河北矾山超钾质岩浆氧逸度及含水性特征的磷灰石地球化学记录  

Apatite geochemical records of oxygen fugacity and water content of the Fanshan ultrapotassic magmas,Hebei Province

作　　者：舒小超 孟凡巍 陶瑞 王坤 SHU Xiaochao;MENG Fanwei;TAO Rui;WANG Kun(School of Resources and Geosciences,China University of Mining and Technology,Xuzhou,Jiangsu 221116,China;Department of Geology,Northwest University,Xi'an,Shannxi 710068,China;Ningxia Mineral and Geological Survey Institute(Autonomous Region Institute of Mineral Geology),Yinchuan,Ningxia 750021,China)

机构地区：[1]中国矿业大学资源与地球科学学院,江苏徐州221116 [2]西北大学地质学系,陕西西安710068 [3]宁夏回族自治区矿产地质调查院(自治区矿产地质研究所),宁夏银川750021

出　　处：《地质学报》2023年第12期4101-4116,共16页Acta Geologica Sinica

基　　金：中央高校基本科研业务费(编号2022QN1057);中国博士后科学基金(编号2022M723380);国家重点研发计划政府间国际科技创新合作项目(编号2023YFE0104000)联合资助的成果。

摘　　要：产于华北克拉通北缘的晚三叠世矾山超钾质碱性岩杂岩体主要由单斜辉石岩和辉石正长岩等组成,以发育内生磷灰石-磁铁矿矿床而闻名。本文选取矾山杂岩体中的磷灰石为研究对象,通过测试磷灰石的主微量元素及原位Sr-Nd同位素组成以探讨母岩浆的氧逸度及含水性特征。研究表明,矾山磷灰石以轻稀土元素富集、重稀土元素亏损为特征,显示出缺乏显著Ce异常的“右倾”的稀土配分型式,且具有较低的Ga含量及较高的δEu值,暗示杂岩体具有较高的岩浆氧逸度,这也得到了磷灰石Mn氧逸度计的支持。所有磷灰石以高Sr、低Y为特征,具有较高的Sr/Y(>30)和La/Yb(>90)比值,且Sr/Y与La/Yb存在明显的正相关关系,表明矾山超钾质岩浆具有较高的含水量。矾山磷灰石的原位Sr-Nd同位素投点位于地幔序列,明显偏离地壳趋势。母岩浆的“氧化且富水”特征是矾山杂岩体得以发生磷灰石-磁铁矿矿化的重要因素,其主要与板片俯冲过程中蚀变洋壳对岩石圈地幔的交代作用有关。通过对比矾山杂岩体与全球范围内其他地质环境中磷灰石的成分数据,发现具有“高Sr、低F、少Cl”特征的磷灰石可作为识别超钾质碱性岩杂岩体(及其相关金属矿床)的特征矿物。The Late Triassic Fanshan ultrapotassic alkaline complex in the northern margin of the North China Craton is mainly composed of clinopyroxene and pyroxene syenite,and is famous for the development of endogenetic apatite-magnetite deposits.Apatites in the Fanshan complex were selected as the research object in this paper to clarify the oxygen fugacity and water content of the parent magmas by measuring their major-and trace-elements and in-situ Sr-Nd isotopic compositions.The study shows that the Fanshan apatites are characterized by enrichment of light rare earth elements and depletion of heavy rare earth elements.The distribution patterns of rare earth elements exhibit the characteristics of right-deviation with low Ga levels and highδEu values.These characteristics indicate that the complex has high magmatic oxygen fugacity,which is also supported by apatite Mn oxygen fugacity meter.All apatites are characterized by high Sr and low Y,and their high ratios of Sr/Y(>30)and La/Yb(>90)exhibit an obvious positive correlation,suggesting a high water content for the Fanshan ultrapotassic magmas.The in-situ Sr-Nd isotopic compositions of the Fanshan apatites plot in the mantle field,obviously deviating from the crustal trend.The characteristics of“oxidation and water-rich”of the parent magmas is an important factor for the apatite-magnetite mineralization in the Fanshan complex,which is probably related to the metasomatism of the altered oceanic crust on the lithospheric mantle during the plate subduction.Comparing the composition data of apatites in the Fanshan complex with those in other geological settings worldwide,it is considered that the apatites with the characteristics of“high Sr,low F and little Cl”can serve as a characteristic mineral for identifying ultrapotassic alkaline rock complexes(and their related metal deposits).

关 键 词：超钾质碱性岩 磷灰石 氧逸度 含水性 矾山杂岩体 

分 类 号：P618.31[天文地球—矿床学]