雷州半岛第四纪火山岩激光40Ar/39Ar等时线定年研究

Abstract

雷州半岛是我国新生代火山岩最重要的分布地区之一,火山活动主要集中在中晚更新世。前人对雷州火山岩的年代学研究以K-Ar法为主。研究表明,雷州火山岩测年结果大致分布在0.38～3.04Ma范围内。根据地层和火山岩层的叠置关系,雷州第四纪火山岩由于覆盖在被确定是1.87Ma和0.76Ma沉积的地层之上,故火山岩年龄应小于该地层年龄。K-Ar法定年结果与雷州地区地层叠置关系存在矛盾。本文通过对雷州半岛第四纪火山岩进行野外考察及采样,利用激光40Ar/39Ar年代学方法进行了精细定年。结果表明,雷州火山岩的喷发主要集中18万年前后。定年结果还表明,对于年轻样品,基于尼尔值计算的K-Ar年龄及40Ar/39Ar表观年龄偏老,等时线年龄相对较为可靠。对同一样品的斑晶、基质作斑晶-基质等时线计算,只有在斑晶基质满足同源条件时才有意义。本文首次提出,通过对比未照射样品的初始36Ar/38Ar值的均一性,以检验样品是否同源,确认斑晶-基质等时线年龄的可信度。据此,等时线的处理方法可以推广应用于特定区域内全部同源同时样品。

Leizhou area is one of the most important distribution areas of Cenozoic volcanic rocks. Volcanic activities in this area concentrate in mid to late Pleistocene. Researchers dated Leizhou volcanic rocks mainly by K-Ar method and the results indicate the isotopic ages vary from 0. 38Ma to 3. 04Ma. However, according to the stratigraphic sequence, Leizhou Quaternary volcanic rocks overlie the layers of 1. 87Ma and 0. 76Ma. Therefore, the ages of volcanic rocks must be younger than the underlying layers, which indicates the error of ages dated by K-Ar method. This paper dates samples taken from Leizhou area by laser 40Ar/39 Ar dating method. The results show that ages mainly center on 0. 18Ma, indicating the eruptions occurred approximately 180000a ago. We can also infer that, to young samples, the isotopic ages dated by K-Ar method and apparent ages dated by 40Ar/39 Armethod are all older than the real ages. By comparison, isochron ages by Ar/Ar method are more reliable. Furthermore, reliable ages calculated with phenocryst and matrix from the same sample (phenocryst-matrix isochron) must base on the fact that the phenocryst and matrix are congenetic. To determine whether they are congenetic or not, this paper proposes as the first time to compare the uniformity of initial Ar/Ar value of samples before irradiated by neutron, thus testing the reliability of phenocryst-matrix isochron. Based on that, isochron dating method could be extended to all the other datings on congenetic and isochronic samples from the same district.