大兴安岭北段三根河林场新民组流纹岩锆石U-Pb定年及地球化学特征

ZIRCON U-Pb DATING AND GEOCHEMISTRY OF THE RHYOLITES FROM XINMIN FORMATION IN SANGENHE FOREST FARM,NORTHERN DAXINGANLING MOUNTAINS

三根河林场新民组火山岩主要由流纹岩、流纹质熔结凝灰岩组成,U-Pb定年结果表明流纹岩形成于165.71±0.83 Ma的中侏罗世.岩石地球化学分析结果显示,SiO2含量71.05%~76.77%,Na2O+K2O含量6.66%~8.49%,铝饱和指数(A/CNK)1.02~1.17,相对富硅、碱、过铝质,稀土元素总量中等,轻、重稀土元素分馏明显,具有较强的Eu负异常(δEu=0.39~0.66)和Dy负异常,大离子亲石元素Ba、Sr相对亏损,高场强元素Th、U相对富集,Nb、Dy、Ti相对亏损,岩石具有A型流纹岩的特征.岩石地球化学研究表明,岩浆来源于地壳物质的部分熔融作用.空间上,新民组沿大兴安岭火山岩带中北段均有分布,显示大兴安岭火山岩带于中侏罗世处于张性构造环境.

The volcanic rocks of Xinmin Formation in Sangenhe forest farm are mainly composed of rhyolites and rhyolitic ignimbrites.The U-Pb dating results indicate the rhyolites were formed in Middel Jurassic,with the age of 165.71±0.83 Ma.The petrogeochemical analysis shows that the rhyolites are characterized by relatively high silicon,alkali and peraluminous,with the SiO2 content of 71.05%-76.77%,Na2O+K2O of 6.66%-8.49%and A/CNK of 1.02-1.17.The rocks present medium total REE content,significant fractionation of HREEs and LREEs,strong negative Eu anomaly(δEu=0.39-0.66)and negative Dy anomaly,relative depletion of LILEs(Ba and Sr),enrichment of HSFEs(Th and U)and relative depletion of Nb,Dy and Ti,with the characteristics of A-type rhyolites.The study shows the magma is derived from partial melting of crustal materials.The Xinmin Formation is spatially distributed along the middle-north section of Daxinganling volcanic belt,revealing the volcanic belt was in a tensional tectonic environment in Middle Jurassic.