Discovery of Early Cretaceous Antuoling Alkaline Basalt in North Taihang Mountains,and Its Geologic Implications

Antuoling basalt occurred as basaltic dyke with 1.0 ~ 2.5m in depth,and was formed in early Cretaceous(122.31Ma)by K-Ar whole rock dating.Peridotite xenolith,assemblage of carbonate and megacrysts of anorthoclase,corundum and phlogopite were found in Antuoling basalt.Antuoling basalt with 46.50 ~ 50.20% in SiO2 content has suffered slightly alternation of carbonatization,chloritization and serpentinization of olivine,and chloritization and carbonatization of clinopyroxene.The compositional points of Antuoling basalt in Zr/TiO2-Nb/ Y diagram fall into alkaline basalt field.The depleting in Nb,Ta and Ti is the major characters of Antuoling basalt in trace elements.In smoothed REE patterns,highly enriched LREE and depleted HREE with(La/Yb)PM=33.4~ 40.1.The valuse of 87Sr/86Sr for two basaltic samples in Antuoling are 0.711300 and 0.706233,and 143Nd/144Nd values are 0.511848 and 0.511897 with negative eNd(122Ma)values(13.73 and 12.76,respectively).Compositional change of Antuoling basaltic magma is controlled mainly by mantle source compositions and fraction of partial melting,rather than obvious crystallization fraction and crustal assimilation during basaltic magma ascending.According to mantle melting column model,the melting column locates from 82.5km to 75.3km in depth,which means that the lithosphere depth under Antuoling area was 75.5km when Antuoling basaltic magma erupted in 122.31Ma.So Antuoling basalt is production of lithosphere delamination of North Taihang Mountains in Mesozoic.On the basis of integrated comparison of melting column position in depth for Antuoling,Nandaling and Nankou basaltic magma,lithosphere depth under Antuoling-Nandaling-Nankou area in north Taihang Mountains undergoes the process of little size thinning(J2)-thickening(J2-K1)-delamination(K1)-nearly stable(K1-today)since the lithosphere thinning took place in Mesozoic.Antuoling porphyric Mo deposit was formed as a result of Antuoling lithosphere delamination in Mesozoic.