四川盆地东部海相下—中三叠统界线的锶同位素年龄标定

中国西南地区海相下—中三叠统以火山碎屑岩为其直观界线,该年龄的标定可为区域地层年表和国际地层委员会全球标准年表研究提供重要的下—中三叠统界线年龄参考值.基于此,本文测试了四川盆地东部华蓥山下—中三叠统界线火山碎屑岩附近的石膏和硬石膏样品87Sr/86Sr比值,以及相应的Si、Ca、Mg、Mn、Sr以及SO42-的含量,根据锶同位素地层学的原理,尝试对下—中三叠统界线进行了年龄标定;同时评估了样品的成岩蚀变性、对海水信息的代表性以及年龄标定结果的可靠性.火山碎屑岩附近5个样品的87Sr/86Sr比值变化在0.708243~0.708391之间,标定的年龄值在240.5~242.5Ma的范围内,2个紧靠火山碎屑岩样品的年龄值在242Ma左右(分别为241.9Ma和242.5Ma),从而为华南地区以火山碎屑岩作为海相下—中三叠统界线的合理性提供了重要的化学地层学依据,也为全球下—中三叠统界线提供了一个新的年龄值.

Age Calibration for the Boundary between Lower and Middle Triassic by Strontium Isotope Stratigraphy in Eastern Sichuan

In Southern China, Geologists normally take the volcanic ash bed (with the thickness from 10 to 50 cm) as the boundary for marine Lower and Middle Triassic (i.e. the boundary between Anisian and Olenekian). The volcanic ash is just interbeded in evaporates (gypsum and anhydrite) of the Jialingjiang Formation (below the volcanic ash) and the Leikoupo Formation (above the volcanic ash), and has been alternated to illite clay stone now in Sichuan Province and Chongqing Municipality. In order to determine the age of the volcanic ash bed (also for the boundary between the Jialingjiang Formation and the Leikoupo Formation), we measured the ~ 87 Sr/~ 86 Sr ratios of marine sulfate samples near the boundary collected from Eastern Sichuan. The XRD analysis results, Sr, Mn, Ca, Mg, Si and SO~~ 2- __4 concentrations indicate that the sulfate generally contains information on the original seawater strontium isotope composition. According to the principle of strontium isotope stratigraphy, we attempt to calibrate the ages of the boundary between the Jialingjiang Formation and the Leikoupo Formation. The ~ 87 Sr/~ 86 Sr ratios of the samples with the thickness range 50cm above and 30 cm below the volcanic ash are from 0.708243 to 0.708312, and the ages calibrated are from 240.8Ma to 242.5 Ma. The erupting time of the volcano is about 242.5Ma by SIS age of top and foot samples. The results of age calibration shows that the volcanic ash as the boundary between Lower and Middle Triassic is reasonable. A new age for the boundary between Lower and Middle Triassic has been provided in this paper.