Chronology and Tectonic Implications of Neoproterozoic Volcanics from Chiniot Deposit, Pakistan
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摘要: 罗迪尼亚超大陆的裂解及与之相关的吉尼奥德玢岩型铁矿的形成是目前研究的热点,对安山岩锆石进行了U-Pb年龄、Hf同位素和微量元素分析.所有锆石都属岩浆成因,具有一致的稀土配分型式以及明显的Ce正异常、Eu负异常和重稀土元素富集特征.锆石年龄主要分为两组,分别为947.8±4.0 Ma和883.0±5.1 Ma,代表两期安山岩的成岩年龄,指示安山岩为两期岩浆活动的混合产物;此外,捕获的基底锆石年龄为1 523.0±66 Ma,属于中元古代.εHf(t)值变化范围较大(-4.67~+13.10),指示其为壳幔物质混合的产物.安山岩产生于罗迪尼亚超大陆时期,是由高温幔源岩浆通过底侵作用,使得中元古代花岗质岩石组成的下地壳发生熔融,壳幔熔体混合形成的,与伸展裂谷有关的构造热事件及地幔柱的活动有着密切关系.吉尼奥德铁矿与凯特里铜矿在成矿地质背景方面具有诸多相似性,暗示其有大型IOCG型矿床的成矿潜力.在960~880 Ma期间,印度板块西北部与华北-刚果-圣弗朗西斯科板块可能连接在一起.Abstract: This is a hotpoint that the study the breakup of Rodinia supercontinent and the formation of Chiniot iron oxide-apatite deposit, we report zircon U-Pb ages, Hf isotopic composition and REE contents of andesite. All the zircons are magmatic origin with the similar REE patterns which are enriched in HREE with a positive Ce anomaly and a negative Eu anomaly. The zircon U-Pb isotopic analyses yields 206Pb/238U ages of 947.8±4.0 Ma and 883.0±5.1 Ma, which can be interpreted as the crystallization ages of two stages of andesites. Besides, the inherited zircons with U-Pb age of 1 523.0±66 Ma may indicate a Mesoproterozoic basement. The igneous zircons have highly variable εHf(t) values ranging from -4.67 to +13.10, indicating that the andesites were derived from partial melting of a mixed mantle sourse. The geochemical and isotopic variation suggest that mantle-derived basic magmas have went through some degree of crustal commination during migration through Mesoproterozoic granitoids. The formation of the andesites were induced by a series of tectono-thermal events, associated with extensional rift and mantle plume during Rodinia supercontinent. There are many similarities between Chiniotiron deposit and Khetri Copper Belt in geological setting of ore-forming processes, indicating that the metallogenetic potentiality of giant IOCG is very well. The northwest of Indian craton may have connection with North China-Congo-São Francisco craton between 960 Ma and 880 Ma.
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Key words:
- Pakistan /
- andesite /
- mantle-derived /
- extensional rift /
- Rodinia supercontinent /
- geochronology /
- petrology
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0. 引言
巴基斯坦位于南亚次大陆西北部,东与印度为邻,东北与中国新疆接壤,西北与阿富汗毗邻,西与伊朗交界,其东部为印度河流域,北部与西部为山地和高原,南部为阿拉伯海.巴基斯坦位于印度板块、欧亚板块和阿拉伯板块交汇部位,处于特提斯构造域中部,其东部和东南部为印度板块的一部分,北部、西部和西南部为欧亚板块与印度板块的碰撞带.由于巴基斯坦位于印度板块的西北部,处于大型板块的边界部位,对其进行深入研究有助于恢复古板块构造,重建古大陆并对探讨与裂谷环境有关的IOCG矿产资源效应有着重要意义.
在罗迪尼亚超大陆时期,印度板块东部从990 Ma开始就已经与澳大利亚板块西部和东南极洲连接在一起了(Mezger and Cosca, 1999;Boger, 2000;Kelly, 2002),而关于印度板块西北部,即巴基斯坦的研究则较为薄弱.在Liu et al.(2009)构建的罗迪尼亚超大陆的模型中,印度板块处于超大陆的西北边界,而1 000~820 Ma期间缺乏有关罗迪尼亚超大陆的古地磁记录,因此,印度板块是否就是超大陆的板块边界,在其西部是否还有其他板块与之相连还存有诸多疑问.在这一时期,刚果-圣弗朗西斯科板块和华北板块的位置同样存在许多不确定的地方.Evans et al.(2015)研究发现,刚果-圣弗朗西斯科板块位于南北纬30°~60°之间的8个可能的位置.Li et al.(2008)研究认为华北板块处于罗迪尼亚超大陆的东北边界位置,刚果-圣弗朗西斯科板块则处于西南边界位置,然而近年来有诸多研究指出,华北板块与刚果-圣弗朗西斯科板块存在较为密切的联系,二者极有可能连接在一起(Peng et al., 2011a;Kouyate et al., 2013;Wang et al., 2015).因此,关于印度板块、华北板块以及刚果-圣弗朗西斯科板块在罗迪尼亚超大陆时期的确切位置同样存在诸多疑问.
IOCG型矿床即铁氧化物铜金矿床的简称,狭义上讲,该类型矿床为岩浆热液型矿床,主要产于前寒武纪,其周围的侵入体主要来自地幔源区.大型矿床(>100 t)主要产出于距边界100 km左右的太古代或古元古代克拉通内部,形成时期通常是超大陆拼合后的100~200 Ma之间,产出的构造环境是非造山型的,通常与地幔底侵有关的岩浆或热液活动有关.对于大型IOCG型矿床来讲,其主要产出于与次大陆岩石圈地幔(SCLM)有关的广泛地幔底侵交代作用过程中(Groves et al., 2010).在印度板块阿拉瓦利山脉(Aravalli Range)最北端发现的凯特里铜矿属于该类型矿床,其产出于罗迪尼亚超大陆时期,在其内部发现了A型花岗岩及钠长岩等与裂谷环境有关的岩石,表明其产出与罗迪尼亚超大陆的裂解有着密切关系(Ray,1990;Knight et al., 2002;Kaur et al., 2006).而吉尼奥德铁矿位于阿拉瓦利山脉向北弧形延伸段,在地理位置、形成时间、构造环境等多方面与凯特里铜矿有着密切联系,对吉尼奥德铁矿与凯特里铜矿的对比研究有助于更好地理解IOCG型矿床的成矿地质背景.
吉尼奥德矿区新元古代火山岩前人研究薄弱,区域地质资料较少,第四系覆盖严重,样品获取难度较大.本文属于首次报道,主要从锆石LA-ICP-MS U-Pb定年、Lu-Hf同位素及锆石微量元素分析上着手进行研究,利用最新研究数据为重建罗迪尼亚超大陆及研究IOCG型矿床成矿地质背景提供了新的证据.
