2016, Vol. 32
华北克拉通西北缘的孔兹岩带是一条古元古代碰撞构造带，是由西部陆块内北部的阴山陆块和南部的鄂尔多斯陆块碰撞形成的(Zhao et al., 1999，2003，2005，2012; Zhai and Santosh， 2011)，该孔兹岩带西段贺兰山-千里山、中段乌拉山-大青山以及东段集宁-凉城等地区出露大面积变泥质岩石(卢良兆等， 1992，1996)。有关孔兹岩带变泥质岩石的变质作用、同位素年代学等方面的研究已取得了大量的研究进展(卢良兆等， 1992，1996；Lu and Jin， 1993; Liu et al., 1998，2014; 刘福来等，2002；Wan et al., 2006，2009，2013; Dong et al., 2007，2013，2014; 董春艳等，2009；Yin et al., 2009，2011，2014; Jiao et al., 2013a，b，2015; Guo et al., 2012; Ma et al., 2012; 刘平华等，2013；蔡佳等， 2013a，b，2014，2015；Cai et al., 2014，2015)。其中，通过变质作用研究，普遍得到了近等温减压的顺时针P-T轨迹(卢良兆等，1992；Liu et al., 1993; Lu and Jin， 1993; 周喜文等，2010；Wang et al., 2011; Jiao et al., 2013a，2015; 蔡佳等，2013a；Cai et al., 2014，2015)。锆石或独居石的同位素年代学研究表明孔兹岩带变泥质岩和变基性岩中的变质锆石记录了1950~1850Ma的变质年龄(Wan et al., 2006，2009，2013; Dong et al., 2007，2013，2014; 周喜文和耿元生，2009；Li et al., 2011; Yin et al., 2009，2011；刘平华等，2013；蔡佳等，2015；Wang et al., 2015; Qiao et al., 2016)，其中~1950Ma可能代表了北部的阴山陆块和南部鄂尔多斯陆块碰撞拼合的时代(Yin et al., 2009，2011; 赵国春，2009；周喜文和耿元生，2009；Zhao et al., 2010; Dong et al., 2013)，1920~1850Ma则可能是碰撞后折返的时代(Yin et al., 2009; Jiao et al., 2013b)。
然而，前人对于孔兹岩带出露的古元古代变泥质岩石的地球化学性质和形成构造环境尚存在不同的认识。部分研究者认为其沉积于稳定大陆边缘环境(金巍等，1991；Condie et al., 1992; 卢良兆等， 1992，1996；Liu et al., 1993; Lu and Jin， 1993)，而Dan et al.(2012)提出孔兹岩带变泥质岩石的沉积物源来自2180~2000Ma的大陆弧，沉积于活动大陆边缘环境。董晓杰(2012)则认为其原岩是在大陆裂谷环境下沉积，为一伸展的构造背景。因此基于前人的研究，本文选取孔兹岩带中段乌拉山-大青山地区的变泥质岩石，包括夕线(堇青)石榴二长片麻岩和石榴黑云二长片麻岩，通过详细的野外地质观察，结合室内岩相学、岩石地球化学特征等综合研究，通过主量、微量元素分析及其地球化学特征，以恢复其原岩类型、探讨其形成时的构造环境及物源区性质等，为进一步揭示华北克拉通孔兹岩带的演化历史提供新的科学依据。
2 区域地质背景华北克拉通西北缘的孔兹岩带是位于西部陆块内的一条E-W向展布、长达1000km的古元古代碰撞构造带，由北部的阴山陆块和南部的鄂尔多斯陆块于~1950Ma碰撞拼合形成，西部陆块与东部陆块在~1850Ma碰撞形成中部造山带(Zhao et al., 1999，2002，2005; Zhao and Zhai， 2013)。孔兹岩带由西向东沿千里山-贺兰山，乌拉山-大青山和集宁-卓资-丰镇一带展布，分别出露西段千里山-贺兰山变质杂岩，中段乌拉山-大青山变质杂岩以及东段集宁变质杂岩，北邻阴山陆块，南侧与鄂尔多斯陆块相接(图 1a，b)，东侧紧邻中部造山带。
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图 1 孔兹岩带在华北克拉通的分布和构造位置图(据Zhao et al., 2005) Fig. 1 Distribution of the Khondalite Belt in the North China Craton(after Zhao et al., 2005) |
