岩石学报  2020, Vol. 36 Issue (9): 2765-2784, doi: 10.18654/1000-0569/2020.09.10   PDF    
锆石轻稀土富集与Hf同位素异常成因:以滇西卓潘碱性杂岩体为例
苗壮, 赵志丹, 雷杭山, 武精凯, 杨逸云, 佟鑫     
中国地质大学地质过程与矿产资源国家重点实验室, 中国地质大学地球科学与资源学院, 北京 100083
摘要: 卓潘碱性杂岩体位于思茅地块的西缘,在岩体的霞石正长岩、正长岩、辉石岩中发现多种具不同CL发光特征的锆石。其中CL图下呈灰白色的锆石(Ⅰ类)的Th、U含量低,轻稀土元素亏损,重稀土元素富集,具典型碱性岩岩浆锆石特征,U-Pb年龄约35.7Ma,代表岩体的成岩年龄;不发光或边部具微弱环带的锆石(Ⅱ类)其Th、U、稀土元素含量高,U-Pb年龄为34.2~35.1Ma;呈杂乱海绵状结构的锆石内部不透明,无法获得谐和的U-Pb年龄;震荡环带发育的锆石为捕获锆石,具有较老的U-Pb年龄。本文依据稀土元素含量与配分模式将Ⅱ类锆石进一步分为三种不同类型:A型锆石轻稀土含量低,有明显左倾特征,其LREE < 1170×10-6,(La/Gd)N < 0.09,(Tm/Gd)N>2.2;B型锆石轻稀土亏损但中-重稀土分馏较小,其(La/Gd)N < 0.009,(Tm/Gd)N < 2;C型锆石轻稀土元素含量明显升高且中-重稀土分馏较小,其LREE>1150×10-6,(Tm/Gd)N < 2。这些异常的稀土元素特征并非由分析到磷灰石或榍石等矿物包裹体导致,而是与热液作用过程中流体成分与反应条件有关。由于晶格损伤导致放射性Pb丢失,热液锆石的年龄略小于岩浆锆石,没有明确的地质意义。尽管在本文中两类不同CL发光特征的锆石的年龄相差不大,但不加区分地计算平均年龄可能无法获得准确的成岩时代。卓潘碱性岩体热液锆石的Hf同位素变化范围极大(εHft)=0.1~100),这种异常的Hf同位素特征可能是由于流体与围岩反应过程中溶解了富Lu矿物(如磷灰石)或具高176Hf/177Hf值的矿物(如石榴子石)所导致的。
关键词: 锆石年代学    锆石微量元素    热液锆石    Hf同位素    卓潘碱性杂岩体    滇西    
Genesis of LREE-enriched zircons and their highly radiogenic Hf compositions: A case study from Zhuopan alkaline complex in western Yunnan
MIAO Zhuang, ZHAO ZhiDan, LEI HangShan, WU JingKai, YANG YiYun, TONG Xin     
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China
Abstract: The Zhuopan alkaline complex is located at the western Simao block. From nepheline syenite, syenite, and pyroxenite in this intrusion, two different types of zircons are recognized according to their CL images. Zircons with bright CL images (type Ⅰ) have low Th and U abundances, depleted LREE and enriched HREE, which is typical for magmatic zircons, and they give a weighted mean 236Pb/238U age of 35.7Ma, representing the intrusive age. In contrast, grains that show dark CL images with weakly oscillatory zoning in the outer rims occasionally (type Ⅱ) contain comparatively higher Th, U and sum REE concentrations and yield the U-Pb ages of 34.2~35.1Ma. Opaque zircons display spongy internal textures and failed to obtain concordia U-Pb ages. Moreover, some inherited zircons with clear oscillatory zoning in CL images can also be observed. The chondrite-normalized REE patterns of type Ⅱ zircons in this study can be further divided into three groups: type A zircons show LREE-depleted pattern with LREE < 1170×10-6, (La/Gd)N < 0.09 and (Tm/Gd)N>2.2; type B zircons have similar increasing slope from La to Sm but no distinct fractionation between MREE and HREE, and they have (La/Gd)N < 0.009 and (Tm/Gd)N < 2; and type C zircons are characterized by the flat REE patterns and elevated LREE concentrations, with LREE>1150×10-6 and (Tm/Gd)N < 2. These differences of REE characteristics are result from various compositions of hydrothermal fluids and alteration conditions, rather than involvement of mineral inclusions such as apatite and titanite. Hydrothermal zircons are younger than magmatic ones because of radiogenic Pb loss caused by radiation damage. Therefore, average U-Pb ages calculated from unfiltered zircon data may not give an accurate intrusive age, although the impact is limited in this study. A wide range of εHf(t) (0.1~100) are observed in Zhuopan hydrothermal zircons. The extremely radiogenic Hf compositions can be generated by dissolution of Lu-rich (such as apatite) or high 176Hf/177Hf minerals (such as garnet).
Key words: Zircon geochronology    Zircon trace elements    Hydrothermal zircon    Hf isotopes    Zhuopan alkaline complex    Western Yunnan    