1. 地质背景与样品特征
巴基斯坦地质演化历史悠久,出露地层较全,其中新生界最为发育,由于其独特的地理位置和复杂的构造环境,巴基斯坦具有良好的成矿条件.以14条断裂为界,可将巴基斯坦划分为10个主要构造单元(图 1).断裂从北向南,自东向西分别为主卡喇昆仑断裂(MKT)、主地幔逆冲断裂(MMT)、主边界断裂(MBT)、主前缘断裂(MFT)、杰赫勒姆断裂(JF)、盐岭断裂(SRT)、库拉姆断裂(KF)、苏莱曼断裂(ST)、佐布断裂(ZF)、吉尔塔尔断裂(KF)、番普断裂(PF)、加萨邦德断裂(GT)、奥纳希纳尔断裂(ONF)和杰曼断裂(CF).构造单元主要为:(1)印度河陆块、(2)贝拉-佐布蛇绿岩带、(3)苏莱曼-基尔塔尔造山带、(4)喜马拉雅造山带、(5)科伊斯坦-拉达克岛弧、(6)南帕米尔地块、(7)喀喇昆仑地块、(8)呼罗珊-莫克兰复理石盆地、(9)卡兰盆地和(10)查盖火山岩浆岩带(Kazmi and Rana, 1982;林传仙,1984;吴良士,2010;吕鹏瑞等,2016).
吉尼奥德(Chiniot)矿区位于印度板块西北边缘地带,地质演化历史悠长.矿区所在旁遮普省属于印度河流冲积平原,矿产较为丰富,主要发育古生代、中生代和新生代的沉积岩地层,出露第四系冲积层并且大面积分布,下伏前寒武纪基底岩石,岩石组成复杂,主要为灰绿色页岩、板岩、千枚岩、石英岩夹凝灰岩、安山岩、流纹岩等火山岩系,被粗玄岩和辉绿-辉长岩等侵入.
在旁遮普平原上,主要矿产为火山岩型铁矿、粘土矿、建筑集料和天然气等.火山岩型铁矿主要分布在萨戈达(Sargodha)、吉尼奥德等地,赋存于前寒武纪火山岩系之中,其中吉尼奥德铁矿储量估计在一亿吨以上,已构成大型矿床;粘土矿分布较广,共有18处产地,赋存于第四系表土层中;在第四系冲积层之下,存在丰富的天然气资源;建筑材料碎石矿(用来制作混凝土集料)赋存在基拉那(Kirana)-吉尼奥德-桑格拉山(Sangla Hill)一线以孤立的山丘形式断续出现的前寒武纪岩石露头中.
地球物理资料显示,前寒武系界线是呈隐伏的隆坳构造,属于印度板块向北延伸部分,吉尼奥德矿区位于中部隆起部位,轴部呈北西-南东走向大致沿萨戈达-吉尼奥德-拉合尔(Lahore)一线展布.在巴基斯坦构造分区图上,矿区所处构造单元称为萨戈达-沙赫布尔埋藏山脊(Sargodha-Shahpur ridge,编号1a)(图 1).
矿区火山岩中富含铁质矿物,成岩之后构造变动、热液作用以及区域变质作用,致使含铁物质进一步迁移富集成矿.旁遮普平原冲积层之下的前寒武纪基底岩石分布广泛,铁矿化普遍.萨戈达-沙赫布尔埋藏山脊区域内,成矿地质条件有利,该区无疑是寻找铁矿的最佳场所.地球物理勘探成果进一步表明,在萨戈达-吉尼奥德-费萨拉巴德(Faisalabad)一带,都发现了非常强的磁异常,证实该区域具有巨大的找矿潜力(图 2).
此次在矿区内采集的为岩芯样品,岩性为安山岩(16PA),样品呈灰黑色,斑状结构(图 3a、3b),块状构造,斑晶矿物为斜长石(40%),粒径0.2~1.2 mm;基质矿物主要为斜长石(45%),可见鳞片状绢云母和碳酸盐矿物呈定向分布,晶体间有玻璃质充填,可见少量石英不均匀分布;副矿物主要为磁铁矿.
2. 分析方法
用于锆石测年研究的样品共1件,为安山岩.先将样品粉碎至80~100目,再先后采用常规浮选和电磁选方法进行分选,然后在双目镜下挑选出晶型和透明度较好的锆石颗粒,接着将这些有代表性的锆石颗粒固定在无色透明的环氧树脂上,对环氧树脂表面抛光使锆石完全暴露以待测试.在固化于样品靶上的锆石颗粒当中选取测试点时,分别进行了阴极发光(CL)(图 4)和透、反射光照相,反复对比CL图像和显微镜下锆石照片,力求避开其内部裂隙和包裹体等干扰因素,据此选定锆石测试点位,以期获得较准确的年龄信息.
锆石U-Pb定年测试分析在武汉上谱分析科技有限责任公司完成,锆石定年分析仪器为四级杆质谱Agilent 7700及与之配套的193 nm准分子激光剥蚀系统(GeoLasPro).激光剥蚀斑束直径为32 μm,每个分析数据包括20~30 s的空白信号和50 s的样品信号,激光剥蚀深度为20~40 μm.锆石年龄计算采用标准锆石91500作为外标,元素含量采用美国国家标准物质局人工合成硅酸盐玻璃NIST610作为外标,29Si作为内标元素进行校正.数据处理采用ICPMSDataCal(Liu et al., 2008)程序,锆石年龄协和图用Isoplot3.75程序获得(Ludwig,2003).
原位微区锆石Hf同位素比值测试在中国地质大学(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),MC-ICP-MS为Neptune Plus(Thermo Fisher Scientific,德国).该系统配备了本实验室自主研发的信号平滑装置.采用该装置即使激光脉冲频率降到1 Hz,还可以获得平稳的信号(Hu et al., 2012b).对于193 nm的激光,在给定的仪器条件下,使用氦气作为载气比使用氩气的信号灵敏度提高了2倍(Hu et al., 2008b).我们研究还表明,少量氮气的引入还可进一步提升大部分元素的灵敏度(Hu et al., 2008a).相对于Neptune Plus的标准锥组合,新设计的X截取锥和Jet采样锥组合在少量氮气加入的条件下能分别提高Hf、Yb和Lu的灵敏度5.3倍、4.0倍和2.4倍.激光输出能量可以调节,实际输出能量密度为5.3 J/cm2.采用单点剥蚀模式,斑束固定为44 μm.详细仪器操作条件和分析方法可参照Hu et al.(2012a).
采用LA-MC-ICP-MS准确测试锆石Hf同位素的难点在于176Yb和176Lu对176Hf的同量异位素的干扰扣除.研究表明,Yb的质量分馏系数(βYb)在长期测试过程中并不是一个固定值,而且通过溶液进样方式测试得到的βY并不适用于激光进样模式中的锆石Hf同位素干扰校正(Woodhead et al., 2004).βYb的错误估算会明显地影响176Yb对176Hf的干扰校正,进而影响176Hf/177Hf比值的准确测定.在本次试验中,我们实时获取了锆石样品自身的βYb用于干扰校正.179Hf/177Hf=0.732 5和173Yb/171Yb=1.132 685(Fisher et al., 2014)被用于计算Hf和Yb的质量分馏系数βHf和βYb.179Hf/177Hf和173Yb/171Yb的比值被用于计算Hf(βHf) and Yb(βYb)的质量偏差.使用176Yb/173Yb=0.796 39(Fisher et al., 2014)来扣除176Yb对176Hf的同量异位干扰.使用176Lu/175Lu=0.026 56(Blichert-Toft et al., 1997)来扣除干扰程度相对较小的176Lu对176Hf的同量异位干扰.由于Yb和Lu具有相似的物理化学属性,因此在本实验中采用Yb的质量分馏系数βYb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ICPMSDataCal9.0(Liu et al., 2009)完成.
