岩石学报  2021, Vol. 37 Issue (1): 113-128, doi: 10.18654/1000-0569/2021.01.08   PDF    
华北克拉通中东部新太古代晚期变质火山岩及动力学体制
刘树文1, 包涵1, 高磊1, 孙国正1, 王伟2, 郭荣荣3, 郭博然4, 付敬浩5, 胡雅璐1, 白翔6,7, 胡方泱8     
1. 北京大学造山带与地壳演化教育部重点实验室, 地球与空间科学学院, 北京 100871;
2. 中国地质大学(北京)地质过程与矿产资源国家重点实验室, 地球科学与资源学院, 北京 100083;
3. 东北大学深部金属矿山安全开采教育部重点实验室, 资源与土木工程学院地质系, 沈阳 110819;
4. 北京矿产地质研究院, 北京 100012;
5. 西南石油大学地球科学与技术学院, 成都 610500;
6. 中国地震局地质研究所, 吉林长白山火山国家野外科学观测研究站, 北京 100029;
7. 中国地震局地震与火山灾害重点实验室, 北京 100029;
8. 中国科学院地质与地球物理研究所岩石圈演化国家重点实验室, 北京 100029
摘要: 热状态和壳幔岩浆作用是理解早期地壳形成演化动力学机制的关键。华北克拉通是世界范围内为数不多的保存有大量新太古代晚期(约26~25亿年)变质火山岩记录的克拉通之一,对揭示全球新太古代晚期壳-幔动力学演化过程具有重要的指示意义。在我们研究组近期关于华北克拉通中东部中新太古代热状态和地壳厚度研究基础上,本文收集并整理了726个华北克拉通中东部(包括中部带)新太古代晚期变质火山岩样品的有效地球化学资料。按照现代通用岩石地球化学标准来分类,这些样品主要包括超铁镁质岩石(其中含苦橄岩、苦橄质玄武岩和科马提岩,~7%)、稀土未分异型玄武岩(~14%)、稀土分异型玄武岩(~27%)、玻安岩(~4%)、高镁安山岩(~12%)、低镁安山岩(~26%)和英安岩-流纹岩(~10%)。然而不同区块之间火山岩岩石组合及其量比存在较大差异,其中吉林南部和赞皇等地区以大量稀土分异型玄武岩、高镁和低镁安山岩为主,含有少量的长英质火山岩;胶东、登封和阜新等地区以稀土未分异和稀土分异型玄武岩占有绝对优势,存在少量安山岩和长英质火山岩;冀北、冀东北部、冀东南部(迁安-滦县)、五台-云中山、辽北、辽南和鲁西等地区岩石组合比较复杂,最突出的特点是出现不同比例的玻安岩,组合有稀土未分异和大量稀土分异型玄武岩、高镁和低镁安山岩,出现少量超铁镁质岩石和长英质火山岩。岩石成因研究揭示稀土未分异型和分异型玄武岩、高镁安山岩和玻安岩主要形成于俯冲板片流体、熔体和沉积物熔体交代地幔的部分熔融,而低镁安山岩、英安岩和其它长英质火山岩则大都经历了上述俯冲相关初始岩浆的结晶分异或地壳物质熔融和地壳混染等过程。新太古代晚期胶东地区表现为相对较薄的地壳厚度和较高的地热梯度(18℃/km),而冀东地区表现为厚的地壳厚度和低的地热梯度(最低8.7℃/km),满足现代俯冲地热梯度需求,其它区域的地温梯度介于热俯冲和现代冷俯冲之间。综合以上资料,我们认为新太古代晚期板块构造体制已经是最主要的壳-幔动力学体制,地幔柱构造体制和板块构造-地幔柱联合作用体制可能仍然在局部地区存在,但其作用范围和强度已经明显减小。因此,随着地幔温度的下降,中太古代到新太古代晚期地幔柱和板片俯冲的转化可能是相互关联、此消彼长的动力学过程,而不是一个突变过程。
关键词: 新太古代晚期    变质火山岩及其成因    地壳热状态    动力学体制    华北克拉通中东部    
Late Neoarchean metavolcanics and geodynamics regime in central and eastern North China Craton
LIU ShuWen1, BAO Han1, GAO Lei1, SUN GuoZheng1, WANG Wei2, GUO RongRong3, GUO BoRan4, FU JingHao5, HU YaLu1, BAI Xiang6,7, HU FangYang8     
1. MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China;
2. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;
3. MOE Key Laboratory on Safe Mining of Deep Metal Mines, Department of Geology, School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China;
4. Beijing Institute of Geology for Mineral Resources, Beijing 100012, China;
5. School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China;
6. National Observation and Research Station of Jilin Changbaishan Volcano, Institute of Geology, China Earthquake Administration, Beijing 100029, China;
7. Key Laboratory of Seismic and Volcanic Hazards, China Earthquake Administration, Beijing 100029, China;
8. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Abstract: Thermal state and crust- and mantle-sourced magmatism are crucial to understanding geodynamic regime of early formation and evolution of continental crust. The North China Craton (NCC) is one of a few Archean cratons with records of Late Neoarchean (ca. 2.6~2.5Ga) volcanic rocks in the world, which has important significance for revealing Late Neoarchean global crust-mantle dynamic processes. In this paper, based on our newly obtained Meso-to Neoarchean crustal thermal state and thickness, we selected 726 samples of Neoarchean meta-volcanic rocks from Central transitional zone and Eastern Block of the NCC. These samples include ultramafic rocks (i.e. komatiites, picrites and picrobasalts, ~7%), REE-undifferentiated basalts (~14%), REE-differentiated basalts (~27%), boninites (~4%), high-Mg andesites (~12%), low-Mg andesites (~26%) and dacites-rhyolites (~10%). There are obvious differences in the lithological assemblages of metamorphic volcanic rocks and their lithological proportions in different areas of the NCC. Southern Jilin and Zanhuang areas mainly developed a lot of REE-differentiated basalts, andesites and minor dacites; Jiaodong, Dengfeng and Fuxin areas are dominated by REE-undifferentiated and -differentiated basalts with minor andesites and dacites; Eastern and northern Hebei, Wutai-Yunzhongshan, northern Liaoning, southern Liaoning and western Shandong areas are featured by REE-undifferentiated and -differentiated basalts, andesites with minor ultramafic rocks and dacites, especially by some boninites. The REE-undifferentiated and -differentiated basalts, boninites and high-Mg andesites and high-Mg dacites were mainly derived from partial melting of the mantle materials metasomatized by fluids and melts from subducted slabs and sediments, whereas the low-Mg andesites, dacites and other felsic volcanic rocks from either fractionation of the initial magmas or partial melting of pure crustal materials. Our studies on the thickness and thermal state of Neoarchean crust shows thin thickness and high geothermal gradient (18℃/km) in Jiaodong area, but thick thickness and low geothermal gradient (8.7℃/km), meeting the geothermal gradient of present-day subduction zone in eastern Hebei area. The geothermal gradients in the other areas are between those of hot subduction and modern cold subduction. Based on the above lines of evidence, we propose that plate tectonic regime had become the dominant crust-mantle dynamic mechanism in the Late Neoarchean. Though the mantle plume and plate-mantle plume interaction mechanisms might still exist in local areas, their scale and intensity significantly decreased in the Late Neoarchean. Therefore, with the decreasing mantle temperature, the Mesoarchean to Late Neoarchean transformation of mantle plume to plate subduction regimes may be an interrelated and reciprocal dynamic process, rather than a sudden transition.
Key words: Late Neoarchean    Metavolcanics and petrogenesis    Crustal thermal state    Geodynamic regime    Central and eastern North China Craton    

