藏东玉龙斑岩铜矿带扎拉尕含矿斑岩体锆石U-Pb年龄及其地质意义

何国朝, 王广强, 黄文婷, 邹银桥, 伍 静, 梁华英, 张玉泉, Charllote M ALLEN



藏东玉龙斑岩铜矿带扎拉尕含矿斑岩体锆石U-Pb年龄及其地质意义

何国朝1,2, 王广强2,3, 黄文婷2,3, 邹银桥2,3, 伍 静2*, 梁华英2, 张玉泉2, Charllote M ALLEN4

1. 广西有色金属集团 资源勘查有限公司, 广西 南宁 530022; 2. 中国科学院 广州地球化学研究所 中国科学院矿物学与成矿学重点实验室, 广东 广州 510640; 3. 中国科学院大学, 北京 100049; 4. Research School of Earth Sciences, Australian National University, Canberra ACT0200, Australia

玉龙斑岩铜矿带扎拉尕斑岩铜钼矿床位于玉龙斑岩铜矿带中北部, 赋矿岩体侵入下二叠统火山岩及三叠系砂泥岩中, 主要由早阶段为二长花岗斑岩及晚阶段正长花岗斑岩组成。分析了早阶段二长花岗斑岩及晚阶段正长花岗斑岩锆石LA-ICP-MS U-Pb年龄。早阶段二长花岗斑岩该年龄为(38.5±0.2) Ma, MSWD=1.12, 晚阶段正长花岗斑岩该年龄为(38.5±0.2) Ma, MSWD=1.08, 早阶段和晚阶段含矿斑岩体锆石U-Pb年龄完全一致。这表明早晚两阶段成矿岩体是在很短的时间间隔内形成的。扎拉尕赋矿斑岩体形成年龄为(38.5±0.2) Ma。据扎拉尕斑岩矿床形成时代及藏东地区在始新世至渐新世地质构造背景, 提出扎拉尕斑岩矿床和玉龙斑岩铜矿带的形成与印度板块-欧亚板块碰撞在藏东地区形成的走滑构造活动诱发的岩浆活动有关, 为陆陆碰撞走滑构造环境的斑岩矿床。

斑岩铜钼矿床; 成岩成矿时代; 碰撞与成矿; 西藏东部

0 引 言

西藏东部玉龙斑岩铜矿带是世界上重要的碰撞造山环境斑岩铜矿带, 该斑岩铜矿带长约300 km, 宽10~30 km, 含一个超大型斑岩铜矿床(玉龙)、两个大型斑岩铜钼矿床(多霞松多和马拉松多)、两个中型斑岩铜矿床(扎拉尕、莽总)及数十个矿化点。自该矿带发现以来, 国内外学者对玉龙斑岩铜矿带及其南侧沿哀牢山-红河巨型走滑断裂带分布的新生代斑岩铜钼(金)矿床开展了大量的研究工作[1–34]。目前, 对藏东喜山期富碱(钾质)岩带及玉龙含矿岩体形成的构造环境、岩浆来源及岩体的属性看法不一, 主要有: (1) 印度板块与亚洲板块碰撞后, 软流圈对流循环导致岩石圈减薄, 软流圈上拱引致岩石圈地幔融熔形成[34]; (2) 古近纪以来陆内俯冲作用导致软流圈上拱引起地幔混合层部分融熔形成[16]; (3)玉龙铜矿带含矿斑岩和俯冲作用有关, 为火山弧型, 成岩物质为壳幔混合物[3–4]; (4) 形成于板内非造山构造背景[6,7,22], 成岩物质来自交代富集地幔[6–7]。对含矿岩体属性也存在不同的认识: 有的认为其属于钙碱性岩石系列[3–5], 有的则认为其属碱性岩[6,7,22,26]。Hou.提出玉龙含矿斑岩有三个活动期, 与走滑断裂有关[26]。陈文明提出玉龙含矿斑岩体及其中的斑晶并非岩浆直接结晶的产物, 而是深源的富碱硅热水流体交代、熔融上部地壳含铜岩石形成的[24]。过去对玉龙斑岩铜矿带的工作多主要集中在玉龙超大型斑岩铜矿床及多霞松多大型斑岩矿床上, 而对其他斑岩矿床的工作则较少。

