峨眉地幔柱轴部的榴辉岩_地幔岩源区_主元素_痕量元素及Sr_Nd_Pb同位素证据

注:本文为国家重点基础研究规划973项目“印度—亚洲大陆主碰撞带成矿作用”(编号2002CB 412610)资助的成果。

收稿日期:2004207219;改回日期:2004212224;责任编辑:郝梓国。作者简介:侯增谦,男,1961年生。研究员,主要从事岩石学、造山带成矿作用研究工作。通讯地址:100037,北京西城区百万庄大街26号,中

国地质科学院矿产资源研究所;Em ail :hzq @cags .net .cn 。

峨眉地幔柱轴部的榴辉岩-地幔岩源区:

主元素、痕量元素及Sr 、Nd 、Pb 同位素证据

侯增谦,卢记仁,林盛中

中国地质科学院矿产资源研究所,北京,100037

内容提要:峨眉山大陆溢流玄武岩(ECFB )的西南部以丽江、大理和攀枝花三角区为中心的苦橄岩分布区,面积约5×104km 2,为峨眉地幔柱的轴部区。Sr 、N d 、Pb 同位素和痕量元素研究表明,大部分火山岩样品落在洋岛火山岩成分范围内,并存在类似FO Z O 、H I M U 和E M 2 的三个端元。这说明它们是在地幔柱轴部,由地幔岩和榴

辉岩(古玄武质洋壳)组成的源区产生的岩浆形成的。岩浆源区再循环玄武质洋壳的存在可能是该区超大型钒钛磁铁矿床形成的根本原因。少部分分布在洋岛火山岩成分范围之外的样品,一部分属于地幔柱岩浆与地壳混染产物,另一部分低T i 岩石可能与岩石圈反应有关。地幔端元的地球化学特征如下:FO Z O 端元以白林山苦橄玄武岩(YB 2

01)为代表,低87Sr 86Sr (0.7036),高143N d 144N d (0.5127),中等206Pb 207

Pb (18.5693);N b U =36.67,T h N b =

01082,L a N b =0.91,Zr N b =6.23。H I M U 端元以丽江苦橄岩(JL 229)为代表,高206Pb 204Pb (20.6412)和207Pb 204Pb (15.7489),低87Sr 86

Sr (0.7048)。E M 2 端元包括两部分:①以二滩苦橄岩2玄武岩(R 21、

3、5、8)为代表,高87

Sr 86

Sr (0.7073),低143N d 144N d (0.5123),低206Pb 204Pb (17.9968)和208Pb 204Pb (37.9450);N b U =27,T h N b =

118,L a N b =1.25,Sm Yb =5.4~6.3,Zr Y =11~13,N b Y =1.7

~2.1。②以宾川苦橄岩2玄武岩(BH 28、10、11)为代表,同位素成分与E M 相似,87Sr 86Sr =0.7052,143N d 144N d =0.5124,208Pb 3 206Pb 3=1.02;L R EE 和T h 丰

度较高,H FSE (N b 、T a 、Zr 、H f 、T i )呈负异常;N b U 低,仅为22.53,T h N b =0.24,L a N b =1.62,R b Sr =01025,Sm Yb =3.4~5.2,Zr Y =4.4~6.2,N b Y =1.5

~1.8。后一部分可能受到岩石圈反应的影响。关键词:大陆溢流玄武岩;峨眉地幔柱;地幔柱轴部区;地幔端元

大洋玄武岩,特别是洋岛玄武岩(O I B )研究已经确定了一些与地幔柱有关的地幔地球化学端元。它们包括亏损地幔端元(DMM ),富集地幔端元

(E M ,E M ),高238U 204

Pb 地幔端元(H I M U )以及Focu s Zone 端元(FO Z O ),Comm on 端元(C )和P ri m itive helium m an tle (PH E M )端元等(Zindler et al

.,1986;H art ,1988;H art et al .,1992;Farley et al

.,1992;H anan et al .,1996)。最近的研究,特别是R e 2O s 同位素研究使早期提出的关于H I M U 与E M 端元的再循环洋壳及相伴沉积物成因假说获得广泛赞同(Hofm ann et al .,1982;W eaver ,1991;Chauvel et al .,1992;H au ri et al .,1993;

R eiberg et al .,1993;Roy 2B ar m an et al

.,1995;Hofm ann ,1997)。但是,关于大陆溢流玄武岩(CFB )成因的争论仍然很激烈,因为它涉及大陆岩石圈地幔与地壳问题。部分研究者主张CFB 直接源于岩石圈地幔(Carlson ,1984,1991;

H aw kes w o rth et al

.,1984,1988;H ergt et al .,1991;Gallagher et al .,1992;L igh tfoo t et al .,

1993;T u rner et al .,1995;T u rner et al .,1996);

另一些作者指出这种方案难于克服热源和岩浆产生速率等困难(M enzies ,1992;A rndt et al .,1993;

Coffin et al

.,1994;W h ite et al .,1995;Cam p bell ,1998)。目前,多数人主张CFB 为地幔柱成因,源于热边界层(2900km 或660km ),并与O I B 同源。但是,如何评价地壳混染和岩石圈反应的影响仍然是个难题。

大陆溢流玄武岩省的苦橄岩2玄武岩组合被视为地幔柱轴部物质的熔融产物,可以和O I B 类比(Cam pbell et al .,1990)。不同的是,前者是地幔柱头部产物,后者为尾部产物,分别属于地幔柱岩浆活动的前后两个阶段。地幔柱头部熔融时,从地幔柱轴部产生的高温岩浆很可能包含有O I B 中已经确定的各种地幔端元及 或其混合产物。前人的研究表

第79卷 第2期

2005年4月 地 质 学 报 A CTA GEOLO G I CA S I N I CA V o l .79 N o.2

A p r. 2005

明,CFB的化学和同位素特征虽然变化较大,但某些CFB与洋内的热点玄武岩极为相似,富集端元,特别是E M1端元相当普遍(Carlson,1991)。CFB 中的H I M U虽然很少见到,但是随着研究工作的进展,H I M U或类H I M U亦不断有所报导(Sch illing et al.,1992;M arty et al.,1993;Stew art et al., 1996;F ranz et al.,1999;Rogers et al.,2000)。

峨眉山溢流玄武岩省西部是重要的苦橄岩分布区。20世纪80年代曾陆续对该区的苦橄岩2玄武岩组合进行过主元素,痕量元素分析(林建英,1981;张云湘等,1988),90年代以来从地幔柱角度研究的文章渐多(Chung et al.,1995;卢记仁,1996; Chung et al.,1998;宋谢炎等,1998,1999, 2001a,b;侯增谦等,1999;张诚江等,1999;徐义刚等,2001;肖龙等,2003;张招崇等,2004),但尚缺少比较系统的同位素和元素地球化学研究。为了进一步查明这些岩石的地球化学特征,阐明岩浆源区性质及其与地幔柱的成因关系,对该区的苦橄岩2玄武岩作了系统的主元素,痕量元素和Sr、N d、Pb 同位素分析。结果表明,这些熔岩包含有类似FO Z O,H I M U和E M 2 等地球化学端元,很可能由地幔柱内的榴辉岩与地幔岩熔融而成。

1 地质背景

峨眉山溢流玄武岩面积约为5×105km2,位于华南地区扬子板块的西南部。扬子板块的西面是典型的特提斯造山带,两者以金沙江缝合线为界。主要火山岩系覆盖在中二叠统茅口灰岩顶部剥蚀面之上,并被上二叠统龙潭组或宣威组含煤建造覆盖,推测主喷发期约为259±2M a(卢记仁,1996;Cou rtillo t et al.,1999;宋谢炎等,2001)。这次大规模火山喷发之前的岩石圈隆升和喷发之后的板块边缘裂解都很明显,三者在时间、空间和成因上有着密切关系(张祖圻,1983;黄汲清等,1987;张云湘等,1988;罗志立等,1988;冯少南,1991;梁定益等,1994)。这种关系被解释为地幔柱活动产生的结果(卢记仁,1996; Chung et al.,1998;宋谢炎等,1998)。

该火山岩系西厚东薄,两地的岩石类型也有较大差别。在康滇隆起带以西,大致以丽江、大理和攀枝花之间的地区为中心有火山相和次火山相苦橄岩出露,后者分布范围更大,面积估计达5×104km2,约占整个火成岩省面积的1 10。在苦橄岩分布区内,玄武岩平均厚度超过2000m,其中宾川上仓剖面的最大厚度达5384m,是整个火成岩省已知的最大厚度(张云湘等,1988)。该地火山岩系底部岩层夹灰岩透镜体,其中所含海相化石说明属茅口晚期喷发产物,喷发时间相对较早。康滇隆起带及其以东广大地区的玄武岩主要产于茅口灰岩顶部侵蚀面之上,以拉斑玄武岩为主。攀枝花—西昌地区产出同时期的超大型钒钛磁铁矿床。

苦橄岩产于两种层位,一种位于层序底部如丽江和攀枝花市附近;另一种位于层序中部,如宾川地区。它们通常在玄武岩中呈互层或夹层产出,含少量橄榄石斑晶。玄武岩类岩石包括拉斑系列,弱碱性系列和碱性系列。次火山相苦橄岩主要呈岩脉或小岩体产出,其中某些苦橄岩或苦橄玄武岩型岩体有Cu、N i、P t矿化。本次所采样品的产地和岩石类型在图1和表1中说明。

