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1 大陆下地壳研究意义及现状
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区别于其他类地行星的大陆地壳,是地球最重要的标志之一(Rudnick et al.,2014),与地球宜居环境形成密切相关。它为人类提供了丰富的矿产和油气资源,因此对大陆地壳的结构、组成与演化研究是人类掌握并利用大陆地壳的前提之一。然而大陆地壳随时间演化的机制和规律至今仍存在争议(Taylor et al.,1985;Windley,1995;Condie,1997),其下地壳作为连接上地幔和中上地壳的纽带,是大陆地壳生长与分异、壳幔物质交换的重要场所,因此对大陆下地壳演化的了解是认识地壳组成及演化的基本前提之一。
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依据地震波速结构,大陆地壳可以分为上地壳、中地壳和下地壳(Rudnick et al.,1995;Christensen et al.,1995)。上地壳的样品可以直接获得,进而能够估算它的地球化学成分。与上地壳相比,大陆下地壳不可直接观察。目前对它的研究方法有多种(Brown et al.,1981;Kay et al.,1981;Rudnick et al.,1995, 2014;高山,1999;侯青叶等,2010),主要包括:① 地震波速探测与深部岩石物理性质的高温高压实验测定结果之间的拟合(Taylor et al.,1985;侯青叶等,2010);② 暴露地表的下地壳剖面或高级变质地体(Brown et al.,1981;Fountain et al.,1981);③ 火成岩携带的下地壳捕虏体(Rudnick et al.,1992;Zheng Jianping et al.,2012),如麻粒岩捕虏体;④ 地表热流值测定(Rudnick et al.,1998;Jaupart et al.,2014)。通过上述方法的综合研究,获得了有关下地壳可能的岩石组合、地球化学组成、地震波速特征以及地球动力学演化等方面的认识(Rudnick et al.,2014;Hacker et al.,2015),大力促进了对大陆下地壳组成及演化的深入认识。
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本文旨在通过总结天然火山作用携带的华北克拉通下地壳捕虏体的已有研究成果,从定深、定性及定年等角度,理解华北克拉通典型地区下地壳的组成、结构及其形成过程。
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2 天然火山作用探测方法
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由于浅部地壳的覆盖,最初对于大陆下地壳的研究很大程度依赖地球物理探测,并辅以地质资料的证据(Rudnick et al.,1995;Wedepohl,1995;Mooney,2007)。地球物理在大尺度下地壳的研究方面发挥着至关重要的作用,主要包括地震波速、区域重力、电导率、地磁和地表热流等研究(Kay et al.,1981;Rudnick et al.,1995)。这些方法给出了大陆下地壳不同的物理属性,然而它仍然存在一些不足之处。首先,地震波速与岩石化学组成存在对应关系,但它还受温度、压力、岩石组构和流体等多方面因素的制约。因此,在利用地震波速研究下地壳时需要考虑各种因素的影响(Wedepohl,1995)。其次,尽管地表热流是地壳组分函数的唯一地球物理参数,但是它对下地壳组成的限制非常宽泛(Rudnick et al.,2014)。最后,地球物理方法适合下地壳区域大尺度上且现今状态的研究,而对于其小尺度上精细结构(例如岩性和年龄等)和地质历史演化的研究仍存在局限性。
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由于构造作用而暴露地表的下地壳剖面是研究大陆下地壳宝贵而直接的样品(Kay et al.,1981;Fountain et al.,1981;Percival et al.,1992;Mezger,1992)。因此,国际岩石圈委员会于1985年8月开展了全球地学大断面(Global Geoscience Transects,简称GGT)计划,旨在将断面范围内地质、地球物理和地球化学资料综合到一个解释性的地壳剖面图上,来反映地壳的垂向大地构造图(Monger,1986)。这一计划很大程度上促进了下地壳剖面在大陆下地壳研究中的应用。然而下地壳剖面在研究下地壳方面仍然存在着一些不足之处,最大的问题在于下地壳在抬升过程中经历了降温减压作用,且在暴露地表之后经历了一系列的地表过程(例如风化和蚀变等)(Rudnick et al.,1992),它能否代表下地壳的原始信息还有待进一步检验。
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正如下地壳剖面一样,下地壳捕虏体也是下地壳宝贵而直接的样品(Rudnick et al.,1992),二者的不同之处在于:前者出露面积大,因构造作用而暴露;后者出露少,被岩浆快速携带至地表。相比大陆科学深钻而言,火山岩携带的下地壳捕虏体(图1)具有经济且深度更深等优点而广受地质学家的青睐。尽管下地壳捕虏体出露少,但它是直接来源于下地壳的天然岩石样品,保存有下地壳的原始信息,是揭示大陆下地壳组成及演化不可替代的材料(Kay et al.,1981;Rudnick et al.,1992, 1995;Zheng Jianping et al.,2012)。
