广东三岗山二长花岗岩体形成时代、地球化学特征及地质意义

Ages and Geochemical Characteristics of the Sangangshan Monzogranite Intrusion in Guangdong Province and Their Geological Significances

为探讨广东三岗山二长花岗岩体的形成年代及地球化学特征,对该岩体进行了LA-ICP-MS锆石U-Pb同位素年龄及主、微量元素和稀土元素分析。结果显示,中粗粒黑云母二长花岗岩和细粒斑状黑云母二长花岗岩的成岩年龄分别为(96.2±1.3)Ma和(99.5±1.2)Ma,为晚白垩世。二者均具有富硅、K>Na、准铝质-弱过铝质、低钙、低镁、较贫铁等特征;二者均明显富集大离子亲石元素(Rb、Th、U),强烈亏损元素Ba、Sr、Ti、P,略有亏损Nb;二者稀土元素总量较低-中等,中粗粒黑云母二长花岗岩稀土总量相对较高,轻重稀土分馏较为明显,稀土配分曲线总体呈较陡的右倾式"V"型,具有中等-弱的负Eu异常(δEu为0.64~0.77)。岩石学与地球化学特征表明,二者均为准铝质-弱过铝质的I型花岗岩,为壳幔混合成因。构造图解反映三岗山岩体形成于火山弧花岗岩环境,推断其与太平洋板块俯冲引起的弧岩浆作用有关,该岩体可能形成于太平洋板块俯冲和后缘拉张导致的伸展构造环境。

In order to explore the chronological and geochemical characteristics of the Sangangshan monzogranite intrusion,the LA-ICP-MS zircon U-Pb isotopic dating and analysis of major and trece elements and REE of the monzogranite have been conducted in this paper.The dating results show that the medium-coarse sized biotite monzogranite and fine porphyritic biotite monzogranite have ages of(96.2±1.3)Ma and(99.5±1.2)Ma,respectively,indicating that they were formed in the late Cretaceous.Their chemical compositions are characterized with rich silicon,K>Na,meta-aluminum to weakly peraluminous,low calcium,low magnesium,and relatively poor iron,.with obvious significant enrichment of large ionophile elements(Rb,Th,U),strong depletion of Ba,Sr,Ti,and P,and slight depletion of Nb.They have relatively low to medium REE contents,with obvious fractionation of LREE and HREE,steeply right-diclined"V"type REE distribution patterns,medium to weak negative Eu anomalies(δEu values of 0.64~0.77).The medium-coarse biotite monogranite has relatively high REE contents.Petrological and geochemical characteristics show that they belong to meta-aluminium to weakly peraluminium I type granite with the crust-mantle mixed origin.The tectonic setting discrimination diagrams show that the Sangangshan intrusion was formed in the volcanic arc granite environment.It is inferred that it’s formation could be related to the arc magmatism caused by subduction of Pacific plate.Therefore,it is believed that the Sangangshan intrusion could be formed in the extensional tectonic environment caused by the Pacific plate subduction and trailing edge tension.