攀西地区二叠纪赋存铌钽矿的正长岩脉的成因探讨*
2015-03-15王汾连赵太平王焰
王汾连 赵太平 王焰
WANG FenLian1,2,ZHAO TaiPing3 and WANG Yan3
1. 国土资源部海底矿产资源重点实验室,广州海洋地质调查局,广州 510075
2. 中山大学海洋学院,广州 510006
3. 中国科学院广州地球化学研究所矿物学与成矿学重点实验室,广州 510640
1. MLR Key Laboratory of Marine Mineral Resources,Guangzhou Marine Geological Survey,Guangzhou 510075,China
2. School of Marine Sciences,Sun Yat-sen University,Guangzhou 510006,China
3. Laboratory of Mineralogy and Metallogeny,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China
2014-09-03 收稿,2014-12-29 改回.
~260Ma 地幔柱活动形成的峨眉山大火成岩省(Chung and Jahn,1995;Xu et al.,2001;Zhou et al.,2002)主要由大陆溢流玄武岩及共生的镁铁-超镁铁质岩体、花岗岩和正长岩组成。此外,该区沿着安宁河断裂带发育大量的正长岩脉,其中一部分岩脉富含铌钽等稀有金属元素。锆石LAICP-MS 法U-Pb 年代学表明这些正长岩脉形成于258 ~256Ma(王汾连等,2013),与峨眉山大火成岩省活动时间相一致,但是对于其源区性质以及它们与区内大型花岗岩体和正长岩体的成因联系并不清楚。如果正长岩脉的成因与峨眉山岩浆活动有密切关系,那么对该区铌钽矿床勘查具有一定的指导意义。因此我们选取了炉库和白草两个矿区相关的正长岩脉及岩体进行了详细的野外考察和大量的岩石学及同位素地球化学分析。本文在作者及前人对于本区正长岩脉和长英质岩体的岩石学和形成时代的基础上,并结合主微量地球化学特征,主要根据炉库和白草矿区富矿正长岩脉、无矿正长岩脉及相关正长岩体的Sr-Nd 同位素特征,探讨本区富含铌钽等稀有金属的正长岩脉的物质来源及其与峨眉山大火成岩省的成因联系。
1 地质背景
攀西地区(攀枝花-西昌地区)位于四川省西南部,自北起四川冕宁,南至德昌、米易和攀枝花,直至云南元谋,南北绵延300 多千米。在大地构造上处于扬子板块西缘,峨眉山大火成岩省的内部带(图1)。峨眉山大火成岩省覆盖面积超过2.5 ×105km2,所形成的火山岩地层厚度从几百米到5千米不等,主要分布在中国的西南部和越南的北部,包括出露广阔的大陆溢流玄武岩以及在时空上关系密切的镁铁-超镁铁质岩体和少量花岗岩体和正长岩体,被认为与地幔柱活动有关(Chung and John,1995;Shellnutt et al.,2009a,b;Xu et al.,2001;Zhou et al.,2002;Zhong et al.,2007)。
在峨眉山大火成岩省其它地区广泛分布的二叠纪玄武岩在攀西地区则出露较少(主要分布在米易龙帚山一带),但该区却发育有众多的中酸性侵入岩体,表现为南北向构造-岩浆活动带(从柏林,1988;张云湘等,1988;四川省地质矿产局,1991)。区内断裂以南北向安宁河断裂带为主,沿此断裂带由北至南,断续发育几个大型的层状镁铁-超镁铁质岩体,分别是太和、新街、红格和攀枝花岩体。本区除了广泛发育的镁铁-超镁铁质岩体外,还伴随有大量的花岗岩和少量正长岩。张云湘等(1998)将攀西地区玄武岩、层状侵入岩和长英质岩体在时空上紧密伴生的关系总结为“三位一体”。长英质岩体集中分布在50km 宽、200km 长的一个狭长带上,从北往南主要的花岗质岩体依次分布有太和花岗岩体、黄草正长岩体、茨达花岗岩体及矮郎河花岗岩体。在这些长英质岩体附近发育有正长岩脉,其中部分正长岩脉富含铌钽等稀有金属元素(贺金良,2004)。
炉库和白草地区出露的正长岩脉中富含铌钽等稀有金属元素,有些达到工业开采标准。这两个地区的矿床均位于盐边县境内,安宁河断裂带西侧(图1)。白草矿区位于炉库矿区北东方向5km 左右,其西侧为碱性正长岩体,东侧为矮郎河花岗岩体。矿区内正长岩脉侵入于二叠纪镁铁-超镁铁质岩体中(图2、图3),距离正长岩体0.5 ~1km,大体上分为贫矿正长岩脉和富矿正长岩脉两类。各类正长岩脉在垂直方向上则呈平行或窄束的放射状排列,产出严格受断裂控制。
