2017, Vol. 33
2. 中国地质科学院地质研究所, 中国地质调查局大陆动力学研究中心, 北京 100037;
3. 中国地质大学, 北京 100083;
4. 四川省地质调查院, 成都 610081;
5. 重庆市地质矿产勘查开发局川东南地质大队, 重庆 400038
2. Centre for Continental Dynamics, China Geological Survey, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
3. China University of Geosciences, Beijing 100083, China;
4. Sichuan Institute of Geological Survey, Chengdu 610081, China;
5. Southeast Sichuan Geological Team, Chongqing Municipal Bureau of Geology and Mineral Exploration and Development, Chongqing 400038, China
花岗岩在造山带中具有重要地位，其成因与造山带的形成和演化有着密切的联系，通过对花岗岩的研究可以更深入地了解汇聚板块边缘和大陆内部壳幔相互作用及地球动力学过程(Castro et al., 1991, 1995; 王涛, 2000; 肖庆辉等, 2007; 张旗等, 2008; Jia et al., 2016)，同时对探讨造山带构造演化和大陆地壳的生长、演化等方面具有非常重要的意义(Rudnick and Gao, 2003; Ma et al., 2004; Yu et al., 2014)。
北祁连造山带是祁连造山带的一部分，是一条典型的早古生代大洋型缝合带，完整地记录了北祁连洋的俯冲过程，及随后祁连地块与阿拉善地块的碰撞造山作用(许志琴等, 2006; 吴才来等, 2006; 于胜尧等, 2007; Song et al., 2013)。前人对北祁连造山带内花岗岩所做的大量研究工作主要集中在中段和东段，较少涉及靠近阿尔金断裂的西段花岗岩(吴才来等, 2010; 秦海鹏, 2012; Chen et al., 2014; Yu et al., 2015a)。李建锋等(2010)认为石包城地区出露的多个面积较小的花岗质岩体均为435Ma的花岗闪长岩，形成于岛弧环境，并且地球化学数据显示其具有高Sr(平均416×10-6)低Yb(平均4.2×10-6)的埃达克质特征。但笔者认为石包城地区西侧的岩体存在明显的两期岩浆活动，岩性分别为花岗闪长岩和钾长花岗岩，东侧的小岩体为花岗岩，并在岩相学、年代学和地球化学上(Sr含量平均为241×10-6)与西侧的复式岩体和李建锋等(2010)认为的花岗闪长岩有明显的差别。因此本文将对北祁连造山带内石包城地区的一类花岗岩进行年代学和成因方面的研究，希望可以为重建俯冲-增生/碰撞造山格架提供线索，为明确古生代期间该地区的构造发展演化时限，重建东亚和冈瓦纳古陆的构造演化做出贡献。
2 地质背景及岩相学祁连造山带作为中央造山带的重要组成部分，多年来一直备受中外地质学家关注(图 1a, b)。祁连造山带为早古生代造山带，具有完整的古生代板块俯冲、碰撞等岩石学记录。祁连造山带(及相邻地区)由北往南可分为阿拉善地块、北祁连造山带、祁连陆块、柴北缘超高压变质带和柴达木盆地，其中北祁连造山带被夹持于阿拉善地块和祁连地块之间(Yu et al., 2012, 2015b; Song et al., 2013)。
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图 1 北祁连造山带早古生代花岗岩分布和区域地质简图(b, 据Song et al., 2013) Fig. 1 Distribution of Early Paleozoic granites and geological sketch map of the North Qilian orogenic belt (b, after Song et al., 2013) |
