四川省石棉县滨东富磁铁矿角闪石岩的地球化学特征及锆石U-Pb年代学研究

Geochemical Characteristics and Zircon U-Pb Isotope Chronology of the Bindong Magnetite-Rich Hornblendite in Shimian County,Sichuan Province

为揭示四川省石棉县滨东磁铁矿床的成因,对出露于该区的镁铁-超镁铁岩体中的角闪石岩进行了岩石学、地球化学及同位素年代学等研究。结果显示,该岩石极端富铁贫硅,低MgO,高Al2 O3和CaO,强烈富集REE(ΣREE为116.65×10^-6~772.99×10^-6),轻重稀土强烈分异,普遍具有明显的正Eu异常(δEu为1.27~1.59),大离子亲石元素Th、U富集,而Rb、Sr、K则相对亏损,高场强元素Ti、Nb、Hf等均有不同程度的亏损,同时还出现Zr-Hf及Nb-Ta元素分馏。锆石U-Pb定年表明,该岩石形成于(260.7±4.3)Ma,与峨眉山大火成岩省的玄武岩的主体喷发时代基本一致。分析认为,滨东富磁铁矿角闪石岩应是大陆边缘裂谷伸展构造背景下地幔柱活动引发亏损地幔部分熔融的幔源岩浆,在低氧逸度条件下多次分异后形成晚阶段超基性富铁熔浆,伴随着温压的降低和氧逸度的升高,于超铁镁岩体下部边缘相与围岩接触带部位熔离结晶成岩成矿的产物。

In order to reveal metallogenesis of the Bindong magnetite deposit in Simian County of Sichuan Province,petrology,geochemistry and isotopic chronology of the hornblendite within the Bindong mafic-ultramafic intrusion in the area have been studied in this paper.The results show that samples of the hornblendite are characterized with extremely rich in Fe but poor in Si,with low MgO contents,high Al2 O3 and CaO contents,extremely enriched REE(ΣREE=116.65×10^-6-772.99×10^-6),strong differentiation of LREE and HREE,obviously positive Eu anomalies(δEu=1.27-1.59),enriched LILE of Th and U,relatively depleted LILE of Rb,Sr and K,and HFSE of Ti,Nb and Hf,and elemental fractionation characteristics of Zr-Hf and Nb-Ta.The zircon U-Pb dating shows that the rock was formed at(260.7±4.3)Ma,corresponding to the main eruption age of Emeishan large igneous province.Based on the above analyses,it is considered that the Bindong magnetite-rich hornblendite in the contact between marginal facies of bottom part of the ultramafic intrusion and the wallrock could be produced by segregation of the late stage ultrabasic Fe-rich melt,which was derived from the several stages evolution,at low oxygen fugacity,of the mantle-derived magma originally from the partial melting of depleted mantle caused by the mantle-plume activity in the continental margin rift extensional tectonic setting,due to the decrease of temperature and pressure and increase of oxygen fugacity.