2 分析方法

主元素和痕量元素成分由中国地质科学院国家地质实验测试中心测定。主元素样品用偏硼酸锂碱熔,等离子光谱(I CP2A ES,型号JA21160)测定。稀土元素样品用N a2O2熔融,经分离富集后用I CP2 A ES测定,其中低含量样品则用等离子质谱(I CP2 M S)测定。N b、T a、Zr、H f样品经N a2O2熔融后,水提酸化,用I CP2M S测定。其他痕量元素样品经四酸(HC l、HNO3、H F、HC l O4)溶解后用I CP2A ES或I CP2M S测定。

Sr、N d同位素成分由中国科学院地质研究所同位素实验室测定。粉末岩样用H F+HC l O4在T eflon容器中低温溶解,采用A G50W×8(H+)阳离子交换柱和P5O7萃淋树脂分离出纯净的R b、Sr 和Sm、N d,并在V G354固体同位素质谱计上测定。Sr同位素质量分馏用86Sr 88Sr=0.1194校正,R b2 Sr全流程空白本底约为2×10-10~5×10-10g。N d 同位素质量分馏用146N d 144N d=0.7219校正,全流程空白本底约为5×10-11g。

Pb同位素在中国地质科学院同位素实验室测定。粉末样品用H F+HNO3溶解,用离子交换柱分离Pb,在M A T261质谱计测定Pb同位素。仪器用NB S981校准。

3 主元素与痕量元素成分

主元素与痕量元素丰度见表1。

3.1 主元素

本区的所有样品分属4类岩石(图2),按M gO

102

第2期 侯增谦等:峨眉地幔柱轴部的榴辉岩2地幔岩源区:主元素,痕量元素及Sr、N d、Pb同位素证据

图1 峨眉山玄武岩、苦橄岩分布区与主要构造单元的关系(a)和采样位置图(b)

F ig.1 T he distributi on of Em eishan basalts and p icrites and the relati ons w ith the tectonic fram ew o rk(a)

and samp ling locati ons(b)

苦橄岩区相当于地幔柱的轴部区,可能还是三联点区。①—金沙江—哀牢山缝合线;②—甘孜—理塘缝合线;③—龙门山—锦屏山裂陷槽; YZ—扬子板块;SG—松潘—甘孜地块;ZZ—中咱地块;T TD—三江特提斯构造域。采样点:1—丽江;2—宾川;3—二滩;4—永宁—泸沽湖; 5—白林山;6—永胜;7—平川;8—米易;9—乌龙坝、普和、汉易;10—金宝山

T he p icrite field co rresponding to the axial area of the p lum e,and m aybe also a tri p le juncti on.①—J inshajiang—A ilao shan suture zone;

②—Garze—L itang suture zone;③—L ongm enshan—J inp ingshan aulacogen.YZ—Yangtze p late;SG—Songpan—Garze B lock;ZZ—zhongzan B lock;T TD—Sangjiang T ethyan T ectonic Dom ain.Samp ling locati ons:1—L ijiang;2—B inchuan;3—E rtan;4—Yongning—L uguhu;5—Bailinshan;6—Yongsheng;7—P ingchuan;8—M iyi;9—D ali induding W ulongba,Puhe and H anyi;10—J inbao shan

含量由高到低依次为橄榄岩、苦橄质岩石、玄武质岩石和粗面岩2流纹岩。橄榄岩采自金宝山含铂岩体下部岩相,粗面岩2流纹岩产于二滩玄武岩剖面上部层位,两者都是高度分异的岩石。粗面岩的标准矿物石英Q≤13.33%。

苦橄质岩石包括苦橄岩与苦橄2玄武岩,M gO 含量为27.23%~13.62%。喷出相苦橄质岩石样品以丽江(JL)和宾川(BH)两地为主,前者M gO含量较高,二滩(R)和白林山(YB)两地各有1件苦橄质岩石样品。浅成相苦橄质岩石包括乌龙坝(W K)小岩体和普和(PH)、汉易(H Y)两处的岩脉。从M gO 与Si O2、A l2O3、Fe2O3+FeO、T i O2、C r和N i的关系判断,苦橄2玄武岩应属苦橄岩类,与玄武岩类的差别比较明显。

玄武质岩石M gO≤11.20%,其中5件样品M gO<6%(5.55%~3.46%)。T i O2含量变化范围较大,约为1%~4%。按照Si O2对N a2O+K2O分类,大多数样品都属于弱碱性或拉斑玄武岩。但是永宁的3件样品(L Y206,07,08)属于富钾的强碱性的碧玄岩2碱玄岩以及钾质粗面玄武岩,其中L Y206, 07的标准矿物橄榄石O1依次为23.13%和3. 44%。二滩剖面自下而上为苦橄岩2玄武岩2粗面玄武岩(R25,29)并具有向粗面岩(R222,27,28)和流纹岩(R230)演化的趋向,但缺少中间的粗安岩类,相当于双峰式的弱碱性系列。剖面下部的玄武岩Si O2含量较高,某些样品(例如R23,11)Si O2达50%。

3.2 痕量元素丰度

本节主要叙述各类岩石中痕量元素的相对丰度和异常特征,它们的比值将在后面讨论。

3.2.1 苦橄岩2玄武岩类

根据不相容元素丰度可分为两个成分组:第 组高度不相容元素丰度比较高(图3a,b;图4a2d), (L a Yb)N=8.5~21.0;第 组高度不相容元素丰度较低(图3c,图4e),(L a Yb)N=3.6~5.4。几乎所有样品都具有不同程度的正Pb异常。

202地 质 学 报2005年

表1 峨眉山CFB省苦橄岩-玄武岩类岩石的主元素(%)与痕量元素(×10-6)成分

Table1 M ajor(%)and trace ele m en t(×10-6)co m position s of p icr itic-basaltic rocks fro m the Em e ishan CFB Prov i nce 样品号JL23JL24JL213JL217JL218JL220JL221JL223JL226JL228JL229JL231BH21BH27岩石类型P P B P P P B P B B P P B B Si O242.0941.9547.0841.9440.4640.3345.6940.2746.1045.2142.1639.9648.3045.82 T i O21.191.211.941.231.141.372.571.042.082.761.391.142.394.09 A l2O36.126.1013.026.165.788.1313.155.9414.2212.327.045.7613.4912.73 Fe2O33.083.523.904.205.172.815.203.176.805.945.504.4710.285.51 FeO7.657.347.367.166.268.247.157.733.936.616.366.633.279.63 M nO0.160.160.260.170.210.180.200.170.550.220.220.200.180.20 M gO25.5225.739.1325.1822.5721.518.1027.237.818.2920.4423.934.625.21 CaO7.286.7310.306.337.937.809.946.2210.0210.517.716.448.627.84 N a2O0.510.442.760.500.250.372.470.322.492.920.590.244.723.37 K2O0.140.140.880.120.020.031.590.180.370.660.120.030.101.47 P2O50.110.110.17<0.1<0.1<0.10.280.120.630.290.13<0.10.300.47烧失4.965.702.345.969.628.002.286.724.663.047.649.982.762.82总量98.8199.1399.1498.9599.4198.7798.6299.1199.6698.7799.3098.7899.0399.16

C s0.30.30.30.50.30.40.30.40.40.20.40.40.50.5

R b4.17.217.05.83.02.527.08.610.011.04.74.23.027.0 Sr118834978394681100545964868177313706 Ba2071566011401071251100150449341154109329570 Sc15.0320.5027.2220.8024.0324.2121.9617.6619.7226.0522.4719.0424.3225.10 V175173335172172217288154240312193153311436

C r2500220027825002600140044229003922972200230072114

Co4542464243624142454141403942 N i8557311097177124551647948611267472412975 Cu10297706857697372246879045207331 Zn6964586863607554101716562109141 Y8.1310.7816.1011.1810.2013.0618.498.7619.9721.0310.799.6728.3636.84 Zr8688126798494190781451989297127274

H f2.32.33.62.12.12.34.61.93.45.02.42.53.67.4

N b1211181210103112422614111338 T a1.61.61.51.41.10.92.81.02.71.91.30.91.02.6 U0.40.40.40.30.30.30.70.30.60.60.40.20.51.1 T h1.41.41.61.41.11.23.11.13.12.51.41.01.54.4 Pb3.34.17.7111.95.04.79.44.6182.82.19.76.9 L a10.2610.6013.3812.408.509.2820.829.9823.8924.6911.378.5215.5937.88 Ce20.3420.6426.7524.0117.4219.2341.0718.7143.5749.4822.3017.3228.1473.93 P r2.602.533.432.972.212.465.082.355.616.252.852.254.009.76 N d11.2511.7915.7713.7510.2911.9222.2410.1622.7827.6712.2810.1718.9540.14 Sm2.882.864.183.392.703.165.372.315.436.853.092.555.5410.12 Eu0.910.991.431.130.941.121.740.821.822.141.030.881.933.04 Gd2.772.974.713.152.643.245.762.555.066.523.042.646.5210.16 T b0.360.420.540.430.360.440.70<0.30.670.840.400.370.911.30

