2012, Vol. 28
2. 中国科学院研究生院地球科学学院, 北京 100039
2. College of Earth Sciences, Graduate University of Chinese Academy of Sciences, Beijing 100039, China
恒山-五台-阜平地区位于华北克拉通中部造山带 (Trans-North China Orogen, Zhao et al., 2001) 中段, 它主要由恒山和阜平高级变质杂岩带以及五台花岗岩-绿岩带组成 (图 1)。恒山和阜平高级变质杂岩带以2523~2475Ma的TTG片麻岩为主, 并含少量~2.7Ga的古老陆壳物质 (Guan et al., 2002；Zhao et al., 2002, 2004；Kröner et al., 2005a, b)。五台花岗岩-绿岩带主要由晚太古代花岗岩类 (2566~2517Ma, Wilde et al., 1997, 2005；Wilde, 2002) 和角闪岩相-绿片岩相的变质火山岩-沉积岩 (2533~2516Ma, Wilde, 2002；Wilde et al., 2004a) 组成。
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图 1 恒山-五台-阜平地区地质简图 (据Wang et al., 2004) Fig. 1 Geological map of the Hengshan-Wutai-Fuping area (after Wang et al., 2004) |
在五台杂岩的原南北金刚库组中出露有一系列的镁铁-超镁铁岩块 (体), 大致有三条带呈NE-SW向分布 (图 1)。李继亮等 (1990)和白瑾等 (1992)曾提出这些镁铁-超镁铁质岩可能是古洋壳的残片。王凯怡等 (1997)进一步认为这些镁铁-超镁铁质岩可能代表了太古宙蛇绿混杂岩带。Polat et al.(2005) 对原南金刚库组内的一些橄榄岩类进行了地球化学研究, 并基于其U型REE配分模式认为它们可能是形成于弧前环境的太古宙蛇绿岩。可以看出, 这些镁铁-超镁铁质岩对认识华北克拉通中部造山带构造演化具有重要的制约价值, 但我们对其成因和潜在的大地构造意义尚缺乏深入的了解。
本文研究的雁门关岩体位于北带的雁门关附近, 岩体沿NE向侵入于五台群底部原北金岗库组的片麻岩内, 主要由二辉橄榄岩、橄榄辉石岩、辉石岩、角闪石岩和辉长岩等组成 (图 2)。辉长岩中岩浆锆石SHRIMP测年结果表明, 该岩体形成于~2.2 Ga (Wang et al., 2010)。本文在详细的岩石学、矿物学和元素地球化学研究基础上, 对其成因和大地构造意义进行了探讨。
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图 2 雁门关镁铁-超镁铁岩体剖面图 (修改自穆书汉, 1983①) Fig. 2 Cross section of the Yanemenguan mafic-ultramafic intrusion |
①穆书汉.1983.山西省超基性岩.山西榆次:山西省地质矿产局区域地质调查队一分队地质调查报告 (内部报告, 未正式出版), 1-192
2 区域地质背景华北克拉通由多个陆块组成 (Zhai et al., 2000), 通常被分为三个部分：东部陆块、西部陆块及分隔其间的中部造山带 (Zhao et al., 2001)。东部陆块和西部陆块以太古宙TTG片麻岩为主, 发育少量的表壳岩, 中部造山带主要由晚太古代TTG片麻岩和花岗岩-绿岩带组成 (Zhao et al., 2001)。恒山-五台山-阜平杂岩带位于华北克拉通中部造山带的中段, 是该造山带研究程度最高的地区, 绝大多数的大地构造演化模式都是在这一地区基于不同的研究角度而提出的 (如Wilde et al., 2002；Kusky and Li, 2003；Kröner et al., 2005a；Polat et al., 2005；Zhao et al., 2004, 2007；Faure et al., 2007；Trap et al., 2007；Wang, 2009)。恒山-五台山-阜平杂岩带各岩石构造单元之间为构造接触, 在剖面上构成向SE逆冲的叠瓦状堆垛岩席 (Wang et al., 2004; Wang, 2009；Trap et al., 2007)。