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图1 天然火山与地球物理探测下地壳结构与组成
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Fig.1 Natural volcanoes and geophysical exploration of the structure and composition of lower crust
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地震学Moho面指P波速度大于7.8km/s的界面;岩石学Moho面指地幔橄榄岩与地壳岩石之间的分界面
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Seismological Moho surface refers to the interface with P-wave velocity greater than 7.8km/s; petrological Moho surface refers to the interface between mantle peridotite and crustal rocks
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前人对下地壳捕虏体已开展了大量的研究(Rudnick et al.,1992, 2014;Zheng Jianping et al.,2012;Hacker et al.,2015),总结归纳起来主要包含三个方面:① 下地壳捕虏体的来源确定(定深);② 下地壳捕虏体的性质确定(定性);③ 下地壳捕虏体的形成年龄确定(定年)。定深研究包含两方面的含义:一是确定火山岩中捕虏体是真正意义上的下地壳岩石捕虏体;二是判定这些捕虏体在下地壳剖面中的相对深浅或可能的准确来源深度。定性研究也包含两方面的含义:一是化学性质的研究,涉及下地壳捕虏体的化学属性,如它们的主量元素、微量元素和同位素(Sr-Nd-Pb-Hf-O)等特征,以及这些岩石地球化学特征所反映的下地壳捕虏体的初始物质来源、成因演化过程和形成时的可能构造背景等;二是物理性质的测定,包括这些捕虏体的波速(Vp)、密度(D)和磁性特征等参数。下地壳捕虏体的定性研究是进行地质学、地球化学与地球物理学等多学科成果交叉联结的纽带。下地壳捕虏体的定年研究主要是厘定下地壳捕虏体的形成及后期演化过程的时代,为下地壳的演化提供时间坐标。
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3 华北下地壳组成、结构特征
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华北克拉通北连中亚造山带,南接祁连-秦岭-大别-苏鲁造山带,东邻西太平洋构区(图2),是我国最大的克拉通。它也是地球上最古老的克拉通之一,在辽宁鞍山和河北曹庄等地存在≥3.8Ga的地表岩石(Liu Dunyi et al.,1992;Song Biao et al.,1996;Wan Yusheng et al.,2005;Wilde et al.,2008),以及河南信阳存在≥3.6Ga的下地壳岩石(Zheng Jianping et al.,2004a)。华北克拉通主要由东部陆块、西部陆块和分隔这两大陆块的中部造山带三部分组成(Zhao Guochun et al., 2001, 2005, 2012)(图2)。它主要由新太古代基底组成,于古元古代时期发生了多期次的块体拼合,并于~1.85Ga发生东、西陆块整体碰撞,形成了统一的克拉通,随后进入晚古元古代(1.84~1.62Ga)时期的伸展有关的岩浆活动阶段(Xia Linqi et al.,2013)。自中元古代开始,直至复县、蒙阴金伯利岩所代表的早古生代稳定克拉通之前,华北进入稳定的沉积盖层阶段,中生代时期其东部发生了显著的克拉通破坏,并伴随非常强烈的构造-岩浆-成矿活动(徐义刚等,2009;郑建平,2009;朱日祥等,2011, 2012a;Zhu Rixiang et al.,2012b;Li Jianwei et al.,2012)。
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伴随显生宙岩浆作用,丰富而重要的下地壳捕虏体被携带至地表(Zheng Jianping et al.,2012;路凤香等,2012),它们包括古生代(480~457Ma)的辽宁复县金伯利岩(郑建平等,1999)和山西应县煌斑岩(Zheng Jianping et al.,2012),中生代的河南信阳~160Ma中-基性火山角砾岩(胡宝群,2001;Zheng Jianping et al.,2003, 2004a, 2006, 2008)、安徽夹沟~130Ma闪长斑岩(Xu Wenliang et al.,2006)、辽宁阜新~100Ma玄武岩(张宏福等,2003)、山东青岛~82Ma基性岩墙(Zhang Hongfu,2012)和莒南~67Ma玄武岩(Ying Jifeng et al.,2006),新生代的河北平泉~45Ma(Zheng Jianping et al.,2012)和汉诺坝~22Ma(Liu Yongsheng et al.,2001)、河南鹤壁~4Ma(Zheng Jianping et al.,2001)和安徽女山~0.63Ma(Huang Xiaolong et al.,2004)玄武岩(图2)。这些下地壳捕虏体为我们了解华北下地壳组成、结构提供了关键材料(Zheng Jianping et al.,2012)。对于含有丰富下地壳捕虏体的典型地区,依据定深,定性和定年的综合研究,可以构建这些地区下地壳结构剖面模型。这些典型地区包括华北南缘的信阳地区,华北东部的莒南地区,华北北缘的汉诺坝地区和华北东南缘的女山地区。以下将重点展开对这四个典型地区下地壳结构剖面模型的介绍。
