青海南祁连巴龙贡噶尔组变火山岩LA-ICP-MS锆石U-Pb年龄及其地质意义

摘要

The Balonggongge’er Formation is widely distributed in the South Qilian Mountain. Due to the absence of the fossils and available geochronologic data, its geological epoch has aroused much attention. Based on 1∶50000 regional geological survey, the au⁃thors identified some volcanic flows, which consist of dacites within the Balonggongge ’er Formation. LA-ICP-MS zircon U-Pb analyses indicate that a dacite sample was formed at 457.6±2.4Ma, belonging to the Late Ordovician period. This is the first precise ra⁃diometric age for the Balonggongge’er Formation, which provides some persuasive geochronologic evidence for the disintegration and the epoch of the formation. The new discoveries and results are of importance to the Early Paleozoic stratigraphic sequence and tectonic evolution in the South Qilian Mountain.%广泛分布于南祁连的巴龙贡噶尔组一直缺少精确的化石和同位素测年依据，其地质时代备受关注。1∶5万区域地质调查在青海省天峻县织合玛乡南部的该套地层中新发现了英安质火山熔岩。采用LA-ICP-MS锆石U-Pb定年方法，获得1件英安岩样品的锆石U-Pb年龄为457.6±2.4Ma，表明该套火山岩形成于晚奥陶世。这是首次获得的有关巴龙贡噶尔组的高精度测年数据，为该组的解体及其时代归属提供了可靠的同位素年代学依据。这一新发现和成果对于研究南祁连的早古生代地层层序及构造演化具有重要意义。