华南陆块东部晚中生代岩浆作用的深部动力学过程

郭 锋, 赵 亮, 张晓兵, 吴扬名, 张 博, 张 峰, 4

华南陆块东部晚中生代岩浆作用的深部动力学过程

郭 锋1, 2, 赵 亮1, 2, 张晓兵1, 2, 吴扬名3, 张 博1, 张 峰1, 4

(1. 中国科学院 广州地球化学研究所, 同位素地球化学国家重点实验室, 广东 广州 510640; 2. 中国科学院 深地科学卓越研究中心, 广东 广州 510640; 3. 中山大学 地球科学与工程学院, 广东 珠海 519082; 4. 中国科学院大学 地球与行星科学学院, 北京 100049)

本文总结和回顾了过去几年来本研究团队在华南陆块东部(主要是东南沿海地区)开展的火成岩岩石地球化学研究进展, 并重点探讨了晚中生代(白垩纪)岩浆作用的深部动力学过程, 取得了以下主要认识: ①华南晚中生代镁铁质岩浆作用记录了古太平洋板块从俯冲到后撤‒撕裂的深部动力学过程, 其地幔源区的富集组分从进俯冲时期的板片上覆沉积物逐渐过渡到后撤‒撕裂阶段的下部镁铁质洋壳; ②长英质火山岩的地壳源区从俯冲阶段的低温(700～810 ℃)含水下地壳转变为后撤‒撕裂阶段的高温(790～860 ℃)贫水陆壳; ③东南沿海地区晚中生代经历了强烈的弧地壳增生和置换作用, 形成了具有“等同位素效应”的双峰式侵入杂岩体。我们提出的板片俯冲‒后撤‒撕裂模式同样适用于解释华南陆块东部早中生代构造‒岩浆演化的深部的动力学机制。

板片俯冲‒后撤‒撕裂作用; 地壳演化; 古太平洋; 晚中生代岩浆作用; 华南陆块

0 引 言

大洋板块俯冲作用、弧下地幔楔富集改造和俯冲板片的归宿是探索地球层圈相互作用和物质再循环过程的重要内容, 也是当今板块构造学说的奠基石之一。目前地球大约有长达43000 km的俯冲带, 每年有大量的大洋岩石圈和上覆沉积物被带到弧下地幔中, 其中俯冲沉积物通量每年在2.5～3.0 km3之间(von Huene and Scholl, 1991; Clift et al., 2009; Scholl and Von Huene, 2010)。如此巨量的再循环地壳物质如何改造地幔楔？这些被俯冲的大洋岩石圈归宿在哪里？俯冲带与板内岩浆存在何种成因联系？这些问题都是地球演化研究的主要内容(Hofmann, 1997; Van der Lee and Nolet, 1997; Wu et al., 2009)。

早中生代以来古太平洋板块经历了初期的洋底扩张、增生、俯冲到最后消亡的全过程, 不同时期其与周边板块的相互作用存在异同(Sun et al., 2007; Seton et al., 2012; Müller et al., 2016)。中生代期间, 中国东部为古太平洋俯冲作用下形成的活动大陆边缘(Engebretson et al., 1985; Maruyama et al., 1989; Faure and Natal’in, 1992; Zheng et al., 2013; Guo et al., 2015; 郭锋, 2016; 唐杰等, 2018; 朱日祥和徐义刚, 2019; Ma and Xu, 2021), 记录了古太平洋板块俯冲、地壳增生、俯冲板片再循环等深部过程。华南地区位于东亚大陆边缘的东南部, 记录了古太平洋板块从俯冲、后撤到撕裂的完整过程(Jahn et al., 1990; Charvet et al., 1994; Lapierre et al., 1997; 董传万等, 1997; Chen and Jahn, 1998; Xu et al., 1999; Zhou and Li, 2000; Zhou et al., 2006; Li and Li, 2007; Li et al., 2007, 2019; 李献华等, 2007; 张国伟等, 2013; Wang et al., 2013; Lin et al., 2018; Guo et al., 2021; Shu et al., 2021; Mao et al., 2021)。在国家自然科学基金委员会‒广东省联合基金等项目的资助下, 我们针对华南陆块东部尤其是中国东南沿海地区晚中生代岩浆作用开展了系统的岩石学、年代学、元素‒同位素地球化学和数值模拟等综合研究。结合前人的研究基础, 本文将从大洋板块俯冲、后撤、撕裂的动力学过程来理解深部岩石圈地幔演变、地壳热结构与成分的变化, 从而为活动大陆边缘镁铁质‒长英质岩浆作用形成机制提供新视角。

