地质力学学报  2020, Vol. 26 Issue (5): 759-790
引用本文
赖绍聪, 朱毓. 扬子板块西缘新元古代典型中酸性岩浆事件及其深部动力学机制:研究进展与展望[J]. 地质力学学报, 2020, 26(5): 759-790.
LAI Shaocong, ZHU Yu. Petrogenesis and geodynamic implications of Neoproterozoic typical intermediate-felsic magmatism in the western margin of the Yangtze Block, South China[J]. Journal of Geomechanics, 2020, 26(5): 759-790.
扬子板块西缘新元古代典型中酸性岩浆事件及其深部动力学机制:研究进展与展望
赖绍聪1,2, 朱毓1,2    
1. 大陆动力学国家重点实验室, 陕西 西安 710069;
2. 西北大学地质学系, 陕西 西安 710069
摘要:华南板块发育有巨量新元古代岩浆岩,因而是研究罗迪尼亚(Rodinia)超大陆演化期间华南板块地幔属性、地壳演化和壳幔相互作用最理想的场所。虽然在扬子西缘新元古代镁铁质和酸性岩浆作用方面已有大量的研究,但是在系统研究中酸性花岗岩类所代表的不同深部动力学意义的方面还较为薄弱。文章基于团队近期对于扬子板块西缘新元古代典型花岗岩类的研究成果,系统揭示不同深度层次的岩浆作用。最新研究支持扬子西缘新元古代受控于俯冲构造背景,除发生俯冲流体和板片熔体交代地幔作用外,最新识别的ca.850~835 Ma高Mg#闪长岩指示俯冲沉积物熔体也参与了地幔交代作用。Ca.840~835 Ma过铝质花岗岩的发现说明扬子西缘新元古代时期不仅存在新生镁铁质下地壳的熔融,也发生了俯冲背景下成熟大陆地壳物质的重熔。Ca.780 Ma Ⅰ型花岗闪长岩-花岗岩组合揭示了俯冲阶段后期板片回撤断离后软流圈地幔瞬时上涌引发的不同地壳层次的岩浆响应。从ca.800 Ma的增厚下地壳来源的埃达克质花岗岩到ca.750 Ma的酸性地壳来源的A型花岗岩的出现,表明扬子西缘新元古代时期经历了俯冲有关的地壳增厚到俯冲后期弧后扩张背景下的区域性地壳减薄。
关键词扬子西缘    中酸性岩浆事件    俯冲背景    不同深度层次岩浆作用    
DOI10.12090/j.issn.1006-6616.2020.26.05.062     文章编号:1006-6616(2020)05-0759-29
Petrogenesis and geodynamic implications of Neoproterozoic typical intermediate-felsic magmatism in the western margin of the Yangtze Block, South China
LAI Shaocong1,2, ZHU Yu1,2    
1. State Key Laboratory of Continental Dynamics, Xi'an 710069, Shannxi, China;
2. Department of Geology, Northwest University, Xi'an 710069, Shannxi, China
Abstract: The South China Block preserves voluminous Neoproterozoic magmatism, it is thus an ideal site for understanding the mantle nature, crustal evolution, and crust-mantle interaction during the assembly and breakup of the Rodinia supercontinent. Although the previous studies have paid more attention to the mafic and felsic rocks, the systematic deep dynamics of intermediate-felsic intrusive rocks is unsubstantial. Based on the recent studies on the Neoproterozoic typical granitoid magmatism, this study provides a systematic insight on the magmatic response of different depth under subduction setting. The new study reveals that the western margin of the Yangtze Block was located at the subduction setting. Apart from the subduction fluids- and slab melts-related mantle metasomatism, the newly recognized ca.850~835 Ma high-Mg# diorites suggest that there existed the subducted sediment melts-related mantle metasomatism. In addition, the identification of ca.840~835 Ma peraluminous granites indicates that the western margin of the Yangtze Block underwent not only the melting of the juvenile mafic lower crust but also the reworking of the mature continental crust during the Neoproterozoic. Moreover, the ca.780 Ma Ⅰ-type granodiorites-granites stand for the magmatic response of different crustal depth induced by the upwelling of asthenosphere mantle. The occurrence from ca.800 Ma thickened lower crust-derived adakitic granites to ca.750 Ma felsic crust-derived A-type granites suggest the geodynamic transition from regionally crustal thickening to extensional thinning under subduction background.
Key words: western margin of the Yangtze Block    intermediate-felsic magmatism    subduction setting    magmatic response of different depth    
0 引言

超大陆的汇聚和裂解在全球地质演化过程中扮演着重要的角色(Cawood et al., 2016),直接影响着地球内外部圈层的演化和相互联系,如岩浆作用、沉积盆地、生物演化、气候变化等(郑永飞,2003; Zheng et al., 2013)。罗迪尼亚(Rodinia)超大陆的重建对于理解前寒武纪时期全球大陆构造格局至关重要(Zheng et al., 2004; Li et al., 2008a, 2008b; Zhao and Cawood, 2012; Cawood et al., 2016)。作为东亚地区最大的板块之一(Zhao and Cawood, 2012),华南板块发育有大量新元古代中酸性岩浆岩和镁铁质-超镁铁质岩石,这些岩石被认为是晚中元古—新元古代时期Rodinia超大陆汇聚与裂解过程的产物(Wang et al., 2013, 2014a; Zheng et al., 2013; Zhao et al., 2018),记录着该时期华南板块的地幔属性、地壳演化和壳幔相互作用信息,从而成为探索Rodinia超大陆演化进程的重要载体。近年来,不同学者从岩浆岩、碎屑锆石、地球物理、显微构造分析等多方面对扬子板块构造背景及其在Rodinia超大陆演化过程中的作用进行了深入的研究,但关于地幔柱模式、板片-裂谷模式和岛弧模式的深部动力学机制依然存在较大争议。

花岗岩类是大陆地壳演化过程中分布最广的岩浆岩类型,对其源区性质和成因机制的深入研究有助于系统了解不同时期大地构造演化中岩浆作用的时空分布、大陆地壳的物质组成和演化规律(Hawkesworth and Kemp, 2006; Kemp et al., 2007; Clemens et al., 2009, 2016, 2017; Clemens and Stevens, 2012; Castro, 2013, 2014; 王孝磊,2017; Cawood and Hawkesworth, 2019; 王涛等,2019)。来源于深部地壳、俯冲陆壳、洋壳以及交代地幔源区的闪长质-花岗闪长质岩石为理解不同构造背景下地幔属性和壳幔相互作用过程提供了机遇(Defant and Drummond, 1990; Smithies and Champion, 2000; Kamei et al., 2004; Martin et al., 2005; Karsli et al., 2007, 2017; Qian and Herman, 2010; Chappell et al., 2012; Clemens et al., 2016, 2017)。因此,对于不同构造层次的花岗岩类的研究,能够有效地揭示地球不同深度层次的物质属性和相互关联。

长期以来,学者们虽然对扬子板块西缘新元古代岩浆岩进行了大量研究,但对于中酸性花岗岩类或者具有特殊构造指示意义的花岗岩岩石组合以及它们所代表的深部动力学意义的系统研究相对较少。

本文基于团队近期对扬子西缘新元古代典型花岗岩体和花岗岩类岩石组合系统的岩石学、地球化学、锆石U-Pb-Hf同位素研究,结合实验岩石学理论和区域地质背景分析,对不同类型花岗岩类岩石和岩石组合的成因机制进行了详细探讨,旨在揭示Ronidia超大陆演化期间扬子板块西缘不同深度源区(地幔—镁铁质下地壳—成熟地壳)的岩浆响应,并综合了已有研究成果,为扬子西缘新元古代时期深部动力学背景提供进一步约束。

1 区域地质概况

作为东亚地区最大的克拉通之一,华南板块由西北部的扬子板块和东南部的华夏板块在新元古代时期经江南造山带碰撞拼合而成(图 1; Zhao et al., 2011; Zhao and Cawood, 2012; Wang et al., 2013, 2014a)。扬子板块北缘以东西向秦岭-大别-苏鲁造山带为界与华北克拉通相邻(图 1a),西北缘以龙门山断裂带为界与松潘-甘孜地块相邻(图 1b),西南缘以哀牢山-红河断裂为界与印支板块相邻(图 1b; Gao et al., 1999; Zhao and Cawood, 2012; Zhao et al., 2018)。

a—华南地理位置;b—华南区域地质简图 图 1 华南地理位置与区域地质简图(据Zhao and Cawood, 2012Zhao et al., 2018修改) Fig. 1 Geological position and simplified geological map of South China (modified after Zhao and Cawood, 2012; Zhao et al., 2018)

扬子板块前寒武基底主要为元古代岩石,太古宙基底仅少量出露于扬子北缘的崆岭地区(Gao et al., 1999; 2011)。太古宙崆岭杂岩分布范围大约为360 km2,主要由酸性片麻岩、变沉积岩、角闪岩和镁铁质麻粒岩组成,其最古老的年龄为ca.3.45 Ga(Guo et al., 2014)。崆岭杂岩经历了高角闪岩相—麻粒岩相变质,其北部被古元古代(ca.1.85 Ga)圈椅埫花岗岩侵入,南部被新元古代(ca.820~750 Ma)黄陵花岗岩侵入(Xiong et al., 2009; Zhang et al., 2010; Peng et al., 2012; Zhao et al., 2013)。

扬子西缘古元古代—中元古代结晶基底主要出露在西南缘的河口群、东川群和大红山群。河口群主要分布于扬子西南缘会理县城周边,由变沉积岩和变火山岩组成。岩石经历高绿片岩相—低角闪岩相变质,其内部不同层位的变凝灰岩指示河口群沉积年龄为ca.1.7 Ga(Chen et al., 2013)。东川群主要分布于扬子西南缘云南省东川地区,其底部因民组凝灰岩锆石U-Pb年龄显示东川群形成时间晚于古元古代(ca.1742 Ma)(Zhao et al., 2010)。大红山群主要分布在扬子西南缘云南大红山—沙漠地区,岩石经历了高绿片岩相—低角闪岩相变质作用,主要由变沉积岩和上覆火山角砾岩、凝灰岩和大理岩组成;锆石U-Pb年龄显示大红山群原岩形成于ca.1711~1659 Ma,变质作用发生于ca.850 Ma(杨红等,2012; Wang et al., 2014b)。