孔兹岩带出露大面积高级变质表壳岩系岩石(变泥质岩石)，主要包括(含墨)夕线石榴斜长(二长)片麻岩、黑云斜长(二长)片麻岩、黑云石榴斜长(二长)片麻岩、石榴石英岩、大理岩、磁铁石英岩、钙硅酸盐岩和石墨片麻岩，这些变质程度达到高角闪岩相-麻粒岩相的以高铝质岩石为主的变沉积岩石组合常被称为孔兹岩系(卢良兆等， 1992，1996；Liu et al., 1993; Lu and Jin， 1993; 李树勋等，1994；Li et al., 2000)。与孔兹岩带变质表壳岩系共同产出的岩石还包括少量闪长质-花岗质片麻岩、长英质副片麻岩、基性麻粒岩、斜长角闪岩以及同构造紫苏花岗岩和S型花岗岩(金巍，1989；卢良兆等， 1992，1996；刘喜山， 1994，1996；Zhao et al., 1999; 钟长汀等，2007)。近年来大量的SHRIMP与LA-ICP-MS 锆石U-Pb定年结果表明区内变泥质岩石的原岩沉积时代为2.1~2.0Ga，而变质时代为1950~1850Ma(Wan et al., 2006，2009; 吴昌华等，2006；Xia et al., 2006a，b; Dong et al., 2007，2013，2014; Yin et al., 2009，2011; 周喜文和耿元生，2009；Jiao et al., 2013b; Santosh et al., 2009，2013; 蔡佳等，2015；Wang et al., 2015; Qiao et al., 2016)。此外，孔兹岩带东段的土贵乌拉、和林格尔、土贵山和徐武家地区以及大青山地区的东坡一带零星出露超高温变质岩石(Santosh et al., 2007a，b，2012; Guo et al., 2012; Liu et al., 2012; Jiao et al., 2015)，其变质温度可达950℃以上，变质时代为1930~1920Ma(Santosh et al., 2007b)，有研究者认为区域内的超高温变质事件发生在阴山和鄂尔多斯陆块碰撞拼合之后，与碰撞后伸展环境下的地幔岩浆底侵有关(赵国春，2009；Guo et al., 2012; Wan et al., 2013)，而Peng et al.(2010，2011)所报道的土贵乌拉附近的徐武家出露~1930Ma辉长苏长岩墙群也进一步佐证了这一观点。
研究区位于孔兹岩带中段的乌拉山-大青山地区，出露的岩石类型主要为古元古代变质表壳岩系岩石(孔兹岩系)，少量古元古代(石榴)基性麻粒岩和斜长角闪岩(卢良兆等， 1992，1996；Lu and Jin， 1993; Zhao et al., 1999)、TTG质片麻岩、紫苏花岗岩、石榴黑云母花岗岩和少量(石墨)大理岩等。主要分布于柳坝沟、包头以北的哈德门沟、桃儿湾、忽鸡沟、大南沟、庙沟、五当召和鸡灯湾等地(图 2)。
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图 2 孔兹岩带乌拉山-大青山地区的地质简图及采样位置(据卢良兆等，1992) Fig. 2 Geological map of the Wulashan-Daqingshan area of the Khondalite Belt and sample localities(after Lu et al., 1992) |
在庙沟和哈德门沟等地赋存中小型石墨矿床。古元古代变质表壳岩系岩石主要包括(石榴)黑云二长/斜长片麻岩、夕线石榴堇青黑云二长/斜长片麻岩、长英质粒状岩石、(石墨)大理岩，多以互层状产出。少量古元古代(石榴)基性麻粒岩呈宽数十厘米到数米的变形岩墙(宽3~20m，长约10m)和不规则透镜体形式赋存于富铝片麻岩或(紫苏)花岗质片麻岩中(刘平华等，2013；Liu et al., 2014)。该地区的变泥质岩石普遍记录了顺时针P-T演化轨迹，峰期和峰后近等温减压变质阶段的温压条件分别为840~880℃和9~11kbar和800~870℃和5.0~7.5kbar(Cai et al., 2014)。
3 野外产状和岩相学特征孔兹岩带乌拉山-大青山地区的夕线(堇青)石榴二长片麻岩和石榴黑云二长片麻岩出露于巴彦花、哈德门沟、忽鸡沟、鸡灯湾和石拐区一带，均以似层状方式产出，其中夕线堇青石榴二长片麻岩可以夹层状产出于长英质片麻岩中，二者片麻理产状近一致。角闪二辉麻粒岩以大小不等的透镜状或似层状产出于夕线石榴二长片麻岩或(石榴)黑云斜长片麻岩中，两者的接触界线清晰(图 3a)。局部可见(含榴)长英质浅色体呈条带状、细脉状、团块状或网脉状，沿片麻理方向分布或切穿片麻理，其中石榴石呈浑圆状且粒径变化较大(0.1~0.8cm)(图 3b，c)。在夕线堇青石榴二长片麻岩中的堇青石围绕石榴石边部形成“黑眼圈”结构，长柱状的夕线石集合体呈近定向分布(图 3d)。代表性样品的显微结构特征见图 4。本文所有矿物代号均采用Whitney and Evans(2010)的资料。
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图 3 乌拉山-大青山地区变泥质岩石野外露头照片
(a)夕线石榴二长片麻岩与二辉麻粒岩呈似层状产出；(b)石榴黑云二长片麻岩；(c)夕线堇青石榴二长片麻岩与含石榴石的长英质浅色体；(d)夕线堇青石榴二长片麻岩 Fig. 3 Outcrops of the metapelitic rocks in the Wulashan-Daqingshan area (a)sillimanite-garnet gneiss interlayered with mafic granulite;(b)garnet-biotite gneiss;(c)sillimanite-cordierite-garnet gneiss associated with garnet-bearing leucosome;(d)sillimanite-cordierite-garnet gneiss |