五台晚太古代绿岩带主要为角闪岩相-绿片岩相的变质火山岩-沉积岩并部分被早元古代的滹沱群不整合覆盖。前人将五台绿岩带中的角闪岩相变质岩部分称为“金刚库组”(田永清, 1991), 并进一步按其成因和形成构造环境的差异分为南、北金刚库组 (李继亮等, 1990；Wang et al., 1996；王凯怡等, 1997)。南金刚库组位于五台绿岩带的东-东南部 (图 1), 与阜平杂岩之间为断层接触；北金刚库组出露于恒山杂岩带的南缘和五台绿岩带的北缘 (图 1)。恒山杂岩带南缘的角闪岩相变质火山岩-沉积岩因为分布于恒山地区, 常被认为是恒山杂岩的表壳岩 (如Zhao et al., 2004), 但下面的一些地质事实使我们对这一认识持保留态度：(1) 恒山杂岩带南缘和五台绿岩带北缘中的斜长角闪岩具有一致的地球化学特征 (Wang et al., 2004)；(2) 恒山杂岩带南缘角闪岩相变质岩的构造变形特征与五台绿岩带北缘的角闪岩相变质岩协调一致, 而与恒山杂岩的主体部分-TTG片麻岩明显不同 (Trap et al., 2007；Wang, 2009)。在本文中, 我们所说的“原南、北金刚库组”不具有地层的含义, 主要指五台绿岩带两侧的角闪岩相变质岩部分。
雁门关岩体位于代县雁门关附近 (图 1), 侵入原北金岗库组片麻岩内, 形态呈北东端扬起西南端倾伏的透镜体。岩体与围岩的接触界限平直, 局部稍有变化, 接触带产状大体与围岩一致, 局部斜交。岩体走向55°, 倾向SE, 上盘围岩为黑云角闪斜长片麻岩, 下盘围岩为斜长角闪岩和黑云角闪斜长片麻岩 (图 2)。岩体内部为超基性岩, 其中橄榄岩在中心, 边部为辉石岩和橄榄辉石岩, 平面和剖面上构成一环带状构造。超镁铁岩中可见少量方辉橄榄岩析离体, 局部有辉石岩脉和透闪石脉穿插。基性岩分布于超镁铁岩两侧, 多集中在西北侧, 边缘为角闪石岩, 内部为粗粒和细粒辉长岩。因此雁门关岩体是由橄榄岩-橄榄辉石岩-辉石岩-角闪石岩-辉长岩等构成的镁铁-超镁铁岩体 (图 2)。
3 岩石学和矿物学 3.1 岩石学特征橄榄岩分布于超基性岩内部, 主要为二辉橄榄岩, 绿黑色, 半自形粒状结构和补堆晶结构 (包含嵌晶), 常见网状结构、反应边结构 (图 3a), 块状构造。主要矿物成分为残晶橄榄石 (15%~30%, 贵橄榄石, Fo=78~82), 纤维蛇纹石 (25%~40%), 单斜辉石 (15%~20%, 透辉石), 角闪石 (15%~20%, 位于单斜辉石内, 主要为韭闪石), 斜方辉石 (10%~15%, 古铜辉石), 及少量镁铁尖晶石、铬尖晶石、磁铁矿和碳酸盐等。橄榄石多被辉石局部或全部包围, 多有蛇纹石化。尖晶石类分布于橄榄石、辉石和蛇纹石等矿物的内部和边缘以及间隙基质内。
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图 3 雁门关镁铁-超镁铁岩体代表岩石的显微结构照片 (a)-二辉橄榄岩的半自形-他形粒状结构和网状结构；(b)-角闪辉长岩的辉长结构 Fig. 3 Typical textures of the Yanmenguan mafic-ultramafic intrusion (a)-hypidiomorphic to allotriomorphic granular and reticular texture in the peridotites; (b)-gabbroic texture in the mafic rocks |
橄榄辉石岩较少, 与橄榄岩成渐变过渡, 主要集中在北侧近底部, 地表未出露。岩石为黑绿色, 块状构造, 半自形粒状结构。主要矿物成分为橄榄石 (~10%)、辉石 (50%~60%) 和角闪石 (10%~20%), 以及少量镁铁尖晶石、铬尖晶石和磁铁矿等。橄榄石均蛇纹石化, 辉石多角闪石化, 尖晶石类分布于辉石的内部和边缘。
辉石岩分布于橄榄岩边缘, 主要集中于北侧。浅绿色, 块状构造, 半自形粒状结构。主要矿物成分为辉石 (50%~80%)、角闪石 (10%~15%) 及少量铬尖晶石、磁铁矿等。辉石多绿泥石化和角闪石化。
角闪石岩分布于基性岩的南北两侧, 深绿色, 粒状结构, 蚀变较强。主要矿物为角闪石 (~15%, 主要为镁普通角闪石, 少量浅闪石)、绿泥石 (50%)、蛇纹石 (15%) 及少量滑石、铬尖晶石、磁铁矿和碳酸盐等。
辉长岩主要为角闪辉长岩, 蚀变中到强, 褐绿色和绿色, 岩石以辉长结构 (图 3b)、粒状镶嵌结构 (补堆晶) 为主, 也见包含嵌晶结构。主要组成为角闪石 (30%~70%, 镁普通角闪石为主, 部分浅闪石, 少量钙镁闪石、阳起石) 和斜长石 (20%~60%, An=43.3~30.1), 及少量磁铁矿等。角闪石为绿色或黄绿色, 多色性明显, 多他形粒状和不规则状, 部分也见柱状, 少见自形晶, 粗巨粒角闪石中包含大量其他细粒矿物。斜长石为厚板状、粒状, 半自形-他形, 不同程度蚀变和粘土化。该辉长岩类的矿物组成与斜长角闪岩类似, 但岩浆结构明显, 故仍称为角闪辉长岩。
3.2 矿物学特征矿物成分测试分析在中科院地质与地球物理研究所的CAMECA SX51电子探针上进行, 分析精度1%~5%, 分析数据见表 1-表 5。
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表 1 雁门关岩体二辉橄榄岩中的橄榄石成分 (wt%) Table 1 Composition of olivine in the peridotites from the Yanmenguan intrusion (wt%) |
