2018, Vol. 34
2. 中国地质大学地球科学与资源学院, 北京 100083
2. School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
前弧玄武岩(FAB)又称MORB-like玄武岩，是在洋内俯冲前弧环境形成的具有特殊地球化学特征的拉斑玄武岩(Ishiwatari et al., 2006；Ishizuka et al., 2014；Ohara et al., 2008；Reagan et al., 2010；肖庆辉等，2016)，近年来受到岩石学家和岩石地球化学家们的广泛关注。前弧玄武岩最早于1980年由Meijer等在伊豆-小笠原-马里亚纳(IBM)俯冲带岛弧前弧关岛东南部的马里亚纳前弧发现，深海钻探揭示该玄武岩位于玻安质火山熔岩之下，地球化学特点总体上与洋中脊相似，又位于现代前弧位置，而且这些玄武岩是前弧中最广泛的岩石，因此把它命名为前弧玄武岩(FAB)(Meijer，1980；Meijer et al., 1982)，随后前弧玄武岩又在小笠原岛前弧(DeBari et al., 1999)和母岛海山附近(Ishiwatari et al., 2006)等地陆续被发现。近年来随着对西太平洋伊豆-小笠原-马里亚纳(IBM)洋内弧的深入研究，总结出俯冲带岛弧前弧地区发育前弧玄武岩、玻安岩、埃达克岩、富铌玄武岩(NEB)和高镁安山岩(HMA)等火成岩组合(Ishikawa et al., 2005, Ishizuka et al., 2006, 2009, 2011a, b，2014；邓晋福等, 2007, 2010; Tatsumi，2008；Reagan et al., 2010, 2013；冯艳芳等, 2011, 2013)。它们是洋陆转化形成大陆的初始弧火成岩组合，代表了洋陆转化形成大陆的雏体。其中，前弧玄武岩是在洋内俯冲开始不久以后发生的伸展作用的首次喷发的熔岩(Ishizuka et al., 2006, 2010；Reagan et al., 2010；肖庆辉等，2016)。这些熔岩以前被认为是弧拉斑玄武岩，近年来，人们发现，这些熔岩的地球化学特征比弧拉斑玄武岩更像扩张中心洋中脊喷发的拉斑玄武岩，具有类似洋中脊一样的轻稀土亏损型式，但具有大离子亲石元素(如Ba、Sr、Pb)稍微富集和高场强元素(如Nb、Ta、Ti)微弱亏损，却又像弧拉斑玄武岩，它兼有洋中脊与岛弧双重特性，而更像洋中脊，但它不是产在扩张中心洋中脊而是产在前弧的喷发玄武岩，故称它为MORB -like玄武岩(Ishizuka et al., 2006, 2009；Reagan et al., 2010；肖庆辉等，2016)；由于它是洋内初始俯冲作用形成的首次岩浆作用产物，代表了洋内初始和洋陆初始转变的时代，并分布在洋内弧前弧岩石组合的底部层位，对确认古大洋洋内弧洋内初始俯冲作用和洋陆转化岩浆作用的存在和起始时间，以及洋内弧前弧岩石组合具有特殊的地质意义，近年来受到人们广泛关注(Ishikawa et al., 2005, Ishizuka et al., 2006, 2009, 2011a, b，2014；Reagan et al., 2010, 2013；肖庆辉等，2016)。
但是，前弧玄武岩除了在现代前弧伊豆-小笠原-马里亚纳(IBM)一带被发现，在古造山带中，这类岩石在国内外的报道并不多。近年来，陆续发现和报到洋陆转化特征岩石组合中的玻安岩(Kerrich et al., 1998；张旗等，1998；Manikyamba et al., 2005；李英杰等，2013；Zhao and Asimow, 2014)、埃达克岩(张旗等，2004；Castillo, 2006, 2012；张旗，2008；王冬兵等，2016；吴新伟和徐仲元，2016)和富铌玄武岩(Sajona et al., 1993；Hollings and Kerrich, 2000；Hollings, 2002; Ujike et al., 2007；李永军等，2014)和高镁安山岩(Kay，1978；Yogodzinski et al., 2001；Guivel et al., 2006；Wang et al., 2008a, b；唐功建和王强，2010；李玮等，2016)，但至今在中国未见发现洋内初始俯冲形成的前弧玄武岩的报道。
