湘南宜章地区辉绿岩、花岗斑岩、安山岩的形成时代和成因——锆石U-Pb年龄和Hf同位素组成

湘南宜章县长城岭地区广泛出露以辉绿岩、花岗斑岩为主的燕山早期岩体,而宜章平和地区主要分布安山岩体,其年龄和成因有待进一步研究。分别用SHRIMP和LA-ICP-MS法测试了它们的年龄和Hf同位素组成,长城岭辉绿岩的SHRIMP锆石U-Pb年龄为153.7Ma±3.1Ma、227.0Ma±4.2Ma;长城岭花岗斑岩的LA-ICP-MS锆石U-Pb测年结果为153Ma±14Ma和231.58Ma±0.67Ma;平和安山岩的SHRIMP锆石U-Pb测年结果为159Ma±14Ma和229.3Ma±7.6Ma,LA-ICP-MS锆石U-Pb测年结果为161Ma±1Ma,显示为燕山早期岩浆侵位和印支期基性岩浆底侵。锆石Lu-Hf同位素原位分析结果表明,εHf（t）值在正值和负值范围内变化,指示岩浆为壳幔混合来源。结合长城岭花岗岩体中继承锆石的年龄信息,认为花岗质岩石可能来源于中元古代基底的重熔。岩体岩浆很可能是由元古宙火成岩石部分熔融形成的,并伴有年轻或新生幔源物质的加入,岩浆上升侵位的过程中发生了混合作用。

Formation epoch and origin of the Yizhang andesite,diabase and granite-porphyry in Yizhang County,southern Hu′nan Province： Zircon U-Pb age and Hf isotopes

Early Yanshanian（Jurassic） rock masses dominated by andesite,diabase and granite-porphyry are widely exposed in Yizhang County,Hunan Province.However,the formation age and petrogenesis of these rock masses remain controversial.The U-Pb ages and Hf isotopes of zircons have been discussed in this paper by using in situ zircon analysis of the SHRIMP and LA-ICP-MS.Zircon SHRIMP ages for Changchengling diabase are 153.7Ma±3.1Ma and 227.0Ma±4.2Ma;LA-ICP-MS ages for Changchengling granite-porphyry are 153Ma±14Ma and 231.58Ma±0.67Ma.SHRIMP and LA-ICP-MS dating of Pinghe andesite yielded ages of 159Ma±14Ma and 229.3Ma±7.6Ma as well as 161Ma±1Ma,which are consistent with the major formation time of the ＂Nanling-series＂ rock bodies in neighboring areas,suggesting Indo-Chinese underplating of mantle-source mafic magma and emplacement of early Yanshanian magma.The εHf（t） values vary from slightly positive to negative,suggesting that the rock masses were derived at least from two sources instead of being formed by the magmas derived from a single source.Based on several age data of inherited zircons from granites in the neighboring Nanling Range,it can be concluded that the granitites were produced mainly by remelting of the Mesoproterozoic basement rocks.They were probably derived from partial melting of the Proterozoic igneous protoliths with minor addition of the juvenile crust or newly mantle-derived magmas,accompanied by assimilation and fractional crystallization.