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表 2 雁门关岩体二辉橄榄岩中的辉石成分 (wt%) Table 2 Composition of pyroxene in the peridotites from the Yanmenguan intrusion (wt%) |
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表 3 雁门关岩体二辉橄榄岩中的尖晶石成分 (wt%) Table 3 Composition of spinel in the peridotites from the Yanmenguan intrusion (wt%) |
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表 4 雁门关岩体中的角闪石成分 (wt%) Table 4 Composition of amphibole from the Yanmenguan intrusion (wt%) |
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表 5 雁门关岩体辉长岩中的斜长石成分 (wt%) Table 5 Composition of plagioclase in the gabbros from the Yanmenguan intrusion (wt%) |
橄榄石 (Olv) 橄榄石主要分布在二辉橄榄岩中, Fo值为78.5~81.8(表 1、图 4a, c), 比地幔橄榄岩 (98~92, Wilson, 1989)、深海橄榄岩 (平均90.5, Dick et al., 1984) 以及阿拉斯加型镁铁-超镁铁岩中的橄榄岩 (91.2~84.6, Himmelberg and Loney, 1995) 低。橄榄石中CaO ( < 0.07%)、Cr2O3( < 0.01%)、Al2O3( < 0.01%)、TiO2( < 0.06%) 较低, 而NiO (0.19%~0.36%) 和MnO (0.17%~0.33%) 较高。Fo-NiO关系图上雁门关橄榄石落在层状镁铁-超镁铁岩区 (图 4a)。
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图 4 雁门关镁铁-超镁铁岩体矿物化学成分关系图 (a)-二辉橄榄岩中橄榄石的Fo-NiO关系图；(b)-辉石成分图 (Deer et al., 1992)；(c)-橄榄石成分图；(d)-基性岩中斜长石的成分图；(e)-单斜辉石Al2O3-Mg#(DeBari and Coleman, 1989)；(f) Al2O3-TiO2关系图 (Loucks, 1990).其中图 (a) 地幔橄榄岩数据引自Krause (2007), 层状基性岩体数据引自Simpkin and Smith (1970)；图 (b) 虚线区分别为阿拉斯加型超镁铁岩中Cpx和Opx成分区 (Irvine, 1974；Himmelberg et al., 1986；Himmelberg and Loney, 1995；Batanova et al., 2005；Pettigrew and Hattori, 2006；Krause, 2007), 图 (e-f) 西南阿拉斯加岩体据Himmelberg et al.(1986) 和Himmelberg and Loney (1995), 中乌拉尔山岩体据Krause (2007) Fig. 4 Mineral chemical compositions of the Yanmenguan mafic-ultramafic intrusion (a)-Fo vs. NiO plot of the olivine; (b)-the compositions of pyroxene (Deer et al., 1992); (c)-the compositions of olivine; (d)-the compositions of plagioclase in the gabbros; (e)-plots of Mg/(Mg+ Fe2+) vs. Al2O3 (DeBari and Coleman, 1989); (f)-TiO2 vs. AlIV (%) (Loucks, 1990) for clinopyroxene in the peridotites. The global mantle array in (a) is after Krause (2007) and the data for layered intrusions are after Simpkin and Smith (1970). The Alaskan type complexes in (b) are after Irvine (1974), Himmelberg et al.