扎拉尕斑岩体位于玉龙斑岩铜矿带北部(图1), 前人曾对扎拉尕赋矿斑岩体作过3个岩石样品钾长石和1个岩石样品黑云母K-Ar测年, 所得的年龄变化较大, 在33.9~41.0 Ma之间[1,5,6]。为了深入分析藏东玉龙斑岩铜带时空分布特征及成矿演化, 很有必要对玉龙矿带一些中型规模矿床成矿岩体作精确的同位素定年。为此, 本文以扎拉尕含矿岩体为主要对象, 开展含矿岩体锆石LA-ICP-MS U-Pb年代学研究并分析其形成背景。

图1 藏东玉龙斑岩铜矿带(a)及扎那尕斑岩铜钼矿床地简图(b) (据文献[4–5]修改)

1–下二叠统火山岩; 2–上三叠统砂岩; 3–岩脉; 4–扎拉尕矿化斑岩; 5–地质界线; 6–断层。

1 扎拉尕斑岩矿床地质及矿化特征

扎拉尕斑岩铜钼矿床位于西藏东部玉龙斑岩铜矿带北部玉龙斑岩铜矿床南侧(图1), 含矿岩体侵入下二叠统火山岩和上三叠统砂泥岩中, 地表出露面积约0.6 km2。岩体的空间形态为岩株, 产状陡立[5]。

扎拉尕斑岩铜钼矿床含矿岩体具多阶段活动特征, 张玉泉等[6–7]据在ZK3孔348.8 m和ZK4孔68 m见到正长花岗斑岩和二长花岗斑岩的侵入接触关系而把岩体分为早晚两阶段: 早阶段为二长花岗斑岩, 晚阶段为正长花岗斑岩。扎拉尕含矿斑岩斑晶主要为钾长石、斜长石、石英及云母和少量角闪石, 斑晶矿物粒度主要在0.31~3.0 mm之间, 基质为显晶质, 基质矿物组成和斑晶的矿物组成相似。扎拉尕早阶段及晚阶段斑岩都发生了蚀变及矿化, 蚀变由内向外可分为钾硅化带、黄铁绢英岩化带、黏土化带及青磐岩化带[4–5]。钾化带主要分布于岩体内, 黄铁绢英岩化带主要分布于岩体内外接触带, 其余2个蚀变带以接触带为中心依次向外展布, 各蚀变带之间呈渐变过渡关系[4–5]。铜钼矿化主要发育于钾硅化带和黄铁绢英岩化带叠加部位。Cu含量在垂向变化上变化不大, Mo则有由上往下增强的趋势[4–5]。

扎拉尕斑岩矿床为中型矿床, 其Cu金属量为0.3 Mt, Cu平均含量在0.36%左右, Mo含量约为0.03%, Au约0.03 μg/g[27]。矿化主要为细脉浸染状发育于岩体和接触带围岩中, 矿化体为柱状, 钻孔未穿透矿化岩体, 控制矿化厚度约540 m, 直径约200 m[5]。主要金属矿物黄铜矿、辉钼矿、黄铁矿、磁铁矿、辉铜矿等, 偶见方铅矿、闪锌矿等。

2 分析方法

两个锆石LA-ICP-MS U-Pb定年样品分选自岩芯, 早阶段二长花岗斑岩采自ZK3孔345.3 m处, 晚阶段正长花岗斑岩采自ZK4孔200 m处(图1)。样品经碎样、磁选及重液选等选出锆石, 镜下挑选纯后装入环氧树脂靶中并打磨抛光至锆石内部结构充分暴露。根据光学显微镜透反射特征和扫描电镜阴极发光(CL)照片综合选出晶形较好, 没裂纹及包裹体不发育的锆石颗粒对其进行LA-ICP-MS U-Pb年代学测试。锆石定年在澳大利亚国立大学地球科学研究院ICP-MS实验室完成, 分析流程见文献[35]。为了减少继承铅、铅丢失等对年龄的影响, 在207Pb/235U-206Pb/238U图中和谐度低于95%的年龄数据点在统计年龄时将被剔除。

