个旧老厂钾质煌斑岩矿物学特征及其锆石U-Pb和黑云母40Ar-39Ar年龄

曹华文, 裴秋明, 张寿庭, 王长明, 王光凯, 王 亮

(1. 中国地质调查局 成都地质调查中心, 四川 成都 610081; 2. 中国地质大学 地球科学与资源学院, 北京 100083; 3. 山东正元建设工程有限责任公司, 山东 济南 250100)

0 引 言

钾质-超钾质岩浆岩富集轻稀土元素、大离子亲石元素、挥发分、Cl和F等卤族元素[1–2]。Müller et al.[2]认为富钾岩浆岩主要形成于五种构造背景: 大陆弧、后碰撞弧、晚期洋弧、初始洋弧和板内弧环境[3–4]。富钾岩浆岩具有多种成因观点: (1) 富集地幔的部分熔融, 没有地壳物质的混染[5–6]; (2) 富集地幔的部分熔融, 且经历地壳物质同化混染作用[7–8]; (3) 基性岩浆的部分熔融和结晶分异, 并经历地壳物质的混染[9]。目前, 富钾岩浆岩的起源较广泛接受的观点是:早期的俯冲板片交代地幔楔, 形成的富集地幔在板内后碰撞环境下再部分熔融的结果[3,10–12]。此外, 富钾岩浆岩还与多种矿产密切相关, 比如 Cu-Au矿[1,13]、Cu-Ni矿[14]和 Sn-W-Mo-Ag-U 矿[15–17]等。

钾质煌斑岩(即钙碱性煌斑岩)属富钾岩浆岩中重要的中-基性岩类。煌斑岩是富集地幔低程度部分熔融的产物[18], 其化学成分与源区成分近似, 能够揭示地幔信息[5,19]。因此, 钾质煌斑岩不仅能提供地球动力学背景的信息, 如洋壳俯冲作用[20–21]、大陆(岩石圈)伸展减薄作用[14,22–25]、克拉通破坏/岩石圈拆沉作用[5,6,26]和大陆裂解与碰撞作用[27–28]等; 也能由其识别地幔交代富集过程[8,29]和大陆岩石圈地幔组成的信息[30–31]等。

造岩矿物的化学组成通常保存了其形成时周围岩浆介质的物理化学信息[32–35], 是一种直接研究岩浆结晶演化及成岩温压条件的理想对象[33]。因此,对岩浆岩进行系统的矿物学研究对探讨个旧煌斑岩演化具有重要的意义[36–37]。个旧锡铜多金属矿床东区老厂花岗岩岩体周缘发育不同类型岩脉, 包括煌斑岩脉、花岗斑岩脉、辉长岩脉和细晶-伟晶岩脉等[38]。老厂煌斑岩脉是个旧杂岩体的重要组成部分, 能够反映个旧地区深部构造-岩浆作用过程, 所以老厂煌斑岩脉的研究能够对个旧杂岩体的成因演化提供重要的证据。前人对老厂煌斑岩的研究主要集中在元素地球化学[39]、同位素地球化学[40]和锆石 U-Pb年代学[41]等, 尚缺少对煌斑岩造岩矿物的系统分析及更加详细的年代学研究。

本文主要以个旧老厂东区迎风山、羊坝底、仙人坡和干坡坡煌斑岩脉为研究对象, 开展辉石、黑云母和长石等的电子探针矿物组成分析、锆石TIMS U-Pb年龄和黑云母40Ar-39Ar年代学研究。在前人研究的基础上, 综合对比区域内岩浆岩活动,揭示区内煌斑岩脉的成因及其与区内 Sn-Cu多金属矿的关系, 并进一步探讨煌斑岩形成的地球动力学背景等。