(1986), Himmelberg and Loney (1995), Batanova et al.(2005), Pettigrew and Hattori (2006) and Krause (2007). The data of SE Alaska complexes in (e) and (f) are after Himmelberg et al.(1986) and Himmelberg and Loney (1995), and the Central Ural complexes from Krause (2007) |
斜方辉石 (Opx) 二辉橄榄岩中的斜方辉石为古铜辉石 (Wo0.7~2.3En81.5~82.7Fs16.2~17.2), Mg#(100Mg/(Mg+Fe2+))=82.9~83.7(表 2、图 4b), 与岩石中橄榄石的Mg#=78.7~79.9接近。
单斜辉石 (Cpx) 样品中的单斜辉石主要为透辉石 (Wo47.3~49.8En47.1~52.6Fs0.1~3.8) 和普通辉石 (Wo39.3~44.7En55.1~56.6Fs0.2~4.1), Mg#=92.5~1(表 2、图 4b)。其矿物成分与阿拉斯加型镁铁-超镁铁岩和岛弧超镁铁杂岩 (图 4b) 的辉石成分相近。
在Al2O-Mg#协变图上 (图 4e), 雁门关镁铁-超镁铁岩体中的单斜辉石与弧堆晶岩演化特征相似 (DeBari and Coleman, 1989), 并与西南阿拉斯加岩体的单斜辉石 (Himmelberg et al., 1986；Himmelberg and Loney, 1995) 接近。在TiO2-AlⅣ关系图上, 雁门关岩体的单斜辉石完全沿弧堆晶岩演化线分布 (Loucks, 1990), 与西南阿拉斯加岩体 (Himmelberg et al., 1986；Himmelberg and Loney, 1995) 和中乌拉尔山岩体 (Krause, 2007) 中的单斜辉石相当一致 (图 4f)。
尖晶石 (Spl) 雁门关二辉橄榄岩中有大量尖晶石类矿物, 多为不透明的铬尖晶石和铬铁矿, 呈自形-他形的细粒状分布于橄榄石和辉石晶粒间或内部, 部分分布于蛇纹石中。尖晶石类富Fe (特别是Fe2+)、Al和Ti, 贫Mg和Cr (表 3)。在橄榄石Fo-尖晶石100Cr/(Cr+Al) 关系图上 (未图示), 雁门关橄榄岩中的橄榄石-尖晶石类的演化关系因Fo偏小而明显偏离于地幔演化序列, 表明雁门关橄榄岩的成因不同于地幔橄榄岩、大洋橄榄岩和蛇绿岩等地幔单元。在100Mg/(Mg+Fe2+)-100Cr/(Cr+Al)(即Mg#-Cr#) 图中, Mg#和Cr#呈良好的负相关 (图 5a)。在Cr-Al-Fe3+图解上, 随着Al的减小, Cr显著增高, 而Fe3+则增加幅度较微弱 (图 5b)。从二者的关系图上也说明雁门关橄榄岩的成因不同于大洋橄榄岩和蛇绿岩等地幔单元, 而较接近于Tonsina岛弧堆晶岩 (图 5a)。
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图 5 雁门关二辉橄榄岩尖晶石类Cr#-Mg#图 (a) 和Cr-Al-Fe3+(b) 图 数据来源：(a) 中阿拉斯加型镁铁-超镁铁岩引自Burns (1985), Himmelberg et al.(1986) 和Himmelberg and Loney (1995), Tonsina弧堆晶岩 (大陆弧) 据DeBari and Coleman (1989)；(b) 中阿拉斯加型镁铁-超镁铁岩引自Zhou et al.(1997)；(a, b) 中其它数据引自Barnes and Roeder (2001) Fig. 5 The diagram of Mg/(Mg+Fe2+) vs. Cr/(Cr+Al) (a) and Cr-Al-Fe3+ triangle (b) for spinel in the Yanmenguan peridotites The Alaskan-type complexes in (a) are after Burns (1985), Himmelberg et al.(1986) and Himmelberg and Loney (1995), and the Tonsina arc cumulates (continent marginal arc) from DeBari and Coleman (1989). The Alaskan-type complexes in (b) are after Zhou et al., (1997). The other data in the diagram are after Barnes and Roeder (2001) |