北祁连造山带呈NW-SE走向(图 1b)，由蛇绿岩、高压(HP)/低温(LT)变质岩(榴辉岩和蓝片岩)、中酸性火山岩、少量基性火山岩、侵入岩，志留纪复理石和泥盆纪磨拉石建造组成(夏林圻等, 2003; 张建新等, 2010; Zhang et al., 2007, 2012; Song et al., 2007, 2013; Yu et al., 2015a)。南北两条呈NW带状展布的蛇绿岩带主要岩石类型包括橄榄岩、基性-超基性堆晶岩、辉绿岩墙、枕状玄武质熔岩等(史仁灯等, 2004; 侯青叶等, 2005; Tseng et al., 2007; Song et al., 2009b, 2013; 夏小洪和宋述光, 2010; Xia et al., 2012; Chen et al., 2014)。北祁连造山带广泛出露的花岗岩时代自520Ma至383Ma，岩石组合类型多样，闪长岩、二长岩、英云闪长岩、花岗闪长岩、花岗岩和钾长花岗岩等均有发育。
石包城地区的多个花岗质岩体位于阿尔金主断裂南侧，北祁连造山带西段，被榆林河谷中新生代沉积物分为若干个出露面积较小的岩体(图 1b)。作为研究对象的花岗岩体呈EW向展布，周围为新生代沉积物覆盖，岩体长约2km，宽约0.5km，局部见混合岩化。花岗岩呈肉红色，块状构造(图 2a, b)，主要矿物为钾长石(45%)、斜长石(25%)、石英(20%)、黑云母(5%)、少量角闪石和白云母(图 2b-d)。钾长石呈自形-半自形，可见格子双晶，具有不同程度风化蚀变，可见高岭土化(图 2d)；斜长石自形程度高，呈板状，可见简单双晶和双晶纹细且密的聚片双晶，轻度绢云母化(图 2c)；石英呈他形充填于其它矿物之间，另可见呈滴状交代钾长石和斜长石(图 2c)；黑云母多色性较强，呈片状，有聚集现象(图 2c)；角闪石晶型破碎，正交偏光下呈棕色、褐色和深绿色，并遭受蚀变(图 2d)；白云母呈片状，部分呈现典型次生特征(图 2d)。
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图 2 北祁连造山带石包城地区花岗岩野外(a、b)及镜下(c、d)照片 Kfs-钾长石；Pl-斜长石；Qz-石英；Bt-黑云母；Amp-角闪石；Ms-白云母(据Whitney and Evans, 2010) Fig. 2 Outcrops (a, b) and photomicrographs (c, d) of Shibaocheng granite in the North Qilian orogenic belt Kfs-K-feldspar; Pl-plagioclase; Qz-quartz; Bt-biotite; Amp-amphibole; Ms-muscovite (after Whitney and Evans, 2010) |
锆石分选工作由河北廊坊区调院完成，样品破碎至80~120目，经淘洗粉尘、去除磁性矿物、重液分选等程序，在双目镜下人工挑出锆石; 由中国地质科学院地质研究所大陆动力学实验室完成锆石与标样环氧树胶浇铸，制成薄片、抛光，拍透反射照片及阴极发光照片等程序。测试分析在中国地质科学院地质研究所完成，锆石U-Pb定年工作所用的MC-ICP-MS为美国Thermo Fisher公司最新一代Neptune Plus型多接收等离子体质谱仪。采用的激光剥蚀系统为美国Coherent公司生产的GeoLasPro 193nm。激光剥蚀所用斑束直径为32μm，频率为10Hz，能量密度约为2.5J/cm2，以He为载气。结合锆石反射、透射照片，避开锆石内部裂隙和包裹体。实验标样为91500，206Pb/238Pb年龄的加权平均值误差为±1σ。详细实验测试过程可参见侯可军等(2009)。数据处理采用ICPMSDataCal程序(Liu et al., 2010)，测量过程中绝大多数分析点206Pb/204Pb>1000，未进行普通铅校正，204Pb由离子计数器检测，204Pb含量异常高的分析点可能受包体等普通Pb的影响，对204Pb含量异常高的分析点在计算时剔除，锆石年龄谐和图用Isoplot(Ludwig, 2003)程序获得。