1 地质概况

个旧是世界级规模的超大型锡铜多金属矿集区, 其有色金属总储量超过1000万t, Sn资源量超过 300万 t[42–44], 是我国重要的 Sn、Cu、Pb和 Zn多金属资源储备基地[45]。个旧矿区位于扬子板块西缘[46], 即义敦岛弧、思茅地块和华夏板块的交汇部位(图 1a)。早新元古代(950～820 Ma), 扬子板块东缘与华夏板块碰撞[50]; 晚二叠世到早三叠世(260～250 Ma), 扬子板块西缘发育地幔柱, 峨眉山玄武岩喷发; 晚三叠世(约 230 Ma[51–52])古特提斯洋封闭, 扬子板块西缘与思茅地块拼合; 中生代侏罗纪(200 Ma)至今, 研究区进入板内演化阶段。

个旧矿区内断裂和褶皱发育, 南北向个旧断裂将矿区划分为东、西两区, 锡多金属矿床(点)多位于个旧东区。北北东向的五子山复式背斜及东西向的5条压扭性大断裂(图1b)将东区矿带自北向南分为马拉格、松树脚、高松、老厂和卡房五个矿田[49](图 1b)。

矿区出露的地层主要为三叠系碳酸盐岩、碎屑岩及基性火山岩(244.4 Ma[42])等, 厚达 5000余 m;主要由三叠系上统火把冲组(T3h)、鸟格组(T3n), 中统法郎组(T2f)、个旧组(T2g)和三叠系下统永宁镇组(T1y)、飞仙关组(T1f)组成(图 1a)。区内岩浆活动十分强烈, 是该区形成大型、超大型矿床的重要条件之一, 其中以燕山期中酸性岩浆侵入岩(85～78 Ma[44])分布最为广泛(图1b)。

煌斑岩脉主要在个旧东区老厂矿田东部出露(图1c), 围岩地层为中三叠统个旧组碳酸盐岩地层。目前地表见有四条煌斑岩脉, 分别出露于迎风山、羊坝底、仙人坡和干坡坡。岩脉主要受北东向和东西向断裂控制, 沿走向呈斜列式分布, 倾向变化较大(320°～360°), 倾角 50°～80°。

2 煌斑岩岩相学和地球化学特征

图1 个旧地区地质简图及煌斑岩脉分布图Fig.1 Sketch map showing the geology and distribution of lamprophyres in the Gejiu area

图2 个旧煌斑岩显微镜下矿物照片(正交偏光)Fig.2 Micro-textures of minerals from lamprophyres in the Gejiu area (Cross-polarized light)

老厂煌斑岩呈深灰绿色, 变余煌斑结构, 块状构造。岩石由斑晶、微晶(基质)和少量杏仁体组成。研究区煌斑岩矿物含量的特征如下, 斑晶包括蚀变单斜辉石(＜ 5%)、橄榄石(＜ 5%)、黑云母(＜ 5%)、石英(约 10%)和斜长石(约 5%), 呈自形半自形晶, 粒径0.5～8.6 mm(图2)。橄榄石几乎全为假象晶, 蛇纹石化严重, 偶见包橄结构。单斜辉石呈无色—浅黄绿色柱状, 有的核部被绿泥石、黑云母交代。黑云母含Ti较高, 呈红褐色片状。石英和斜长石具熔蚀现象, 斜长石为中长石, 具聚片双晶结构, 部分具麻点状边环及碳酸盐化。研究区煌斑岩中的石英斑晶与哀牢山缝合带北衙地区[53]的特征类似, 可能属于基性岩浆与中酸性岩浆混合的标志, 或者是富含挥发分的煌斑岩浆快速上升侵位的结果。微晶由钾长石(30%～35%)、蚀变单斜辉石(约 20%)、黑云母(15%～20%)和少量石英(＜ 5%)组成, 粒径多小于0.3 mm。钾长石呈他形—半自形板状微晶, 单斜辉石、黑云母特征及蚀变现象同斑晶, 石英呈他形粒状充填上述矿物粒间。少量杏仁体呈不规则形状, 被绿泥石、碳酸盐、石英充填。副矿物主要有磁铁矿、磷灰石及锆石等。据此矿物学特征, 研究区煌斑岩应为蚀变橄辉云斜煌斑岩[32,54,55], 矿物成分特征与辽东半岛钾玄质云斜煌岩[6]类似。在岩石化学组成上[39], 老厂煌斑岩为钾质钙碱性煌斑岩, 属超钾质岩石[56]系列。

