桂东南云开地区回龙岩体的年代学、地球化学特征及其地质意义

Geochronological,Geochemical and Geological Significance of Huilong Gabbroic Pluton in Southeastern Guangxi

华南地区早中生代的构造属性是地学界研究争论的焦点之一,主要有洋陆俯冲/碰撞与陆内构造演化两种观点;以往的研究主要以酸性花岗质岩石为主,导致其构造背景具有一定的多解性。为进一步探讨桂东南早中生代时期大地构造背景和深部壳幔作用过程,对桂东南云开地区回龙岩体角闪辉长岩进行了年代学、地球化学以及Sr-Nd同位素研究。桂东南回龙岩体角闪辉长岩的LA-ICP-MS锆石U-Pb年龄为(245.8±1.7)Ma,表明岩体形成于早三叠世;样品具较低的SiO2(47.42%~48.80%)、MgO(2.24%~2.62%)和较高的TFe2O3(18.24%~21.49%)、P2O5(0.38%~0.88%)和TiO2(1.66%~2.04%)含量;富集大离子亲石元素(LILE,如Ba、Rb和U),亏损Nb-Ta、Zr-Hf、Ti等高场强元素,(La/Yb)N=3.85~5.99,具明显正铕异常(δEu=1.04~1.48)。(87Sr/86Sr)i=0.717658~0.718405,εNd(t)=-10.30^-10.71,显示EM II型富集地幔的Sr-Nd同位素特征,表明云开地区存在受古老俯冲事件影响所形成的富集地幔源区。结合前人研究成果,认为回龙岩体是在古太平洋板块俯冲背景下的弧后伸展环境中由古老俯冲事件形成的富集地幔源区部分熔融的产物,且岩浆经历了镁铁质矿物的结晶分异作用。

Early Mesozoic tectonics of South China has been disputed for decades.There are two prevailing view points for tectonic attributes:oceanic-continental subduction/collision or intracontinental tectonics.Previous studies mainly focus on the granitic rocks,causing multiple interpretations on its tectonic setting.The LA-ICP-MS zircon U-Pb age of Huilong gabbros is(245.8±1.7)Ma,showing that Huilong pluton was formed in the Early Triassic.The Huilong gabbros have relatively low contents of SiO2(47.42%-48.80%)and MgO(2.24%-2.62%),and relatively high contents of TFe2O3(18.24%-21.49%),P2O5(0.38%-0.88%)and TiO2(1.66%-2.04%).The rocks are enriched in large ion lithophile elements(LILEs,such as Ba,Rb,and U)and depleted in high field strength elements(HFSEs,such as Nb,Ta,Zr,Hf,and Ti,(La/Yb)N=3.85-5.99),showing a significant positive Eu anomaly(δEu=1.04-1.48).Sr-Nd isotope characteristics,including(87Sr/86Sr)i=0.717,658-0.718,405 andεNd(t)=-10.30 to-10.71,demonstrate affinity to the EM II enriched mantle,and imply an enriched mantle source formed under the influence of ancient subduction events at Yunkai.Based on previous studies,the Huilong pluton was probably generated from the partial melting of an enriched mantle source under the backarc spreading of paleo-Pacific subduction beneath South China,with fractionation of mafic minerals.