D y1.882.273.492.582.202.814.031.864.004.872.402.065.707.84

Ho0.400.470.690.490.440.550.780.360.800.900.440.421.161.60

E r0.721.121.651.291.151.521.870.942.112.361.181.023.133.95

Tm<0.10.150.210.160.150.200.240.120.270.270.150.130.370.56 Yb0.590.881.270.870.861.061.480.751.741.580.850.772.232.96 L u<0.10.140.190.130.150.170.24<0.10.260.210.11<0.10.350.49注:P—苦橄岩;P icb—苦橄玄武岩;B—玄武岩;T b—粗面玄武岩;T—粗面岩;R—流纹岩;B s—碧玄岩;Tph—碱玄岩;P r—橄榄岩。

第 成分组的丽江苦橄岩与互层的玄武岩相比,稀土元素丰度稍低,但两者的(L a Yb)N基本一致,约为8.5。宾川苦橄岩的稀土元素丰度比丽江苦橄岩的稍高,与该区玄武岩的稀土丰度相近,但(L a Yb)N较高,约为14.8(图3,a)。丽江、宾川和二滩3个地区苦橄岩的不相容元素相比,丽江的不相容元素较低(图4a),这与它的M gO较高是一致的。

丽江玄武岩具有正N b异常和弱的负P异常

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续表1 样品号BH28BH210BH211BH215BH217BH219BH220R21R23R25R28R211R215R222岩石类型P icb P B P P icb P P P B B B B B T Si O243.3843.6247.2344.1445.3443.6241.3943.8049.9848.6647.8450.2046.5666.11 T i O21.030.801.202.142.261.781.641.882.432.692.442.562.760.65

A l2O310.959.5613.528.799.178.204.656.2310.9911.9510.5812.2513.8612.62 Fe2O36.055.186.614.986.924.556.115.483.933.912.753.774.582.82 FeO5.135.444.896.795.567.075.956.377.197.318.647.388.834.62

M nO0.180.170.180.170.150.160.160.160.160.170.180.170.230.25

M gO14.6618.528.0117.3713.6218.6026.2121.668.728.2610.357.407.110.26 CaO11.739.6611.168.709.428.805.836.427.718.019.939.9710.281.50

N a2O1.011.071.191.401.341.150.530.722.053.001.062.562.685.68 K2O0.360.232.080.701.080.580.290.762.412.002.471.010.423.82

P2O50.500.310.470.310.330.240.250.200.360.320.250.290.340.10烧失4.424.661.863.683.823.966.264.942.842.362.321.560.620.48总量99.4099.2298.4099.1799.0198.7199.2798.6298.7798.6498.8199.1198.2798.91

C s1.91.70.51.91.21.31.11.90.30.20.20.20.20.7

R b159.4672222191431463560228.780 Sr6484172200371408319142281734502515566540132 Ba75945241005941200508298299619564619486377692 Sc26.3628.4430.5521.5821.0522.2018.4217.6121.1822.1026.9226.1527.054.54 V2311902402322482231602192692742902753202.3

C r94016001681100120013001800200040338370529016816

Co593444677069454142435140451.7 N i4085021135475726341100750114111160867915 Cu104743110961109394121698912115034 Zn5660621001038410110488939374106224 Y18.8017.0426.5015.6016.0015.4112.3912.8722.2721.0518.7520.7524.1378.20 Zr11675118166176135152169267235209210116759

H f2.61.92.84.34.63.43.73.76.05.64.85.02.918

N b33264229322420274136323230110 T a1.91.42.22.22.31.71.51.82.52.42.02.01.96.4 U1.61.11.80.90.90.80.70.91.51.41.20.70.34.4 T h8.26.0103.73.83.52.43.87.16.75.52.80.523 Pb118.514146.26.74.416111610185.418 L a56.2139.9168.0828.6831.3023.6920.4428.3049.5346.1238.9732.2327.17109.4 Ce94.5964.55111.252.3356.5743.8937.8953.9693.9287.2073.5062.6052.70189.4 P r10.827.3212.926.677.115.394.856.7412.0510.939.248.177.2623.02 N d37.3524.9141.0526.3628.2220.9119.7827.7945.4943.0236.4132.0929.9787.06 Sm7.664.988.616.116.374.924.836.079.929.407.987.817.4019.91 Eu2.151.482.411.831.941.461.401.752.662.612.282.302.444.67 Gd6.454.437.105.425.284.683.784.507.247.306.657.076.8715.39 T b0.710.620.970.690.740.620.490.580.970.830.740.840.882.73

D y4.463.545.703.643.873.442.963.135.194.864.354.695.2415.70

Ho0.830.691.150.710.720.660.600.560.950.860.820.931.063.22

E r2.131.853.021.731.861.791.601.512.562.322.122.282.748.12

Tm0.270.250.380.220.230.240.210.170.320.280.260.290.351.12 Yb1.471.482.301.221.211.250.970.971.801.661.481.641.996.82 L u0.250.220.310.140.150.190.150.140.270.230.220.240.280.96

(图4b),苦橄岩N b的正异常很弱或无异常,但负的Sr、P异常明显。宾川苦橄岩无N b异常,个别样品(BH220)具负Sr异常。二滩苦橄岩(R21)的负N b 异常极弱,但负P异常尚明显。

二滩玄武岩的稀土元素和强不相容元素丰度较高(图3a,图4c),(L a Yb)N=13.6(平均值),具有弱的负N b异常和清楚的负P异常,但没有负Sr异常。

图3b和图4d显示出宾川的苦橄岩2玄武岩(BH28、10、11)比较富含轻稀土(L R EE)和T h,并且

402地 质 学 报2005年

续表1 样品号R224R227R228R229R230L Y202L Y206L Y207L Y208YB201YB204LL204LL209YC204岩石类型B T T B R B B s Tph T b P icb B B B Si O247.49.66.6264.1748.6070.2147.1841.3348.6943.7745.3046.0748.5047.3648.53 T i O22.120.630.642.420.613.832.332.442.101.401.693.793.211.61

A l2O312.0113.9113.7913.9710.3412.9310.9812.4210.1510.1914.9013.4413.2614.54 Fe2O35.123.134.664.584.792.633.624.133.325.026.546.444.882.72 FeO7.462.582.807.402.8110.988.055.605.726.566.687.687.2510.13 M nO0.220.240.370.210.250.200.180.140.130.170.200.220.170.20

M gO8.330.590.476.530.125.5511.206.607.7915.386.434.246.206.56 CaO10.851.591.398.181.558.4212.729.7814.8810.2011.057.969.358.06

N a2O3.015.415.423.063.423.690.561.431.281.372.343.653.243.07 K2O0.744.264.932.123.160.462.324.823.430.110.801.221.471.32

P2O50.220.120.130.26<0.10.440.350.400.320.140.300.550.450.22烧失1.780.280.601.741.442.465.202.826.083.422.201.821.962.10总量99.3599.3699.3799.0798.7098.7798.8499.1898.9799.2699.2099.5098.8099.06

C s0.60.40.41.30.71.30.30.20.40.90.20.51.12.6

R b2592705985146352372.214334274 Sr55279486581261300169829681191328565790399 Ba2956417141400319479603643678325534473559484 Sc32.153.994.3724.861.6925.2531.8121.7128.9629.2228.0921.2523.2129.54 V2464.12.03201.8409267259250253288294306281

C r45228251552910477719269012001142533257

Co462.01.5401.1394635366046373749 N i1331714897.86718688140454652611152 Cu194223417028271111947939316819315740 Zn41168157102340127686267736014211880 Y19.9472.6736.6422.61112.935.1921.0720.7818.3116.6320.3335.7732.3322.34 Zr1397563101648842641712021567492310273117

H f3.7208.14.524674.55.04.02.22.27.16.03.0

N b18944725102353339301116464510 T a1.66.42.91.95.62.42.32.72.10.82.03.02.90.9 U0.84.91.50.95.21.10.71.20.80.30.31.61.20.9 T h2.5216.23.3254.23.23.83.00.91.27.75.63.2 Pb8.013160.19%48105.68.37.85.6111134017 L a21.69116.950.2230.53147.338.1932.7534.0427.0710.0512.4356.7549.1419.14 Ce40.90203.391.6056.13264.574.6457.1462.8951.7418.5224.57103.987.3040.61 P r5.2323.7211.447.3332.949.657.337.456.232.363.0712.5910.834.63 N d22.1885.2644.8628.14124.740.7228.9929.1424.6810.7113.9049.7843.2419.42 Sm5.5919.0710.346.7930.379.936.566.525.762.964.0711.3210.125.16 Eu1.793.983.372.086.683.131.971.941.741.061.443.313.051.58 Gd5.7216.719.416.5627.569.685.455.325.273.764.689.319.215.05 T b0.802.471.270.744.431.270.780.800.700.610.661.421.250.84

