山东济南下寒武统朱砂洞组软沉积变形中燧石的Sm-Nd年龄测定及其意义

在济南南部下寒武统朱砂洞组丁家庄段硅质白云岩中,发育地震引起的硅质触变脉、硅质触变沉陷构造和触变楔等软沉积变形构造。从同一地震触变沉陷构造中,采得一组灰黑色燧石样品。使用英国VG354同位素质谱仪和同位素质谱分析方法,测得每个样品的147Sm/144Nd和143Nd/144Nd同位素值。采用Sm-Nd等时线法,对燧石进行了测年。获得燧石的成岩年龄为530.8±6.1Ma,由于软燧石(硅胶体)成岩至少经历了0.1Ma的时间,所以,地震沉积事件发生在530.7±6.1 Ma,属早寒武世早期。由于测年结果与沧浪铺阶的下界年龄接近,山东朱砂洞组丁家庄段无化石且河南省标准地层剖面中的整个朱砂洞组属沧浪铺阶,这暗示济南地区朱砂洞组丁家庄段属沧浪铺阶。本次测年得到了鲁西地区第1个下寒武统的同位素年龄值,这对确定该地区早寒武世地震沉积事件发生的时间具有参考价值和意义,也为深入研究鲁西地区寒武纪地层提供了新资料。

Sm-Nd age dating and its significance of chert in soft sediment deformation structures from the Lower Cambrian Zhushadong Formation,Jinan of Shandong Province

In southern Jinan area,some soft sediment deformation structures triggered by earthquake developed in siliceous dolostone layers of the Dingjiazhuang Member of the Lower Cambrian Zhushadong Formation,such as siliceous thixotropic veins,siliceous thixotropic subsidence and wedge, etc. A set of gray black cherts were sampled from the same soft-siliceous subsidence structure. Using the VG354 isotopic mass spectrometer(made in Britain)and the method of isotope mass spectrometry, the ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd isotope ratios of each samples were measured. By means of the Sm-Nd isochron,the cherts have been dating. The diagenetic ages of the 530.8±6.1 Ma were obtained for the set of gray black cherts,belonging to the early stage of the Early Cambrian.As the minimum time for a soft flint (colloidal silica) to form a lithified rock is 0.1 Ma, it can be inferred that the seismic sedimentary events occurred at 530.7±6.1 Ma.Since the dating result is close to the lower bound age of the Canglangpuan Stage,and there is exists no fossil in the Dingjiazhuang Member of the Zhushadong Formation of Shandong Province,and the whole Zhushadong Formation belongs to the Canglangpuan Stage in the key profile of the Zhushadong Formation of Henan Province. So it is implied that the Dingjiazhuang Member of the Zhushadong Formation belongs to the Canglangpuan Stage. Through dating,the first isotopic age of the Lower Cambrian of the western Shandong is got,which is possessed of some reference value and significance to define the time of seismic depositional events in the Early Cambrian,and also provides a new datum for the further study of the Cambrian strata in the area.