扬子地块西缘古—中元古代地层划分对比研究: 来自通安组火山岩锆石U-Pb年龄的证据

提要：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207Pb/206Pb年龄(1833±2) Ma和四段底部变凝灰岩锆石207Pb/206Pb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。

Division and correlation of Paleo-Mesoproterozoic strata on the western margin of Yangtze block: Evidence from the U-Pb age of tuff zircon in the Tongan Formation

Abstract: The Tongan Formation is one of the most ancient basement strata on the western margin of Yangtze block; nevertheless, accurate stratigraphic diagenetic age of this formation has had no related reports yet, with only the intrusion age of ~1.5 Ga from zircon of gabbro-diorite dyke and the oldest deposition time of ~1.8 Ga from detrital zircons. Based on petrographic research and using tuff as the study object, the authors dated the volcanic zircons from meta-basic volcanics tuff of the lower first period and meta-tuff of the bottom fourth period of the Tongan Formation by using LA-ICP-MS zircon situ U-Pb dating method. As a result,, the authors respectively acquired the 207Pb/206Pb age of 1833Ma for tuffaceous basic volcanic rock and 1508±15Ma for meta-volcanic tuff. Some conclusions have been reached: (1) ~1.8Ga represents the depositional age of the lower first period, which defines the oldest deposits of the Tongan Formation, consistent with the Dongchuan Group and Dahongshan Group; (2) the depositional age of the bottom fourth period is ~1.5Ga, in accord with the ages of the third period of the Tongan Formation and the Heishan Formation of the Dongchuan Group, as shown by data obtained by previous researchers; (3) the authors further define the Tongan Formation, which should be under the Huili Group, in accordance with the Dongchuan, Hekou and Dahongshan groups, as a heteropic product.