The genesis of Cenozoic basalts from the Jining area, northern China: Sr–Nd–Pb–Hf isotope evidence

Abstract

The Jining Cenozoic basalt province, located at the northern margin of the North China Craton, is one of the major Cenozoic basaltic provinces in northern China and Mongolia. In order to constrain the petrogenesis of the Jining basalts and its implications for the geodynamic evolution of the region, four basalt sections in Jining province, the Baiyinxiang, Hanqingba, Xin’anzhan, and Chahanmiao sections, were analyzed for Sr–Nd–Pb–Hf isotopic compositions. These basalts are predominantly composed of alkaline basalts and tholeiites. The tholeiites are overlain by the alkaline basalts in all but the Chahanmiao section. In general, the Jining basalts are similar to oceanic island basalts in terms of their incompatible element patterns.The εHf and εNd values of the Jining basalts range from +2.5 to +9.1 and −3.6 to +4.7, respectively. They display an εNd–εHf trend nearly parallel to, but above the mantle-array defined by oceanic basalts. Their 87Sr/86Sr ratios vary from 0.7036 to 0.7054, 206Pb/204Pb isotopic ratios vary from 16.9 to 18.0, 207Pb/204Pb varies from 15.3 to 15.5, and 208Pb/204Pb varies from 37.1 to 38.1. These data suggest the mixing of an isotopically depleted (i.e., MORB-like) mantle component and an enriched (i.e., EMI-like) component. Our data, in combination with studies of mantle xenoliths in the region, suggest that the subcontinental lithospheric mantle may not have made a significant contribution to the EMI component observed in the Jining basalts. The deviation of the εHf–εNd trend from the mantle array may reflect the involvement of ancient recycled continental crust in the enriched component. The mixing of melts derived from (a) recycled continental crust and (b) asthenospheric mantle peridotite may be responsible for the geochemical variations observed in the magmatic sequence. This is not only so for the Jining basalts, but also for the commonly occurring inter-layered lava flows found between xenolith-bearing alkaline basalts and tholeiites in the nearby Hannuoba region and many other basaltic provinces. The upwelling of deep mantle material may have been triggered by the western Pacific subduction and derived from above the barrier of the stagnant slab at the ∼600-km mantle transitional zone. The location of the upwelling is likely in the vicinity of Jining and Hannuoba as evidenced by the large magmatic volumes in the North China Craton and the abundant mantle xenoliths hosted by the Hannuoba basalts.