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沿着东亚大陆边缘,新生代伸展盆地及相关火山岩自西伯利亚到中国东部绵延超过4000km(Zou Haibo et al., 2000),其中中国东部地区,新生代玄武岩广泛分布于黑龙江五大连池至海南岛以南以及南海沿岸省份及邻近的近海大陆架上,构成中国东部火山岩带(Zhou Xinghua et al., 1982; Peng et al., 1986; 池际尚, 1988; Xu Yigang et al., 2005; Zhang Junjun et al., 2009; Zeng Gang et al., 2010)。中国东部新生代玄武岩是典型的大陆裂谷系玄武岩,具有与洋岛玄武岩(OIB)类似的地球化学特征(Zou Haibo et al., 2000; 徐义刚等,2015),与中国东部中生代辉长岩、辉绿岩、玄武岩等基性岩石表现出的弧形微量元素分布特征明显不同(Zhao Junhong et al., 2007; Liu Shen et al., 2008; 齐有强等,2011;徐峥,2012;Yu Jiejiang et al., 2012; 冯光英等,2022)。前人的研究表明,中国东部华南和华北地区新生代玄武岩的地幔源区与古太平洋板片中生代俯冲作用相关。俯冲大洋地壳(包括洋壳镁铁质火成岩、海底沉积物和大陆下地壳三种组分)部分熔融产生的熔体与地幔相互作用形成的交代地幔源区,在新生代由于太平洋板片后撤引起中国东部大陆岩石圈拉张和软流圈地幔上涌,致使交代成因的地幔源区发生部分熔融形成了中国东部新生代玄武岩(徐峥,2012;Hoang et al., 2018; Pang Chongjin et al., 2019; 徐峥等,2019)。华北及东北地区新生代玄武岩具有相似的地球化学特征,其源区可能为DM和EMI二端元组分混合而成(徐峥,2012;徐峥等,2019),而华南地区同南海海盆及其周边地区(如雷琼半岛、南海北缘以及中南半岛)新生代玄武岩具有相似的地幔源区特征,都显示DM和EMII二端元组分混合的结果(Zou Haibo et al., 2000; Yan Quanshu et al., 2018)。海南地幔柱的提出为南海及周边新生代玄武岩的形成提供了新的解释(Lebedev et al., 2000; 鄢全树等,2008a, 2008b; Lei Jianshe et al., 2009; Wang Xuance et al., 2012, 2013; Huang Zhouchuan et al., 2015)。南海北部大陆边缘位于华南褶皱带与南海洋陆过渡带之间。大陆边缘不仅是物质和能量循环的主要场所,也是火山、地震等活动最为密集的区域,是了解固体地球循环的关键(张培震等,2003;郑永飞等,2015)。南海及其周边新生代玄武岩广泛发育,对玄武岩岩石成因的研究不仅可以反映地幔源区物质组成和地球化学性质,同时可以反映源区物质组成的不均一性以及同构造演化相关的地幔性质演化。对此,前人已开展了大量的研究,但是对一些科学问题的探讨还比较薄弱:如源区物质组成及其演化、地幔部分熔融程度及其残留相、同华南新生代玄武岩是否具有不同的动力学机制,两者为何具有相似的地球化学组成,为何中国东部中生代镁铁质岩浆岩多显示弧形微量元素特征,而新生代玄武岩具有OIB型微量元素特征等。
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南海及周边新生代玄武岩分布区域包括南海、雷琼半岛、北部湾盆地、珠江口盆地以及中南半岛等地(图1a;Yan Quanshu et al., 2018),由拉斑玄武岩逐渐演化为碱性玄武岩,在雷琼地区广泛发育的晚新生代玄武岩是南海停止扩张后区域性事件的一部分,其中的石山组橄榄玄武岩是南海扩张停止后形成的最晚一期火山岩,也是琼北火山岩由拉斑玄武岩转化为碱性玄武岩的关键时期(牛晓露等,2022),本文将通过对石山组橄榄玄武岩详细的岩石地球化学和矿物学研究,结合中国东部新生代玄武岩研究已取得的丰硕成果对上述科学问题作进一步的讨论。
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1 地质背景及岩相学特征
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南海位于多个大构造板块(印度板块、欧亚板块、太平洋板块和菲律宾海板块)相互作用区域,地质现象丰富,地球动力学过程复杂(Yan Quanshu et al., 2018)。在周围大构造板块的作用下,从新生代早期以来,南海北缘经历了从主动到被动的转变,同时发生了从大陆裂解到海底扩张的构造演化过程,并在扩张期(扩张期为32~16Ma)后发生了一期岩浆活动,这期板内岩浆活动的影响范围较广泛,可达南海海盆、雷琼半岛、北部湾以及中南半岛等地区(Flower et al., 1992; Tu Kan et al., 1992; 贾大成等,2003;李昌年等, 2004; 樊祺诚等,2004, 2008;韩江伟等, 2009; Wang Xuance et al., 2013)。