兴蒙造山带位于西伯利亚板块和华北板块之间的中亚造山带的东南缘(图 1a)，是目前已知发展历史长、构造岩浆活动复杂的一条巨型增生型造山带(Çengör et al., 1993；Ruzhentsev and Mossakovskiy, 1996；Buchan et al., 2002；Xu et al., 2003；Cao et al., 2004；Khain et al., 2003；Jahn et al., 2000, 2004；Kovalenko et al., 2004；Windley et al., 2001, 2007；Kröner et al., 2008；朱永峰和徐新，2006；徐新等，2007；Xiao et al., 2009, 2010, 2013；Xiao and Santosh, 2014)。据近年来的研究成果，兴蒙造山带从南向北可分为5个ENE-WSW的构造带：南方造山带、索伦缝合带、北方造山带、二连浩特-贺根山蛇绿岩带和乌里雅斯太陆缘(图 1b)(Xiao et al., 2003；Jian et al., 2012)。内蒙古迪彦庙蛇绿岩位于二连浩特-贺根山蛇绿岩带的东部(图 1b)。到目前为止，二连浩特-贺根山蛇绿岩带的研究已经积累了大量资料，但其形成时代和构造环境仍存在较大争议(Miao et al., 2008；Jian et al., 2012；Zhang et al., 2015)。二连浩特-贺根山蛇绿岩带早期多被认为形成于洋中脊环境(MORB型)(曹从周等，1986；梁日暄，1994；包志伟等，1994；Nozaka and Liu, 2002)，但大多数学者通过对玄武岩、铬铁矿等地球化学研究显示二连-贺根山蛇绿岩具有俯冲带特征(SSZ型)，如弧后拉张洋盆和前弧等(Robinson等，1995; Robinson et al., 1999；Zhou et al., 2004；Miao et al., 2008；王树庆等，2008；李英杰等，2013；Zhang et al., 2015)。
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图 1 研究区大地构造位置示意图(a, 据Jahn, 2004修改; b, 据Badarch et al., 2002；Miao et al., 2008；Xiao et al., 2009；Jian et al., 2010, 2012；Zhang et al., 2015修改)和迪彦庙蛇绿岩带南部孬来可吐蛇绿岩亚带地质简图(c) 一些古生代蛇绿混杂岩的同位素年龄来自Jian et al., 2008, 2010, 2012；Xiao et al., 2009；Li et al., 2013 Fig. 1 Geological map showing the tectonic units of study area (a, modified after Jahn, 2004; b, modified after Miao et al., 2008; Xiao et al., 2009; Jian et al., 2010, 2012; Zhang et al., 2015) and simplified geological map of the southern Naolaiketu ophiolite sub-belt in the Diyanmiao ophiolite (c) Some isotopic ages of ophiolitic melanges from Jian et al., 2008, 2010, 2012; Xiao et al., 2009; Li et al., 2013 |
