鲁玉龙，彭建堂，阳杰华，李玉坤，陈宪佳，周 溪，李干龙



湘中包金山矿区花岗闪长斑岩的锆石U-Pb年龄、Hf-O同位素组成及其地质意义

鲁玉龙1, 3，彭建堂1, 2，阳杰华2，李玉坤1，陈宪佳1，周 溪1, 3，李干龙3

(1. 中南大学地球科学与信息物理学院有色金属成矿预测教育部重点实验室，长沙 410083；2. 中国科学院地球化学研究所矿床地球化学国家重点实验室，贵阳 550002；3. 湖南省有色地质勘查局二总队，湘潭 411102)

包金山金矿床是近年在湘中地区探获的以金为主的多金属矿床，其成矿与印支晚期岩浆活动关系密切。利用高精度SIMS锆石U-Pb定年方法得到矿区两条花岗闪长斑岩年龄分别为(225.1±1.5) Ma和(223.3±1.4) Ma，在误差范围内一致，属印支晚期大规模岩浆活动的产物；矿区两条花岗闪长斑岩的176Hf/177Hf值均集中分布在0.2825附近，对应的Hf()值介于−5.9~3之间，平均为−3.98，两阶段模式年龄DM2为0.96~1.45 Ga，平均为1.35 Ga；δ18O值为0.824%~0.973%；Hf-O同位素研究表明花岗闪长斑岩主要为中元古代下地壳岩石重熔形成，且有部分幔源物质参与成岩作用；与紫云山花岗岩体对比分析，表明两者为同源岩浆演化的产物，且间接指示包金山金矿成矿时间为227~223 Ma；花岗闪长斑岩形成于多板块汇聚的动力学背景，为印支地块向北挤压和太平洋板块向西俯冲共同作用的结果。

SIMS 锆石U-Pb年龄；Hf-O同位素示踪；花岗闪长斑岩；包金山矿床；湘中

随着一系列典型印支期成矿年龄的确认，如荷花坪锡多金属矿((224±1.9) Ma)[1]、云头界钨钼多金属矿((216.8±7.5) Ma)[2]、锡田锡铅锌多金属矿((212.8±3.0) Ma)[3]，华南印支期的成矿作用越来越值得重视。在湘中地区，金属(多金属)矿床多分布于印支期花岗岩体周边，如白马山杂岩体附近的古台山石英脉型金锑矿[4]和高家坳微细浸染型金矿床[5−6]；天龙山岩体附近的大新破碎蚀变岩夹石英脉型金矿[7]；芙蓉岩体附近的廖家坪金锑钨矿；紫云山岩体附近的包金山金矿等；其成矿与印支期的岩浆活动有着密切的关系。

白马山−龙山−紫云山隆起带为湘中重要的金多金属成矿带，其上的金属矿床主要赋存于受岩浆上侵穹窿作用形成的放射状或环状断裂中[7−11]。包金山金矿位于该成矿带的东段(见图1(b))，为典型的受紫云山穹窿环状断裂和地层联合控制的矿床，以石英脉型为主，其成矿流体属岩浆期后热液，与矿区内的花岗闪长斑岩有着密切联系[12−19]。但是以往地质工作对于矿区内花岗闪长斑岩的研究还处于空白，严重制约了对该区成矿作用的认识。

本文作者在野外实地调查的基础上，通过高精度 SIMS 锆石 U-Pb 测年及锆石原位 Hf-O同位素分析，精确构建区内花岗闪长斑岩脉的年代学格架，深入探讨了花岗闪长斑岩的源区特征及其形成的动力学背景，不仅为研究紫云山地区成岩成矿作用奠定了基础，而且有助于进一步研究华南地区早中生代构造−岩浆演化过程。

1 矿区地质概况

包金山金矿床位于湘中盆地的东缘，紫云山岩体北侧3 km处。在大地构造位置上，该区处在扬子地块与华夏地块的对接带(见图1(a))，长期以来经历了复杂的构造岩浆活动，并伴有大规模的金属成矿活动。矿区出露的地层为元古代板溪群马底驿组浅变质岩系，赋矿地层为第二段倾向北的钙质板岩夹钙质条带状板岩及灰岩透镜体。矿区主构造线方向为近东西向(见图2)，以断裂构造为主，近东西向断裂为紫云山穹窿环状断裂的一部分；近南北向、北东向及北西向破矿断裂。区内花岗闪长斑岩分布广泛，但产出面积较小，多为脉状，呈NW走向，倾向北东。本区主成矿期与印支晚期中酸性岩浆的热液活动有关[16−18]。矿区内的花岗闪长斑岩与矿体相互穿插，在其转弯处常有富矿体存在，其对成矿具有重要的叠加改造作用，并为成矿物质活化运移及矿体的进一步富集提供了热量和能量，并提供了部分的成矿物质[13, 17−19]。围岩蚀变以绢云母化、硅化、黄铁矿化与金成矿的关系最为密切。