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海南岛是我国南海海域中最大的陆缘岛,位于华南板块、澳大利亚板块和印支板块的交汇部位,以琼州海峡与华南内陆相连。特殊的大地构造位置使其既受到特提斯构造域的影响,又受滨太平洋构造域的控制。海南岛自北向南包括四条近东西向断裂带,即王五-文教断裂、昌江-琼海断裂、尖峰-吊罗断裂和九所-陵水断裂(王智琳等,2013)。其中位于王五-文教断裂以北的海南岛北部地区广泛发育晚新生代玄武岩(图1b),具有大陆板内玄武岩的特征,是认识南海周边地区新生代以来构造演化及地幔深部过程的重要窗口(韩江伟等,2009)。
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海南岛北部的第四纪火山岩经历了多次喷发活动,最大厚度可达~1000m,喷发面积可达4000km2(黄振国等,1993;樊祺诚等,2004),是中国东南部玄武岩出露面积最大的地区(刘建强等,2013)。前人通过系统的工作,建立了火山地层层序,根据岩石特征、期次划分及形成的地质构造背景将这些基性火山岩自下而上划分为五个火山喷发期(陈哲培等,1991)。其中最上部的石山组橄榄玄武岩形成于全新世早期(＜11.8ka),是由陆相玄武质熔岩局部底部角砾熔岩组成,厚度约95m,分布在石山、马鞍岭、永兴和雷虎岭一带。
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本文样品采自海口市秀英区永兴镇,采样GPS坐标为:N19°52′48.95″,E110°12′31.61″。地表为低矮山丘,周围被第四纪土壤和植被覆盖,露头处可见多期喷发的特点,表现为气孔状构造玄武岩被块状构造玄武岩覆盖(图2a、b),采集其中的块状样品进行室内分析(图2c、d)。石山组橄榄玄武岩呈斑状结构,斑晶为橄榄石(约占20%,粒度可达1~3mm),基质主要为斜长石和单斜辉石,副矿物为含钛磁铁矿等,其中斜长石晶形较好,单斜辉石主要呈他形结构充填于斜长石间架结构中,组成玻基辉绿结构(图3)。
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图1 南海及周边地区新生代晚期火山岩分布图 (a)(据Yan Quanshu et al., 2018)和海南岛北部晚新生代玄武岩分布图(b)(据刘建强等,2013)
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Fig.1 Distribution of Late Cenozoic (＜16Ma) intraplate volcanism in the South China Sea region (a) (after Yan Quanshu et al., 2018) and distribution and sampling locations of the Late Cenozoic basalts in northern Hainan Island (b) (after Liu Jianqiang et al., 2013)
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2 分析方法
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代表性矿物电子探针分析在中国地质科学院地质研究所深地动力学实验室完成,仪器型号为JXA-8100。仪器工作条件为加速电压15kV,探针束流20nA,束斑直径为5 μm,峰值计数时间为20s,背景计数时间为10s。
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全岩主微量元素和Sr-Nd-Pb同位素分析均在南京宏创地质勘查技术服务有限公司开展。主量元素利用帕纳科Axios MAX XRF分析完成,分析精度优于3%。微量元素利用Elan DRC-e ICP-MS分析完成,称取烘干后的样品50mg置于Teflon溶样弹中,加入高纯酸,待溶样完成后将提取液转移至100mL干净的PET(聚酯)瓶中,加入1mL的(Rh+Re)双内标溶液(浓度1mg/L),用Milli-Q稀释至100.00g,使得Rh和Re在溶液中的浓度为10ng/mL,待上机测定,同时分析1个岩石标样(BHVO-2)和一个平行样以保证分析结果的准确度,分析精度优于5%。
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图2 海南岛北部石山组橄榄玄武岩野外照片(a,b,c) 及岩石手标本照片(d)
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Fig.2 Field pictures (a, b, c) and sample picture (d) of olivine basalt from Shishan Formation in northern Hainan Island
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图3 海南岛北部石山组橄榄玄武岩显微镜下照片(a, b)
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Fig.3 Photomicrographs of the Shishan Formation olivine basalts (a, b) in northern Hainan Island
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Ol—橄榄石;Pl—斜长石;Cpx—单斜辉石
