Ar-Ar dating and Sr-Nd-Pb isotopic character of Paleogene basalts from the Xialiaohe Depression, northern Bohai Bay Basin: implications for transformation of the subcontinental lithospheric mantle under the eastern North China Craton

Paleogene basalts are widely distributed in the Xialiaohe Depression, which lies in the northern part of the Bohai Bay Basin, the second largest petroleum-bearing basin of China, in the northeastern part of the North China Craton. The basalts mainly occur in three formations: the Paleocene Fangshenpao Formation (PFF), the Eocene Shahejie Formation (ESF), and the Oligocene Dongyin Formation (ODF). The PFF is dominated by tholeiites, whereas the ESF and ODF are characterized by alkaline basalts with minor tholeiites. These basalts contain generally lower contents in large-ion lithophile elements (LILEs) and most high-field-strength elements (HFSEs) relative to ocean-island basalts (OIBs), except for positive anomalies for Ba, Sr, Eu, and Ti, and are characterized by OIB-like Sr and Nd isotopic compositions and by abnormally low radiogenic lead isotopic composition. They display a positive correlation between Pb-206/Pb-204 and Nd-143/Nd-144, and a negative correlation between 206Pb/204Pb and Sr-87/Sr-86. The geochemical characteristics of these basalts are quite different from that expected from magmas derived from crustal contamination or melting from a uniform asthenospheric mantle source, but is consistent with derivation from newly formed lithospheric mantle. Combined with the geochemical character of the ESF and ODF basalts, we ascribe the abnormally low radiogenic lead isotopic composition for the Paleocene PFF basalts to newly formed lithospheric mantle that originated from recycling of delaminated thickened lithosphere in Late Mesozoic, including a lower crustal component.