1 华南陆块东部晚中生代岩浆作用的时空格架

华南陆块尤其是东部地区广泛分布了中生代火成岩(图1a), 这些火成岩岩浆作用的时空格架一直备受关注。Charvet et al. (1994)将中国东南沿海地区火山作用分为两个阶段: 晚侏罗世‒早白垩世以酸性火山岩喷发为主的第一阶段, 晚白垩世以拉斑玄武岩及酸性火山岩为主的第二阶段。Zhou et al. (2006)将东南沿海地区火山作用也划分为两个主要阶段: 燕山早期和燕山晚期, 或称之为上、下火山岩系。Guo et al. (2012)将粤东‒福建东南部中生代火成岩划分为三个阶段: 168～145 Ma、143～130 Ma和104～95 Ma。Wang et al. (2013)则把华南地区岩浆作用与构造变形结合起来, 将华南地区中生代岩浆作用细分为六个峰期: 240 Ma、220 Ma、175 Ma、158 Ma、125 Ma和93 Ma, 其中前面两个峰期与印支期变质‒变形年龄相吻合, 集中在华南内部, 空间上对应于印支期花岗岩的分布范围; 最后一个峰期与燕山晚期的变形年龄基本一致, 主要分布在东南沿海地区。Cao et al. (2021b)根据华南地区780多个侏罗纪‒白垩纪火成岩年代学数据, 将岩浆作用分为四个主要阶段: 190～175 Ma、165～155 Ma、145～125 Ma和105～95 Ma, 对应的岩浆活动峰值时间为160 Ma、130 Ma和100 Ma(图1b), 其中前面两个阶段主要为侏罗纪侵入岩, 集中分布于华南内部, 与古海沟的距离在550～1200 km之间; 后两个阶段主要为白垩纪火山岩, 集中分布在东南沿海地区, 与古海沟的距离为400～800 km。

本次研究统计了长英质火山岩和花岗岩锆石Hf同位素组成, 结果显示, 无论是喷出岩还是侵入岩, 锆石Hf同位素组成与其形成年龄呈现出一定的负相关关系, 反映了随着时间推移, 岩浆源区中亏损幔源组分不断增加(图1c、d), 而且岩浆温度随岩石年龄变年轻而逐渐增高(图1e)。长英质火山岩的Hf(t)变化趋势略优于花岗质岩石。总体上, 华南地区中生代岩浆作用呈现出多期次幕式特点, 尤其是晚中生代火成岩的分布与古太平洋板块俯冲作用之间存在密切联系(Zhou et al., 2006)。

2 镁铁质岩浆作用记录的俯冲‒后撤‒撕裂过程

华南陆块东部地区广泛发育白垩纪弧岩浆和板内镁铁质岩浆作用(图2; 董传万等, 1997; Xu et al., 1999; Wang et al., 2003, 2008; Zhao et al., 2007; Meng et al., 2012; Li et al., 2014; Zhang et al., 2019, 2020a; 秦社彩等, 2019; Wu et al., 2020)。毫无疑问, 这两类岩浆作用与当时的古太平洋板块俯冲作用存在着成因联系(Charvet et al., 1994; Lapierre et al., 1997; Xu et al., 1999; Zhou and Li, 2000), 但是它们之间存在何种联系, 仍不清晰。为此, 我们对该区白垩纪(120～70 Ma)镁铁质火成岩进行全面的地球化学数据汇编和重新分类, 并结合俯冲板片熔融的二维数值模拟, 构建了古太平洋俯冲、后退和撕裂作用的统一构造动力学模型, 并以此为基础来阐述俯冲带和板内镁铁质岩浆作用的内在成因联系(Guo et al., 2021)。