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图2 华北克拉通深源岩石捕虏体出露点(据Zhang Hongfu et al., 2012; Zhao Guochun et al., 2012; Zheng Jianping et al., 2012修改)
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Fig.2 The localities of deep-seated xenoliths in the North China Craton (modified after Zhang Hongfu et al., 2012; Zhao Guochun et al., 2012; Zheng Jianping et al., 2012)
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1 —东部陆块和西部陆块出露的基底;2—东部陆块和西部陆块被覆盖的基底;3—中部造山带内被覆盖的基底;4—中部造山带内出露的基底;5—西部陆块孔兹岩带内出露的基底;6—东部陆块胶-辽-吉带内出露的基底;7—熊耳群火山岩带;8—主要断层
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1 —Exposed basement in the Eastern and Western Blocks; 2—covered basement in the Eastern and Western Blocks; 3—covered basement in the Trans-North China Orogen; 4—exposed basement in the Trans-North China Orogen; 5—exposed basement in the khondalite belt; 6—exposed basement in the Jiao-Liao-Ji belt; 7—Xiong'er volcanic belt; 8—major fault
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信阳:晚中生代(~160Ma)信阳火山碎屑岩携带有丰富的下地壳捕虏体,主要包括变辉长岩、辉石岩、长英质麻粒岩、中性麻粒岩、基性麻粒岩和榴辉岩等(胡宝群,2001;路凤香等, 2003; Zheng Jianping et al., 2003)。通过对这些捕虏体定深,定性及定年的研究,Zheng Jianping et al.(2008)和Ping Xianquan et al.(2015)建立了晚中生代时期信阳地区下地壳精细的垂向分层结构:上部(约25~30km)主要由3.4~3.6Ga不含石榴子石的长英质麻粒岩和少量辉石岩组成,中部(30~35km)由1.8~2.2Ga含石榴子石的长英质麻粒岩和变辉长岩组成,下部(35~45km)由1.7~2.0Ga中性和基性麻粒岩及古生代榴辉岩组成(图3)。地震波速探测显示信阳地区现今下地壳Moho面深度~32km(郑晔和滕吉文,1989),明显浅于由下地壳捕虏体所获得的晚中生代下地壳深度,暗示了该区下地壳自晚中生代以来经历了强烈的改造过程而发生了明显的减薄。
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莒南:晚白垩世(67Ma)莒南玄武岩携带有二辉石麻粒岩、含石榴子石麻粒岩、二辉辉石岩和石榴辉石岩等下地壳捕虏体(Ying Jifeng et al., 2006, 2010, 2013; Tang Huayun et al., 2014)。在总结莒南地区前人研究成果的基础上,Tang Huayun et al.(2014)构建了该区下地壳组成、结构模型(图4):下地壳上部由镁铁质-中性二辉麻粒岩和含石榴子石麻粒岩组成;下地壳下部由堆晶的石榴辉石岩和二辉辉石岩组成。锆石U-Pb年龄和Hf同位素表明,莒南下地壳经历了复杂的演化,包括最初4.0~3.0Ga和2.7~2.5Ga的生长,到早古元古代(~2.3Ga)转换为麻粒岩相的下地壳,而后经历了古元古代晚期(2.0~1.8Ga)、新元古代、早古生代、晚三叠世和晚侏罗世的幕式热事件,最后早白垩世玄武质岩浆底侵堆晶形成辉石岩(Tang Huayun et al., 2014)。
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汉诺坝:新生代(22~14Ma)玄武岩携带有丰富的下地壳捕虏体,主要包括变泥质岩、长英质麻粒岩、中性麻粒岩、基性麻粒岩、辉石岩和石榴辉石岩等(樊祺诚等, 1996; Liu Yongsheng et al.,2001;Wei Ying et al., 2015)。综合地质温压计、地质年代学和汉诺坝地区下地壳地震反射相关的研究成果,Wei Ying et al.(2015)构建了该区下地壳组成、结构模型(图5):下地壳上部(24~33km)主要由太古宙(~2.5Ga与少量2.7Ga)长英质麻粒岩以及在140~128Ma被改造的长英质麻粒岩组成;前寒武纪和中生代晚期镁铁质麻粒岩是下地壳中部(33~38km)的重要组成部分;下地壳最底层(38~42km)主要发育晚中生代(140~120Ma)和新生代(47~45Ma)麻粒岩和辉石岩。汉诺坝地区下地壳的分层结构说明了华北克拉通北缘经历了复杂的演化过程:包括新太古代晚期(~2.5Ga)的增生、随后的古元古代(1.9~1.8Ga)的幕式增生或者再造作用和显生宙(220~45Ma,140~120Ma, 47Ma)可能与克拉通边缘俯冲-碰撞和华北克拉通的破坏相关的事件(Wei Ying et al., 2015)。
