南秦岭镇安西部晚三叠世煌斑岩脉地球化学特征及其对构造环境的指示  

Geochemical characteristics of Late Triassic lamprophyres from the western Zhen’an,South Qinling and its indicative significance for tectonic environment

作　　者：孙万龙 韩奎 鲁麟 薛旭平 王明志 贾忠胜 刘能 孟磊 刘坤 田科 SUN Wanlong;HAN Kui;LU Lin;XUE Xuping;WANG Mingzhi;JIA Zhongsheng;LIU Neng;MENG Lei;LIU Kun;TIAN Ke(Shaanxi Institute of Geological Survey,Xi’an 710054,Shaanxi,China;Shaanxi Mineral Resources and Geological Survey,Xi’an 710068,Shaanxi,China;Qinba Research Branch of China Society of Natural Resources,Xi’an 710054,Shaanxi,China)

机构地区：[1]陕西省地质调查院,陕西西安710054 [2]陕西省矿产地质调查中心,陕西西安710068 [3]中国自然资源学会秦巴研究分会,陕西西安710054

出　　处：《地质通报》2022年第11期1982-1995,共14页Geological Bulletin of China

基　　金：陕西省公益性地质调查项目《陕西省镇安县杨泗镇-东川镇钨钼多金属矿产地质调查》(编号:202106);陕西省自然科学基础研究计划资助项目《白钨矿矿物学对南秦岭镇安西部石英脉型钨矿岩浆-热液演化的约束》(编号:2020JQ-974);陕西省地质勘查基金项目《陕西省镇安西部整装勘查区基础地质调查》(编号:61201707318);《陕西典型小岩体成矿预测与勘查示范》(编号:61201908334)。

摘　　要：煌斑岩是反映深部构造-岩浆作用和源区地球化学性质的良好地质体。对南秦岭镇安西部地区出露的煌斑岩脉进行系统研究,用LA-ICP-MS测定了煌斑岩中锆石U-Pb同位素,得到其206 Pb/238 U年龄为222.2±1.2 Ma(MSWD=0.38,n=15),此年龄代表脉体的形成年龄,属晚三叠世。地球化学分析表明,岩石属钙碱性钾质-超钾质系列,轻稀土元素和Rb、Ba等大离子亲石元素富集,重稀土元素和Nb、Ti等高场强元素亏损,具有俯冲带幔源岩石的成分特征;岩石具有较高的(87 Sr/86 Sr)i值(0.7066~0.7076)和较低的εNd(t)值(-5.77~-4.62);锆石176 Hf/177 Hf值为0.28250~0.28287,εHf(t)值为-4.75~1.89,锆石二阶段Hf模式年龄与全岩Nd二阶段模式年龄主要集中于中元古代(1.3~1.5 Ga)。综合分析表明,煌斑岩的源区为中元古代富集地幔,形成于后碰撞伸展构造环境。在晚三叠世(220 Ma左右)后碰撞伸展构造环境下,勉略洋壳俯冲过程中形成的流体交代富集地幔在构造减压、深部物质上涌提供热和流体的共同作用下,发生部分熔融,形成具EMⅡ型同位素特征的镁铁质岩浆,镁铁质岩浆沿后碰撞伸展阶段形成的裂隙上升侵位形成煌斑岩脉。The lamprophyres are good geological bodies reflecting deep tectonomagmatism and geochemical properties of source.In this paper,a systematic study has been carried out on the lamprophyre veins in the west of Zhen'an,South Qinling Mountains.Zircon U-Pb isotopes of the lamprophyre have been determined by LA-ICP-MS.The ^(206)Pb/^(238)U age of the lamprophyre is 222.2±1.2 Ma(MSWD=0.38,n=15),which represents the formation age of the vein body and belongs to the Late Triassic.Geochemical data show that the rocks belong to the calc-alkaline potassium-superpotassic series,enriched in LREE and large ion lithophile elements such as Rb and Ba,and deficient in HREE and high field strength elements such as Nb and Ti,which have the composition characteristics of mantle derived rocks in subduction zones.The isotopic compositions of the rocks are characterized by high initial(^(87)Sr/^(86)Sr)_(i) rations(0.7066~0.7076)and negativeε_(Nd)(t)values(-5.77~-4.62);Furthermore,the ^(176)Hf/^(177)Hf rations of zircons range from 0.28250 to 0.28287,withε_(Hf)(t)values from-4.75 to 1.89,The two-stage Hf model ages(T_(DM2))and the whole-rock Nd isotopic model ages,are mainly concentrated in mesoproterozoic(1.3~1.5 Ga).Comprehensive analysis shows that the source of lamprophyre is Mesoproterozoic enriched mantle and formed in post-collision extensional tectonic environment.At the Late Triassic(approximately at 220 Ma),the enriched lithospheric mantle experienced metasomatism by subduction fluid,which occurred to partial melting under the conditions of tectonic decompression,heat and fluid from the upwelling deep materials,and produced mafic magma,the magma emplaced to shallow crust along tectonic fracture and formed the lamprophyres.

关 键 词：煌斑岩脉 LA-ICP-MS锆石U-PB年龄 地球化学 SR-ND-HF同位素 南秦岭 

分 类 号：P534.51[天文地球—古生物学与地层学] P588.13[天文地球—地质学]