2 岩相学特征
两个矿区富矿正长岩脉主体呈灰色至灰白色,多为粗粒-伟晶不等粒结构(图4a)。主要组成矿物为钾长石(30%~50%,主要是条纹长石和微斜长石,部分正长石),钠长石(10% ~30%)和霓石(5% ~15%)、钠铁闪石(5% ~10%)及黑云母(1% ~2%)。烧绿石(主要赋存Nb2O5,少量Ta2O5)是主要赋矿矿物(图4b),其次为褐钇铌矿。副矿物包括锆石、榍石和少量钛铁矿、磁铁矿、萤石等。正长岩体和无矿正长岩脉多为细粒-中粒结构(图4c,d),矿物组成与含矿岩脉相似,但在含量上有差异。相比富矿正长岩脉,无矿正长岩脉含有更多的钾长石(60% ~80%)、斜长石(5% ~10%)、黑云母(2%)和及较少的钠长石(5%)和霓石(5%),而副矿物含量如榍石、钛铁矿等明显高于富矿正长岩脉,烧绿石等矿石矿物极少。
3 分析方法和结果
3.1 分析方法
图1 攀西地区玄武岩、辉长岩及长英质岩体分布图及部分赋存在正长岩脉中的铌钽矿床(据贺金良,2004;Pang et al.,2009)矿点7 和8 分别为白草和炉库矿区Fig.1 Distribution of balast,gabbro and felsic intrusions and some Nb-Ta ore deposits hosted in syenitic dikes in Panxi area (after He,2004;Pang et al.,2009)The number 7 and 8 representative Baicao and Luku Nb-Ta deposit respectively
Sr-Nd 同位素分析测试在中国科学院广州地球化学研究所同位素年代学与地球化学国家重点实验室的Micromass ISOPROBE 型多接收电感耦合等离子体质谱仪上进行。详细的实验流程和分析方法见梁细荣等(2003)和韦刚健等(2002)。实验所用的Nd 标样为国际标样Shin-Etsu JNdi-1(Tanaka et al.,2000)。标样溶液均用体积比约为2% 的HNO3溶液稀释,浓度为100 ~200ng/mL。样品化学处理所用的HNO3试剂均经二次蒸馏,水溶剂为电阻>18MΩ 的高纯水。Nd-Ce 混合试样由Nd-GIG 溶液与高纯Ce 标样溶液混合而成,其Ce/Nd 质量比分别为0.05、0.10、0.50、1.00 和1.50,溶液浓度为100 ~200ng/mL。Nd-Sm 混合试样由Nd-GIG 溶液与高纯Sm 溶液混合而成的,其Sm/Nd 质量比分别为0.05、0.10、0.20、0.30、0.50 及0.70,溶液浓度为200ng/mL。关于同质异位素干扰的校正,142Ce 对142Nd 的同质异位素干扰是通过测量无干扰的140Ce 强度并使用142Ce/140Ce =0.125424 进行校正。校正之前,142Ce/140Ce 比值的质量分馏通过145Nd/146Nd=0.482639 进行校正,146Nd/144Nd 比值以及其它非放射成因的Nd 同位素比值采用O’Nions et al.(1977)的推荐值,其中,146Nd/144Nd =0.72190。144Sm 对144Nd的同质异位素干扰通过测量无干扰的144Sm 强度,并使用144Sm/147Sm =0.20504 进行校正,144Sm/147Sm 比值的质量分馏通过149Sm/147Sm = 0.92160 进行校正。144Sm/147Sm 及149Sm/147Sm 比值采用Walder et al. (1993)和Wasserburg et al. (1981)的测量值。
图2 攀西地区炉库和白草铌钽矿区图(据四川省地质局403 地质队,1965①四川省地质局403 地质队. 1965. 会理路枯烧绿石伟晶岩矿区详细普查报告.内部资料. 注:原地名为路枯,归属四川省会理县,现改名为炉库,归属四川省盐边县;四川省地质局西昌地质队,1962②四川省地质局西昌地质队. 1962. 会理白草铌钽矿区详细普查报告. 内部资料. 白草现归属四川省盐边县改编)Fig.2 Geolgoical maps of the Luku and Baicao Nb-Ta ore deposits in the Panxi district