D y4.3414.647.734.9224.687.674.554.513.983.434.077.767.064.52

Ho0.863.141.620.994.951.560.920.900.760.680.851.531.400.91

E r2.367.674.062.5811.783.912.442.362.091.742.393.873.472.37

Tm0.281.090.570.341.560.470.320.300.290.240.320.510.450.33 Yb1.646.643.481.889.452.791.711.771.501.481.842.872.512.04 L u0.230.850.450.281.270.320.260.270.240.210.220.370.340.32

高场强元素(H FSE:N b,T a,Zr,H f和T i)具有明显的负异常。永宁的3件样品(L Y206、07、08)H FSE 没有异常。米易的玄武岩(M Y202)属于高度演化的熔岩,M gO含量为3.46%,亦富含L R EE和T h,并且有明显的负N b异常,但是没有Zr、H f,T i负异常。永胜玄武岩(LL204、09)的不相容元素特征与永宁的碱性玄武质岩石比较相似。

第 成分组包括火山相和次火山相,前者以白林山(YB201,04)和宾川(BH21)的苦橄2玄武岩为代表,后者有乌龙坝(W K)小岩体和普和(PH)岩脉

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续表1 样品号M Y202PH202H Y202W K202W K205W K210W K212W K215J21J23J24J27J212J215

岩石类型B P P icb P icb P P P P P r P r P r P r P r P r Si O251.1044.3844.6948.5643.5544.1842.4141.3436.9136.5037.2237.7835.5735.74 T i O22.991.021.661.161.290.980.900.980.400.390.370.370.400.50

A l2O315.109.228.859.609.349.498.018.002.522.632.392.682.593.25 Fe2O34.993.906.392.844.674.064.242.647.377.787.036.659.138.66 FeO6.446.795.765.886.786.536.637.595.365.314.424.764.745.66

M nO0.190.170.150.150.190.160.160.160.140.130.140.170.120.19

M gO3.4619.4713.9314.4618.4719.1023.6321.7434.5034.7734.8834.7832.7232.85 CaO8.158.6510.009.709.118.327.037.980.660.600.920.651.491.32

N a2O3.801.081.231.521.271.140.870.160.040.040.040.040.110.10 K2O0.560.500.500.170.310.220.200.030.170.180.130.130.140.18

P2O50.440.150.200.180.170.180.110.17<0.1<0.1<0.1<0.1<0.1<0.1烧失1.503.825.864.243.424.204.968.1610.5410.3611.1810.5811.229.82总量98.7299.1599.2298.4698.5798.5699.1598.9598.6198.6998.7298.5998.2398.27

C s0.25.36.31.73.13.93.71.12.01.11.91.22.10.6

R b1424165.11617222.77.47.06.16.06.78.7 Sr7452483131131701649423141422133035 Ba464341383158249298151122941149096108115 Sc19.0122.6726.0622.7323.7923.6219.5716.888.467.216.707.728.124.92 V234185226200197181219250101978093114118

C r481500150020001600150023002100580062005500660075007800

Co2664605362614538726347677777 N i13529614520561574857704925948832995900966 Cu408891112120891091033115125172217 Zn10765793862725752636459897497 Y31.5315.0815.2314.9515.8614.4512.2111.443.563.723.303.813.245.53 Zr3177810279106745964484347464452

H f7.52.02.52.02.81.81.51.71.21.01.21.11.11.3

N b528.8155.9105.64.64.95.25.34.84.95.86.9 T a3.31.11.40.50.80.60.50.51.90.60.60.60.51.0 U2.30.60.50.30.40.50.30.50.30.30.30.20.30.4 T h121.91.60.81.40.70.90.91.11.11.00.90.91.5 Pb209.28.64.65.94.83.52.55.9118.99.5128.7 L a69.209.8214.935.7610.775.175.005.506.285.645.984.644.718.82 Ce123.818.0829.9711.5520.7310.919.6510.8710.199.5010.828.688.7615.95 P r14.732.423.841.632.541.461.421.401.251.131.191.040.991.83 N d54.069.7416.507.4811.917.066.617.664.204.534.384.614.156.83 Sm11.322.764.202.653.412.552.382.271.011.131.181.130.991.58 Eu3.010.951.371.011.160.970.820.860.270.250.430.270.360.53 Gd8.573.014.053.343.933.452.712.861.241.281.371.191.011.51 T b1.310.540.630.480.520.500.420.38<0.3<0.3<0.3<0.3<0.3<0.3

D y6.793.053.412.993.292.942.632.430.790.840.880.870.751.25

Ho1.350.610.640.630.660.610.530.490.160.160.150.150.150.21

E r3.461.651.691.621.861.761.421.210.470.500.420.480.480.62

Tm0.450.220.230.210.240.230.180.14<0.1<0.1<0.1<0.1<0.1<0.1 Yb2.731.381.251.311.341.311.090.910.370.390.320.400.330.51 L u0.370.210.200.210.190.200.150.13<0.1<0.1<0.1<0.1<0.1<0.1

(图3c)。前人曾经用中子活化分析对宾川玄武岩测得类似BH21的数据(张云湘等,1988),这组曲线是有代表性的。它们与前一组曲线相交切,可能反映了两种深度,两种压力状态下的部分熔融产物,前一组曲线代表了石榴子石稳定区,后一组曲线则代表尖晶石稳定区。这意味着岩浆产生过程伴随着岩石圈减薄作用。

3.2.2 橄榄岩和粗面岩2流纹岩类

金宝山岩体的橄榄岩稀土元素和不相容元素丰度最低,具有负N b异常和明显的负Sr异常,其中

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图2 峨眉山溢流玄武岩某些主元素氧化物及C r、N i对M gO关系图

F ig.2 M gO content p lo tted against abundances of som e m aj o r elem ent oxides,C r and N i in the ECFB

表示4类岩石的成分特征和相互关系。JL—丽江;BH—宾川;R—二滩;L Y—永宁2泸沽湖;YB—白林山;LL—永胜;YC—平川;M Y—米易; W K—乌龙坝;J—金宝山;PH—普和;H Y—汉易

Show ing the compo siti on and the relati ons of four types of rock.JL—L ijiang;BH—B inchuan;R—E rtan;L Y—Yongning2L uguhu;YB—Bailinshan;LL—Yongsheng;YC—P ingchuan;M Y—M iyi;W K—W ulongba;J—J inbao shan;PH—Puhe;H Y—H anyi

3件样品显负Eu异常(图3d,图4f)。二滩粗面岩和流纹岩稀元素和不相容元素丰度最高,而且具有强烈的Sr、P和T i负异常,其中3件样品具负Eu异常。但是样品R228无N b异常,不相容元素丰度也较低,与宾川高T i玄武岩(BH27,T i O2=4.019%)以及泸沽湖玄武岩(L Y202)几乎完全一致。

以上的痕量元素丰度曲线表明,N b的丰度可从弱正异常变为明显的负异常,其他一些元素如Zr、H f、T i、P、Sr等的负异常不同程度地出现在某些样品中。

3.3 蚀变作用对某些元素的影响

岩浆期后蚀变作用有可能使某些活动性元素

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图3 峨眉山溢流玄武岩球粒陨石标准化稀土元素丰度曲线图

F ig.3 Chondrite2no r m alized rare2earth2elem ent patterns of the ECFB

(a)—表示丽江苦橄岩(P I C2JL,8件),宾川与二滩的苦橄岩(P I C2BH,5件)和二滩玄武岩(B2R,7件)的R EE曲线范围以及丽江的玄武岩曲线(4件);(b)—高(L a Yb)N R EE曲线,包括:(1)苦橄岩2玄武岩(BH28、10、11);(2)钾质碱性玄武岩(L Y206、07、08);(3)进化型玄武岩(LL204、09,M Y202);(c)—低(L a Yb)N R EE曲线,包括浅成相苦橄岩和喷出相苦橄玄武岩2玄武岩;(d)—橄榄岩、粗面岩2流纹岩的R EE 曲线与玄武岩R EE曲线比较。标准化值据(M c Donough et al.,1995)

(a)—T he range is fo r the L ijiang p icrites(P I C2JL,8samp les),B inchuan and E rtan p icrites(P I C2BH,5samp les),and E rtan basalts(B2 R,7samp les);(b)—R EE patterns of h igh(L a Yb)N rock s,including p icrite2basalt(BH28,10and11),po tassic alkali basalt(L Y206,07 and08)and evo luted basalt(LL204and09,M Y202);(c)—R EE patterns of low(L a Yb)N rock s,including hypabyssal p icrite and erup tive p icritic basalt2basalt;(d)—Comparison of R EE patterns fo r perido tite,trachyte2rhyo lite and basalt.Chondrite values after(M c Donough et al.,1995)

(例如:R b、Sr、B a、U、Pb)的丰度发生变化。为了评价这种变化,本文采用Yang等(1998)的方法,利用N b的丰度对某些活动性元素的丰度作图(图5),以便滤去某些明显偏离正相关趋势的数据,尽量减少蚀变作用的影响。

正如图5a至5c所示,R b与N b的相关性较差,没有明显的线性趋势,Sr、B a与N b相关性较好,但高度演化的粗面岩、流纹岩(R222、27、28、30)和其他少数样品例外。由于斜长石分离结晶的影响,粗面岩和流纹岩的Sr、B a含量明显偏低。其他少数样品对相关趋势的偏离可能与蚀变有关。例如BH211,L Y202和JL2 21三件玄武岩的Sr明显偏高,而L Y206的Sr偏低。与此类似,BH211、17,JL213,21和R229的B a偏高,而JL226、LL204和M Y202的B a稍低。