扬子西缘中元古代地层主要包括分布在西南缘滇中地区的昆阳群、川西南地区的会理群和滇北地区的苴林群。昆阳群大约10 km厚,主要由陆源碎屑岩、碳酸盐岩和火山岩组成(Greentree et al., 2006),碎屑锆石约束上昆阳群的沉积年龄最年轻为ca.960 Ma(Greentree et al., 2006; Sun et al., 2009)。会理群厚度大于10 km,主要由变碎屑岩、变质碳酸盐岩和变火山岩组成(Li et al., 2013; Zhu et al., 2016);上会理群由力马河组、凤山组和天宝山组组成,天宝山火山岩形成于ca.1025~1021 Ma(Zhu et al., 2016),侵位于下天宝山组的镁铁质岩墙形成于ca.1023 Ma,侵入上会理群会东花岗岩SIMS锆石U-Pb年龄为ca.1048~1043 Ma(Wang et al., 2019)。苴林群分布面积大约800 km2, 主要由下部的片麻岩、火山岩和大理岩以及上部的石英岩、砂岩和碳酸盐岩组成,元谋地区苴林群的变质玄武岩LA-ICP-MS锆石U-Pb年龄为ca.1050~1043 Ma(Chen et al., 2014a)。除此之外,扬子西南缘分布大量晚中元古代(ca.1050~1020 Ma)A型英安岩、流纹岩、花岗岩和S型花岗岩(Chen et al., 2018; Zhu et al., 2020b)。

扬子西缘新元古代地层主要为攀枝花地区的盐边群,自下而上分为荒田组、渔门组、小坪组和乍古组。盐边群是一套绿片岩相变质的火山-沉积序列,分布面积大约300 km2。碎屑锆石显示盐边群年龄为ca.1000~865 Ma,峰值年龄为ca.920 Ma和ca.900 Ma(Sun et al., 2008)。最早的(ca.860 Ma)关刀山岩体侵入于盐边群,限定盐边群沉积时限早于ca.860 Ma(Li et al., 2003; Du et al., 2014; Sun and Zhou, 2008)。此外,新元古代高家村-冷水箐镁铁质岩石(Zhao et al., 2019; Zhou et al., 2006a)、同德岩体和岩脉(Munteanu et al., 2010; Li and Zhao, 2018; Zhao et al., 2019)及大渡口镁铁质岩石侵入盐边群(Zhao and Zhou, 2007b; Zhao et al., 2019)。

2 构造模式

大量的地质年代学和地球化学研究表明,华南板块周缘新元古代中酸性岩石以及相关的镁铁质-超镁铁质岩石可能受控于地幔柱模型(Li et al., 1995, 1999, 2002, 2003, 2006, 2008a, 2008b; Ling et al., 2003; Zhu et al., 2008; Wang et al., 2008, 2011; 李献华等,2012; Wu et al., 2019)、岛弧模型(Zhou et al., 2002, 2006a, 2006b; Sun et al., 2007; Li et al., 2008a, 2008b; Zhao et al., 2008a, 2011, 2017, 2018, 2019; Wang et al., 2013, 2014a; 赖绍聪等,2015Lai et al., 2015; 朱毓等,2017; Li and Zhao, 2018; Zhu et al., 2019a, 2019b, 2019c, 2020a)和板块裂谷模型(Zheng et al., 2007, 2008)。

地幔柱模型认为,华南与Rodinia超大陆内部诸多大陆同时发育有广泛的ca.830~795 Ma和ca.780~745 Ma的双峰式岩浆活动,这些岩浆作用受控于超级地幔柱的上涌,该模式进一步指出华南板块位于超大陆的中心。有学者对比华南桂北地区ca.828 Ma基性岩脉和澳大利亚地幔柱成因的Gairdner岩墙群(ca.827 Ma),提出ca.825 Ma的地幔柱引发华南新元古代大陆裂谷和岩浆活动(Li et al., 1999)。此外,一些与地幔柱成因相关的特征性镁铁质—超镁铁质岩石的发现也支持华南地幔柱的存在。Li et al.(2002)对扬子西缘康滇裂谷苏雄组火山岩进行详细的地球化学研究发现,这些火山岩由碱性玄武岩和粗面岩以及流纹岩组成,显示双峰式火山岩特征。主量及微量元素数据表明其具有类似于夏威夷OIB(板内洋岛玄武岩)和埃塞俄比亚CFB(大陆溢流玄武岩)的地球化学特征,进而认为苏雄组双峰式火山岩形成于超级地幔柱诱发的大陆裂谷环境。Li et al.(2006)进一步指出扬子西缘盐边地块并没有ca.830~740 Ma的弧岩浆和蛇绿岩报道,因而ca.830~740 Ma的镁铁质-超镁铁质侵入体形成于大陆裂谷环境。此外,扬子东南缘益阳玄武岩被认为具有与科马提岩类似的地球化学特征,这进一步指出超级地幔柱引发的高温(> 1500 ℃)地幔熔体的存在。Wang et al.(2008)对扬子北缘碧口玄武岩进行系统研究发现,ca.821~811 Ma碧口玄武岩具有CFB的特征,数值模拟显示地幔潜在温度为1400~1550 ℃,明显高于MORB(大洋中脊玄武岩)源区的地幔温度。Zhu et al.(2008)指出盐边地区新元古代镁铁质岩墙具有板内玄武岩的地球化学属性,形成于与地幔柱有关的陆内裂谷环境。Wang et al.(2011)对华南新元古代岩浆锆石O同位素进行研究后指出,华南新元古代低的锆石δ18O岩浆岩是地幔柱和大陆裂谷构造背景高温环境下水与岩浆相互作用的结果。此外,Wu et al.(2019)对碧口地区酸性火山岩进行了系统的岩石地球化学分析,这些ca.820~810 Ma酸性火山岩部分显示A型花岗岩的特征,指示一个扩张的背景。总之,巨量特征性的镁铁质岩石与基性岩脉的研究支持Rodinia超级地幔柱的存在。

岛弧模型则认为,新元古代时期扬子板块周缘受大洋板块俯冲作用的影响,华南广泛的新元古代岩浆作用受控于攀西-汉南弧和江南弧,华南板块处于Rodinia超大陆的边缘位置。Zhou et al.(2002)指出,扬子板块西缘康定—丹巴—米易地区ca.860~760 Ma花岗片麻岩具有明显的弧地球化学属性,并进一步识别出了盐边地区ca.840 Ma的弧后沉积盆地以及侵入其中的ca.812~806 Ma高家村—冷水箐基性-超基性岩,结合印度与华南在古地磁和岛弧体系方面的相似性,认为华南位于Rodinia超大陆的边缘(Zhou et al., 2006a)。此外,他们还识别出了ca.750 Ma雪隆堡埃达克质英云闪长岩和花岗闪长岩,认为这些富钠的埃达克质花岗岩来源于俯冲大洋板片的部分熔融,进而为华南新元古代时期大洋板片俯冲提供了直接的证据(Zhou et al., 2006b)。此后,Zhao and Zhou(2007b)对攀枝花地区ca.740 Ma的橄榄辉长岩和角闪辉长岩进行了详细的地球化学研究,结果显示其来源于与俯冲流体和板片熔体有关的交代地幔源区的部分熔融;他们也指出攀枝花地区ca.760 Ma大田和大尖山埃达克质花岗岩类来源于俯冲板片的部分熔融(Zhao and Zhou, 2007a)。Munteanu et al.(2010)认为同德岩体展现出钙碱性特征和弧地球化学属性,扬子板块西缘在中—新元古代时期处于一个安第斯型大陆边缘环境。Du et al.(2014)对关刀山岩体进行详细的研究发现,该岩体具有明显亏损的全岩Nd同位素组分,结合微量元素特征指示其来源于受俯冲板片流体交代的地幔源区,并进一步指出ca.860 Ma的关刀山岩体可能代表扬子板块西缘俯冲大洋板片初始俯冲的岩浆产物。

板块裂谷模式认为,晚中元古代时期扬子板块周缘洋壳俯冲导致岛弧岩浆岩的形成和陆壳增生,随后弧后盆地关闭,大量ca.960~860 Ma的弧陆碰撞和同碰撞事件使岛弧岩石重熔,在ca.830~800 Ma,碰撞加厚造山带的构造垮塌使得中元古代地壳活化,弧下地幔熔融产生了高镁玄武岩。Zheng et al.(2007, 2008)对扬子板块内部江南造山带以及攀西、康定地区新元古代火成岩进行详细的锆石Hf-O同位素研究,认为华南早期(ca.825 Ma)的岩浆岩形成于弧陆碰撞造山带拉张垮塌熔融,而晚期(ca.750 Ma)为大陆裂谷岩浆作用产物。

总之,已有的研究工作侧重于对扬子西缘镁铁质岩浆的系统研究(朱维光,2004林广春,2006李奇维,2018),而对于中酸性岩石和具有特殊构造意义的花岗岩岩石组合以及它们代表的深部动力学意义的系统研究不足。而且,之前对于中酸性岩石的研究更多的是对新生镁铁质下地壳源区的讨论,而缺乏其他深度源区的岩浆信息。此外,对于扬子西缘中—晚新元古代时期构造转换进程的研究同样有待加强。基于此,文章结合团队近期对于扬子西缘不同地区典型花岗岩类和特征性岩石组合(图 2)的研究,系统探究其岩浆成因和地质意义,旨在揭示扬子西缘从地幔到地壳不同深度源区(地幔—镁铁质下地壳—成熟地壳)的岩浆响应,并进一步为扬子西缘新元古代深部动力学机制提供有益的见解和约束。