锆石是岩石中广泛存在的副矿物,因其抗风化能力强,U、Th含量高及U-Th-Pb体系封闭温度高等特征而被广泛用于放射性同位素定年。锆石微量元素也被用于建立地质温度计(Watson et al., 2006)、划分岩石类型(Belousova et al., 2002; Wang et al., 2012)、判断构造背景(Grimes et al., 2015; 赵振华, 2016)、追溯岩浆和热液过程(Li et al., 2018; Yan et al., 2018)等方面。此外,锆石的Hf-O同位素也可以示踪地壳混染与地壳再循环等地质过程(Kemp et al., 2007; Liu et al., 2014)。

典型的岩浆成因锆石具有轻稀土元素(LREE)亏损,重稀土元素(HREE)极度富集的特征,以花岗岩为代表的岩浆锆石常具有明显的正Ce异常和负Eu异常(赵志丹等, 2018)。不同于直接从岩浆中结晶的岩浆锆石,热液锆石可结晶于锆饱和的热液流体(Hoskin, 2005)或由流体与蜕晶质化锆石之间发生离子交换以及构造恢复形成(Geisler et al., 2003b)。受流体作用的影响,热液锆石常具有高U、低Th/U、高稀土元素(REE)总量的特征(Ayers and Peters, 2018)。许多热液锆石的LREE含量明显偏高(Hoskin, 2005; Rayner et al., 2005; Li et al., 2018),然而也有多地报道了具有其他REE配分形式的热液锆石(Pettke et al., 2005; Pelleter et al., 2007; Toscano et al., 2014),这些变化可能受流体成分、体积、蚀变温度和时间等多种因素的影响(Cavosie et al., 2006)。此外,当测试过程中分析到纳米级的微小矿物包裹体时也会得到异常的锆石微量元素含量(Zhong et al., 2018)。滇西卓潘碱性杂岩体中发育多种不同类型的锆石,为探究锆石不同微量元素特征的成因提供了良好的研究对象。

滇西地区沿着哀牢山-红河断裂带发育大量年龄在32~38Ma的钾质碱性侵入岩(Lu et al., 2012; Tong et al., 2019),其中卓潘碱性杂岩体由于其位于兰坪-思茅盆地西缘且硅不饱和而区别于其他岩体(董方浏等, 2007; 杜斌等, 2018)。目前已报道卓潘岩体辉石正长岩的黑云母40Ar/39Ar坪年龄为36.7±0.2Ma(董方浏等, 2005)和38.0±0.6Ma(张玉泉和谢应雯, 1997);霞石正长岩的霞石40Ar/39Ar坪年龄为36.9±0.2Ma(王江海等, 2002); 杜斌等(2018)对岩体中的正长岩和正长辉石岩进行锆石U-Pb年龄测试,获得加权平均年龄分别是33.4±0.4Ma和34.2±0.3Ma。由以上可见岩体不同岩性之间年龄差距较大,缺乏准确的定年。本文选取卓潘岩体中的霞石正长岩、正长岩和辉石岩样品进行锆石U-Pb定年,锆石微量元素及Hf同位素研究,在厘定岩体成岩年龄的同时进一步判断不同微量元素特征锆石的形成原因。

1 地质背景与样品采集

思茅地体包含由变火山岩和变沉积岩组成的元古代变质基底,古生代海相沉积岩,以及不整合于其上的中生代-古近纪陆源沉积物(云南省地质矿产局, 1990)。该板块夹持于保山地块与扬子板块之间,西侧为昌宁-孟连构造带,东侧为哀牢山-红河构造带(Metcalfe, 2013)(图 1a)。这两条构造带分别是古特提斯洋的主洋盆(澜沧江洋)和支洋盆(金沙江-哀牢山洋)俯冲、闭合与造山作用的产物(Jian et al., 2009; Deng et al., 2014; Lai et al., 2014a, b)。思茅地体在构造属性上属于印支地体向北延伸的部分(Metcalfe, 2013),且是分离地块,直到三叠纪才与印支地体拼合(Rossignol et al., 2016),但这一观点至今仍然存在争议。随着印度板块向欧亚板块的持续挤压,在青藏高原东南缘沿哀牢山-红河断裂带附近产生大量始新世至渐新世(38~32Ma)的钾质火山岩(Xu et al., 2001; Huang et al., 2010)、富碱侵入岩(Lu et al., 2013; He et al., 2016; Chen et al., 2017)和煌斑岩脉(Li et al., 2002; Xu et al., 2007; Lu et al., 2015),形成一条近700km的钾质碱性岩带(图 1a)。