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Ol—Olivine; Pl—plagioclase; Cpx—clinopyroxene
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Sr-Nd-Pb同位素分析在Nu Plasma II MC-ICP-MS上完成,测定过程中,采用86Sr/88Sr=0.1194内部校正仪器质量分馏,Sr同位素国际标准物质NIST SRM 987作为外标校正仪器漂移;采用146Nd/144Nd=0.7219内部校正仪器质量分馏,Nd同位素国际标准物质JNdi-1作为外标校正仪器漂移;采用205Tl/203Tl=2.3885校正仪器质量分馏。Pb同位素国际标准物质NIST SRM 981作为外标,校正仪器漂移。美国地质调查局USGS地球化学标准岩石粉末(玄武岩BCR-2、玄武岩BHVO-2、安山岩AGV-2、流纹岩RGM-2等)作为质控盲样,经过化学前处理与质谱测定,其同位素比值结果在误差范围内与推荐值一致(Weis et al., 2006)。
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3 分析结果
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3.1 矿物组成
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石山组橄榄玄武岩中代表性橄榄石电子探针分析结果见表1。橄榄石颗粒的组成分为两种,大的橄榄石斑晶内部成分均匀,无环带(图4a、b),SiO2和MgO含量较高(分别为38.71%~39.17%和43.84%~44.28%),FeO含量较低(15.54%~16.27%);基质中小颗粒橄榄石SiO2和MgO含量较低(分别为34.79%~36.75%和24.67%~33.81%),FeO含量较高(27.85%~38.80%)。相应的,前者的Fo值较高(82.8~83.5),而后者的Fo值较低(53.1~68.4),与全岩Mg#(55~58)基本一致,说明基质与周围熔体基本达到平衡,而斑晶结晶较早,MgO含量较高。
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石山组橄榄玄武岩代表性单斜辉石和斜长石电子探针分析结果分别见表2和表3。单斜辉石的MgO含量为13.51%~16.11%,CaO含量为21.19%~22.69%,Al2O3含量为2.60%~4.55%,FeO含量为6.63%~8.00%, En含量为39.38~45.70,Fs和Wo值分别为10.67~13.08和43.20~47.54,属于普通辉石,或者透辉石。斜长石SiO2含量变化于53.04%~56.31%,Al2O3含量26.77%~28.69%,Na2O＞K2O, Na2O+K2O=4.83%~6.57%, 其An端元组分变化于44.0~58.3,属于中长石和拉长石。
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3.2 主微量元素组成
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石山组橄榄玄武岩主微量元素组成见表4和表5。7个玄武岩样品的LOI变化于0~0.33%之间(-0.2%和-0.15%在误差内同0),表明样品基本没有受到岩浆期后蚀变作用的影响。所有的样品都具有较为一致的SiO2组成(48.37%~49.12%),Na2O＞K2O, Na2O+K2O=4.93%~5.64%,在TAS图解中落于粗面玄武岩范围内(图5),属于碱性岩系列。此外,Al2O3=13.48%~13.72%,而CaO含量介于9.35%~9.47%之间,MgO含量在6.41%~7.27%之间,Mg#=55~58。在Harker图解中(图6),MgO与SiO2、K2O、CaO、Na2O和TiO2 之间为负相关关系,而与Ni和Cr之间为正相关关系。TAS图解中(图5),我们将石山组橄榄玄武岩和华南新生代玄武岩对比,发现两者都为碱性玄武岩,华南玄武岩碱性更强,大部分属于粗面玄武岩和碧玄岩。
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图4 海南岛北部石山组橄榄玄武岩电子探针Mg和Fe元素面扫描图像(a, b)及背散射图像(c, d)
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Fig.4 Scanning images of Mg and Fe elements (a, b) and the backscattered images (c, d) of the olivine basalts from Shishan Formation in northern Hainan Island
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图5 海南岛北部石山组橄榄玄武岩TAS图解(据Middlemost, 1994;华南新生代玄武岩数据引自Zou Haibo et al., 2000)
