摘要:贺兰山汝箕沟地区上三叠统延长组顶部发育一套基性火山岩，为我们探讨贺兰山地区晚三叠世的构造属性提供了重要依据。该套火山岩为一套拉斑玄武岩，主量元素以相对富Na2O （1.29%～3.66%）、贫K2O （# （43%～58%）为特征。稀土元素总含量较低（65.13×10-6～94.52×10-6），（La/Yb）N = 3.46～4.71，为轻稀土富集型。富集Cs、Rb、Ba等大离子亲石元素和高场强元素，结合构造环境判别图解，该套火山岩形成于陆内裂谷环境。岩浆喷溢时间晚于盆地裂陷沉积时间，具被动裂谷性质。综合分析表明，汝箕沟玄武岩岩浆是拉张作用下岩石圈地幔尖晶石二辉橄榄岩与石榴石二辉橄榄岩过渡区域部分熔融的产物，并经历了一定程度的橄榄岩、单斜辉石的结晶分异，表明晚三叠世—早侏罗世阿拉善地块与东部华北板块之间曾经历过一次伸展裂解事件。
关键词: 地球化学/
拉斑玄武岩/
被动裂谷盆地/
汝箕沟/
贺兰山
Abstract:The mafic lavas, exposed in the Rujigou area, Helan Mountain, showed a conformable contact with the Upper Triassic Yanchang Formation. Here, we present new major and trace elements of these lavas in order to reveal the geodynamics background and the tectonic setting. The lavas are mainly composed of tholeiite basalts characterized by low K2O（#（43%～58%）. All samples exhibit LREE-enriched patterns without obvious Eu anomalies（La/Yb）N = 3.46～4.71, δEu = 0.96～1.05）. In the primitive mantle-normalized spider diagram, the basalts are enriched in large ion lithophile elements（LILE）and high field strength elements（HFSE）. In the multiple tectonic environment discrimination diagrams, the high field strength element abundances indicate that the rocks were emplaced in an environment similar to a continental rift setting. The volcanic eruption occurred in the terminal stage of the basin rifting, which is likely a kind of passive rifting. The comprehensive analysis of geological and geochemical characteristics in this area suggests that the tholeiite rocks were formed by partial melting of mantle peridotite at the spinel-garnet transition zone, and experienced crystallization of olivine and clinopyroxene, which provides an evidence of the rifting event between the Alax massif and the North China plate in the Late Triassic-Early Jurassic.
Key words:Geochemical characteristics/
Tholeiite basalt/
Passive rift basin/
Rujigou/
Helan Mountain
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