3. 分析结果
3.1 锆石U-Pb同位素
根据锆石CL图像和透、反射光特征,选择晶形比较完整、粒度较大的锆石颗粒进行锆石U-Pb同位素组成测试分析(图 4).16PA锆石形状不规则,多为半自形,颜色较暗,晶形较完整,透反射图像中裂纹不发育,部分锆石具有较清晰的环带结构,粒径在60~100 μm,ω(Th)/ω(U)集中在0.22~1.32之间,平均为0.82,均大于0.10,显示了岩浆锆石的特征.共分析了20个锆石颗粒测点,除去协和度较低的2个点外,其余18个有效测点(表 1)均落在谐和线上,出现3组年龄(图 5a).其中第一组5个点的206Pb/238U年龄集中分布于880.3~884.7 Ma之间,谐和年龄为883.0±5.1 Ma,MSWD=0.47;加权平均年龄为883.0±10.0 Ma,MSWD=0.02(图 5d).第二组10个点的206Pb/238U年龄范围为943.7~970.3 Ma,谐和年龄为947.8±4.0 Ma,MSWD=0.06;加权平均年龄为948.2±8.0 Ma,MSWD=0.56(图 5c).第三组3个点的207Pb/206Pb年龄范围为1 510.8~1 528.7 Ma,206Pb/238U谐和年龄为1 539.0±11.0 Ma,MSWD=0.08;207Pb/206Pb加权平均年龄为1 523.0±66 Ma,MSWD=0.02(图 5b).
表 1 吉尼奥德安山岩锆石LA-ICP-MS U-Pb年代学测试结果Table Supplementary Table LA-ICP-MS U-Pb isotopic compositions of zircons for Chiniot andesite测点号 含量(10-6) Th/U 同位素比值 年龄(Ma) 谐和度 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 16PA-01 435 464 0.94 0.070 4 0.002 1 1.558 2 0.046 8 0.157 8 0.001 9 938.9 59.3 953.8 18.6 944.4 10.7 99% 16PA-02 272 339 0.80 0.072 0 0.002 4 1.578 9 0.048 6 0.157 8 0.002 2 987.0 66.7 962.0 19.2 944.3 12.2 98% 16PA-03 159 173 0.92 0.073 7 0.003 1 1.646 6 0.071 4 0.162 2 0.002 7 1 031.5 87.0 988.3 27.4 968.8 15.0 98% 16PA-04 519 525 0.99 0.070 2 0.002 4 1.549 2 0.050 9 0.157 6 0.002 1 1 000.0 73.2 950.2 20.3 943.7 11.4 99% 16PA-05 45 57 0.79 0.068 2 0.005 9 1.600 4 0.106 3 0.159 2 0.004 6 873.8 179.6 970.4 41.5 952.2 25.8 98% 16PA-06 198 342 0.58 0.068 3 0.002 8 1.493 3 0.056 9 0.157 7 0.002 3 875.9 87.0 927.7 23.2 943.8 13.0 98% 16PA-07 1160 583 1.99 0.066 6 0.002 1 1.466 5 0.044 7 0.157 7 0.002 1 833.3 61.0 916.7 18.4 943.8 11.9 97% 16PA-08 133 595 0.22 0.067 1 0.002 9 1.554 5 0.055 7 0.162 4 0.002 6 842.6 88.9 952.3 22.2 970.3 14.5 98% 16PA-10 186 757 0.25 0.069 4 0.002 5 1.530 9 0.054 1 0.157 8 0.001 8 922.2 73.3 942.8 21.7 944.4 9.9 99% 16PA-11 153 245 0.63 0.094 1 0.003 8 3.620 4 0.135 0 0.275 0 0.005 1 1 510.8 75.9 1 554.0 29.7 1 566.0 25.7 99% 16PA-12 413 458 0.90 0.069 0 0.002 7 1.523 7 0.077 2 0.157 7 0.003 1 898.2 86.1 940.0 31.1 944.2 17.2 99% 16PA-13 357 461 0.78 0.067 7 0.002 2 1.381 4 0.048 3 0.146 8 0.002 2 861.1 68.5 881.0 20.6 882.9 12.5 99% 16PA-15 232 534 0.44 0.067 1 0.002 1 1.363 0 0.043 7 0.146 5 0.002 1 838.9 66.7 873.1 18.8 881.3 12.1 99% 16PA-16 376 289 1.30 0.094 8 0.002 6 3.559 8 0.100 8 0.271 7 0.003 8 1 524.1 51.9 1 540.6 22.5 1 549.7 19.4 99% 16PA-17 306 468 0.65 0.070 1 0.002 2 1.420 6 0.046 8 0.146 3 0.002 3 931.5 64.8 897.6 19.7 880.3 13.0 98% 16PA-18 460 535 0.86 0.069 4 0.001 7 1.408 9 0.035 0 0.147 1 0.002 0 909.3 51.9 892.7 14.8 884.7 11.2 99% 16PA-19 335 385 0.87 0.069 0 0.002 2 1.400 6 0.042 0 0.146 7 0.001 8 898.2 64.0 889.2 17.8 882.7 9.9 99% 16PA-20 277 317 0.87 0.095 0 0.002 7 3.481 1 0.094 0 0.265 3 0.003 7 1 528.7 53.7 1522.9 21.3 1516.9 18.9 99% 3.2 锆石稀土元素组成
吉尼奥德安山岩样品(16PA)锆石稀土元素(表 2)总量为(∑REE)在780×10-6~4 045×10-6之间,轻重稀土比值(∑LREE/∑HREE)为0.007~0.064.Eu表现为亏损(δEu为0.038~0.482),Ce表现为强烈富集(δCe为3.03~56.22),稀土元素配分型式非常一致(图 6).锆石的稀土元素特征可以用来鉴别其来源及类型(Hoskin and Ireland, 2000;Rubatto,2002;Belousova et al., 2002),由于HREE的离子半径较小,更接近Zr,使锆石具有LREE亏损、HREE富集的特点,而岩浆锆石又具有明显的Eu负异常和Ce正异常的特征(Belousova et al., 2002),这与本文吉尼奥德安山岩的锆石特点一致.(Sm/La)N为15.8~467,La为0.01~1.30,在锆石结晶机制判别图解中(图 7),数据点主要落在岩浆锆石区域附近,锆石基本没有受后期流体影响,以上特征均表现出吉尼奥德安山岩锆石具有典型岩浆锆石特征(Hoskin,2003).