根据Nb含量以及Nb/Y、Nb/U、Ba/Nb和Zr/Nb值等, 华南陆块东部白垩纪(120～77 Ma)镁铁质岩石可分为拉斑‒钙碱性岛弧型玄武岩(Nb<20´10−6)、低Nb(Nb<40´10−6)的弱碱性玄武岩(OIB型)和高Nb(Nb>50´10−6)碱性玄武岩(OIB型)(图3a)。随着Nb含量变化, 白垩纪镁铁质岩石不仅在岩石类型上变化明显, 同时在微量元素地球化学特征方面也有所反映: 从岛弧型玄武岩Nb-Ta相对亏损, 到低Nb玄武岩无高场强元素(Nb、Ta、Zr、Hf)亏损, 到高Nb玄武岩Nb-Ta正异常; 相应地, Pb从正异常逐渐向无异常到负异常转变(图4), 反映了再循环陆壳物质对岩浆成因的贡献逐渐降低(Sun and McDonough, 1989; Rudnick and Gao, 2014)。

同位素组成上(图5), 岛弧型玄武岩显示出相对富集的Sr-Nd-Pb-Hf同位素组成: (87Sr/86Sr)i>0.705,Nd()<0,Hf()<+4, (206Pb/204Pb)i>18.10, (207Pb/204Pb)i> 15.57, (208Pb/204Pb)i>38.20。高Nb玄武岩则显示出最亏损的Sr-Nd-Hf同位素组成, (87Sr/86Sr)i<0.704,Nd()>+4,Hf()>+8。而低Nb玄武岩介于二者之间, 同位素组成变化较大, 如Nd()变化在−1.7～+6.8之间, (87Sr/86Sr)i的变化范围为0.7036～0.7075。但在Pb同位素组成上, 三类岩石则较相似。

根据Lu/Hf对Th/La、Th/Yb判别图解(图略), 华南陆块东部弧岩浆具低Lu/Hf值、高Th/La和Th/Yb值, 反映它们形成于活动大陆边缘或者陆缘弧(Zhao et al., 2019), 其源区主要为俯冲陆源沉积物熔体改造的地幔楔, 与现代小安德烈斯和西南日本俯冲带相似(White and Dupré, 1986; Hanyu et al., 2006; Labanieh et al., 2010), 为相对较热的俯冲带(Plank et al., 2009; Zhang et al., 2019及其中参考文献)。

锆石U-Pb年龄数据来源: Cao et al., 2021b及其中参考文献。锆石Hf同位素数据来源: Guo et al., 2012; Liu et al., 2012, 2014, 2016a, 2016b, 2018; Huang et al., 2015; Jiang et al., 2015; Zhao et al., 2015, 2016a, 2016b, 2021; Wang et al., 2015; Li et al., 2016a, 2016b, 2017; Xia et al., 2016; Yan et al., 2016。岩浆温度为全岩锆饱和温度数据来源: Lapierre et al., 1997; 余明刚等, 2008; He et al., 2009, 2012; Guo et al., 2012; Jiang et al., 2013, 2015; Li et al., 2013, 2016a, 2016b, 2017; Liu et al., 2014, 2016a, 2018; Song et al., 2016; Wang et al., 2016a; Yan et al., 2016; Zhang et al., 2018。

早、晚白垩世A型花岗岩分布据Peng et al. (2021)。

汇编的详细数据见Guo et al. (2021)附表1。

图b中镁铁质侵入岩母岩浆的微量元素含量根据Guo et al. (2015, 2016)的方法进行了重新计算。原始地幔标准化数据来自Sun and McDonough (1989)。