3.2 主、微量元素分析本项研究所选新鲜样品的岩石粉末碎样、化学全分析工作分别在河北廊坊区调院和河北廊坊物化探研究所(实验室)完成，氧化物用X荧光光谱仪3080E测试，执行标准分别为：Na2O、MgO、Al2O3、SiO2、P2O5、K2O、CaO、TiO2、MnO、Fe2O3按GB/T 14506.28—1993标准；H2O+按GB/T 14506.2—1993标准；CO2按GB 9835—1988标准；LOI按LY/T 1253—1999标准。分析的相对标准偏差小于2%~8%。稀土元素La、Ce、Pr、Nd、Sm、Eu、Gd、Tb、Dy、Ho、Er、Tm、Yb、Lu、Y和微量元素Cu、Pb、Th、U、Hf、Ta、Sc、Cs、V、Co、Ni用等离子质谱(ICP-MS)Excell测试，执行标准为DZ/T 0223—2001；微量元素Sr、Ba、Zn、Rb、Nb、Zr、Ga用X荧光光谱仪2100测试，执行JY/T 016—1996标准。分析精度大多数元素可达到10-8，少量元素为10-6(Zr、Ba)和10-7(Hf、Nb)，其相对标准偏差小于10%。
3.3 LA-ICP-MS Hf同位素锆石Hf同位素的分析是在前述锆石U-Pb同位素基础上完成的，测试在中国地质科学院地质研究所Neptune Plus型多接收等离子质谱和GeoLasPro 193nm激光剥蚀系统(LA-MC-ICP-MS)上进行的，实验过程中采用He作为剥蚀物质载气，根据锆石大小，剥蚀直径采用44μm，测定时使用锆石国际标样GJ-1作为参考物质。相关仪器运行条件及详细分析流程见侯可军等(2007)。分析过程中锆石标准GJ-1的176Hf/177Hf测试加权平均值分别为0.282007±0.000025(2σ)。计算初始176Hf/177Hf时，Lu的衰变常数采用1.865×10-11a-1 (Scherer et al., 2001)，εHf(t)值的计算时采用球粒陨石Hf同位素值176Lu/177Hf=0.0336，176Hf/177Hf=0.282785(Bouvier et al., 2008)。在Hf的地幔模式年龄计算中，亏损地幔176Hf/177Hf现在值采用0.28325，176Lu/177Hf采用0.0384(Griffin et al., 2000)，地壳模式年龄计算时采用平均地壳的176Lu/177Hf=0.015(Griffin et al., 2002)。
4 分析结果 4.1 LA-ICP-MS锆石U-Pb定年石包城地区花岗岩用于进行锆石U-Pb定年的样品编号为14CL101-3、14CL101-6和14CL101-9。阴极发光图像中，三者的锆石形态一致，呈灰黑色，主要为长柱状，少数呈短柱状，锆石长度基本在80~180μm，长宽比1.5：1~2：1；可见清晰岩浆振荡环带，应属岩浆成因(图 3d)。一般认为，锆石中某一区域的U、Th和REE等微量元素含量越高，对应的阴极发光图象越暗(Hanchar and Miller, 1993; Rubatto and Gebauer, 2000; Crofu et al., 2003)。
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图 3 北祁连造山带石包城地区花岗岩锆石U-Pb谐和曲线图(a-c)和阴极发光图像(d) 图(d)中：实线和虚线分别代表LA-ICP-MS U-Pb定年和Hf同位素分析测点 Fig. 3 Zircons U-Pb concordia plots (a-c) and cathodoluminescence (CL) images (d) of representative zircon grains from Shibaocheng granite in North Qilian orogenic belt In Fig. 3d: Solid and dashed circles indicate the locations of LA-ICP-MS U-Pb and Hf analyses, respectively |
样品14CL101-3中Th含量为87×10-6~1102×10-6，U含量为275×10-6~2894×10-6，Th/U比值平均为0.29(表 1)，可能为岩浆成因(吴元保和郑永飞, 2004)。通过锆石内部的206Pb/238U进行年龄计算，加权平均年龄为463±4Ma(MSWD=1.4，n=21)(图 3a)。此外，测点7、16和27的U-Pb年龄分别为1159Ma、1025Ma和1083Ma，结合阴极发光图像分析，上述年龄来自于锆石继承核(图 3d)。
