Geochronology of the Dahongshan Group

Abstract

The Dahongshan Group is divided into five formations from the bottom to the top: the Laochanghe Formation; the Manganghe Formation; the Hongshan Formation, the Feiweihe Formation and the Potou Formation. As can be seen from 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 ^+13.56_-10.86 Ma. Sm-Nd dating of four whole-rock samples and one hornblende from the Manganghe and Hongshan formations yielded an isochron age of 1657 ± 82 Ma. This result is in good agreement with the zircon U-Pb age and thus can represent the formation age of the Dahongshan Group. The initial¹⁴³Nd/¹⁴⁴Nd ratio is estimated at 0.510646. with ENd (T) = + 3.1 ± 1.8. In addition the Dahongshan Group rocks are characterized 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 ∼ 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 Group should be assigned to the Middle Proterozoic.