朝鲜半岛北部定州与铁山稀土矿相关碱性岩和花岗岩的岩石学和SIMS锆石U-Pb年代学特征研究

朝鲜半岛北侧的狼林地块是中朝克拉通的重要组成部分,狼林地块具有与辽南地块以及辽吉活动带类似的早前寒武纪岩石组成、变质和变形特征。我们在分布有稀土矿的朝鲜定州-铁山一带(狼林地块西北部),采集了与稀土矿密切相关的碱性岩(粗粒正长岩和萤石化霓辉正长岩)和花岗岩(似斑状花岗岩)样品。岩石样品的岩相学观察表明,定州与铁山地区的碱性岩和花岗岩并不显示明显的岩浆期之后的热液蚀变特征。SIMS锆石U-Pb定年结果显示粗粒正长岩的岩浆结晶年龄为1865±6Ma,萤石化霓辉正长岩的岩浆结晶年龄为1868±8Ma,铁山地区的四件似斑状花岗岩的年龄结果分别为1871±13 Ma、1866±8 Ma、1872±6 Ma和1873±7 Ma,这些年龄值一方面代表了岩浆活动时代,另一方面可能也代表了朝鲜定州和铁山稀土矿的成矿时代。狼林地块的这些碱性岩与花岗岩的时代与华北克拉通辽吉活动带以及狼林地块其它地区的高级变质作用同期,并且在辽南地块,也有类似碱性岩的报道。综合现有的证据表明,辽吉活动带、辽南地块以及狼林地块在古元古代可能属于同一个大型构造带,并且这个构造带(或者构造带的一部分)当时处于伸展的构造背景。通过全面对比野外观测结果、岩石的岩相结构、岩浆结晶时代可知,朝鲜定州和铁山一带的稀土矿与中朝克拉通的世界级稀土矿——白云鄂博,在岩石组合、稀土矿物组成、岩石蚀变特征、成矿时代等方面存在较大的区别。关于这一稀土矿的具体稀土矿物组成、矿床成因以及储量等众多问题,未来还需要进行大量的工作。

Study on the petrographic and SIMS zircon U-Pb geochronological characteristics of the magmatic rocks associated with the Jongju and Cholsan REE deposits in northern Korean Peninsula.

Rangrim massif in the northern part of the Korean Peninsula is an importance constituent part of the Sino-Korea Craton. Its Early Precambrian rock associations, metamorphic and deformation characteristics are similar to those of the Liaonan terrane and the Liaoji Belt in North China Craton. Some REE deposits are distributed in the Jongju and Cholsan regions and they are closely related to alkaline rocks and porphyraceous granites outcropped in these regions. Coarse-grained syenite, fluoritized aegirine-augite syenite and porphyraceous granite samples were collected from these regions. Petrographic observation results show that the rocks don''t show obvious alteration related to later hydrothermal fluids except those derived from the same episode of magmatism of the rocks. SIMS zircon dating results give magmatic ages for the weathered (coarse-grained) syenite of 1865±6Ma, for the fluoritized aegirine-augite syenite of 1868±8Ma and for the four porphyraceous granite samples of 1871±13Ma, 1866±8Ma, 1872±6Ma and 1873±7Ma, respectively. Synchronous magmatism and high grade metamorphism are widely distributed along the three Paleoproterozoic mobile belts in the North China Craton and similar alkaline rocks have also been reported from the Liaonan terrane. These lines of evidence suggest that the Liaonan terrane, the Liaoji mobile belt and the Rangrim terrane probably represent different parts of a single huge tectonic belt during Paleoproterozoic and this tectonic belt (or part of this belt) is in an extensional environment. After a thorough synthesis of the results from field and petrographic observations and zircon U-Pb dating, and based on a simple comparison, it can be seen that the REE deposits in the Jongju and Cholsan regions are different from the world class Bayan Obo REE deposit in many different ways, like the rock assemblage, the REE minerals, the alteration features of rocks and the time of REE metallogenesis. More work will be needed s in order to fully understand the REE mineral assemblages and the metallogenesis of these deposits.