Helium and argon isotopes of the Tertiary basic igneous rocks from Shandong Peninsula and implications for the magma origin

Helium (He) and Argon (Ar) isotopic compositions of the Tertiary basic igneous rocks were determined by the high temperature melting extraction method. The selected samples for the studies included alkaline basalts and diabases from the Jiyang basin, and the surrounding Shanwang and Qixia outcrops in the Shandong Peninsula, eastern China. The results show that the Paleogene basalts and diabases from the Jiyang basin yielded a wide range of 4 P He abundance of (73.70-804.16) x 10(-8) cm(3) STP(.)g(-1), with He-3/He-4 ratios of 0.374-2.959 Ra, which was lower than the MORB but evidently higher than the continental crust value. The Neogene alkaline basalts from the Jiyang basin, Shanwang and Qixia outcrops have variable He-4 abundances ((42.34- 286.72) x 10(-8)cm(3) STP(.)g(-1)), and "continental crust- like" He-3/He-4 ratios (0.013-0.074 Ra). All of them contain atmospheric- like Ar-40/Ar-36 ratio (395.4-1312.7), reflecting the mantle sources with air components. Their low He-3/He-4 ratios are interpreted as the enrichment of the radiogenic He-4 mainly inherited from the mantle. He and Ar systematics show the mixing of MORB-type, air and a He-4 enriched member in the mantle source, suggesting that these igneous rocks originated from the depleted asthenospheric mantle mixed with an EMI component. Therefore, the present He and Ar isotopes do not support the viewpoints that the Cenozoic igneous rocks of Eastern North China were the products of mantle plume(s) activities.