表 2 吉尼奥德安山岩锆石稀土元素分析结果(10-6)Table Supplementary Table Rare earth element (10-6) data of zircons for Chiniot andesite测点号 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu δEu δCe ∑REE LREE/HREE 16PA-01 0.06 15.25 0.45 11.54 16.91 3.23 118.06 38.93 475.33 179.74 759.92 153.49 1 389.16 232.20 0.162 9.96 3 394.26 0.014 16PA-02 0.31 15.51 0.41 5.32 11.79 2.03 68.36 23.72 295.59 112.68 482.85 101.98 933.16 159.14 0.171 8.98 2 212.86 0.016 16PA-03 0.71 25.53 0.33 6.29 10.48 2.32 64.54 21.38 272.11 104.13 463.67 101.93 994.03 187.42 0.209 12.84 2 254.86 0.021 16PA-04 0.90 24.13 0.76 12.50 24.13 2.85 139.48 45.73 572.62 212.61 899.18 182.75 1 581.89 273.34 0.118 6.71 3 972.87 0.017 16PA-05 0.29 32.48 0.06 0.79 4.68 1.80 20.23 6.48 79.61 28.76 137.65 34.83 358.46 73.96 0.482 56.22 780.08 0.054 16PA-06 0.01 12.49 0.07 2.71 3.32 1.40 16.97 6.70 92.75 41.94 238.14 67.16 815.94 188.11 0.463 50.18 1 487.70 0.014 16PA-07 0.27 105.00 1.36 20.47 30.08 7.76 143.59 44.17 515.59 187.53 817.33 176.16 1 648.31 297.43 0.299 22.19 3 995.07 0.043 16PA-08 0.03 3.07 0.21 2.94 7.55 0.44 71.06 25.78 314.13 114.45 486.21 100.27 918.33 155.39 0.038 4.34 2 199.86 0.007 16PA-10 0.24 8.04 0.34 2.74 11.03 1.65 92.97 38.81 510.64 175.96 721.34 148.77 1 323.80 227.57 0.109 5.78 3 263.90 0.007 16PA-11 0.13 23.19 0.27 4.56 11.63 1.55 78.58 24.43 311.12 120.59 529.44 114.26 1 039.62 185.62 0.117 22.07 2 445.00 0.017 16PA-12 2.21 74.69 2.26 13.05 10.52 3.66 45.50 17.28 213.17 85.31 446.10 113.93 1 274.24 267.31 0.435 7.37 2 569.23 0.043 16PA-13 0.24 20.15 0.27 3.41 9.77 1.21 57.55 21.39 286.23 108.49 485.47 103.23 968.87 171.14 0.121 17.19 2 237.43 0.016 16PA-15 0.36 8.92 0.77 6.32 13.95 2.53 47.38 12.69 114.42 36.67 152.60 33.66 381.21 73.90 0.271 3.03 885.39 0.039 16PA-16 0.96 138.69 0.47 5.92 11.60 1.75 59.34 22.46 286.47 113.28 523.86 118.96 1 174.24 205.44 0.166 50.15 2 663.46 0.064 16PA-17 0.76 16.80 0.96 11.65 16.49 2.24 92.55 38.05 458.43 173.49 785.64 171.08 1 613.61 272.53 0.138 4.12 3 654.29 0.014 16PA-18 0.14 19.75 0.34 9.04 21.23 2.28 124.82 44.49 531.59 200.99 877.82 190.20 1 731.93 290.67 0.105 15.30 4 045.29 0.013 16PA-19 1.30 16.90 0.70 7.86 13.24 2.27 85.81 27.12 338.57 127.22 552.66 113.93 1 062.94 177.21 0.155 4.30 2 527.73 0.017 16PA-20 0.11 57.14 0.73 11.93 23.68 4.02 116.90 36.54 437.20 166.56 729.25 159.37 1 527.51 262.17 0.192 23.05 3 533.10 0.028 图 7 吉尼奥德安山岩锆石(Sm/La)N vs. La判别图解灰色区域:Boggy Plainzoned pluton的岩浆和热液锆石分布范围(Hoskin,2005); 据Kirkland et al.(2009)修改Fig. 7. Discrimination plots of chondrite-normalized Sm/La ratio vs. La of Chiniot andesite zircons3.3 锆石Lu-Hf同位素
安山岩共计20个测点(表 3),各点的176Lu/177Hf比值变化于0.001~0.016之间,176Lu/177Hf值均小于0.02,说明锆石形成后的放射性成因Hf积累十分有限,所测定的176Lu/177Hf可以较好地反映其形成过程中Hf同位素的组成特征(吴福元等,2007).除了2个测点的εHf(t)在-15以下,176Hf/177Hf比值较小外,其余18个测点的176Hf/177Hf比值主要分布于0.281 955~0.282 569之间,对应的εHf(t)为-4.67~+13.10,平均值为+3.49,具有壳幔混合特征,总体以幔源为主.单阶段模式年龄TDM1=970~1 837 Ma,平均值为1 392 Ma,两阶段模式年龄TDM2=978~1 952 Ma,平均值为1 546 Ma.
表 3 吉尼奥德安山岩锆石Lu-Hf同位素组成Table Supplementary Table Zircon Lu-Hf isotopic compositions for the Chiniot andesite测点号 年龄(Ma) 176Yb/177Hf 176Lu/177Hf 176Hf/177Hf 1σ εHf(0) εHf(t) tDM1(Ma) tDM2(Ma) fLu/Hf 16PA-01 948 0.039 999 0.001 5 0.282 193 0.000 017 -20.5 -0.47 1 513 1 726 -0.95 16PA-02 948 0.028 189 0.001 1 0.282 169 0.000 014 -21.3 -1.04 1 529 1 757 -0.97 16PA-03 948 0.018 261 0.000 7 0.281 689 0.000 018 -38.3 -17.80 2 171 2 670 -0.98 16PA-04 948 0.032 286 0.001 4 0.282 473 0.000 016 -10.6 9.50 1 114 1 178 -0.96 16PA-05 948 0.030 862 0.001 1 0.282 138 0.000 014 -22.4 -2.19 1 575 1 820 -0.97 16PA-06 948 0.004 009 0.000 1 0.282 354 0.000 017 -14.8 6.10 1 240 1 365 -1.00 16PA-07 948 0.008 419 0.000 4 0.282 531 0.000 017 -8.5 12.22 1 003 1 027 -0.99 16PA-08 948 0.016 916 0.000 8 0.282 457 0.000 016 -11.1 9.34 1 118 1 186 -0.98 16PA-09 1523 0.034 658 0.001 3 0.281 955 0.000 016 -28.9 4.02 1 837 1 952 -0.96 16PA-10 883 0.030 309 0.001 1 0.282 179 0.000 015 -21.0 -2.12 1 517 1 765 -0.97 16PA-11 883 0.040 152 0.001 6 0.282 337 0.000 018 -15.4 3.23 1 311 1 471 -0.95 16PA-12 883 0.006 677 0.000 3 0.282 318 0.000 012 -16.0 3.31 1 294 1 467 -0.99 16PA-13 883 0.040 720 0.001 6 0.282 114 0.000 016 -23.3 -4.67 1 626 1 904 -0.95 16PA-14 1523 0.040 086 0.001 6 0.282 051 0.000 019 -25.5 7.12 1 717 1 783 -0.95 16PA-15 883 0.025 434 0.001 0 0.282 190 0.000 016 -20.6 -1.64 1 496 1 739 -0.97 16PA-16 948 0.026 902 0.001 1 0.281 739 0.000 014 -36.5 -16.27 2 125 2 587 -0.97 16PA-17 948 0.025 834 0.001 1 0.282 569 0.000 014 -7.2 13.10 970 978 -0.97 16PA-18 948 0.039 073 0.001 5 0.282 211 0.000 013 -19.8 0.21 1 486 1 688 -0.96 16PA-19 948 0.025 073 0.001 0 0.282 416 0.000 012 -12.6 7.75 1 181 1 274 -0.97 16PA-20 948 0.016 708 0.000 6 0.282 163 0.000 013 -21.5 -1.00 1 521 1 755 -0.98 4. 讨论
4.1 火山岩成岩年龄
旁遮普平原火山岩属于印度西北部阿拉瓦利(Aravalli)火山岩的北延部分,在阿拉瓦利山脉,火山岩分布非常广泛,这些岩石从拉贾斯坦邦(Rajasthan)穿过焦特布尔(Jodhpur)经哈里亚纳邦(Haryana)一直延伸至基拉那(Shah,1977;Farah and DeJong, 1979;Kumar and Virdi, 1997).基拉那地区火山岩主要分布在哈希(Hachi)岩群中,少量火山熔岩流出现在萨拉班(Sharaban)岩群中,这反映了火山活动的长期性和多期性.这些火山岩沿德里(Delhi)-桑格拉山埋藏脊展布,组成隆起的核部.哈希火山岩代表双向裂谷克拉通喷发,这些岩石以流纹岩流和火山碎屑(凝灰岩、角砾岩、集块岩和火山岩灰)为主,伴有次生玄武安山岩,英安岩和流纹英安岩数量较少.前人对上述不同区域的火山岩进行了同位素Rb-Sr和锆石U-Pb定年,获取的年龄值范围在990~750 Ma之间(Davies and Crawford, 1971;Crawford and Compston, 1973;Choudhary et al., 1984;Eby and Kochhar, 1990;Deb, 2001;吴良士,2010).