3 样品分析方法

测试样品选自老厂迎风山(LCD01、LCD02)、羊坝底(LCD04、LCD06)、仙人坡(LCD07、LCD08)和干坡坡(LCD13)的新鲜煌斑岩, 采样位置如图 1c所示。电子探针(EPMA)矿物学组成的测试由中国地质大学(北京)电子探针实验室完成。样品制备成0.05mm厚的光薄片, 测试仪器为EPMA-1600型电子探针; 测试条件: 加速电压 15kV, 束流 20nA, 束斑直径1μm; 分析误差一般小于1%; 测试矿物为辉石、黑云母、长石、磷灰石和磁铁矿。

融合DPI设备应对移动通信网络的省集中核心网的信令面（S6aa、S1-MME、S110/S11,Gx/RRx、Radius、L2TP、Mw/MMg/Mj/ISC/GGm、Cx/Dx/SSh/Zh等接口）及用户面（S1-U、S5//8、S2a、Gnn/Gp等接口）所涉及的链路进行采集。如下图所示：

煌斑岩样品锆石和黑云母单矿物的分选通过全岩粉碎、过筛、手工淘洗、重液分离和实体显微镜检查等工序完成, 锆石和黑云母单矿物样品的纯度大于99%。

煌斑岩颗粒锆石TIMS U-Pb定年由核工业北京地质研究院分析测试研究中心完成。测试仪器为ISOPROBE-T 热电离质谱, 详细测试流程和实验方法参见文献[57–59], 实验全流程Pb空白为0.005～0.03 ng,U空白为0.002 ng。

煌斑岩黑云母单矿物样品与北京房山花岗岩黑云母(ZBH-25)和美国鱼谷凝灰岩中的透长石(FCT-01)一起送往中国原子能研究院的原子反应堆 B4孔道进行快中子照射。照射时间为8.06 h, 快中子通量为1.86×1017n/cm2。样品测量在中国地质大学(北京)科学研究院实验中心同位素地质年代室MM-5400质谱仪上完成, ZBH-25标准样品年龄为(133.2±0.2) Ma,FCT-01标准样品年龄为(27.8±0.1) Ma。实验流程详见王瑜等[60]。Ar-Ar坪年龄谱及等时线年龄计算应用Isoplot 3.75程序[61]分析, 误差精度为2σ。Ar-Ar年龄测试的详细测试方法及步骤见文献[62]。

4 分析结果

4.1 锆石TIMS U-Pb年龄

个旧煌斑岩中的锆石为浅黄色、透明、自形长柱状、柱状晶体。锆石的U-Pb同位素测定及表面年龄计算结果列于表 1和图 3。钾质煌斑岩中的锆石通常较少[8], 仅挑选出极少量岩浆锆石。本次实验对2颗锆石进行了U-Pb同位素测定。其普通铅含量较低,经对实验空白校正后的206Pb/204Pb值较高, 说明其测试精度较高[59]。2个测点均位于协和线上, 具有一致的206Pb/238U 表观年龄, 因此其加权平均值(87.1±1.3) Ma(MSWD = 0.002)代表锆石的形成年龄。该年龄与老厂似斑状花岗岩锆石((83.3±1.6) Ma[44])和老厂等粒花岗岩锆石年龄((85.8±0.8) Ma[44])基本一致(图1b)。