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女山:新生代(0.63Ma)玄武岩携带有丰富的下地壳捕虏体,主要包括长英质麻粒岩、中性麻粒岩和镁铁质麻粒岩等(Huang Xiaolong et al.,2004; Zheng Jianping et al., 2012; Ping Xianquan et al., 2019)。通过对女山新生代玄武岩中长英质到镁铁质麻粒岩捕虏体和锆石捕虏晶的研究,建立显示华北克拉通东南部深部地壳精细结构与演化的下地壳剖面柱状图(图6)。来自中地壳底部到上地壳顶部的长英质麻粒岩的计算纵波速度为6.32km/s,它们中残留的岩浆锆石给出了2716±32Ma的上交点年龄和3.1~2.9Ga的亏损地幔Hf模式年龄。中性麻粒岩形成于古元古代的2.1~1.9Ga,具有3.4Ga的全岩Nd球粒陨石模式年龄和3.1~2.8Ga的锆石Hf亏损地幔模式年龄(Huang Xiaolong et al., 2004; Zheng Jianping et al., 2012)。它们的计算波速Vp=6.53km/s,大于长英质麻粒岩,因此被认为是来自长英质麻粒岩层位之下。镁铁质麻粒岩具有的计算纵波波速为6.86km/s,它们代表的是下地壳的底部。镁铁质麻粒岩中的大部分锆石都形成于晚中生代(约115Ma)。综合看来,女山下地壳的底部更年轻,且主要由长英质、中性和镁铁质麻粒岩组成。这一趋势反映地壳的垂向增生,包括中太古代和新太古代新生物质的加入,新太古代、古元古代和晚中生代的再造(Ping Xianquan et al., 2019)。
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图3 华北克拉通南缘信阳地区岩石圈结构剖面(据Zheng Jianping et al., 2008; Ping Xianquan et al., 2015修改)
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Fig.3 Cartoon showing the suggested lithospheric architecture of the Xinyang area in the southern North China Craton (modified after Zheng Jianping et al., 2008; Ping Xianquan et al., 2015)
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图4 华北克拉通东部莒南地区岩石圈结构剖面 (据Tang Huayun et al., 2014修改)
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Fig.4 Cartoon showing the suggested lithospheric architecture of the Junan area of the eastern North China Craton (modified after Tang Huayun et al., 2014)
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4 华北下地壳形成演化过程
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下地壳结构剖面模型显示了华北下地壳结构分层的空间特征,而在时间维度上,下地壳也经历自形成以来的复杂演化过程。通过对下地壳捕虏体定年(锆石U-Pb年龄)和定性(锆石Hf同位素)的分析,可以揭示华北下地壳的形成与演化过程。
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4.1 始太古代—冥古宙(＞3.6 Ga)下地壳成分
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华北出露的最古老岩石在河北东部的曹庄和辽宁中部的鞍山(图1)。曹庄太古宙变质铬云母石英岩中,碎屑锆石年龄为3.85~3.5Ga(Liu Dunyi et al., 1992),解释为来自含有始太古代岩石的地体。最近,万渝生等(2021a, 2021b)在冀东卢龙地区发现3.8Ga TTG(Tonalite-Trondhjemite-Granodiorite)岩石和4.0~3.9Ga碎屑锆石。鞍山片麻岩中锆石具有3804Ma的207Pb/206Pb加权平均年龄,代表了片麻岩原岩的形成时代(Liu Dunyi et al., 1992)。
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华北下地壳捕虏体中最古老的锆石来自其南缘的信阳地区(图1)。该区长英质麻粒岩捕虏体中锆石给出了3.82~3.69Ga的原岩年龄(Zheng Jianping et al., 2004a; Ping Xianquan et al., 2018; Ma Qiang et al., 2020)。锆石的Hf同位素研究(图7)显示,信阳地区下地壳捕虏体记录了4.31~3.93Ga的两阶段模式年龄(Ping Xianquan et al., 2018)。同时华北北部鞍山和曹庄也发现有4.4~3.95Ga的锆石两阶段Hf模式年龄记录(Wu Fuyuan et al., 2005, 2008; Liu Dunyi et al., 2008)。这些共同表明华北下地壳最老组分可以追溯到＞4.0Ga的冥古宙,并于3.82~3.69Ga的始太古代发生部分熔融形成长英质麻粒岩捕虏体的原岩。