这些火成岩的年龄与罗迪尼亚超大陆出现时间非常一致(图 8).随着中元古代晚期全球范围的格林威尔-基巴拉-纳曼卡(Grenville-Kibaran-Namaquan)造山带的产生,罗迪尼亚超大陆在1 000 Ma左右完成拼合(Torsvik et al., 1996;Dalziel,1997;Kröener, 2000;McCourt et al., 2006;Li et al., 2008).在超大陆形成的120 Ma期间,其内部的扩张型裂谷盆地开始形成,其中一部分演化为大洋盆地,并最终导致罗迪尼亚超大陆在新元古代中期裂解为分离的大陆板块(Unrug,1998;Li et al., 2008;Wendorff and Key, 2009).由于超大陆的形成,导致了其周围板片在地幔过渡带的崩塌式下沉以及超大陆的热隔绝现象,从而在超大陆形成40~60 Ma后,产生了超级地幔柱,造成了全球大范围同时期的岩浆事件并最终导致了罗迪尼亚超大陆于750 Ma左右发生裂解(Lin et al., 2007;Li et al., 2008).
印度次大陆东部在990~960 Ma期间已经与南极洲大陆东部发生碰撞拼合,印度东部的东高止山脉(The Eastern Ghats)、南极洲东部的北查尔斯王子山脉(Prince Charles Mountains)和肯普地(Kemp Land)在990~900 Ma期间发生了大量高级变质事件(Mezger and Cosca, 1999;Boger, 2000;Kelly, 2002).在印度西部阿拉瓦利-德里造山带西部边缘的阿姆巴吉(Ambaji)-森德拉(Sendra)带形成于克拉通内部的扩张裂谷盆地,并发展成弧-沟系统(Sen,1981),洋壳沿着南德里岛弧带西部边缘向东俯冲,最终岛弧地体与阿拉瓦利大陆边缘发生碰撞,Basantgarh型Cu-Zn矿床在临近俯冲带的钙碱性玄武岩夹层中的沉积单元中形成.而在发生碰撞前,弧后盆地发生的扩张构造事件产生了阿姆巴吉和德里层状Zn-Pb-Cu沉积型矿床(Deb and Sarkar, 1990).对阿姆巴吉-森德拉岛弧地体中变质岩石的研究发现,在990~836 Ma期间发生了一系列构造热事件,而这些与1 600~1 500 Ma期间发生的古老的阿拉瓦利-德里造山带无关,在该地体中还发现了990 Ma的流纹岩,其中的锆石具有来自裂谷环境的特征(Deb, 2001),因而,在此期间发生的岩浆活动与裂谷等伸展构造环境有着密切关系.
在900~830 Ma的时间段内,由于缺少相应的古地磁资料,全球范围内关于罗迪尼亚超大陆的地质记录较少.例如,900~850 Ma期间,在斯堪的纳维亚、苏格兰和泰米尔发现的双峰式裂谷岩浆活动被认为是罗迪尼亚超大陆开始裂解的征兆(Dalziel and Soper, 2001;Paulsson and Andreasson, 2002);880~870 Ma期间,在非洲卡拉哈里(Kalahari)板块北部及刚果-坦桑尼亚-班韦乌卢(Congo-Tanzania-Bangweulu)板块南部开始出现与裂谷相关的岩浆活动,并且一直持续到750 Ma(Johnson et al., 2005);在中国南部扬子板块发现的一系列花岗质和镁铁质-超镁铁质侵入体年龄范围在830~820 Ma,被认为与罗迪尼亚超大陆裂解期间的地幔柱有关(Li, 2003).而近十年来,关于罗迪尼亚超大陆各板块中与裂谷相关岩浆活动的研究日益增多(Johnson et al., 2007;Evans et al., 2010;Peng et al., 2011a;Peng et al., 2011b;Wang et al., 2011;Liu et al., 2012;Delpomdor et al., 2013;Kouyate et al., 2013;Evans et al., 2015;Peng,2015;Wanget al., 2015;Zhang et al., 2016).这些岩浆活动均被解释为罗迪尼亚超大陆开始发生裂解,Li(2003)则认为这是罗迪尼亚超级地幔柱的初始迹象,而大范围的地幔柱活动直到825 Ma才开始出现,并且在780 Ma和750 Ma相继发生了多次幕式地幔柱事件.
本文获得的吉尼奥德安山岩锆石U-Pb成岩年龄有两组,分别为947.8±4.0 Ma(MSWD=0.06)和883.0±5.1 Ma(MSWD=0.47).测试结果与前人获得的同位素Rb-Sr及锆石U-Pb年龄范围一致,成岩时代属于新元古代拉伸纪.从990 Ma开始,印度次大陆东部已经与南极洲大陆东部发生了碰撞拼合,开始成为罗迪尼亚超大陆的一部分;在印度西部,开始出现一系列与裂谷有关的构造热事件,从全球范围来看,罗迪尼亚超大陆从1 000 Ma完成拼合后开始,在整个超大陆内部就已经开始出现扩张型裂谷盆地以及与伸展构造有关的岩浆活动.此外,研究区还发现了与裂谷环境有关的A型花岗岩(徐剑南等,未发表).因而,本文认为吉尼奥德安山岩产生于948 Ma和883 Ma左右发生的两次裂谷火山喷发活动,是由一系列与伸展裂谷有关的构造热事件引发,并与地幔柱的活动有着密切关系.