本区金矿以石英脉型为主，主要有脉状和筒状两类矿体。其中脉状矿体多呈群脉雁形排列，走向NW，倾向SW，倾角45°~67°。该类矿体走向延伸较短，一般在20 m左右，倾向延深相对较长，矿体厚度0.2~2.0 m。筒状矿体的走向和倾向与脉状矿体相近，但规模相对较大，厚度几米到十几米，倾向延深可达百米。金矿化主要位于石英脉与围岩的接触部位，近围岩的石英脉内常见明金，金品位变化较大。

图1 湘中区域地质略图

图2 包金山矿区地质略图

2 岩石岩相学特征

通过系统的野外地质观察及镜下显微鉴定分析，本次所采集的样品均为花岗闪长斑岩。岩石呈灰黑色，斑状结构，块状构造(见图3(a)和(b))，其浅色矿物为长石、石英，暗色矿物以黑云母为主，斑晶含量约20%~25%，主要由长石(15%~20%)和石英(5%~8%)组成，基质含量约75%~80%，主要由它形粒状石英、长石和黑云母。副矿物主要为锆石、磁铁矿等。长石主要是斜长石、次为正长石。长石常沿边缘、粒间发生不同程度的绢云母化，整体蚀变强烈，长石可呈残余状产出，局部甚至全部蚀变为绢云母而保留原长石的晶体外形(见图3(d)、(e)和(f))；石英分布零星，主要呈椭圆状、圆状以斑晶的形式分布，集合体粒度一般0.5~1.5 mm(见图3(c)和(e))。

3 样品采集及测试分析

用于锆石测年研究的花岗闪长斑岩样品共有2件，样品BJS-1采自钻孔，样品BJS-2采自10中段的44线附近，样品均未发生风化。

3.1 锆石U-Pb年龄测定

锆石的挑选由河北省廊坊市诚信地质服务有限公司完成，样品破碎至0.3 mm以下，用人工淘洗和电磁选方法富集锆石，再在双目镜下精选锆石，未使用任何化学药剂。将精选的锆石用无色透明环氧树脂固定并进行抛光，使内部暴露，进行透射光和反射光下照相，并用阴极发光扫描电子显微镜照相，以了解锆石的内部结构，系统对比的基础上选出最理想的供分析的锆石颗粒。锆石阴极发光(CL)内部结构照相在中国科学院地质与地球物理研究所Cameca电子探针仪器上完成，分析电压为50 kV，电流为15nA。锆石U-Pb定年在中国科学院地质与地球物理研究所离子探针实验室的Cameca IMS-1280型二次离子质谱仪(SIMS)上进行，详细的分析流程见文献[20]。锆石样品的Pb/U比值用标准锆石 TEMORA 2(417 Ma)[19]的ln(206Pb/238U)与ln(238U16O2/238U)之间的线性关系校正[22]；Th和U含量用标准锆石91500[23]计算；普通Pb用测量的204Pb进行校正；用现代地壳的平均Pb同位素组成[24−25]作为普通Pb组成进行校正。单点分析的同位素比值及年龄误差为1(为绝对误差)，U-Pb平均年龄误差为95%置信度。

3.2 锆石原位Hf、O同位素分析

锆石微区原位Hf同位素分析在中国地质大学(武汉)地质过程与矿产资源国家重点实验室(GPMR)利用激光剥蚀多接收杯等离子体质谱(LA-MC-ICP-MS)完成。激光剥蚀系统为GeoLas 2005 (Lambda Physik，德国)，配备了信号平滑装置，使用氦气作为载气，并引入少量氮气来提升元素的灵敏度[26]。详细仪器操作条件和分析方法参照文献[27]。锆石样品自身的Yb用于干扰校正[28]；179Hf/177Hf = 0.7325和173Yb/171Yb= 1.132685[29]用于计算Hf和Yb的质量分馏系数Hf和Yb；179Hf/177Hf 和173Yb/171Yb的比值用于计算Hf(Hf) 和Yb (Yb)的质量偏差；使用176Yb/173Yb=0.79639[29]来扣除176Yb对176Hf的同量异位干扰；使用176Lu/175Lu=0.02656[30]来扣除干扰程度相对较小的176Lu对176Hf的同量异位干扰；采用 Yb 的质量分馏系数Yb来校正Lu的质量分馏行为。分析数据的离线处理(包括对样品和空白信号的选择、同位素质量分馏校正)采用软件ICP-MS Data Cal完成[31]。