4.2 黑云母40Ar-39Ar年龄

经过 13～14个阶段的逐级加热后得到研究区 6个煌斑岩黑云母斑晶样品40Ar-39Ar年龄数据(表 2),从而获得样品的40Ar-39Ar年龄图谱和40Ar-39Ar反等时线年龄(图4)。

迎风山(LCD01)黑云母的坪年龄为(71.5±0.4) Ma,反等时线年龄为(71.8±0.3) Ma (MSWD = 1.17) (图4a和 4b); 迎风山(LCD02)黑云母的坪年龄为(74.2±0.4) Ma, 反 等 时 线 年 龄 为 (75.5±1.3) Ma(MSWD = 2.3) (图 4c和 4d)。羊坝底(LCD04)黑云母的坪年龄为(74±0.3) Ma, 反等时线年龄为(74±1.3) Ma(MSWD = 6.1) (图4e和4f); 羊坝底(LCD06)黑云母的坪年龄为(73.8±0.4) Ma, 反等时线年龄为(74.3±1.1) Ma (MSWD = 4.4) (图 4g 和 4h)。仙人坡(LCD07)黑云母的坪年龄为(70.2±0.4) Ma, 反等时线年龄为(71.2±0.4) Ma (MSWD = 0.58) (图4i和4j);仙人坡(LCD08)黑云母的坪年龄为(71.3±0.4) Ma,反等时线年龄为(73.1±2.2) Ma (MSWD = 7.6) (图4k和 4l)。

上述6个样品的坪年龄和反等时线年龄在误差范围内均一致, 且40Ar/36Ar初始比值(图4)和现代大气 Ar比值(295.5±0.5)较接近, 说明测试的样品中不存在过剩的Ar, 并且也无显著的Ar丢失。指示样品自结晶作用以来对K和Ar保持封闭体系, 受后期地质热事件的扰动较弱, 均为可信的地质年龄。所以, 老厂煌斑岩形成时代为 75～70 Ma, 平均年龄为 72.9 Ma, 该年龄与老厂-卡房(老卡)花岗岩体钾长石40Ar-39Ar年龄近一致(71.6 Ma[63])。

图3 煌斑岩中锆石U-Pb年龄协和图Fig.3 U-Pb concordia diagram of zircons from lamprophyres

表1 个旧煌斑岩中锆石TIMS法U-Pb同位素年龄分析结果Table 1 Analytical results of zircons from lamprophyres in the Gejiu area

表2 个旧老厂煌斑岩脉黑云母在加热过程中的40Ar/39Ar释放数据Table 2 40Ar/39Ar stepwise heating data for biotite from lamprophyres

(续表 2)

4.3 辉石的矿物学特征

本次研究采集的样品中单斜辉石发生了部分蚀变; 此外, 辉石在结晶以后易与基质中甚至相邻的矿物发生元素交换, 这会导致辉石成分的改变, 故辉石成分变化较大。单斜辉石电子探针分析结果(表 3)显示, 煌斑岩中的单斜辉石斑晶和微晶成分近一致,总体表现为贫Ti、Al、Na, 富Ca、Fe和Mg特征。

表3 研究区单斜辉石化学成分(%)及端元组分计算Table 3 Chemical composition (%) and end-member proportion of clinopyroxene in the study area

(续表 3)

(续表 3)

Ca∶Mg∶∑(Fe+Mn)平均为 43.65∶47.75∶8.6, 属 Ca-Fe-Mg辉石族(图 5a); 在辉石 Wo-En-Fs分类图中[64],主要位于普通辉石范围内, 靠近透辉石区域(图 5b)。其Ti-Al特征类似于超钾质岩-过渡带辉石[66](图5c)。辉石 Mg#值分布于 79.7～100之间, 在单斜辉石SiO2-Al2O3图解中[67](图6a), 主要落入亚碱性区, 少部分落入碱性区; 在Si-AlⅣ图解[68](图 6b)和 Ti-(Ga+Na)图解[69]中(图 6c), 均位于拉斑玄武岩区。上述特征暗示其母岩浆属于碱性-亚碱性系列, 拉斑玄武岩浆; 并且,辉石单矿物与全岩成分[39]的岩浆系列判别结果一致。