1—水陆地区高Mg#闪长岩;2—米易地区过铝质花岗岩;3—大陆地区Ⅰ型花岗质岩石;4—攀枝花—盐边地区辉长闪长岩-埃达克花岗岩-A型花岗岩
a—扬子西缘地理位置;b—扬子西缘区域地质简图
图 2 扬子板块西缘区域地质图及研究岩体地理位置(据Zhao et al., 2019修改) Fig. 2 Simplified geological map of the western margin of the Yangtze Block and the geological position of the studied plutons (modified after Zhao et al., 2019)
3 扬子西缘新元古代典型花岗岩类岩浆事件 3.1 扬子西缘新元古代俯冲流体与沉积物交代地幔岩浆作用:来自ca.850~835 Ma水陆高Mg#闪长岩的约束

作为高镁安山岩的侵入等同体,高镁闪长岩具有中等SiO2含量、高MgO含量或者Mg#值(Kelemen et al., 2004, 2007; Tatsumi, 2008; Qian and Hermann, 2010)。因为含有类似于平均大陆地壳的地球化学组分,高镁的中性岩石对于评估大陆地壳演化具有极其重要的意义(Smithies and Champion, 2000; Tatsumi, 2006; Qian and Hermann, 2010)。此外,高镁中性岩石具有的地壳和地幔属性的双重地化特征,同样引起人们广泛的兴趣(Kamei et al., 2004; Tatsumi, 2006; Qian and Hermann, 2010)。一方面,它们的地幔指标值高(例如高的Mg#值和MgO含量以及Cr、Ni含量),显示出亲原始幔源岩浆的特性(Smithies and Champion, 2000; Tatsumi, 2006);另一方面,它们展现出富集大离子亲石元素和亏损高场强元素的特征,显示明显的镁铁质下地壳熔体属性(Smithies and Champion, 2000; Martin et al., 2005; Qian and Hermann, 2010)。大部分高镁中性岩石代表了上覆地幔楔熔体与俯冲组分(俯冲流体、俯冲沉积物熔体和俯冲大洋板片熔体)的平衡(Stern and Kilian, 1996; Shimoda et al., 1998, 2003; Martin et al., 2005; Hanyu et al., 2006; Tatsumi, 2006)。因此,研究高镁中性岩石可以为探索俯冲带岩浆作用详细过程提供重要的帮助。

对扬子板块西缘新元古代大量基性岩的研究已经表明扬子西缘新元古代存在俯冲流体和板片熔体交代的地幔源区(Zhou et al., 2006a; Zhao and Zhou, 2007b; Sun and Zhou, 2008; Zhao et al., 2008a; Munteanu et al., 2010; Du et al., 2014; Meng et al., 2015; Zhu et al., 2019b)。但是,对于俯冲沉积物熔体交代的地幔岩浆作用的研究较少。因此,文章选取扬子西缘米易地区的水陆闪长岩岩体(图 3; Zhu et al., 2020a)进行综合的岩石地球化学研究。该岩体为一套中细粒的石英闪长岩,呈南北向展布,这些闪长质岩石包含的矿物主要有斜长石(40%~45%)、角闪石(20%~35%)、石英(10%~20%)以及很少的黑云母、辉石、磁铁矿和锆石。LA-ICP-MS锆石U-Pb加权平均年龄显示这些闪长岩形成于ca.850~835 Ma;主量元素特征表明它们属于钙碱性岩石,具有中等的SiO2含量(57.08%~61.12%),高的MgO含量(3.36%~4.30%)和Mg#值(56~60,>50),显示高镁闪长岩的特征(图 4a4d; Smithies and Champion, 2000; Kelemen et al., 2014)。水陆高Mg#闪长岩属于正常的安山岩-英安岩-流纹岩系列(图 4e4f)。微量元素显示水陆高Mg#闪长岩具有富集的轻稀土元素和大离子亲石元素(Rb、Ba、Th、Sr和K),以及亏损的高场强元素(Nb、Ta、Zr和Hf)(图 5)。此外,它们含有高的相容性元素含量(例如,Cr含量60.2×10-6~107×10-6,Ni含量27.0×10-6~47.6×10-6)。全岩Sr-Nd同位素研究表明水陆高Mg#闪长岩具有低的全岩(87Sr/86Sr)i比值(0.7034~0.7042)和正的εNd(t)值(+3.26~+4.26)(图 6a)。锆石Hf同位素显示出明显亏损的特征(εHf(t)=+8.43~+13.6)(图 6b)。

a—华南区域地质简图; b—扬子西缘区域地质简图; c—水陆岩体区域地质简图 图 3 扬子板块西缘新元古代水陆岩体地理位置与区域地质简图(据Zhu et al., 2020a修改) Fig. 3 Geological position and simplified geological sketch map of the Neoproterozoic shuilu pluton of the Yangtze Block(modified after Zhu et al., 2020a)

a—Na2+Ka2O vs. SiO2图解(Middlemost, 1994);b—K2O vs. SiO2图解(Roberts and Clemens, 1993);c—A/NK vs. A/CNK图解(Frost et al., 2001);d—Mg# vs. SiO2图解;e—Sr/Y vs. Y图解;f—(La/Yb)N vs. (Yb)N图解(Defant and Drummond, 1990) 图 4 扬子板块西缘新元古代水陆高Mg#闪长岩主微量图解(据Zhu et al., 2020a修改) Fig. 4 Major and trace elements diagrams for the Neoproterozoic Shuilu high-Mg# diorites in the western margin of the Yangtze Block (modified after Zhu et al., 2020a)

a—水陆高Mg#闪长岩球粒陨石标准化图;b—水陆高Mg#闪长岩原始地幔标准化微量元素蛛网图 图 5 扬子板块西缘新元古代水陆高Mg#闪长岩球粒陨石标准化蛛网图和原始地幔标准化微量元素蛛网图(Sun and McDonough, 1989;据Zhu et al., 2020a修改) Fig. 5 Diagrams of chondrite-normalized REE patterns and primitive mantle-normalized trace-element patterns for the Neoproterozoic Shuilu high-Mg# diorites in the western margin of the Yangtze Block (Sun and McDonough, 1989; modified after Zhu et al., 2020a)

a—水陆高Mg#闪长岩全岩εNd(t) vs. (87Sr/86Sr)i图解;b—水陆高Mg#闪长岩锆石εHf(t) vs.锆石U-Pb年龄图解 图 6 扬子板块西缘新元古代水陆高Mg#闪长岩全岩Sr-Nd同位素和锆石Hf同位素图解(据Zhu et al., 2020a修改) Fig. 6 Diagrams of whole-rock Sr-Nd isotopes and zircon Hf isotopes for the Neoproterozoic Shuilu high-Mg# diorites in the western margin of the Yangtze Block (modified after Zhu et al., 2020a)

水陆高Mg#闪长岩具有中等的Sr含量(470×10-6~606×10-6)和Y含量(16.2×10-6~20.5×10-6)以及低的Sr/Y比值(26.6~32.3),低的(La/Yb)N比值(6.26~13.5)和高的(Yb)N值(9.36~11.7)。这说明它们不是典型的板片熔体来源的埃达克岩石(图 4e4fDefant and Drummond, 1990)。均一的全岩Nd同位素和锆石Hf同位素特征以及缺少镁铁质包体和不平衡矿物对说明水陆高Mg#闪长岩不属于镁铁质与酸性岩浆混合的产物(Kemp et al., 2007; Karsli et al., 2017)。它们的Mg#值(56~60,超出常规的40~45范围)明显高于镁铁质下地壳组分部分熔融产生的熔体(图 4d)(Rapp and Watson, 1995; Rapp et al., 1999),进一步指示它们不属于下地壳来源的闪长岩。事实上,水陆高Mg#闪长岩具有接近于亏损地幔的Sr-Nd-Hf同位素组成(图 6),说明其来源于地幔源区。高的Cr(60.2×10-6~107×10-6)、Co(45.2×10-6~131×10-6)、Ni(27.0×10-6~47.6×10-6)含量以及低的Nb/La比值(0.10~0.26)也支持一个亏损的岩石圈地幔源区。因此,地球化学特征说明水陆高Mg#闪长岩可能来源于亏损的岩石圈地幔。需要注意的是,虽然这些闪长岩表现出地幔属性,但它们同样显示出明显富集的特征,即富集轻稀土元素、大离子亲石元素、Pb和亏损Nb、Ta和Ti。考虑到明显亏损且均一的Nd-Hf同位素特征,这些富集的特征并非来源于岩浆上升就位过程的地壳混染,而很可能产生于地幔源区发生部分熔融之前的交代作用。实验研究已经表明上地幔橄榄岩的含水熔融是产生高镁岩浆的合理机制(Tatsumi, 2006)。来源于俯冲岩石圈的富集大离子亲石元素的含水流体能够为地幔楔的含水熔融提供条件(Crawford, 1989; Hanyu et al., 2006)。水陆高Mg#闪长岩属于钙碱性岩石,并且含有大量的角闪石矿物,这说明其原始地幔源区是含水的(Grove et al., 2002; Smith et al., 2009)。类似于俄罗斯Kamchatka地区的Golovin和Belaya弧火山岩(Kepezhinskas et al., 1997),水陆高Mg#闪长岩具有高的Rb/Y比值(1.37~2.60)和Ba含量(441×10-6~1000×10-6)以及低的Nb/Y比值(0.18~0.24)(图 7a7b),这显示它们的原始熔体经历了俯冲流体有关的富集作用。更重要的是,这些高Mg#闪长岩显示轻微的Nd-Hf同位素解耦特征(图 7c),相较于亏损地幔,它们具有较低的Nd-Hf同位素。因为板片流体对于Sm、Nd、Lu和Hf具有低的相容系数(Bau, 1991, Hanyu et al., 2006),所以低的Nd-Hf同位素(相较于亏损地幔)并不是由于地幔源区只经历了俯冲流体的交代作用,俯冲沉积物熔体可能也参与了地幔交代进程(Guo et al., 2015; Zhao et al., 2018, 2019)。这种由于俯冲沉积物交代地幔作用而产生的Nd-Hf同位素解耦现象已经在新元古代镁铁质-中性岩石有所报道(Zhao et al., 2008a, 2018, 2019)。Th元素在俯冲流体中是不流动的,但可以跟随俯冲沉积物熔体从俯冲板片转移到上覆地幔源区(Hawkesworth et al., 1997; Johnson and Plank, 2000; Woodhead et al., 2001; Hanyu et al., 2006)。因此,富集的Th元素可以指示俯冲沉积物熔体的贡献。在原始地幔蛛网图上,不同于扬子西缘俯冲流体交代地幔来源的镁铁质-中性岩石(Du et al., 2014; Zhu et al., 2019a),水陆高Mg#闪长岩显示出明显富集的Th元素。相较于平均N-MORB组分(Th/Ce=0.016)(Sun and McDonough, 1989)和全球俯冲沉积物(Th/Ce=0.12)(Plank and Langmuir, 1998),它们显示明显高的Th/Ce比值(0.08~0.27)。它们也具有比N-MORB(Th/Yb=0.04)(Sun and McDonough, 1989)较高的Th/Yb比值(1.79~8.59)。此外,变化的Th/Ce(0.08~0.27)和Th/Sm(0.90~3.80)比值也能够证明俯冲沉积物的明显加入(图 7d)。最新的对于扬子西缘新元古代ca.850~840 Ma辉长岩的研究(Zhao et al., 2019)指出,这些辉长岩显示出高的锆石δ18O值和变化的εHf(t)值,这是由于地幔源区经历了俯冲沉积物熔体的交代作用。这进一步指出扬子西缘新元古代时期存在俯冲沉积物交代的地幔源区。因此,文章提出俯冲流体和沉积物熔体同时交代地幔源区,随后上覆地幔源区部分熔融产生了水陆高Mg#闪长岩。变化的Th/Yb(1.79~8.59)和Ba/La(24.5~53.6)比值同样支持水陆高Mg#闪长岩的地幔源区受俯冲流体和沉积物熔体的共同交代作用(图 7eHanyu et al., 2006)。