LILE/Nb与REE/Nb、Zr/Nb等协变关系可以用来约束再循环地壳组分的性质。协变关系图解显示, 华南陆块东部高Nb玄武岩源区再循环地壳组分主要为脱水的洋壳, 而低Nb玄武岩的源区中则包含了一定量的海洋沉积物(图6)。此外, Wu et al. (2020)研究发现江西吉安地区螺丝山晚白垩世玄武岩中橄榄石斑晶具有相对地幔值更低的O同位素组成, 反映其再循环洋壳经历了高温水‒岩反应(Eiler, 2001)。而且华南陆块东部高Nb玄武岩总体上具一定的Eu、Sr正异常和高Ba/Th值, 以及初始岩浆贫水等特征(Wang et al., 2003, 2008; Wu et al., 2020), 说明再循环物质很可能来自辉长岩等堆晶岩组成的下部洋壳(Bach et al., 2001; Kelley et al., 2003), 并在俯冲过程中经历了强烈脱水作用, 因而呈现出强烈的Pb负异常(图4c)。部分低Nb玄武岩表现出低Ca同位素组成, 也反映了俯冲沉积物改造的地幔源区(Zhang et al., 2020a)。

华南陆块东部白垩纪镁铁质岩石地球化学特征和成因解释揭示, 区域地幔的改造介质从洋壳上部的海洋沉积物转变到洋壳下部的基性地壳部分, 这些镁铁质洋壳在熔融之前经历了强烈的俯冲脱水过程(Zhang et al., 2019, 2020a; Wu et al., 2020)。因此无论是岛弧玄武岩还是板内OIB型玄武岩, 其源区都包含了来自俯冲的古太平洋板片组分, 只是代表了不同的洋壳部分。

根据华南陆块东部白垩纪镁铁质岩石的地球化学变化趋势, 采纳有限元计算方法(Gerya and Yuen, 2003), 对脱水洋壳熔融过程进行二维热力学模拟研究。模拟结果显示, 当俯冲脱水板片在软流圈下沉过程中, 未减薄或者撕裂的完整脱水板片不会发生熔融; 只有当俯冲板片发生撕裂、破碎或者碎片化, 脱水的洋壳才能沿着俯冲岩石圈的撕裂面发生部分熔融作用, 其熔融的程度与板片的宽度、厚度呈负相关关系(图7; Guo et al., 2021)。

综合华南陆块东部白垩纪镁铁质岩石地球化学变化趋势、时空分布特征和热力学数值模拟结果, 我们提出了古太平洋板块俯冲到后撤‒撕裂过程与镁铁质岩石成因模式: 俯冲沉积物熔体交代的地幔楔为弧岩浆的源区; 不均一的板片后撤很可能导致了板片撕裂和脱水板片的部分熔融, 熔体与软流圈反应形成OIB型板内玄武岩的地幔源区(图8; Guo et al., 2021)。

最近, 华南东南部也发现了早白垩世OIB型基性脉岩(Yan et al., 2021)和同时期的A型花岗岩(Peng et al., 2021), 被认为与俯冲板片的后撤‒撕裂作用相关。Peng et al. (2021)对白垩纪A型花岗岩进行了统计, 识别出两条A型花岗岩带, 其中早白垩世A型花岗岩带从西南向东北方向侵位时代逐渐变年轻, 可能与古太平洋板片由西南向东北后撤过程有关; 晚白垩世A型花岗岩则主要分布在东南沿海地区, 与古太平洋板片向东后撤过程相关(图2)。这与Sun et al. (2007)认为古太平洋板块向欧亚板块运动方向在125 Ma左右发生了转变相吻合。

3 白垩纪长英质火山作用记录的板片俯冲‒后撤过程

相对于镁铁质岩浆, 中国东部发育更为广泛的中生代长英质火山岩和花岗质岩石(图9), 尽管这些长英质岩浆活动普遍被认为与古太平洋板块俯冲作用相关(Xu et al., 1999; Zhou and Li, 2000; Zhou et al., 2006), 但古太平洋板块的俯冲方式(如平板俯冲、洋脊俯冲以及周期性的俯冲与后撤)及其对长英质岩浆作用成因的影响目前还存在争议(Li and Li, 2007; Li et al., 2007; Sun et al., 2007; Guo et al., 2012; Liu et al., 2012, 2014, 2016b)。前人研究表明, 早白垩世晚期‒晚白垩世, 中国东南部经历了两期火山作用, 这两期火山岩表现出明显的地球化学成分变化, 暗示其可能来自于不同的构造环境(Guo et al., 2012; Liu et al., 2012, 2014, 2016; Li et al., 2014)。通过对浙江东南部白垩纪长英质火山岩详细的年代学和岩石地球化学研究, 我们识别出古太平洋板块俯冲与后撤过程在地壳演化中的记录, 建立了长英质岩浆作用与古太平洋板块俯冲之间的成因联系(Zhao et al., 2021)。

