Abstract: To deepen the understanding on tectonic evolution of Beishan Moutain, Gongpoquan granite pluton in middle Beishan, Gansu Province, is studied in geochronology and geochemistry. The Gongpoquan granite is located in the west part of the middle Beishan belt between the Mingshui-Xiaohuanshan and Hongliuhe-Niujuanzi- Xichangjing ophiplitic belts. Based on mineralogy and geochemistry, the Gongpoquan goanite can be divided into two groups. 1) The leucogranites which are characterized by garnet, primary muscovite and tourmaline, exhibit slightly enrichments in LREE with (La/Yb)_N=1.38?1.93, and negative Eu anomalies (δEu =0.19?0.28). They also exhibit enrichments in Cs, Rb, Th, U, K, P and Hf, and depletions in Ba, Nb, Nd, and Ti. 2) The monzogranites show features of peraluminous S-type granites, with A/CNK>1.1 (on average). The monzogranites also have weak fractionations between LREE and HREE ((La/Yb)_N=2.74?22.78), but unobvious Eu anomalies (δEu =0.83?1.07). They display enrichments in Cs, Rb, Ba, Th, U, K, and depletions in Nb, Ta, P and Ti. According to the mineral assemblage and geochemical characteristics, it can be concluded that the Gongpoquan granite was formed in the post-collisional setting. The LA-ICP MS U-Pb zircon age of Gongpoquan granite is 402.2±3.0 Ma, which represents its crystallization age, and the ε_Hf(t) values of zircons vary from 0.1 to 2.9, indicating the additions of mantle-derived materials. The Gongpoquan post-collisional granite, together with adjacent grantiods, confirms that the paleo-ocean basin closed before Early Devonian in the west part of Beishan, meanwhile the whole area was in the setting of post-collisional environment.

Key words: Beishan, leucogranite, post-collision, LA-ICP MS, Hf isotope

摘要： 为加深对北山地区古生代构造演化的认识, 选取甘肃省北山公婆泉花岗岩体进行年代学和地球化学研究。公婆泉花岗岩体位于北山中带, 明水?小黄山蛇绿岩带与红柳河?牛圈子?洗肠井蛇绿岩带之间。根据花岗岩矿物组成及地球化学特征将其分为两种不同类型: 1) 淡色花岗岩含石榴石、原生白云母和电气石等特征矿物, 轻、重稀土元素分异不明显, (La/Yb)_N=1.38~1.93, 有明显的Eu负异常(δEu =0.19~0.28), 同时富集Cs, Rb, Th, U, K, P和 Hf, 亏损Ba, Nb, Nd和Ti; 2) 二长花岗岩具有过铝质S型花岗岩的特征, A/CNK值平均大于1.1, 轻、重稀土元素分异不明显, (La/Yb)_N=2.74~22.78, 具轻微的Eu异常(δEu =0.83~1.07), 同时富集Cs, Rb, Ba, Th, U和K, 亏损Nb, Ta, P和Ti。公婆泉两类花岗岩的矿物学和地球化学特征表明其形成于后碰撞环境。公婆泉花岗岩体的锆石LA-ICP MS U-Pb年龄为402.2±3.0 Ma, 代表其岩浆结晶年龄, ε_Hf(t)值介于0.1~2.9之间, 暗示幔源物质的加入。公婆泉后碰撞花岗岩体与北山造山带西段相关花岗岩类的地球化学和年代学特征证实, 北山中带的俯冲过程于早泥盆世之前在西段结束, 并且此时在区域上处于后碰撞环境。

关键词: 北山, 淡色花岗岩, 后碰撞, LA-ICPMS, Hf同位素