Isotopic geochronology of the Xianghongdian alkaline complex, northern margin of the Dabie Mountains, China

To the north of the Qinling-Dabie orogen there exists a Mesozoic (K-1) gran ite-syenite belt called the Beihuaiyang granite-syenite belt, in which the Xianghongdian alkaline complex outcrops in the Devonian Foziling schist and also intruded into the slightly earlier volcanic rocks. This alkaline comp lex is composed of syenite, alkaline syenite and nepheline syemite, but the nepheline syenite occurs inside the alkaline syenite in the pod-like, irre gular, lenticular or layered shape. Researches on the petrogenesis of the a lkaline complex and the relationship between the alkaline syenite and the n epheline syenite must rely on precise isotopic ages. Previous researches ha ve reported isotopic dating results merely by the Ar-40/Ar-39 method on hor nblende and biotite separated from the alkaline syenite and nepheline syeni te. In view of this, the authors have made fairly detailed isotopic geochro nological studies of the alkaline syenite and nepheline syenite by using Rb -Sr isochron for the whole rock and minerals, U-Pb for single zircon separa te and the Ar-40/Ar-39 method for hornblende and potash feldspar. The resul ts of this research are completely different from those of previous researc hes and show that the nepheline syenite was formed far earlier than the alk aline syenite.