近年来新发现的迪彦庙蛇绿岩各单元出露齐全，并且蛇绿岩原始层序保留较为完整(李英杰等，2012)。已有研究显示迪彦庙蛇绿岩为SSZ型(李英杰等，2012；2013)，可能为弧前环境，但缺乏对广泛分布的玄武岩形成时代和构造环境的详细研究，制约了对二连浩特-贺根山洋盆构造演化的认识。笔者近期在SSZ型迪彦庙蛇绿岩带西侧达哈特一带新发现前弧枕状拉斑玄武岩，本文拟通过对达哈特前弧枕状拉斑玄武岩的系统的野外地质特征、岩石学、地球化学及锆石U-Pb年代学研究，并与世界典型现代马里亚纳前弧玄武岩进行详细对比，探讨迪彦庙SSZ型蛇绿岩带中达哈特弧枕状拉斑玄武岩的大地构造意义，为兴蒙造山带构造演化提供进一步的约束。
1 地质概况和岩石学特征内蒙古迪彦庙蛇绿岩分为南北两个亚带：北部白音布拉格蛇绿岩亚带和南部孬来可吐蛇绿岩亚带(图 1b，c)，各带宽约3km，延伸约100km，近EW向转NEE向展布(图 1c)，蛇绿岩各单元出露齐全，岩性主要为：方辉橄榄岩、层状辉长岩、中粗粒-细粒均质块状辉长岩、枕状玄武岩、角砾状玄武岩、角斑岩和石英角斑岩，以及上覆岩系硅质岩和硅质泥岩，蛇绿岩原始岩石组合层序保留较为完整，与世界典型SSZ型蛇绿岩如特罗多斯蛇绿岩(Robinson et al., 2003；Osozawa et al., 2012)和阿曼Semail蛇绿岩(Goodenough et al., 2014)可以对比，表现为超镁铁质岩分布于蛇绿岩的最下部，向上依次为辉长杂岩和基性火山熔岩等，薄层硅质岩覆盖在枕状玄武岩之上。超镁铁质岩主要为方辉橄榄岩；辉长杂岩主要包括层状辉长岩、均质辉长岩和斜长岩等；基性火山熔岩比较丰富，主要为块状玄武岩、枕状玄武岩、细碧岩、角砾状玄武岩、玻安岩、角斑岩和石英角斑岩(李英杰等，2012；2013)。地质图上，迪彦庙蛇绿岩呈轴向近EW向转NE向的穹窿构造，因穹窿构造和剥蚀作用的叠加，最下部的超镁铁质岩分布在蛇绿岩带的中心海拔最高的部位，外围依次是辉长杂岩、火山熔岩和上覆岩系，断续呈对称分布(图 1c)。迪彦庙蛇绿岩带发育韧性剪切带，蛇绿岩局部变形变质强烈，形成糜棱岩化和片理化岩石及构造片岩。整个蛇绿岩带呈构造岩片构造就位于下二叠统寿山沟组与下二叠统大石寨组之中，寿山沟组为海相复理石建造，大石寨组岩性主要为流纹岩、安山岩夹碎屑岩。蛇绿岩带周围的寿山沟组与大石寨组地层普遍发生一定范围和强度的密集劈理化、碎裂岩化、千枚岩化和糜棱岩化。
本次工作在迪彦庙蛇绿岩带的南部孬来可吐蛇绿岩亚带西侧达哈特一带新发现枕状玄武岩，野外地质观察该枕状玄武岩发育典型的枕状构造(图 2a)和球颗结构(图 2c)。单一岩枕呈椭球状(图 2a，b)，大小悬殊，小者尤如鸡蛋，大者为1×2m，多为10×20cm~30×50cm。其长轴大体定向。岩枕紧密堆积，枕间胶结物较少，为熔岩物质。岩枕的横断面为椭圆形或近圆形，常具有同心环状生长线。岩枕边缘常具有薄的2~5cm的冷凝边(图 2b)，反映了岩枕在水下形成的特点。岩石普遍受到风化和蚀变作用，呈现不同程度的灰绿色。枕状玄武岩以球颗结构和中空骸晶结构为特征(图 2c-e)，岩石由斑晶和基质构成，斑晶由斜长石、橄榄石假像构成，粒径一般0.05~0.8mm。斜长石主呈半自形细长板状，杂乱状排列，具绿帘石化。个别斑晶呈骸晶状。基质主由纤状、针柱状辉石及斜长石构成。其中纤状、针柱状辉石、斜长石主呈束状、帚状、放射状排列构成球颗结构(图 2d，e)，这些辉石和斜长石的中心常被绿泥石或隐晶质所充填，即构成中空骸晶结构(图 2e)，为水下熔岩急剧萃冷条件下迅速结晶的产物。受风化作用和蚀变作用的球颗玄武岩表面显现出许多凸起的球颗(图 2c)，风化强者球颗可脱落。在岩枕的横断面上，球颗的分布大体呈同心环状。沿岩枕边缘球颗小而多，在岩枕的中央球颗大而少。
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图 2 迪彦庙蛇绿岩带中达哈特枕状玄武岩野外产状及显微镜下照片 (a)枕状玄武岩宏观野外露头；(b)单个枕状玄武岩野外露头；(c)枕状玄武岩的球颗结构野外露头；(d)枕状玄武岩的球颗结构显微镜下照片；(e)枕状玄武岩中空骸晶结构显微镜下照片 Fig. 2 Field occurrence and microtextures of the Dahate pillow basalt in Diyanmiao ophiolite (a)outcrop of the Dahate pillow basalt; (b)field feature of a pillow basalt; (c)field feature of variolitic texture in pillow basalt; (d)microphoto of variolitic texture in pillow basalt; (e) microphoto of the central absent skelecton crystal texture in pillow basalt |
