内蒙古艾勒格庙强过铝质花岗岩定年、地球化学特征及其地质意义
Age and geochemistry of strongly peraluminous granite in Airgin Sum area, Inner Mongolia, and its geological significance
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摘要: 内蒙古艾勒格庙二长花岗岩岩体分布在锡林浩特-艾勒格庙前寒武纪微陆块的西部,利用LA-MC-ICP-MS锆石UPb法测得其形成年龄为418.5±1.1Ma,侵入时代为晚志留世末期。岩石地球化学特征显示高硅(SiO2 74.22%~75.51%),富钾(K2O 5.41%~5.78%),铝饱和指数A/CNK在1.17~1.21之间,属于高钾钙碱性强过铝质花岗岩类。微量元素特征为富集Rb、Th、U、Pb、K和重稀土元素(Yb 3.74×10-6~9.65×10-6),强烈亏损Nb、Ta、P、Ti等元素。该岩体形成于后碰撞构造环境,是地壳增厚后折返减压部分熔融的产物。它的存在揭示,艾勒格庙苏左旗北部造山带在晚志留世之前已经发生弧陆碰撞,之后的顶志留世-中泥盆世为后碰撞-后造山的伸展背景。Abstract: Airgin Sum monzogranite plution is located in western Xilin Gol-Airgin Sum Precambrian block, Inner Mongolia. Its age of 418.5±1.1Ma obtained by LA-MC-ICP-MS U-Pb zircon dating means that Airgin Sum plution was formed at the end of Late Silurian. Its petrology and geochemistry show that Airgin Sum monzogranite is characterized by high silicon (SiO2 74.22%~75.51%) and rich potassium (K2O 5.41%~5.78%)in association with A/CNK ratios ranging from 1.17 to 1.21, suggesting high-K and calc-al-kaline strongly peraluminous granite. The monzogranite is strongly enriched in Rb, Th, U, Pb, K and heavy rare earth elements (Yb 3.74×10-6~9.65×10-6) and is also characterized by depletion of Nb, Ta, Ti and P. It is suggested that the strongly peraluminous mon-zogranite was formed in the post-collisional setting and resulted from partial melting of crustal material after exhumation of overthickened crust. It is thus concluded that the northern orogen in Airgin Sum-Sonidzuoqi had collided between arc and block before Late Silurian, followed by an post-collision and post-orogenic extension setting during Upper Silurian-Middle Devonian.
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