山东省微山稀土矿正长岩类锆石U-Pb年代学 及地球化学特征研究

为了加深对微山稀土矿碱性杂岩体岩浆活动期次及与成矿关系的理解,对微山稀土矿床碱性杂岩体进行岩相学、地球化学分析和锆石U-Pb年代学测定。结果表明,微山稀土矿碱性杂岩体正长岩类地球化学特征相似,具有同源性,为偏铝质碱性岩。稀土总量高,轻重稀土元素分馏明显,呈轻稀土高度富集模式。微量元素组成具有幔源岩浆岩的特征,富集Rb,Ba,Sr等大离子亲石元素,亏损Nb,Ta,Zr等高场强元素,Eu异常不显著。利用LA-ICP-MS U-Pb年代学方法,测得含矿石英正长岩、霓辉石英正长岩锆石U-Pb年龄分别为122.4±2.0 Ma(MSWD=5.2)和130.1±1.4 Ma(MSWD=9),存在2536±6.1 Ma(MSWD=1.6)的继承锆石,表明地壳物质参与成岩过程。结合鲁西燕山期构造演化过程,认为微山正长岩类形成于华北克拉通中生代构造体制转折后的伸展背景下,成矿作用与碱性杂岩体的演化有关。

Zircon U-Pb Ages and Geochemical Characteristics Study of Syenite from Weishan REE Deposit,Western Shandong

In order to enhance the understanding of the stage of magmatism and the relationship between Weishan alkaline complex and REE mineralization, the zircon U-Pb age and geochemistry of Weishan alkaline complex are conducted. The geochemical result shows that syenite series has the same geochemical characteristics, proving that they are from the same resource, and the syenite belong to metaluminous and alkaline series. They have high∑REE, and distinct fractionation between HREE and LREE, exhibiting depleted in HREE, Nb, Ta, Zr, but enriched in LREE, Rb, Ba, Sr, and have slight Eu abnormality, which is consistent with the characteristics of mantle-derived magmatic rocks. The element-geochemical features reflect that the magma comes from enriched mantle. LA-ICP-MS zircon U-Pb ages of the ore-bearing quartz syenite and the aegirine-augite syenite are separately 122.4±2.0 Ma (MSWD=5.2) and 130.1±1.4 Ma (MSWD=9), and the presence of 2536±6.1 Ma (MSWD=1.6) inherited zircon is also an evidence that crustal materials have been involved in the magmatism process. It is concluded that Weishan syenites had come into being in an extensional setting after the transformation of the tectonic regime in the NCC in the Mesozoic area, when a large-scale enriched mantle melted to cause the generation of alkaline magma and a massive upwelling of the mantle materials from the asthenosphere to form finally the REE deposit after the formation of the alkaline rock complex.