Geochronology of the Dahongshan Group

The Dahongshan Group is divided into five formations from the bottom to the top: the Laochanghe For-mation; the Manganghe Formation; the Hongshan Fomation, the Feiweihe Formation and the Potou For-mation. As can be seen from the U-Pb concordia plot for zircon fractions collected from the metamorphicsodic lava of the Hongshan Fomation, the upper intersect age with the concordia is 1665.55Ma.Sm-Nd dating of four whole-rock samples and one hornblende from the Manganghe and Hongshan foma-tions yielded an isochron age of1657±82 Ma. This result is in good agreement with the zircon U-Pb ageand thus can represent the formation age of the Dahongshan Group. The initialNd/Nd ratio is esti-mated at 0.510646, with End (T)=+3.1±1.8. In addition the Dahongshan Group rocks are character-ized by low REE contents, with δEu>1. All this goes to show that the original rocks of the DahongshanGroup were derived from a depleted mantle. The calculated Sm-Nd model ages range from~1900 to~2000 Ma, representing the time of crust/mantle differentiation in the area studied. From the above results, in conjunction with the time-scale scheme for China, the Dahongshan Groupshould be assigned to the Middle Proterozoic.