图3 攀西地区正长岩脉野外照片图(a)富矿正长岩脉侵入至辉长岩体中;(b)贫矿正长岩脉呈平行状侵入辉长岩体中Fig.3 Outcrop pictures showing the syenitic dikes (mineralized and barren syenitic dikes)intruding the gabbroic intrusion in sharp contact
3.2 分析结果
本文测得的炉库和白草矿区富矿正长岩脉和贫矿正长岩脉及相关正长岩体的Sr 和Nd 同位素分析结果见表1。可以看出,总体上,两矿区岩石的εNd(t)非常均一,正长岩体εNd(t)= -0.3 ~+0.4,贫矿正长岩脉的εNd(t)= -0.3 ~+0.7,富矿正长岩脉的εNd(t)= -0.2 ~+0.2。富矿正长岩脉和贫矿正长岩脉比正长岩体具有更高的143Nd/144Nd 初始比值(岩脉的143Nd/144Nd 初始比值0.512263 ~0.512316,多数大于0.512300,正长岩体的143Nd/144Nd 初始比值为0.512290 ~0.512326,多数小于0.512300)。fSm/Nd值均为较大的负值,变化于-0.52 ~-0.18 之间。两矿区岩石初始86Sr/87Sr 值同位素表现出宽泛的范围。正长岩体和贫矿正长岩脉的(86Sr/87Sr)i分别变化于0.7032 ~0.7090 和0.7044~0.7064,富矿岩脉的(86Sr/87Sr)i变化于0.7049 ~0.7091。在εNd(t)vs. (86Sr/87Sr)i图解上(图5),两矿区岩石同位素数据几乎水平分布,且初始86Sr/87Sr 比值偏离地幔演化线。
表1 攀西地区炉库和白草铌钽矿区含矿岩脉、无矿岩脉及正长岩体Sr-Nd 同位素组成(LK-炉库矿区;BC-白草矿区)Table 1 Whole-rock Sr-Nd isotopes of the mineralized syenitic dikes,barren senitic dikes and syenitic plutons from Luku and Baicao deposits in the Panxi district
图4 攀西地区炉库和白草矿区富矿正长岩脉和贫矿正长岩脉矿物显微照片(a、b)富矿正长岩脉,伟晶-粗粒结构,主要矿物为钾长石、霓石、钠闪石及矿石矿物烧绿石;(c、d)贫矿正长岩脉Fig.4 Photomicrographs of minerals from the mineralized (a,b)and barren syenitic dikes (c,d)in the Panxi district The mineralized syenitic dikes have pegmatic-coarse grains. Major minerals include K-feldspar,albite,aegirine and pyrochlore
图5 攀西地区炉库和白草铌钽矿区富矿正长岩脉、贫矿正长岩脉及正长岩体Sr-Nd 同位素组成地幔演化线来自于Zindler and Hart (1986);OIB 数据来自Sun and McDonough (1989);扬子中/上地壳和下地壳数据来自Chen and Jahn (1998). 峨眉山玄武岩和镁铁质侵入体来自于Xu et al.(2001),Zhong et al. (2003,2004),Xiao et al. (2004)(t =260Ma). 数字表示地壳和地幔物质混染百分比. Northern Vietnam苦橄岩(母岩浆)的计算参数Nd (×10 -6),εNd(t),Sr (×10 -6)和(87Sr/86Sr)i分别为4.4,+7,102 和0.704;扬子中/上地壳两端元组分分别为20,-22,220,0.715 和20,-10,220,0.715Fig.5 The (87 Sr/86 Sr)i vs. εNd (t)of the mineralized,barren syenitic dikes and syenitic plutons in the Panxi districtMantle array are after Zindler and Hart (1986). Date sources:OIB from Sun and McDonough (1989),the Yangtze middle/upper and lower crust from Chen and Jahn (1998). Emeishan basalts and mafic intrusions from Xu et al. (2001),Zhong et al. (2003,2004),Xiao et al. (2004). The numbers indicate the percentages of participation of the crustal materials. The calculated parameters of Nd (×10 -6),εNd(t),Sr (×10 -6)and (87 Sr/86 Sr)i are 4.4,+7,102 and 0.704 from picrites in Northern Vietnam as parental magmas;20,-22,220,0.715 and 20,-10,220,0.715 as two components of the Yangtze middle/upper crust
4 讨论与结论
4.1 物质来源
由于地壳岩石比较富集轻稀土,Sm/Nd 值低于球粒陨石均一储库值,其εNd(t)<0,亏损地幔富集重稀土,Sm/Nd 值高于球粒陨石均一储库值,其εNd(t)>0。因此,如果某一火成岩体的εNd(t)<0,表明他们来源于地壳物质,或至少在他们形成的过程中与地壳物质发生过相当明显的混染。混染程度越明显,岩石的εNd(t)值越为负值。相反,如果火成岩的εNd(t)>0,表明他们来源于幔源物质。所以岩石的钕同位素组成可以用来推断其物质来源。炉库和白草矿区富矿正长岩脉和贫矿正长岩脉及相关正长岩体具有相似的且均一的钕同位素组成,其中两矿区富矿正长岩脉εNd(t)值为-0.2 ~+0.2,贫矿正长岩脉εNd(t)值为-0.3 ~+0.7,正长岩体εNd(t)值为-0.3 ~+0.4,各类岩石的Sr 同位素则表现出较大的变化范围,富矿正长岩脉的(86Sr/87Sr)i变化于0.7049 ~0.7091,贫矿正长岩脉和正长岩体的(86Sr/87Sr)i分别变化于0.7044 ~0.7064 和0.7032 ~0.7090,但是绝大多数岩石样品的(86Sr/87Sr)i<0.706,表明样品即含有地幔来源组分,也含有地壳岩石组分,其物质来源可能具有壳-幔混源的特点。一些样品具有较高的(86Sr/87Sr)i值,也显示出其可能遭受到地壳混染。如果岩石样品遭受地壳混染,那么岩石样品不仅区域上Sr 同位素组成(86Sr/87Sr)i会升高,Nd 同位素组成也会发生相应的变化。事实上,在研究区无论是富矿正长岩脉还是贫矿正长岩脉及正长岩体的εNd(t)基本一致,表明其来自于Sm/Nd 值比较均一的源区。因此本区富矿正长岩脉较高的(86Sr/87Sr)i值可能是由于岩石遭受风化所致。
区域上,本文两矿区的富矿正长岩脉、贫矿正长岩脉及相关正长岩体与攀西地区红格镁铁质/超镁铁质层状侵入体一致(εNd(t)= - 2.7 ~ + 1.0,(86Sr/87Sr)i= 0.7058 ~0.7064,Zhong et al.,2003),且在(86Sr/87Sr)i-εNd(t)图上(图5),样品数据点多数落于峨眉山玄武岩和镁铁质侵入体Sr-Nd 同位素组成范围内。王汾连等(2013)认为本区正长岩脉和岩体中锆石εHf值多为0.1 ~9.5,仅在炉库正长岩体中测得一粒锆石的εHf(t)(t =255.6Ma)为-6 和白草矿区富矿正长岩脉中一粒锆石的εHf(t)最低值为-0.2,说明该区正长岩脉和岩体的源区以幔源为主,且与峨眉山玄武岩及镁铁质侵入体来自于相同的地幔源区。两矿区正长岩脉和正长岩体的La/Nb 值多数小于1 亦说明受壳源混染的程度较低,所以该区正长岩脉为幔源岩石,可能有极少量地壳物质加入。
4.2 岩石成因机制
炉库和白草矿区富矿正长岩脉和正长岩体为过碱性的正长岩,贫矿正长岩脉为准铝质正长岩(Wang et al.,2014)。前人对攀西地区广泛发育的长英质岩体(正长岩和花岗岩)做了大量详细的研究工作。Shellnutt and Zhou(2007)、Shellnutt et al.(2008,2009b)认为攀西地区过碱性花岗岩是峨眉山玄武质岩浆结晶分异的产物,而准铝质花岗岩为底侵的辉长质岩体再次熔融的产物。炉库和白草矿区的富矿正长岩脉及相关正长岩体为准铝-过碱性的岩石,其形成是否源自幔源的结晶分异,亦或是底侵的辉长质岩体再次熔融的产物?或是还有其他的机制导致正长岩脉的形成?