主、微量元素分析测试均在中国地质调查局天津地质调查中心实验测试室完成，主元素分析采用Panalytical公司PW440型X荧光光谱仪(XRF)测定，分析误差低于2%，微量元素和稀土元素采用Thermo Fisher公司X-Series p型电感藕合等离子质谱仪(ICP-MS)测定，分析精度和准确度一般也优于5%。
达哈特枕状玄武岩锆石的分选在河北省区域地质矿产调查研究所实验室完成，采用重液浮选和电磁分离方法进行挑选，在双目镜下对分选出的锆石进行人工挑选，尽量挑选无包裹体、无裂纹和透明度高的晶形完好的锆石颗粒作为测定对象，从约80kg的样品中选出了近50粒锆石，锆石的阴极发光(CL)图像分析由北京锆年领航科技有限公司的高分辨热场发射能谱阴极发光室(SEM-EDS-CL)完成，仪器由热场发射扫描电镜(Nano SEM450)、阴极荧光谱仪(Gatan MonoCL4)和牛津电制冷能谱仪(INCA XMax50 EDS)组成，可提取高精度全光、单光阴极发光图像。锆石原位LA-ICP-MS U-Pb同位素年龄分析在中国地质调查局天津地质调查中心实验测试室完成，锆石定年分析所用仪器为Thermo Fisher公司Neptune型MC-ICP-MS及与之配套的Newwave UP 193激光剥蚀系统。激光剥蚀斑束直径为32μm，激光剥蚀样品的深度为20~40μm，锆石年龄计算采用国际上通用的标准锆石GJ-1作为外标，元素含量采用美国国家标准物质局人工合成硅酸盐玻璃NIST SRM610作为外标，29Si作为内标元素进行校正。数据处理采用ICPMSDataCal 8. 4程序(Liu et al., 2008)，并采用Andersen(2002)方法对测试数据进行普通铅校正，年龄计算及谐和图绘制采用ISOPLOT(3.0版)(Ludwig, 1991, 2003；Yuan et al., 2004)完成，U-Pb同位素测试方法及流程见李怀坤等(2009)。
3 分析结果 3.1 地球化学特征本次工作分析了迪彦庙蛇绿岩带达哈特枕状玄武岩的6个样品。从表 1可以看出，枕状玄武岩主量元素变化范围较小，SiO2 42.97%~50.90%，Al2O314.22%~16.30%，CaO 7.67%~20.33%，MgO 3.76%~7.42%，Mg#=49.7~60.3，K2O 0.04%~0.44%，Na2O 1.58%~4.34%，TiO20.59%~0.94%，P2O50.07%~0.13%，MnO 0.14%~0.16%。样品Na2O+K2O含量变化于1.62%~4.66%，平均为3.66%，Na2O/K2O为9.59~47.3，平均为27.0，总体为富钠贫钾岩系。主量元素特征显示，达哈特枕状玄武岩与N-MORB、马里亚纳前弧玄武岩基本一致，略有差异，更接近于马里亚纳前弧玄武岩(FAB)(表 1)。
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表 1 迪彦庙蛇绿岩达哈特枕状玄武岩和马里亚纳前弧玄武岩(据Reagan et al., 2010)及N-MORB的主量元素(wt%)和微量元素(×10-6)含量比较 Table 1 Major (wt%) and trace (×10-6) element analyzing results of the Dahate pillow basalt in the Diyanmiao ophiolite, Mariana fore-arc basalt(after Reagan et al., 2010) and N-MORB |