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4.2 新太古代(2.8~2.5 Ga)下地壳的增生和改造
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大量的锆石U-Pb年龄数据表明,华北克拉通东部地块变质地质体主要由形成于新太古代晚期的TTG片麻岩组成 (Zhai Mingguo, 2011;Zhai Mingguo and Santosh,2011;Geng Yuansheng et al., 2012)。TTG片麻岩中还存在少量以条带、透镜体和捕虏体形式存在的变质镁铁质火山岩、沉积物和条带状铁建造(BIF)等表壳岩。例如,在山东西部,变质岩出露在泰山杂岩的早期新太古代(2.80~2.65Ga)变质火山沉积建造中,并与TTG片麻岩一起强烈变形(2.75~2.65Ga;Jahn et al., 1988)。该杂岩中的绿岩带由科马提岩和枕状玄武岩组成,年龄也在2800~2700Ma(Polat et al.,2006)。该带被辉长岩、闪长岩和高铝TTG等深成岩体侵入,并在大约2600Ma的角闪岩相条件下发生变质(Ren Peng et al., 2016)。它们被解释为地幔柱-克拉通相互作用的产物(Polat et al., 2006)或形成于大洋环境(Dong Chunyan et al., 2021)。西部地块(图1)由鄂尔多斯地块、孔兹岩带和阴山地块组成(Zhao Guochun et al., 2000;Santosh et al., 2007)。最近的研究已将孔兹岩带确定为阴山和鄂尔多斯地块之间的碰撞缝合线,并将该带重新定义为内蒙古缝合带(Kusky and Li Jianghai, 2003; Kusky et al., 2007)。鄂尔多斯地块主要被中生代至新生代盖层覆盖,地球物理探测推断盖层之下存在变质基底。阴山地块由新太古代乌拉山杂岩、色尔腾山群和各种正片麻岩组成。这些正片麻岩包括英云闪长岩、花岗闪长岩和闪长片麻岩、紫苏花岗岩、角闪二长花岗岩和角闪花岗岩,它们是通过改造较老的(2.8~2.7Ga)组分形成的(Geng Yuansheng et al., 2012)。
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图5 华北克拉通北缘汉诺坝地区岩石圈结构剖面(据Wei Ying et al., 2015修改)
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Fig.5 Cartoon showing the suggested lithospheric architecture of the Hannuoba area of the northern margin of North China Craton (modified after Wei Ying et al., 2015)
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新太古代(2.8~2.5Ga)增生事件广泛记录在华北地区的下地壳捕虏体/捕虏晶中(图7),如古生代蒙阴和复县金伯利岩、晚中生代信阳火山岩以及新生代平泉、汉诺坝和鹤壁玄武岩。这一事件与华北地表露头中出现的广泛地壳增生事件(Jahn et al., 1988;Wu Fuyuan et al., 2005; Li Jianghai and Kusky, 2007;Jiang Neng et al., 2010; Kusky, 2011; Zhai Mingguo, 2011)相吻合,并与约2.7Ga(Condie, 2000; Zheng Jianping et al., 2004d, 2009a)的全球地壳形成重大事件相吻合。
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4.3 古元古代(2.3~1.8 Ga)下地壳的改造和增生
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古元古代构造热事件广泛记录在华北克拉通的变沉积岩、高压麻粒岩、高压角闪岩、紫苏花岗岩、S型花岗岩和片麻岩中,尤其是在横贯华北的中部造山带、孔兹岩带和胶-辽-吉带 (图1; Guo Jinhui et al., 2002; Guan Hong et al., 2002; Santosh et al., 2007; Zhao Guochun et al., 2010)。在华北克拉通内显生宙砂岩(Yang Jinhui et al., 2006)和现代碎屑沉积岩(Yang Jie et al., 2009)的锆石中也发现了古元古代U-Pb年龄峰值。古元古代变质锆石颗粒和古老岩浆锆石核部周围的薄生长边缘常见于阜平杂岩(Zhao Guochun et al., 2000; Guan Hong et al., 2002)、恒山杂岩(Kröner et al., 2005)以及怀安和宣化杂岩(Guo Jinhui et al., 2002),表明主要的区域变质事件发生在约1.95~1.82Ga(Zhao Guochun et al., 2000, 2010; Kusky, 2011;Zhai Mingguo, 2011)。