冥古宙和太古宙占据了地球一半以上的形成与演化历史,其壳幔动力学体制是地球科学的关键问题。长期以来研究者们基于地球早期比太古宙以后更高的地幔潜能温度和大陆长英质地壳起源于早期玄武质地壳的认识,提出各种各样的太古宙壳幔动力学体制模型,其代表性模型包括:(1)滞留-盖层模型(Stagnant-lid),该模型认为冥古宙到太古宙时期,随着地球的冷却,早期地壳表面玄武质岩石渐渐冷却形成滞留盖层,盖层之下不同规模的地幔对流导致对流室汇聚、沉降边缘对接增厚向下拖曳,发生高压变质甚至榴辉岩化、部分熔融形成最早的TTG岩浆。与玄武质岩浆结晶分异形成最早的长英质地壳的认识一样,该模型需要地壳中绝大部分的地质记录是拉斑玄武岩,而TTG片麻岩只沿着汇聚边界分布,然后这一汇聚边界后退形成更多的长英质片麻岩(Moyen and Laurent, 2018);(2)地幔柱模型认为铁镁质和超铁镁质岩浆起源于上下地幔边界或者核幔边界未亏损地幔的高温部分熔融,可以形成多级地幔柱体系,产生地球早期具有代表性的科马提岩、科马提质玄武岩、苦橄岩和富集的拉斑玄武岩等地质记录,其中地幔柱头部和边部的拉斑玄武岩成分略有差别。该模型以巴布顿绿岩带为典型代表,大量的铁镁质-超铁镁质岩出现是该模型的关键地质记录(Bédard, 2018; Zhao and Zhai, 2013)。这种模型的关键是超高温和干的伸展构造体系,基本不亏损的高度熔融的幔源岩浆,出现铁镁质岩墙群、高温A型钾质花岗质岩石和少量壳幔混合成因的中性岩浆岩石记录。这种体制下长英质岩石通常形成于干体系高温和低压热动力学过程;(3)重力沉降模型(Sagduction tectonics)主要是基于太古宙的地幔温度高于现代地幔温度250℃以上,地壳岩石处于高热和高度塑性状态,地壳表层高密度铁镁质岩石因重力不稳定而下沉,到地壳底部熔融形成TTG岩浆(Lin and Beakhouse, 2013)。这种构造体制主要受控于地壳不同层次的岩石密度,主要下沉的是高密度的铁镁质岩石和BIF,形成重力卵型底劈构造和高角度构造面理和线理,当沉降深度的围岩与沉降岩石密度接近时运动停止。由于其它变质沉积岩与围岩的密度差很小,难于发生大规模沉降,尤其是变质泥质岩和长英质岩石几乎不可能发生重力沉降,且被重力沉降的岩石很难折返回到地表。沉降后形成一些类拗拉槽型上部沉积充填带;(4)滴落构造模型(Dripping tectonics)是结合滞留盖层思路,通过热-力学数值模拟,获得岩石圈下部在地幔对流粘滞力带动下物质粘性流动,岩石圈上部保持基本稳定的滞留层,形成岩石圈底部不规则向下突出、沉降到拆沉-滴落,导致下部岩石圈物质再循环进入地幔,熔体通过沉降管道上升形成TTG岩浆(Capitanio et al., 2019a, b; Cawood et al., 2018; Nebel et al., 2018),但是地表物质尤其是长英质沉积物很难通过这种模型进入到地幔,且这种构造机制形成再循环型岩浆的过程需要非常长的时间周期;(5)包括我们在内的很大一部分研究者将太古宙构造体制,尤其是新太古代构造体制与现代主体构造体制类比,用板块构造体制来解释新太古代壳幔作用的记录。这种观点通常认为从古太古代时就存在壳幔再循环(Wang et al., 2020)。这种壳幔再循环导致的地幔含水量和硅-碱-铝质组分有所增加,流体-壳源熔体交代地幔,导致其降温并部分熔融,形成新太古代岩浆记录的多样性和明显的中新太古代地幔的不均一性,且氧逸度明显增加。这种作用的机制与现代板块构造体制有明显差别,从中太古代晚期到新太古代晚期,以存在较为刚性的块体之间的挤压碰撞、热的板片俯冲和流体-熔体交代地幔产生的相关壳幔岩浆作用记录为主要标志。这种俯冲机制包括高地热梯度条件下块体对接(Buckling)、中等地热梯度下小规模高角度浅俯冲然后俯冲板片翻转和断离(Rolling back and break-off)(Bai et al., 2014, 2016; Fu et al., 2017, 2018; Guo et al., 2013, 2015a, b, 2017a, b, 2018; Kusky, 2020; Liu et al., 2002, 2004, 2019; Wang et al., 2013, 2015; Zhai and Santosh, 2011; 刘树文等, 2018a)。近年来的研究进展表明,至少前三种模式可能从冥古宙、经始太古代、古太古代到中太古代都存在,在不同演化阶段可能不同的动力学体制都起到过主导作用。不同阶段都存在多种方式的动力学体制的联合作用,从地球早期地幔柱与滞留盖层、与沉降构造联合动力学体制到中太古代滞留-盖层或者地幔柱和板块俯冲联合动力学体制逐渐演化过渡为太古代晚期以热俯冲的动力学体制为主(Capitanio et al., 2019a, b; Cawood et al., 2018; Hawkesworth et al., 2019; Nebel et al., 2018)。

本文整理了我们研究组未发表的和众多同行们已经发表的华北克拉通中东部新太古代晚期变质火山岩资料,我们认为这些资料较好地记录了新太古代晚期的壳幔作用过程。结合我们研究组最近关于华北克拉通中东部新太古代热状态和地壳厚度的研究,这些火山岩特征、成因及其反映的壳幔动力学过程对于理解新太古代晚期的壳幔动力学体制可能具有重要意义,供同行们批评和讨论。