图 1 滇西新生代钾质火成岩分布图(a, 据曾普胜等, 2002; Huang et al., 2010修改)及卓潘碱性杂岩体地质图(b, 据云南省地质局, 1979修改) Fig. 1 Simplified geological map showing the outcrops of Cenozoic potassic igneous rocks in western Yunnan (a, modified after Zeng et al., 2002; Huang et al., 2010) and geologic map of Zhuopan alkaline complex (b)

① 云南省地质局. 1979.永平幅(G-47-ⅩⅩⅡ) 1:20万区域地质调查报告

卓潘岩体位于思茅地体西缘,哀牢山-红河剪切带以西80km处。岩体为近东西向的椭圆状岩株,长约15km,宽约5km,侵位于白垩系砂岩中(图 1b)。岩体主要由西部的辉石岩和辉石正长岩(65%),东部的正长岩(30%)以及围绕岩体边缘产出的少量霞石正长岩(5%)组成,不同岩性之间呈渐变接触。岩体内第四纪沉积物覆盖严重,与围岩的接触带上可见硅化、绢云母化与角岩化(图 1b)。

本文锆石分别取自卓潘碱性杂岩体的辉石岩(ZP1616)、正长岩(ZP1612、ZP1618、ZP1620)和霞石正长岩(ZP1602、ZP1607)样品中。辉石岩具堆晶结构,主要矿物包括单斜辉石(95%)、磷灰石(3%)、黑云母(2%)及少量Ti-Fe氧化物和锆石(图 2a)。正长岩为中粒结构,矿物以碱性长石(55%~70%)、单斜辉石(5%~35%)、黑云母(1%~10%)为主,以及少量角闪石、磷灰石、锆石和榍石(图 2b, c)。霞石正长岩具中粒结构,主要矿物有钾长石(70%)、霞石(10%)、霓辉石(10%)、霓石(7%)及黑云母(3%);副矿物包括磷灰石、锆石、榍石和Fe-Ti氧化物(图 2d)。

图 2 滇西卓潘碱性杂岩体的镜下照片 Agt-霓辉石; Amp-角闪石; Ap-磷灰石; Bi-黑云母; Di-透辉石; Ne-霞石; Or-正长石 Fig. 2 Microphotographs of Zhuopan alkaline rocks in western Yunnan Agt-aegirine-augite; Amp-amphibole; Ap-apatite; Bi-biotite; Di-diopside; Ne-nepheline; Or-orthoclase
2 样品制备及测试方法

锆石颗粒由破碎后的新鲜岩石粉末经重液法、磁选法、目视挑选法等技术分选得到,并粘在环氧树脂盘上抛光制成锆石靶。锆石阴极发光(CL)图像采集在北京锆年领航科技有限公司完成。锆石微量元素与U-Pb同位素测年在中国科学院海洋研究所海洋地质与环境重点实验室利用LA-ICP-MS分析完成。激光剥蚀系统使用Photo-machines公司的EXCITE193nm,电感耦合等离子质谱(ICP-MS)为Agilent7900。激光束斑直径32μm,工作电压1.35kV。实验过程中每5个样品点分别用2个91500标样进行校正,仪器状态监测使用GJ-1和Plesovice锆石标样。锆石U-Pb数据利用ICPMSDataCal10.2进行离线处理(Liu et al., 2008, 2010),并采用Andersen(2002)的方法进行普通Pb校正。锆石微量元素以29Si作为内标校正。

锆石原位Hf同位素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使用LA-MC-ICPMS(Neptune Plus)完成。激光剥蚀系统为GeoLas 2005,激光束斑直径44μm。同位素分馏校正使用91500锆石标样,仪器状态监测使用GJ-1和TEM锆石标样。详细实验方法见Hu et al. (2012)。数据的离线处理使用ICPMSDataCal10.2(Liu et al., 2010)。

3 结果 3.1 锆石形态与CL图像特征

样品锆石多为不规则粒状或柱状,不同样品之间锆石形态差异明显。依据CL发光特征可将锆石分为四种类型(图 3a-d):灰白色锆石;暗色不发光锆石;海绵结构不透明锆石;震荡环带发育的捕获锆石。各类锆石的特征和在样品中的分布如下:正长岩样品ZP1612和ZP1618中锆石粒度65~390μm,表面较为平整,内部透明-半透明。锆石在CL图中呈均一的灰白色,部分可见不规则的环带区域,记为Ⅰ类锆石(图 3a, i, j)。