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表 1 北祁连造山带石包城地区花岗岩锆石LA-ICP-MS U-Th-Pb数据 Table 1 LA-ICP-MS zircon U-Pb isotopic analyses for the samples of Shibaocheng granite in North Qilian orogenic belt |
样品14CL101-6中Th的含量为69×10-6~2985×10-6，U的含量为437×10-6~4396×10-6，Th/U平均比值为0.27(表 1)。该样品的锆石U-Pb加权平均年龄为464±4Ma(MSWD=0.6，n=16)(图 3b)。同时，测点11、17、20、27和29的锆石U-Pb年龄分别为1112Ma、1358Ma、765Ma、599Ma和1472Ma，为中-新元古代产物，锆石8、10、18分别为525Ma、540Ma和510Ma，为早古生代产物，这些可能为继承性锆石或捕掳的碎屑锆石，此外，测点5的锆石U-Pb年龄为378Ma，可能是在后期因热事件等原因而发生了一定程度的铅丢失(表 1)。
样品14CL101-9中Th的含量为75×10-6~2519×10-6，U的含量为481×10-6~3365×10-6，Th/U平均比值为0.32(表 1)。该样品的锆石U-Pb加权平均年龄为465±4Ma(MSWD=0.24，n=17)(图 3c)。同时，测点7、10、11、19和23的锆石U-Pb年龄分别为612Ma、1476Ma、810Ma、508Ma和516Ma，属中-新元古代和早古生代，结合阴极发光图像，判断这些锆石可能反映了锆石继承核、锆石捕掳晶或混合年龄，测点21、22、24、26、28和29等多属晚古生代至中生代(表 1)，阿尔金断裂北侧的敦煌地块曾发生过同时代的岩浆-构造热事件(王楠等, 2016a, b)，因此上述年龄可能反映了北祁连造山带曾受到晚古生代中国西部广泛的构造热事件影响。
4.2 地球化学特征石包城地区的花岗岩地球化学分析样品编号为14CL101-2、14CL101-5和14CL101-8。其SiO2含量为72.94%~73.92%，FeOT含量为0.98%~1.41%，Na2O含量为3.95%~4.50%，K2O含量为1.81%~1.95%，Na2O+K2O含量为5.90%~6.31%，Na2O/K2O比值为2.03~2.48(表 2)。根据SiO2-Na2O+K2O分类图(图 4a)，样品落入了花岗岩区域内，根据SiO2-K2O图(图 4b)，该类花岗岩属钙碱性系列。Al2O3含量为14.53%~14.73%，A/CNK为1.15~1.21，A/NK为1.57~1.69。根据A/CNK-A/NK分类图(图 5)，石包城花岗岩样品全部落入了过铝质区域，属强过铝质。总体上，具有高Si、Al，富全碱，低Mg、Fe的特征。
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表 2 北祁连造山带部分花岗岩和石包城地区花岗岩主量(wt%)和微量元素含量(×10-6) Table 2 Major (wt%) and trace (×10-6) elements compositions of some granites and Shibaocheng granite in North Qilian orogenic belt |
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图 4 北祁连造山带石包城地区花岗岩SiO2-Na2O+K2O图(a, 据Middlemost, 1994)和SiO2-K2O图(b, 据Martin et al., 2005) Fig. 4 Whole-rock SiO2 vs. Na2O+K2O (a, after Middlemost, 1994; ) and SiO2 vs. K2O (b, after Martin et al., 2005) classification diagrams of the granite phases from Shibaocheng granite in North Qilian orogenic belt |