1 新太古代晚期变质火山岩大数据收集和分类原则

本研究使用样品的选取原则是:(1)样品需具有配套的常量元素、微量元素(包括稀土元素)数据;(2)必须是常量元素主要为XRF方法分析(少量湿法分析),微量元素(包括稀土元素)使用电感耦合等离子体质谱(ICP-MS)进行分析,且分析数据符合精度要求;(3)每一地区同一批次样品有可靠的定年数据和年龄;(4)同一批次样品有与年龄配套的锆石Lu-Hf同位素或者全岩Sm-Nd同位素数据进行成因限定。因此,我们共收集了华北克拉通中东部ca. 2.6~2.49Ga的有主量、微量和稀土元素分析结果的变质火山岩样品726件,其中232件为我们项目组尚未发表的资料,其它样品数据引自近年来在国内外学术刊物上已经发表的有效分析资料。所有的726件分析样品中的地区分布如下:胶东地区20件(Shan et al., 2015; Tang et al., 2007),吉林南部39件(Guo et al., 2016; 李承东等, 2014),辽北地区212件(Li and Wei, 2017; Peng et al., 2015, 2019; Wang et al., 2017a; 我们未发表的资料),辽南地区66件(Gao et al., 2020b; Guo et al., 2017b; Han et al., 2014; Zhu et al., 2015),鲁西地区48件(Gao et al., 2019, 2020a; Li et al., 2016; Peng et al., 2013; Shi et al., 2019; Yu et al., 2019),阜新地区42件(Wang et al., 2011, 2015),冀东青龙南-迁安-滦县地区50件(Lv et al., 2012; 郭荣荣等, 2014; 刘树文等, 2018a;我们未发表的资料),青龙-上营-洒河桥-遵化地区91件(Guo et al., 2013, 2015b, 2017c; Wang et al., 2019a),冀北平泉-承德-赤城-宣化-怀安地区68件(Ge et al., 2015; Liou et al., 2017; Wang et al., 2017b),五台山-云中山地区54件(Gao and Santosh, 2019; Liu et al., 2016; et al., 2006; Polat et al., 2005; Wang et al., 2004, 2014, 2019b),赞皇地区16件(Deng et al., 2013; Zhang et al., 2019),登封地区20件(Diwu et al., 2011; Zhang et al., 2018)。

对这些变质火山岩的分类和命名是一个很大的问题。长期以来研究者们主要是依据一些统计现代火山岩获得的微量元素分类图对所研究的变质火山岩进行分类,忽略了这些变质火山岩的主量元素特征,尤其是能够直接反映矿物组成特征的SiO2、Al2O3、MgO、FeO(Fe2O3)、碱质成分和TiO2等关键组分,导致一些命名的变质火山岩与其实际岩石种类不完全一致。由于一些富镁贫硅的岩石常常表现明显蚀变,导致较高的烧失量,这也会导致岩石化学分类时岩石名称的一些不确定性。鉴于目前这种研究现状,本文的岩石化学分类主要参考IUGS推荐的Pearce and Robinson (2010)分类和Pearce and Reagan (2019)研究全球玻安岩时的火山岩分类,即将全岩主量岩石化学分析成分去掉烧失量,按照全球范围内太古宙铁镁质岩石和辽东地区新太古代岩石中Fe3+:Fe2+=2:8(离子比, Pearce and Reagan, 2019; Peng et al., 2015),将全铁成分换算成Fe2O3和FeO,然后归一化到100%,重新核算每一个主量元素的组分含量(用下角标C加以注释)。在此基础上,为了大数据统计处理方便起见,将SiO2C≤45%的岩石命名为变质超铁镁质岩石、将45%≤SiO2C < 52%且MgOC≥18%时的科马提岩和12%≤MgOC < 18%的变质苦橄岩也归于超铁镁质岩石。45% < SiO2C≤52%的作为玄武岩类,进一步根据MgOC=8%为界划分为低镁玄武岩和高镁玄武岩。根据Condie (1981)在太古代绿岩带中划分了TH1和TH2型玄武岩平均化学成分,本文得到球粒陨石标准化的TH1型的(La/Yb)N < 1.3,TH2型和平均岛弧拉斑玄武岩的(La/Yb)N>1.3,因此选用(La/Yb)N=1.3将变质玄武岩类划分为稀土未分异型(相当于TH1型)和稀土分异型(相当于TH2型,包括拉斑玄武岩、钙碱性玄武岩和二者过渡型)变质玄武岩。变质安山岩类则定义为在52% < SiO2C≤63%之间,当MgOC≥5%时称为高镁安山岩。变质玻安岩定义为SiO2C>52%、MgOC≥8%和TiO2C < 0.5%(Pearce and Reagan, 2019)。对既符合高镁安山岩又符合玻安岩分类标准的岩石一律称为变质玻安岩,对除变质玻安岩外、剩余满足SiO2C>52%和MgOC≥5%的安山岩在统计分析时放进高镁安山岩类一起讨论。当SiO2C>63%时称为变质英安岩、流纹岩或者变质长英质火山岩,未细分。

2 新太古代晚期变质火山岩类型及岩石组合与分布

从现有分析资料不难看出华北克拉通中东部新太古代晚期变质火山岩岩石组合非常复杂,保存有很少量的变质超铁镁质岩石和变质玻安岩记录,最主要的岩石分布是拉斑玄武岩和玄武安山岩,其次是安山岩、英安岩和流纹岩(图 1a)。一些变质火山岩也表现了非常高的总碱质(Na2O+K2O)变化量,从小于1%到超过9%,尤其是辽北和冀东北部地区的少量玄武质-玄武安山质岩石表现了很高的总碱质含量,落入了偏碱性岩石系列范围(图 1a, b)。我们假设收集到的样品分析结果没有问题,那么这种高度变化的总碱质含量可能一方面与原岩初始化学组分变化有关,这一点也被微量元素分类图解上部分样品落入碱性玄武岩、粗面玄武岩、粗面安山岩样品所证实(图 1b);另一个重要原因就是变质变形过程中部分样品受到了较为强烈的碱质流体交代作用。在MgOC-SiO2C分类图上(图 1c),这些样品表现为两个重要的岩石组合,一是分布量最大的高铁低镁岩石组合,包括很少量的苦橄质玄武岩、最大量的玄武岩和玄武安山岩-安山岩和少量的长英质火山岩,另一个就是高镁岩石组合,包括科马提岩、苦橄岩、硅质高镁玄武岩、玻安岩和高镁安山岩(图 1c黄色范围)。与显生宙玄武岩相比,所有的这些变质铁镁质和超铁镁质岩石总体上表现为低钛特征,只有很少量的样品TiO2含量大于2%,尤其是高镁岩石系列表现了从拉斑玄武岩到钙碱性岩石系列的渐变过渡关系(图 1d)。

图 1 变质火山岩的地球化学分类 (a) IUGS火山岩TAS分类(Le Bas et al., 1986);(b)微量元素分类图(Winchester and Floyd, 1976);(c) MgOC-SiO2C分类图(Pearce and Reagan, 2019);(d) La-Yb岩石系列分布图(Ross and Bédard, 2009). LSB-低硅玻安岩;HSB-高硅玻安岩;SHMB-硅质高镁玄武岩;HMA-高镁安山岩;A-安山岩;D-英安岩;Ol-橄榄石;Opx-斜方辉石 Fig. 1 Geochemical classification diagrams for metavolcanic rocks (a) International Union of Geological Sciences (IUGS) total alkalis-silica (TAS) diagram of volcanic rocks (Le Bas et al., 1986); (b) Zr/TiO2×0.0001 vs. Nb/Y diagram (Winchester and Floyd, 1976); (c) MgOC-SiO2C diagram (Pearce and Reagan, 2019); (d) La vs. Yb diagram (Ross and Bédard, 2009). LSB, low-Si boninite; HSB, high-Si boninite; SHMB, siliceous high-Mg basalt; HMA, high-Mg andesite; A, andesite; D, dacite; Ol, olivine; Opx, orthopyroxene