日本西南部数据来自Hanyu et al., 2006。

高Nb玄武岩地幔源区的改造组分主要是脱水洋壳, 低Nb玄武岩地幔源区的改造组分中包括脱水洋壳和较大比例的沉积物。模拟参数来源: Zhang et al., 2020a; Guo et al., 2021。模拟参数中元素含量单位: ×10−6。

两期火山岩均表现出高SiO2、富K2O和中等‒强过铝质的特征, 并且具有相似的Nd和Hf同位素组成(早期火山岩:Nd()=−9.5～−7.5,Hf()=−8.5～−0.7; 晚期火山岩:Nd()=−7.1～−6.1,Hf()=−7.8～−2.2; Zhao et al., 2021)。Sr-Nd同位素模拟计算结果显示, 其来源于华夏地块古老基底岩石和中生代新生地壳的混合源区(Chen and Jahn, 1998; Zhang et al., 2019)。

由于云母类矿物富集Rb而具高Rb/Sr值, 因此地壳熔融过程中, 水致熔融(云母稳定而在源区残留)和脱水熔融(云母不稳定发生分解进入熔体)(Patiño Douce and Beard, 1995; Gao et al., 2017)会使熔体中Rb含量和Rb/Sr值产生差异。两期火山岩虽然具有相似的同位素组成, 但早期火山岩具有高Sr、Ba含量和低Rb/Sr值特征, 暗示其为含水条件下的熔融产物, 而黑云母在源区残留; 晚期火山岩则表现出低Sr、Ba含量和高Rb/Sr值特征, 反映其为相对贫水状态下黑云母脱水熔融的产物(图10a、b; Gao et al., 2017)。同时, 全岩锆饱和温度计算显示, 早期火山岩显示较低的熔融温度(700～810 ℃); 而晚期火山岩具有较高的熔融温度(790～860 ℃; 图10c)。此外, 早期火山岩显示较高的Sr/Y和(La/Yb)CN值, 暗示其来源于较深的熔融源区; 而晚期火山岩具有较低的Sr/Y和(La/Yb)CN值, 暗示其来源于较浅的地壳深度(图 10c、d; Profeta et al., 2015)。

板片越薄, 越有利于脱水洋壳熔融; 板片越宽, 下沉变慢, 加热充分, 熔融范围增大。

(a) 相对较热古太平洋板块俯冲导致了沉积物大量熔融, 其熔体交代上覆地幔楔, 被沉积物熔体交代地幔楔熔融形成了区域上弧岩浆; (b) 俯冲板块回卷后撤导致海沟后退和板片撕裂, 被撕裂的脱水板片熔融形成的熔体交代软流圈地幔形成OIB型玄武岩的地幔源区。

晚白垩世A型花岗岩年代学数据来源: 邱检生等, 1999; 肖娥等, 2007; 林清茶等, 2011; Chen et al., 2013, 2019。

因此, 白垩纪两期火山岩矿物学和地球化学组成特征差异可能是由其源区不同的熔融条件造成的, 即早期火山岩来源于较深、相对湿‒冷的地壳源区; 而晚期火山岩来源于较浅、相对干‒热的地壳源区。熔融源区--H2O条件改变暗示, 华南陆块东部地壳在早白垩世晚期(约110～100 Ma)经历了从俯冲挤压环境向板内伸展环境的转变(图11), 这与古太平洋板块从俯冲到板片后撤的动力学过程相吻合。根据区域镁铁质和长英质岩浆的时空演变趋势, 华南陆块东部地区从俯冲到后撤‒撕裂的转折时间大体发生在110～100 Ma之间, 与区域上开始出现A型花岗岩和OIB型镁铁质岩浆作用的时间大体一致。