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古元古代事件在复县、应县、信阳、莒南、汉诺坝和女山的下地壳中也被广泛识别(图7)。在复县,一些麻粒岩包体的大量证据,表明在1.9~1.8Ga时发生了重大的热事件,并代表了较老(2.7~2.5Ga)事件的完全重置。然而,在应县、信阳和汉诺坝,该事件标志着来自软流圈的物质的加入。古元古代事件还在华北克拉通产生了广泛分布的~1.85Ga S型花岗岩和相关基性岩,以及类似年龄的高压镁铁质麻粒岩(Zhao Guochun et al., 1999)。根据Zhao Guochun et al.(2000, 2010)的建议,在华北南缘东部(即莒南和女山),古元古代(2.3~2.2Ga)事件发生的时间略早于东部地块和西部地块之间的碰撞(1.9~1.8Ga)。它们既包括来自亏损地幔的岩浆加入,也包括较老(＞3.0Ga)地壳成分的改造。因此,下地壳捕虏体的数据不足以区分华北拼合的实际过程(Zhao Guochun et al., 2010; Kusky, 2011),需要在更大的区域开展进一步的研究工作。
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图6 华北克拉通东南缘女山地区岩石圈结构剖面(据Ping Xianquan et al., 2019修改)
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Fig.6 Cartoon showing the suggested lithospheric architecture of the Nushan area of the southeastern North China Craton (modified after Ping Xianquan et al., 2019)
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4.4 新元古代(0.8~0.6 Ga)下地壳改造
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复县麻粒岩中一些较年轻的锆石似乎代表了新元古代新锆石生长的另一幕,表明先前存在的硅酸盐和/或氧化物分解,以释放Zr和放射性Hf。这一事件被大致限定在约600Ma,与华北克拉通中已知的主要构造热事件均不吻合。然而,在新元古代至早寒武世(620~600Ma;Piper and Zhang Qirui, 1997),沉积中存在广泛的间断,表现为克拉通大部分地区的区域不整合。我们认为,这一区域性隆升反映了一个热事件(地幔柱?)影响(或改造)下地壳,并记录在复县镁铁质麻粒岩的一些年轻锆石中(Zheng Jianping et al., 2004b)。在汉诺坝岩石圈地幔(645±225Ma;Yu Jinhai et al., 2003)和信阳附近周庄辉长岩的锆石中也发现了类似的年龄。汉诺坝岩石圈地幔的年龄可能受到间隙硫化物和封闭硫化物定义的等时线的限制,这些硫化物在岩石圈地幔形成后由于Re的加入而具有更高的Re-Os同位素比值,表明下地壳和岩石圈地幔之间可能存在耦合事件(Yu Jinhai et al., 2003; Zheng Jianping et al., 2004d)。
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图7 华北克拉通下地壳捕虏体中锆石Hf同位素组成特征
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Fig.7 176Hf/177Hf versus U-Pb age for zircons in xenoliths from lower crust in the North China Craton
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数据来源:(a)复县镁铁质捕虏体来自Zheng Jianping et al.(2004b), 复县和蒙阴捕虏晶来自Zheng Jianping et al.(2009a),应县捕虏体来自Zheng Jianping et al.(2012);(b)信阳镁铁质捕虏体、长英质捕虏体和捕虏晶来自Zheng Jianping et al.(2004a, 2008); 阜新捕虏体来自Zheng Jianping et al.(2004c),莒南麻粒岩捕虏体来自Ying Jifeng et al.(2010)和Tang Huayun et al.(2014),青岛镁铁质麻粒岩捕虏体来自Zhang Hongfu(2012);(c)平泉长英质麻粒岩捕虏体来自Zheng Jianping et al.(2004c, 2012),女山和鹤壁麻粒岩捕虏体来自Zheng Jianping et al.(2012),辽东捕虏晶来自Zhang Hongfu et al.(2012),汉诺坝捕虏体来自Zheng Jianping et al.(2004d, 2009b), Jiang Neng et al.(2010, 2011)和Wei Ying et al.(2015);鞍山片麻岩来自Wu Fuyuan et al.(2008)