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图 5 北祁连造山带石包城地区花岗岩A/CNK-A/NK图(据Maniar and Piccoli, 1989) Fig. 5 A/CNK vs. A/NK diagram of Shibaocheng granite in North Qilian orogenic belt (after Maniar and Piccoli, 1989) |
石包城地区的花岗岩REE总量为40.7×10-6~71.4×10-6，配分模式表现为轻稀土富集重稀土相对亏损的右倾型(图 6a)，其中LREE总量为37.6×10-6~65.9×10-6，LREE/HREE比值为11.1~12.0，(La/Yb)N为14.0~20.4，表明轻重稀土之间分馏程度较高(表 2)。(La/Sm)N为5.08~6.18，(Gd/Lu)N为1.59~2.47，反映了LREE元素内部分馏程度较高，而HREE元素内部分馏不明显。此外，该类花岗岩具有中等-轻的负Eu异常，Eu/Eu*为0.48~0.74。微量元素蛛网图中(图 6b)，石包城花岗岩主要富集大离子亲石元素(LILEs)Rb、Ba、K和Sr等，相对亏损高场强元素(HFSEs)Nb、Ta、P和Ti等，并且不同样品在Th、U含量上表现出了一定的差异性。
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图 6 北祁连造山带石包城地区花岗岩全岩球粒陨石标准化稀土配分模式(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. 6 Chondrite-normalized REE patterns (a) and primitive-mantle normalized spiderdiagrams (b) for Shibaocheng granite in North Qilian orogenic belt (normalization values after Sun and McDonough, 1989) |
此外，根据Miller et al. (2003)提出的锆元素饱和浓度温度计进行计算，石包城地区花岗岩的TZr为759~761℃，平均温度为760℃；根据Rapp and Watson (1986)和Montel (1993)提出的稀土元素饱和浓度温度计进行计算，石包城地区花岗岩的TREE为714~756℃，平均温度为732℃(表 2)。
4.3 Hf同位素特征对石包城地区花岗岩中23颗已测U-Pb年龄的锆石进行原位Hf同位素分析(表 3)。分析结果显示，176Yb/177Hf和176Lu/177Hf比值范围分别为0.023978~0.072957和0.000813~0.002436，176Hf/177Hf范围为0.282713~0.282878，对应的εHf(t)变化于+7.6~+13.5，根据锆石U-Pb年龄计算的二阶段模式年龄(tDM2)变化于581~957Ma之间(图 7、图 8、表 3)。
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表 3 北祁连造山带石包城地区花岗岩锆石Lu-Hf同位素数据 Table 3 LA-ICP-MS zircon Hf isotopic analyses for the samples of Shibaocheng granite in North Qilian orogenic belt |
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图 7 北祁连造山带石包城地区花岗岩锆石εHf(t)频率直方图(a)和tDM2频率直方图(b) Fig. 7 Histograms of εHf(t) (a) and tDM2 (b) of zircons of Shibaocheng granite in North Qilian orogenic belt |
