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通安组作为扬子地块西缘最古老的基底地层之一,是前人通过1.8 Ga最大沉积时限的碎屑锆石年龄和第三段已获得辉绿岩1.5 Ga的侵入年龄来限定的;但未见其更精确地层年龄的相关报道。笔者以通安组一段和四段地层中发现的火山岩样品为研究对象,采用LA-ICP-MS方法对其锆石进行原位微区U-Pb测年,获得了通安组一段下部变基性火山岩锆石~(207)Pb/~(206)Pb年龄(1833±2)Ma和四段底部变凝灰岩锆石~(207)Pb/~(206)Pb年龄(1508±15)Ma。该年龄结果表明:(1)通安组下部层位的沉积时代为1.8 Ga,基本限定了通安组的最大沉积时限,与东川群因民组、大红山群底部大致相当;(2)通安组四段底部沉积时代为1.5 Ga,与前人获得的通安组三段及东川群黑山组顶部层位沉积时代大致相当,为同期异相沉积;(3)进一步确定了通安组在地层柱上应位于会理群之下,与东川群、河口群、大红山群为可对比的同期异地异相沉积地层。
As one of the oldest basement strata in the western margin of the Yangtze block, the Tong’an Formation was defined by the age of detrital zircons dating from the maximum depositional age of 1.8 Ga and the age of invasion of 1.5 Ga by diabase. But no more accurate report on the age of the strata. Based on the volcanic rock samples found in the first and fourth strata of Tong’an Formation, the authors used in-situ U-Pb dating of zircons by LA-ICP-MS method to obtain the lower subbase of the Tong’an Formation (207) Pb / 206 Pb ages (1833 ± 2) Ma and the zircon 207 Pb / 206 Pb ages (1508 ± 15 Ma) for the four volcanic rocks. The age results show that: (1) The depositional age of the lower strata of the Tong’an Formation is 1.8 Ga, which basically limits the maximum deposition time of the Tong’an Formation, which is roughly the same as that of the East Sichuan Forint Group and the bottom of the Dahongshan Group; (2) ) The depositional age of the bottom of the fourth member of the Tongan Formation is 1.5 Ga, which is approximately the same as that of the third member of the Tong’an Formation and the top strata of the Heishan Formation of the Dongchuan Group, and is the same-phase deposition in the same period. (3) The safety group should be located below the Huili group in the stratigraphic column, and can be distinguished from the Dongchuan group, estuarine group and Dahongshan group during the same period by different types of heterogeneous sedimentary strata.