综上所述, 中国东南部早白垩世晚期‒晚白垩世经历了两期地壳熔融。117～113 Ma, 古太平洋板块俯冲在华南陆块东部形成活动大陆边缘(Zhang et al., 2019), 俯冲板片脱水引起地壳发生熔融, 形成早期相对富水、冷的长英质火山岩; 同时俯冲沉积物熔体交代地幔楔, 形成同期的弧镁铁质岩浆。110～93 Ma期间, 俯冲的古太平洋板片发生后撤和撕裂, 引起弧后伸展和软流圈物质上涌, 大量来自软流圈的热能引起下地壳物质的熔融, 形成晚期相对贫水、热的长英质火山岩以及同期的A型花岗岩。

(a) Rb/Sr-Ba; (b) Sr-Rb(据Gao et al., 2017); (c) (La/Yb)CN-TZr(据Profeta et al., 2015); (d) Sr/Y-La/Yb(据Wang et al., 2016b)。

WPG. 板内花岗岩; VAG. 火山弧花岗岩; syn-COLG. 同碰撞花岗岩; ORG. 洋中脊花岗岩。浙东南114～113 Ma和100～93 Ma火山岩数据来源: Zhao et al., 2021。东南沿海114～90 Ma火山岩数据来源: Lapierre et al., 1997; 余明刚等, 2008; He et al., 2009; Guo et al., 2012; Jiang et al., 2013, 2015; Yan et al., 2016。

4 “等同位素效应”镁铁质‒长英质侵入杂岩记录的地壳演化过程

在中国东南沿海地区, 经常可以看到一些镁铁质‒长英质侵入杂岩中镁铁质岩石与长英质岩石具有相似的同位素组成, 被称为“等同位素效应”(薛怀民等, 1996; 邢光福和陶奎元, 1998; Xing et al., 2004), 如平潭和泉州侵入杂岩(图12)。平潭和泉州的花岗岩都具有高分异I型花岗岩特点, 而伴生的镁铁质侵入岩为角闪石辉长岩, 二者在Hf和Nd同位素组成上非常相似(图13)。与此同时, 花岗岩中锆石具有类似地幔的O同位素组成(图14), 反映其来源于幔源的镁铁质原岩, 且很少有变沉积岩组分的参与。关于这些花岗岩成因, 主要有以下三种可能性。

(1) 花岗岩来自镁铁质岩浆的分异作用。由于泉州和平潭花岗岩与辉长岩具有几乎一致的形成时间(～115 Ma), 二者可能为同源岩浆不同阶段分异的产物。在东南沿海地区, 镁铁质岩浆的分布规模相对于长英质岩石小很多, 如果二者之间是分异关系, 那么应该存在更多的镁铁质岩石。另外, 镁铁质与长英质岩石之间表现出双峰式特点, 几乎没有中性过渡成分岩石, 因此也很难用分异作用来解释。

图12 平潭和泉州侵入杂岩体地质图(据张博等, 2020)

图13 平潭和泉州的花岗岩与辉长岩Nd(a)和Hf(b)同位素组成对比(据Zhang et al., 2019; 张博等, 2020)

其他东南沿海的锆石Hf-O同位素数据来自Chen et al. (2013, 2019)及其中参考文献。地幔锆石的O同位素组成据Valley (2003)。

(2) 花岗岩为镁铁质岩浆与壳源岩浆混合的产物。Griffin et al. (2002)根据平潭花岗岩中锆石较大的Hf同位素变化范围, 认为其为壳‒幔源岩浆混合作用的结果。但是我们最近的研究结果显示无论是平潭花岗岩还是泉州花岗岩, 其锆石Hf同位素组成的变化范围较小, 不能用源区的不均一性来解释(张博等, 2020)。平潭杂岩体中可观察到基性微粒包体(MME), 反映了局部岩浆混合作用。对这些岩体不同岩性(辉长岩、花岗闪长岩和花岗岩)的磷灰石进行详细地球化学研究, 结果显示它们三者之间并不存在岩浆混合关系, 而是各自母岩浆分异与流体作用或者脱气作用的结果(Zhang et al., 2020b, 2021)。