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图 4 乌拉山-大青山地区变泥质岩石显微结构照片
(a)石榴石变斑晶与基质黑云母、长石和石英共生(单偏光)；(b)石榴石边部围绕柱状夕线石、黑云母和钛铁矿(单偏光)；(c)石榴石与基质夕线石、黑云母和石英共生，部分石榴石边部围绕堇青石(单偏光)；(d)石榴石边部围绕堇青石和黑云母，堇青石内包含细粒夕线石、黑云母和石英(正交偏光) Fig. 4 Photomicrographs of the metapelitic rocks from the Wulashan-Daqingshan area (a)photomicrograph(PPL)of porphyroblastic garnet associated with matrix biotite，feldspar，and quartz;(b)photomicrograph(PPL)of garnet porphyroblast surrounded by prismatic sillimanite，biotite，and ilmenite;(c)photomicrograph(PPL)of garnet associated with matrix sillimanite，biotite，and quartz，and garnet rimmed by a corona of cordierite;(d)garnet mantled by symplectites of cordierite，and minute sillimanite，biotite and quartz enclosed in cordierite |
夕线(堇青)石榴二长片麻岩的主要组成矿物有石榴石、长石、石英、堇青石、黑云母和夕线石等，并含少量尖晶石、褐红色金红石、锆石、独居石、磁铁矿和钛铁矿。石榴石变斑晶呈浑圆状或拉长状，粒径为0.2~0.8cm(图 4b-图 d)，可包裹细粒的石英、长石、黑云母和细长针状夕线石等矿物。部分石榴石边部被大量粗粒黑云母和夕线石围绕(图 4c)，石榴石边部可形成含堇青石的后成合晶结构，堇青石内含有细粒夕 线石、黑云母、石英和Fe-Ti氧化物等(图 4b-d)。基质中的 夕线石呈针-柱状集合体，与黑云母、长石和石英共生(图 4b，c)。
石榴黑云二长片麻岩的主要组成矿物有石榴石、长石、石英、黑云母，副矿物为锆石、磁铁矿和钛铁矿等(图 4a)。其中石榴石变斑晶呈浑圆状，粒径变化较大(0.1~1.2cm)，含量6%~8%。部分石榴石可包裹黑云母等矿物(图 4a)。
4 岩石地球化学特征选取13件代表性样品，包括12件夕线(堇青)石榴二长片麻岩(BH1-1、BH3-3、BH28-2、BH27-1、BH27-2、BH39-4、BT16-6、BT20-1、BT35-1、P01-2、P01-6、P01-10)和1件石榴黑云二长片麻岩(BH23-1)。考虑到研究区岩石普遍遭受麻粒岩相变质作用(卢良兆等，1992)，部分活动性较强的元素如Cs、Rb、K、Ba、U、Th等容易受到熔体、晚期流体作用等的影响(Rollinson，1993)。然而，有研究者认为部分元素比值如K2O/Na2O和TiO2/Al2O3受高级变质作用的影响相对较小(Fralick and Kronberg， 1997; Roser and Korsch， 1986)。Mg、Fe、Ca、Mn、Cr、Ni、V、Co、Sc、Nb、Ta、Y及HREE属于不活动元素，不易受到化学风化作用的影响，Si、Ti、Al活动性较弱，可视为不活泼元素，而Na、Sr、Zr、Hf的地球化学行为介于活动元素和不活动元素之间，属于准不活泼元素(Rudnick et al., 1985; McLennan，1989; Rollinson，1993)。因此，本文尽量使用不活动元素进行原岩恢复和构造环境的讨论，而避免使用活动性强的元素，并结合多种构造判别图解。
选取样品新鲜的部分，在河北省区域地质调查所矿物分选实验室将全岩粉末样品磨制至200目，随后利用国家地质实验测试中心3080E型荧光光谱仪XRF和等离子质谱仪(ICP-MS)进行全岩主量元素、微量元素和稀土元素的测试分析，其中主量元素测试采用常规的XRF方法分析，微量和稀土元素均采用等离子体质谱仪(ICP-MS)进行分析。具体测试条件和分析步骤可参阅靳新娣和朱和平(2000)的论述。
4.1 主量元素特征
