滇中昆阳群富良棚组凝灰岩锆石U-Pb年代学及其地质意义

ZIRCON U-Pb CHRONOLOGY AND GEOLOGICAL SIGNIFICANCE OF THE FULIANGPENG FORMATION TUFF IN KUNYANG GROUP,CENTRAL YUNNAN

为约束滇中地区中元古界昆阳群富良棚组的形成时代及其区域地层对比关系,对采自该组上部的玄武质凝灰岩进行了LA-ICP-MS锆石U-Pb同位素和Hf同位素测试分析。其锆石U-Pb同位素年龄为1050.4±5.3Ma(MSWD=0.71)。结合已有年龄数据,揭示富良棚组火山岩的形成时代范围为1050~1030Ma。Hf同位素分析结果显示其εHf(t)值介于-4.9与-5.9之间,加权平均值为-5.4±0.5,对应的二阶段模式年龄(TDM2)为2670~2760Ma,指示其形成自新太古代地壳物质的重熔。综合扬子陆块西南缘中元古代晚期沉积演化序列及岩浆活动记录,认为昆阳群富良棚组可与会理群天宝山组以及苴林群普登组为同期异相产物,并推测昆阳群富良棚组应为罗迪尼亚超大陆聚合时构造体制转换阶段的火山作用产物。

U-Pb and Hf isotopes of zircon to the basaltic tuff collected from the upper part of the Fuliangpeng Formation in central Yunnan area have been analyzed by LA-ICP-MS so as to constrain its formation age and carry out regional stratigraphic correlation of the middle Proterozoic Kunyang Group.U-Pb zircon dating reveals that the formation age of Fuliangpeng Formation tuff is 1050.4±5.3Ma(MSWD=0.71)and the formation age of Fuliangpeng Formation,in consideration of previous work,ranges from 1050to 1030Ma.The analytical results of the Hf isotopes reveals that the zirconεHt(t)values of the tested tuff range from-5.9to-4.9,with a weighted average of-5.4±0.5and two-stage model ages from 2670to 2760Ma respectively,indicating that the tuff is derived from remelting of Neo-Archean crustal materials.Based on the late Mesoproterozoic sedimentary evolution sequence and magmatic activity records,researchers have reached a conclusion that the Fuliangpeng Formation of Kunyang Group,the Tianbaoshan Formation of Huili Group and Puden Formation of Julin Group are the contemporaneous heterophase products.And it is speculated that the Fuliangpeng Formation of Kunyang Group was the volcanic product of the tectonic system transformation stage during the integration of Rodinia supercontinent.