角闪石 (Amp) 角闪石在雁门关镁铁-超镁铁岩体中比较常见, 在辉长岩和角闪石岩中为主要矿物。角闪石的成分类型见详细分类图解 (图 6a, b, Leake et al., 1997, 2004)。辉长岩中角闪石绝大多数为镁 (普通) 角闪石, 部分为浅闪石、钙镁闪石、阳起石和韭闪石。Mg#=66~77(表 4), 不同于阿拉斯加型辉长-苏长岩类的角闪石 (韭闪石为主, Mg#=59~78), 但后者角闪石岩中的角闪石也主要为镁 (普通) 角闪石 (Mg#=76~82)。橄榄岩中角闪石主要为韭闪石, Mg#=89~95(表 4), 成分与阿拉斯加型超镁铁岩 (韭闪石为主, Mg#=60~89, Himmelberg and Loney, 1995) 接近。雁门关岩体中角闪石成分与岛弧堆晶岩的演化趋势比较一致 (Beard and Barker, 1989), 例如西南阿拉斯加杂岩体 (图 6c, Himmelberg et al., 1986；Himmelberg and Loney, 1995)。
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图 6 雁门关镁铁-超镁铁岩体各岩类中角闪石的成分分类图解 (a, b) 和Si-(Na+K)A图解 (c) 角闪石名词术语及分类图解引自Leake et al., 1997, 2004.(c) 中其他数据来源：西南阿拉斯加杂岩体 (Himmelberg et al., 1986；Himmelberg and Loney, 1995), 岛弧堆晶岩 (Beard and Barker, 1989) Fig. 6 Composition diagrams of the amphibole (a, b), and plot of Si against cations in A-site of the amphibole (c) in the Yanmenguan mafic-ultramafic intrusion Nomenclature are after Leake et al.(1997, 2004). The amphibole data of the Alaskan-type intrusions in (c) are after Himmelberg et al.(1986), Himmelberg and Loney (1995), and those of the arc cumulates after Beard and Barker (1989) |
斜长石 (Plg) 斜长石主要见于雁门关基性岩中, 是组成辉长岩的主要矿物。斜长石的An较低 (An30.1~43.3), 为中长石 (表 5、图 4d), 比阿拉斯加型镁铁-超镁铁杂岩中的辉长-苏长岩类 (An53~82, Himmelberg and Loney, 1995) 低, 但与东比利牛斯Albera地块Hercynian期基性侵入杂岩中的部分低An斜长石相近 (Vilà et al., 2005)。
综合所述, 雁门关镁铁-超镁铁岩体中主要组成矿物的成分明显不同于地幔橄榄岩、深海橄榄岩和蛇绿岩中的同类矿物, 但与阿拉斯加型杂岩和岛弧基性侵入岩基本可以类比。
4 元素地球化学主量元素分析采用X-射线荧光光谱法 (XRF-1500 sequential spectrometer, Shimadzu), 分析精度1%~3%。稀土和微量元素分析先使用HF-HNO3熔样, 后采用ICP-MS法在VG Plasma Quad PQ2 Turbo质谱仪上分析, 标样GSR-1和GSR-3, 空白Ba和Ni < 5×10-9, Cr < 10×10-9, 其他元素均为 < 1×10-9, 分析精度REE < ±5%, 其它微量元素 < ±5%~10%。测试分析在中科院地质与地球物理研究所完成, 分析数据见表 6。
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表 6 雁门关岩体各岩石的主量 (wt%) 和微量 (×10-6) 元素成分 Table 6 Major (wt%) and trace element (×10-6) compositions of the Yanmenguan intrusion |
雁门关岩体中二辉橄榄岩的主量元素含量比较一致, SiO2为41.0%~41.6%, MgO为29.1%~29.4%, Mg#为88.4~88.9(表 6)。角闪石岩的SiO2为48.3%, TiO2为0.57%, MgO为15.7%, Mg#=82.7(表 6)。辉长岩的SiO2为49.2%~52.0%, TiO2为0.4%~1.1%, MgO为3.7%~16.4%, Mg#为56.8~83.9(表 6)。