a—Rb/Y vs. Nb/Y图解(Kepezhinskas et al., 1997);b—Ba vs. Nb/Y图解(Kepezhinskas et al., 1997);c—锆石εHf(t) vs.全岩εNd(t)图解(Zhao et al., 2019);d—Th/Ce vs. Th/Sm图解(Guo et al., 2015; Zhang et al., 2019);e—Ba/La vs. Th/Yb图解(Hanyu et al., 2006修改) 图 7 扬子板块西缘新元古代水陆高Mg#闪长岩俯冲组分判别图(据Zhu et al., 2020a修改) Fig. 7 Discriminant diagrams of subduction components for the Neoproterozoic Shuilu high-Mg# diorites in the western margin of the Yangtze Block (modified after Zhu et al., 2020a)

水陆高Mg#闪长岩属于钙碱性岩石(图 4b),显示富集的轻稀土和大离子亲石元素,亏损的高场强元素(图 5)。这些地球化学特征类似于新元古代ca.860~810 Ma弧属性的镁铁质-中性岩石(Munteanu et al., 2010; Du et al., 2014; Zhu et al., 2019a),充分说明它们具有弧岩浆属性。已有研究表明,不同的锆石微量元素比值可以反映不同的岩浆环境(Grimes et al., 2015)。水陆高Mg#闪长岩的锆石微量元素显示中等的Hf含量(6393×10-6~11731×10-6)以及高的U/Yb(0.17~1.54)值(图 8a)和Nb/Yb(17.7~68.6)值(图 8b),其高的U/Yb(0.17~1.54)值显示大陆弧岩浆环境(0.1~4.0),进一步支持这些岩浆锆石来源于大离子亲石元素富集的含水熔体(Grimes et al., 2015)。此外,与来源于地幔柱环境(例如,夏威夷和冰岛)的岩浆锆石(图 8b)不同,水陆高Mg#闪长岩的锆石明显处于地幔锆石序列之上,显示与新元古代俯冲组分交代地幔来源的辉长岩相似的微量元素特征(图 8Zhao et al., 2019)。因此,水陆高Mg#闪长岩形成于俯冲背景下,而并非地幔柱环境。水陆高Mg#闪长岩的识别,为扬子板块西缘新元古代俯冲构造环境提供了进一步的证据,并且从岩石地球化学角度揭示了扬子西缘俯冲背景下俯冲沉积物熔体有关的地幔交代作用。

a—水陆高Mg#闪长岩锆石U/Yb vs. Hf图解;b—水陆高Mg#闪长岩锆石U/Yb vs. 10000*Nb/Yb图解(Grimes et al., 2015; Zhao et al., 2019) 图 8 扬子板块西缘新元古代水陆高Mg#闪长岩锆石微量元素图解(Zhu et al., 2020a) Fig. 8 Diagrams of zircon trace elements for the Neoproterozoic Shuilu high-Mg# diorites in the western margin of the Yangtze Block (Zhu et al., 2020a)
3.2 扬子西缘新元古代成熟大陆地壳的不平衡熔融:关于ca.840~835 Ma宽裕-茨达过铝质花岗岩的见解

过铝质花岗岩具有高的A/CNK值(>1.0),广泛出现在各种构造环境中(Chappell and White, 1992; Kemp et al., 2007; Patiño Douce, 1995)。大多数过铝质花岗岩被认为是在较为成熟地壳环境下,幔源岩浆上升引发沉积物(变泥质岩和变质杂砂岩)发生部分熔融的产物(Clemens, 2003; Chappell et al., 2012; Clemens et al., 2016)。虽然有报道显示一些过铝质花岗岩的形成也涉及到准铝质的火成岩原岩(玄武质到安山质岩石)(Chappell and White, 1992, 2001; Clemens, 2003; Chappell et al., 2012),但是成熟的沉积物组分在它们的岩浆进程中同样起到了关键的作用(Kemp et al., 2007; Chappell et al., 2012; Zhao et al., 2015; Clemens, 2018)。由此看来,探索过铝质花岗岩的岩石成因能够为了解成熟大陆地壳组分的熔融提供至关重要的帮助。

扬子西缘新元古代镁铁质岩石、中性岩石、埃达克质岩石和钠质花岗岩已经被广泛研究,并用于评估地壳演化、地幔熔融和分异(Zhou et al., 2002, 2006a, 2006b; Sun et al., 2007; Zhao et al., 2008a, 2008b, 2010; Du et al., 2014; Lai et al., 2015; Li and Zhao, 2018; Zhu et al., 2019a, 2019b)。Ca. 860~740 Ma的镁铁质-中性岩石被认为主要来源于俯冲流体或熔体交代的地幔源区(Zhou et al., 2006b; Zhao and Zhou, 2007b; Sun and Zhou, 2008; Zhao et al., 2008a; Du et al., 2014);Ca. 800~750 Ma的埃达克质花岗岩被解释为来自于俯冲板片(Zhou et al., 2006a; Zhao and Zhou, 2007a)或增厚下地壳的部分熔融(Huang et al., 2009; Zhu et al., 2019b);而ca. 800~750 Ma的钠质花岗岩类被认为主要是镁铁质下地壳源区的产物(Lai et al., 2015; Zhao et al., 2008b; Zhu et al., 2019a)。由此看来,扬子西缘新元古代广泛的地幔和新生镁铁质下地壳的熔融已经被报道。然而,对于成熟大陆地壳物质部分熔融的研究仍然是空白。因此,文章选取扬子西缘米易地区宽裕-茨达过铝质花岗岩进行详细研究(图 9),旨在揭示扬子西缘新元古代成熟大陆地壳岩浆作用(Zhu et al., 2019c)。

图 9 扬子板块西缘新元古代宽裕-茨达过铝质花岗岩体区域地质简图(据Zhu et al., 2019c修改;研究区位置见图 3b) Fig. 9 Simplified geological sketch map of the Neoproterozoic Kuanyu and Cida peraluminous granitic plutons in the western margin of the Yangtze Block (modified after Zhu et al., 2019c. The geological positon of the study area is shown in Fig. 3b)

宽裕-茨达花岗岩位于米易西北部的花园镇附近,主要为中粒—中粗粒的黑云母花岗岩。宽裕花岗岩主要由20%~30%钾长石,20%~25%斜长石,20%~25%石英,20%~15%黑云母和0%~3%磁铁矿以及锆石组成;茨达花岗岩包含矿物主要为15%~20%钾长石,0~10%条纹长石,20%~25%斜长石,30%~35%石英,0~5%黑云母,磁铁矿和锆石。锆石U-Pb年龄显示这些花岗岩形成于ca. 840~835 Ma,具有高的SiO2含量(66.9%~75.6%)、K2O/Na2O值(1.44~3.25)、A/CNK值以及低的Mg#值(17~33)(图 10)。微量元素特征表明宽裕-茨达过铝质花岗岩显示类似中上地壳属性,具有富集的Rb、Th、U、K和Pb,以及亏损的Nb、Ta、Sr和Ti。全岩锆饱和温度计显示这些过铝质花岗岩具有高的结晶温度(790~850 ℃)。不同于之前新元古代花岗岩类的同位素特征,宽裕-茨达过铝质花岗岩具有明显富集的Nd同位素特征(εNd(t)=-5.1~-2.9),锆石Hf同位素也主要显示负的εHf(t)值(-7.75~+3.31)(图 11)。

a—A/NK vs. A/CNK图解(Frost et al., 2001);b—(Na2O+K2O - CaO) vs. SiO2图解(Frost et al., 2001);c—K2O/Na2O vs. SiO2图解(Moyen and Martin, 2012);d—Mg# vs. SiO2图解 图 10 扬子板块西缘新元古代宽裕-茨达过铝质花岗岩主量元素图解(据Zhu et al., 2019c修改) Fig. 10 Major elements diagrams for the Neoproterozoic Kuanyu and Cida peraluminous granites in the western margin of the Yangtze Block (modified after Zhu et al., 2019c)

a—宽裕-茨达过铝质花岗岩全岩εNd(t) vs. (87Sr/86Sr)i图解;b—宽裕-茨达过铝质花岗岩锆石εHf(t) vs.锆石U-Pb年龄图解 图 11 扬子板块西缘新元古代宽裕-茨达过铝质花岗岩全岩Sr-Nd同位素和锆石Hf同位素图解(据Zhu et al., 2020c修改) Fig. 11 Diagrams of whole-rock Sr-Nd isotopes and zircon Hf isotopes for the Neoproterozoic Kuanyu and Cida peraluminous granites in the western margin of the Yangtze Block (modified after Zhu et al., 2020c)