3 分析结果

扎拉尕斑岩铜钼矿床赋矿岩体早阶段二长花岗斑岩和晚阶段正长花岗斑岩锆石U-Pb同位素组成特征见表1及表2, 扎拉尕赋矿斑岩矿床早阶段二长花岗斑岩锆石LA-ICP-MS U-Pb年龄比较集中, 分布在37.4~39.6 Ma之间, Th/U在0.28~1.26之间; 晚阶段正长花岗斑岩锆石LA-ICP-MS U-Pb年龄主要分布在至37.6~39.0 Ma之间, Th/U在0.32~0.79之间。

扎拉尕赋矿斑岩体锆石CL图韵律环带发育(图2), 具岩浆锆石的一般特征, 此外, 扎拉尕斑岩锆石Th/U比值较大, 在0.28~1.26之间, 也具岩浆锆石的特征。因此, 可以认为分析锆石为岩浆结晶作用过程中形成的锆石, 锆石U-Pb主群年龄代表岩浆侵位年龄。LA-ICP-MS U-Pb测年具有快速及相对较经济的优点, 为了获得较精确的年龄值, 我们分析较多锆石颗粒, 然后用累积概率统计图分析锆石年龄分布特征。正态分布数据在累积概率统计图上呈一条斜率为正值的直线分布, 落在直线沿伸方向上方的较大年龄被解释为继承锆石年龄, 而分布在直线沿伸方向下方的年龄被解释为铅丢失锆石年龄。在累积概率统计图上呈线性分布的年龄代表锆石主群年龄[29,35]。

图2 扎拉尕斑岩铜钼矿床赋矿岩体锆石CL图

表1 扎拉尕斑岩早阶段二长花岗斑岩锆石LA-ICP-MS U-Pb年龄

4 讨 论

4.1 扎拉尕斑岩体形成时代

我们对扎拉尕早阶段二长花岗斑岩共做了31点锆石U-Pb同位素组成分析, 所有分析点在207Pb/235U-206Pb/238U图中和谐度都高于95%, 因此, 所有的分析点数据都符合计算年龄要求。31个分析点数据在累积概率统计图中为直线分布(图3a, 内插), 其统计年龄代表主群年龄。这31个分析点得出的统计年龄值为(38.5 ±0.2) Ma, MSWD = 1.12, 因此, 扎拉尕早阶段二长花岗斑岩是在(38.5±0.2) Ma侵位形成的。

在扎拉尕晚阶段正长花岗斑岩锆石29个分析点中, 1个分析点在207Pb/235U-206Pb/238U图中和谐度小于95%, 其余28个分析点谐度大于95%, 因此, 1个分析点在计算年龄时被剔除(表2)。其余28个分析点数据在累积概率统计图中为直线分布(图3b, 内插), 其统计年龄代表主群年龄。扎拉尕晚阶段正长花岗斑岩锆石28个分析点U-Pb统计年龄值为(38.5±0.2) Ma, MSWD = 1.08, 因此, 扎那尕晚阶正长花岗斑岩是在(38.5±0.2) Ma侵位形成的。

表2 扎拉尕斑岩晚阶段正长花岗斑岩锆石LA-ICP-MS U-Pb年龄

注: (1) 141-14号分析点因和谐度低于95%而剔除。

虽然在钻孔中见到早阶段二长花岗斑岩与晚阶段正长花岗斑岩呈侵入接触关系[6–7], 但我们的分析结果表明, 扎拉尕赋矿斑岩早阶段二长花岗斑岩和晚阶段正长花岗斑岩锆石LA-ICP-MS U-Pb年龄却一致。这表明扎拉尕早阶段二长花岗斑岩和阶段正长花岗斑岩是在很短时间隔间内形成的, 目前同位素定年精度难以区分早晚两阶段岩浆侵位时代的差异, 扎那尕赋矿斑岩形成时代为(38.5±0.2) Ma。