表2 辉长岩体稀土元素部分熔融模拟计算结果Table 2 Partition coefficients and presumed source for rare earth element modeling
本区两个矿床的贫矿正长岩脉具有明显的Sr 正异常(Wang et al.,2014),这表明它不可能是峨眉山玄武岩结晶分异的产物。因为峨眉山玄武岩一般具有明显的Sr 负异常(Xu et al.,2001;Xiao et al.,2003,2004),如果峨眉山玄武岩再经过大量斜长石的结晶分异其所形成的长英质岩体就会更缺失Sr 元素。前已叙述,富矿正长岩脉、贫矿正长岩脉和相关正长岩体在时空上紧密联系,且岩石学、元素地球化学和Sr-Nd 同位素特征的相似性都表明他们来自于相同的源区,所以炉库和白草两矿区富矿正长岩脉同样不可能是由峨眉山玄武质岩浆结晶分异形成的。贫矿正长岩脉正的Sr异常暗示其源区应该是富斜长石的,地震成像研究揭示在峨眉山大火成岩省内带的下地壳有一个P 波高速带(7.1 ~7.8km/s,刘建华等,2000),前人解释为可能是随峨眉山地幔柱上升的地幔岩石在柱头处部分熔融,熔融产物玄武质岩浆再底侵于下地壳而形成(Xu et al.,2004;Xu and He,2007),对地震波速的岩石学限定,也说明底侵层由辉长岩和辉石岩组成(Zhu et al.,2003)。该底侵层的存在为本区正长岩体及岩脉的形成提供了前提条件。那么本区富矿正长岩脉及相关正长岩体是否为地幔柱岩浆活动造成的底侵在下地壳底部的辉长质岩石再次部分熔融形成的产物?
图6 辉长岩体部分熔融结果无矿正长岩脉稀土数据来自Wang et al. (2014)Fig.6 REE modeling of partial melting of gabbros Parameters are from Table 2The REE data of barren syenitic dikes are from Wang et al. (2014)
攀西地区猫猫沟正长岩体与两矿区贫矿正长岩脉形成时代相一致,且地球化学特征及岩相学特征相似。罗震宇等(2006)设定该区具有堆晶特征的辉长岩(LJ-12,矿物组成为斜长石65%,辉石18%,磁铁矿12%,角闪石5%)为源岩,利用主量元素的最小方差质量平衡计算结果认为该辉长质堆晶岩低程度部分熔融(~5%)可形成猫猫沟霞石正长岩体。那么,本区与铌钽有关的正长岩脉是否也是由该辉长质堆晶熔融形成的呢?我们选择了同样的源岩利用稀土元素进行部分熔融模拟计算发现其不能形成具有本区正长岩脉稀土元素特征的岩石。但是攀西地区同样具有较高含量斜长石(40%)的辉长岩(LJ-7,其矿物组成为辉石25%,角闪石17%,磁铁矿17%和副矿物;Zhou et al.,2005)经过低程度部分熔融则可以形成本区贫矿正长岩脉(图6)。该模拟计算所用的分配系数及熔融组分见表2。由于该区贫矿正长岩脉明显富Sr 和Eu 异常,表明源区岩石斜长石残留相较少,我们认为辉长岩部分熔融后残留相为50%角闪石,30%辉石和20%斜长石。因此本区贫矿正长岩脉可以由攀西地区底侵的辉长质岩石经过低程度部分熔融(5% ~10%)而形成。前已叙述,本区富矿正长岩脉和贫矿正长岩脉具有相同的物质来源,其不同于贫矿正长岩脉的负Eu 异常是由于岩浆高度分异演化的结果(Wang et al.,2014),但其原始的物质来源仍旧是底侵的辉长质岩体低程度部分熔融形成的岩浆。
综上所述,攀西地区赋存铌钽等稀有元素的正长岩脉与峨眉山大火成岩省具有成因联系,~260Ma 大火成岩省岩浆活动造成本区铌钽矿脉的发育,这为该地区铌钽矿床的勘查起到一定的指导意义。
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