宽裕-茨达过铝质花岗岩形成于ca. 840~835 Ma,显示高的A/CNK值(1.04~1.18),属于过铝质到强过铝质花岗岩(图 10a)。实验研究表明,过铝质中酸性熔体可以产生于地壳环境下准铝质玄武岩到安山岩的部分熔融(Rapp and Watson, 1995; Sylvester, 1998; Chappell, 1999; Sisson et al., 2005; Chappell et al., 2012)。然而,这样的源区产生的花岗岩一般具有低的K2O含量和K2O/Na2O值(< 1)。相反,宽裕-茨达过铝质花岗岩显示高的K2O含量和K2O/Na2O值(>1)(图 10c),因此可以排除准铝质火成岩源区。之前大量的研究也指出,过铝质富硅熔体可以由成熟地壳源区变沉积物(泥质岩和杂砂岩)发生部分熔融形成(Sylvester, 1998; Clemens, 2003)。类似的,宽裕-茨达过铝质花岗岩含有高的SiO2和K2O含量以及高的A/CNK比值,说明源区主要为变质沉积岩而并非变火成岩。这些过铝质花岗岩显示轻微分异的HREE,低的(Gd/Yb)N(1.85~4.66)和Sr/Y(1.31~7.62)值,说明它们来源于石榴石稳定区域之上的较浅的地壳源区(Patiño Douce, 1996; Rossi et al., 2002)。宽裕-茨达过铝质花岗岩具有变化的CaO/Na2O值(0.09~0.65)和Al2O3/TiO2值(25.3~88.4)以及中等的Rb/Ba(1.68~3.86)和Rb/Sr(0.32~0.85)值(图 12a12b),指示它们来源于不均一的变沉积物源区(变泥质岩和变质杂砂岩)的部分熔融。不同于镁铁质下地壳源区的花岗岩,宽裕-茨达过铝质花岗岩它们高的摩尔Al2O3/(MgO+FeOT)值(2.04~5.23)和低的摩尔CaO/(MgO+FeOT)值(0.15~0.48)同样说明它们来源于不均一的变沉积物源区(Altherr et al., 2000)(图 12c)。此外,富集的全岩εNd(t)(-5.1~-2.9)值和负的为主的锆石εHf(t)值(-7.75~+3.31)以及古老的二阶段Hf模式年龄(1512~2210 Ma)都支持一个演化的大陆地壳源区。因此,宽裕-茨达过铝质花岗岩来源于演化的地壳环境下不均一变沉积物的部分熔融。

a—CaO/Na2O vs. Al2O3/TiO2图解(Sylvester, 1998);b—Rb/Ba vs. Rb/Sr图解(Patiño Douce 1999);c—molar Al2O3/(MgO+FeOT) vs. molar CaO/(MgO+FeOT)图解(Altherr et al., 2000) 图 12 扬子板块西缘新元古代宽裕-茨达过铝质花岗岩岩浆源区图解(据Zhu et al., 2019c修改) Fig. 12 Discriminant diagrams of magma source for the Neoproterozoic Kuanyu and Cida peraluminous granites in the western margin of the Yangtze Block (modified after Zhu et al., 2019c)

需要注意的是,宽裕-茨达过铝质花岗岩显示部分亏损的Hf同位素特征(εHf(t)高达+3.31)(图 11b)。考虑到缺少幔源岩浆混合的直接岩石矿物学证据(如镁铁质包体和不平衡的矿物现象)以及宽裕-茨达过铝质花岗岩特殊的地球化学证据(如高SiO2含量,低Mg#值和明显负的εNd(t)值)(Vernon, 1984; Tang et al., 2014; Jiang and Zhu, 2017),它们不均一的锆石Hf同位素特征可能是由于不平衡熔融进程造成的。作为Hf的主要携带者,锆石控制着源区的Hf含量,因此它的溶解过程能够支配熔体中Hf同位素的演化(Tang et al., 2014; Wang et al., 2018; Kong et al., 2019)。Flowerdew et al.(2006)提出锆石溶解速率的不均一性能够导致单一源区内不同批次熔体产生不均一的锆石εHf(t)值,进而使得Hf同位素体系与其他放射性同位素体系发生不同程度的解耦。宽裕-茨达过铝质花岗岩中显示存在一些锆石捕虏晶,说明源区中一些残余的锆石被夹带进而发生部分熔融(Kong et al., 2019)。这些夹带而来的锆石会赋存Hf,使得产生的宽裕-茨达花岗质熔体具有高的Nd/Hf值(6.17~9.76)和低的Hf含量(5.00×10-6~8.43×10-6)。因此,未溶解的锆石会在源区保留大量的177Hf,使得后续产生的熔体含有较高的177Hf/176Hf值,进而与143Nd/144Nd值发生解耦(Tang et al., 2014; Kong et al., 2019)。而宽裕-茨达过铝质花岗岩含有负的εNd(t)(-5.1~-2.9)和变化的εHf(t)(-7.75~+3.31),显示Nd-Hf同位素的轻微解耦。锆元素在大陆地壳中是较为丰富的,从下地壳平均值的68×10-6变化到上地壳平均值的193×10-6(Rudnick and Gao, 2003),意味着地壳熔融过程中锆石的缓慢不平衡熔融是较为常见的。因为源区中高的锆元素含量能够使得部分熔融过程锆石矿物中的锆元素迅速饱和,在更多的熔体产生来消耗源区中多余的锆元素之前,锆石的溶解会停止,因此源区中锆的含量是影响锆石溶解速率的重要因素(Tang et al., 2014; Kong et al., 2019)。宽裕-茨达过铝质花岗岩高的锆含量(159×10-6~304×10-6)暗示其源区可能含有更高的初始的锆含量。Tang et al. (2014)指出当源区中初始锆含量足够高时(>100×10-6),不同批次岩浆的Hf会从低含量高放射性特征转变为高含量低放射性,这样会使得来自同一地壳源区强烈熔融之后的熔体由早期的亲地幔同位素特征(亏损的)转变为晚期的亲地壳同位素特征(富集的)。来自于同一地壳源区的不同批次的岩浆会产生类似于壳幔岩浆混合的同位素特征(Tang et al., 2014)。因此,源区中高的锆含量以及不平衡熔融作用可以解释宽裕-茨达过铝质花岗岩中Hf同位素不均一性。

宽裕-茨达过铝质花岗岩形成于ca. 840~835 Ma。它们来源于不均一的变沉积物(变质杂砂岩+变沉积岩)的不平衡熔融。过铝质花岗岩能够产生于裂脊俯冲背景下的地壳重熔、陆陆碰撞背景下的地壳增厚与熔融、弧后盆地环境下后碰撞垮塌与先存沉积物的熔融以及俯冲早期阶段大陆地壳的部分熔融(Sylvester, 1998; Collins, 2002; Collins and Richards, 2008; Cai et al., 2011; Liu and Zhao, 2018; )。裂脊俯冲环境产生过铝质花岗岩的同时可能伴生区域性的高温低压变质作用(Cai et al., 2011; Jiang et al., 2010),这在扬子西缘新元古代时期并未被发现。因此,裂脊俯冲背景下的地壳熔融不能产生宽裕-茨达过铝质花岗岩。宽裕-茨达过铝质花岗岩形成年龄明显早于扬子周缘增厚下地壳来源的花岗岩(Huang et al., 2009; Zhu et al., 2019b),并且也没有地质证据显示早新元古代时期扬子西缘米易地区处于弧后盆地环境。因此,大陆碰撞与弧后盆地环境有关的构造模式是不合理的,而俯冲环境下早期阶段的地壳重熔更能解释宽裕-茨达过铝质花岗岩的形成。Chen et al.(2014b)提出北祁连地区寒武纪Chaidanuo过铝质花岗岩形成于俯冲早期阶段地壳物质的重熔。同样的,扬子西缘新元古代时期俯冲环境也被广泛提出。新元古代早期,随着大洋板片的东向俯冲,强烈的垂向板片回转能够增加板片俯冲速率(Niu et al., 2003),造成强烈的海底扩张(Zhu et al., 2009; Gerya, 2011),这样在前弧地区会发生海沟的回撤和上覆板片的扩张。紧接着上覆板片会在弧岩浆上涌作用下发生流变学性质的减弱,因而导致俯冲早期上覆板片的区域性减薄(Gerya and Meilick, 2011)。这一进程同时会使得地幔物质减压熔融产生镁铁质火山岩或者侵入岩(例如新元古代早期ca. 860 Ma关刀山岩体,ca. 842 Ma下村镁铁质岩体和ca. 840 Ma MORB型盐边玄武岩)(Sun et al., 2007; 郭春丽等,2007; Du et al., 2014)。与此同时,广泛的幔源岩浆上升并加热上覆地壳,当温度达到固相线时发生地壳内部成熟沉积物的不平衡熔融,进而产生宽裕-茨达过铝质花岗岩。因此,文章提出宽裕-茨达过铝质花岗岩代表新元古代时期扬子西缘俯冲早期阶段成熟地壳物质的不平衡熔融。扬子西缘新元古代时期不仅经历新生镁铁质下地壳的熔融,也发生了成熟大陆地壳物质的重熔。

