Ar-Ar dating and geochemical characteristics of Qinglongtai diabase from the Liaohe depression, North China: finishing time restriction on thinning process of the eastern North China's lithosphere

Ar-Ar dating for whole rock from the Qinglongtai diabase returns a plateau age of 35.5±1.7 Ma, which is in good accordance with their isochronal age within the uncertainty. It shows that the diabase has intruded in the Paleogene Shahejie period. Geochemically, these diabases have low SiO2, high Na2O+K2O and TiO2 contents, with intensive differentiation between light REE and height REE, slight Eu positive anomaly, and generally enrich in large iron lithophile elements (LILE, such as Sr and Ba) and high field strength elements (HFSE, such Nb and Ta), and can be comparable to those typical ocean island basalts (OIB). Previous researches on basalts and lithospheric evolution of the eastern North China Craton (NCC) have indicated that basalts during late Cretaceous and Paleogene were dominated by tholeiites and derived by high degrees of partial melting of a spinel lherzolite at low pressure, while the Neogene basalts were dominated by alkaline basalt derived from low degrees of partial melting of a garnet lherzolite at high pressure. High LaCN/YbCN and Ce/Y ratios with positive anomalies of Sr, Ba and Eu of the Qinglongtai diabase imply that they could be derived from low partial melting of transitional zone between spinel lherzolite and garnet lherzolite. Consequently, the intruding age of the Qinglongtai diabase from the Liaohe depression could be seem to the exact time for lithosphere thickness transposition of the eastern NCC from thinning to thickening.