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Data sources: (a) Fuxian mafic xenoliths (Zheng Jianping et al., 2004b); Fuxian and Mengyin xenocrysts (Zheng Jianping et al., 2009a); Yingxian xenolith (Zheng Jianping et al., 2012); (b) Xinyang mafic xenoliths, felsic xenoliths and xenocrysts (Zheng Jianping et al., 2004a, 2008); Fuxin Monzonite (Zheng Jianping et al., 2004c); Junan granulit exenoliths (Ying Jifeng et al., 2010; Tang Huayun et al., 2014); Qingdao mafic granulitexenoliths (Zhang Hongfu, 2012); (c)felsic granulites from Pingquan (Zheng Jianping et al.,2004c, 2012) and from Nüshan and Hebi (Zheng Jianping et al., 2012); Liaodong xenocrysts (Zhang Hongfu et al., 2012b); Hannuoba xenoliths (Zheng Jianping et al.,2004d, 2009b; Jiang Neng et al., 2010, 2011; Wei Ying et al.,2015).Anshan gneiss (Wu Fuyuan et al., 2008)
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4.5 古生代—早中生代下地壳改造
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辽宁南部新生代辽东玄武岩(靠近古生代复县金伯利岩)中的捕虏晶锆石具有εHf=-16,反映了太古宙基底在古生代时期的改造(462Ma; Zhang Hongfu et al., 2011)。这种下地壳改造可能对应于复县岩石圈地幔中记录的交代事件和复县金伯利岩活动(Lu Fengxiang et al., 1998)。信阳(榴辉岩中365Ma; Zheng Jianping et al., 2008)和汉诺坝(麻粒岩捕虏体中约315Ma; Zhang Hongfu et al., 2012)也发现了下地壳捕虏体/捕虏晶中的其他古生代年龄。
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信阳和汉诺坝地区分别位于华北克拉通的南部和北部边缘,也记录了古生代至早中生代下地壳的改造。信阳地区毗邻秦岭-大别-苏鲁造山带,由早古生代(~440Ma;Yang Jingsui et al., 2005)和早中生代(240~220Ma;Li Shuguang et al., 1993)扬子板块与华北克拉通的深度俯冲和碰撞形成。该带西部(即秦岭段)记录了古生代碰撞,东部(即大别-苏鲁段)记录了中生代碰撞。与信阳地区相邻的桐柏段数据较少;这是秦岭带和大别-苏鲁带之间的过渡区域。信阳地区榴辉岩捕虏体中80%的锆石给出了古生代(377~263Ma)U-Pb年龄,记录了由于晚古生代秦岭-大别-苏鲁造山带桐柏段的俯冲和碰撞,华北克拉通南缘最底部的地壳的强烈变形。汉诺坝地区毗邻东中亚造山带南缘。该带从哈萨克斯坦延伸至西太平洋,是一个构造拼贴,将北部的西伯利亚克拉通与华北克拉通和南部的塔里木地块分隔开来。褶皱带的主要构造单元包括蛇绿岩、岛弧、洋岛、增生楔和一些前寒武纪微大陆(Khain et al., 2003)。大量地球物理和地球化学研究(Khain et al., 2003; Windley et al., 2007)表明,该造山带具有复杂的历史,包括晚古生代洋壳俯冲、古海洋闭合和长期广泛的岩浆作用,尽管造山带的碰撞时间和构造样式长期以来一直存在争议。俯冲可能会改变华北北缘(即汉诺坝)的岩石圈。
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4.6 晚中生代—新生代增生和改造
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河北汉诺坝麻粒岩、平泉镁铁质麻粒岩和捕虏晶中年龄为120~45Ma的锆石(图7)可能反映了伴随玄武岩底侵(增生)和现有下地壳在晚中生代—中新生代再熔融(改造)的热事件。这种底侵作用可能与大陆裂谷作用阶段有关,这是冈瓦纳大陆离散、亚洲增生(Wilde et al., 2003)和华北克拉通岩石圈地幔破坏的结果(Zheng Jianping et al., 2007;郑建平, 2009)。在白垩纪末期挤压构造引起的隆升和剥蚀之后,古近纪大陆裂谷作用在中国东部产生了广泛的大陆边缘盆地(Ren Jianye et al., 2002)。
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汉诺坝下地壳捕虏体中丰富的锆石年龄为47~45Ma,可能与伴随这一阶段大陆裂谷作用的热事件有关。除两个古近纪锆石颗粒外,所有颗粒的εHf均为正值。相比之下,中生代锆石的εHf为负值(Zheng Jianping et al., 2009b)。Hf同位素的这种时间变化可能与岩石圈减薄过程中的壳幔相互作用过程有关。在岩石圈减薄的早期阶段,底侵岩浆要么来自富集的岩石圈地幔,要么底侵岩浆对地壳的同化程度大于后期侵位的岩浆。汉诺坝(即古近纪)的持续岩浆底侵与玄武岩地球化学推断的华北克拉通西部岩石圈减薄基本一致(Xu Yigang et al., 2005),但与新生代中国大陆东部推断的岩石圈增厚形成对比(Zheng Jianping et al., 2009b)。
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5 主要认识
