福建武平地区桃溪群混合岩U-Pb定年及其Hf同位素组成:对桃溪群时代及郁南运动的约束

摘要

桃溪群混合岩位于南岭东段武夷山褶皱带南段.本文通过采自桃溪附近帽村-田园两个代表性花岗片麻岩样品进行LA-ICP-MS锆石U-Pb 测年获得494～496Ma的结晶年龄,641～657Ma和799Ma两组继承锆石的206Pb/238U 表观年龄.同时对样品FJ-137进行的Hf同位素测定表明其εHf(t)变化于-2.3与-18.2之间,其TDM2变化于1.84～2.65Ga之间.结合前人的资料我们推断该样品源区很可能经太古代或古元古代地壳物质的再循环形成晚新元古代花岗质岩石或晚新元古代沉积地层再部分熔融的产物,我们倾向于认为原划属桃溪群的岩石单元更可能是一套杂岩,其形成时代可能在新元古代;我们的资料表明在武夷-云开及其东南地区可能存在奥陶-寒武纪之间 (约490Ma左右)的构造-岩浆事件,该事件对应于以往所认为的郁南运动.%This paper presents LA-ICPMS zircon U-Pb geochronology and Hf isotopic composition for two migmatitic and gneissic samples from the Taoxi Complex at the Aocun-Tianyuan areas (Wuping). Zircons from the two samples ( FJ-137 and F J-138) yield the weighted mean 206 pb/238 U ages of (496 ± 4) Ma ( n = 8 ) and (494 ± 6) Ma ( n = 17), respectively, which are interpreted as the formation age. Some grains give Neoproterozoic ages (641 ～657Ma and 799Ma), which can be considered to be inherited grains. The Hf isotopic compositions for FJ-137 sample range from -2.3 to -18.2 with the Hf model age of 1.84 ～ 2.65Ga. Our results along with other geological data,show that (1) the Mesoproterozoic Taoxi "Group" is most likely to be the Complex; (2) these rocks probably represent the products of the reworked Neoproterozoic-early Paleozoie sedimentary sequences; (3) the 494 ～ 496Ma magmatism in the Wuping area might be correlated with the Yu'nanian orogenic event that were previously discussed in other studies .