我们获得扎拉尕赋矿斑岩体锆石LA-ICP-MS U-Pb年龄((38.5±0.2) Ma)和前人钾长石和黑云母K-Ar年龄(33.9~41.0 Ma)[1,5,6]明显不同。我们认为两种同位素体系所得的同位素年龄不同, 主要原因是两种同位素体系抗后期地质热事件干扰能力不同。钾长石或黑云母K-Ar同位素体系封闭温度较低, 在300 ℃左右[36], 易受后期地质事件干扰。如前所述, 扎拉尕岩体发生了温度多大于300 ℃的钾化及硅化[32], 岩体中的钾长石和黑云母K-Ar同位素封闭体系会在岩体发生钾硅化蚀变时受到干扰, 发生放射性成因Ar的增加或丢失, 导致K-Ar同位素年龄发生变化而不能精确反映岩体形成年龄; 而锆石U-Pb同位素系统封闭温度在800 ℃左右[36], 抗后期干忧能力强, 后期钾硅化及其他地质热事件一般不会破坏锆石U-Pb同位素封闭系统, 因此, 扎拉尕赋矿斑岩锆石LA-ICP-MS U-Pb法年龄能更精确地反映岩体形成年龄。

4.2 含矿岩体形成动力学背景分析

印度板块与欧亚板块在约65 Ma左右时发生的陆陆碰撞在藏东地区形成了红河-哀牢山巨型左旋走滑断裂[37–38]。沿红河-哀牢山巨型左旋走滑断裂系统及其北延断裂发育一系列新生代钾质碱性岩及和钾质碱性岩相关的斑岩型铜钼(金)矿床, 形成三江新生代与钾质碱性岩有关的成矿带。玉龙斑岩铜矿带位于红河-哀牢山巨型断裂带北延断裂系统中[39]。目前对三江新生代钾质碱性岩成矿带形成动力学背景有不同的看法, Chung.[34]提出藏东新生代钾质碱性岩与藏东地区40 Ma 左右发生的地幔对流导致岩石圈减薄有关; 张玉泉等[40]提出哀牢山-金沙江新生代富碱侵入岩形成于裂谷构造背景; Wang.[39]提出哀牢山-红河始新世至渐新世富碱侵入岩和沿着红河-哀牢山深大走滑深大断裂活动产生的局部俯冲有关。

图3 扎拉尕斑岩铜钼矿床含矿岩体锆石LA-ICP-MS U-Pb年龄(内插图为累积概率统计图)

红河-哀牢山走滑断裂带两侧晚三叠纪到始新世陆相红层发生褶皱及沿红河-在哀牢山断裂及其北延断裂发育一系列始新世至早渐新世扭压盆地[41], 表明红河-哀牢山走滑断裂系统在始新世至早中新世处于扭压构造背景[32,39]。红河-哀牢山巨型左行走滑断裂带水平错距(700±200) km, 切穿岩石圈地幔[37–38]。切穿岩石圈地幔断裂活动引起软流圈上涌, 从而会导致岩石圈地幔部分熔融[38]。

玉龙斑岩铜矿带主要赋矿岩体微量元素及Sr-Nd-Pb同位素组成特征[40]和沿红河-哀牢山巨型左行走滑断裂分布的铁镁质至硅铝质钾质碱性岩的相似[38,41–43], 都具交代富集地幔源区的地球化学特征[33,42,44,45], 显示它们具相似的形成机制。前人成果表明, 沿着红河-哀牢山左行走滑深大断裂活动带分布的铁镁质至硅铝质钾质碱性岩与该深大断裂左行走滑活产生的局部俯冲有关[39]。玉龙斑岩铜矿带位于红河-哀牢山左行走滑深大断裂北延断裂系统, 而玉龙斑岩铜矿带从北西往南东五个主要赋矿岩体从玉龙(锆石U-Pb年龄(41.3±0.2) Ma[12])、扎拉尕(锆石U-Pb年龄(38.5±0.2) Ma)、莽总(锆石U-Pb年龄(37.6±0.2) Ma[26])、多霞松多(锆石U-Pb年龄(37.5± 0.2) Ma[30])至马拉松多(锆石U-Pb年龄(36.9±0.6) Ma[13])年龄逐渐降低, 显示出左行断裂构造控制的特征。因此, 有理由认为玉龙斑岩铜矿带赋矿岩体的形成与印度板块与欧亚板块在65 Ma左右时发生的陆陆碰撞, 在藏东三江地区产生切穿岩石圈地幔的红河-哀牢山左行走滑断裂及其北延断裂系统活动, 产生局部俯冲, 诱发地幔上涌, 发生强烈壳幔物质交换及引发的岩浆活动有关, 玉龙斑岩铜矿带形成于陆陆碰撞走滑构造背景。