图 3 卓潘碱性杂岩体锆石阴极发光图像 Fig. 3 Cathodoluminescence (CL) images of zircon from Zhuopan alkaline complex

霞石正长岩样品ZP1602、ZP1607和正长岩样品ZP1620中锆石粒度70~370μm,边部发育不同程度的溶蚀。部分锆石表面粗糙,内部不透明,CL图中明暗杂乱呈海绵状结构(图 3c, e-g);另一部分颗粒相较而言表面平整,内部透明或半透明,CL图中不发光或边部微弱环带(图 3b, e-g),记为Ⅱ类锆石。此外,样品ZP1607中偶见CL图呈灰白色的Ⅰ类锆石(图 3f);ZP1602中还发育少量粒度小,磨圆好且具明显震荡环带特征的捕获锆石(图 3d)。

辉石岩样品ZP1616中仅分选出52颗锆石,粒度75~175μm。该样品同时具有上述Ⅰ类、Ⅱ类两种特征的锆石,此外还有部分自形短柱-长柱状且环带发育的捕获锆石(图 3d, h)。

3.2 锆石U-Pb年龄与微量元素

对6件卓潘碱性杂岩体的锆石样品进行了年代学和微量元素分析,测试结果见表 1表 2图 4。正长岩样品ZP1612、ZP1618中发育Ⅰ类锆石,获得206Pb/238U加权平均年龄分别为35.7±0.3Ma(2σn=20)和35.7±0.4Ma(2σn=20)(图 4a, b)。这些锆石Th含量为123×10-6~2928×10-6,U含量为116×10-6~3520×10-6,Th/U比值为0.4~3.0(表 1)。锆石稀土元素在球粒陨石标准化后呈现LREE亏损且分馏明显的左倾特征((La/Ga)N=0.0001~0.02)(图 5a)。

表 1 卓潘碱性杂岩体的锆石U-Pb定年结果 Table 1 Zircon U-Pb analytical results of Zhuopan alkaline complex

表 2 卓潘碱性杂岩体的锆石微量元素含量(×10-6) Table 2 Zircon elemental compositions of Zhuopan alkaline complex (×10-6)

图 4 卓潘碱性杂岩体锆石U-Pb年龄谐和图 Fig. 4 Zircon U-Pb concordia diagrams for Zhuopan alkaline complex

图 5 卓潘碱性杂岩体锆石球粒陨石标准化REE配分模式(标准化值据Sun and McDonough, 1989) 阴影部分数据引自Hoskin (2005), Pelleter et al. (2007), Toscano et al. (2014), Li et al. (2018) Fig. 5 Chondrite-normalized REE patterns for zircons from Zhuopan alkaline complex (normalizing data from Sun and McDonough, 1989) Shadowed areas from Hoskin (2005), Pelleter et al. (2007), Toscano et al. (2014), Li et al. (2018)

霞石正长岩样品ZP1602、ZP1607中不透明且CL图呈现海绵状结构的锆石无法获得稳定的U-Pb同位素信号,因此仅对CL图不发光的Ⅱ类锆石进行测试,实验过程中避开裂隙与包裹体。2件样品的锆石206Pb/238U加权平均年龄分别为34.9±0.3Ma(2σn=13)和35.1±0.2Ma(2σn=16)(图 4c, d)。锆石具有高的Th(115×10-6~18908×10-6)、U(971×10-6~17430×10-6)含量和变化较大的Th/U比值(0.01~2.7)。其(La/Ga)N值在0.0001~4.6之间变化,表明锆石LREE的分馏程度变化明显,甚至呈现负斜率(图 5c)。

正长岩ZP1620中发育Ⅱ类锆石,其206Pb/238U加权平均年龄为34.2±0.3Ma(2σn=18)(图 4e)。该样品锆石有极高的Th(1528×10-6~41624×10-6)、U(6489×10-6~20028×10-6)含量,Th/U比值为0.2~2.1,(La/Ga)N值为0.00002~0.09(表 2)。该样品呈现明显的HREE富集体特征,且重稀土的分馏程度变化较大((Ho/Lu)N=0.5~1.1)(图 5a, b)。

