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.
All Science Journal Classification (ASJC) codes
- Atmospheric Science
- Earth and Planetary Sciences (miscellaneous)