由岩浆岩的碱性指数Nb/Y比值来看，达哈特枕状玄武岩的样品Nb/Y比值集中在0.03~0.04，平均为0.04%，略低于N-MORB和马里亚纳FAB(平均分别为0.08和0.07)。在不活动性元素Zr/TiO2-Nb/Y分类图上(图 3a)，达哈特枕状玄武岩6个样品均落入安山岩/玄武岩区，并位于安山岩/玄武岩区和亚碱性玄武岩区的边界附近，N-MORB和马里亚纳FAB样品落入安山岩/玄武岩区和亚碱性玄武岩区的边界上，三者近一致。在K2O-SiO2图解中(图 3b)，达哈特枕状玄武岩6个样品均落入低钾拉斑玄武岩系列区，与马里亚纳FAB的5个样品和N-MORB样品近于重合。总之，上述图解判别表明，达哈特枕状玄武岩与马里亚纳FAB和N-MORB岩性较为一致，同属于拉斑系列玄武岩。
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图 3 达哈特枕状玄武岩Zr/TiO2-Nb/Y岩石分类图(a，据Winchester and Floyd, 1977)和K2O-SiO2图(b，据Middlemost，1985) Fig. 3 Zr/TiO2 vs. Nb/Y diagram(a, after Winchester and Floyd, 1977) and K2O vs. SiO2 diagram(b, after Middlemost, 1985)of the Dahate pillow basalt |
达哈特枕状玄武岩稀土总量(∑REE)较低(27.28×10-6~38.62×10-6)，平均为32.36×10-6，略低于N-MORB和马里亚纳FAB(39.11×10-6和33.86×10-6)，更为接近马里亚纳FAB。∑LREE较低(17.63×10-6~24.28×10-6)，平均为20.21×10-6，略高于马里亚纳FAB(平均为17.81×10-6)，略低于N-MORB(22.27×10-6)；重稀土元素较低(9.65×10-6~14.34×10-6)，平均为12.15×10-6，略低于N-MORB和马里亚纳FAB(平均分别为16.84×10-6和16.05×10-6)，轻重稀土含量相差不大，轻稀土略微富集，LREE/HREE较低(1.57~1.83)，平均为1.67，略高于N-MORB和马里亚纳FAB(平均分别为1.32和1.09)，稀土元素球粒陨石标准化图(图 4a)显示轻稀土(LERR)亏损型曲线((La/Yb)N=0.74~1.03)，与N-MORB的稀土配分形式接近(图 4a)，并位于马里亚纳FAB样品的区域内。稀土分馏不明显，δEu=0.87~1.11。
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图 4 达哈特枕状玄武岩球粒陨石标准化稀土元素配分曲线图(a，标准化值据Boynton，1984)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. 4 Chondrite-normalized REE distribution patterns (a, normalizing values after Boynton, 1984) and primitive mantle-normalized trace element spider diagram (b, normalizing values after Sun and McDonough, 1989) of the Dahate pillow basalt |
达哈特枕状玄武岩中相容元素Cr和Ni含量较高，分别为452×10-6~671×10-6(平均为269.5×10-6)和112×10-6~207×10-6(平均为269.5×10-6)。高场强元素(HFSE)Zr、Hf、Nb、Ta含量平均值分别为53×10-6、1.53×10-6、0.72×10-6和0.06×10-6，均低于N-MORB，而接近马里亚纳FAB样品，表明其岩浆源区有俯冲板片脱水流体输入。岩石中Th含量平均为0.17×10-6，接近马里亚纳FAB样品(0.16×10-6)，稍高于N-MORB (0.12×10-6)，Th一般富集于沉积物中，高Th含量表明其岩浆源区受到俯冲板块表层沉积物的影响。