3.3 扬子西缘新元古代地壳演化:来自ca. 780 Ma大陆Ⅰ型花岗闪长岩-花岗岩组合的证据

作为最常见的花岗岩类型,Ⅰ型花岗岩是理解壳幔关联与地壳分异进程的重要窗口(Hawkesworth and Kemp, 2006; Kemp et al., 2007)。Ⅰ型花岗岩可以形成于多种岩浆成因环境,包括纯的地壳或者地幔源区(Collins, 1996; Hawkesworth and Kemp., 2006; Kemp et al. 2007; Clemens et al. 2016)、壳幔岩浆的混合作用(Kemp and Hawkesworth 2014; Weidendorfer et al. 2014; Liu et al., 2018)和古老的或新生的火山岩(Chappell et al. 2012; Lu et al. 2016, 2017)。最新的一些研究也指出沉积物的加入对Ⅰ型花岗岩形成也起着至关重要的作用(Chappell et al., 2012; Zhao et al., 2015; Clemens, 2018)。因此,厘清Ⅰ型花岗岩的岩石成因对于了解区域地壳属性、壳幔相互作用和地壳增生与重熔过程具有重要的意义。

结合已报道的对于扬子西缘Ⅰ型花岗岩(康定花岗闪长岩和石棉Ⅰ型花岗岩)的研究(Zhao et al., 2008b; Lai et al., 2015),文章选取扬子西缘大陆Ⅰ型花岗岩体进行详细研究(图 13),旨在探索不同地壳源区的岩浆响应(Zhu et al., 2019a)。大陆花岗岩体为一复式花岗岩体,中心相由花岗闪长岩组成,边缘相主要为花岗岩。花岗闪长岩为灰色中粒结构,主要由斜长石(30%~35%)、石英(20%~25%)、钾长石(10%~15%)、角闪石(10%~15%)、黑云母(5%~10%)、磁铁矿和锆石等矿物组成。花岗岩为中粒结构,主要矿物包含钾长石(25%~30%)、斜长石(22%~27%)、石英(30%~35%)、角闪石(2%~5%)、黑云母(3%~5%)、磁铁矿(0~2%)和锆石。锆石U-Pb年龄显示大陆花岗闪长岩与花岗岩形成于ca. 780 Ma。大陆花岗闪长岩与花岗岩显示出完全不同的主微量元素特征(图 14)。大陆花岗闪长岩具有中等的SiO2(60.88%~68.07%)和K2O(1.47%~2.18%)含量以及高的Na2O/K2O值(2.27~3.65),属于钙碱性准铝质到轻微过铝质(A/CNK值为0.94~1.08)岩石(图 14b14c),并且显示中等的MgO含量(1.21%~2.00%)和Mg#值(40~47)(图 15a)。大陆花岗岩显示明显高的SiO2含量(71.80%~75.34%,除去一个样品SiO2含量65.74%)和K2O含量(2.85%~5.31%)以及低的Na2O/K2O值(0.58~1.51),属于高钾钙碱性过铝质花岗岩类(A/CNK值为1.05~1.20)(图 14b14c)。同位素特征显示大陆花岗闪长岩具有低的全岩(87Sr/86Sr)i值(0.7032~0.7034)、正的全岩εNd(t)值(+1.1~+2.3)和锆石εHf(t)值(+2.16~+7.39)以及较年轻的地壳Hf模式年龄(1214~1544 Ma);而大陆花岗岩显示相对高的全岩(87Sr/86Sr)i值(0.7034~0.7037),负的全岩εNd(t)值(-0.8~-0.6),不均一的锆石εHf(t)值(-4.65~+5.80)和相对古老的地壳Hf模式年龄(1310~1968 Ma)。

图 13 扬子板块西缘新元古代大陆Ⅰ型花岗岩体区域地质简图(据Zhu et al., 2019a修改;研究区位置见图 3b) Fig. 13 Simplified geological sketch map of the Neoproterozoic Dalu Ⅰ-type granitic pluton in the western margin of the Yangtze Block (modified after Zhu et al., 2019a. The geological positon of the study area is shown in Fig. 3b)

a—Q-A-P-F图解(Middlemost, 1994);b—K2O vs. SiO2图解(Roberts and Clemens, 1993);c—A/NK vs. A/CNK图解(Frost et al., 2001);d—Na2O+K2O vs. SiO2图解(Middlemost, 1994);e—Rb-Ba-Sr 图 14 扬子板块西缘新元古代大陆Ⅰ型花岗闪长岩-花岗岩主微量元素图解(据Zhu et al., 2019a修改) Fig. 14 Major and trace elements diagrams for the Neoproterozoic Dalu Ⅰ-type granodiorites-granites in the western margin of the Yangtze Block (modified after Zhu et al., 2019a)

a—Mg# vs. SiO2图解;b—Nb/Y vs. Rb/Y图解;c—CaO/Na2O vs. Al2O3/TiO2图解(Sylvester, 1998);d—Rb/Ba vs. Rb/Sr图解(Patiño Douce 1999);e—(Na2O+K2O)/(FeOT+MgO+TiO2) vs. Na2O+K2O+FeOT+MgO+TiO2图解(Patiño Douce 1999);f—CaO/(MgO+FeOT+TiO2) vs. CaO+MgO+FeOT+TiO2图解(Patiño Douce 1999) 图 15 扬子板块西缘新元古代大陆Ⅰ型花岗闪长岩-花岗岩岩体岩浆源区判别图解(据Zhu et al., 2019a修改) Fig. 15 Discriminant diagrams of magma source for the Neoproterozoic Dalu Ⅰ-type granodiorites-granites in the western margin of the Yangtze Block (modified after Zhu et al., 2019a)

大陆花岗闪长岩-花岗岩组合显示明显的Ⅰ型花岗岩特征属性。大陆花岗闪长岩属于典型的钙碱性Ⅰ型花岗岩类,考虑到岩体附近并无巨量的镁铁质岩石与包体出露,幔源岩浆的分异与壳幔岩浆的混合很难解释大陆Ⅰ型花岗闪长岩的形成(Kemp et al., 2007; Clemens et al., 2011; Clemens and Stevens, 2012)。它们正的全岩εNd(t)值(+1.1~+2.3)、锆石εHf(t)值(+2.16~+7.39)以及较年轻的地壳Hf模式年龄(1214~1544 Ma)指示大陆花岗闪长岩可能来源于中元古代新生下地壳源区;低的Nb/Y值(0.20~0.30)和Rb/Y值(0.57~3.61)指示下地壳源区(图 15b)(Rudnick and Fountain, 1995)。它们具有低的Mg#值(40~47,多数小于45)(图 15a),说明其母质熔体来源于镁铁质岩石的部分熔融(Rapp and Watson, 1995)。此外,中等的CaO/Na2O值(0.38~0.84)和Al2O3/TiO2值(28.79~49.20)以及低的Rb/Ba值(0.03~0.09)和Rb/Sr值(0.10~0.20)都证明大陆花岗闪长岩来源于玄武质熔体源区(图 15c-15f)。之前研究表明,玄武质岩石在角闪岩相边界发生20%左右至40%的部分熔融后能产生富Al2O3和Na2O的花岗闪长质熔体(Rapp and Watson, 1995),大陆花岗闪长岩的主量元素含量同样显示出类似于角闪石实验熔体的特征,因此认为,大陆花岗闪长岩形成于新生镁铁质下地壳的部分熔融。相较而言,大陆花岗岩具有高的SiO2、K2O和A/CNK值,属于高钾钙碱性过铝质Ⅰ型花岗岩;地球化学特征表明其来源于混合的变质沉积物(泥质岩+杂砂岩)源区(图 15c—15f)。此外,它们显示富集的全岩εNd(t)值(-0.8~-0.6)、不均一的锆石εHf(t)值(-4.65~+5.80)和古老的地壳Hf模式年龄,同样说明它们来源于相对古老的地壳岩石的部分熔融。事实上,涉及沉积物源区的Ⅰ型花岗岩已经被广泛报道。Kemp et al.(2007)提出,来自东澳大利亚地区典型的含角闪石Ⅰ型花岗岩受幔源岩浆的加热,来源于沉积物的部分熔融,而并非古老的变火成岩。Chappell et al.(2012)指出,无论是部分熔体还是全岩同化,沉积物的加入是形成过铝质Ⅰ型花岗岩的重要机制。Zhao et al.(2015)也提出沉积物能够以含水熔体的形式参与到Ⅰ型花岗岩的形成过程。Clemens(2018)提出,澳大利亚东南地区的Harcourt岩基属于高钾钙碱性岩体,它们虽然显示出Ⅰ型花岗岩属性,但主要来源于变沉积物源区。由此看来,大陆高钾钙碱性Ⅰ型花岗岩来源于变质沉积物熔体是合理的。需要注意的是,它们显示正的与负的锆石εHf(t)值(-4.65~+5.80),考虑到大陆Ⅰ型花岗岩与花岗闪长岩的共生关系,这说明在沉积物发生部分熔融之前锆石已经在镁铁质熔体中发生结晶(Kemp et al., 2007)。它们正的锆石εHf(t)值代表镁铁质熔体的信息,而负的εHf(t)值指示沉积物组分。因此,大陆Ⅰ型花岗岩主要来源于镁铁质下地壳熔体引发的变质沉积物的部分熔融。

大陆Ⅰ型花岗闪长岩-花岗岩组合具有明显的弧岩浆特征,即明显的富集大离子亲石元素,亏损高场强元素。大陆Ⅰ型花岗闪长岩为典型的活动大陆边缘环境下的钙碱性岩浆作用,而大陆Ⅰ型花岗岩为俯冲背景下镁铁质岩浆上涌诱发中上地壳的岩浆响应。它们代表了扬子板块西缘新元古代时期地壳的增生与重熔进程。

