摘要
辽西北票市四合屯及周边地区赋存长毛恐龙及早期鸟类化石的地层时代,存在晚侏罗世、早白垩世和晚侏罗世-早白垩世多种观点。同样,同位素年龄数据亦存在很大差异。作者曾报道四合屯含脊椎动物化石层位的地层综合剖面第6层所夹YL31凝灰岩中锆石的U-Pb年龄为125.2±0.9 Ma,并指出四合屯脊椎动物生存时代属早白垩世。本文报道两组 Ar-Ar定年数据,作为这一观点的又一新的证据。YL29玄武安山岩的层位属综合地层剖面第2层下部,它的K-Ar年龄是129.0±2.6 Ma。此样品的Ar-Ar阶段加热实验给出一条平坦的年龄谱线,其坪年龄是128.2±0.8Ma,对应的39Ar析出量达90％以上;15个阶段的总气体年龄为128.4±1.4 Ma;相关性很好的Ar-Ar等时线给出的年龄是128.4±0.2 Ma。YL30辉绿岩侵入到综合地层剖面5-9层,其Ar-Ar年龄谱也呈平坦状,坪年龄是122.3±0.5 Ma,对应的39Ar析出量高达93％以上;12个阶段的总气体年龄为122.1±1.2 Ma;相关性很好的Ar-Ar等时线给出的年龄是121.8±1.3Ma。国际上关于晚侏罗世～早白垩世界线年龄一直存有争议,其观点主要有135Ma和144Ma两种方案,中国年代学工作者倾向性135±5Ma。无论采用哪种方案,四合屯脊椎动物生存时代均属早白垩世。
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 Cretaceous or the Late Jurassic - Early Cretaceous. Even the dating data are also very 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 Yixian Formation in Sihetun and its neighboring area, indicated that the Sihetun vertebrate belonged to the Early Cretaceous. This paper provides new evidence 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 yields a K-Ar age of 129. 0±0. 9 Ma. The Ar-Ar step-heating experiment performed on the YL29 basalt-andesite yields a remarkably simple pattern. Apparent ages from step 6-13, comprising over 90 percent of the release of 39Ar, 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 the subvolcanic rock, which intruded into beds 5-9. The Ar-Ar step-heating experiment on this sample yields a virtually concordant set of ages over 93 percent of the release of 39Ar (step 7-12) with a plateau age of 122. 3±0. 5 Ma. The Ar-Ar total gas age (all 12 steps) on the diabase is 122. 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 plateau age. About the Jurassic-Cretaceous boundary age, there are continuous disputations internationally and both 135 Ma and 144 Ma are proposed for the boundary. The Chinese geochronologists prefer the forme one. According to above Ar-Ar age data of YL29 basalt-andesite and YL30 diabase, the Sihetun vertebrate assemblage should be the Early Cretaceous in age.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2001年第4期663-668,共6页
Acta Petrologica Sinica
基金
国家自然科学基金(编号:49873023)资助项目.
