The Age of the Potassic Alkaline Igneous Rocks along the Ailao Shan–Red River Shear Zone: Implications for the Onset Age of Left‐Lateral Shearing

Abstract

The potassic alkaline igneous rocks along the Ailao Shan–Red River (ASRR) shear zone range in composition from ultramafic to felsic and are characterized by similar trace element patterns and radiogenic isotope ratios. Their distribution, and in some cases their structure and outcrop outline, suggest a genetic relationship to the ASRR shear zone. Zircons separated from 12 potassic alkaline bodies along the ASRR zone have been dated by an in situ U‐Pb method to temporally constrain the left‐lateral movements along the shear zone. The ages of the alkaline bodies range from 34.0 to 36.3 Ma. These new zircon ages, together with previous data, suggest that the onset age of the left‐lateral movements along the ASRR shear zone began at or slightly before 36 Ma and that the movements lasted from ≥36 to 17 Ma. When uncertainties are taken into account, the duration of motion on the ASRR shear zone coincides with the opening of the South China Sea, implying a genetic relationship between these two events.