在FeOT-MgO-Na2O+K2O图上, 雁门关岩体的大部分样品落在弧堆晶岩区 (图 7a)。在Al2O3-MgO-CaO图上, 雁门关橄榄岩、角闪石岩和辉长岩分别投在超镁铁堆晶岩和镁铁堆晶岩范围内 (图 7b)。在主量元素协变图解上 (图 8), SiO2、TiO2、Al2O3、P2O5、Na2O和K2O与MgO呈较好的负相关, FeOT则略为正相关, MnO相对稳定, 而CaO变化大, 显示各类岩石化学成分受分离结晶作用控制, 特别是受橄榄石和单斜辉石结晶的影响。
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图 7 雁门关镁铁-超镁铁岩FeOT-MgO-Na2O+K2O图 (a, 据Irvine and Baragar, 1971) 和Al2O3-MgO-CaO图 (b, 据Coleman, 1977) 图 (a) 中弧火山岩、弧堆晶岩和弧非堆晶岩区域引自Beard, 1986；图 (b) 中MP-地幔构造岩；UC-超镁铁堆晶岩；MC-镁铁堆晶岩 Fig. 7 The diagrams of FeOT-MgO-Na2O+K2O (a, after Irvine and Baragar, 1971) and Al2O3-MgO-CaO (b, after Coleman, 1977) In Fig. 7b: the fields of arc vocanics, arc cumulates and non-cumulates are after Beard, 1986 |
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图 8 雁门关镁铁-超镁铁岩MgO-其它主量元素协变图 图中各矿物分离结晶演化趋势由分离结晶公式计算而得, 主量元素分配系数根据干国樑 (1993)、Torres-Alvarado et al.(2003)、Liu et al.(2008) 和EarthRef Databases (http://www.earthref.org/databases/main.htm) 的数据统计得到 Fig. 8 Plots of MgO vs. other oxides of the Yanmenguan mafic-ultramafic intrusion showing major-element variations with fractional crystallization The trends of fractional crystallization of minerals are calculated by the formula of fractional crystallization, using bulk F=0.25 (5 steps with each step F=0.05) and the distribution coefficients obtained from the data in Gan (1993), Torres-Alvarado et al.(2003), Liu et al.(2008) and EarthRef Databases (http://www.earthref.org/databases/main.htm) with a statistic method |
雁门关二辉橄榄岩Ni含量为1210×10-6~1223×10-6, Co为123×10-6~130×10-6, REE含量为球粒陨石 (CN) 的2~8倍, REE和HFSE为原始地幔的0.8~2.8倍 (表 6)。在球粒陨石标准化的REE配分图上 (图 9a), 具微弱分异的右倾REE分布型式 ((La/Yb)CN=2.18~3.25, (La/Sm)CN=1.22~1.75, (Gd/Yb)CN=1.52~1.90), LREE轻微富集, 而HREE相对亏损, 无明显的Ce和Eu异常 ((Eu/Eu*)CN=~0.94, (Ce/Ce*)CN=1.03~1.07)。该橄榄岩的稀土特征明显不同于五台山东部原南金刚库组中的方辉橄榄岩和纯橄岩 (高过渡金属元素而极低的REE和HFSE以及明显的U型REE配分型式, Polat et al., 2005)。在原始地幔标准化的微量元素图解上 (图 9b), 略富部分LILE, Nb、Ta、Y和Ti弱负异常, Zr中等负异常。
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图 9 雁门关镁铁-超镁铁岩稀土元素球粒陨石标准化配分图 (a) 和原始地幔标准化微量元素蛛网图 (b) 阿拉斯加型岩体各类岩石数据引自Himmelberg and Loney (1995), 标准化数据引自Sun and McDonough (1989) Fig. 9 C1 chondrite-normalized REE patterns (a) and primitive mantle-normalized trace-element patterns (b) for the Yanmenguan mafic-ultramafic rocks The data of Alaskan-type mafic-ultramafic rocks are after Himmelberg and Loney (1995), and of primitive mantle and C1 chondrite after Sun and McDonough (1989) |
