黑龙江穆棱新生代玄武岩锆石巨晶的微量元素及Hf同位素研究

Trace element and Hafnium isotopes of Cenozoic basalt-related zircon megacrysts at Muling,Heilongjiang province,northeast China

穆棱古近纪(第三纪)碱性玄武岩位于郯庐断裂东北段分支敦化～密山深大断裂带的中北部,与吉林敦化、汪清,黑龙江镜泊湖、牡丹江等新生代火山岩一起分布于兴蒙造山带和太平洋板块俯冲带结合带上;本文利用电子探针、激光剥蚀等离子质谱技术(LA-ICP-MS)及多接收器电感耦合等离子体质谱仪(MC-ICP-MS)分析了来自玄武岩风化冲积物中锆石巨晶的主、微量元素及其Hf同位素组成。这些锆石巨晶具有明显的熔蚀结构,锆石电子探针分析其Zr/Hf比最高可达217,平均150,是目前东部新生代玄武岩锆石巨晶的最高值。它们具有相似的微量元素特征,具有明显的Ce正异常,富集重稀土和HFSE元素,锆石Hf/Y比值大多小于10,Th/U均小于1(一个数据例外),Yb/Sm为21～59。与岩浆锆石和变质锆石相比,显示过渡的特征;锆石^(176)Hf/^(177)Hf变化为0.283105～0.283151,ε_(Hf)变化在+11.8～+13.4之间,显示亏损地幔来源,反映锆石形成时岩石圈地幔主体具有"新生"的特征,表明岩石圈减薄或置换事件在东北地区东北部穆棱地区也同样存在。穆棱新生代碱性玄武岩有关锆石巨晶的成因特征与中国东部海南文昌、福建明溪及山东昌乐等地新生代玄武岩的锆石巨晶相同,可能与有下地壳物质参与的近期岩石圈地幔的交代作用有关。

Abstract Major and trace element and Hf isotopes were reported for zircon megacrysts from the alluvium of Tertiary alkali basalts at Muling in the Heilongjiang Province, which, together with the Cenozoic basalts at Dunhua and Wangqing in the Jilin Province and Jingpowu and Mudanjiang in the Heilongjiang Province, are distributed along the middle-northern section of Dunhua-Mishan fault-the northern branch of Tanlu fault between Xing-Meng Orogen Belt and Pacific subduction zone. Generally, the zircons have clear surface melting feature and display transitional features compared with igneous or metamorphic zircons. Electronic microprobe analysis shows that they have an average Zr/Hf ratio of 150 with the maximum of 217 which is the highest value among the Cenozoic basalt-related megacryst zircons in the eastern China. They have same origin because of their similar trace element compositions, such as distinct positive Ce anomalies, enrichment in HREEs and HFSEs, most Hf/Y ratios of less than 10, Th/U ratios less than 1 with one exception, and Yb/Sm ratios of 21~59. They have a range of ^176Hf/^177Hf ratios from 0.283105 to 0.283151 with БHf Values ranging from ＋ 11.8 to ＋ 13.4, similar to those from other locations in East China, e.g. Wenchang in the Hainan Province, Mingxi in the Fujian Province, and Changle in the Shandong Province. The Hf isotopic characteristics of depleted mantle origin, combining with other major and trace element features, indicate that Muleng Cenozoic basalt-related megacryst zircons may be developed during Phanerozoic metasomatism in upper mantle related to lithospheric thinning or replacement at Muling in the Heilongjiang Province.