锆石微区原位 O 同位素分析在中国科学院地质与地球物理研究所离子探针实验室的Cameca进行。将做过SIMS锆石U-Pb定年的样品靶再次磨去约5 μm，以消除前期在U-Pb 定年时造成的氧污染，详细的分析流程参见文献[20]。仪器质量分馏校正采用 91500 锆石标准，其中 91500 标准锆石的18O= 0.99%[32]，测量的18O/16O比值通过VSMOW值 (18O/16O= 0.0020052)校正后，加上仪器质量分馏校正因子IMF即为该点的δ18O值：(18Ο)=((18O/16O)M/0.0020052−1)×100%；IMF= (18O)M(standard)−(18O)VSMOW；18OSample=(18O)M+IMF。

图3 花岗闪长岩岩相学特征

4 分析结果

4.1 锆石U-Pb年龄

供U-Pb同位素定年的2件花岗闪长斑岩样品中的锆石呈灰色长柱状或短柱状，颗粒较大，长度100~200 μm，长宽比一般为1.5~4，晶体自形程度较好，大部分具有清晰韵律环带结构(见图4)，并且锆石的Th/U比值主要介于0.10~0.90之间(见表1和表2)，具有岩浆锆石的特点[33]。利用Isoplot[34]绘制锆石的谐和曲线及谐和年龄的投影图(见图5)，数据点多分布在谐和线上或在谐和线附近呈线状分布，表明所测的锆石颗粒在形成后U-Pb同位素体系是封闭的，基本未发生U或Pb的加入和丢失[35−37]。

样品BJS-1进行了25个点的定年分析，其中有4个测点(8、15、17、18)年龄为离群值，其余21个测点206Pb/238U数据得到加权平均年龄(225.5±3.3) Ma(95%置信度，MSWD=0.21)，谐和年龄为(225.1±1.5) Ma(95%置信度，MSWD=0.40)(见图5(a))。

图4 包金山金矿床花岗闪长斑岩锆石CL图像及分析点位图

图5 包金山金矿床花岗闪长斑岩锆石U-Pb年龄谐和图

样品BJS-2进行了25个点的定年分析，其中有4个测点(1、9、18、21)年龄为离群值，其余21个测点206Pb/238U数据得到的加权平均年龄为(223.5±3.2) Ma(95%置信度，MSWD=0.25)，谐和年龄为(223.3±1.4) Ma(95%置信度，MSWD=1.9)(见图5(b))。

因此，包金山矿区两条花岗闪长斑岩的形成年龄分别为(223.3±1.4) Ma 和(225.1±1.5) Ma，其形成时间在误差范围内一致，属印支晚期的产物。

4.2 锆石的Hf、O同位素组成特征

对样品BJS-1的锆石进行微区原位Hf同位素分析，初始176Hf/177Hf比值为0.282494~0.282729之间，平均为0.282538，对应的Hf()为−5.3~3(见图6(a))，平均为−3.4，两阶段模式年龄DM2为0.96~1.41 Ga，平均为1.32 Ga。锆石微区原位O同位素分析，18O为0.824%~0.973%(见图6(b))，平均为0.907%。

对样品BJS-2的锆石进行微区原位Hf同位素分析，初始176Hf/177Hf比值为0.282286~0.282592，平均为0.282498，对应的Hf()为−5.9~−2.3(见图6(a))，平均值−4.5，两阶段模式年龄DM2为1.25~1.45 Ga，平均为1.37 Ga。锆石微区原位O同位素分析，18O为0.866%~0.948%(见图6(b))，平均为0.916%。