角闪石岩与辉长岩微量元素成分接近 (表 6), 在球粒陨石标准化的REE配分图上 (图 9a), 具右倾REE分布型式 ((La/Yb)CN=3.7~5.1, (La/Sm)CN=1.3~2.1, (Gd/Yb)CN=~1.88), LREE较富集, 而HREE相对亏损, 无明显的Ce和Eu异常 ((Eu/Eu*)CN=0.86~0.93, (Ce/Ce*)CN=0.84~1.04)。在微量元素图解上, 富集部分LILE, 中等Nb、Ta、Sr、Y和Zr亏损, 弱的Ti负异常 (图 9b)。
在REE配分图上 (图 9a), 辉长岩具右倾REE分布形式 ((La/Yb)CN=2.69~4.40, (La/Sm)CN=1.51~1.59, (Gd/Yb)CN=1.40~2.11), LREE相对HREE富集, 无明显的Ce和Eu异常 ((Eu/Eu*)CN=0.83~1.07, (Ce/Ce*)CN=0.96~1.08)。富集LILE, Nb、Y、Zr和Ti负异常明显 (图 9b)。
雁门关岩体各类岩石的REE和微量元素的特征可与阿拉斯加型杂岩体 (西南阿拉斯加镁铁-超镁铁杂岩, Himmelberg and Loney, 1995)(图 9) 和岛弧堆晶岩 (如共生于Boromo绿岩带的古元古代岛弧镁铁-超镁铁堆晶杂岩, Béziat et al., 2000) 类比。雁门关岩体各岩类明显的Nb、Ta、Y、Zr和Ti负异常显示出与俯冲带相关岩浆的特性, 因此可以推断雁门关岩体为岛弧岩浆分离结晶形成的。
5 岩石成因雁门关镁铁-超镁铁岩的大部分元素含量 (特别是REE和HFSE) 与MgO和Zr具较好的相关性 (未图示), 各类岩石具有近一致和相互平行的REE和微量元素配分型式, 因此其元素地球化学反映了其岩浆源区特征, 我们可以据此来研究其岩石成因和形成的大地构造背景。
5.1 结晶分离雁门关镁铁-超镁铁岩体各类岩石的主量元素与MgO呈良好的线性相关 (图 8)。FeOT和MgO与SiO2负相关、FeOT与MgO正相关表明橄榄石的结晶分离。在基性岩中CaO与MgO正相关、Al2O3与MgO负相关表明辉石的结晶分离。Ni、Co和Cr与MgO正相关反映了橄榄石和辉石结晶分离。岩石中普遍无明显的Eu异常说明斜长石的结晶分离不显著。
5.2 岩体成因及其大地构造背景雁门关岩体在矿物成份上, 特别是单斜辉石、尖晶石和角闪石, 具有阿拉斯加型侵入岩体和俯冲带或弧相关的岩浆特征。从其元素地球化学来看, 雁门关岩体也与阿拉斯加型侵入杂岩体具有可类比的的微量和REE特征, 并具有明显的Nd (Ta), Zr (Hf), Y和Ti负异常。从微量元素的构造环境判别图给出的信息来看, 雁门关岩体主要为与俯冲相关的板缘成因 (如Ti/Y-Zr/Y图解, Pearce and Gale, 1977；Zr-Nb/Zr图解, Thiélemont and Tegyey, 1994；未图示), 具有大陆边缘弧的性质 (图 10a, b)。因此, 雁门关岩体是侵入在大陆边缘弧环境的岩浆分离结晶的产物, 而不是蛇绿岩组分。
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图 10 Th-Hf-Ta (a, 据Wood, 1980) 和 (La/Sm)CN vs. Nb/La (b, 据John et al., 2003) 判别图解 Fig. 10 Trace element discrimination diagrams of Th-Hf-Ta (a, after Wood, 1980) and (La/Sm)CN vs. Nb/La (b, after John et al., 2003) |
在五台绿岩带的原南、北金岗库组内分布有三条NEE向的镁铁-超镁铁岩带 (图 1), 对其时代和成因主要有以下几种观点：1) 晚太古的顺层侵入体 (层状变镁铁-超镁铁质岩), 为基性玄武质岩浆 (或混染岩浆) 在上地幔高度分异后经深大断裂上升到地壳浅部侵位并就地分异冷凝而成 (穆书汉, 1983)。2) 可能为科马提岩 (白瑾, 1986；田永清, 1991)。3) 古老大洋壳的残片或太古宙蛇绿混杂岩带 (李继亮等, 1990；白瑾等, 1992；王凯怡等, 1997；Li et al., 2002, 2004；Kusky and Li, 2003；Kusky et al., 2004；Polat et al., 2005)。王凯怡等 (1997)认为该镁铁-超镁铁质岩为变质橄榄岩-辉长岩-辉绿岩墙-枕状玄武岩组合, 代表了太古宙蛇绿混杂岩带, 可能形成于弧前、弧间和弧后小洋盆。Polat et al.(2005) 对原南金岗库组中的超镁铁岩 (纯橄岩和方辉橄榄岩) 做了地球化学研究, 根据岩石强烈亏损REE和HFSE以及U型REE配分模式, 认为可能是形成于弧前环境的晚太古代蛇绿岩。4) 岛弧相关杂岩体。程素华和李江海 (2006①)对五台山东南部原南金岗库组中李福沟超镁铁质岩做了地球化学初步研究, 其岩石富镁, REE平坦, 具负Eu异常, 微量元素配分图右倾, 各元素具微弱的分异, Nb、Zr负异常, 认为可能形成于与岛弧相关的环境。