3.4 扬子西缘新元古代构造转换:俯冲背景下的区域性地壳增厚到减薄

一般而言,埃达克质岩石具有高的Sr含量、Sr/Y和La/Yb值,以及低的Y和Yb含量(Defant and Drummond, 1990; Martin et al., 2005)。这类岩石具有多种岩石成因,其中包括加厚下地壳的部分熔融(Wang et al., 2006, 2012; Huang et al., 2009; Zhang et al., 2018)。另一方面,A型花岗岩(碱性、含水和非造山)是一类较为特殊的花岗岩,它们一般具有高的SiO2含量、Na2O+K2O含量和Ga/Al值(Whalen et al., 1987),普遍形成于各种构造背景(大陆裂谷、俯冲带和后碰撞环境)下的区域扩张环境(Eby, 1992)。由此看来,在造山带演化过程中,A型花岗岩紧随着埃达克质花岗岩的出现能够为探索区域性的地壳增厚到减薄提供一个窗口。

鉴于特殊的构造指示意义,文章选取扬子西缘攀枝花—盐边地区新元古代辉长闪长岩-埃达克质花岗岩(黑云母花岗岩)-A型花岗岩(钾长花岗岩)组合(图 16; Zhu et al., 2019b)进行详细的岩石成因与构造意义的解读。大尖山辉长闪长岩为灰色中粒结构,块状构造;主要包含矿物有斜长石(45%~50%)、角闪石(25%~35%)、被改造的辉石(约10%)、石英(< 5%)、磁铁矿(1%~2%)和锆石。大尖山黑云母花岗岩为中细粒结构,主要包含矿物有斜长石(40%~50%)、石英(20%~30%)、钾长石(15%~20%)、黑云母(5%~10%)和锆石。钾长花岗岩为肉红色花岗岩,中粒结构,块状构造,主要矿物有斜长石(20%~30%)、石英(25%~30%)、钾长石(30%~40%)、条纹长石(5%~10%)、很少的黑云母以及锆石和磷灰石。LA-ICP-MS锆石U-Pb定年结果显示,大尖山辉长闪长岩形成于ca. 810 Ma,大尖山黑云母花岗岩形成于ca. 800 Ma,而攀枝花钾长花岗岩形成于ca. 750 Ma(Zhu et al., 2019b)。大尖山辉长闪长岩具有低的SiO2含量(52.62%~53.87%)、中等的MgO含量(2.67%~3.41%)和Mg#值(46~52),显示富集的轻稀土元素和大离子亲石元素(Rb、Ba和Sr),亏损的高场强元素(Nb、Ta和Ti)。此外,这些辉长闪长岩含有中等的Cr(20.7×10-6~26.4×10-6), Co(45.9×10-6~60.1×10-6)和Ni(8.86×10-6~11.1×10-6)含量。大尖山辉长闪长岩显示低的(87Sr/86Sr)i值(0.705184~0.705392)和亏损的Nd同位素组分(εNd(t)=+1.0~+1.5)以及亏损的锆石Hf同位素组分(εHf(t)=+3.66~+8.18)。大尖山黑云母花岗岩显示高的SiO2(74.08~74.82%)和Na2O(4.76~5.60%)含量,低的MgO(0.25%~0.30%)含量和Mg#(36~41)含量。微量元素特征显示它们具有高的Sr含量(335×10-6~395×10-6)、明显低的Y含量(7.04×10-6~9.71×10-6,小于18×10-6)和Yb含量(0.78×10-6~1.07×10-6,小于1.9×10-6)以及高的Sr/Y值(38.9~54.3),显示出埃达克质花岗岩属性。此外,它们具有低的Cr(2.94×10-6~3.59×10-6)和Ni(1.32×10-6~1.55×10-6)含量。大尖山埃达克花岗岩显示轻微亏损的Nd(εNd(t)=+0.5~+0.6)同位素和锆石Hf(εHf(t)=+1.62~+8.07)同位素组分。攀枝花钾长花岗岩具有非常高的SiO2(76.61%~77.14%)、Na2O(3.13%~4.34%)、K2O(4.82%~5.68%)含量和分异指数(95.3~96.9),以及低的Al2O3(11.43%~11.86%)、CaO(0.25%~0.37%)和MgO(0.02%~0.10%)含量,显示出高分异A2型花岗岩特征。攀枝花A型花岗岩含有负的全岩εNd(t)值(-1.2~-1.6)和低的锆石εHf(t)值(-1.50~+6.78)。

a—攀枝花—盐边地区地理位置;b—攀枝花—盐边地区区域地质简图 图 16 扬子板块西缘攀枝花—盐边地区地理位置和区域地质简图(据Zhu et al., 2019b修改) Fig. 16 Geological position and simplified geological sketch map of the Panzhihua-Yanbian region in the western margin of the Yangtze Block (modified after Zhu et al., 2019b)

大尖山辉长闪长岩显示高的Sr(631×10-6~796×10-6)、Y(22.0×10-6~29.5×10-6)、Yb(2.21×10-6~2.95×10-6)含量和低的(La/Yb)N值(2.61~7.87),显示正常的弧火山岩特征(图 17a17b)(Defant and Drummond, 1990),显示富集的Rb、Cs、Sr和Ba以及亏损的Nb、Ta和Ti,展现出俯冲带弧岩浆特征(Sun and McDonough, 1989)。正的εNd(t)(+1.0~+1.5)和εHf(t)(+3.66~+8.18)值说明大尖山辉长闪长岩来源于亏损的地幔源区(图 17c)。此外,它们高的Th/Yb、Th/Zr和Rb/Y值以及低的Nb/Zr和Nb/Y值显示原始的地幔源区经历了俯冲组分(主要为俯冲流体)的交代作用(图 17d17f)(Kepezhinskas et al., 1997)。大尖山埃达克花岗岩有高的SiO2含量,低的MgO和Mg#值以及Cr,Ni含量,说明它们形成于增厚镁铁质下地壳的部分熔融(图 17g17h)。正的全岩εNd(t)(+0.5~+0.6)和εHf(t)(+1.66~+8.10)值同样指示镁铁质下地壳源区。攀枝花A型花岗岩有高的SiO2含量和分异指数,显示出高分异的A型花岗岩特征(图 18a18b),Sr vs. Rb和Sr vs. Ba微量元素图解显示这些花岗岩经历明显的长石的分异(图 18c18d)。负的全岩εNd(t)值(-1.2~-1.6)指示攀枝花A型花岗岩来源于地壳源区。极度低的MgO(0.02%~0.10%)、Cr(3.25×10-6~7.13×10-6)、Ni(1.35×10-6~2.98 ×10-6)含量显示很少的幔源组分的贡献。考虑到它们低的CaO/(FeO+MgO+TiO2)和高的FeOT/(FeOT+MgO)值,攀枝花A型花岗岩形成于低压环境之下长英质地壳的部分熔融(图 18e18f)(Patiño Douce, 1997)。

a—(La/Yb)N vs. (Yb)N图解(Defant and Drummond, 1990; Martin et al., 2005);b—Sr/Y vs. Y图解(Defant and Drummond, 1990; Martin et al., 2005);c—全岩εNd(t) vs. (87Sr/86Sr)i图解;d—Th/Yb vs. Nb/Yb图解(Pearce, 2008);e—Nb/Zr vs. Th/Zr图解(Kepezhinskas et al., 1997);f—Rb/Y vs. Nb/Y图解(Kepezhinskas et al., 1997);g—MgO vs. SiO2图解;h—Mg# vs. SiO2图解 图 17 扬子板块西缘新元古代大尖山辉长闪长岩和埃达克花岗岩主微量元素图解(据Zhu et al., 2019c修改) Fig. 17 Major and trace elements diagrams for the Neoproterozoic Dajianshan gabbro-diorites and adakitic granites in the western margin of the Yangtze Block(modified after Zhu et al., 2019c)

a—Nb vs.10000*Ga/Al图解(Whalen et al., 1987);b—Nb-Y-Zr/4图解(Eby, 1992);c—Sr vs. Rb图解(Sami et al., 2018);d—Ba vs. Rb图解(Sami et al., 2018);e—FeOT/(FeOT+MgO) vs. SiO2图解(Patiño Douce, 1997);f—CaO/(FeO+MgO+TiO2) vs. CaO+FeO+MgO+TiO2图解(Patiño Douce, 1999) 图 18 扬子板块西缘新元古代攀枝花高分异A2型花岗岩主微量元素图解(据Zhu et al., 2019c修改) Fig. 18 Major and trace elements diagrams for the Neoproterozoic Panzhihua highly fractionated A2-type granites in the western margin of the Yangtze Block(modified after Zhu et al., 2019c)

大尖山辉长闪长岩形成于ca.810 Ma,它们来源于俯冲流体交代的岩石圈地幔的部分熔融。这些辉长闪长岩属于典型的钙碱性岩石,并且显示富集的轻稀土和大离子亲石元素,属于典型的俯冲带岩浆作用(Wilson, 1989)。结合ca.810 Ma的俯冲组分交代地幔来源的高家村-冷水箐镁铁质侵入体的存在(Zhou et al., 2006a),扬子西缘新元古代时期经历了大洋板片的俯冲作用。大尖山埃达克花岗岩形成于ca.800 Ma,它们产生于增厚下地壳源区(角闪石为主,石榴石存在,斜长石缺少)的部分熔融,同样富集轻稀土和大离子亲石元素,显示俯冲背景下活动大陆边缘源区的特征(Defant and Drummond, 1990; Wang et al., 2012; Tang et al., 2016)。扬子西缘经历了长期俯冲环境下幔源岩浆的抽离,这些幔源岩浆在上升侵位过程中形成ca.860~810 Ma镁铁质侵入体,与此同时大量的幔源岩浆也使得扬子西缘下地壳逐渐增厚(Zhao et al., 2008a),进而在ca.800 Ma之后发生加厚下地壳的部分熔融,产生埃达克质花岗岩(大尖山埃达克花岗岩)。攀枝花A型花岗岩形成于ca.750 Ma,来源于浅部低压环境下壳源长英质岩石的部分熔融,指示区域扩张的环境(Eby, 1992; Whalen et al., 1987)。这些花岗岩属于A2型花岗岩,恰好对应于弧岩浆作用的消亡阶段(Eby, 1992)。攀枝花A2型花岗岩的浅层源区恰好反映了非挤压的构造背景,而这种环境下地壳趋向减薄,岩浆热能足以到达地表进而使得相对浅层的岩石发生部分熔融(Patiño Douce, 1997)。由此看来,攀枝花A型花岗岩代表了扬子西缘新元古代俯冲进程后期弧后扩展阶段浅部地壳在相对低压环境下发生部分熔融。