辉石岩样品ZP1616中两类锆石的206Pb/238U加权平均年龄为35.6±0.3Ma(2σn=15)(图 4f)。锆石Th含量为554×10-6~11498×10-6,U含量为564×10-6~5458×10-6,Th/U比值为0.4~3.0,(La/Ga)N值为0.00002~0.005(表 2)。此外,测得其他短柱状环带锆石的加权平均年龄为98.7±1.6Ma(2σn=6)(图 4f),此类捕获锆石的微量元素特征不在本文的讨论范围之内。

所有被测试的锆石具有不同程度的正Ce异常(Ce/Ce*=1.3~403),未见明显的Eu异常。从图 5表 2中可以看出,Ⅰ类锆石发育的样品其锆石微量元素均一(如P1612、ZP1618),而Ⅱ类锆石的微量元素特征变化较大(如ZP1602、ZP1620)。为方便讨论,本文进一步将Ⅱ类锆石的不同稀土配分形式分为3种类型:A型锆石轻稀土含量低,有明显左倾特征(图 5a),其LREE < 1170×10-6,(La/Gd)N < 0.09,(Tm/Gd)N>2.2;B型锆石轻稀土亏损但中-重稀土分馏较小(图 5b),其(La/Gd)N < 0.009,(Tm/Gd)N < 2;C型锆石轻稀土元素含量明显升高且中-重稀土分馏较小(图 5c),其LREE>1150×10-6,(Tm/Gd)N < 2。可以看出所有Ⅰ类锆石的稀土配分形式与Ⅱ类锆石中的A型相似(图 5a)。

3.3 锆石Hf同位素

本文对发育Ⅱ类锆石的正长岩、霞石正长岩和同时具有两种锆石的辉石岩样品进行锆石Lu-Hf同位素测试,结果见表 3(仅保留信号稳定的数据)。杜斌等(2018)对卓潘岩体中Ⅰ类锆石的测试获得其176Yb/177Hf比值为0.010074~0.104147,εHf(t)值为-10.6~-4.3。而本文测试的锆石具有变化较大的176Yb/177Hf值(0.02~0.3)、176Hf/177Hf值(0.282788~0.285617)和εHf(t)值(0.9~100.9)。这些数据已无法代表成岩过程中原始的Hf同位素值,也无法计算出有意义的地壳模式年龄,其形成原因将在后文论述。

表 3 卓潘碱性杂岩体的锆石Lu-Hf同位素测试结果 Table 3 Analytical results of zircon Lu-Hf isotopes for Zhuopan alkaline comple
4 讨论 4.1 轻稀土富集锆石的成因

卓潘碱性杂岩体中Ⅱ类C型锆石富集的轻稀土元素与典型岩浆锆石特征有明显不同。常见的引起锆石中LREE含量升高的原因有以下几种:(1)一些情况下锆石结晶于岩浆演化后期时会出现富集LREE的特征(Pettke et al., 2005),如早期熔体中富集HREE的矿物(如角闪石)的结晶会导致锆石结晶时熔体富含LREE(Whitehouse and Kamber, 2002);(2)测试过程中分析到细小的矿物包裹体,如磷灰石、榍石、褐帘石、磷钇矿、独居石等(Jain et al., 2001; Cavosie et al., 2006; Xia et al., 2010; Zhong et al., 2018);(3)由于放射性元素(Th、U)含量高,锆石晶格产生放射性损伤(Holland and Gottfried, 1955; Murakami et al., 1991),并引起局部轻稀土元素含量升高(Whitehouse and Kamber, 2002; Cavosie et al., 2006);(4)锆石形成于热液流体中或被热液作用改造(Hoskin, 2005; Pettke et al., 2005; Schaltegger, 2007)。

卓潘碱性杂岩体的不同岩性由同源岩浆结晶分异而来(杜斌等, 2018),但全岩的MgO和SiO2含量与LREE含量之间没有明显的相关关系(陈喜峰等, 2015; 杜斌等, 2018)。如果锆石富集LREE的特征是因为其结晶于岩浆作用后期,则这种锆石在数量上应该与早期结晶的锆石相近,但事实上绝大多数的岩浆锆石都为LREE亏损的模式(Zhong et al., 2018)。此外,卓潘岩体的矿物组合中富集HREE的矿物并非主要的矿物相,仅发育少量由辉石变化而来的角闪石。因此,锆石结晶于岩浆演化后期或富含HREE矿物的早期结晶不是锆石富集LREE的主要原因。

锆石中Th、U等元素的衰变会使锆石产生晶格损伤(lattice damage),而锆石LREE的富集程度可能与损伤程度相关(Whitehouse and Kamber, 2002; Cavosie et al., 2006)。使用参数Dα代表锆石形成以来经受α粒子轰击的程度(Holland and Gottfried, 1955; Murakami et al., 1991),以(La/Gd)N代表LREE富集程度,从Dα-(La/Gd)N图解中可以看出损伤程度较高的锆石并非一定富集轻稀土元素(图 6a),因此可以排除晶格损伤的决定性影响。

