广西大瑶山朴全岩体形成时代、成因及地质意义

Geochronology,Petrogenesis and Geological Significance of the Puquan Granite Pluton in Dayaoshan Area,Guangxi

广西大瑶山隆起区广泛分布加里东期岩浆岩,相关研究对于认识大瑶山隆起区的地球动力学背景、演化机制及找矿工作具有指示意义。对大瑶山隆起区西北部的朴全岩体进行详细的年代学、元素与Hf同位素地球化学研究。LA-ICP-MS锆石U-Pb定年结果显示,朴全岩体的成岩年龄为(441.1±1.2)Ma(MSWD=0.14),属晚奥陶世。朴全岩体为I型中-细粒花岗岩,具高硅(SiO2含量为68.50%~72.83%)、中铝(Al2O3含量为13.66%~14.42%)和中碱(K2O/Na2O比值为0.75~1.33,全碱含量为7.17%~8.34%)等特征,属高钾钙碱性系列,A/CNK值为0.89~1.18,整体属于准铝质-弱过铝质花岗岩。岩体富集Rb、Th、U、Pb等元素,亏损Ba、Sr、Nb、Ti等元素。稀土总量为97.41×10^-6~178.80×10^-6,具有强烈的负Eu异常(δEu=0.12~0.14)特征。该岩体的锆石εHf(t)值大多数为负值,整体变化于-14.92~2.75之间,平均值为-2.67。在εHf(t)-t图解和(^176Lu/^177Hf)-t图解上,样品值均落在球粒陨石演化线之下,两阶段模式年龄tDM2=1.2~2.4 Ga,平均值1.6 Ga,指示其可能来源于中元古代地壳物质的部分熔融,并受到了幔源组分的影响,表明朴全岩体形成于扬子陆块与华夏陆块碰撞挤压的构造背景下。

Caledonian magmatic rocks are widespread in the Dayaoshan uplift zone in Guangxi Province,and research on them has instructional significance for understanding the geodynamic setting,evolution mechanism and prospecting work of Dayaoshan uplift.This paper present the first detailed geochronological,elemental and Hf isotope study on the Puquan intrusion in the northwestern uplift zone.LA-ICP-MS zircon U-Pb dating on the Puquan intrusion yielded Late Ordovician age of(441.1±1.2)Ma(MSWD=0.14).The intrusion comprises medium-fine-grained I-type granite,and is characterized by high SiO2(68.50%-72.83%)and medium Al2O3(13.66%-14.42%)contents and medium alkalinity(total alkali=7.17%-8.34%,K2O/Na2O=0.75-1.33)with A/CNK=0.89-1.18,suggesting its metaluminous to weakly peraluminous nature.The rocks are enriched in LILEs(such as Rb,Th,U,and Pb)and depleted in Ba,Sr,Nb,and Ti.The total REE contents are 97.41×10^-6-178.80×10^-6,and the rocks have distinctly negative Eu anomalies(δEu=0.12-0.14).The magmatic zirconεHf(t)values are mostly negative(-14.92 to 2.75),with an average of-2.67.In theεHf(t)vs.age and the(^176Lu/^177Hf)vs.age diagrams,the samples fall below the chondrite evolution line,and the two-stage model ages(tDM2)are of 1.2 Ga to 2.4 Ga(average 1.6 Ga),indicating that the rocks may have derived from partial melting of the Mesoproterozoic crustal material with possible mantle-derived components.This shows that the Puquan intrusion was formed under syn-collisional compression between the Yangtze and Cathaysia Plate.