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矿床地质  2010 

西藏冈底斯造山带几乎同时形成的两套埃达克岩为什么一套含矿一套不含矿?

Keywords: 地球化学,岩浆源区,含矿埃达克岩,不含矿埃达克岩,冈底斯碰撞造山带,西藏高原

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Abstract:

野外地质调查结合锆石U-Pb年龄测定和岩石化学成分分析,发现西藏冈底斯碰撞造山带晚中新世发育两套埃达克岩。一套呈NS向岩墙产出,锆石U-Pb-LA-ICP-MS年龄为15.6~16.8Ma,地球化学上以富Na2O、贫K2O、Sr/Y比值高为特点,不含矿;另一套呈岩株产出,锆石U-PbSHRIMP年龄为14.0~15.3Ma,地球化学上以富K2O、贫Na2O、Sr/Y比值低为特点,伴随着大规模的斑岩型铜钼矿化。两套埃达克岩都高度富集大离子不相容元素(LILE)Rb、Ba、Th、U、K、Sr、Pb,强烈亏损高场强元素(HFSE)Nb、Ta、Ti,反映出俯冲组分对岩浆源区的明显影响,具备岛弧岩浆作用的基本特征。比较而言,含矿埃达克岩更富集Rb、Th、U、K、Pb,而不含矿埃达克岩更富集Sr;不含矿埃达克岩的高场强元素Nb、Ta、Ti亏损更加强烈,Zr、Hf相对富集。两套埃达克岩都是形成于碰撞后地壳伸展的同一地球动力学背景下,其岩石地球化学差异主要与岩浆源区有关。研究表明,含矿埃达克岩的岩浆源区较浅,位于下地壳底部,参与岩浆作用的俯冲组分以沉积物熔体为主,岩浆的氧化性较强,并且经历了更多的分异过程。不含矿埃达克岩的岩浆源区较深,位于岩石圈地幔上部,参与源区岩浆作用的俯冲组分主要是板片流体,岩浆氧化性较低,岩浆分异过程较少。两套埃达克岩含矿与不含矿的原因包含了源区物质组成和岩浆生成条件两方面的差异。

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