图5 个旧煌斑岩辉石分类图解Fig.5 Classification diagrams for clinopyroxenes from lamprophyre veins

图6 单斜辉石岩浆系列判别图解Fig.6 Discrimination diagram of the clinopyroxene series in lamprophyre veins

由辉石 Al2O3-Mg#图解[70](图 7a)和 FeOt/MgO(辉石)-FeOt/MgO(全岩)图解[71](图 7b)可知, 辉石总体是沿平衡结晶的趋势, 表明辉石与其寄主岩石达到了平衡。少部分辉石颗粒 KD(cpx)大于 0.4, 指示可能为早期结晶的产物。因此, 煌斑岩结晶时的岩浆系列同母岩浆系列一致, 这证明岩浆源区熔融程度低, 且岩浆在演化过程中没有经历充分的分离结晶作用。

随着单斜辉石的结晶, FeOt与MgO、CaO均呈负相关关系, 表明发生了Fe2+对Mg2+和Ca2+的替代;这种结晶趋势[72]表明单斜辉石形成于中高温环境(＞ 1000 )℃。应用辉石-熔体温压计[73]可以估算岩浆形成的温度和压力条件。据此计算出的温度稍高于二辉石温度计(图 5b)估算的结果。单斜辉石斑晶和微晶形成温度分别为1219～1282 (℃平均1255 )℃和1159～1215 (℃平均1199 ); ℃压力分别为0.3～1.1 GPa(平均 0.86 GPa)和 0.1～1.3 GPa (平均 0.62 GPa); 深度估算采用的静岩压力梯度为 30.3 MPa/km, 则其对应的结晶深度分别为 10～50 km (平均 29 km)和4～44 km (平均20 km)。辉石形成压力和深度变化较大, 从下地壳到中上地壳均有辉石结晶形成, 而形成温度较高且稳定; 这证明其不是在深部岩浆房中结晶后再随岩浆侵入地表时带出, 而是随岩浆快速上升过程中逐步结晶而成。

在岛弧环境中, 母岩浆富水, 具有较高的氧逸度, 单斜辉石主要采用ⅣMgⅣSi =ⅥFe3+ⅣAl方式替代, 形成 CaFe3+AlSiO6分子, 因此相对于非造山环境的单斜辉石, 相对富Si, 贫Al和Ti, 且Alz/Ti比值远远大于非造山环境[74]。老厂煌斑岩样品从辉石斑晶到辉石基质, 存在微弱的由透辉石向普通辉石演化的趋势(图5b), 表明存在Fe对Mg的替代关系; 这同时可从辉石FeO与MgO呈负相关性得到验证。这种替代关系证明研究区煌斑岩浆起源于岛弧环境[75], 岩浆体系为高温, 中等大小氧逸度的特点。

单斜辉石 Alz-TiO2图[74]中(图 8a), 依靠斜率可以判别老厂煌斑岩为与弧有关的玄武岩。单斜辉石主要氧化物含量百分数 F2-F1双因子图解[76]中(图8b), 老厂煌斑岩单斜辉石样品投入“火山弧+洋底玄武岩区”, 更靠近火山弧玄武岩区域。表明其母岩浆的形成与大洋板块向大陆板块的消减过程中的洋壳板块的熔融有关。

4.4 黑云母的矿物学特征

黑云母中的 Fe3+和 Fe2+含量采用林文蔚和彭丽君[77]的待定矿物学式阳离子数法确定。黑云母的主元素探针分析结果列于表 4。黑云母中的Fe2+/(Fe2++Mg)比值均一, 说明老厂煌斑岩未遭受后期流体的显著改造[78]。老厂煌斑岩黑云母斑晶和黑云母微晶具有较明显的差异, 微晶黑云母更富TiO2、MgO, 相对贫 FeO。两者镁铁比值(MF)分别平均为 1.02和 1.28; 含铁系数(Fe’)分别为 0.52和0.38, 含镁系数(Mg’)分别为 0.48和 0.62, Mg#分别为51.1和 64.0。黑云母成分分类图解中(图 9a), 斑晶属铁质黑云母和镁质黑云母; 微晶云母属镁质黑云母。