图 6 卓潘碱性杂岩体锆石微量元素特征图解 含磷灰石包裹体锆石引自Zhong et al. (2018);文献岩浆锆石与热液锆石引自Yang et al. (2014)Li et al. (2018) Fig. 6 Trace elements discrimination plots of zircons from Zhuopan alkaline complex Zircons with apatite inclusions are from Zhong et al. (2018); magmatic and hydrothermal zircons are from Yang et al. (2014) and Li et al. (2018)

在分析过程中测试到亚微米(sub-micrometer)级的磷酸盐包裹体同样可以引起锆石LREE的富集,且此时可以明显观察到P与(La/Gd)N之间的线性关系(图 6b)。在样品数据的离线处理过程中,C型锆石中没有观察到P元素信号强度的明显峰值,而且LREE富集程度与P之间没有相关关系(图 6b),进一步排除了分析到磷灰石、磷钇矿等矿物包裹体的可能。同理,锆石的(La/Gd)N值与Th之间没有明显的相关关系(图 6c),排除了分析到(富Th)独居石等矿物包裹体的可能。

卓潘岩体锆石的轻稀土富集程度与Nb、Ta呈现不同程度的正相关关系(图 6d, e)。常见的富含Nb、Ta元素的锆石矿物包裹体有金红石和榍石,然而金红石在硅酸盐熔体中亏损REE(Foley et al., 2000; Klemme et al., 2005),因此可以排除;榍石则富集轻稀土(Deng et al., 2015; 范裕等, 2017)或中稀土元素(MREE)(Green and Pearson, 1983; Tiepolo et al., 2002),可能会造成锆石LREE的富集。Xu et al.(2015)报道了滇西富碱侵入岩带中榍石的矿物化学特征,发现寄主岩石发生铜矿化的榍石具有更高的REE含量。本文选取其中稀土元素最富集的榍石与卓潘锆石进行混合模拟。锆石样品选自ZP1607中的一颗Ⅰ类锆石,该样品还同时发育轻稀土亏损(A型)和轻稀土富集(C型)的Ⅱ类锆石。结果显示(图 7a),轻稀土最富集的锆石需要50%体积分数的榍石加入,且此时重稀土元素拟合程度较差,Ti含量远高于卓潘锆石中的实际含量。当有少量榍石加入时会无法观测到明显的正Ce异常,这也与实际观察不符。此外,锆石的(La/Gd)N值与Ti之间也没有正相关关系,因此锆石LREE的富集由榍石等微小矿物包裹体引起的可能性较小。

图 7 锆石与榍石包裹体混合模拟(a)与锆石成因判别图解(b, 底图据Hoskin, 2005; c, d, 底图及数据引自Li et al., 2018) 图a模型中锆石数据取自本文样品ZP1607的13号测试点,榍石数据取自Xu et al. (2015)中的样品TC904,百分数代表榍石混入的体积分数;图b-d的图例同图 6 Fig. 7 Modeling of mixing between zircon and titanite inclusions (a) and discriminant diagrams for zircons in Zhuopan (b, after Hoskin, 2005; c, d, after Li et al., 2018) Spot 13 from sample ZP1607 in this study and sample TC904 from Xu et al. (2015) represent the initial magmatic zircon composition and titanite end-member, respectively; the percentage represents the volume of titanite; Symbols in Fig 7b-d are same as in Fig. 6

锆石中Nb,Ta元素含量取决于熔体成分(Nardi et al., 2013),实验岩石学证明锆石结晶时熔体中的P会阻止Ta进入锆石晶格,而Al则会促进该过程(Van Lichtervelde et al., 2011)。卓潘锆石的平均Nb、Ta含量与其寄主岩石的Al含量成正相关,与P含量呈负相关,因此富含Nb和Ta的锆石可能是由于结晶于富Al贫P的岩浆。此外,在富F的碱性流体中,Th、U、Nb、Hf、REE等不相容元素具有强的流动性(Rubin et al., 1993),这种流体中沉淀的锆石也会富集相应的元素。澳大利亚Boggy Plain带状侵入岩体(Hoskin, 2005),内蒙巴尔哲碱性花岗岩(Yang et al., 2014),冀西北东坪碱性杂岩体(Li et al., 2018)等多处都被报道有轻稀土富集且U含量高的热液锆石(图 5c),这些热液锆石相对于岩浆锆石也具有更高的Nb、Ta含量(图 6d, e)。在(Sm/La)N-Ce/Ce*图解中C型锆石与热液锆石特征一致(图 7b),这些证据表明轻稀土富集的锆石还可能与热液流体作用有关。此外,赵振华等(2010)报道了水泉沟碱性正长岩中的锆石具有平滑的稀土配分模式,并且在轻稀土形成似M型四分组效应。卓潘岩体中的C类锆石也呈现了相似的特征(图 5c)。