(3) 同期底侵镁铁质岩石的熔融作用。在成分上, 花岗岩具高钾钙碱性、强烈的Sr-Eu负异常以及偏铝到过铝质特征, 反映了其壳源成因特点(Sisson et al., 2005)。但在同位素组成上, 花岗岩与辉长岩呈现出一致性, 反映了其同源性。且二者在空间上的密切共生, 也反映了相互之间紧密的成因联系。

通常, 镁铁质岩石相对变沉积岩具有高得多的熔融温度, 因此只要地壳源区中存在这些变沉积岩组分, 它们会首先发生部分熔融, 形成过铝质的长英质岩浆以及类似华夏基底非常富集的Nd-Hf同位素组成(Chen and Jahn, 1998; Yu et al., 2009, 2010)。这显然与花岗岩较华夏地块基底高得多的Nd-Hf同位素组成相矛盾。花岗岩中锆石具类似地幔的O同位素组成特征, 从另一个侧面也说明花岗岩的熔融源区几乎没有古老华夏基底岩石的贡献。

近些年来, 陆续报道华南陆块东部存在古老华夏基底(于津海等, 2006; Xu et al., 2007; Yu et al., 2009, 2010及其中参考文献)。那么一种可能性是, 这些基底岩石被加厚、榴辉岩化而拆沉到地幔中, 但是区域上几乎没有出现强烈的挤压变形、超高压变质岩和埃达克质岩浆作用等白垩纪地壳加厚的地质学和岩石学记录。另一种可能就是, 早先存在的地壳岩石被底侵的幔源岩浆破坏、稀释和置换(Guo et al., 2019), 变成了新增生的弧地壳; 这与地球物理观察到区域中下地壳存在4～5 km的低速层相吻合(Zhang et al., 2008)。

综合分析表明, 区域“等同位素效应”双峰式侵入杂岩体的成因: 古太平洋板块俯冲背景下, 幔源岩浆不断底侵到中、下地壳, 破坏并置换了原有的华夏古老地壳, 部分幔源岩浆上升侵位于上地壳, 形成了杂岩的镁铁质组分(图15)。固结的幔源岩浆在中、下地壳发生部分熔融作用形成长英质岩浆, 这些岩浆发生不同程度的分异、自混合作用和脱气过程形成了花岗岩和花岗闪长岩(张博等, 2020; Xu et al., 2021; Cao et al., 2021a; Zhang et al., 2021)。由于长英质岩浆与镁铁质岩浆是同源的, 因此在同位素组成上具有一致性。这种新生弧地壳对古老地壳的置换在活动大陆边缘广泛存在, 如中国东北、俄罗斯远东、澳大利亚塔斯马尼亚造山带等地(Guo et al., 2019), 并形成了众多的“等同位素效应”火成杂岩。

5 总结与展望

(1) 根据Nb含量, 华南陆块东部白垩纪镁铁质岩石可以划分为岛弧型玄武岩、低Nb玄武岩和高Nb玄武岩, 其地幔源区记录了从俯冲洋壳上覆沉积物到下洋壳蚀变辉长岩的熔体改造过程, 反映了古太平洋板片从俯冲到后撤‒撕裂的深部动力学过程, 转折的时间主要发生在110～100 Ma之间。

(2) 古太平洋板片从俯冲到后撤‒撕裂过程改变了区域地壳的热‒化学结构, 尤其是在东南沿海地区, 长英质岩浆的地壳源区从俯冲阶段的富水低温状态转变到后撤‒撕裂阶段的贫水高温状态。

(3) 古太平洋板块俯冲作用在华南陆块东部形成了大量的新生弧地壳, 它们破坏和置换了早先存在的华夏地块古老地壳, 新生弧壳在幔源岩浆的不断底侵过程中发生熔融形成了广泛的中酸性岩浆, 它们与上侵的镁铁质岩浆形成了区域上的“等同位素效应”双峰式火成杂岩。

