere component with small but variable degrees of contribution by the continental lithospheric mantle. The generation of alkali olivine basalt requires addition of a two-pyroxene granulite component, thus we suggest it was derived from a basanitic parental magma that underwent minor amounts of lower crustal contamination during ascent to the surface.