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前人通过对华北克拉通下地壳捕虏体定深,定性及定年的综合研究,建立了以信阳、莒南、汉诺坝和女山等地区为代表的中—新生代下地壳结构与组成剖面。这些剖面显示,华北下地壳垂向上具有分层的特征,且上老下新,暗示可能存在底侵作用。其中捕虏体的锆石U-Pb年龄和Hf同位素研究,揭示了华北克拉通下地壳的复杂演化过程。该克拉通下地壳最古老的物质组成可能老至冥古宙 (~4.0Ga),此后经历了古太古代(3.80~3.65Ga)的再造作用,新太古代(2.8~2.5Ga)和古元古代(2.3~1.8Ga)的增生与再造共存,同时还经历了显生宙以来(包括462~220Ma,140~90Ma和47~45Ma)的增生与再造事件。
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致谢:程裕淇先生对我国地质科学,特别是前寒武纪地质学作出杰出贡献,谨以此文纪念程裕淇先生诞辰110周年!感谢万渝生研究员的邀稿,以及评审人黄小龙研究员和英基丰研究员的宝贵修改意见。
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摘要
与中上地壳相比,对下地壳组成、结构的认识受限于样品的获取,然而天然火山作用携带的下地壳捕虏体可以为了解下地壳提供关键样品。华北克拉通是世界上最古老的克拉通之一,显生宙以来的火山作用携带有丰富的下地壳捕虏体,为探测华北下地壳组成、结构及其形成过程提供了可能。通过对这些捕虏体定深、定性及定年的综合研究,构建了以信阳,莒南,汉诺坝和女山等典型地区为代表的下地壳组成、结构剖面模型。这些剖面表明,华北克拉通下地壳具有分层的特点,且上老下新,暗示可能与底侵作用有关。其中捕虏体的锆石U-Pb年龄和Hf同位素的研究,揭示了该克拉通下地壳复杂的形成与演化过程:最古老的组成部分可能老至~4.0 Ga冥古宙,此后经历了3.80~3.65 Ga古太古代的再造作用,2.8~2.5 Ga 新太古代和2.3~1.8 Ga 古元古代的增生与再造共存,同时还经历了显生宙以来包括462~220 Ma,140~90 Ma和47~45 Ma的增生与再造事件。
Abstract
Compared with the middle and upper crust, the understanding of the composition and structure of the lower crust is limited by the availability of samples. However, lower crustal xenoliths captured by natural volcanism can provide key samples. The North China Craton is one of the oldest cratons in the world, and volcanism in this craton since Phanerozoic had carried abundant lower crustal xenoliths, which provide a possibility to explore the composition, structure and formation processes of the lower crust beneath North China Craton. Based on the comprehensive study of the determinations on the depths, nature and ages of these xenoliths, the composition and structural profiles of the lower crust represented by Xinyang, Junan, Hannuoba and Nushan areas are constructed. These profiles indicate that the lower crust beneath North China Craton is stratified with vertical architecture with old upper part and young lower part, which may be related to magma underplating. Zircon U-Pb ages and Hf isotopes of those xenoliths revealed the complex formation and evolution processes of the lower crust of the craton. The oldest component may have formed in Hadean(~4.0 Ga), and then experienced Eoarchean (3.80~3.65 Ga) reworking. During Neoarchean (2.8~2.5 Ga) and Paleoproterozoic (2.3~1.8 Ga) periods, crustal accretion and reworking coexisted. Since Phanerozoic (462~220 Ma, 140~90 Ma and 47~45 Ma), the lower crust has also experienced multistage accretion and reworking events.
Keywords
xenoliths ； volcanism ； lower crust ； structure and composition ； North China Craton