乌拉山-大青山地区代表性变泥质岩石样品的主量元素化学成分和元素比值的分析结果(表 1)显示，岩石明显富Al2O3(15.14%~25.70%，平均为18.11%)，铝指数A/CNK为1.26~5.40，属于过铝质岩石。SiO2含量为52.13%~68.66%(平均61.61%)，Fe2O3T含量为3.31%~11.37%(平均6.88%)、MgO含量为1.32%~4.67%(平均2.99%)、Na2O含量为1.43%~3.95%(平均2.91%)、K2O的含量为1.51%~5.38%(平均3.08%)。K2O/Na2O 比值为0.45~2.24，SiO2/Al2O3比值变化为2.03~4.54(平均3.43)。其地球化学特征与华北克拉通其它高级变泥质岩石十分相似(卢良兆等，1996)。Mg#=100×MgO/(MgO+FeO)在28~43之间变化。总体来看，上述样品的平均化学成分与上地壳平均值(UCC；Taylor and McLennan， 1995)较为接近，较北美页岩(NASC)的SiO2、CaO和K2O含量略低，而Al2O3、MgO和Na2O含量略高(Gromet et al., 1984)。此外，TiO2、Al2O3、Fe2O3T、MgO与SiO2均显示出明显的负相关关系(图 5)，反映随着研究区变泥质岩原岩成熟度的增加，不稳定组分(如岩屑成分和长石)含量逐渐减少(Bhatia， 1983，1984; Camiré et al.， 1993)。K2O、Na2O、CaO与SiO2均无相关性。
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表 1 乌拉山-大青山地区变泥质岩石主量元素(wt%)、稀土元素和微量元素(×10-6)含量 Table 1 Major elements(wt%)and trace elements(×10-6)of the metapelitic rocks in the Wulashan-Daqingshan area |
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图 5 乌拉山-大青山地区变泥质岩石的SiO2与TiO2、Al2O3、Fe2O3T、MgO的相关图解 Fig. 5 Plots of SiO2 vs. TiO2，Al2O3，Fe2O3T and MgO for the metapelitic rocks in the Wulashan-Daqingshan area |
研究区典型的孔兹岩系样品在野外多以似层状产出，含有特征变质矿物夕线石和堇青石等，表明其原岩中粘土矿物含量较高。进一步利用A-C-FM图解(图 6，转王仁民等，1987)进行原岩恢复，样品点落在富铝粘土岩和杂砂岩区。Simonen(1953)的尼格里参数(al+fm)-(c+alk)-Si原岩判别图解(图 7a)显示，样品点主要落在泥质沉积岩的区域。Herron(1988)提出的碎屑岩类型地球化学分类图解(图 7b)表明样品的原岩主要为页岩和硬砂岩，以及少量铁页岩。La/Yb-∑REE原岩判别图解(图 7c；Gromet et al., 1984)也显示样品点落于页岩粘土岩区和砂岩区。La-Th图解(图 7d；McLennan et al., 1990)指示样品的原岩主要为晚太古代沉积岩。
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图 6 乌拉山-大青山地区变泥质岩石的A-C-FM原岩恢复图解(转王仁民等，1987) Fig. 6 A-C-FM classification diagram for the metapelitic rocks from the Wulashan-Daqingshan area(secondary source after Wang et al., 1987) |
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图 7 乌拉山-大青山地区变泥质岩石的地球化学图解
(a)(al+fm)-(c+alk)-Si尼格里参数图解(Simonen，1953)；(b)log(Fe2O3/K2O)-log(SiO2/Al2O3)图解(据Herron，1988)；(c)La/Yb-∑REE图解(Gromet et al., 1984)；(d)La-Th图解(McLennan et al., 1990) Fig. 7 Classification diagrams of the metapelitic rocks in the Wulashan-Daqingshan area (a)(al+fm)-(c+alk)-Si(Simonen，1953);(b)log(Fe2O3/K2O)vs. log(SiO2/Al2O3)(Herron，1988);(c)La/Yb vs. ∑REE(Gromet et al., 1984);(d)La-Th(McLennan et al., 1990) |
研究区变泥质岩石样品的稀土元素(REE)分析数据列于表 1。结果显示稀土元素总量(∑REE)在187.1×10-6~389.3×10-6之间，具有中等程度的轻重稀土元素分异，其中，轻稀土明显富集，重稀土相对亏损，(La/Yb)N比值变化于8.89~28.5之间，具有负铕异常，δEu(Eu/Eu*)值变化于0.42~0.82之间(表 1；图 8a)。样品的稀土元素配分型式，与北美页岩(NASC；Gromet et al., 1984)、太古宙后澳大利亚平均页岩(PAAS；McLennan，1989)和上地壳平均成分(Taylor and McLennan， 1995)相似，表明研究区变泥质岩石成分类似上地壳岩石。负铕异常表明其物源中可能有成熟度较低且年轻的弧物质加入(Taylor and McLennan， 1985)。样品的稀土元素总量与Al2O3、K2O无明显的相关性，表明粘土矿物不是稀土元素主要的赋存矿物，稀土元素可能主要赋存于副矿物中(Gromet et al., 1984; Bhat and Ghosh， 2001)。