图15 东南沿海早白垩世“等同位素效应”双峰式杂岩的形成模式(据张博等, 2020; Xu et al., 2021; Cao et al., 2021a修改)

华南陆块东部地区中生代岩浆作用期次非常多, 成分复杂, 岩石组合多样, 前人曾经提出了多种深部动力学成因模式。基于我们提出的板块俯冲‒后撤‒撕裂模型, 可以将区域上广泛发育的中生代早期(比如印支期和中‒晚侏罗世)壳源岩浆作用理解为平板俯冲作用导致地壳缩短加厚和深熔作用的产物, 而OIB型玄武岩或其对应的镁铁质侵入岩和A型花岗岩组合(早侏罗世、早白垩世和晚白垩世)则可以视作板片后撤‒撕裂或断离作用的岩石学记录。然而, 除了早白垩世古太平洋板块俯冲作用有清晰的地质学和岩石学记录外, 有关中生代其他各个时期的俯冲带位置和俯冲方式仍然存在较大的争议。理论上, 古太平洋板块的平板俯冲作用势必导致地壳的缩短加厚, 并促进板片的高压熔融, 从而产生类似于南美安第斯造山带的埃达克质岩浆, 但是这类岩浆在华南陆块东部地区中生代火成岩中罕见, 因此需要更详细的岩石学和地球化学研究。综上所述, 关于古太平洋与华南大陆之间的相互作用动力学过程仍需要更多的来自古地磁学、古板块恢复、地质学、岩石学和地球化学甚至是动力学模拟等多学科的交叉和集成研究, 以期取得更为可靠的研究成果。

致谢:成文过程中得益于与中山大学王岳军教授和中国科学院广州地球化学研究所黄小龙研究员的讨论, 中国地质大学(武汉)郑建平教授和云南大学王选策教授在评审过程中提出了宝贵意见和建议, 在此一并致以诚挚的谢意。

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Geodynamics of Late Mesozoic Magmatism in the Eastern South China Block: An Overview

GUO Feng1, 2, ZHAO Liang1, 2, ZHANG Xiaobing1, 2, WU Yangming3, ZHANG Bo1, ZHANG Feng1, 4

(1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China; 2. Center of Excellence of Deep Earth Sciences, Guangzhou 510640, Guangdong, China; 3. School of Earth Sciences and Engineering, Sun Yat-sen University, Zhuhai 519082, Guangdong, China; 4. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China)

In this paper, we review the petrological and geochemical studies of the late Mesozoic igneous rocks in the eastern part of South China (mainly the southeast coastal area) conducted by our group during the past five years, focusing mainly on the geodynamic process of the Cretaceous magmatism. The main advancements include: (1) The late Mesozoic mafic magmatism in South China recorded the geodynamic processes from subduction to rollback and tearing of the paleo-Pacific slab. During these processes, the mantle sources for the mafic magmas were enriched by the subducted sediments of the upper oceanic crust during the advanced subduction and were then metasomatized by the lower oceanic crust in response to the slab retreat and tearing. (2) The crustal sources for felsic volcanic rocks changed from the low-temperature (700–810 ℃) water-rich crust in the advanced subduction stage to the high-temperature (790–860 ℃) water-poor continental crust in the rollback-tearing stage. (3) The late Mesozoic crust in the southeast coastal area experienced extensive crustal accretion and subduction-induced replacement, forming the ‘equal-isotope’ and petrochemically bimodal intrusive complexes. We propose that the slab subduction-rollback-tearing model may also be applicable to the geodynamics of the early Mesozoic tectonic-magmatic evolution in the eastern South China Block.

slab subduction and rollback-tearing; crustal evolution; the paleo-Pacific; late Mesozoic magmatism; South China Block

2021-12-10;

2022-02-25

国家自然科学基金项目(U1701641、41525006、42073032、42021002)资助。

郭锋(1971–), 男, 研究员, 主要从事岩石学和大地构造学研究。E-mail: guofengt@263.net; fengguo@gig.ac.cn

P581; P511.4

A

1001-1552(2022)03-0416-019

10.16539/j.ddgzyckx.2022.03.002