卓潘岩体的Ⅱ类锆石除了LREE富集的C型锆石外,还具有所谓典型“岩浆锆石”稀土特征的A型锆石和总稀土含量高但HREE平坦的B型锆石。这些暗色锆石在REE-Eu/Eu*图解(图 7c)和REE-LREE图解(图 7d)中与热液锆石具有相同的稀土元素特征。尽管当岩浆源区残留大量石榴子石也可以产生重稀土元素平坦的B型锆石,但这种特征仅在一件正长岩样品的部分锆石中发现,所以其并非继承自源区。因此,不能仅依靠LREE富集特征准确区分热液锆石与岩浆锆石。例如,在澳大利亚Mole花岗岩(Pettke et al., 2005)和摩洛哥Tamlalt-Menhouhou金矿的钠长岩(Pelleter et al., 2007)中可见到具有“岩浆锆石”稀土特征的热液锆石(图 5a);而在西班牙Aznalcóllar矿区的流纹岩(Toscano et al., 2014)中发育有同B型锆石类似的热液锆石(图 5b)。这些锆石微量元素的变化可能体现了流体成分以及蚀变时反应温度和持续时间的不同(Cavosie et al., 2006)。热液锆石可以直接从锆饱和流体中结晶(Hoskin, 2005),也可以由流体与蜕晶质化锆石之间发生离子交换形成(Geisler et al., 2003a)。卓潘岩体中发育大量不透明且CL图下呈杂乱的海绵状结构的锆石(图 3e-j),部分还残留有上一代锆石的核部(图 3e),可能是热液流体与岩浆锆石反应形成(Li et al., 2014)。本文测试的Ⅱ类锆石一部分在透射光下呈半透明暗褐色,CL图中不发光,既无环带也没有残留锆石核部,推测为锆饱和流体中直接沉淀形成(Hoskin, 2005; Li et al., 2014);另一部分同时具有前述两种锆石的特征,可能是在过渡过程中形成。

4.2 成岩年龄及地质意义

卓潘碱性杂岩体的Ⅱ类锆石具有高的U含量(3231×10-6~14739×10-6),对高U锆石(U>2000×10-6)进行U-Pb定年时,表观年龄随U的升高而明显的偏高现象被称为“高U效应(High-uranium matrix effect)”(White and Ireland, 2012; Gao et al., 2014; 李秋立, 2016)。但是这种现象一般出现在SIMS或SHRIMP测试过程中,且对较为年轻的锆石(约20~50Ma)影响不明显(White and Ireland, 2012)。本文使用LA-ICP-MS可以为高U锆石提供可靠的U-Pb年龄(Zhao et al., 2014),且样品属于年龄较小的新生代锆石(约35Ma),在锆石U含量与年龄图解中没有发现年龄随U含量的增加而升高的现象(图 8a)。

图 8 卓潘锆石U含量与年龄协变图解(a)与锆石α衰变强度与年龄协变图解(b) Fig. 8 Variation diagrams for U vs. Age (a) and Dα vs. Age (b) of zircons in Zhuopan

由于U在流体中活动性较高,受热液作用影响的锆石一般具有较高的U含量。Th、U的衰变会造成锆石的晶格损伤,从而引起放射性成因Pb丢失,使获得的U-Pb表观年龄相对偏低(Geisler and Schleicher, 2000; Mathieu et al., 2001)。从图 8b中可以看到,放射性损伤高的锆石年龄相对较小。卓潘岩体中热液锆石发育的ZP1602、ZP1607、ZP1620样品的加权平均年龄(34.9Ma、35.1Ma、34.2Ma)略小于岩浆锆石发育的样品ZP1612和ZP1618(均为35.7Ma),而两类锆石均有发育的辉石岩样品ZP1616的加权平均年龄(35.6Ma)则正好位于二者之间(图 9)。因此,正长岩ZP1612和ZP1618的U-Pb年龄更能代表卓潘岩体的成岩时间(约35.7Ma),这与相同样品中榍石的U-Pb年龄是一致的(马倩等,未发表数据)。Ⅱ类锆石发育的样品其年龄没有明确的地质意义,可能代表热液活动时间或者与成岩年龄的混合年龄。本文获得的成岩年龄略大于杜斌等(2018)测得的正长岩(33.4Ma)和正长辉石岩(34.2Ma)的岩浆锆石年龄,卓潘岩体中可见大量岩脉侵入,年龄差距是可能是由岩浆的持续活动造成。