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图 8 乌拉山-大青山地区变泥质岩石的球粒陨石标准化稀土元素配分图(a)和原始地幔标准化微量元素蛛网图(b)(标准化值据Sun and McDonough， 1989) Fig. 8 Chondrite-normalized rare earth element patterns(a)and primitive mantle-normalized trace element patterns(b)for the metapelitic rocks in the Wulashan-Daqignshan area(normalization values after Sun and McDonough， 1989) |
大离子亲石元素(Rb、Cs、Ba和Sr)研究区变泥质岩石中大离子亲石元素Rb的含量为40.6×10-6~171×10-6、Cs含量为0.39×10-6~8.04×10-6、Ba和Sr含量均变化较大，分别为550×10-6~1492×10-6和106×10-6~707×10-6(表 1)。除去样品石榴黑云二长片麻岩(BH23-1)和夕线(堇青)石榴二长片麻岩(BH39-4)的Rb含量较高，其它样品的Rb含量相比北美页岩(125×10-6，Gromet et al., 1984)均偏低。此外，除去夕线(堇青)石榴二长片麻岩(BH39-4)的Cs含量较高，其它样品的Cs含量相比北美页岩(5.16×10-6，Gromet et al., 1984)亦偏低。Ba和K2O具有较好的正相关性，表明其原岩中富钾的粘土矿物，如伊利石和绢云母等控制了该元素的含量(Nesbitt et al., 1980; Feng and Kerrich， 1990)。
亲镁铁矿物微量元素(Cr、Ni、Co和Sc)Cr、Co、Ni和V为相容元素，具有相似的地球化学性质，在风化沉积过程中常发生分异(Bhatia and Crook， 1986; Feng and Kerrich， 1990)。样品中的Cr(97×10-6~386×10-6)和Ni(37.6×10-6~149×10-6)含量变化较大，Co和Sc的含量分别为11.5×10-6~36.8×10-6和10.6×10-6~27.2×10-6(表 1)。Cr/Ni比值为1.28~6.68，Ni/Co比值为1.54~4.05，Sc/Ni比值为0.18~0.69、Sc/Cr比值为0.05~0.19，变化范围均较大。其中，Cr和Ni之间无相关性，表明两者在风化过程中发生分异。Cr、Ni与MgO的相关性均较明显，而Cr、Ni与Al2O3无相关性，表明Cr和Ni的含量在风化过程中可能受镁铁质矿物的控制。而Sc与Al2O3呈明显正相关性，表明Sc可能是吸附在粘土矿物上发生沉积的。
高场强元素(Zr、Hf、Nb、Ta、Y、Th和U)高场强元素在变泥质岩原岩的风化沉积过程中相对稳定，可以较好地反映源区特征(Winchester and Floyd， 1977; Taylor and McLennan， 1985; Bhatia and Crook， 1986)。样品中的高场强元素Zr(159×10-6~290×10-6)、Hf(4.47×10-6~8.51×10-6)、Nb(6.42×10-6~33.2×10-6)、Ta(0.22×10-6~2.94×10-6)、Y(13.5×10-6~38×10-6)、Th(9.64×10-6~31.6×10-6)、U(1.04×10-6~4.55×10-6)的平均值与上地壳平均值(Taylor and McLennan， 1995)接近。其中Nb、Ta、P和Ti在微量元素蛛网图上显示出明显亏损的特征(图 8b)。此外，Zr与Hf、Nb与Ta之间有很好的相关性，表明这些元素对有相似的地球化学行为，且在风化和沉积过程没有发生分异。Zr、Nb、Y与SiO2、Al2O3无相关性，表明风化、分选和沉积作用对其影响很小，代表了沉积物源区的特征(Bhatia and Crook， 1986)。Th与K2O、Al2O3无明显的相关性，表明粘土矿物并非Th的赋存矿物，而U与K2O有较好的相关性，Zr与Th无相关性。轻稀土元素与Th有较好的相关性，表明独居石可能是Th的赋存矿物。Th和U的相关性较好，Th/U比值为5.84~17.57(平均值为10.06)，高于上地壳平均值(3.8；Taylor and McLenann， 1995)，表明二者具有相似的地球化学性质。