4.2 岩石成因与源区性质探讨
旁遮普平原前寒武系岩石在萨戈达、拉布瓦、吉尼奥德、斯哈赫科特和桑格拉山等地以孤立的山丘形式出露.这些孤山被平坦的冲积平原分隔开来,岩石呈北西-南东走向,倾角45°~85°.重力数据(Farah,1973)表明,该地区存在裂谷断层和地垒、地堑构造.表层构造表明,存在范围广泛的不对称背斜,其呈现北西-南东走向,并向西北方向倾伏.该背斜由于裂谷作用以及地垒与地堑的形成而遭受破坏.
吉尼奥德安山岩εHf(t)主要分布在-4.67~+13.10之间,平均值为+3.49,变化范围较大,其对应的176Hf/177Hf值变化也较大(图 10),这显示出锆石的Hf同位素的不均一性,可能为开放体系导致熔体发生这种变化(Kemp et al., 2007).此外,还有2个测点的εHf(t)值在-15以下,在岩浆混合作用发生时,由于这两个锆石较早结晶,从而使得其Hf同位素特征得以保留,说明其来自古老陆壳物质.
由于锆石Hf同位素体系具有很高的封闭温度,其Hf同位素体系不会随部分熔融而发生变化,因此可以推断锆石Hf同位素的不均一性是由于亏损地幔和地壳这两种不同端元之间相互作用的结果(Bolhar et al., 2008).在t-εHf(t)图解(图 9a)中,数据点主要落在球粒陨石附近及上方,在t-176Hf/177Hf图解(图 9b)中,数据点主要落在亏损地幔与下地壳之间,相对集中,说明吉尼奥德安山岩具有壳幔混合特征,总体以幔源为主.幔源岩浆在喷发过程中,捕掳有中元古代时期的岩浆锆石并保留下来,因而出现了εHf(t)值非常低的来自古老地壳的锆石;与此同时,在岩浆上升侵位过程中,与古老地壳发生了混合作用,由于混合程度不同,εHf(t)值的变化范围也非常大,总体上呈现出幔源岩浆特征.这与前人研究的在阿姆巴吉-森德拉岛弧地体中发生的与裂谷相关的构造热事件相一致(Deb, 2001).本文认为吉尼奥德安山岩是由高温幔源岩浆通过底侵作用,使得由中元古代花岗质岩石组成的下地壳发生熔融,壳幔熔体混合形成的.
4.3 找矿意义
凯特里铜矿带(Khetri Copper Belt,KCB)形成于阿拉瓦利山脉最北端,从东北部的辛卡纳(Singhana)一直延伸到西南部的桑贾瓦(Sangarva),总长约80 km.在铜矿带内部发现了一系列具有基底属性的同构造花岗质深成岩体,Kaur et al.(2006)对其中的Biharipur和Dabla岩体进行了分析,发现其侵位于区域扩张的构造环境下,年龄跨度在1 800~1 500 Ma之间,这些岩石被认为是与阿拉瓦利-德里超群同时期产生的(Gopalan et al., 1979;Choudhary et al., 1984;Sivaraman and Raval, 1995;Pandit and Khatatneh, 1998;Sinha-Roy,2000;Biju-Sekhar et al., 2003).沿着170 km长的铜矿带中还发现了广泛分布的钠长岩,被认为产生于陆内裂谷环境中(Ray,1990;Kaur et al., 2006).地球物理数据表明,沿着德里构造带展布的高密度地幔物质,是元古代裂谷扩张时期地幔上涌产生的(Vijaya Rao et al., 2000;Mishra et al., 2000).因此,Kaur et al.(2006)认为Biharipur-Dabla A型花岗岩与皮尔瓦拉(Bhilwara)拗拉槽裂谷和岩石圈扩张有着密切关系.在Madhan-Kudhan铜矿中矿物集合体的石榴石裂变径迹年龄为897±125 Ma(Singh,1988),研究发现铜矿的矿化主要为后生的,与同一时期(850~750 Ma)的构造后A型花岗岩侵位有着密切关系(Knight et al., 2002).
旁遮普平原的吉尼奥德火山岩位于基拉那基底重力异常高值区(Kirana Basement High,KBH),异常带呈弧形沿萨戈达-吉尼奥德-拉合尔一线展布,与萨戈达-沙赫布尔埋藏山脊走向相一致,属于印度西部拉贾斯坦邦阿拉瓦利山脉向北弧形延伸段.吉尼奥德火山岩的形成与凯特里铜矿带附近的火山岩有着紧密联系,结合本文的3组锆石U-Pb年龄结果,发现1 523 Ma的锆石可能来自于中元古代陆内裂谷扩张产生的同构造花岗质深成岩体中,948 Ma和883 Ma的锆石可能产于罗迪尼亚超大陆拼合后的拉张裂谷环境中,这3组年龄与凯特里铜矿发现的两期A型花岗岩年龄一致,说明印度西北部拉贾斯坦邦一直延伸至巴基斯坦旁遮普平原的火山岩有可能为同一套岩石,吉尼奥德铁矿和凯特里铜矿在岩石年龄、构造环境、成矿条件等多方面有着许多相似的地方,暗示着吉尼奥德铁矿存在大型IOCG型矿床的潜力.
4.4 罗迪尼亚超大陆的重建
在华北板块东南部地区发现了925 Ma左右的放射状岩墙群、960~890 Ma的褚兰-大连-沙里院(Chulan-Dalian-Sariwon)镁铁质岩席、830 Ma左右的栾川辉长岩以及代表着三联点一部分的933~890 Ma的徐淮陆内裂谷系统(Peng et al., 2011a;Peng et al., 2011b;Wang et al., 2011;Liu et al., 2012;Peng,2015;Zhang et al., 2016).
在刚果板块西部发现了930~920 Ma的大陆溢流玄武岩和流纹质熔岩,在东部发现了与大陆裂谷活动有关的880~850 Ma的岩浆活动以及882 Ma左右的辉绿岩岩席,在南部发现了与裂谷有关的880 Ma左右的流纹岩(Franssen and André,1988;Porada and Berhorst, 2000;Tack, 2001;Johnson et al., 2007;Delpomdor et al., 2013).在圣弗朗西斯科板块发现了926~918 Ma的一系列镁铁质岩墙群(Heaman,1991;Correa-Gomes and Oliveira, 2000;Evans et al., 2010, 2015).