U、Pb与N b的含量也是大致呈正相关的,其可能与区域性特征有关。

4 Sr、N d、Pb同位素

Sr、N d、Pb同位素结果列于表2,并在图6,7中说明。

87Sr 86Sr值的变化范围较大,一般从0.70362到0.70747。碧玄岩(L Y206)的87Sr 86Sr值高达0.70928,但是,它的N d和Pb同位素比值与同一地点产出的碱玄岩(L Y207)非常接近,据此推断,该比值可能是海水蚀变所致。另外,两件粗面岩的87Sr 86 Sr值很高(0.71098和0.71785),接近地壳值,图上未标出。

143N d 144N d值的变化范围从0.51277到0.51231。Sr,N d同位素排列比较规则,与Kerguelen 的排列很相似,高87Sr 86Sr端介于E M 与E M I1之

802地 质 学 报2005年

图4 峨眉山溢流玄武岩原始地幔标准化痕量元素丰度曲线图

F ig.4 P ri m itive m antle2no r m alized incompatible2elem ent patterns of the ECFB

(a)—丽江苦橄岩(P I C2JL,8件)与宾川、二滩苦橄岩痕量元素丰度比较;(b)—丽江玄武岩,具有正N b异常;(c)—二滩玄武岩,具有负的N b、P异常;(d)—岩石类型同图3b,其中BH28、10、11的H FSE为负异常;(e)—与图3c相似,但浅成相岩石(W K,PH)具有负N b异常;

(f)—表示金宝山橄榄岩(PER2J)与二滩粗面岩2流纹岩的痕量元素丰度特征,强烈的Sr,P和T i负异常可能是斜长石,磷灰石和Fe2T i氧化物晶出造成的。原始地幔值据Sun等(1989)

(a)—R ange fo r L ijiang p icrites(P I C2JL,8samp les)compared w ith the patterns of B inchuan and E rtan p icrites;(b)—L ijiang basalts w ith po sitive N b anom alies;(c)—E rtan basalts w ith negative N b and P anom alies;(d)—Rock types si m ilar to F ig.3b,H FSE of BH28,10and 11show ing negative anom alies;(e)—Si m ilar to F ig.3c,but hypabyssal rock s(W K,PH)show ing negative N b anom alies;(f)—Show ing the characteristics of trace elem ents fo r J inbao shan perido tite(PER2J)and E rtan trachyte2rhyo lite.Significantly negative Sr,P and T i anom alies are p robably due to crystallizati on of p lagi oclase,apatite and Fe2T i oxides.P ri m itive m antle value after Sun et al.(1989)

间。有2件样品(BH210,11)的Sr,N d同位素比值非常接近E M1(表2和图6a),相应的平均值为0170513和0151241。

206Pb 204Pb值变化范围较大,从1719598~2016412,其中二滩的3件样品(R21,5,28)比值最低,丽江的JL229、3比值最高(图6b,c)。207Pb 204Pb 值都位于北半球参考线(N HRL)之上,而且大多数样品都落在N HRL与地球等时线之间,只有二滩的4件样品位于地球等时线左侧。这几件样品的208 Pb 204Pb值也是最低的,而且JL23,29该比值显著低于N HRL(图6c)。

以上资料表明可能存在3个同位素端元:①以白林山YB201,04为代表的低87Sr 86Sr,高143N d 144N d 端元;②以丽江JL229、3为代表的高206Pb 204Pb,

低87Sr 86Sr端元;③以二滩R21、5为代表的高87Sr 86Sr,低143N d 144N d,206Pb 204Pb和208Pb 204Pb端元。根据Sr2N d2Pb同位素构成的一对类双曲线图,这3个端元的同位素特征及其相互关系更加醒目(图7a,b)。

5 地幔源性质:三元混合系统

5.1 端元的成分和性质

上述Sr2N d2Pb同位素资料表明可能存在三个端元,不相容痕量元素比值也反映出这一特征。在图8中,分析值都落在由O n tong Java洋台玄武岩, H I M U型洋岛玄武岩和大陆地壳物质构成的三角形之内,而且绝大部分数值都在H aw aii苦橄岩区及其下延线左侧。不过富集端元样品可细分为两组,它们的同位素和痕量元素特征既有共同性,也有明显差异。

902

第2期 侯增谦等:峨眉地幔柱轴部的榴辉岩2地幔岩源区:主元素,痕量元素及Sr、N d、Pb同位素证据

图5 N b的丰度对某些活动性元素和T h的丰度图

F ig.5 A bundance of N b vs abundances of som e mobile elem ents and T h

在图5b,c中,粗面岩2流纹岩的Sr、Ba含量明显偏低,受斜长石早期晶出控制。它们与某些受蚀变影响的样品标明了样号。在图5f中,样品BH210、8、11和M Y202随着M gO含量降低,T h与N b含量呈线性增高,但T h N b值不变,约为0.24。图例同图2

In F ig.5b and5c,Sr and Ba contents of trachyte2rhyo lite are obvi ously low due to early crystallizati on of p lagi oclase.T hey and som e altered samp les are labeled w ith samp le num bers.In F ig.5f,T h and N b contents of samp les BH210,8,11and M Y202increase linearly w ith decreasing of M gO contents,but T h N b rati o s are relatively constant,about0.24.Sym bo ls as given in F ig.2

表2 峨眉山CFB省岩石样品的同位素比值

Table2 Isotope ratios for selected sam ples fro m

the Em e ishan CFB Prov i nce

采样地点样品

样品

种类

87Sr

86Sr

143N d

144N d

206Pb

204Pb

207Pb

204Pb

208Pb

204Pb

丽江JL23苦橄岩0.7046680.51265319.429915.700438.964 JL213玄武岩0.7048900.51268718.525315.616938.626 JL226玄武岩0.7045570.51262118.793315.592738.8329 JL229苦橄岩0.7048360.51259020.641215.748938.600

宾川BH210苦橄岩0.7051760.51239518.746315.547839.0141 BH211玄武岩0.7050780.51242618.677615.669239.0970 BH215苦橄岩0.7052500.51257618.592815.555338.6444 BH219苦橄岩0.7056290.51258018.846715.590539.010

二滩R21苦橄岩0.7071320.51233317.959815.564737.9450 R25玄武岩0.7074700.51231018.033815.574738.0774 R211玄武岩0.7060600.51249318.335115.613638.5404 R222粗面岩0.7109760.51259418.577315.510038.6925 R228粗面岩0.7178500.51258018.084115.548838.0641

永宁L Y206碧玄岩0.7092850.51246918.529015.508438.5516 L Y207碱玄岩0.7060520.51247518.474315.525438.5032

白林山YB201

苦橄

玄武岩

0.7036220.51276918.569315.568838.4480 YB204玄武岩0.7044340.51274018.518315.639038.5521

乌龙坝

W K2005苦橄岩0.7053530.51259318.365115.566138.3923

W K2010苦橄岩0.7055120.51263318.765515.578238.4515

5.1.1 低87Sr 86Sr端元

如前所述,此端元以白林山YB201为代表,

87Sr 86Sr=0.70362,143N d 144N d=0.51277(当t=

259M a时,ΕN d(t)=+3.9),206Pb 204Pb=

18.5693,207Pb 204Pb=15.5688,208Pb 204Pb=

38.4480。这些比值与FO Z O的相应比值一致(H art

et al.,1992;H au ri et al.,1994),也与O n tong

Java洋台玄武岩的相应比值一致(M ahoney et al.,

1991;M ahoney et al.,1993;Cam pbell,1998)。

Cam p bell认为后者与FO Z O一样,代表下地幔成

分。

在图8中,O n tong Java Zr N b和L a N b的平

均值分别为17.65和1.03,比原始地幔值(15.96和

0.98)略为亏损。由于O n tong Java熔岩的熔融程度

高达20%~30%(N eal et al.,1997),这些比值受

分馏作用的影响可能很小,大致可代表幔源岩石的比

值。与它们相比,YB201的Zr N b和L a N b值(6.73

和0.91)较低,推测是分馏作用所致。前一比值与社会

(Society)岛低87Sr 86Sr端元的Zr N b值(约7)

相近,该端元亦被视为FO Z O C端元(W h iteetal., 012地 质 学 报2005年

图6 峨眉山溢流玄武岩Sr 、N d 和Pb 同位素变化图

F ig .6 V ariati on diagram s fo r Sr ,N d and

Pb iso topes of the ECFB

符号同图2。地幔端元(据Zindler et al .,1986;H art ,1988;H art et

al

.,1992;Farley et al .,1992;H auri et al .,1994;H anan et al .,1996);St H elena 的平均值(据Sun et al .,1989)。洋岛成分区的资

料来源:Kerguelen (据W eis et al .,1993;Yang et al .,1998);H eard

(据Barling et al .,1994);Society (据W h ite et al ,1996);P itcairn (据W oodhead et al

