【摘要】 基性岩脉是岩石圈伸展作用的产物,对研究地幔性质和地球动力学演化具有十分重要的意义。粤北下庄铀矿田是我国最大的花岗岩型铀矿田之一,区内发育了大量与铀矿化作用密切相关的基性岩脉。前人从地球化学和年代学方面,对基性岩脉和铀矿床做了不同程度的研究,但有关铀矿床的成因及其与基性岩脉内在联系仍有不同认识。本研究新获得一批下庄铀矿田基性岩脉的角闪石40Ar-39Ar年代学数据,识别出一期形成于200～190Ma的基性岩脉,标志着华南地区在印支期碰撞造山作用结束后岩石圈伸展裂解作用可能至少在200～190Ma已经开始。结合前人已有的研究结果,粤北下庄至少发育三期基性岩脉:200～190Ma、～180Ma和145～140Ma,与华南地区在此期间广泛的岩石圈伸展作用相对应。结合成岩成矿作用的时差以及铀矿体与基性岩脉的空间关系,笔者认为准确的获得基性岩脉的侵位时代与铀的成矿作用的年龄,是探讨基性岩脉与铀成矿作用关系的前提。当基性岩脉与铀的成矿作用年龄接近或具有对应关系时,铀矿床中基性岩脉可能不仅可提供幔源流体（∑CO₂矿化剂和He）参与铀的成矿作用,也可为铀的沉淀富集提供理想场所（还原障）;当基性岩脉与铀的矿化作用在时间上存在较大的时差时,基性岩脉也可为后期铀的沉淀富集提供条件,且与基性岩脉相关的深大断裂可为幔源流体（∑CO₂矿化剂）参与铀成矿过程提供运移通道。基于此,笔者认为无论基性岩脉的侵位与铀的矿化作用是否存在时差,基性岩脉均可以为后期铀的沉淀富集提供场所,进而促进铀的成矿作用。因此,本文深化了花岗岩型铀矿区内铀成矿作用与基性岩脉内在联系的认识,为该区下一步找矿勘查工作提供重要理论依据。更多还原

【Abstract】 Mafic dyke is the product of lithospheric spreading and crustal extension,which is of great significance for studying the nature of the mantle and the evolution of continental dynamics. Abundant and widely distributed mafic dykes are commonly and universally interpreted to be a manifestation of extensional tectonism. In the northern Guangdong Province of the Nanling Mountain,the Xiazhuang uranium deposits are considered as one of the largest granite-related uranium ore field in China. Mesozoic mafic dykes are widespread and regularly developed,which are closely associated with granite-related uranium deposits in the Xiazhuang uranium ore field. The geochronology and geochemistry of these mafic dykes and uranium deposits have been extensively studied. However,the link between mafic dykes and uranium mineralization are considerable controversy. The new Ar-Ar dating of the hornblende of the diabase,dioritic porphyrite and diabase porphyrite from the Xiazhuang uranium ore field yields the plateau ages of 200 ～ 190 Ma,179. 6 ± 2. 8 Ma and 145. 1 ± 12. 1 Ma,respectively. These results coupled with published geochronological data show that at least three major episodes of mafic dykes occurred in the study area at 200 ～ 190 Ma, ～ 180 Ma and 145 ～ 140 Ma,indicating that South China Block probably occurs the wide extension at 200 ～ 190 Ma and 145 ～ 140 Ma. The identification of mafic dykes at 200 ～ 190 Ma in the northern Guangdong Province of the Nanling metallogenic province represents the extensional tectonism in South China block,which have been started at 200 ～ 190 Ma after the Indosinian orogeny. Based on the fact that closely spatial and temporal relationships between uranium ore body and mafic dykes in the Xiazhuang uranium ore field,age gap between ore-forming and emplacement of mafic dykes is key to evaluate the role of mafic dyke in uranium mineralization. When the ore-forming event is slightly younger than the occurrence of mafic dykes,it is considered that magmatic activities represented by mafic dykes could not only supply mantle-derived ∑ CO₂（ noble gas He）,which is indispensable for uranium mineralization,but also provide a favorable reducing environment to promote the deposition and enrichment of uranium from hydrothermal fluids. Accordingly,the emplacement of mafic dykes is much older than age of uranium mineralization,mafic dykes associated with deep faults could be still favorable factors for uranium deposition and fluid migration.Therefore,this paper reevaluates the role of mafic dyke in uranium mineralization,and provides some reliable suggestions for further prospecting in the Xiazhuang uranium ore filed and other similar deposits in the region which may have formed in a similar model.更多还原