图7 单斜辉石平衡演化判别图解(底图分别据文献[70]和[71])Fig.7 Discrimination diagram of the evolution for clinopyroxenes from lamprophyre veins

图8 辉石构造背景判别图解Fig.8 Tectonic environment discrimination diagrams for clinopyroxenes from lamprophyre veins

表4 研究区黑云母化学成分组成Table 4 Chemical composition of biotite in the study area

(续表 4)

(续表 4)

图9 煌斑岩黑云母命名、氧逸度和温度判别图解Fig.9 Discrimination diagram of the classificatiton, temperature, pressure and oxygen fugacity for biotites from lamprophyre veins

富Mg、高Fe3+/Fe2+比值的黑云母属于典型的钾玄质岩石系列[2]。与磁铁矿、钾长石共生的黑云母成分可以估算岩浆结晶时的氧逸度和温度[80–82]。个旧煌斑岩黑云母样品大致落在 Fe3+-Fe2+-Mg图解中Ni-NiO(NNO)缓冲线上(图 9b)。结合其黑云母稳定度(100×Fe/(Fe+Mg))投影到黑云母 lgfO2-t图解(图9d), 得到黑云母斑晶和微晶其形成时大致的温度分别为 780～900 ℃和 880～1020 ℃, 氧逸度 lgf(O2)分别为–11～ –15 和–10～ –13。此方法获得的黑云母形成温度与Henry et al.[81]设计的“变质岩中黑云母Ti温度计”投影的结果(图 9c)近似, 但稍偏高。黑云母全铝压力计可以估算其结晶形成时的压力和深度[85],个旧煌斑岩黑云母斑晶和微晶的结晶压力分别为33～116 MPa (平均 82 MPa)和 35～135 MPa (89 MPa),其对应的结晶深度分别为 1.2～4.4 km(平均 3.1 km)和 1.3～5.1 km (平均 3.4 km)。

FeOt/(FeOt+MgO)-MgO物质来源判别图解[83](图10a)中, 黑云母斑晶和微晶均位于典型壳幔混源和典型壳源范围分界线附近, 表明其物源以壳源物质为主, 微晶黑云母中幔源物质混入更多。在MgOFeOt-Al2O3图解[84](图 10b)中, 主要属造山带钙碱性岩浆岩系列。综上所述, 黑云母斑晶和黑云母微晶之间成分、形成温度和深度均存在较大的差异;微晶更偏幔源, 斑晶更偏壳源。

4.5 长石的矿物学特征

长石的主元素探针分析结果列于表 5。老厂煌斑岩长石斑晶为斜长石(An25～40Ab57～69Or3～6), 具有较明显的环带结构和溶蚀结构。长石微晶为碱性长石(An1～6Ab21～38Or59～78)。在长石分类图上斑晶和微晶分别属于中长石-更长石和透长石(图11)。中长石斑晶的SiO2与Al2O3、CaO、An呈良好的负相关关系,而与 Na2O呈正相关关系, 表明斜长石遵循岩浆结晶分异的演化规律, 分异完全。从中长石斑晶到透长石微晶, Al2O3、CaO、An和Na2O均急剧降低, SiO2和K2O急剧升高。透长石微晶的化学成分与卡房花岗岩中的碱性长石接近[86]。

