LA-ICP-MS锆石U-Pb同位素定年:对闽北地区稻香组形成时代的制约

闽北地区地区基岩的露头连续较差,但其独特的岩石组合及变质变形特征表明稻香组确实存在,稻香组在前人的研究工作中一直作为寒武纪浅变质岩系,曾测得其Sm-Nd模式年龄值为(500±35) Ma,但是由于测试方法的局限性,这一形成年龄的可靠性值得商榷。激光剥蚀-电感耦合等离子体质谱(LA-ICP-MS)U-Pb同位素年代学测试技术能为研究地层的形成时代提供最直接、精确的证据。本次研究对稻香组气孔状玄武岩和硅质岩中的锆石进行LA-ICP-MS U-Pb同位素年代学测试,为研究稻香组的层位归属提供了精确的地质年代学证据。结果表明,稻香组中气孔状玄武岩最新的一组年龄为(111.4±2.0) Ma,表明其形成于晚中生代早白垩世,这一年龄应代表稻香组形成年龄的下限,为稻香组形成的最早年龄提供了约束,这一结果也与硅质岩中最小一组年龄124 Ma相一致(硅质岩中最新的一组年龄代表了其形成年龄的下限)。气孔状玄武岩和硅质岩的年龄表明稻香组很可能包括了元古代-古生代-中生代的物质信息,需要进一步将其解体,至少形成于中生代的气孔状玄武岩和硅质岩应单独划分出来。稻香组年龄的获得丰富了闽西北地区前寒武纪基底演化资料,为研究该区区域构造演化提供了新的年龄支持。

LA-ICP-MS Zircon U-Pb Isotopic Dating: Constraints on the Daoxiang Formation Age in North Fujian Province

Although there is poor outcrop in the area, the Daoxiang formation exposed in North Fujian has unique rock assemblage and metamorphism, and deformation characteristics show that the Daoxiang group does exist, and belongs to metamorphic rock of the Cambrian period in previous research with Sm-Nd model age as (500±35) Ma. However, the limitations of the test method, adversly influence the the reliability of the formation age. LA-ICP-MS U-Pb dating provides reliable and precise evidence of the formation age of the rocks. Zircon LA-ICP-MS U-Pb data of basalt and siliceous rocks carried out by this group show that the newest age of basalt is (111.4±2.0) Ma, i.e. belonging to Early Cretaceous, which represents the lower limit of the formation age. The result is consistent with that of siliceous rocks, the newest of which is 124 Ma. The inherited zircon ages indicate proterozoic-paleozoic-mesozoic materials may be included in the source region, so the Daoxiang formation should be further disintegrated, the Vesicular basalt and Siliceous rocks which formed at Mesozoic should be separated from the Daoxiang formation. The discovery of all these ages enriches research work of tectonic evolution of Daoxiang formation and provides valuable data of the isotopic geochronological age.