桂东北大宁岩体锆石SHRIMP年代学和地球化学研究

Zircon SHRIMP U-Pb dating and geochemical characteristics of Daning batholith in northeast Guangxi

大宁岩体位于扬子板块与华夏板块结合带,岩性组合为二长闪长岩-花岗闪长岩-二长花岗岩-钾长花岗岩。岩体中广泛存在壳幔混合包体。本文利用锆石SHRIMPU-Pb法,获得大宁岩体成岩年龄上限为(419.1±6.4)Ma(MSWD=1.03),属加里东晚期。岩体的地球化学研究显示,大宁岩体属弱过铝高钾钙碱性岩石,富大离子亲石元素和轻稀土,显Nb、Ba、Sr、Eu组合异常。岩石具有较高的ISr值(0.7112～0.7196),较低的εNd(t)值(-6.77～-7.53)和TDM值(1.40～1.71Ga)。结合邻区加里东花岗岩壳幔混合特征,本文认为,大宁岩体为幔源岩浆底侵诱发的下地壳变砂屑岩部分熔融形成,形成过程中有显著的幔源组分加入。

Daning batholith,located in the border area between Yangtze plate and Cathaysia plate,consists mainly of monzodiorite,granodiorite and monzogranite with minor granite.In this paper,zircon SHRIMP U-Pb dating and geochemical characteristics of Daning batholith are reported to discuss the magma source and genesis of this Caledonian intrusion in northern Guangxi of South China.Zircon SHRIMP U-Pb dating of the late stage granite yielded an age of（419.1±6.4）Ma（MSWD=1.03）,which provides the upper constraint of the batholith formation in Late Caledonian.The major element compositions show that the granites are weakly peraluminous and strongly potassic-calc-alkaline.Geochemical characteristics of the granites indicate that this pluton was derived from partial melting of metamorphosed feldspathic clastic lower crust.The granites are commonly enriched with large ion lithophile elements and light rare earth elements with high Rb/Sr and low Nb/Ta ratios,showing behaviors of high Sr and low Nd isotopic combination.However,TDM of the granites are obviously lower than the average value of normal granite in this area,which is consistent with the regional linear distribution of lower TDM of Caledonian granites.Combined with field observation of widely spread mafic microgranular enclaves in the batholith,the authors consider that an intense mantle-crust interaction probably occurred during the anatectic event in Late Caledonian.Mantle constituents were obviously involved in the formation of anatectite.