在微量元素原始地幔标准化蛛网图(图 4b)上，达哈特枕状玄武岩样品为轻稀土亏损型，类似N-MORB，但是，相对N-MORB，大离子亲石元素(LILE)K、Rb、Ba、U稍微富集，高场强元素Nb、Ta、Ti微弱亏损，分布趋势与马里亚纳FAB样品相似，但Ba和Sr较马里亚纳FAB样品稍高(图 4b)。
3.2 锆石U-Pb年龄锆石呈淡黄色，或无色透明，大部分锆石晶型较好，多为短柱及等粒状(长宽比在1左右)，粒径为30~50μm，锆石阴极发光图像(CL)显示锆石晶形多呈半自形，部分颗粒比较破碎，呈现扇状、补丁状、平坦状生长环带，内部无残留核，外部无变质边，具有基性岩浆成因锆石的特征(吴元保和郑永飞，2004；Jian et al., 2012)(图 5a)。从表 2可以看出，8个测点具有低的Th含量(44×10-6~321×10-6)和相对高的U含量(65×10-6~331×10-6)，Th/U比值变化较小(Th/U=0.6~1.08)，均大于0. 4(表 1)，Th和U具有良好的正相关关系，也属于典型的岩浆锆石特征(Pidgeon et al., 1998；Claesson et al., 2000)。LA-ICP-MS锆石U-Pb定年结果(表 2)显示，8个测点的谐和度均较高，206Pb/238U年龄集中于324±6Ma~341±6Ma之间，所有8个数据点集中分布在谐和曲线附近(图 5b)，加权平均年龄为333.4±8.5Ma，MSWD=2.8(95%置信度)(图 5b)，认为可以代表玄武岩岩浆结晶年龄。
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图 5 达哈特枕状玄武岩中典型锆石的CL图象(a)和LA-ICP-MS U-Pb谐和图及加权年龄平均值(b) Fig. 5 Representative zircon CL images (a), concordia curves and weighed mean age (b) of LA-ICP-MS U-Pb data from the Dahate pillow basalt in the Diyanmiao ophiolite |
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表 2 迪彦庙蛇绿岩达哈特枕状玄武岩的LA-ICP-MS锆石U-Pb同位素分析结果 Table 2 LA-ICP-MS zircon U-Pb isotopic analysis of the Dahate pillow basalt in the Diyanmiao ophiolite |
本次工作在迪彦庙蛇绿岩带达哈特枕状玄武岩中获得LA-ICP-MS锆石U-Pb年龄为333.4±8.5Ma，锆石发育扇状、补丁状、平坦状生长环带，为基性岩浆岩特征，333.4±8.5Ma代表了达哈特枕状玄武岩的结晶年龄，属于早石炭世，与侵入枕状玄武岩的细脉状大洋斜长花岗岩的LA-ICP-MS锆石U-Pb年龄(328.2±1.5Ma)较为一致(作者未发表)。
在区域上，迪彦庙蛇绿岩中早石炭世达哈特枕状玄武岩的结晶年龄，与二连浩特-贺根山蛇绿岩带蛇绿岩的形成时代为石炭纪(早、中、晚石炭世)基本吻合或较为一致。例如，Jian et al.(2012)在小坝梁蛇绿岩中获得2个锆石U-Pb年龄为辉长岩354±7Ma和斜长花岗岩333±4Ma，Zhang et al.(2015)在二连浩特东部蛇绿岩中获得3个锆石U-Pb年龄，辉长岩为354±4.5Ma和353±3.7Ma，斜长花岗岩345±5.5Ma，李英杰等(2015)在北部梅劳特乌拉蛇绿岩中辉长岩的LA-ICP-MS锆石U-Pb年龄为308.5±2.2Ma，以及黄波等(2016)在崇根山蛇绿岩中获得的玄武岩锆石U-Pb年龄359±5Ma等与本文获得的年龄较为一致。综上所述，二连浩特-贺根山蛇绿岩带为一条早石炭世开始发育的石炭纪(早、中、晚石炭世)的大型蛇绿岩带，并主体在晚二叠世、最晚早三叠世侵位碰撞。这些年代学资料也佐证了哈特枕状玄武岩形成时代为早石炭世。
