桂东南云开地区南渡钾玄质侵入岩的年代学、地球化学及其地质意义

Chronology,geochemistry and its geological significance of Nandu shoshonitic pluton in Yunkai area,southeastern Guangxi

为约束桂东南地区燕山早期的构造环境,对南渡杂岩体开展岩相学、岩石地球化学和LA-ICP-MS锆石U-Pb年代学研究。南渡杂岩体由谭福、六廪、新村和石河4个岩性单元构成,主要岩石类型为石英二长岩、二长闪长岩、正长岩、石英二长斑岩、角闪正长斑岩等。2件样品的LA-ICP-MS锆石U-Pb年龄分别为155.2±1.7 Ma和155.9±1.8 Ma,表明南渡杂岩体形成于燕山早期的晚侏罗世。南渡杂岩体的常量元素具有高硅铝、低铁镁和富碱的特征;里特曼指数为3.56~6.77,全碱(Alk)含量6.33%~11.59%,K_(2)O/Na_(2)O值为1.02~1.63。南渡杂岩体呈现轻稀土富集的右倾型球粒陨石标准化配分模式,轻微负Eu异常;富集大离子亲石元素、Zr和Hf,中等Sr、P和Ti亏损,无明显Nb、Ta异常。在Harker图解上,TiO_(2)、^(T)Fe_(2)O_(3)、MgO、CaO、MnO和P_(2)O_(5)均与SiO_(2)呈良好的负相关性,而Al_(2)O_(3)和Na_(2)O与SiO_(2)无明显相关关系;结合Sr、P和Ti的亏损,以及无负Eu异常的特征,暗示可能存在磷灰石、钛铁矿和单斜辉石的分离结晶作用,而无斜长石的分离结晶作用。南渡杂岩体的岩相学和岩石地球化学特征表明,其为典型的板内钾玄质岩石,形成于板内伸展构造环境,可能与华南大陆内部燕山早期的软流圈地幔物质上涌,岩石圈伸展和裂谷作用有关。

In order to constrain the tectonic enviromment of Early Yanshan in southeastern Guangxi,the petrog-raphy,petrogeochemistry and LA-ICP-MS zircon U-Pb geochronology of Nandu pluton were studied.The Nandu pluton is composed of four lithologic units,including Tanfu,Liulin,Xincun and Shihe,of which main rock types are quartz monzonite,monzonite,syenite,quartz monzonite porphyry and hornblende syenite porphy-ry and so on.The LA-ICP-MS zircon U-Pb ages of two samples are 155.2±1.7 Ma and 155.9±1.8 Ma,re-spectively,indicating that the Nandu pluton was formed in the Late Jurassic of Early Yanshanian period.The major elements of Nandu pluton are characterized by high silicon and aluminum,low iron and magnesium,rich alkali,with Rietmann index ranging from 3.56 to 6.77,the total alkali content ranging from 6.33%to 11.59%,and K2O/Na_(2)O value ranging from 1.02 to 1.63.The Nandu pluton has a right-leaning chondrite standardized distribution pattern which is enriched in light rare earth elements and the negative Eu anomaly is not obvious.It is enriched in large ion lithophile elements,Zr and Hf,with moderate depletion of Sr,P and Ti,and no obvious Nb and Ta anomalies.In the Harker diagrams,TiO_(2),TFe2O_(3),MgO,CaO,MnO and P2O_(5) are negatively correlated with SiO_(2),while Al_(2)O_(3) and Na_(2)O are not significantly correlated with SiO_(2).Combined with the depletion of Sr,P and Ti,and no negative Eu anomaly,it is suggested that there might be separation crys-tallization of apatite,ilmenite and clinopyroxene,but no fractional crystallization of plagioclase.The petrograph-ic and geochemical characteristics of Nandu pluton indicate that it is the typical intraplate shoshonite rocks that formed in an intraplate extensional tectonic environment,and maybe it is related to the asthenosphere of mantle upwelling,lithospheric extension and rifting in the Early Yanshan period in South China continent.