内蒙古乌拉特中旗北七哥陶辉长岩SHRIMP锆石U-Pb年龄、地球化学特征及其地质意义

SHRIMP U-Pb dating,geochemistry and tectonic implications of the Beiqigetao gabbros in Urad Zhongqi area,Inner Mongolia

内蒙古乌拉特中旗温更地区北七哥陶辉长岩岩体由橄榄辉长岩和角闪辉长岩组成,主要造岩矿物为橄榄石、辉石、斜长石及角闪石等。锆石SHRIMPU-Pb同位素定年结果表明,橄榄辉长岩形成于269±8Ma,为晚古生代华北板块北缘岩浆活动的产物。橄榄辉长岩和角闪辉长岩具有相似的稀土元素、微量元素以及同位素特征,即:轻稀土元素(LREE)富集的右倾平滑配分模式;大离子亲石元素(LILE)K、Sr、Ba明显富集,高场强元素(HFSE)Nb、Ta、U、Th相对亏损,但Zr-Hf不亏损;87Sr/86Sr(t)平均值分别为0.706776和0.706960,εNd(t)平均值分别为-8.8和-8.9,206Pb/204Pb、207Pb/204Pb、208Pb/204Pb比值的平均值分别为17.2019和17.2277、15.4252和15.4524、37.3082和37.4724,在87Sr/86Sr(t)-143Nd/144Nd(t)图解上,北七哥陶辉长岩都落在了EMI型富集地幔附近。根据以上事实,结合区域资料表明,橄榄辉长岩和角闪辉长岩产于后碰撞的构造环境,并且来自相同的岩浆源区,主要继承了华北陆块内部EMI型富集地幔,可能受到了古亚洲洋俯冲消减过程中再循环下地壳组分对岩石圈地幔的改造。

Two types of cumulate gabbros are exposed in Wengeng area,Urad Zhongqi,Inner Mongolia,including olive gabbro and amphibole gabbro.The major petrogenetic minerals in the Beiqigetao gabbros are olive,pyroxene,plagioclase and hornblend.The SHRIMP U-Pb dating from olive gabbro give a concordant age of 269±8Ma.Both the gabbros show right-oriented linear REE patterns with LREE enrichment,and are characterized by LILE（K,Sr,Ba） enrichment and negative HFSE（Nb,Ta,U,Th） anomalies.The average 87Sr/86Sr（t）,εNd（t）,206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb of olive gabbro are 0.706776,-8.8,17.2019,15.4252,and 37.3082,and those of amphibole gabbro are 0.706960,-8.9,17.2277,15.4524,and 37.4724,Respectively.Beiqigetao gabbros lie to EMI end member in the 87Sr/86Sr（t）-143Nd/144Nd（t） diagram.All the above evidences suggest that olive gabbro and amphibole gabbro were derived from the same mantle sources,which had undergone subduction-related metasomatism except inheriting the mantle source characteristics of the North China Block.