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Fig.5 TAS diagram for the olivine basalts from Shishan Formation in northern Hainan Island (after Middlemost, 1994; data for Cenozoic basalts in Southeast China from Zou Haibo et al., 2000)
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石山组橄榄玄武岩具有基本一致的稀土元素及微量元素组成,在稀土元素球粒陨石标准化图解中(图7a),轻稀土高度富集,轻重稀土分异明显,无明显的Eu异常,δEu=0.99~1.00,同OIB的稀土配分模式图基本重合。在微量元素蛛网图中(图7b),其元素组成同样与OIB高度吻合,富集大离子亲石元素Rb和Ba,高场强元素Nb、Ta、Zr和Hf明显富集,Ti无亏损。
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3.3 Sr-Nd-Pb同位素组成
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石山组橄榄玄武岩形成于全新世,其年代较新,放射性同位素累积可以忽略不计,因此可以用测试值代替岩石形成时的原始同位素比值。石山组橄榄玄武岩具有非常一致的Sr-Nd-Pb同位素组成(表6),87Sr/86Sr变化于0.70437~0.70451之间,143Nd/144Nd变化于0.512833~0.512855之间,εNd(t)=3.8~4.2,在87Sr/86Sr-143Nd/144Nd同位素图解中(图8),所有样品都落于OIB区域内。
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图6 海南岛北部石山组橄榄玄武岩Harker图解(a~i)
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Fig.6 Harker diagram of the Shishan Formation olivine basalts (a~i) in northern Hainan Island
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图7 海南岛北部石山组橄榄玄武岩稀土元素球粒陨石标准化图解(a)和微量元素原始地幔标准化蛛网图(b) (据Sun et al., 1989; 华南新生代玄武岩据Zou Haibo et al., 2000)
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Fig.7 Chondrite-normalized REE distribution pattern (a) and primitive mantle normalized spidergram (b) for the olivine basalts from Shishan Formation in Hainan Island (after Sun et al., 1989; data for Cenozoic basalts in SE China from Zou Haibo et al., 2000)
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续表5
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7个样品的206Pb/204Pb比值在18.643~18.655之间,207Pb/204Pb和208Pb/204Pb的比值范围分别为15.639~15.643和38.977~38.996。在206Pb/204Pb-207Pb/204Pb和206Pb/204Pb-208Pb/204Pb图解中(图9)同样落入OIB区域内,同时位于北半球参考线之上,同南海及南海北部边缘新生代玄武岩具有一致的Pb同位素组成。
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4 讨论
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4.1 地壳混染和结晶分异
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海南岛玄武岩属于大陆玄武岩,玄武岩浆喷发至地表的过程中会经过地壳,由于大陆岩石圈较厚的地壳组成,导致地壳物质的混染很可能在岩石成因过程中发挥重要作用。相比于幔源岩浆,地壳物质通常具有较高的SiO2含量,较高的87Sr/86Sr比值和较低的143Nd/144Nd比值,因此地壳混染会引起SiO2含量的升高,同时使SiO2与87Sr/86Sr呈正相关关系,而与143Nd/144Nd呈负相关关系。石山组橄榄玄武岩Sr-Nd同位素比值变化范围极小,同SiO2之间没有相关性,因此,地壳物质的混染基本可以排除。进一步的证据来自微量元素组成,地壳物质一般明显亏损Nb、Ta和Ti,因此地壳混染会导致相关元素明显的亏损,而石山组橄榄玄武岩Nb和Ta为弱富集,且Ti没有明显亏损。此外,石山组玄武岩的Nb/U比值在43.0~46.4之间,与OIB和MORB一致(47±10;Hofmann et al., 1986),远高于平均大陆地壳的Nb/U比值(6.15;Rudnick et al., 2003);另外,地壳物质的混染会导致幔源岩浆岩La/Sm比值迅速升高到5以上(Lassiter et al., 1997),石山组橄榄玄武岩的La/Sm比值在4.5~4.6之间,也说明地壳物质的混染不明显。综上所述,石山组橄榄玄武岩基本没有遭受明显的地壳物质混染,其微量元素和同位素特征可以代表其地幔源区特征。
