Further discussion on the geologic age of Sihetun vertebrate assemblage inwestern Liaoning, China: evidence from Ar-Ar dating.

The geologic age of the feathered dinosaurs and early birds in Sihetun and its neighboring area in Beipiao, western Liaoning is appointed to different epoches of the Mesozoic Era, such as the Late Jurassic, the Early Cretaceo us or the Late Jurassic - Early Cretaceous. Even the dating data are also v ery different. Recent report of the U-Pb age of 125.2 +/- 0.9 Ma of zircon separated from tuff in the bed 6 of the composite stratigraphic column of Y ixian Formation in Sihetun and its neighboring area, indicated that the Sih etun vertebrate belonged to the Early Cretaceous. This paper provides new e vidence from Ar-Ar dating. YL29 basalt-andesite was collected from the low part of the bed 2 of the composite stratigraphic column, and the sample yie lds a K-Ar age of 129.0 +/- 0.9 Ma. The Ar-Ar step-heating experiment perfo rmed on the YL29 basalt-andesite yields a remarkably simple pattern. Appare nt ages from step 6 similar to 13, comprising over 90 percent of the releas e of Ar-39, are virtually concordant, and give a plateau age of 128.2 +/- 0 .8 Ma. The Ar-Ar total gas age (all 15 steps) on the sample is 128.4 +/- 1. 4 Ma. When all steps data are plotted on a correlation or isochron diagram, an age of 128.4 +/- 0.2 Ma is obtained. YL30 diabase was collected from th e subvolcanic rock, which intruded into beds 5 similar to9. The Ar-Ar step- heating experiment on this sample yields a virtually concordant set of ages over 93 percent of the release of Ar-39 (step 7 similar to 12) with a plat eau age of 122.3 +/- 0.5 Ma. The Ar-Ar total gas age (all 12 steps) on the diabase is 121.1 +/- 1.2 Ma. The age obtained from the isochron plot using all steps data points is 121.8 +/- 1.3 Ma, indistinguishable from the plate au age. About the Jurassic-Cretaceous boundary age, there are continuous di sputations internationally and both 135 Ma and 144 Ma are proposed for the boundary. The Chinese geochronologists prefer the forme one. According to a bove Ar-Ar age data of YL29 basalt-andesite and YL30 diabase, the Sihetun v ertebrate assemblage should be the Early Cretaceous in age.