4.2 构造环境与意义蛇绿岩可以产在大洋中脊、弧前、弧间、弧后盆地等多种环境中(如，Dilek and Robinson, 2003；Dilek and Furnes, 2009，2011；Dilek and Furnes, 2014)，近年来的研究显示，由于消减带仰冲构造运动有利于蛇绿岩的保存就位，绝大多数蛇绿岩应该产于消减带有关的环境中(Pearce et al., 1984；Shervais，2001；Pearce，2008；Whattam and Stern, 2011)。李英杰等(2012, 2013)通过对迪彦庙蛇绿岩的火山熔岩发育IAT和玻安岩，初步认为其形成于SSZ的弧前环境。从野外特征看，新发现的哈达特枕状玄武岩的基底为SSZ型蛇绿岩，即以方辉橄榄岩为主的超镁铁质岩、辉长岩和斜长岩等，与现代伊豆-小笠原-马里亚纳(IBM)前弧可以进行对比(图 6)。在伊豆-小笠原-马里亚纳(IBM)地区，初始弧火成岩组合剖面底部为前弧玄武岩，其基底为SSZ型蛇绿岩，即席状岩墙、辉长岩、地幔橄榄岩(图 6)。深海钻探揭示小笠原岛前弧初始弧火成岩组合剖面底部为前弧枕状玄武岩，其基底为SSZ型蛇绿岩层序，由底到顶依次为：地幔橄榄岩、辉长岩和席状岩墙(图 6)。基底为SSZ型蛇绿岩或洋壳是前弧玄武岩的一个特征识别标志(Reagan et al., 2010；肖庆辉等，2016)。
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图 6 迪彦庙蛇绿岩哈达特枕状玄武岩剖面、伊豆-小笠原-马里亚纳前弧岩石地层剖面(据Ishizuka et al., 2014)和小笠原岛前弧层序岩石组合(据Reagan et al., 2013)对比 Fig. 6 The comparison between the Dahate pillow basalt in the Diyanmiao ophiolite, Izu-Bonin-Mariana forearcrock stratigraphic section(after Ishizuka et al., 2014) and fore-arc sequence of Boniface Island(after Reagan et al., 2013) |
从地球化学特征方面，上述哈达特枕状玄武岩的主量元素类似于N-MORB，更接近于马里亚纳FAB；从稀土配分模式上看，哈达特枕状玄武岩主要显示为亏损型，说明其源区是亏损的，类似N-MORB源区的亏损组分(图 4a)；从原始地幔标准化的微量元素蛛网图上可以看出(图 4b)，哈达特枕状玄武岩总体是平坦的，类似于N-MORB。但是，从表 1和原始地幔标准化的微量元素蛛网图上(图 4b)，大离子亲石元素(LILEs)含量比洋中脊稍高，显示弱富集；高场强元素Nb和Ta含量比洋中脊稍低，显示弱亏损，呈现出俯冲带岛弧拉斑玄武岩的特征，暗示少量俯冲流体的参与。总体上哈达特枕状玄武岩兼有洋中脊与岛弧双重特性，更像洋中脊，与典型的前弧玄武岩相似。在球粒陨石地幔标准化的稀土配分模式图(图 4a)和原始地幔标准化的微量元素蛛网图上(图 4b)，哈达特枕状玄武岩与马里亚纳FAB样品相吻合。以上地球化学特征表明俯冲作用不强烈，可能处于俯冲刚开始阶段。另外，与N-MORB相比，哈达特枕状玄武岩具有较低的轻稀土和重稀土比值(LREE/HREE)，重稀土元素较低，反映明地幔楔以熔融为主。此外，一些强不相容元素对在岩浆活动过程中由于其分配系数远小于部分熔融比例，其比值不随熔融比例的变化而变化(Hofmann，1997)，可以反映源区的性质，因此可以作为判别其形成环境的一个依据。岩石的Ti/V值为11.37~21.56，多数小于20，平均17.97，普遍低于MORB(Ti/V一般大于20)。岩石Ta/Hf为0.035~0.042(< 0.1)、Nb/Zr为(0.013~0.014)、Th/Nb(0.1~0.4)，La/Nb(2.8~4.0)等比值均与板块汇聚边缘弧玄武岩表现一致，而不同于板块发散边缘及板内玄武岩(李永军等，2015)。因此，从强不相容元素的微量元素比值上可以看出，虽然哈达特前弧玄武岩具有和N-MORB一样的稀土配分模式和类似的微量元素蛛网图，但是其形成环境和N-MORB还是不同的。在不相容元素对的相关图Th/Yb-Nb/Yb图解(图 7a)上，哈达特枕状玄武岩3个样品落在大洋岛弧区域，另外3个落在弱俯冲区和马里亚纳FAB的3个样品近于重合，表明哈达特枕状玄武岩可能形成于洋内弧前弧初始俯冲环境。