.,1993);M angaia (据W oodhead ,1996)。PM —原始地幔成分(M c Donough et al .,1995);CC —大陆地壳成分(R udnick

et al

.,1995);H d —H eard 岛趋势线(F rey et al .,2000);N HRL —北半球参考线(H art ,1984);Geoch ron =4.55Ga 地球等时线

Sym bo ls as in F ig .2.D ata sources as fo llow s :M antle end 2m em ber compo siti ons (after Zindler et al

.,1986;H art ,1988;H art et al .,1992;Farley et al .,1992;H auri et al .,1994;H anan et al .,1996);St H elena (after Sun et al .,1989);Kerguelen (after W eis et al

.,1993;Yang et al .,1998);H eard (after Barling et al .,1994);Society (after W h ite et al ,1996);P itcairn (after W oodhead et al .,1993);M angaia (after W oodhead ,1996).PM —P ri m itive M antle

(M c Donough et al .,

1995);

CC —Continental C rust

(R udnick et al .,1995);H d —H eard Island trend line (F rey et al .,2000);N HRL —N o rthern H em isphere R eference L ine (H art,1984);Geoch ron =4.55Ga isoch

ron

图7 峨眉山溢流玄武岩206Pb 204Pb 2143N d 144

N d (a )

和206Pb 204Pb 287Sr 86

Sr (b )图

F ig .7 D iagram s of

206

Pb 204Pb vs .143N d 144

N d (a )and vs .87Sr 86

Sr (b )of the ECFB

样品及符号同图6。注意,L Y 206、07的N d 和Pb 同位素很接近,但

L Y 206的Sr 同位素显著偏高,可能因海水蚀变所致,图中未标出Sym bo ls as in F ig .6.N o te :N d and Pb iso topes of samp les L Y 206and 07are very clo se ,but Sr iso tope of L Y 206is significantly h igher ,p robably due to seaw ater alterati on ,and it is no t p lo tted

1996)。

YB 201的T h N b 和N b U 值(0.082和36.

67)与O n tong Java 的平均值(0.082和35.15)基本

1

12第2期 侯增谦等:峨眉地幔柱轴部的榴辉岩2地幔岩源区:主元素,痕量元素及Sr 、N d 、Pb 同位素证据

一致,其中T h N b值是最低的(图9a)。在图9b中可见,它的N d Sm值很低,R b Sr值(0.012)也与社会岛和O n tong Java的相应比值(~0.02和0. 069)近似。这些痕量元素比值排除了该端元由亏损地幔源(DMM)与陆壳物质混合而成的可能性。

因此,这一端元无论Sr2N d2Pb同位素还是痕量元素比值都与O n tong Java洋台玄武岩及社会岛低87Sr 86Sr端元相当,应属于FO Z O端元,代表下地幔弱亏损橄榄岩部分熔融产物。

5.1.2 高206Pb 204Pb端元

到目前为止在本区,这一端元仅见于丽江苦橄岩中,以JL229为代表。它的206Pb 204Pb和207Pb 204 Pb值较高,分别为20.6412和15.7489;87Sr 86Sr值较低,为0.70484(图6b,7b)。

这些特征说明它属于H I M U型苦橄岩。在大陆溢流玄武岩中,这种类型岩石很少出现(Carlson, 1991)。但是最近在非洲的大陆溢流玄武岩中已经鉴定出这种类型玄武岩(Stew art et al.,1996;F ranz et al.,1999;Rogers et al.,2000)。德干西部某些高206Pb 204Pb,高87Sr 86Sr玄武岩是由地壳混染而成的(Peng et al.,1994),两者以87Sr 86Sr的高低相区别。

在Zr N b对L a N b图中,JL229位于H I M U 区顶端与E M 、 交汇处,属于H I M U2FO Z O混合系列中靠近H I M U的样品。事实上丽江的8件苦橄岩和3件玄武岩都局限在该混合系列的P itcairn 与Societly重叠区及其周围,只有JL226位置较低在H I M U范围之中。丽江玄武岩N b和T h丰度明显高于苦橄岩(表1,图5f),但它们的T h N b值都很相近,平均为0.10,这也正好是JL229的T h N b 值。JL229的N b U值(35.00)与O n tong Java的平均值(35.15)一致(图9a)。它的R b Sr值(0.058)和N d Sm值(3.97)与O I B相近(图9b)。这些痕量元素比值可能反映了本区H I M U端元的特征。

5.1.3 高87Sr 86Sr端元

此端元以二滩剖面底部的苦橄岩和玄武岩为代表,它们的Sr,N d和Pb同位素比值十分相似(表2,图6、7)。R25的87Sr 86Sr值最高,为0.70747; 143N d 144N d最低为0.51231(当t=259M a时,ΕN d (t)=23.6)。R21的206Pb 204Pb和208Pb 204Pb都是最低的,分别为17.9598和37.9450。如前所述,这些特征表明,该端元介于E M 与E M 之间,而且相对富含地壳物质。

痕量元素比值以玄武岩(R23、5、8)为代表,它们的Zr N b和L a N b值基本一致,平均值分别为6.52和1.24(图8)。它们的T h N b和N b U值也很近似,平均为0.18和26.57(图5d、f,9a);R b Sr 和N d Sm平均值为0.083和4.58(图9b)。前面已经提及该端元样品稀土丰度高(图3a)并具有负N b、负P异常(图4c),连同这些比值一起明确地反映出它们相对富含地壳物质,而且似乎与Society 岛的E M 型玄武岩更为相似。

5.1.4 中等87Sr 86Sr、低143Nd 144Nd端元

宾川剖面的一组苦橄岩2玄武岩样品(BH28、10、11)具有特定的同位素和痕量元素特征。BH210、11的Sr,N d和Pb同位素比值十分相近,其中Sr, N d同位素比值非常接近E M ,平均值为0.70513和0.51241(ΕN d(t)=-1.1~-1.8)(表2,图6)。它们的206Pb 204Pb和208Pb 204Pb值也十分相近,而208 Pb3 206Pb3值是本批样品最高的,为1.01~1.03,落入E M 区(Hofm ann,1997),并与它们的T h U值较高(未列出)相一致。

它们的痕量元素比值很相近而且有别于其他样品。BH28、10、11的Zr N b值很低,但L a N b值较高,平均值分别为3.07和1.62(图8)。虽然随着M gO 降低,它们的T h、U和N b含量增高,但是T h N b和N b U值变化不大,平均值依次为0.24和22.53。米易地区一件高度演化的玄武岩样品(M Y202)也是如此(图5f,9a)。R b Sr和N d Sm的平均值为0.025和

4.88(图9b)。这些比值与它们的L R EE和T h较高,

H FSE,特别是Zr、H f较低并呈负异常是一致的。虽然这些比值,特别是N b U值与典型的E M 型O I B不完全相同,而且还可能受到地壳混染或岩石圈反应的影响,但是它们也与远洋沉积物的地球化学特征相一致(Pachett et al.,1984;W eaver,1991;W h ite et al.,1996;P lank et al.,1998)。

5.2 高87Sr 86Sr端元的成因

大陆溢流玄武岩高87Sr 86Sr端元的成因成为当前争论的焦点。关键问题是如何鉴别它究竟是地幔柱本身产生的,还是地幔柱岩浆与地壳混染或与岩石圈反应的结果。前面的论述已经确定了在本区的同期岩浆中存在FO Z O和H I M U端元,而它们分别代表了下地幔和古洋壳;本区的岩浆成分又是受FO Z O2H I M U2CC成分约束的。因此这一端元很可能与双源岩(地幔橄榄岩+榴辉岩)熔融有关。

5.2.1 榴辉岩存在的证据

由于重稀土(HR EE)和Y是石榴子石的相容元

212地 质 学 报2005年

图8 峨眉山溢流玄武岩Zr N b vs.L a N b图

F ig.8 D iagram of Zr N b vs.L a N b fo r the ECFB

样品符号同图2。资料来源如下:O ntong Java(据M ahoney et al., 1993);H I M U型O I B(据W eaver,1991;Kogiso et al.,1997); Society(据W h ite et al.,1996);P itcairn(据W oodhead et al., 1993);N.Kerguelen(据Yang et al.,1998);H aw aii(据N o r m an

et al.,1999)。Siberian;N d1,2代表典型的地壳混染熔岩(据

L igh tfoo t et al.,1990;W oodent et al.,1993)。PM—原始地幔

值(据M c Donough et al.,1995);CC和U CC为大陆地壳和大陆

上地壳平均成分(据R udnick et al.,1995;Barth et al.,2000);

箭头指向N2M ORB的平均值:Zr N b=31.76,L a N b=1.07(据Sun et al.,1989)

Sym bo ls as in F ig.2.D ata sources as fo llow s:O ntong Java (M ahoney et al.,1993);H I M U O I B(W eaver,1991;Kogiso et al.,1997);Society(W h ite et al.,1996);P itcairn(W oodhead

et al.,1993);N.Kerguelen(Yang et al.,1998);H aw aii (N o r m an et al.,1999).Siberia,N d1,2rep resent crustal contam inati on lavas typ ically(L igh tfoo t et al.,1990;W oodent

et al.,1993).PM refers to P ri m itive M antle values (M c Donough et al.,1995).CC and U CC m ark average compo siti ons of bulk and upper continental crust respectively (R udnick et al.,1995;Barth et al.,2000).T he arrow po ints to the average M ORB values:Zr N b=31.76,L a N b=1.07(Sun

et al.,1989)