按照上述分类命名原则,华北克拉通中东部新太古代晚期各地区的岩石组合特征如下(图 2b):胶东地区主要为稀土未分异型变质玄武岩(30%)和稀土分异型变质玄武岩(65%),含有少量的高镁安山岩;吉林南部地区主要由稀土分异型变质玄武岩(31%)、高镁安山岩和安山岩(46%)、变质长英质火山岩(21%)和少量超铁镁质岩石组成;辽北地区出现超铁镁质岩石(12%),包括变质蚀变橄榄岩、辉橄岩、蛇纹岩和异剥钙榴岩(图 2a, c),这些岩石是堆晶岩还是洋壳残片尚不清楚,最主要的岩石类型是变质玄武岩和安山岩,包含稀土未分异玄武岩(14%)和稀土分异型变质玄武岩(25%)、变质高镁安山岩和安山岩(44%),含有少量的变质玻安岩和长英质火山岩;辽南地区超铁镁质岩石15%,稀土未分异和稀土分异型变质玄武岩为32%,变质玻安岩占9%,变质高镁安山岩和安山岩占27%,但变质长英质火山岩相对较多(17%)。鲁西地区主要由稀土未分异和稀土分异型变质玄武岩(44%)、变质高镁安山岩、安山岩(42%)为主,含有较多的变质玻安岩(10%),并含有少量的变质超铁镁质岩石(4%);阜新地区缺少变质超铁镁质岩石和玻安岩,以稀土未分异和稀土分异型变质玄武岩(50%)、变质高镁安山岩和安山岩(42%)为主,含有少量的变质长英质火山岩(7%);冀东青龙南-迁安-滦县地区以大量的变质长英质火山岩(48%),稀土未分异和稀土分异型变质玄武岩(22%)及变质高镁安山岩和安山岩(22%)为主,含有少量玻安岩(6%)和少量的变质超铁镁质岩石;冀东北部青龙-上营-洒河桥-遵化地区以稀土未分异和稀土分异型变质玄武岩(58%)、变质高镁安山岩、安山岩(20%)和变质长英质火山岩(13%)为主,出现少量变质玻安岩和变质超铁镁质岩石,包括变质角闪石岩和易熔岩等;与冀东北部地区相似,冀北平泉-承德-赤城-宣化-怀安地区以稀土未分异和稀土分异型变质玄武岩(35%)、变质高镁安山岩和安山岩(47%)、变质长英质火山岩(9%)为主,并少量变质玻安岩和变质超铁镁质岩石;五台山-云中山地区主要火山岩岩性为大量稀土分异型变质玄武岩(19%)、变质高镁安山岩和安山岩(71%),含有少量稀土未分异变质玄武岩、变质玻安岩和长英质火山岩;与五台地区类似,赞皇地区主要火山岩岩性为稀土分异型变质玄武岩(63%)、变质高镁安山岩和安山岩(37%);登封地区缺少变质超铁镁质岩石和玻安岩,主要为稀土未分异变质玄武岩(55%)、稀土分异型变质玄武岩(15%)和变质高镁安山岩(20%),含有少量变质安山岩和长英质火山岩。

图 2 华北克拉通中东部陆块变质火山岩区域分布及岩石组合特征 (a)华北克拉通地质简图(据Zhao et al., 2005修改);(b)不同区域变质火山岩岩石组合. DF-登封;FX-阜新;JD-胶东;N-EH-冀东北部;NH-冀北;NL-辽北;S-EH-冀东南部(青龙南部-迁安-滦县地区);SJ-吉南;SL-辽南;TH-太华;WS-鲁西;WT-YZ-五台-云中;ZH-赞皇;ZT-中条 Fig. 2 Regional distributions and lithological assemblages of metamorphic volcanic rocks in the basement terranes of Central and Eastern Block of NCC (a) geological sketch map of the North China Craton (modified after Zhao et al., 2005); (b) the lithological assemblages of metamorphic volcanic rocks in different areas. DF, Dengfeng; FX, Fuxin; JD, eastern Shandong; N-EH, northern part of eastern Hebei; NH, northern Hebei; NL, northern Liaoning; S-EH, southern part of eastern Hebei (including Qianan, luanxian and southern Qinglong areas); SJ, southern Jilin; SL, southern Liaoning; TH, Taihua; WS, western Shandong; WT-YZ, Wutai and Yunzhong areas; ZH, Zanhuang; ZT, Zhongtiao

从上述各地区不同岩石类型组合和各类岩石分布量比(图 2)不难看出,新太古代变质超铁镁质岩石在辽北、辽南、冀东、冀北和吉南地区均有少量分布,所占火山岩样品总量比仅有~7%,稀土未分异型玄武岩占~14%,稀土分异型玄武岩占~27%,变质玻安岩只少量分布(~4%)。而高镁安山岩和安山岩比例为~38%,变质长英质火山岩仅占~10%。

鲁西、辽南、辽北、冀东北部、冀东南部(迁安-滦县)、冀北和五台-云中地区可大致划归为同一种岩石组合类型,以稀土未分异型玄武岩(REE-undifferentiated basalts)、稀土分异型玄武岩(REE-differentiated basalts)、高镁安山岩(high-Mg andesites)、安山岩(andesites)组合为主体,出现少量超铁镁质岩(ultramafic rocks)、玻安岩(boninites)和长英质火山岩(felsic volcanic rocks),我们以Ultramafic rock+Basalt+Boninite+Andesite作为其标志性岩石组合,称为UBBA型(图 2),最突出的特点是出现变质玻安岩,且出现大量的稀土未分异和稀土分异型玄武岩,高镁和低镁安山岩,并组合有少量英安岩,存在超铁镁质岩石。胶东、登封和阜新地区主要保存了大量的稀土未分异、稀土分异型玄武岩和不等量安山岩,可以称为Tholeiite-Basalt+Andesite(TBA)型组合(图 2b)。吉林南部和赞皇地区岩石组合为稀土分异型拉斑玄武岩、高镁和低镁安山岩,只有少量的长英质火山岩,我们可以称其为Basalt-Andesite(BA)型组合(图 2b)。