5 讨论 5.1 源区风化程度风化作用对乌拉山-大青山地区变泥质岩石原岩中的碎屑矿物含量和元素地球化学组成有重要影响(Nesbitt and Young， 1982，1984; McLennan，1993; Fedo et al., 1995)。上地壳岩石在化学风化作用下，长石容易发生蚀变并形成伊利石和高岭石等粘土矿物，因而长石中的碱性元素如Ca、Na、K和大离子亲石元素易被带走，最终导致Al2O3和TiO2等元素的比例增加(Nesbitt et al., 1980; McLennan et al., 1993)。因此，可以利用CIA(Chemical Index of Alteration)指数来评价源岩风化程度，它反映了碎屑沉积岩中长石和粘土矿物比例的变化(Nesbitt and Yong， 1982)，CIA指数越大，表明源岩的风化程度越高，计算公式为CIA=(Al2O3/(Al2O3+CaO*+Na2O+K2O))×100(采用摩尔数计算)，其中CaO*为岩石中硅酸盐组分所含CaO的摩尔数，可以采用P2O5来校正磷酸盐中的CaO，CaO* *=CaO-(P2O5×10/3)(用摩尔数计算)(McLennan，1993; Panahi et al., 2000)。
如校正后的CaO* *的摩尔数小于Na2O的摩尔数，则采用校正的CaO* *作为CaO*的摩尔数；如果校正后的CaO* *的摩尔数大于Na2O的摩尔数，则采用Na2O的摩尔数作为CaO*的摩尔数(Bock et al., 1998)。除样品BH28-2(CIA值为83.25)外，其他样品的CIA值为52.99~69.89(平均值为59.41)，基本低于太古宙后澳大利亚平均页岩和北美页岩的CIA值(70和68)，反映其源岩经历了中等程度的风化作用(Nesbitt and Young， 1982; Taylor and McLennan， 1985; Fedo et al., 1996)。
Al2O3-(CaO*+Na2O)-K2O(A-CN-K)图解(图 9；Fedo et al., 1995)可以反映沉积岩的风化特征，理想的风化趋势近平行于A-CN边，结果表明多数样品的风化程度明显低于平均页岩，表明其风化阶段主要位于斜长石向粘土矿物转变(LaMaskin et al., 2008)，并且样品点基本沿A-CN边分布，表明变泥质岩源岩的成岩作用发生之前，风化作用并未造成钾组分的大量流失，基本保持原始沉积环境中的钾含量。
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图 9 乌拉山-大青山地区变泥质岩石的Al2O3-(Na2O+CaO*)-K2O图解(据Fedo et al., 1995) Fig. 9 Ternary plot of Al2O3-(Na2O+CaO*)-K2O for the metapelitic rocks in Wulashan-Daqingshan area(after Fedo et al., 1995) |
一般认为SiO2/Al2O3和La/V比值可以有效的判别沉积物的成熟度(Roser and Korsch， 1986)。样品的SiO2/Al2O3(2.03~4.54)和La/V(0.27~1.19)比值均较低，表明变泥质岩原岩的成熟度较低，为近源沉积。Cox et al.(1995)提出成分变化指数(Index of Compositional Variation，缩写为ICV)可以用于指示碎屑岩的成分成熟度，ICV越高，碎屑岩的成分成熟度越低，反映构造背景趋于不稳定。具体的计算公式为ICV=(Fe2O3T+Na2O+K2O+CaO+MgO+MnO+TiO2)/Al2O3(以重量百分比计算)。研究区样品的ICV值为0.77~1.32(平均值为1.04)，表明其成熟度较低，可能形成于构造活动区。
5.3 物源区性质A-CN-K图解除了可以直观地反映源岩风化程度，还可以限定源岩的成分。在没有K交代的情况下，数据点的延长线与钾长石-斜长石连线的交点可以反映未风化的源岩中两种长石的比例，以获得源岩的成分(Fedo et al., 1995)。如图 9所示，样品的源岩主要以花岗质岩石为主(英云闪长岩-花岗闪长岩-花岗岩)，为其提供主要的碎屑沉积物。
样品的Al2O3/TiO2为16.48~38.25(平均值为23.91)，指示其主要来源于长英质岩石而非铁镁质岩石(Girty et al., 1996; Hayashi et al., 1997)。此外，碎屑岩中的稀土元素、微量元素(如Th、U、Sc和高场强元素)对物质源区也有指示意义(Taylor and McLenann， 1985; Bhatia and Crook， 1986; McLennan et al., 1990，1995)，这些元素在沉积过程中具有弱的活动性且难溶于水，在风化搬运过程中几乎没有变化，可以较好地反映物源区性质(Bhatia and Crook， 1986)。此外，基性岩和长英质源岩的Eu/Eu*、La/Sc和Th/Sc比值不同，可以据此指示沉积岩的物源(Taylor and Mclennan， 1985; McLennan and Taylor， 1991)。除样品BH23-1(Th/Sc比值较高，为2.98)外，其它样品的Th/Sc比值在0.47~1.34之间(平均值为1.01)，接近上地壳的Th/Sc值(为1.0)，表明源区物质以长英质为主。为进一步揭示乌拉山-大青山变泥质岩石的物源区性质，将样品投入La/Th-Hf源区判别图解(图 10，Floyd and Leveridge， 1987)，样品的La/Th比值(2.54~5.20)和Hf含量(4.47~8.51)较低，均落入长英质源区附近，靠近上地壳平均成分，说明其源岩物质来自上地壳，以长英质岩石为主，可能有古老沉积物的加入(Floyd and Leveridge， 1987; Gu et al., 2002)。