素,因此地幔源岩在石榴子石稳定区部分熔融时,熔体中不相容元素与它们的比值(例如Sm Yb,N b Y和Zr Y)将因石榴子石比例增高而增大。如果原生岩浆中这些比值超过石榴二辉橄榄岩低度熔融比值的某一限度,它们就可能成为榴辉岩存在的证据。

图10a,b的低比值样品相当于按痕量元素丰度区分的第 成分组,中、高比值样品相当于第 成分组。本区有13件样品的Sm Yb值在4.02~6.3之间,其中高87Sr 86Sr端的样品(R21、3、5、8)Sm Yb 值最高,为5.4~6.3。它们仍然落在Society的范围之内(图10a),但比西伯利亚,

德干和哥伦比亚河玄

图9 峨眉山溢流玄武岩T h N b2N b U图(a)

和R b Sr2N d Sm图(b)

F ig.9 D iagram s of T h N b vs.N b U(a)and

R b Sr vs.N d Sm(b)of the ECFB

O I B平均值据(Sun et al.,1989),其他符号及资料来源同图8。注意,在图9b中,浅成岩体(金宝山,乌龙坝和普和)以及粗面岩2流纹岩类都因斜长石晶出使R b Sr值增高;此外,明显受地壳混染的玄武岩(YC204)和海水蚀变的碧玄岩(L Y206)R b Sr值也较高

T he average values fo r O I B are from Sun et al.(1989),o ther sym bo ls and data source are as in F igure8.N o te R b Sr rati o s of hypabyssal rock s(J inbao shan,W ulongba an Puhe)and trachyte2 rhyo litic rock s increase due t crystallizati on of p lagi oclase;in additi on,one basalt(YC204)affected by obvi ous crustal contai m inati on and one basanit(L Y206),affected by seaw ater alterati on also have h igh R b Sr rati o s

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第2期 侯增谦等:峨眉地幔柱轴部的榴辉岩2地幔岩源区:主元素,痕量元素及Sr、N d、Pb同位素证据

图10 峨眉山溢流玄武岩L a Sm2Sm Yb图(a)

及N b Y2Zr Y图(b)

F ig.10 D iagram s of L a Sm vs.Sm Yb(a)and

N b Y vs Zr Y(b)of the ECFB

大陆岩石圈地幔(CLM)的中值据M c Donough(1990),图b中双

斜线为冰岛玄武岩成分的上、下边界线(据F itton et al.,1997)。其他符号和资料来源同前

M edian compo siti on of continental litho spheric m antle(CLM)is from M c Donough(1990).I low er panel,the parallel lines m ark the upper and low er bounds of the Iceland basalt data(F itton et al.,1997).O ther sym bo ls and data sources are the sam e as above

武岩的Sm Yb值(<3)高得多,也高于H aw aii和R eun i on的相应值(<4)(L assiter et al.,1997)。Zr Y值约在4.5~13之间,N b Y值在0.4~2.1之间,其中R21、3、5、8的Zr Y值也是最高的,大致在11~13之间(图10b)。图10b的比值变化范围与Kerguelen群岛玄武岩相似(F rey et al.,2000)。图8和图10都表明,本区熔岩不可能由亏损的软流圈物质与大陆地壳混染而成。

值得注意的是,高87Sr 86Sr端样品的这些比值比石榴二辉橄榄岩低度熔融的计算结果高得多。例如石榴二辉橄榄岩非定比分离熔融,当熔融程度为1%时,Zr Y值也仅为10(F itton et al.,1997)。采用批式熔融方程计算的Sm Yb值也是较低的(L assiter et al.,1997)。这些计算结果说明,单纯石榴橄榄岩部分熔融,即使熔融程度很低也不可能产生该端元岩浆,何况从地幔柱轴部高温区产生的岩浆,其熔融程度不可能很低(Cam pbell,1998)。但是,如果源区由榴辉岩和石榴二辉橄榄岩构成,由于石榴子石比例增高就有可能产生这种岩浆。岩浆系统中H I M U端元的确定以及FO Z O2H I M U2CC对本区岩浆成分的约束关系大大地加强了这种可能性。

主元素、痕量元素和同位素特征都说明,这一端元是相对富含地壳物质的,如果它们呈榴辉岩产出,那么很可能是富含长英质的钾长石石英榴辉岩,有别于单纯由古玄武质洋壳形成的榴辉岩。该端元岩石不但Si O2和K2O比较高,还在玄武岩的单斜辉石斑晶中找到英安岩质熔融包裹体以及钾长石晶体(未发表资料)。这说明斑晶晶出时仍然存在没有完全混溶的两种岩浆:玄武岩浆和英安岩浆。这一结果似乎证明榴辉岩的存在,而且说明它很可能是钾长石石英榴辉岩。这与H au ri对H aw aii火山熔岩的解释相类似(H au ri,1996),也与陆壳和洋壳物质的高压实验结果相一致(Irifune et al.,1994;Yasuda et al.,1994,1997;O no et al.,1996;Co rdery et al.,1997;Kogiso et al.,1998;T akahash i et al., 1998;H iro se et al.,1999;W ang et al.,1999; O no et al.,2001)。

高87Sr 86Sr值和中等87Sr 86Sr值、低143N d 144 N d值两组熔岩可能都与再循环陆壳物质有关,并与H eard岛及Kerguelen群岛的熔岩相类似,E M 和E M 之间可能是连续变化的(W eis et al., 1993;W oodhead et al,1993;B arling et al., 1994),故这里统称为E M 2 端元。

5.2.2 源区非均性

根据前面的论述,本区火山岩的同位素和痕量元素比值都具有O I B的富集特征。这意味着它们的源区含有俯冲古洋壳及相伴的沉积物。从图9a可以看出,丽江、宾川和二滩三处的火山岩样品覆盖了以M angaia,P itcairn和Society为代表的H I M U,E M 和E M 型洋岛的成分范围。丽江火山岩样品分布在三种类型洋岛的成分范围内,宾川火山岩分布在E M 与E M 型洋岛的重叠区,二滩的样品只落在Society范围内,但T h(U) N b值较高,表明其相对富含俯冲沉积物。因此,从整体看,地幔柱轴部区由地幔岩和榴辉岩(古洋壳)构成;从空间分布看,

412地 质 学 报2005年

丽江、宾川和二滩三处火山岩的源岩成分不完全相同;就是丽江地区火山岩的源岩成分也是不均匀的。这可能与源区两种岩石熔融的比例不同有关。这种不均匀性可能是地幔柱轴部区的基本特征。最近的洋岛玄武岩研究和大陆溢流玄武岩研究都证实这一点(W h ite et al.,1996;H au ri,1996;Yasuda et al.,1997;Co rdery et al.,1997;T akahash i et al., 1998)。

在主元素方面,与M ORB相比,大多数O I B与CFB都具有富FeO、T i O2和碱性元素的特征(例如, Cam pbell,1998;T akahash i et al.,1998),峨眉山玄武岩也一样(张云湘等,1988)。Kogiso等(1998)的实验证明,与橄榄岩生成的岩浆相比,由橄榄岩和玄武岩混合物部分熔融产生的岩浆具有这些特征(Kogiso et al.,1998)。这表明它们的源岩比地幔橄榄岩更富玄武质组分,这种组分就是再循环玄武质洋壳。这与同位素和痕量元素的研究结果相一致。Cam pbell(1998)指出,含有榴辉岩的O I B源区与M ORB源区相比,不但FeO含量高(榴辉岩的2FeO为10.5%,地幔岩为8%),而且具有比较氧化(氧逸度大约高出1个对数单位)和含水量较高的特征(Cam p bell,1998)。显然,由再循环古洋壳和周围橄榄岩构成的地幔柱轴部源岩部分熔融产生的岩浆不但FeO、T i O2和碱性组分含量比较高,也是比较氧化的。因此,再循环玄武质洋壳(榴辉岩)的存在可能是攀枝花—西昌地区超大型钒钛磁铁矿床形成的根本原因。这些地幔柱成因矿床的岩浆特征:偏碱性,富铁钛并有较高的氧逸度(见卢记仁,1996及相关文献)与这种复合源岩产生的岩浆成分相当吻合。

5.2.3 关于地壳混染及岩石圈反应

地壳混染和岩石圈反应被广泛用来解释玄武岩,特别是高87Sr 86Sr端元玄武岩的成因。但是,如何鉴别这些作用仍然是没有完全解决的问题。下面我们试图利用痕量元素比值图进一步讨论上述岩石的物质来源。

从图8~10可以看出,本区大部分岩石样品都落在洋岛玄武岩范围内,据此判定它们属于地幔柱成因。这些样品中的一部分,T h N b和Sm Yb值较高,落在E M 型洋岛(Society)玄武岩范围内。E M 型源岩代表含有百分之几陆源沉积物的再循环洋壳(Hofm ann,1997;W h ite et al.,1996),它们的成分区与地壳混染区有逐渐过渡趋势,但两者尚有明确界线(图8,9a)。

