准噶尔盆地西北缘晚石炭系-早二叠系火山岩岩石学和地球化学研究

晚石炭纪-早二叠纪,在准噶尔盆地西北缘,发育区域性的火山岩建造,岩性以酸性英安岩和流纹岩为主,同时发育少量中性玄武安山岩和安山岩,多属于中-低钾的钙碱性系列。样品全碱(Na₂O+K₂O)含量为0.99％ ～8.12％,K₂O的含量较低,为3.01％ ～0.05％,稀土元素总量(∑ REE)较低。稀土元素轻重分异,轻稀土元素(LREE)相对于重稀土元素(HREE)富集:(La/Yb)_N=0.76～5.18,且部分样品显示轻微的Eu负异常和Ce正异常:δEu=0.533～1.148,δCe=0.979～1.224。微量元素中大离子亲石元素(LILE)富集,高场强元素Nb、Ta亏损。分析认为石炭系末期-二叠系早期,研究区所处构造环境为洋内弧附近的前弧盆地,发育的火山岩是母源岩浆经历了一定程度分异作用后的产物,以岛弧环境为主,兼具有部分板内大陆环境特征。

The petrological and geochemical characteristics of Late Carboniferous-Early Permian volcanic rocks from northwestern margin of Junggar Basin

Territorial volcanic formation developed in Late Carboniferous-Early Permian period in northwestern margin of Junggar Basin.The volcanic rocks mainly composed of acidic dacite and rhyolites,including a few intermediate basalts and basaltic andesites.They belong to calc-alkalic series of low-medium potassium.Volcanic rock samples have relatively high alkali(Na₂O+K₂O=0.99％～8.12％),low rare earth element contents and relatively low K₂O(3.01％～0.05％)contents.Chondrite normalized REE patterns shows the characteristic of LREE enrichment((La/Yb)_N=0.76～5.18)and differentiation,with weak Eu anomalies(δEu=0.533～1.148)and positive Ce anomalies(δCe=0.979～1.224).These samples are enriched in LILE but relatively depleted in HFSE(Nb,Ta),These characteristics indicate that the Late Carboniferous-Early Permian period volcanic rocks in study area are certain differential production of primitive magma,mainly generated in island arc environments,with in infra-plate continental setting partially.These volcanic rocks are from extensional setting of fore-arc basin nearby the intra-oceanic.