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图 10 乌拉山-大青山地区变泥质岩样品的La/Th-Hf图解(据Floyd and Leveridge， 1987) Fig. 10 La/Th-Hf source rock discrimination diagram for the metapelitic rocks in the Wulashan-Daqingshan area(after Floyd and Leveridge， 1987) |
如前所述，部分研究者认为孔兹岩带出露的古元古代变泥质岩石原岩形成于(1)稳定大陆边缘环境(金巍等，1991；Condie et al., 1992; Liu et al., 1993; Lu and Jin， 1993)；(2)形成于活动大陆边缘环境，物源来自2.18~2.00Ga的大陆弧(Dan et al., 2012)；(3)形成于大陆裂谷环境，为一伸展的构造背景(董晓杰，2012)。
变泥质岩的原岩形成于不同的构造环境会具有不同的地球化学特征，可利用其地球化学特征判别沉积构造环境(Maynard et al., 1982; Bhatia， 1983，1984; Taylor and McLennan， 1985; Bhatia and Crook， 1986; Roser and Korsch， 1986)。其中，Roser and Korsch(1986)根据碎屑岩中K2O/Na2O-SiO2关系，提出区分被动大陆边缘(PM)、活动大陆边缘(ACM)、岛弧(ARC)三种构造环境的判别图解。在K2O/Na2O-SiO2图中，样品点主要位于活动大陆边缘区内，少数样品位于岛弧环境区(图 11a)。在Maynard et al.(1982)提出的SiO2/Al2O3-K2O/Na2O双变量图解(图 11b)中，样品点主要落入活动大陆边缘区，部分样品落入演化的岛弧环境区。考虑到K、Na等主量元素的活动性，很多微量元素如稀土元素、Th、Nb、Y、Zr、Sc等具有不活动性，适合于沉积源区和构造环境的判别(Bhatia，1984; Bhatia and Crook， 1986; McLennan et al., 1993)。利用不活动元素La-Th-Sc(图 11c，Bhitia and Crook， 1986)和Th-Sc-Zr/10(图 11d，Bhitia and Crook， 1986)判别图解进行验证，结果显示样品点同样多位于活动大陆边缘(ACM)和大陆岛弧(CIA)区域。类似的，前人所报道的孔兹岩带变泥质岩石(石榴黑云片麻岩、夕线堇青石榴片麻岩)样品也主要落于活动大陆边缘(ACM)和大陆岛弧(CIA)区域内(Condie et al., 1992; 卢良兆等，1992；李树勋等，1994；董晓杰，2012；蔡佳，未发表数据)。综合研究认为研究区变泥质岩石主要形成于有演化岛弧发育的活动大陆边缘环境。
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图 11 乌拉山-大青山地区变泥质岩石的构造环境判别图解
(a)K2O/Na2O-SiO2图解(Roser and Korsch， 1986)；(b)SiO2/Al2O3-K2O/Na2O(Maynard et al., 1982)；(c)La-Th-Sc图解(Bhatia and Crook， 1986)；(d)Th-Sc-Zr/10图解(Bhatia and Crook， 1986).PM-被动大陆边缘；ACM-活动大陆边缘；ARC，A1-岛弧；A2-演化岛弧；CIA-大陆岛弧；OIA-大洋岛弧 Fig. 11 Discrimination diagrams of the metapelitic rocks in the Wulashan-Daqingshan area (a)K2O/Na2O-SiO2(Roser and Korsch， 1986);(b)SiO2/Al2O3-K2O/Na2O(Maynard et al., 1982);(c)La-Th-Sc(Bhatia and Crook， 1986);(d)Th-Sc-Zr/10(Bhatia and Crook， 1986). PM-Passive Continental Margin; ACM-Active Continental Margin; ARC，A1-Island Arc; A2-Evolved Island Arc; CIA-Continental Island Arc; OIA-Oceanic Island Arc |