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图 8 北祁连造山带石包城地区花岗岩锆石Hf同位素组成 Fig. 8 Hf isotopic compositions of zircons of Shibaocheng granite in North Qilian orogenic belt |
过铝质花岗岩通常被认为是纯变质-沉积岩在造山过程中地壳加厚期间深熔的响应(S型)，或与区域伸展或俯冲有关的环境下的产物(Chappell and White, 1974; Barbarin, 1996; Collins, 1998; Douce and Harris, 1998; Sylvester, 1998; Douce, 1999; Healy et al., 2004; Chen et al., 2014)。但近年研究表明，Ⅰ型花岗岩也可以具有过铝质甚至是强过铝质特征(Chappell, 1999; Chappell et al., 2012)。实验岩石学同样显示过铝质熔体可以由下地壳条件下的(变质)准铝质玄武质-安山质岩石部分熔融而成(Beard and Lofgren, 1991; Douce, 1995; Sisson et al., 2005)，但这种熔体通常具有低K2O/Na2O比值(＜1.0)的特点，因此在根据A/CNK判断成因类型时，应结合K2O/Na2O比值(Zhao et al., 2015)。石包城花岗岩的K2O/Na2O比值仅为0.4~0.49，与Ⅰ型花岗岩特征相符。磷灰石溶解度实验表明，磷灰石在Ⅰ型花岗岩中含量非常低，并且会随着SiO2含量的增加而减少，但在S型花岗岩中呈相反趋势(Chappell, 1999; Wu et al., 2003a, b)。根据SiO2-P2O5关系判断(图 9)，石包城花岗岩与Ⅰ型花岗岩特征更为接近。富铝矿物白云母多产于S型花岗岩中，但后来的研究发现，即便是原生白云母也无法判断是否属于S型花岗岩，相反角闪石和堇青石被视为判断Ⅰ型和S型花岗岩的标志性矿物(Chappell and White, 1974; Miller, 1985)，显微镜下可观察到角闪石(图 2d)，是证明石包城地区的花岗岩为Ⅰ型花岗岩的重要证据。因此，岩相学和岩石地球化学均显示石包城花岗岩具有明显的Ⅰ型花岗岩特征。
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图 9 北祁连造山带石包城地区花岗岩全岩SiO2-P2O5图解 Fig. 9 Whole-rock SiO2 vs. P2O5 diagram of Shibaocheng granite in North Qilian orogenic belt |
同时，石包城地区花岗岩表现出了一定的S型花岗岩特征，如较高的铝饱和指数(A/CNK=1.15~1.21)，而由于岩体遭受轻度蚀变，部分有明显次生特征的白云母和绢云母的出现可能是造成这一现象的重要原因。此外，锆石CL图像中可见中元古代-新元古代的继承锆石(图 3d)，而锆石继承核多见于S型花岗岩中。Ⅰ型花岗岩表现出部分S型花岗岩的地球化学特征，这可能反映了花岗岩浆侵位至沉积覆盖的过程中受到了少量地壳物质的混染，由于沉积组分的加入而造成(Zheng and Hermann, 2014)。大量沉积组分加入到岩浆中往往会使同位素具有较大的波动范围，但石包城花岗岩εHf(t)值全部为正值且变化范围不大(表 3)，这表明地壳沉积物质加入的比例可能不大和/或两种岩浆混合程度较高。同时，tDM2年龄明显小于部分锆石继承核，也进一步说明了岩浆来源的不单一性，含有地壳沉积组分。因此，石包城地区的花岗岩可能是以火成岩物质部分熔融为主的Ⅰ型花岗岩，但由于同时含有部分沉积组分的加入，属于Ⅰ-S过渡型花岗岩。