以上岩墙群和火山岩均指示出这些地区在960~880 Ma期间出现了拉张裂谷作用,Peng et al.(2011a)基于年龄的一致性认为在刚果和圣弗朗西斯科板块发现的岩墙群和火山岩与华北的岩墙群属于同一个大火成岩省,华北板块与刚果-圣弗朗西斯科板块有可能连在一起.Kouyate et al.(2013)通过对西非板块北部摩洛哥885 Ma左右岩墙群的研究,认为华北板块、刚果-圣弗朗西斯科板块与西非板块存在密切联系.Wang et al.(2015)对塔里木板块900~870 Ma的双峰式火山岩进行了研究,并对各板块中相似年龄裂谷作用有关的岩浆活动进行了总结,提出塔里木板块、华北板块、刚果-圣弗朗西斯科板块与西非板块在罗迪尼亚超大陆时期相互之间有可能相互连接在一起,也有可能在罗迪尼亚超大陆时期世界范围内广泛存在这与裂谷活动有关的岩浆活动.
Evans et al.(2015)通过对刚果-圣弗朗西斯科板块的古地磁分析发现,在920 Ma左右该板块处于中-高古纬度(30°~60°).结合本文中吉尼奥德火山岩在948 Ma和883 Ma发生的两次裂谷火山活动,以及旁遮普平原火山岩的NW-SE走向,暗示着该地区的裂谷系统可能属于华北-刚果-圣弗朗西斯科裂谷系统三联点的一部分,在960~880 Ma期间受地幔柱影响,相继发生了一系列与拉张裂谷有关的火山活动,表明印度板块西北部与华北-刚果-圣弗朗西斯科板块在此期间可能连接在一起(图 11),但这还需要进一步的古地磁证据.
5. 结论
(1) 吉尼奥德安山岩具有两组锆石U-Pb年龄,分别为947.8±4.0 Ma和883.0±5.1 Ma,捕获的基底锆石年龄为1 523.0±66 Ma.数据表明,948 Ma和883 Ma左右分别发生的两次裂谷火山喷发活动,是由一系列与伸展裂谷有关的构造热事件引发,并与地幔柱的活动有着密切关系.
(2) 吉尼奥德安山岩εHf(t)主要分布于-4.67~+13.10之间,平均值为+3.49,两阶段模式年龄TDM2=978~1 952 Ma,平均值为1 546 Ma,表现出较明显的壳幔混合特征,吉尼奥德安山岩是由高温幔源岩浆通过底侵作用,使得中元古代花岗质岩石组成的下地壳发生熔融,壳幔熔体混合形成的.
(3) 吉尼奥德铁矿中安山岩及基底锆石的年龄与凯特里铜矿中A型花岗岩及花岗质深成岩体的年龄相一致,并且二者在构造环境、矿床形成条件等成矿地质背景方面具有诸多相似性,暗示其有大型IOCG型矿床的成矿潜力.
(4) 在960~880 Ma期间,华北板块东南部和刚果-圣弗朗西斯科板块西部与南部均发现了大量与裂谷有关的岩浆活动,结合本文中发现的位于印度板块西北部的新元古代安山岩,表明印度板块西北部与华北-刚果-圣弗朗西斯科板块在这一时期可能连接在一起.
致谢: 中国地质大学(武汉)熊启慧硕士和张磊硕士在锆石同位素分析时给予了帮助,对此表示衷心的感谢. -
图 6 吉尼奥德安山岩中锆石稀土元素球粒陨石标准化配分模式图
Fig. 6. Chondrite-normalized REE patterns of zircons from Chiniot andesite
图 7 吉尼奥德安山岩锆石(Sm/La)N vs. La判别图解
灰色区域:Boggy Plainzoned pluton的岩浆和热液锆石分布范围(Hoskin,2005); 据Kirkland et al.(2009)修改
Fig. 7. Discrimination plots of chondrite-normalized Sm/La ratio vs. La of Chiniot andesite zircons
表 1 吉尼奥德安山岩锆石LA-ICP-MS U-Pb年代学测试结果
Table 1. LA-ICP-MS U-Pb isotopic compositions of zircons for Chiniot andesite
测点号 含量(10-6) Th/U 同位素比值 年龄(Ma) 谐和度 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 16PA-01 435 464 0.94 0.070 4 0.002 1 1.558 2 0.046 8 0.157 8 0.001 9 938.9 59.3 953.8 18.6 944.4 10.7 99% 16PA-02 272 339 0.80 0.072 0 0.002 4 1.578 9 0.048 6 0.157 8 0.002 2 987.0 66.7 962.0 19.2 944.3 12.2 98% 16PA-03 159 173 0.92 0.073 7 0.003 1 1.646 6 0.071 4 0.162 2 0.002 7 1 031.5 87.0 988.3 27.4 968.8 15.0 98% 16PA-04 519 525 0.99 0.070 2 0.002 4 1.549 2 0.050 9 0.157 6 0.002 1 1 000.0 73.2 950.2 20.3 943.7 11.4 99% 16PA-05 45 57 0.79 0.068 2 0.005 9 1.600 4 0.106 3 0.159 2 0.004 6 873.8 179.6 970.4 41.5 952.2 25.8 98% 16PA-06 198 342 0.58 0.068 3 0.002 8 1.493 3 0.056 9 0.157 7 0.002 3 875.9 87.0 927.7 23.2 943.8 13.0 98% 16PA-07 1160 583 1.99 0.066 6 0.002 1 1.466 5 0.044 7 0.157 7 0.002 1 833.3 61.0 916.7 18.4 943.8 11.9 97% 16PA-08 133 595 0.22 0.067 1 0.002 9 1.554 5 0.055 7 0.162 4 0.002 6 842.6 88.9 952.3 22.2 970.3 14.5 98% 16PA-10 186 757 0.25 0.069 4 0.002 5 1.530 9 0.054 1 0.157 8 0.001 8 922.2 73.3 942.8 21.7 944.4 9.9 99% 16PA-11 153 245 0.63 0.094 1 0.003 8 3.620 4 0.135 0 0.275 0 0.005 1 1 510.8 75.9 1 554.0 29.7 1 566.0 25.7 99% 16PA-12 413 458 0.90 0.069 0 0.002 7 1.523 7 0.077 2 0.157 7 0.003 1 898.2 86.1 940.0 31.1 944.2 17.2 99% 16PA-13 357 461 0.78 0.067 7 0.002 2 1.381 4 0.048 3 0.146 8 0.002 2 861.1 68.5 881.0 20.6 882.9 12.5 99% 16PA-15 232 534 0.44 0.067 1 0.002 1 1.363 0 0.043 7 0.146 5 0.002 1 838.9 66.7 873.1 18.8 881.3 12.1 99% 16PA-16 376 289 1.30 0.094 8 0.002 6 3.559 8 0.100 8 0.271 7 0.003 8 1 524.1 51.9 1 540.6 22.5 1 549.7 19.4 99% 16PA-17 306 468 0.65 0.070 1 0.002 2 1.420 6 0.046 8 0.146 3 0.002 3 931.5 64.8 897.6 19.7 880.3 13.0 98% 16PA-18 460 535 0.86 0.069 4 0.001 7 1.408 9 0.035 0 0.147 1 0.002 0 909.3 51.9 892.7 14.8 884.7 11.2 99% 16PA-19 335 385 0.87 0.069 0 0.002 2 1.400 6 0.042 0 0.146 7 0.001 8 898.2 64.0 889.2 17.8 882.7 9.9 99% 16PA-20 277 317 0.87 0.095 0 0.002 7 3.481 1 0.094 0 0.265 3 0.003 7 1 528.7 53.7 1522.9 21.3 1516.9 18.9 99% 表 2 吉尼奥德安山岩锆石稀土元素分析结果(10-6)
Table 2. Rare earth element (10-6) data of zircons for Chiniot andesite
