内蒙古敖尔盖铜矿中-酸性侵入岩体SHRIMP锆石U-Pb定年与岩石地球化学特征研究

SHRIMP U-Pb zircon dating and petrogeochemistral characteristics of the intermediate-acid intrusive rocks in the Aoergai copper deposit of Inner Mongolia

敖尔盖铜矿位于内蒙古巴林右旗幸福之路苏木,大地构造归属大兴安岭南段晚古生代增生造山带。本次研究所采样品为矿区大面积出露的中-酸性侵入岩体。锆石SHRIMP U-Pb定年结果表明,岩体的成岩年龄为(245.4±1.8)Ma(MSWD=1.4),为该区印支早期岩浆活动的产物。岩石地球化学特征显示敖尔盖中-酸性侵入岩体具有与埃达克岩类似的地球化学特征,其SiO2≥56%(65.03%-70.11%),Al2O3≥15%(14.45%-15.49%),MgO<3%(1.18%-1.61%),Y<18×10-6(5.85×10-6-6.83×10-6),Yb≤1.9×10-6(0.416×10-6-0.637×10-6),Sr>400×10-6(380×10-6-642×10-6),LREE/HREE=11.60-12.24,δEu=1.00-1.18。说明其来源较深,形成压力较大。Sr-Nd同位素结果表明岩体的εNd(t)值(+3.9-+4.4)较高,同时(87Sr/86Sr)i值(0.70380-0.70412)较低,且Nd同位素模式年龄(TDM)较年轻,为578-657 Ma,指示源岩物质可能来源于新元古代形成的亏损地幔。结合区域构造背景综合分析认为,古亚洲洋在本区消失后,该岩体是在挤压构造背景下,由新元古代亏损地幔部分熔融形成。同时通过对比区域岩浆活动并结合大地构造研究认为,大兴安岭南段印支期岩体随着年龄由老到新,其形成构造环境从挤压转为伸展。

The Aoergai copper deposit is located in Xingfuzhilusumu area of Bairin Right Banner in Inner Mongolia,and its tectonic setting is Late Paleozoic accretion orogenic belt of southern Da Hinggan Mountains.The samples collected in this study were intermediate-acid intrusive rocks.Zircon SHRIMP U-Pb dating results show that the granodiorites were formed at 245.4±1.8 Ma（MSWD=1.4）,being the product of magmatism in early Indosinian period.The granodiorites in the Aoergai copper deposit have similar petrogeochemistral characteristics to adakite.The granodiorites yielded the following data： SiO2≥56 wt.%（65.03-70.11 wt.%）,Al2O3≥15 wt.%（14.45-15.49 wt.%）,MgO3 wt.%（1.18-1.61 wt.%）,Y18μg/g（5.85-6.83μg/g）,Yb≤1.9 μg/g（0.416-0.637μg/g）,Sr400μg/g（380-642 ug/g）,LREE/HREE=11.60-12.24,and δEu=1.00-1.18,indicating that they were formed at a great depth and under strong stress.The granodiorites have high εNd（t） values（＋3.9-＋4.4）,low（87Sr/86Sr）i values（0.70380-0.70412）,and young Nd model ages（578-657Ma）,suggesting that the magma might have been derived from a depleted mantle source in Neoproterozoic.Based on regional tectonic settings,the authors consider that the granodiorites in the Aoergai copper deposit were most likely derived from partial melting of mafic materials in the depleted mantle in a compression tectonic setting after the disappearance of the Paleo-Asian Ocean.Based on regional analysis,the authors suggest that the tectonic setting of the plutons was an environment of conversion from compression to extension in Indosinian period.