石包城地区花岗岩中FeOT、MgO和CaO与SiO2并未表现出负相关性，表明岩浆演化过程中以角闪石为主的镁铁矿物的分离结晶作用并不明显；Al2O3、NaO和K2O同样未与SiO2表现出任何相关性(图略、表 2)，同时Sr和Ba未表现出负异常(图 6)，因此同样可以排除岩浆演化过程中发生过强烈的斜长石分离结晶作用，Eu的负异常很可能表明的是源区残留了部分斜长石。石榴石强烈富集HREE，(Dy/Yb)N-(La/Yb)N和(Dy/Yb)N-Nb/Ta图解(图 10)表明，石包城地区的花岗岩稀土元素分异程度主要受残留石榴石比率控制(He et al., 2011; 李曙光等, 2013)，而相对亏损HREE和Yb的特征同样可能反映了岩浆源区可能有石榴石残留(葛小月等, 2002)。角闪石富集MREE，残留角闪石往往会降低MREE含量，因此石包城地区的花岗岩平坦的MREE-HREE配分暗示基性源岩部分熔融过程中残留了角闪石(Bea et al., 1994)，HoN与YbN大致相当也暗示角闪石可能是重要的残留相矿物(葛小月等, 2002; 李承东等, 2004)，这同时与样品低TiO2、MREE和低的Rb/Sr比值一致。因此，石包城地区花岗岩的源岩在源区发生部分熔融之后的残留相矿物组合可能至少为石榴石+角闪石+斜长石。
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图 10 北祁连造山带石包城地区花岗岩(La/Yb)N-(Dy/Yb)N (a)和(Dy/Yb)N-Nb/Ta (b)图解(据He et al., 2011) Fig. 10 (La/Yb)N vs. (Dy/Yb)N (a) and (Dy/Yb)N vs. Nb/Ta (b) diagrams of Shibaocheng granite in North Qilian orogenic belt (after He et al., 2011) |
较低的熔融温度(＜800℃)条件下产生岩浆往往需要角闪石和云母类等含水矿物发生脱水反应来提供一定的初始水含量来降低熔融温度(Best and Christiansen, 2001; Clemens and Watkins, 2001; 康磊等, 2016)，石包城地区花岗岩较低的K含量和表现出的Ⅰ型花岗岩特征暗示其可能为角闪石脱水分解而诱发的部分熔融产物。对角闪岩脱水熔融的实验表明(Beard and Lofgren, 1991; Wolf and Wyllie, 1994)，含水矿物角闪石的分解会提供熔融需要的初始部分水，并加速角闪岩的脱水熔融，石榴石和辉石的产生和熔体的大量增长也是随着角闪石和斜长石的分解或消失而产生的：斜长石+角闪石+石英=石榴石+辉石+角闪石+斜长石+熔体(Wolf and Wyllie, 1994; Zhao et al., 2007)。因此，石榴石、角闪石和斜长石可能是熔体抽离之后的主要稳定相。
根据张旗等(2006)对花岗岩划分的不同类型，石包城地区的花岗岩属于低Sr低Yb型，与具中等负Eu异常的低Sr低Yb型花岗岩相平衡的残留相为麻粒岩，为中等压力条件下形成，残留矿物组合为斜长石+石榴石+角闪石+辉石，位于石榴石+斜长石稳定区，与本文推测的残留矿物一致。因此，结合残留相矿物和Sr-Yb特征，本文认为石包城地区花岗岩的残留相可能为麻粒岩。结合实验岩石学资料(Drummond et al., 1996; Xiong et al., 2005)，石榴石稳定出现的压力至少0.8~1GPa。
石包城地区的花岗岩FeOT、MgO和CaO等主量元素含量较低，同时伴有较低的Mg#值，富集K、Rb、Sr、Ba和LREE等大离子亲石元素而相对亏损Nb、Ta、P和Ti等高场强元素，均暗示其可能来源于地壳(Rudnick and Gao, 2003)。正的εHf(t)值通常被解释为来源于新生地壳或在形成过程中有地幔物质的加入，石包城地区的花岗岩锆石εHf(t)值变化于+7.6~+13.5，全部为正且相对均一变化范围不大，结合极低的Cr和Ni等地幔元素含量以及野外未见基性岩浆包体现象，基本可以排除其形成过程中有地幔物质的直接加入，表明其更可能源于新生地壳物质的部分熔融。因此，石包城花岗岩物质来源以新生基性地壳熔融为主，同时伴有少量沉积组分。tDM2介于581~957Ma之间，平均为788Ma，属新元古代时期。其中早期的年龄与Rodinia超大陆的聚合时代接近，而峰期年龄则与祁连-阿尔金造山带内前人报道的部分花岗质片麻岩(776~751Ma)、蛇绿岩(780~768Ma)、玄武岩(800~750Ma)和流纹岩(750Ma)等相同(苏建平等, 2004; 夏小洪和宋述光, 2010; Xia et al., 2012; Song et al., 2006, 2009a, b, 2013; 董顺利等, 2013; Yu et al., 2013; Chen et al., 2014)。