分布在洋岛玄武岩之外的样品中有一部分可能属于地幔柱岩浆与地壳混染的产物。它们或者分布在以西伯利亚玄武岩的N d层为代表的地壳混染区之内,或者分布在N d层与Society成分区之间。1件玄武岩样品(YC204)具有明显的地壳混染特征,出现在混染区的中部(图9a)。另外一些浅成相橄榄岩(4件)和粗面岩2流纹岩类岩石(3件)的地壳混染程度较低。它们分布在N d层的低混染端与夏威夷苦橄岩区交汇处附近(图8),或者落在N d层与Society成分区之间(图9a)。在图10a中,它们的分布点从洋岛玄武岩向上地壳平均成分点附近延伸。

分布在洋岛玄武岩范围之外的另外4件样品(BH28、10、11和M y202),它们的地球化学特征已在5.1.4叙述。在图5f中,它们的T h N b值最高,为0.24;在图9a中,它们出现在N d层与Society成分区之间,似乎应当属于地幔柱岩浆与地壳混染产物。但在图8,9b和10a中,这些样品并不在地幔柱岩浆与地壳混染的趋势线上,并以Zr N b和R b Sr 值低,而L a Sm值高为特征。这与地壳混染的预期结果不符,因为地壳混染应使Zr N b和R b Sr值增高,L a Sm值降低。虽然由于元素相容性差别较大,这些比值容易受岩浆分馏影响,但是①BH28、10、11三件样品M gO含量高,为苦橄岩2玄武岩组合,应属于熔融程度较高的原生岩浆,分馏作用影响不会很明显;②即使岩浆分馏起作用也不能解释R b Sr 值低这一特征,因为这种作用应当使岩浆中的R b Sr值增高而不是降低。因此我们认为这些特征不是由岩浆分馏造成的。

BH28、10、11三件样品属于典型的低T i玄武岩,T i O2含量仅为0.8%~1.2%。这种岩石通常被认为是地幔柱岩浆与岩石圈反应的结果。关于低T i 玄武岩的成因还在争论,例如,A rndt等(1993)和Cam p bell(1998)都认为正常的大陆岩石圈地幔不太可能成为低T i玄武岩源区,但是后一作者也注意到比较稀少的石榴金云橄榄岩可作为低T i玄武岩合适的物质来源(见A rndt et al.,1993;Cam pbell, 1998;及其中文献)。目前,许多作者都采用钾镁煌斑岩平均成分作为岩石圈易熔成分的代表。地幔柱熔体与它们混合可使L a N b,L a Sm,T h(U) N b,以及R b Sr值增高。前三种比值增高与本组样品特征相符,但后一比值增高与图9b的情况不符。不过,在低度熔融时,残余金云母可能使进入岩浆的R b 受到抑制,从而使R b Sr值降低。从软流圈顶部向上迁移进入岩石圈的硅酸盐熔体亦是低R b Sr值的(Sun et al.,1989)。许多作者指出,在地幔柱到

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第2期 侯增谦等:峨眉地幔柱轴部的榴辉岩2地幔岩源区:主元素,痕量元素及Sr、N d、Pb同位素证据

达之前,岩石圈下部就存在少量这种熔体。虽然它们已是固态物质,但由于环境温度仅略低于固相线温度,很容易受热熔融(Sun et al.,1989;W h ite et al.,1995;W h ite et al.,1996)。当地幔柱到达岩石圈地幔之下并发生减压熔融时,上升的早期地幔柱岩浆很容易受其影响。因此,这些低T i玄武岩可能是地幔柱岩浆与岩石圈反应的结果。这与前人的研究结果一致(例如,Chung et al.,1995;Song et al.,2001;肖龙等,2003)。

6 结论

(1)峨眉山大陆溢流玄武岩省西南部以丽江、大理和攀枝花市为中心的苦橄岩分布区,面积约5×104km2,为地幔柱的轴部区。

(2)大部分火山岩样品的Sr、N d、Pb同位素和痕量元素比值落在各种类型的洋岛火山岩成分范围之内,并存在类似FO Z O,H I M U和E M 2 的端元成分,说明它们是在地幔柱轴部由榴辉岩和地幔岩组成的源区,通过部分熔融产生的岩浆形成的。岩浆源区古玄武质洋壳(榴辉岩)的存在可能是该区超大型钒钛磁铁矿床形成的关键原因。

(3)分布在洋岛玄武岩成分之外的少部分样品中,一部分属于地幔柱岩浆与地壳混染产物,另一部分可能是地幔柱岩浆与岩石圈反应的结果。前者主要为浅成相橄榄岩和粗面岩2流纹岩类岩石,后者以宾川的3件低T i玄武岩为代表。

(4)根据苦橄岩2玄武岩类两组不相容元素丰度曲线相互交切判断,它们可能是在两种深度,两种压力状态下产生的岩浆形成的,岩浆产生过程伴随着岩石圈减薄作用。

致谢:本项研究得到国土资源部地质行业基金和跨世纪人才基金的支持。在野外调查期间蒙林建英和张如之两位高级工程师帮助,在成文过程中又得到陈伟十工程师和张绮玲副研究员大力协助,在此深表感谢。

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M a jor ,Trace Elem en t and Sr -Nd -Pb Isotope Ev idence

HOU Zengqian ,LU J iren ,L I N Shengzhong

Institu te of M ineral R esou rces ,Ch inese A cad e m y of Geolog ical S ciences ,B eij ing 100037,Ch ina

Abstract

T he p icrite field ,located in the sou thw est of Em eishan con tinen tal flood basalts (ECFB )and w ith the

cen ter in L ijiang —D ali —Panzh ihua triangu lar area ,is abou t 5×104km 2

and regarded as the ax ial area of the Em ei m an tle p lum e .T he studies of Sr ,N d and Pb iso topes as w ell as trace elem en ts indicate that m o st sam p les of vo lcan ic rock s are located in the field of ocean ic island basalt ,and FO Z O 2like ,H I M U 2like and E M 2 2like end 2m em bers ex ist .T he evidences indicate that the m agm as are fo r m ed by m elting of p yro lite and eclogite (ancien t basaltic ocean ic cru st )in the ax ial zone of m an tle p lum e .T he ex istence of recycled basaltic ocean ic cru st in the ax ial zone of m an tle p lum e m ay be the key of the fo r m ati on of gian t V 2T i 2m agnetite depo sits .T he rest sam p les are located ou tside the field of ocen ic island basalt .Som e of them belong to the con tam inati on p roduct betw een m an tle 2p lum e m agm as and cru st ,and som e low 2T i sam p les m ay be related to the reacti on w ith litho sphere .T h ree m an tle end 2m em bers ,FO Z O ,H I M U and E M 2 ,are iden tified by studying of Sr ,N d and Pb iso topes as w ell as trace elem en ts .T he end 2m em ber ,FO Z O

rep resen ted by B ailin shan p icritic basalt (YB 201)is characterized by low 87Sr 86Sr (017036),h igh 143N d

144

N d (015127),m oderate 206Pb 207

Pb (1815693),N b U (36167),T h N b (01082),L a N b (0191)and Zr

N b (6123).T he end 2m em ber ,H I M U rep resen ted by L ijiang p icrite (JL 229)is characterized by h igh

206

Pb 204Pb (2016412)and 207Pb 204Pb (1517489),and low 87Sr 86

Sr (017048).T he end m em ber ,E M 2

com p rises tw o group s :(1)E rtan p icrite 2basalts (R 21,3,5and 8)characterized by h igh 87Sr 86

Sr

(017073)and low 143N d 144N d (015123),206Pb 204Pb (1719968)and 208Pb 204

Pb (3719450),and N b U (27),T h N b (118),L a N b (1125),Sm Yb (514~613),Zr Y (11~13)and N b Y (117~211).(2)B inchuan p icrite 2basalts (BH 28,10and 11)w ith iso tope com po siti on si m ilar to tho se of E M are

characterized by 87Sr 86Sr (017052),143N d 144N d (015124),208Pb 206

Pb (1102),h igh in L R EE and T h abundances ,and negative anom alies in H FSE (N b ,T a ,Zr ,H f and T i ),by low N b U (on ly 22153),and by T h N b (0124),L a N b (1162),R b Sr (01025),Sm Yb (314~512),Zr Y (414~612)and N b Y (115~118).T hese end 2m em bers can be com p ared w ith tho se of ocean ic island basalts .T he end 2m em ber ,FO Z O rep resen ts a sligh tly dep leted pyro lite ,and is p robab ly the en trainm en t from low er m an tle .T he end 2m em ber ,H I M U is the p roduct of p artial m elting of subducted basaltic ocean ic cru st (eclogite )and p yro lite ,w h ile E M 2 that of m elting and m ix ing of subducted ocean ic cru st w ith associated sedi m en ts (K 2feldspar 2quartz eclogite )and pyro lite .

Key words :con tinen tal flood basalts ;Em ei m an tle p lum e ;ax ial zone of m an tle p lum e ;m an tle end 2m em bers

9

12第2期 侯增谦等:峨眉地幔柱轴部的榴辉岩2地幔岩源区:主元素,痕量元素及Sr 、N d 、Pb 同位素证据

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