如前所述，前人对孔兹岩带出露的变泥质岩石的变质作用研究普遍得到近等温减压型顺时针P-T演化轨迹(图 1；周喜文和耿元生，2009; Yin et al., 2014; Jiao et al., 2013a，2015; Cai et al., 2014，2015; Wang et al., 2011)，顺时针P-T轨迹可能与地壳挤压增厚有关，反映碰撞造山过程中，地壳挤压增厚并随后折返至地表的动力学过程(England and Thompson， 1984; Condie et al., 1992; Brown，1993)，进一步支持了孔兹岩带的碰撞拼合模式(Zhao et al., 2005)，表明华北克拉通西部的阴山陆块和鄂尔多斯陆块间的俯冲-碰撞作用造成陆壳加厚。另外，孔兹岩带高级变泥质岩石和变基性岩的锆石同位素年代学研究结果总结(表 2；Wan et al., 2006，2009，2013; 吴昌华等，2006；Dong et al., 2007，2013，2014; 周喜文和耿元生，2009；Li et al., 2011; Yin et al., 2009，2011；刘平华等，2013；Jiao et al., 2013b，2015; 蔡佳等， 2014，2015；Wang et al., 2015; Qiao et al., 2016)以及变泥质岩的继承性碎屑锆石和变质锆石年龄直方图(图 12、图 13)综合显示，孔兹岩带高级变泥质岩的继承性碎屑锆石的主要峰值年龄为~2030Ma，并含有~2320Ma和~2430Ma两个次要峰值年龄，表明中元古代物质为其提供主要物质来源，其最大沉积时代应为~2000Ma。Wang et al.(2015)提出处于活动大陆边缘下的中元古代(2.1~2.0Ga)岩浆弧提供了孔兹岩带东缘变泥质岩石的沉积物源。孔兹岩带高级变泥质岩的变质锆石直方图显示~1950Ma，~1880Ma和~1850Ma三个年龄峰值，其中~1950Ma可能代表了北部的阴山陆块和南部鄂尔多斯陆块碰撞拼合的时代(Yin et al., 2009，2011; 赵国春，2009；周喜文和耿元生，2009；Zhao et al., 2010; Dong et al., 2013)，1920~1880Ma则可能是碰撞后折返的时代，~1850Ma可能代表了进一步折返的时代(Yin et al., 2009; Jiao et al., 2013b)。
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表 2 乌拉山-大青山孔兹岩带变质岩石锆石年龄总结 Table 2 Summary of zircon ages of metamorphic rocks inthe Khondalite Belt |
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图 12 乌拉山-大青山变泥质岩石的继承性碎屑锆石207Pb/206Pb年龄直方图 Fig. 12 Histogram of the 207Pb/206Pb ages of detrital zircons from the metapelitic rocks in the Wulashan-Daqingshan area |
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图 13 乌拉山-大青山变泥质岩石的变质锆石207Pb/206Pb年龄直方图 Fig. 13 Histogram of the 207Pb/206Pb ages of the metamorphic zircons from the metapelitic rocks in the Wulashan-Daqingshan area |
综上表明乌拉山-大青山变泥质岩石形成于有演化岛弧发育的活动大陆边缘环境，中元古代物质(2.1~2.0Ga)为其提供主要沉积物源，随后卷入了~1950Ma华北克拉通西部古老陆块之间的碰撞造山作用，经历了大规模麻粒岩相变质作用，并于1920~1850Ma发生碰撞后折返至地表。
6 结论对乌拉山-大青山地区古元古代变泥质岩石详细的野外地质观察、岩相学以及岩石地球化学等方面的综合分析研究，得出以下几点认识：
(1) 乌拉山-大青山地区夕线(堇青)石榴二长片麻岩和石榴黑云二长片麻岩富集大离子亲石元素，Zr和Hf含量较低，具有轻稀土元素富集和Eu负异常的特征；
(2) 研究区变泥质岩石的原岩为一套粘土岩或杂砂岩，物源以上地壳的长英质成分为主。稀土配分模式与太古宙后澳大利亚平均页岩和上地壳平均成分类似；
(3) 该变泥质岩石的源岩经历了中等程度的风化作用，沉积于有演化岛弧发育的活动大陆边缘环境。
(4) 乌拉山-大青山变泥质岩石原岩形成于有演化岛弧发育的活动大陆边缘环境，随后卷入了华北克拉通西部古老陆块之间的碰撞造山作用，并经历了麻粒岩相变质作用后折返至地表。
致谢 中国地质科学院地质研究所刘超辉副研究员和北京科技大学的肖玲玲老师在野外提供了很大的帮助；审稿人提出了建设性的修改意见；在此一并表示衷心感谢。
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