3 变质火山岩成因

为了进一步探讨这些新太古代晚期变质火山岩所代表的动力学意义,我们必须了解各类岩石组合的岩浆作用特征和大致岩石成因。

3.1 UBBA变质火山岩

这种类型变质火山岩在华北克拉通不仅发育量大而且分布范围广,岩石类型复杂。该类火山岩中含48个超铁镁质岩石样品,包括蛇纹岩、蛇纹石化辉橄岩、富铁的易熔岩、富镁的透闪石角闪石岩、普通角闪石岩、透辉石岩,水铝榴石异剥钙榴岩等,因为各种类型的蚀变改造和交代作用导致该类岩石岩石学特征异常复杂。这些超铁镁质岩石表现了类似于SSZ型蛇绿岩的岩石组合特征。Peng et al. (2015)通过大量的岩石学、岩石地球化学和Sr-Nd同位素研究认为其中超铁镁质岩石直接来源于地幔,其它相关的火山-侵入岩形成于俯冲相关的弧岩浆作用。按照稀土元素特征,这些超铁镁质岩石样品中10个具有稀土未分异型拉斑玄武岩的稀土配分样式,大部分样品具有70~90的Mg#值,一些富铁岩石Mg#变化范围较宽。个别样品表现出Ce负异常(图 3a, Peng et al., 2015),一些大离子亲石元素和P浓度变化较大并呈不明显的Nb和Ti负异常(图 3b),岩石成因研究表明其岩浆来源于原始地幔到亏损地幔较高程度的部分熔融,且地幔源区受到了来自俯冲相关的沉积物的熔体交代作用(图 3e, f),岩石受到了海底流体或者后期流体的强烈交代。另一类超铁镁质岩(38个样品)则表现为明显右斜式稀土图谱和Th、Nb、Ta的负异常,Sr、P和Ti从正异常到明显负异常(图 3c, d),大部分样品TiO2含量小于2%。岩石成因研究表明该类岩石的岩浆起源于原始地幔较高程度部分熔融,源区明显受到了俯冲板片熔体的交代,并存在OIB型地幔端元,只是贡献量很小(图 3e, f)。

图 3 超铁镁质岩石的微量元素特征和成因 稀土未分异型超铁镁质岩石球粒陨石标准化稀土图谱(a)和原始地幔标准化多元素图谱(b);稀土分异型超铁镁质岩石球粒陨石标准化的稀土图谱(c)和原始地幔标准化多元素图谱(d); 球粒陨石和原始地幔值引自Sun and McDonough (1989);(e) La/Yb-Nb/Yb岩石成因鉴别图,反映原始岩浆的地幔端元和熔体交代作用(据Pearce, 2008修改);(f) Th/Yb-Nb/Yb值图(Pearce, 2008). DM亏损地幔;N-MORB正常洋中脊玄武岩;E-MORB富集洋中脊玄武岩;CC大陆地壳;OIB洋岛玄武岩;AC平均地壳 Fig. 3 Chondrite-normalized REE patterns and primitive mantle-normalized spider diagrams, and petrogenetic discrimination diagrams for the ultramafic rocks Chondrite-normalized REE patterns (a) and primitive mantle-normalized spider diagrams (b) for REE-undifferentiated ultramafic rocks; chondrite-normalized REE patterns (c) and primitive mantle-normalized spider diagrams (d) for REE-differentiated ultramafic rocks; the chondrite and primitive mantle values are after Sun and McDonough (1989); (e) La/Yb vs. Nb/Yb diagram (modified after Pearce, 2008); (f) Th/Yb vs. Nb/Yb diagram (Pearce, 2008). DM, depleted mantle; N-MORB, normal-type mid-ocean ridge basalt; E-MORB, enriched-type mid-ocean ridge basalt; CC, continental crust; OIB, ocean island basalt; AC, average crust

该岩石组合中的稀土未分异型高镁拉斑玄武岩表现轻稀土亏损到平坦型稀土配分模式,绝大部分岩石没有明显的Nb、Ta、Ti负异常(图 4a, b),岩石成因研究表明该类岩石岩浆起源于尖晶石橄榄岩熔融程度在5%~30%范围内的部分熔融,与稀土未分异的超铁镁质岩石有明显一致的岩石成因特征,且其岩浆源区同样受到了俯冲沉积物熔体的交代作用(图 4e, f)。与稀土未分异型高镁拉斑玄武岩相比,稀土分异型高镁玄武岩呈现明显高的稀土总量、(La/Yb)N比值和右斜式稀土图谱,从不明显到明显的Nb、Ta和Ti负异常(图 4c, d),TiO2在0.2%到2.1%范围内变化,清楚地表现了岛弧型拉斑玄武岩的地球化学特征。岩石成因研究表明该类岩石岩浆起源于两个初始岩浆端元,一个和稀土未分异型岩浆源完全一致,但是主要受到了俯冲板片流体、熔体交代的地幔尖晶石橄榄岩的部分熔融。另一个端元岩浆为OIB型岩浆源,源区物质中石榴石和尖晶石的比例超过50:50,反映了其来源深度相对较大,该类岩浆源同样受到了来自板片熔体的交代作用或者形成于两者的熔体混合(图 4e, f)。

图 4 变质玄武岩类的微量元素特征和岩石成因 稀土未分异型玄武岩球粒陨石标准化稀土图谱(a)和原始地幔标准化多元素图谱(b) (其中粉红色图标为该类样品中高镁玄武岩样品);稀土分异型玄武岩球粒陨石标准化稀土图谱(c)和原始地幔标准化多元素图谱(d) (其中深蓝色图标为该类样品中高镁玄武岩样品); 球粒陨石和原始地幔值引自Sun and McDonough (1989);(e) Sm/Yb-Sm玄武质岩浆岩石成因鉴别图(Aldanmaz et al., 2000);(f)两类玄武岩源区成分和俯冲熔体交代特征(据Pearce, 2008修改). PM-原始地幔;其余英文缩写含义同图 3 Fig. 4 Chondrite-normalized REE patterns and primitive mantle-normalized spider diagrams, and petrogenetic discrimination diagrams for the meta-basalts Chondrite-normalized REE patterns (a) and primitive mantle-normalized spider diagrams (b) for REE-undifferentiated basalts (blue) and REE-undifferentiated high-Mg basalts (pink); chondrite-normalized REE patterns (c) and primitive mantle-normalized spider diagrams (d) for REE-differentiated basalts (red) and REE-differentiated high-Mg basalts (dark blue); the chondrite and primitive mantle values are after Sun and McDonough (1989); (e) Sm/Yb vs. Sm diagram (Aldanmaz et al., 2000); (f) La/Yb vs. Nb/Yb diagram (modified after Pearce, 2008). PM, primitive mantle; the other abbreviations are the same as Fig. 3

与高镁拉斑玄武岩一样,低镁拉斑玄武岩同样也分为稀土未分异和稀土分异型拉斑玄武岩,它们的微量元素特征与对应的高镁拉斑玄武岩特征类似,主要形成于对应的稀土未分异和稀土分异型高镁拉斑玄武岩的初始岩浆结晶分异,少量样品反映了大陆地壳混染的地球化学特征,反映了它们形成演化过程中存在同化混染和分离结晶(AFC)作用。与高镁拉斑玄武岩相比,低镁拉斑玄武岩类缺少明确的OIB端元,但是存在E-MORB到OIB之间的样品分布,不能完全排除OIB端元岩浆作用的存在。

