The Dahongshan Group is divided into five formations from the bottom to the top:the Laochanghe For-mation;the Manganghe Formation;the Hongshan Formation ,the Feiweihe Formation and the Potou For-mation.As can be seen ...The Dahongshan Group is divided into five formations from the bottom to the top:the Laochanghe For-mation;the Manganghe Formation;the Hongshan Formation ,the Feiweihe Formation and the Potou For-mation.As can be seen form the U-Pb Concordia plot for zircon fractions collected from the metamorphic sodic lava of the Hongshan Formation,the upper intersect age with the Concordia is 1665.55-10.86^+13.56Ma.Sm-Nd dating of four whole-rock samples and one hornblende from the Manganghe and Hongshan forma-tions yielded an isochron age of 1657±82Ma .This result is in good agreement with the zircon U-Pb age and thus can represent the formation age of the Dahongshan Group.The initial^143Nd/^144Nd 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 Dahongshan Group were derived from a depleted mantle.The calculated Sm-Nd model ages range from-1900 to -2000Ma,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 Group should be assigned to the Middle Proterozoic.展开更多
文摘The Dahongshan Group is divided into five formations from the bottom to the top:the Laochanghe For-mation;the Manganghe Formation;the Hongshan Formation ,the Feiweihe Formation and the Potou For-mation.As can be seen form the U-Pb Concordia plot for zircon fractions collected from the metamorphic sodic lava of the Hongshan Formation,the upper intersect age with the Concordia is 1665.55-10.86^+13.56Ma.Sm-Nd dating of four whole-rock samples and one hornblende from the Manganghe and Hongshan forma-tions yielded an isochron age of 1657±82Ma .This result is in good agreement with the zircon U-Pb age and thus can represent the formation age of the Dahongshan Group.The initial^143Nd/^144Nd 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 Dahongshan Group were derived from a depleted mantle.The calculated Sm-Nd model ages range from-1900 to -2000Ma,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 Group should be assigned to the Middle Proterozoic.