测点号 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu δEu δCe ∑REE LREE/HREE 16PA-01 0.06 15.25 0.45 11.54 16.91 3.23 118.06 38.93 475.33 179.74 759.92 153.49 1 389.16 232.20 0.162 9.96 3 394.26 0.014 16PA-02 0.31 15.51 0.41 5.32 11.79 2.03 68.36 23.72 295.59 112.68 482.85 101.98 933.16 159.14 0.171 8.98 2 212.86 0.016 16PA-03 0.71 25.53 0.33 6.29 10.48 2.32 64.54 21.38 272.11 104.13 463.67 101.93 994.03 187.42 0.209 12.84 2 254.86 0.021 16PA-04 0.90 24.13 0.76 12.50 24.13 2.85 139.48 45.73 572.62 212.61 899.18 182.75 1 581.89 273.34 0.118 6.71 3 972.87 0.017 16PA-05 0.29 32.48 0.06 0.79 4.68 1.80 20.23 6.48 79.61 28.76 137.65 34.83 358.46 73.96 0.482 56.22 780.08 0.054 16PA-06 0.01 12.49 0.07 2.71 3.32 1.40 16.97 6.70 92.75 41.94 238.14 67.16 815.94 188.11 0.463 50.18 1 487.70 0.014 16PA-07 0.27 105.00 1.36 20.47 30.08 7.76 143.59 44.17 515.59 187.53 817.33 176.16 1 648.31 297.43 0.299 22.19 3 995.07 0.043 16PA-08 0.03 3.07 0.21 2.94 7.55 0.44 71.06 25.78 314.13 114.45 486.21 100.27 918.33 155.39 0.038 4.34 2 199.86 0.007 16PA-10 0.24 8.04 0.34 2.74 11.03 1.65 92.97 38.81 510.64 175.96 721.34 148.77 1 323.80 227.57 0.109 5.78 3 263.90 0.007 16PA-11 0.13 23.19 0.27 4.56 11.63 1.55 78.58 24.43 311.12 120.59 529.44 114.26 1 039.62 185.62 0.117 22.07 2 445.00 0.017 16PA-12 2.21 74.69 2.26 13.05 10.52 3.66 45.50 17.28 213.17 85.31 446.10 113.93 1 274.24 267.31 0.435 7.37 2 569.23 0.043 16PA-13 0.24 20.15 0.27 3.41 9.77 1.21 57.55 21.39 286.23 108.49 485.47 103.23 968.87 171.14 0.121 17.19 2 237.43 0.016 16PA-15 0.36 8.92 0.77 6.32 13.95 2.53 47.38 12.69 114.42 36.67 152.60 33.66 381.21 73.90 0.271 3.03 885.39 0.039 16PA-16 0.96 138.69 0.47 5.92 11.60 1.75 59.34 22.46 286.47 113.28 523.86 118.96 1 174.24 205.44 0.166 50.15 2 663.46 0.064 16PA-17 0.76 16.80 0.96 11.65 16.49 2.24 92.55 38.05 458.43 173.49 785.64 171.08 1 613.61 272.53 0.138 4.12 3 654.29 0.014 16PA-18 0.14 19.75 0.34 9.04 21.23 2.28 124.82 44.49 531.59 200.99 877.82 190.20 1 731.93 290.67 0.105 15.30 4 045.29 0.013 16PA-19 1.30 16.90 0.70 7.86 13.24 2.27 85.81 27.12 338.57 127.22 552.66 113.93 1 062.94 177.21 0.155 4.30 2 527.73 0.017 16PA-20 0.11 57.14 0.73 11.93 23.68 4.02 116.90 36.54 437.20 166.56 729.25 159.37 1 527.51 262.17 0.192 23.05 3 533.10 0.028 表 3 吉尼奥德安山岩锆石Lu-Hf同位素组成
Table 3. Zircon Lu-Hf isotopic compositions for the Chiniot andesite
测点号 年龄(Ma) 176Yb/177Hf 176Lu/177Hf 176Hf/177Hf 1σ εHf(0) εHf(t) tDM1(Ma) tDM2(Ma) fLu/Hf 16PA-01 948 0.039 999 0.001 5 0.282 193 0.000 017 -20.5 -0.47 1 513 1 726 -0.95 16PA-02 948 0.028 189 0.001 1 0.282 169 0.000 014 -21.3 -1.04 1 529 1 757 -0.97 16PA-03 948 0.018 261 0.000 7 0.281 689 0.000 018 -38.3 -17.80 2 171 2 670 -0.98 16PA-04 948 0.032 286 0.001 4 0.282 473 0.000 016 -10.6 9.50 1 114 1 178 -0.96 16PA-05 948 0.030 862 0.001 1 0.282 138 0.000 014 -22.4 -2.19 1 575 1 820 -0.97 16PA-06 948 0.004 009 0.000 1 0.282 354 0.000 017 -14.8 6.10 1 240 1 365 -1.00 16PA-07 948 0.008 419 0.000 4 0.282 531 0.000 017 -8.5 12.22 1 003 1 027 -0.99 16PA-08 948 0.016 916 0.000 8 0.282 457 0.000 016 -11.1 9.34 1 118 1 186 -0.98 16PA-09 1523 0.034 658 0.001 3 0.281 955 0.000 016 -28.9 4.02 1 837 1 952 -0.96 16PA-10 883 0.030 309 0.001 1 0.282 179 0.000 015 -21.0 -2.12 1 517 1 765 -0.97 16PA-11 883 0.040 152 0.001 6 0.282 337 0.000 018 -15.4 3.23 1 311 1 471 -0.95 16PA-12 883 0.006 677 0.000 3 0.282 318 0.000 012 -16.0 3.31 1 294 1 467 -0.99 16PA-13 883 0.040 720 0.001 6 0.282 114 0.000 016 -23.3 -4.67 1 626 1 904 -0.95 16PA-14 1523 0.040 086 0.001 6 0.282 051 0.000 019 -25.5 7.12 1 717 1 783 -0.95 16PA-15 883 0.025 434 0.001 0 0.282 190 0.000 016 -20.6 -1.64 1 496 1 739 -0.97 16PA-16 948 0.026 902 0.001 1 0.281 739 0.000 014 -36.5 -16.27 2 125 2 587 -0.97 16PA-17 948 0.025 834 0.001 1 0.282 569 0.000 014 -7.2 13.10 970 978 -0.97 16PA-18 948 0.039 073 0.001 5 0.282 211 0.000 013 -19.8 0.21 1 486 1 688 -0.96 16PA-19 948 0.025 073 0.001 0 0.282 416 0.000 012 -12.6 7.75 1 181 1 274 -0.97 16PA-20 948 0.016 708 0.000 6 0.282 163 0.000 013 -21.5 -1.00 1 521 1 755 -0.98 -
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