对花岗闪长斑岩样品BJS-1和BJS-2的锆石颗粒(包括定年的锆石)进行了微区原位Hf-O同位素测定，分析结果分别见表3和4。

5 讨论

5.1 成岩年代及意义

本实验中采用高精度SIMS锆石U-Pb测年方法获得的包金山矿区花岗闪长斑岩年龄为(225.1±1.5) Ma 和(223.3±1.4) Ma，在误差范围内一致。其与临近的紫云山花岗岩体(锆石SIMS U-Pb(227.0±2.2)~ (225.2±1.7) Ma[38]；锆石LA-ICP-MS U-Pb(222.5±1.0) Ma~(222.3±1.8) Ma[39])、歇马花岗岩体(锆石SHRIMP U-Pb(218±3) Ma[40]；锆石SHRIMP U-Pb(214.1±5.9) Ma[41])以及与区域上的南岳花岗岩体(锆石LA-ICP-MSU-Pb(215.5±1.5) Ma[42]、沩山花岗岩体(黑云母Rb-Sr(227.0±13) Ma和(221.9±5.8) Ma[43])、大神山花岗岩体(锆石LA-ICP-MS(224.3±1.0) Ma[44])、白马山花岗岩体(锆石LA-ICP-MS U-Pb(224.3±2.4) Ma、(221.4±4.0) Ma和(226.6±4.1) Ma[45])的成岩时限基本一致，表明矿区内花岗闪长斑岩属于印支晚期(230~200 Ma)大规模岩浆活动的组成部分[37, 46]。该类花岗闪长斑岩在区域上广泛发育，在紫云山岩体主体花岗岩中也偶见出露，其与紫云山岩体应属同期不同阶段的产物，形成时间应略晚于后者。

表1 包金山矿区花岗闪长斑岩样品BJS-1锆石U-Pb(SIMS)分析结果

表2 包金山矿区花岗闪长斑岩样品BJS-2锆石U-Pb(SIMS)分析结果

Test method: SIMS; Test unit: Beijing Nano SIMS Lab, Institute of Geology and Geophysics, Chinese Academy of Sciences.

图6 包金山金矿床花岗闪长斑岩的Hf和O同位素组成

表3 包金山金矿床花岗闪长斑岩BJS-1锆石Hf-O同位素分析结果

包金山金矿产于紫云山岩体入侵形成的环状断裂内，那么其成矿时间不会早于紫云山岩体的侵位时间；而花岗闪长斑岩脉穿插包金山矿脉，表明包金山成矿不会晚于花岗闪长斑岩的形成时间。因此，包金山成矿时间应介于紫云山岩体((227.0±2.2)~(225.2±1.7) Ma[38])和包金山花岗闪长斑岩((225.1±1.5) Ma 和(223.3±1.4) Ma)侵位时间之间，即227~223 Ma。

5.2 岩浆源区及意义

对花岗闪长斑岩锆石原位Hf-O同位素数据统计(见图6)，其Hf-O同位素的组成基本相似，基本可以重叠。初始176Hf/177Hf值均集中分布在0.2825附近，对应的Hf()值在−5.9~3.0之间，平均为−3.98；两阶段模式年龄DM2为0.96~1.45 Ga，平均为1.35 Ga；18O值为0.824~0.973%之间，均大于地幔18O值((0.53±0.03)%)[47]；在Hf()-U/Pb Age图解中(见图7(a))，样品点主要落于下地壳区域，表明花岗闪长斑岩为中元古代下地壳岩石重熔形成。而在18O-Hf()图解中(见图7(b))，Hf-O同位素完全落在以大容山过铝质花岗岩为代表的地壳端元区域以外，表明源区有幔源岩浆的加入，且加入地幔物质的比例至少可达20%[48−49]。样品BJS-1的12号测点，初始176Hf/177Hf比值较大，对应其年龄计算的Hf()值为3.0(见表3和4)，其可能为幔源物质参与的结果。与临近的紫云山岩体相比，两者的Hf同位素组成基本相同，分布范围和峰值基本一致(见图6(a))；两者的O同位素组成也基本相似，不过花岗闪长斑岩18O值的峰值略高一些(见图6(b))，这可能是由于花岗闪长斑岩后期发生蚀变的结果。在图7中，花岗闪长斑岩与紫云山花岗岩落于同一区域，表明两者为同源的 特征。

表4 包金山金矿床花岗闪长斑岩样品BJS-2锆石Hf-O同位素分析结果

Zirons Hf isotopic: LA-MC-ICP-MS, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan) Airons Hf Isotopic; Beijing nano SIMS Lab, Institute of Geology and Geophysics, Chinese Academy of Sciences.