5.2 石包城地区花岗岩的构造背景北祁连造山带完整的记录了北祁连洋俯冲-闭合，弧陆碰撞-陆陆碰撞，以及随后的造山带内调整和伸展等过程，目前带内已知的古生代花岗岩类时代自~520Ma持续到383Ma(吴才来等, 2006, 2010; Song et al., 2013; 秦海鹏等, 2014)。
其中，>470Ma的花岗岩主要主要分布在北祁连造山带的南缘，但前人对此有不同的认识，并引发了对北祁连洋俯冲极性的讨论：(1) 吴才来等(2004, 2006, 2010)和Yu et al. (2015a)认为是北祁连洋双向俯冲过程中早期向南俯冲的证据，并形成了野马咀、柯柯里和牛心山等花岗岩体；(2) Song et al. (2013)和Chen et al. (2014)等则认为是北祁连洋北向俯冲过程的产物。465~440Ma的花岗岩则主要分布在北祁连造山带的中部和北部，大量岩石学证据表明该时期北祁连洋由于受到柴达木板块向北俯冲的影响，俯冲极性转变为向北俯冲，并导致了洋盆的闭合以及阿拉善地块与祁连地块之间的碰撞作用(吴才来等, 2004, 2006, 2010)。而＜435Ma的金佛寺、黄羊河、新开沟-南坝和熬油沟等岩体则标志着北祁连造山带进入了碰撞后内部调整和陆内演化阶段(吴才来等, 2004; 秦海鹏, 2012; 秦海鹏等, 2014; 陈育晓, 2014)。
石包城地区的花岗岩形成时代为465~463Ma，与井子川石英闪长岩-石英二长闪长岩-英云闪长岩(464Ma)、清阳河英云闪长岩(463Ma)、民乐窑沟花岗闪长岩(463Ma)和新开沟-南坝花岗闪长岩(461Ma)时代一致(表 2)，而上述岩体被认为形成于北祁连洋北向俯冲的活动大陆边缘或是弧-陆碰撞的构造背景之下(吴才来等, 2004, 2006, 2010; 陈化奇, 2007; 秦海鹏, 2012; Song et al., 2013)。根据Y-Nb和Yb-Ta构造判别图解(图 11)(Pearce et al., 1984)，石包城地区的花岗岩落入了火山弧花岗岩区域内，结合区域地质背景，很可能是安第斯型活动大陆边缘环境下由于北祁连洋壳的俯冲作用，导致新生下地壳基性火成岩发生部分熔融，并可能在上升侵位的过程中混入了少量沉积组分。
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图 11 北祁连造山带石包城地区花岗岩Y-Nb (a)和Yb-Ta (b)构造判别图解(据Pearce et al., 1984) VAG-火山弧花岗岩；syn-COLG-同碰撞花岗岩；ORG-洋脊花岗岩；WPG-板内花岗岩 Fig. 11 Y vs. Nb (a) and Yb vs. Ta (b) discrimination diagrams of Shibaocheng granite in North Qilian orogenic belt (after Pearce et al., 1984) VAG-volcanic arc granites; syn-COLG-syncollisional granites; ORG-ocean ridge granites; WPG-within-plate granites |
(1) 根据锆石U-Pb定年结果，北祁连造山带西段石包城地区的花岗岩形成时代为465~463Ma；地球化学数据表明其属于钙碱性系列，并具强过铝质特征；TZr和TREE平均温度分别为760℃和732℃。
(2) 石包城地区花岗岩属Ⅰ-S过渡型花岗岩，可能主要是由于地壳内基性火成岩低温条件下由于角闪石脱水分解而发生部分熔融，且岩浆未发生明显的分离结晶作用，并在上升过程中受到沉积组分的混染; 残留相为石榴石、角闪石和斜长石为主的麻粒岩。
(3) 锆石Hf同位素分析显示，石包城地区花岗岩的εHf(t)变化于+7.6~+13.5，二阶段模式年龄变化于581~957Ma。表明其主要源于新生地壳的部分熔融，加入其中的沉积组分源于中-新元古代。
(4) 北祁连造山带于465~461Ma期间经历了洋壳的北向俯冲作用，处于活动大陆边缘背景，石包城地区花岗岩为该背景下形成的产物。
致谢 感谢戚学祥研究员和于胜尧教授为本文提出的意见和建议。
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