5 主要结论

(1) 扎拉尕铜钼矿赋矿斑岩体主要由早阶段二长花斑岩斑和晚阶段正长花岗斑岩组成, 早阶段二长花岗斑岩锆石LA-ICP-MS U-Pb和晚阶段长花岗斑岩锆石LA-ICP-MS U-Pb年龄相同, 都是(38.5± 0.2) Ma。扎拉尕斑岩早晚两阶段岩体是成矿岩浆在很短时间隔间内侵位形成的。

(2) 玉龙斑岩铜矿带斑岩铜钼矿床与陆陆碰撞形成的走滑深大断裂诱发的壳幔相互作用有关, 玉龙斑岩铜矿带形成于陆陆碰撞走滑构造背景。

野外工作期间西藏地质矿产局第一地质大队给予了大力协助; 澳大利亚国立大学地球科学研究院ICP-MS实验室在实验工作中给予了大力支持, 特此致谢!

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Zircon LA-ICP-MS U-Pb age of the Zalaga porphyry associated with Cu-Mo mineralization in the Yulong ore belt and its geological implication

HE Guo-chao1,2, WANG Guang-qiang2,3, HUANG Wen-ting2,3, ZOU Yin-qiao2,3, WU Jing2*, LIANG Hua-ying2, ZHANG Yu-quan2and Charllote M ALLEN4

1. Resource Exploration Co. Ltd of Guangxi Non-Ferrous Metal Group Co., Ltd, Nanning 530022, China; 2. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; 3. University of Chinese Academy of Sciences, Beijing 100049, China; 4. Research School of Earth Sciences, Australian National University, Canberra ACT0200, Australia

The Zalaga porphyry Cu-Mo deposit is located at the northern domain of the Yulong porphyry copper ore belt in the eastern Tibet. The porphyry emplaced into the lower Permian volcanic rock and Late Triassic sandstone and could be divided into early stage monzonite granite porphyry and late stage syenogranite porphyry. Both stages of the porphyry were dated, respectively, using zircon LA-ICP-MS U-Pb method. The early stage monzonite granite porphyry has zircon LA-ICP-MS U-Pb age of (38.5±0.2) Ma, MSWD=1.12 and the late stage syenogranite porphyry has zircon LA-ICP-MS U-Pb age of (38.5±0.2) Ma, MSWD=1.08. The early porphyry has the same zircon LA-ICP-MS U-Pb age as that of the late stage porphyry, suggesting that the early porphyry and late stage porphyry emplaced almost at the same time and that the Zalaga porphyry was emplaced during the Late Eocene. Based on the structural condition of the period during Eocene to Oligocene in eastern Tibet, it is concluded that the Zalaga porphyry Cu-Mo(Au) deposit and the Yulong porphyry copper ore belt has genetic relation to the activities of strike-slip fault zone which transverses the lithosphere mantle and triggered the magmatic activities caused by the collision between Indian and Asia continents.

porphyry Cu-Mo deposit; age of porphyry and associated mineralization; continental collision and mineralization; Eastern Tibet

P597

A

0379-1726(2014)04-0399-09

2013-04-08;

2013-12-09;

2013-12-23

中国科学院战略性先导科技专项(B类) (XDB03010302); 国家自然科学基金(41272099, 41121002)

何国朝(1964–), 男, 教授级高级工程师, 主要从事矿产地质及勘查工作。E-mail: 124737253@qq.com

WU Jing, E-mail: wujing7808@163.com, Tel: +86-20-85290107