① 程素华, 李江海.2006.五台李福沟新太古代超镁铁岩特征及其意义.见:北京大学地球与空间科学学院.构造地质学新理论与新方法学术研讨会论文摘要集.构造地质学新理论与新方法学术研讨会, 北京, 2006.北京:北京大学, 53)
可以看出, 尽管对这些镁铁-超镁铁岩的成因有不同解释, 而且也没有明确的同位素年龄, 但多数研究者认为它们可能形成于晚太古代, 并把其成因与晚太古代五台绿岩带的发育联系起来。本文中的研究表明, 至少有一部分的镁铁-超镁铁岩是早元古代的, 它们具有阿拉斯加型镁铁-超镁铁岩和与俯冲带或弧相关的岩浆特征, 明显不属于太古宙蛇绿混杂岩带。考虑到我们的研究与Polat et al.(2005) 的结果有较大差异, 不排除五台绿岩带中的三条镁铁-超镁铁岩带具有不同的时代和成因。因此有必要作进一步的地质、地球化学和同位素年代学的深入研究, 才能比较全面解释这些镁铁-超镁铁岩带的时代和成因。
6.2 华北克拉通中部造山带~2.2Ga (2.1~2.3Ga) 地质事件已有大量同位素年龄数据表明 (表 7), 在华北克拉通中部造山带的吕梁-恒山-五台-阜平地区普遍存在与雁门关岩体近同时的、ca. 2.2Ga (2.1~2.3Ga) 的岩浆侵入、火山喷发和沉积事件 (Wang et al., 2010；Zhao et al., 2004, 2006, 2007), 代表了该造山带内一期重要的古元古代构造-岩浆热事件。
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表 7 吕梁-恒山-五台-阜平地区~2.2Ga (2.1~2.3Ga) 地质事件 Table 7 Summary of~2.2Ga geological events in the Lüliang-Hengshan-Wutaishan-Fuping area |
在恒山高级变质杂岩带, Kröner et al.(2005a, b) 利用U-Pb SHRIMP和单颗粒锆石蒸发法识别出约2360~2330、2250和2110Ma三期片麻状花岗岩。在五台花岗-绿岩带, 大洼梁花岗岩体的SHRIMP U-Pb年龄为2176±12Ma, 王家会粉色花岗岩三个样品的SHRIMP U-Pb年龄分别为2117±17Ma, 2116±16Ma和2084±20Ma (王凯怡和Wilde, 2002；Wilde, 2002；Wilde et al., 2005)；而滹沱群长英质凝灰岩中两个锆石的SHRIMP 207Pb/206Pb年龄分别为2180±5和2087±9Ma (Wilde et al., 2004b)。在阜平高级变质杂岩带, 南营花岗片麻岩的岩浆锆石SHRIMP U-Pb年龄分别为2024±21(花岗闪长片麻岩) 和2077±13Ma (二长花岗片麻岩)(Zhao et al., 2002)；侵位于2513±13Ma黑云母片麻岩的弱片理化片麻花岗岩的锆石SHRIMP U-Pb年龄为2045±64Ma (Guan et al., 2002)；Zhao et al.(2002) 和Guan et al.(2002) 分别获得湾子表壳岩富铝片麻岩2109±Ma和细粒副片麻岩2097±46Ma年龄。
在吕梁山杂岩带, 赵国春等测得盖家庄斑状花岗片麻岩的侵位结晶年龄为2375±10Ma (SHRIMP锆石U-Pb年龄)；与岛弧相关的赤坚岭-关帝山TTG片麻岩的年龄约为2199~2173Ma (英云闪长片麻岩SHRIMP锆石U-Pb年龄2199±11Ma, 花岗闪长片麻岩2180±7Ma和二长花岗片麻岩2173±7Ma, Zhao et al., 2008)。耿元生等 (2000)测得赤坚岭花岗片麻岩的单颗粒U-Pb年龄为2151±12和2152±35Ma；吕梁群变质流纹岩和野鸡山群含云母石英岩的单颗粒锆石U-Pb年龄分别为2099±41Ma和2124±45Ma。
所有这些年龄数据结合地球化学研究表明, 华北克拉通中部造山带的吕梁-恒山-五台-阜平地区在~2.2Ga (2.1~2.3Ga) 时处于大陆边缘弧的构造环境 (Wang et al., 2010), 这是该造山带自~2.5Ga晚太古代强烈岛弧岩浆活动后又一期重要的地壳增生事件, 这一认识还说明华北克拉通东、西部陆块至少在~2.2Ga时仍未碰撞闭合。
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