在Nb-Zr-Y图解(图 7b)上，哈达特枕状玄武岩均落于N-MORB区域，与马里亚纳FAB和N-MORB样品落在同一区域，在Hf-Th-Ta图解(图 7c)上，哈达特枕状玄武岩3个样品落于岛弧拉斑玄武岩区域，另外3个样品与马里亚纳FAB样品近于重合，落于N-MORB区域；在Ti-V图解(图 7d)上，哈达特枕状玄武岩4个样品落于马里亚纳FAB区域及其边界上，2个样品落于岛弧拉斑玄武岩与N-MORB的边界上。综上所述，哈达特枕状玄武岩的主量、微量和稀土元素地球化学特征表明，哈达特枕状玄武岩为前弧玄武岩，可以与现代的马里亚纳FAB对比，为古大洋洋内初始俯冲的产物。同时，迪彦庙蛇绿岩中发育玻安岩(李英杰等，2013)和埃达克岩(作者未发表)，与哈达特前弧玄武岩一起组成洋内弧前弧岩石组合，为二连-贺根山洋洋陆转化形成大陆的初生弧火成岩组合。
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图 7 迪彦庙蛇绿岩达哈特枕状玄武岩、马里亚纳FAB和N-MORB判别图 (a) Th/Yb-Nb/Yb分类图(Pearce，2008); (b) Nb-Zr-Y构造环境判别图解(Meschede，1986); (c) Hf/3-Th-Ta图解(Wood，1980); (d) Izu-Bonin-Mariana前弧玄武岩Ti/V比值(Ishizuka et al., 2014) Fig. 7 Tectonic discriminant diagrams of the Dahate pillow basalt, Mariana fore-arc basalt and N-MORB (a) Th/Yb vs. Nb/Yb classification diagram(Pearce, 2008); (b) Nb-Zr-Y(Meschede, 1986); (c) Hf/3-Th-Ta diagram(Wood, 1980); (d) Ti/V ratios diagram of the Dahate pillow basalt and Izu-Bonin-Mariana fore-arc basalts(Ishizuka et al., 2014) |
本文报道的哈达特前弧玄武岩分布于二连-贺根山蛇绿岩带典型发育区的迪彦庙SSZ型蛇绿岩之中，其作为洋内初始俯冲作用形成的首次岩浆作用产物，反映了二连-贺根山古洋盆洋内初始俯冲作用和洋陆初始转化岩浆作用。新获得的哈达特前弧玄武岩LA-ICP-MS锆石U-Pb年龄为333.4±8.5Ma，其形成时代为早石炭世，代表了二连-贺根山洋洋内初始俯冲作用和洋陆转化首次岩浆作用的起始时代。进一步表明古亚洲洋二连-贺根山洋盆在早石炭世处于洋内初始俯冲阶段，早石炭世之后的一段时间可能为古亚洲洋二连-贺根山洋盆的洋内弧洋壳俯冲期。因此，哈达特早石炭世前弧玄武岩的发现与研究，对确定二连-贺根山洋在早石炭世存在洋内初始俯冲、洋陆转化和洋内弧前弧岩石组合提供了岩石学、地球化学和年代学信息。
5 结论(1) 哈达特枕状玄武岩具有亏损型稀土配分模式，类似N-MORB；而高场强元素(HFSE)Nb、Ta和Ti等比N-MORB稍低，大离子亲石元素(LILE)K、Rb、Ba、U比N-MORB稍高，与弧拉斑玄武岩相似，指示达哈特枕状玄武岩兼有洋中脊与岛弧双重特性，而更像洋中脊，与马里亚纳前弧玄武岩(FAB)相一致，为前弧玄武岩。
(2) 哈达特前弧玄武岩LA-ICP-MS锆石U-Pb年龄为333.4±8.5Ma，其形成时代为早石炭世，指示了在早石炭世二连-贺根山洋发生了洋内初始俯冲作用和洋陆转化岩浆作用，对确定二连-贺根山洋在早石炭世存在洋内初始俯冲、洋陆转化和洋内弧前弧岩石组合提供了岩石学、地球化学和年代学信息。
致谢 本文在野外调查和写作过程中得到中国地质调查局天津地质调查中心谷永昌、刘永顺、辛后田等的热情指导和帮助; 河北地质大学刘晓磊、李步逊、靳洁敏、刘琳莹、石宏杰、吕沛、秦梦月、朱悦等也做了大量工作; 审稿专家提出了宝贵的修改意见; 在此一并表示衷心的感谢!
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