云开地区早古生代宁潭片麻状花岗质岩体锆石U-Pb定年、岩石成因及构造背景

Zircon U-Pb Dating,Petrogenesis and Tectonic Background of the Early Paleozoic Ningtan Gneissic Granitic Pluton,in the Yunkai Terrane

华南地区广泛发育的早古生代岩体形成的构造背景是近年来的研究焦点.云开地区是华南早古生代构造-岩浆活动最为强烈的地区之一.对本地区宁潭片麻状花岗质岩体进行详细的锆石LA-ICP-MS U-Pb定年和地球化学研究表明,宁潭岩体岩浆锆石发育较为明显的振荡环带,偶见被包裹的继承锆石核;3个样品的LA-ICP-MS锆石U-Pb年龄分别为430±2Ma、426±4Ma、433±2Ma,指示该岩体侵位于早古生代.宁潭岩体富Si、Al,贫Mg、Ca,CIPW标准矿物中出现刚玉分子,属高钾钙碱性系列过铝质花岗岩类.其稀土元素配分曲线呈明显的海鸥型,显示较强负Eu异常,稀土元素分馏程度较低;亏损高场强元素,富集轻稀土元素及大离子亲石元素;全岩(^(87)Sr/^(86)Sr)i值较高(0.711 30~0.718 42),ε_(Nd)(t)值较为均一(-8.6^-8.1),对应的TDM2值为1 827~1 848Ma;岩浆锆石ε_(Hf)(t)值变化范围较大(-10.9^-2.5),对应的TDM2值主要分布于1 600~2 000Ma,峰值明显(~1 800Ma).上述特征表明宁潭岩体属于地壳物质重熔型花岗岩,主要起源于华夏元古宙基底物质的重熔,源区以变质泥岩、变质砂岩为主.宁潭岩体侵位时华南地区陆内造山带处于后造山伸展垮塌阶段,该岩体的侵位与造山后的伸展作用有关.

Numerous researches have focused on the Early Paleozoic magmatic rocks widely developed in South China,because of their ambiguous petrogenesis.We present here the zircon U-Pb dating and geochemical data for the Ningtan gneissic granitic pluton in the Yunkai terrane,which experienced intensive tectonic-magmatic activity during the Early Paleozoic.Cathodoluminescence imaging reveals that these zircon crystals generally have oscillatory zoning and the inherited zircons are exposed.LAICP-MS zircon U-Pb dating of the three samples yielded weighted mean ages of 430±2 Ma,426±4 Ma and 433±2 Ma,respectively indicating that the pluton was emplaced in the Early Paleozoic.The major element geochemistry is characterized by high content of Si,Al,low content of Mg,Ca and the appearance of corundum in CIPW standard mineral.The Ningtan pluton is composed of high-K calc alkaline peraluminous granitoids.The REE distribution curves show the seagull-shape,with strong depletion in Eu anomaly.They exhibit strongly negative high field strength elements and enrichment of light rare earth elements,large ion lithophile elements.The isotope data are characterized by high initial（^87 Sr/^86 Sr）iratio（0.711 30 to 0.718 42）,negative εNd（t）values（-8.6 to -8.1）and negative εHf（t）values（-10.9 to -2.5）.The two stage Nd-isotope model ages（TDM2）are 1 827-1 848 Ma.The two stage Hf-isotope model ages（TDM2）mainly distribute in 1 600-2 000 Ma,with the peak value of -1 800 Ma.These geochemical data indicate that the Ningtan pluton belongs to the crust-derived granite category,originated from the remelting of Cathaysia Proterozoic basement,mainly metamorphic argillaceous rocks and sandstone.The emplacement of the Ningtan pluton,was formed in an extensional background,as a result of the intracontinental orogenic collapse of South China in Early Paleozoic.