黑龙江省萝北地区晚寒武世含钍花岗岩脉的发现及其地质意义  

Discovery and geological significance of Late Cambrian Th-rich granite veins in Luobei area,Heilongjiang Province

作　　者：李丽荣[1] 王正其[1] 肖冠雄 李怀渊[2] 张恩[2] 卢辉雄[2] LI Li-rong;WANG Zheng-qi;XIAO Guan-xiong;LI Huai-yuan;ZHANG En;LU Hui-xiong(State Key Laboratary of Nuclear Resource and Environment,East China University of Technology,Nanchang 330013,China;Airbome Survey and Remote Sensing Centre of Nuclear Industry,Shijiazhuang 050002,China)

机构地区：[1]东华理工大学核资源与环境国家重点实验室,江西南昌330013 [2]核工业航测遥感中心,河北石家庄050002

出　　处：《地球化学》2020年第1期36-49,共14页Geochimica

基　　金：黑龙江省地质勘查基金(SDK2015-026);国家自然科学基金(41040019)。

摘　　要：对侵位于萝北地区麻山群中的含钍花岗岩脉开展了单颗粒锆石U-Pb定年(LA-ICP-MS)和岩石地球化学特征研究,旨在查明岩脉形成时代并探讨其成因,并为佳木斯地块地质历史的重塑提供素材。结果显示,萝北含钍花岗岩形成年龄为(488.4±2.3)Ma,属晚寒武世产物;含钍花岗岩呈细粒花岗结构,出现黑云母和白云母等矿物,富钾、过铝质,富集轻稀土元素、大离子亲石元素(K、Rb)和高场强元素(Th、Ce、Zr、Hf和Sm),亏损Sr、Ba、P和Ti等元素;轻、重稀土分馏显著,Eu强烈亏损;岩石(87Sr/86Sr)i为0.70058~0.70780(平均0.70606),(143Nd/144Nd)i区间为0.51171~0.51175(均值0.51173),εNd(t)=-5.1^-5.8(平均-5.5)。研究认为,含钍花岗岩属S型,是后碰撞构造环境由麻山群副片麻岩部分熔融形成;萝北晚寒武世花岗岩的发现佐证了佳木斯地块在寒武纪-奥陶纪发生碰撞造山事件(约500Ma),推断在晚寒武世(488.4±2.3)Ma,佳木斯地块内部块体及其与北部的西伯利亚克拉通之间已经完成碰撞拼贴过程,形成了佳木斯地块及其与周边地块的基本构造格局;与典型产铀岩体比较,萝北含钍花岗岩具有显著高LREE/HREE比值(25.0~29.3)、(La/Yb)N比值(51.7~67.2)和δCe值(1.20~1.30)特点。This study conducted laser ablation inductively coupled plasma mass spectrometry for zircon U-Pb dating and determined the petrogeochemical characteristics of Th-rich granite veins in the Mashan Group in the Luobei area,Heilongjiang Province,northeastern China.The U-Pb dating results indicated that the Th-rich granite was formed during the Late Cambrian period(488.4±2.3 Ma).The petrogeochemical results showed that this granite has a fine-grained,granitic texture and is dominated by minerals such as biotite and muscovite.Moreover,the granite is alkali-rich,K-rich,peraluminous,and enriched in light rare earth elements(LREEs),large-ion lithophile elements(e.g.,K and Rb),and high-field-strength elements(e.g.,Th,Ce,Zr,Hf,and Sm).Sr,Ba,P,and Ti are present in relatively small amounts,and significant LREE and heavy rare earth element(HREE)fractionation and strong Eu depletion were observed.Isotope studies on the Th-rich granite showed that the(87Sr/86Sr)i values ranged from 0.70058 to 0.70780,whereas(143Nd/144Nd)i andεNd(t)values were 0.51171–0.51175 and-5.1 to-5.8,respectively.Based on the petrogeochemical data,it was concluded that the Th-rich granite is of the S-type,which was formed by partial melting of Mashan Group paragneiss in a post-collisional tectonic environment.The discovery of the Late Cambrian granite veins in Luobei could be evidence of collisional orogenic events involving the Jiamusi Massif in the Cambrian–Ordovician period(~500 Ma).Furthermore,it is speculated that the collision and combining process involving the interior of the Jiamusi Massif and Siberia Craton was completed in the Late Cambrian period(488.4±2.3 Ma);as a result of this collision and combination,the basic structural pattern of the Jiamusi Massif and surrounding blocks was formed.Compared with typical U-bearing rock masses,the Th-rich granite in Luobei has a significantly higher LREE/HREE ratio(25.02–29.34),(La/Yb)N ratio(51.70–67.16),andδCe value(1.20–1.30).

关 键 词：锆石U-PB定年 地球化学 含钍花岗岩 构造环境 萝北地区 黑龙江省 

分 类 号：P595[天文地球—地球化学] P597[天文地球—地质学]