5 讨 论

5.1 煌斑岩的形成时代

在钙碱性基性岩浆中, 辉石、钛铁矿和榍石等的结晶会优先带走 Zr元素; 即, 在通常情况下, 钙碱性基性岩浆中Zr不能达到饱和形成岩浆锆石[8]。所以, 钙碱性基性岩浆岩(如钾质煌斑岩)中的锆石大部分是捕获锆石或继承锆石[88–91]。程彦博等[40–41]测试的老厂煌斑岩锆石 U-Pb年龄谐和曲线图中, 年龄值出现“分堆”现象(图 12b), 测试结果离散(MSWD =20[40–41]), 这可能是由于其包含两类锆石引起的。

图10 煌斑岩构造背景与物质来源图解Fig.10 Tectonic environment discrimination diagrams for biotites from lamprophyre veins

表5 研究区长石化学成分(%)及端元组分计算Table 5 Chemical composition (%) and end-member proportion of feldspar in the study area

一类是捕获锆石, 即老厂似斑状花岗岩体的锆石(约82 Ma), 代表了煌斑岩上侵过程对老厂花岗岩的捕获同化。并且程彦博等[40]测试的煌斑岩(铁镁质岩墙)锆石 εHf(82 Ma)为–9.5～ –5.1(平均值为–7.1),tDM2年龄平均值为 1300 Ma, 证明锆石应属壳源岩浆锆石, 即反映的是捕获的围岩(花岗岩)的锆石特征, 因此该锆石年龄值不能代表煌斑岩侵位的年龄。另一类为少部分岩浆锆石, 即煌斑岩岩浆冷凝结晶时形成的岩浆锆石(约73 Ma), 代表煌斑岩的形成时代。程彦博等[40]测试的第二类锆石U-Pb年龄与本次黑云母Ar-Ar研究结果(75～70 Ma, 平均72.9 Ma)一致(图12), 表明老厂煌斑岩(约73 Ma)晚于个旧杂岩体岩基(中酸性侵入岩)的主要形成时代(85～78 Ma[40]), 属于个旧杂岩体后基性岩墙群的一部分。

图11 煌斑岩长石分类图(底图据文献[86])Fig.11 Classification diagram for feldspars from lamprophyre veins

图12 个旧煌斑岩黑云母Ar-Ar年龄、锆石U-Pb年龄和老厂花岗岩锆石U-Pb年龄Fig.12 Age distribution of lamprophyre veins and granites in the Gejiu ore district

此外, 个旧地区除煌斑岩外的成岩-成矿事件均发生在77 Ma前, 煌斑岩脉黑云母Ar-Ar反映的73 Ma左右的热事件只有代表煌斑岩脉的侵位事件。而且, 这也从侧面印证个旧锡铜多金属成矿热液活动(95～77 Ma[40,63])应与煌斑岩脉的侵位无关,主要受花岗岩浆热液活动(85～78 Ma)影响。

5.2 煌斑岩的地球动力学背景

钾质煌斑岩通常发育于板块汇聚和主动大陆边缘[18], 构造背景为碰撞后环境[8]。老厂煌斑岩单斜辉石的起源环境判别图解(图 8)表明其为与火山弧有关的岩浆源区, 但是个旧地区在晚白垩世(85～78 Ma)已经是一个大陆板内环境, 因此单斜辉石反映的岛弧成分特征是其母岩浆源区继承了先前的岛弧(如特提斯洋闭合时期)地幔源区的特征。中国东部在晚白垩世时期(89～87 Ma)受东侧太平洋板块加速俯冲的影响, 挤压应力和压缩达到峰值[92], 个旧杂岩体(85～75 Ma)的形成稍晚于挤压应力峰值, 形成于挤压应力释放后的拉伸背景。

