冈底斯基性次火山岩地球化学和Sr-Nd-Pb同位素：碰撞后火山作用亏损地幔源区的约束
The geochemistry and Sr-Nd-Pb isotopes of basaltic subvolcanics from the Gangdese:Constraints on depleted mantle source for post-collisional volcanisms in the Tibetan plateau.
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摘要: 青藏高原隆升与伸展构造模式（地幔对流减薄、板片断离和大陆俯冲）认为地幔软流圈上涌为藏南碰撞后火山作用提供了热源,而已有研究成果没有包括碰撞后火山作用软流圈地幔源区的岩石学、地球化学证据,而是集中在富集岩石圈地幔.出露于冈底斯东段达孜-甲马地区的基性次火山岩的^40Ar/^39Ar坪年龄为38.07～42Ma.次火山岩具有高Mg#值和相容微量元素含量（Cr、Ni、V、Sc）,低LILE、和LREE,低LREE/HREE分离.原生样品呈现平坦的稀土配分曲线和LILE亏损的原始地幔标准化曲线,显示出OIB地球化学亲和性.岩石具有低Sr、低Pb和高Nd同位素组成[（^87Sr/^86Sr）i=0.7048～0.7064;（^144Nd/^143Nd） i = 0.5126～0.51286 ;^206Pb/^204Pb = 18.3722～18.5288; ^207Pb/^204Pb = 15.5686～15.6214; ^208 Pb/^204Pb = 38.5203～38.7298],显示出与富集OIB非常相似的Sr-Nd-Pb同位素组成及变化范围.应用我们的分析数据并结合已有研究成果,达孜-甲马基性次火山与adakite、林子宗火山岩、钾质岩-超钾质岩构成了藏南碰撞后火山岩Sr-Nd-Pb同位素组成演化带,基性次火山岩分布于演化带的原生端元,揭示出其岩浆源区与富集岩石圈地幔存在明显的差异.研究结果证明,具有与OIB相似地球化学和Sr-Nd-Pb同位素组成的基性次火山岩的源区是软流圈地幔.由冈底斯带新近纪基性火山作用厘定的软流圈上涌深部过程对洋壳断离模式、基性岩浆底侵作用和西藏高原地壳加厚时限提供了有效的约束.
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Key words:
- Tibetan plateau /
- Post-collissional volcanism /
- Basaltic subvolcanics /
- Asthneosphere
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