因此，本文作者认为区内花岗闪长斑岩由中元古代下地壳岩石熔融，与幔源岩浆形成的壳、幔混合岩浆源区演化而来，其与紫云山花岗岩体应属于同源岩浆不同演化阶段的产物。

5.3 动力学背景探讨

在华南地块内印支晚期花岗岩的逐渐确认，尤其是一系列印支晚期 A 型花岗岩的发现，如湖南的锡田岩体、浙江的大爽岩体、江西蔡江岩体和福建高溪岩体等[39, 50−55]，对于华南印支晚期花岗岩形成于地壳伸展构造体制的认识已经成为共识。但对于华南印支期花岗岩形成的动力学背景还存在分歧，部分学 者[44, 53−60]认为印支运动期间(约260 Ma)，印支地块向北挤压，与华南地块发生碰撞，导致地层加厚，之后10~20 Ma发生热−应力松弛作用，进入伸展阶段，地壳减压熔融，进而形成花岗岩。LI等[61]认为，在中二叠世(约280 Ma)太平洋板块西缘转为活动大陆边缘，开始向华南地块俯冲，到三叠世华南地块发生造山运动，之后进入伸展−减薄构造背景，地壳熔融形成花岗岩[62]。从印支期华南地块内的构造变形来分析，其以NE-NNE向的褶皱及逆冲推覆构造为主，单纯印支地块引起的南北向的挤压应力很难解释这些构造的形成。这些构造的形成应与同期东部太平洋板块的俯冲有着密切的关系。

图7 包金山金矿床花岗闪长斑岩εHf(t)−U/Pb图解(a)和δ18O−εHf(t)图解(b)

所以，本文作者认为华南印支期应该属于多板块汇聚的动力学背景，其构造变形和同期花岗岩的形成应为印支地块向北挤压和太平洋板块向西俯冲共同作用的结果。

6 结论

1) 包金山矿区两条花岗闪长斑岩锆石SIMS U-Pb 年龄分别为(225.1±1.5) Ma和(223.3±1.4) Ma，在误差范围内一致，其与印支晚期(230~200 Ma)大规模的岩浆活动时限一致，并间接的指示了包金山成矿时间为227~223 Ma，属典型印支晚期成矿。

2) 花岗闪长斑岩由中元古代下地壳岩石重熔与部分幔源岩浆组成的壳、幔混合岩浆源区演化而来，其与紫云山岩体为同源不同演化阶段的产物。

3) 花岗闪长斑岩形成于多板块汇聚的动力学背景，为印支地块向北挤压和太平洋板块向西俯冲共同作用的结果。

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(编辑 龙怀中)

Zircon U-Pb ages and Hf-O isotopes of granodiorite -porphyry in Baojinshan mining area and their geological significance

LU Yu-long1, 3, PENG Jian-tang1, 2, YANG Jie-hua2, LI Yu-kun1, CHEN Xian-jia1, ZHOU Xi1, 3, LI Gan-long3

(1.Key Laboratory of Non-ferrous Metals Metallogenic Prediction, Ministry of Education,School of Geosciences and Info-physics, Central South University, Changsha 410083, China;2. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry,Chinese Academy of Sciences，Guiyang 550002, China;3. The Second Corps of Nonferrous Geological Prospecting Bureau in Hunan Province,Geological Exploration Institute, Xiangtan 411102, China)

Baojinshan gold deposit is Au-dominated polymetallic deposit discovered recently in central Hunan, its mineralization is considered to be closely related to the magmatic activities in late Indosinian. High accuracy SIMS zircon U-Pb age determination was used in this study to obtain the ages of two granodiorite-porphyry dykes at (225.1±1.5) Ma and (223.3±1.4) Ma, indicating that this deposit was the product of extensive magma activity in late Indosinian. The176Hf/177Hf values of two granodiorite-porphyry dykes intensively distributed around 0.2825, with the correspondingHf() ranged from −5.9 to 3, averaging −3.98. The TDM2model age was estimated from 0.96 GPa to 1.45 Ga, averaging 1.35 Ga; the18O value of granodiorite-porphyry ranged from 0.824% to 0.973%. Hf-O isotope analysis shows that the granodiorite-porphyry is mainly formed by the rock remelting of Mesoproterozoic lower crust, and mantle-sourced materials are considered to participate the diagenesis. Contrastive analysis on the granodiorite-porphyry in Baojinshan with Ziyunshan granite suggests that both of them are the products of comagmatic evolution, which indirectly indicates that the metallogenetic time of Baojinshan gold deposit ranges in 227−223 Ma. The granodiorite-porphyry is formed under the multi-plates convergent dynamics background, and affected by the northward squeeze of Indochina Block and westward subduction of Pacific Plate.

SIMS U-Pb zircon dating; Hf-O isotopes tracing; granodiorite-porphyry; Baojinshan gold deposit; central Hunan Province

Project (41473043, 41272096) supported by the National Natural Science Foundation of China

2016-06-17; Accepted date: 2016-10-25

PENG Jian-tang; Tel: +86-13787787177; E-mail: jtpeng@126.com

10.19476/j.ysxb.1004.0609.2017.07.16

1004-0609(2017)-07-1441-14

P588.121；P597.3

A

国家自然科学基金资助项目(41473043，41272096)

2016-06-17；

2016-10-25

彭建堂，教授，博士；电话：13787787177；E-mail：jtpeng@126.com