个旧煌斑岩形成时代稍晚于右江盆地东缘(广西河池)的大化陇长煌斑岩(89 Ma[22])和罗城垒洞煌斑岩(约100 Ma[90]) (图1a和表6)。陇长煌斑岩和垒洞煌斑岩均起源于 EM2型岩石圈地幔, 陇长煌斑岩Nd模式年龄较老(2.1～2.2 Ga)。程彦博[40]研究发现老厂煌斑岩的(87Sr/86Sr)i为 0.7099～0.7114 (平均0.7108), εNd(t)值为–8.8～ –6.8 (平均–8.1), 认为其可能来源于富集地幔的部分熔融, 并有显著的地壳混染[40]。个旧煌斑岩与陇长煌斑岩在岩石地球化学和同位素地球化学方面一致, 表明个旧地区与广西陇长地区在晚白垩世时期皆处于伸展背景。EM2型岩石圈富集地幔来源于次大陆岩石圈地幔的交代作用,与古老洋壳俯冲、脱水、交代有关[18]。白垩纪时期,华南地区的陆内伸展作用促使早期(元古宙或晚古生代)俯冲作用形成的交代富集地幔(EM2型)发生部分熔融形成陇长、垒洞和个旧老厂煌斑岩浆; 区域上的壳幔断裂为岩浆侵入提供了良好通道, 并在岩浆上升过程, 捕获花岗岩等围岩, 发生了地壳混染,形成橄辉云斜煌斑岩。即老厂煌斑岩形成于俯冲-碰撞后的板内拉伸构造环境, 可能起源于三叠纪时期古特提斯洋板块俯冲的残留板片对地幔楔的交代富集作用形成的源区[39]。

华南地区该时期表现为大规模的岩石圈伸展(100～80 Ma[38]), 也是滇东南个旧 Sn-Cu、都龙Sn-W-Zn、白牛厂Sn-Ag矿床, 桂西大厂Sn矿床, 滇黔桂卡林型Au矿床和大瑶山-大明山浅成低温热液型Au-Cu矿床形成的动力学背景。因此, 大陆岩石圈地幔伸展作用, 既能形成广泛的富钾岩浆岩(钾质-超钾质)岩浆活动, 也与 Cu-Au-Sn多金属矿床的形成有关。扬子地块西南缘(哀牢山缝合带)分布着众多的与 Cu-Au-Ni矿有关的渐新世(37～26 Ma)煌斑岩;个旧老厂煌斑岩与其岩石地球化学特征相似(图 1a和表 6)。目前较为统一的观点认为这些煌斑岩起源于古特提斯洋俯冲板片交代的富集地幔(EM2型)[98],因此, 从大地构造背景和岩浆起源角度对比, 个旧晚白垩世煌斑岩与西南三江渐新世煌斑岩起源可能一致(表6), 均受古特提斯洋俯冲板片交代作用的控制, 只是侵位时间有差异。

表6 研究区及其附近煌斑岩成因与特征Table 6 Geological characteristics and genesis of lamprophyres from the study area and its vicinity

6 结 论

(1) 煌斑岩 2个锆石 TIMS U-Pb年龄((87.1±1.3) Ma)为捕获老卡花岗岩体锆石的年龄;

(2) 煌斑岩6个黑云母Ar-Ar年龄平均为72.9 Ma,代表老厂煌斑岩侵位时代, 老厂煌斑岩属于个旧中-酸性岩浆活动(85～78 Ma)后基性岩墙群的一部分;

(3) 煌斑岩辉石斑晶和微晶成分近一致, 均属普通辉石, 黑云母斑晶属壳源铁质-镁质黑云母, 而黑云母微晶属镁质黑云母, 中长石斑晶偏基性(An25～40),透长石微晶偏酸性(An1～6);

(4) 老厂煌斑岩形成于俯冲-碰撞后的板内拉伸构造环境, 可能起源于三叠纪时期古特提斯洋板块俯冲的残留板片对地幔楔的交代富集作用形成的富集地幔源区。

黑云母Ar-Ar同位素测试和电子探针测试的过程中得到了中国地质大学(北京)王瑜和尹京武老师的无私帮助和指导; 两位审稿专家和编辑对文章的修改提出了诸多宝贵的意见, 在此一并表示真诚谢意！

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