少量玻安岩(28个样品)和大量高镁安山岩的出现是该类岩石组合的标志性特征。这些岩石表现为高镁含量(图 5a, b)和稀土分异型拉斑玄武岩的稀土图谱特征,呈Nb、Ta和Ti负异常和部分样品的Th负异常(图 5c-f),属于钙碱性岩石系列,只有少数样品表现为稀土未分异型的稀土图谱特征,表现为拉斑玄武岩系列(图 5c)。其中高镁安山岩属于低温高镁安山岩(图 5a)。低镁安山岩表现为与高镁安山岩一致的微量元素地球化学特征(图 5g, h)。在La/Sm对La岩石成因鉴别图(图 5i)上,变质玻安岩样品表现了高的正斜率线性分布,变质高镁安山岩样品表现了与变质玻安岩样品类似的正斜率线性分布但有少数样品呈近水平分布,而低镁安山岩较多的样品呈近水平分布,因此变质玻安岩和高镁安山岩主要为部分熔融成因,变质低镁安山岩主要为结晶分异成因(图 5i)。在La/Yb对Nb/Yb的对数坐标图上,所有变质玻安岩、低镁安山岩样品和绝大部分高镁安山岩样品都表现了俯冲板片熔体交代和岛弧型火山岩的分布趋势,部分高镁安山岩样品表现了俯冲沉积物熔体交代的分布趋势(图 5j)。变质玻安岩样品表现了与IBM型岛弧区近似的地球化学分布,在MgOC对SiO2C分类、成因和构造环境鉴别图上,所有的玻安岩样品都落在了方辉橄榄岩低压下部分熔融的熔体范围内,并且落在了Setouchi proto-arc、arc-basin体系和典型太古代Whudu型岛弧区的岩石成分分布区内,反映了它们的低压下方辉橄榄岩部分熔融的成因和汇聚型板块边缘的构造背景(图 5b)。其中拉斑系列岩石高镁安山岩落在了接近DM和N-MORB范围,体现了它们与高镁拉斑玄武岩的近缘关系,并且其源区受到了俯冲板片流体-熔体的交代作用,落在岛弧-盆地构造环境(图 5b),与澳大利亚太古代Whundu型玻安岩一致。低镁安山岩类岩石主要形成于各类变质玄武岩和高镁安山岩岩浆的分离结晶,即可能存在AFC岩浆演化过程。

图 5 变质玻安岩和各类安山岩的地球化学特征 (a)变质安山岩分类图(据邓晋福等, 2018修改);(b)玻安岩成因分类-成因图(Pearce and Reagan, 2019);变质玻安岩球粒陨石标准化稀土图谱(c)和原始地幔标准化多元素图谱(d);变质高镁安山岩球粒陨石标准化稀土图谱(e)和原始地幔标准化多元素图谱(f);变质低镁安山岩球粒陨石标准化稀土图谱(g)和原始地幔标准化多元素图谱(h); 球粒陨石和原始地幔值引自Sun and McDonough (1989);(i) La/Sm-La岩石成因鉴别图(Treuil and Joron, 1975);(j)岩浆源区成分和俯冲熔体交代特征(据Pearce, 2008修改).缩写同图 1图 3 Fig. 5 Geochemical characteristics of the meta-boninites and andesites (a) MgOC vs. SiO2C diagram for the andesites (modified after Deng et al., 2018); (b) MgOC vs. SiO2C diagram for the boninites (Pearce and Reagan, 2019); Chondrite-normalized REE patterns and primitive mantle-normalized spider diagrams for meta-boninites (c, d), for meta high-Mg andesites (e, f) and for meta low-Mg andesites (g and h); the chondrite and primitive mantle values are after Sun and McDonough (1989); (i) La/Sm vs. La petrogenetic discrimination diagram (Treuil and Joron, 1975); (j) La/Yb vs. Nb/Yb diagram (modified after Pearce, 2008). The abbreviations are the same as Fig. 1 and Fig. 3

该岩石组合中还有少量的变质英安岩-流纹岩样品,其中绝大部分样品表现了较高的(La/Yb)N比值(>25)。这些样品分布在冀北、冀东北部和辽北地区,Mg#值在20~70之间变化,其中符合埃达克岩地球化学特征的样品很少,主要分布在辽北地区,形成于俯冲板片部分熔融,且熔体上升过程中受到了地幔楔物质的混染(Peng et al., 2015)。绝大部分样品尽管Mg#较高,变化范围大,但是Mg#值与较低浓度的Cr和V等过渡族元素没有明显相关性,说明它们起源于下部地壳铁镁质岩石的部分熔融,经历了分离结晶作用(Gao et al., 2019, 2020a, b; Guo et al., 2013, 2015b, 2017c; Li and Wei, 2017; Liu et al., 2012; et al., 2006; Peng et al., 2013; Wang et al., 2004)。

3.2 TBA型变质火山岩

该类变质火山岩主要分布在胶东地区、登封地区和阜新地区,该类岩石组合最突出的特征是稀土未分异型和稀土分异型拉斑玄武岩占有绝对优势,组合有少量安山质岩石和长英质火山岩(图 2)。其中稀土未分异型拉斑玄武岩表现为近于平坦的稀土配分模式,没有明显的Eu异常。在原始地幔标准化微量元素图谱中,大部分元素表现出较为平坦的图谱特征,没有Nb、Ta和Ti负异常(Diwu et al., 2011; Zhang et al., 2018; 王伟等, 2015),其La/Yb、Th/Yb和Nb/Yb比值均接近于N-MORB,并表现为N-MORB-E-MORB-OIB排列趋势,指示其地幔源区没有受到明显的外来物质的扰动,形成于尖晶石二辉橄榄岩10%~15%的部分熔融,即软流圈地幔的低压部分熔融(图 4a, b, e, f)。

稀土分异型拉斑玄武岩,具有轻微右斜式稀土配分模式,没有明显的Eu异常,表现为微弱的Nb、Ta和Ti的负异常,具有相对较高的La/Yb和Nb/Yb比值,表现了拉斑玄武岩向钙碱性玄武岩过渡的特征(图 1d图 4c, d)。它们形成于受到俯冲流体-熔体交代富集,并有不同比例石榴石参与的亏损地幔石榴石尖晶石二辉橄榄岩的部分熔融,形成深度应该高于稀土未分异型拉斑玄武岩(图 4e, f)。它们的原岩可能形成于洋内初始俯冲作用阶段,主要起源于受到微弱俯冲流体作用改造的亏损-弱富集大洋岩石圈地幔的部分熔融(Diwu et al., 2011; Zhang et al., 2018; 王伟等, 2015)。