因此,ca.810 Ma的大尖山辉长闪长岩指示扬子西缘新元古代时期处于俯冲背景,广泛的早—中新元古代交代地幔来源的岩浆(> 810 Ma)在上升侵位过程中加厚了镁铁质下地壳。Ca.800 Ma的大尖山埃达克花岗岩到ca.750 Ma攀枝花A型花岗岩的出现代表扬子西缘俯冲背景下地壳从加厚到区域减薄的过程。区域扩张环境的出现指示着俯冲进程末期的弧后扩张阶段。

4 扬子西缘新元古代俯冲构造环境

如上构造模式所述,虽然地幔柱模式和板片裂谷模式能够解释扬子西缘单一岩性或者某种地化特征,但二者存在着较为明显的缺陷。

地幔柱模式认为华南板块中江南造山带形成于ca.1000 Ma,但大量的沉积-火山证据已经显示江南造山带形成于新元古代时期(Zhao et al., 2011; Zheng et al., 2013; Wang et al., 2014a)。地幔柱模式下的岩浆作用以具有OIB地化特征的镁铁质-超镁铁质岩石大面积分布为主要特征(如澳大利亚Gairdner和Amtata岩墙群,约210000 km2)(Kou et al., 2018),虽然华南地幔柱可以解释一些双峰式火山岩和大陆溢流玄武岩的出露,但是这些基性岩石出露有限,扬子西缘乃至整个华南板块新元古代岩浆岩以中酸性岩石为主,基性岩为辅,并且基性岩中显示典型OIB特征的岩石较少。此外,超级地幔柱诱发的岩浆作用一般持续时间较短(如,峨眉山大火成岩省,< 1 Ma;塔里木大火成岩省,约20 Ma; Shellnutt, 2014; 徐义刚等,2017)。因此,地幔柱模式不能解释扬子西缘长期持续(ca.870~740 Ma)的岩浆作用(Zhao et al., 2018, 2019)。板块裂谷模式的提出更多的来源于江南造山带中的新元古代岩浆岩证据,缺少来源扬子西缘及北缘的地质证据(李奇维,2018)。此外,地幔柱和板片裂谷模式都难以解释扬子西缘中—晚新元古代时期大量的钙碱性TTG岩石和埃达克质岩石。

事实上,近年来越来越多的碎屑锆石、地球物理、岩浆岩、显微构造等综合证据支持扬子西缘新元古代时期主要受控于俯冲构造体制。Sun et al.(2008)提出扬子西缘盐边群中的砂岩和泥岩显示弧环境地化特征和中酸性火山岩源区,他们通过对扬子西缘前寒武地层的碎屑锆石的研究进一步指出,扬子西缘存在ca.1000~740 Ma的新生岩浆作用阶段,这恰好对应于扬子西缘长期的俯冲进程(Sun et al., 2009)。Gao et al.(2016)提出贯穿四川盆地的多接受地震剖面显示出类似于古老俯冲地幔的残留形态,因此他们认为这些最新发现的地震反射剖面指示新元古代俯冲构造体制。Zhao et al.(2017)对石棉蛇绿岩进行精细岩石地球化学研究发现,它们具有SSZ-型蛇绿岩地化特征,指示扬子板块西缘处于巨大的安第斯型大陆边缘弧环境。他们进一步对扬子西缘新元古代辉长岩进行详细的全岩Nd和锆石Hf-O同位素研究(Zhao et al., 2019),指出扬子西缘新元古代地幔源区受长期的俯冲交代作用,涉及俯冲流体、俯冲沉积物熔体和俯冲板片熔体。Zhu et al.(2020a)对扬子西缘ca.850~835 Ma水陆高Mg#闪长岩最新的全岩地球化学与锆石Hf同位素研究表明,它们起源于俯冲流体与沉积物熔体交代的地幔源区,通过对扬子西缘新元古代交代地幔岩浆作用的系统总结,他们同样发现扬子西缘新元古代时期地幔源区经历长期的俯冲组分(从俯冲流体,到沉积物熔体,到俯冲板片熔体)的交代作用(图 19Zhu et al., 2020a)。此外,扬子西缘和西北缘早新元古代(ca.830 Ma)到晚新元古代(< 700 Ma)镁铁质岩石Sr-Nd-Pb-Fe同位素特征的转变(从相对富集的Sr-Nd-Pb和轻的Fe到相对亏损的Sr-Nd-Pb和重的Fe)同样支持扬子周缘处于俯冲环境(李奇维,2018)。张慰(2017)通过对扬子板块西缘—西南缘新元古代杂岩体(冕宁岩浆杂岩,米易-磨盘山岩浆杂岩,元谋岩浆杂岩)进行详细的显微构造分析提出,这些岩体指示岩浆流动面理走向总体为近南北向,说明这些岩浆杂岩体在侵位过程中受到近东西向的挤压,这些近乎南北向的原生岩浆流动面理支持扬子西缘基底自西向东的持续俯冲挤压作用。因此,扬子西缘新元古代俯冲构造体制能够较为完善地解释来自碎屑锆石、岩浆岩、地球物理、显微构造分析等方面所体现的地质学特征现象。

a—扬子西缘ca.870~820 Ma俯冲进程及主要俯冲组分;b—扬子西缘ca.820~740 Ma俯冲进程及主要俯冲组分;c—扬子西缘ca.870~740 Ma地幔源区涉及的俯冲组分 图 19 扬子板块西缘新元古代俯冲背景下地幔交代作用(据Zhu et al., 2020a修改) Fig. 19 A sketch map and summary for the Neoproterozoic metasomatized mantle magmatism under subduction setting in the western margin of the Yangtze Block (modified after Zhu et al., 2020a)
5 扬子西缘新元古代岩浆作用研究展望

基于以上综合的研究,文章认为扬子西缘新元古代时期俯冲背景下不同深度层次的岩浆作用为约束深部动力学机制和探究不同类型花岗质岩石的成因机制提供了窗口。后续的研究工作仍需在以下两个方面进一步加强。

(1) 结合文章对花岗岩类以及其他学者对镁铁质岩浆的研究(朱维光,2004林广春,2006李奇维,2018; Zhao et al., 2018, 2019; Zhu et al., 2019a, 2019b, 2019c, 2020a),扬子西缘新元古代时期俯冲背景下不同深度层次的岩浆作用已经被限定。但不同深度源区岩浆的相互作用(如壳幔岩浆混合作用)是否发生,如何发生的?对于不同源区岩浆相互作用的探讨能够为了解详细的岩浆演化进程提供见解,建立系统的弧岩浆剖面,进而明确从地幔交代作用到最终各类型岩浆产生过程不同深度源区所扮演的物质与能量角色。当然,这需要进行野外典型露头(如与花岗岩共存的镁铁质包体或者镁铁质岩墙)的详细探查以及矿物微区特征(如长石和角闪石的原位主微量和原位Sr-Pb同位素)的深入研究。

(2) 基于俯冲构造背景,扬子西缘中—晚新元古代俯冲阶段可能存在俯冲板片的回撤与断离(Cawood et al., 2016; Zhao et al., 2019)。但是俯冲板片断离的时限仍然未被较为准确的限定。已有研究已经指出扬子西缘俯冲板片的回撤与断离引发大量软流圈地幔岩浆的瞬时上涌,进而加热中上地壳,形成ca.780 Ma大陆Ⅰ型复式花岗岩体(Zhu et al., 2019a)。晚新元古代时期俯冲板片的回撤引发区域性的弧后扩张,进而使得长英质地壳发生部分熔融产生ca.750 Ma攀枝花A型花岗岩(Zhu et al., 2019b)。Zhao et al.(2019)指出扬子西缘地幔源区交代作用涉及俯冲流体、俯冲沉积物熔体和俯冲板片熔体。俯冲板片熔体的出现(ca.820 Ma)同样也能够反映俯冲阶段中后期板片的断离熔融(Zhao et al., 2019)。由此看来,需要进一步对俯冲板片回撤与断离较为精确的时限以及该构造转换下中—晚新元古代时期的岩浆响应进行系统研究。该项工作需要对扬子西缘中—晚新元古代时期TTG岩浆作用及可能共存的OIB型镁铁质岩浆作用进行系统研究。此外,该背景下巨量软流圈地幔岩浆的上涌也会使得壳幔源区发生强烈相互作用,这也是需要关注的问题。

6 结论

扬子板块西缘新元古代中期处于长期俯冲背景之下,地幔源区不仅经历了俯冲流体和板片熔体的交代作用,也经历了俯冲沉积物熔体的交代作用。此外,扬子西缘新元古代时期不仅经历了新生镁铁质下地壳的熔融,也发生了成熟大陆地壳物质的重熔。俯冲进程早—中期阶段交代地幔幔源岩浆的上涌在形成镁铁质侵入体的同时也加厚了下地壳,俯冲中—晚期阶段发生了增厚下地壳的部分熔融和弧后扩张背景下区域性地壳减薄。

花岗岩类岩浆作用的研究对于完善扬子西缘新元古代构造岩浆演化有至关重要的意义,对于不同深度源区的岩浆作用的限定有助于了解岩浆从产生到就位过程全面的信息,从俯冲交代地幔源区到地壳深部热区的系统研究更能为建立弧岩浆剖面提供全方位的支撑。此外,对于地壳深部热区不同批次的岩浆相互作用的研究更能有助于全面了解弧背景下岩浆供给体系。

致谢: 感谢责任主编邢树文、胡健民研究员的邀请撰写本文。感谢李献华院士与另一位审稿人对本文稿提出的建设性意见。感谢编辑部老师对稿件的详细校对修订。

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