Mantle xenoliths from Late Cretaceous basalt in eastern Shandong Province： New constraint on the timing of lithospheric thinning in eastern China

Studies of mantle xenoliths hosted in both the Cenozoic alkali basalt and the Early Paleozoic kimberlite suggest that part of the subcontinental lithosphere as thick as more than 100 km has been lost from the Early Paleozoic to Cenozoic1—8]. Neither the scale and mechanism nor the accurate timing of the lithospheric thinning has been precisely constrained7-12]. One of the reasons for this is that there are only a few Mesozoic basalts cropped out, especially, few containing mantle-derived …

Mantle xenoliths from Late Cretaceous basalt in eastern Shandong Province: New constraint on the timing of lithospheric thinning in eastern China

The age of the alkali basalt from Daxizhuang in Jiaozhou, eastern Shandong, was determined to be 73.5 ± 0.3Ma by40Ar-39Ar technique. The basalt gave high εNd(t) values of +7.5 and +7.6, suggesting that the primitive magma was derived from depleted asthenospheric mantle with a formation depth of about 65-95 km. Spinel-lherzolite xenoliths have been discovered in the basalt. The Fo values of olivine from the xenoliths range from 88 to 89. The estimated equilibrium P-T conditions of spinel-lherzolite xenoliths are about 2.0 GPa and 1010-1140℃, suggesting an equilibrium depth of about 65 km. Geochemical characteristics of the Late Mesozoic (125-115 Ma) mantle-derived rocks in Shahdong Province suggest an enriched lithospheric mantle along the southern margin of the North China block. However,geochemical characteristics of the Late Cretaceous basalts in Daxizhuang and the equilibrium P-T condition estimated from the xenoliths suggest that the lithosphere at 73 Ma ago was as thick as that of the Cenozoic in the region. Additionally, the xenoliths are rich in basaltic-component, suggesting a derivation from a newly-formed lithosphere. Therefor,lithospheric thinning took place at a time interval between about 120 and 73 Ma along the southern margin of the North China block.