本岩石组合中的高镁和低镁安山岩,表现为较宽范围变化的总碱质含量(图 1a),轻重稀土强烈分馏的稀土配分模式,无明显Eu异常,通常(La/Yb)N比值较高,在2.5~25.2之间变化。在原始地幔标准化微量元素图谱中,它们具有明显Nb、Ta和Ti负异常,具有明显高的La/Yb、Nb/Yb、Th/Yb和Nb/Y比值(图 5e-j),以至于部分样品落入碱性玄武岩和粗面安山岩的范围(图 1a, b)。这些特征表现了与赞岐岩类岩石类似的地球化学特征。一些高镁安山岩类主要形成于受俯冲板片流体、俯冲板片和沉积物熔体交代的地幔楔的部分熔融,而低镁安山岩的一部分与高镁安山岩结晶分异有关,另一部分表现了与稀土分异型拉斑玄武岩的亲缘性,经历了AFC演化过程(图 5i, jDiwu et al., 2011; Zhang et al., 2018; 王伟等, 2015)。

本岩石组合中存在很少量的变质英安岩,它们表现出明显高的Sr/Y(>300)和(La/Yb)N(>20)比值,及明显高的MgO含量和Mg#值,轻稀土强烈分馏,重稀土相对分馏较弱,并呈明显的正Eu异常。这些特征表明这类岩石形成过程中可能有幔源物质的卷入,或者形成于俯冲板片熔体受到了地幔物质污染,或者形成于高镁安山质岩浆的结晶分异。

3.3 BA型变质火山岩

就目前已有的资料看,该变质火山岩组合最重要的特征是缺少稀土未分异型变质火山岩组合,保存了大量稀土分异型拉斑玄武岩、大量高镁和低镁安山岩及少量的英安岩(图 1a图 2)。其中吉林南部夹皮沟地区变质高镁安山岩形成于2.49Ga,具有与日本Stouchi地区经典的高镁安山岩一致的高镁钙碱性地球化学特征和类似于赞岐岩的微量元素地球化学特征,具有高的Mg#值(68~71)和Cr、Ni、Co等过渡族元素含量,亏损Nb、Ta和Ti等高场强元素,形成于板片流体-熔体和俯冲沉积物熔体交代的地幔楔物质的部分熔融(李承东等, 2014)。变质稀土分异型拉斑玄武岩形成于2.59~2.54Ga,Mg#的范围为38~64,具有亏损Nb、Ta和Ti的地球化学特征和较宽的锆石Hf同位素范围,并保存了古老的~2.7Ga的捕获锆石,形成于俯冲流体-熔体交代地幔楔物质部分熔融过程,且经历了AFC演化过程(Guo et al., 2016)。与吉林南部新太古代晚期变质火山岩类似,赞皇地区新太古代晚期变质火山岩以稀土分异型变质拉斑玄武岩和安山质岩石为主体,表现为Nb、Ti等高场强元素亏损,形成于大洋岛弧和弧前火山作用(Deng et al., 2013)。在前人分析数据基础上,本文对这些岩石的进一步岩石成因分析表明,其初始未演化玄武质岩浆起源于俯冲板片流体-熔体交代的地幔楔尖晶石石榴石二辉橄榄岩5%~10%的部分熔融,并经历了分离结晶和大陆地壳物质的混染过程,形成了玄武质、玄武安山质和安山质岩石。该壳幔岩浆作用发生于岛弧和活动大陆边缘构造背景,与前人的认识基本一致。

4 新太古代动力学体制

在太古宙滞留盖层、地幔柱还是板块构造体制研究中,太古宙不同阶段与现代地幔温度差(ΔT)是关键。当ΔT < 100℃以内产生规模较大的类现代板块俯冲构造体制;当100℃≤ΔT < 175℃时产生规模较小、高角度的浅俯冲,然后俯冲板片发生翻转和断离,导致频繁发生板片俯冲和后退,形成以侧向增生为主导的壳幔动力学体制;当175℃≤ΔT < 250℃时形成前俯冲构造体制(Pre-subduction),岩石圈以塑性变形增厚和对接为主,形成挤压增厚的构造体制;当ΔT≥250℃不可能形成俯冲构造,以地幔柱、滞留层和重力沉降(Sagduction or dripping tectonics)为主(Capitanio et al., 2019a, b; Cawood et al., 2018; Gerya, 2014; Nebel et al., 2018; Sizova et al., 2010; Van Hunen and Moyen, 2012)。

4.1 变质火山岩指示的动力学体制

华北克拉通中东部新太古代变质火山岩的少量全岩Sm-Nd的同位素资料(Wu et al., 2005)和大量的锆石Lu-Hf同位素资料表明,新太古代经历了~2.5Ga和2.7Ga两期主要的地壳生长事件,铁镁质岩浆起源于亏损地幔,大部分样品都不同程度地受到了老地壳物质混染的影响(Gao et al., 2019, 2020a, b; Guo et al., 2013, 2015a, b, 2016, 2017b, 2018; Liu et al., 2011; Wang et al., 2013, 2015; 万渝生等, 2015; 王伟等, 2015)。本文和前人关于华北克拉通中东部新太古代变质火山岩的岩石成因研究表明,这些来源于地幔的岩浆作用受到了不同程度的俯冲流体、板片熔体和沉积物熔体交代。本文和Peng et al. (2015)研究表明华北克拉通中东部新太古代晚期变质火山岩中包括很少量超铁镁质岩石样品,主要是蛇纹石化的辉橄岩、异剥钙榴岩、易熔岩和各种角闪石岩。超铁镁质和铁镁质岩石,按照Pearce and Reagan (2019)分类,其中6个样品属于科马提岩,15个样品属于变质苦橄岩,9个为苦橄质玄武岩,此外还有3个MgO含量超过30%样品在硅质高镁玄武岩范围(蛇纹石化辉橄岩)。这些铁镁质和超铁镁质岩石(占火山岩样品总量的4.6%)主要分布在辽北、辽南、冀东北部青龙-上营-遵化等地区。

为了确定新太古代晚期地幔岩浆作用的特点,我们主要用MgOC≥8%的样品讨论其岩浆起源,以尽量减少结晶分异和地壳混染作用的影响。结果显示初始岩浆有四个端元,最主要的是稀土未分异型端元和稀土分异型拉斑玄武岩端元(图 1d),以及少量的玻安岩端元和OIB型玄武岩端元。其中OIB型玄武岩样品表现出较高的TiO2含量和Nb/Yb比值。~4%的典型玻安岩和大量高镁玄武岩类样品的存在,表现为明显的Nb、Ta和Ti亏损的大量稀土未分异和稀土分异型玄武岩,岩石成因研究揭示这些岩石主要形成于俯冲板片流体-熔体和沉积物熔体交代地幔的部分熔融,而不是壳幔岩浆混合作用(Tang et al., 2007; Wang et al., 2017b)。这些火山岩成分和成因表明在新太古代晚期板片俯冲相关的壳幔岩浆作用已经在华北克拉通中东部地区成为主导的动力学体制,但是明显存在多元化幔源岩浆作用,尤其是OIB岩浆作用、科马提质玄武岩和苦橄质玄武岩类的存在表明高温-高压地幔岩浆作用在局部是存在的,并且与俯冲板片体制下的岩浆作用相伴产出,最有可能是板块构造体制和地幔柱构造体制联合作用的产物(Gao et al., 2019; Polat, 2009)。