图 9 卓潘碱性杂岩体锆石U-Pb年龄频率直方图 Fig. 9 Histograms of U-Pb ages of the Zhuopan alkaline complex
4.3 高REE锆石Hf同位素异常

在卓潘锆石的Lu-Hf同位素测试过程中,许多暗色锆石无法获得稳定的176Hf/177Hf信号,而排除这些点之后,剩余的锆石仍然具有变化很大的176Hf/177Hf值(εHf(t)=1~100)(图 10表 3),部分数据明显高于普通锆石(εHf(t) < 20)。在测试176Hf的过程中必须扣除同质异位素176Lu和176Yb的干扰,当没有进行足够的干扰校正时,锆石176Hf/177Hf值会随176Yb/177Hf值的增大而比真实值偏高(Fisher et al., 2014)。经过证实对高Yb锆石(Yb/Hf > 0.05)运用173Yb/171Yb=1.132685来计算质量偏移系数(βYb),用176Yb/173Yb=0.79639来进行干扰校正会得到较好的结果(Fisher et al., 2014)。然而在图 10中可以看到,校正后的数据仍然具有极高的176Hf/177Hf值,推测这是由于锆石的Lu-Hf体系受到影响或还有其他因素干扰了测试的准确度。

图 10 卓潘锆石176Yb/177Hf-176Hf/177Hf协变图解 Fig. 10 Plot of 176Yb/177Hf vs. 176Hf/177Hf for zircons in Zhuopan

后期的热液蚀变过程对锆石的原生Hf同位素特征影响很小(Gerdes and Zeh, 2009; Lenting et al., 2010),尽管在蚀变过程中存在Lu和Yb的迁移,但由于锆石的Lu/Hf值很低,对176Hf/177Hf初始值的计算影响不大(Gerdes and Zeh, 2009);而Yb含量的变化可以通过后期计算进行校正。对于直接从流体中结晶的热液锆石,有两种模型可以解释其极高的Hf同位素成因:(1)热液流体溶解围岩中的磷灰石和单斜辉石,部分磷灰石中极高的176Lu/177Hf值会形成大量放射性成因Hf同位素。例如Valley et al.(2010)对美国Adirondack高地的磁铁矿-磷灰石矿床进行矿物化学研究认为,流体中溶解5%高Lu磷灰石即可在约35Myr的时间内使其形成热液锆石的εHf(t)值上升35个单位。卓潘岩体中热液锆石与岩浆锆石的年龄相差很小,在如此短时间内累积100个单位的εHf(t)值则需要溶解更高Lu含量或更多体积的磷灰石。(2)一些变质岩中的矿物有高的176Hf/177Hf值,如榴辉岩中的石榴子石(176Hf/177Hf > 0.29)(Schmidt et al., 2011; Cheng et al., 2018)。流体与变质围岩反应过程中若石榴子石的高放射性Hf同位素释放进入流体,从中结晶出的热液锆石也会有较高的Hf同位素比值(Valley et al., 2010; Li et al., 2018)。卓潘岩体位于昌宁-孟连变质岩带与点苍山-哀牢山变质岩带之间,研究区内丰富的断裂为流体的迁移提供了条件。因此锆石异常高的εHf(t)值可能是相关流体吸收了具高Lu/Hf或高176Hf/177Hf的矿物。

5 结论

(1) 卓潘碱性杂岩体中CL图下呈灰白色的Ⅰ类锆石属于岩浆锆石,其U-Pb年龄为35.7Ma,代表岩体的成岩年龄。

(2) 岩体中CL图呈暗色的Ⅱ类锆石受到热液作用影响。由于热液成分和蚀变条件的不同,锆石的稀土元素呈现不同的配分模式,轻稀土元素的富集并不是识别热液锆石的唯一标志。

(3) 热液锆石异常高εHf值的成因可能是相关流体吸收了具高Lu/Hf或高176Hf/177Hf的矿物成分。

致谢      本研究的野外工作受到喻学惠教授的大力帮助;中国科学院海洋研究所海洋地质与环境重点实验室王晓红老师和中国地质大学(武汉)地质过程与矿产资源国家重点实验室胡兆初教授在锆石U-Pb定年和Hf同位素测试过程中提供了重要帮助;二位审稿人提出了宝贵的修改意见;在此一并表示感谢。

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