Elemental and Sr-Nd-Pb isotopic compositions of late Cenozoic Abaga basalts, Inner Mongolia: Implications for petrogenesis and mantle process

Abstract

The Abaga volcanic field is one of the largest and least known areas of late Cenozoic intraplate igneous activities in eastern China. Twenty-seven Abaga basaltic rocks were analyzed for major and trace element contents, Sr-Nd-Pb isotopic compositions, and K-Ar dating. These basalts predominantly consist of alkali basalt with subordinate transitional olivine tholefite and rare quartz tholeiite. Combining our data of ten K-Ar dates with previously published data showed that the time of the principal volcanic eruption was from the Miocene to the Quaternary (14.57∼0.19 Ma). A large amount of volcanic activity is believed to have begun in the northwestern portion of the AVF and gradually migrated southeast-wards with geological time. The Abaga basalts exhibit chondrite-normalized REE patterns, incompatible elements and isotopic ratios (87Sr/86Sr of 0.703654∼0.704286 and 143Nd/144Nd of 0.512845∼0.512891) affiliated with oceanic island basalts (OIBs). In general, they have relatively homogenous Pb isotopic compositions (206Pb/204Pb of 18.409∼18.521, 207Pb/204Pb of 15.514∼15.546, and 208Pb/204Pb of 38.259∼38.447), indicating that these lava suites have a similar source. But the alkali basalts are relatively enriched in incompatible elements when compared to the tholeiites. This compositional difference may be attributable to the different degrees of partial melting in the mantle source. The Sr-Nd-Pb isotopic data indicated that the mantle sources of the late Cenozoic Abaga basalts display a DMM-EM2 array similar to those of Southeast Asia. The 208Pb/ 204Pb vs. 206Pb/204Pb plot was also significantly displaced above the Northern Hemisphere reference line in a pattern clustering near the less-depleted end of the field for the Indian Ocean mid-oceanic ridge basalt (MORB) that shows Dupal signatures. Therefore, we suggest that the Abaga basaltic magmas were dominantly derived from an Indian Ocean MORB-like depleted asthenospheric source with fewer contributions of an enriched mantle component (EM2) from the subcontinental lithospheric mantle. Copyright © 2008 by The Geochemical Society of Japan.