4.2 新太古代晚期地壳厚度和热状态

近年来数值模拟实验(Number experiments of thermal mechanics)技术的发展为太古宙壳幔动力学体制研究注入了活力。这些实验基于假设的太古宙热状态参数获得的模拟结果,提供了动力学体制、构造样式和动态演进模式。到目前为止,早前寒武纪地球科学研究的基本假设是太古宙地幔温度和莫霍面地热梯度比现代高得多(Gerya, 2014; Sizova et al., 2010)。但是该假设缺少有效的约束,因为太古宙壳幔作用的地球物理记录基本上被后期长期的改造所抹掉,所以要限定早期地壳的热状态只有地质学、岩石学和地球化学方法是目前基本可用的手段,因此我们希望通过这些可用的方法,初步判断地球早期的热状态。目前的一个关键科学问题是太古宙不同时期地壳究竟有多厚,地热梯度究竟有多高,地幔温度究竟比现代高多少?到目前为止这些还是未知数,不同的作者基于不同的模型有各种各样的假设或者推断。基于目前的这种情况,我们研究组发展了利用纯壳源铁镁质岩石部分熔融产生的TTG岩浆实验、微量元素理论模拟和一维热传导,结合长英质地壳放射性热源,初步获得了华北克拉通中东部陆块中-新太古代时期的地壳厚度和莫霍面地热梯度(具体方法、步骤和成果将另文发表)。研究结果表明~2.5Ga华北克拉通中东部各区地壳厚度为35~56km,其中中条山44~47km、鲁西地区40~43km、胶东地区35~38km、辽北-吉南40~43km、登封-太华地区50~53km和冀东-辽西地区53~56km(Sun et al., 2019b, 2020)。~2.5Ga各地区的莫霍面地热梯度分别为中条山13.2℃/km、鲁西地区13.2℃/km、胶东地区18℃/km、辽北-吉南15.1℃/km、登封-太华10.1℃/km和冀东-辽西8.7℃/km(Sun et al., 2019b, 2020)。上述估算结果表明华北克拉通中东部新太古代晚期地壳厚度和地热梯度具有明显的空间分带性,南缘的登封-太华地区和北部的冀东-辽西地区地壳厚度大于50km,而地热梯度低(8.7~10.1℃/km),与现代地热梯度相当或略有不同(按照均变估计约11.7℃/km,比现代地幔温度高不足100K),完全满足现代板块构造起主导作用的物理条件。而中条、鲁西地区略低于13.5℃/km,相当于比现代地幔温度高约100~150K(低于175K),板块构造体制仍可以起主导作用,但是应表现为热俯冲,即规模小、频次高、俯冲角度大,导致俯冲板片回转和断离深度明显低于现代板片俯冲。而辽北-吉南地区和胶东地区明显较高的地热梯度(>14.2℃/km),可能反映了弧后盆地的热状态特征(图 6; Gerya, 2014; Sizova et al., 2010)。图 6反映了这样的热状态均发育在高压麻粒岩和高压-中压麻粒岩相分界线上。空间上的这种热状态与现代板块构造从NW向SE向的俯冲和弧后盆地热状态特征类似(Bai et al., 2016; Fu et al., 2018, 2019; Guo et al., 2013, 2015b, 2017b; Wang et al., 2013, 2015; 刘树文等, 2018b)。这种~2.5Ga时期的热状态说明,新太古代晚期华北克拉通中东部满足类现代板块构造和热俯冲的热状态条件,不能排除东部地区吉南-辽东-胶东等地区局部存在地幔柱(Plume)或者沉降(Sagduction or Dripping)构造体制的可能性。

图 6 华北克拉通中东部新太古代地壳厚度和地热梯度P-T空间图(底图据Brown, 2014修改) Lg-低程度变质, 包括沸石相;Gr-绿片岩相;Am-角闪岩相;Gn-麻粒岩相;UHTM-超高温变质作用;Bl-蓝片岩相;E-HPG-榴辉岩-高压麻粒岩相;UHPM-超高压变质作用;DF-登封;EH-冀东;JD-胶东;NL-辽北;SJ-吉南;TH-太华;WL-辽西;WS-鲁西;ZT-中条 Fig. 6 P-T space for Neoarchean crustal thickness and geothermal gradient of Central and Eastern Block of North China Craton (base map modified after Brown, 2014) Lg, low-grade metamorphism, includes the zeolite facies; Gr, greenschist facies; Am, amphibolite facies; Gn, granulite facies; UHT, ultrahigh temperature metamorphism; Bl, blueschist facies; E-HPG, eclogite to high-pressure granulite facies; UHPM, ultrahigh pressure metamorphism; DF, Dengfeng; EH, eastern Hebei; JD, eastern Shandong; NL, northern Liaoning; SJ, southern Jilin; TH, Taihua; WS, western Shandong; WL, western Liaoning; ZT, Zhongtiao

结合上述的华北克拉通中东部新太古代晚期的热状态和地壳厚度研究结果,区域上辽北、辽南、冀东和鲁西地区与这些变质火山岩伴生有大量的TTG片麻岩和赞岐岩质侵入岩,高镁安山岩广泛存在,典型的玻安岩在部分地区存在,明显与俯冲相关的流体、熔体的地幔交代作用有关(Gao et al., 2019, 2020a, b; Guo et al., 2017c; Sun et al., 2019a, 2020; Wang et al., 2015),说明新太古代晚期地球的壳幔动力学已经由早期地幔柱体制(mantle plume regime)为主转化为以早期板块俯冲动力学体制为主,地幔柱高热未亏损深部软流圈地幔来源的岩浆产物已经很少,说明这种动力学作用在局部还存在,但是已经不是主导的壳幔动力学体制。因此,华北克拉通在新太古代晚期总体表现为以早期板块体制为主导,局部为地幔柱和板块构造联合作用的壳幔动力学体制(Gao et al., 2019)。

5 结语

结合上述地壳厚度和热状态研究,新太古代晚期华北克拉通中东部的地热梯度发生了明显分化,冀东-辽西等地区表现为厚的地壳厚度和低的地热梯度,辽北-吉南地区和胶东地区表现为相对较薄的地壳厚度和较高的地热梯度,局部超出了前板片俯冲的地热梯度范围,而其它地区在现代地热梯度和热俯冲的地幔温度范围之间。结合新太古代晚期变质火山岩组合和成因研究,我们认为在新太古代晚期,板块构造体制已经是最主要的壳幔动力学体制,地幔柱构造体制、板块构造与地幔柱联合作用体制仍然在局部地区存在,但是其作用的范围和强度已经明显减小。因此中太古代到新太古代晚期,随着地幔潜能温度的下降,地幔柱和板片俯冲的动力学体制可能是相互关联的此消彼长的动力学过程,而不是一个灾变事件导致的突变过程。

     谨以此文祝贺沈其韩院士百年华诞,祝愿沈先生健康长寿!

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