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图8 海南岛晚新生代石山组橄榄玄武岩 87Sr/86Sr-143Nd/144Nd同位素图解
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Fig.8 87Sr/86Sr-143Nd/144Nd diagrams of Shishan Formation olivine basalts from Hainan Island
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数据来源:NCC下地壳据Jahn et al.(1999);古老岩石圈地幔据Zhang Hongfu et al.(2002); MORB、OIB、EMI和EMII据Zindler et al.(1986);端元混合建模参数如下,A代表亏损地幔端元:Sr=7.66×10-6, Nd=0.58×10-6, 87Sr/86Sr=0.7026, 143Nd/144Nd=0.51311(据Workman et al., 2005); B代表富集端元:87Sr/86Sr=0.7078, 143Nd/144Nd=0.51258(据Hart et al., 1992)。数字标记代表DM对混合物的贡献百分比 (据Yan Quanshu et al., 2018)。北→南:华南新生代玄武岩的地理分布位置
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Data source: lower crust of the NCC after Jahn et al.(1999); old lithospheric mantle beneath the NCC after Zhang Hongfu et al.(2002); MORB, OIB, EMI and EMII after Zindler et al.(1986); modeling parameters for end-member mixing is as follows, A for a depleted end member: Sr=7.66×10-6, Nd=0.58×10-6, 87Sr/86Sr=0.7026, 143Nd/144Nd=0.51311 (after Workman et al., 2005); B for an enriched end member: 87Sr/86Sr=0.7078, 143Nd/144Nd=0.51258 (after Hart et al., 1992).Tick marks with numbers represent%contributions from the DM to the mixture (after Yan et al., 2018).North→South: the location of the Cenozoic basalts in the Southeast China
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图9 海南岛石山组橄榄玄武岩206Pb/204Pb-207Pb/204Pb (a)及206Pb/204Pb-208Pb/204Pb (b)图解
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Fig.9 206Pb/204Pb-207Pb/204Pb (a) and 206Pb/204Pb-208Pb/204Pb (b) diagrams of the Shishan Formation olivine basalts from Hainan Island
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DM、HIMU、EMI、EMII端元据Zindler et al.(1986); OIB据Castillo (1988);印度洋MORB据Mahoney et al.(1989); NHRL:北半球参考线(据Hart, 1984);北→南:华南新生代玄武岩的地理分布位置
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The approximate fields for DM, HIMU, EMI and EMII are from Zindler et al.(1986), for OIB from Castillo (1988) and for India Ocean-type MORB from Mahoney et al.(1989);NHRL: North Hemisphere reference line (after Hart, 1984); North→South: the location of the Cenozoic basalts in the Southeast China
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石山组橄榄玄武岩的Mg#为55~58,低于原始岩浆的Mg#(65~75;邓晋福,1987),表明岩浆在演化过程中经历了一定程度的分离结晶作用。在石山组橄榄玄武岩Harker图解中(图6),MgO与Ni和Cr之间为正相关关系,表明岩浆演化过程中经历了橄榄石和单斜辉石的分离结晶作用,而MgO同CaO/Al2O3之间没有明显的相关性,说明与橄榄石相比单斜辉石的分离结晶作用较弱,此外,MgO同TFe2O3之间没有明显的相关性,同SiO2、K2O、CaO、Na2O以及TiO2呈负相关关系,说明钛铁氧化物以及斜长石的分离结晶不明显,这也与稀土元素球粒陨石标准化图解中没有明显的Eu负异常一致。
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4.2 地幔源区物质组成及残留相
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石山组橄榄玄武岩的微量元素组成与OIB基本一致,同时在所有的构造判别图解中(图10),石山组橄榄玄武岩都落入板内碱性玄武岩区域。通常玄武岩被认为来自二辉橄榄岩地幔源区的部分熔融(徐义刚等,2015),高压下二辉橄榄岩较低程度的部分熔融可使形成的熔体强烈富碱(Takahashi et al., 1983),然而实验岩石学表明,传统的干的地幔橄榄岩部分熔融形成的熔体对于给定的MgO含量通常具有高于典型OIB组成的Al2O3,此外橄榄岩部分熔融产生的熔体TiO2普遍较低(大多低于1.5%),因此OIB的源区物质组成可能并不是橄榄岩。进一步的实验岩石学证据表明碳酸盐化的橄榄岩(Hirose, 1997)、角闪石岩交代脉(Pilet et al., 2008)、橄榄岩+CO2(Brey et al., 1975; Dasgupta et al., 2007; 刘建强等, 2013)和辉石岩(Hauri, 1996; Hirschmann et al., 2003; Ren et al., 2004)都可以作为板内玄武岩的源区母岩。但是,碳酸盐化的橄榄岩部分熔融形成的熔体具有亏损Zr、Hf、K和Ti的特征(Hirose, 1997; Dasgupta et al., 2007; Zeng Gang et al., 2010), 而角闪石岩部分熔融形成的熔体具有Zr、Hf、K负异常及Ti正异常的特征(Pilet et al., 2008),相反,石山组橄榄玄武岩明显富集Zr和Hf且没有明显K和Ti异常。橄榄岩+CO2部分熔融形成的熔体具有较低的Al2O3,也与石山组橄榄玄武岩不同。因此,上述地幔源区组成都不可能成为石山组橄榄玄武岩的源区母岩。
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实验岩石学证明,辉石岩部分熔融可以形成硅不饱和的碱性岩(Hirschmann et al., 2003)。大量的研究表明,南海和南海北缘新生代玄武岩(Yan Quanshu et al., 2018; 杨文建等,2020)以及夏威夷洋岛玄武岩都起源于辉石岩的源区母岩或者源区有辉石岩的贡献(Ren Zhongyuan et al., 2004, 2006; Sobolev et al., 2005, 2007; Herzberg, 2006; 刘建强等,2013)。同样,我们认为石山组橄榄玄武岩也来自于辉石岩源区部分熔融,证据如下:首先岩浆分离结晶出橄榄石的过程中,Ca为高度不相容元素,但在单斜辉石中Ca属于相容元素 (=1.82~1.95; Pertermann et al.,2002),因此单斜辉石发生部分熔融形成的熔体比橄榄岩部分熔融形成的熔体具有更低的CaO含量。在全岩MgO-CaO图解中(图11c),石山组橄榄玄武岩具有较低的CaO含量而落入辉石岩区域。此外,Ni在橄榄石与熔体之间的分配系数(7.37~11.9;Wang Zhengrong et al., 2008)明显高于其在辉石与熔体中的分配系数(2.44~3.78; Le Roux et al., 2011)。因此,相比橄榄岩地幔,在同等部分熔融条件下,辉石岩部分熔融产生的熔体具有更高的Ni含量。玄武岩中的橄榄石斑晶是幔源岩浆中最先结晶的矿物,它保留了岩浆演化早期的信息,因此可以利用橄榄石斑晶的主元素追溯原始源区组成(罗清晨等,2020)。石山组橄榄玄武岩斑晶Fo值明显高于基质中橄榄石的Fo值,且成分均一,说明斑晶结晶时间较早,并且在形成之后没有同熔体发生平衡反应。在橄榄石斑晶Fo-Ni图解中(图11b),石山组橄榄玄武岩具有较高的Ni含量(0.14%~0.20%),明显高于橄榄岩源区部分熔融形成的橄榄石,同样指示其源区为辉石岩。在全岩MgO-Al2O3图解中(图11a),石山组橄榄玄武岩具有较高的Al2O3含量,同样落入辉石岩区域。广义的辉石岩指的是石榴子石辉石岩,石榴子石富集重稀土元素,含石榴子石源区部分熔融形成的岩浆具有高的Sm/Yb比值,石山组橄榄玄武岩Sm/Yb为4.97~5.08,明显高于尖晶石二辉橄榄岩源区Sm/Yb(0.67;Salters et al.,2004),说明源区残留相含有石榴子石。此外,由于金红石等Ti-Fe氧化物高度富集Nb和Ta,而角闪石富集Nb,因而源区不可能有金红石等Ti-Fe氧化物以及角闪石的残留,这与蛛网图中Nb和Ta的正异常一致。在La-La/Sm图解中(图11d),石山组橄榄玄武岩可以由单纯的石榴子石辉石岩源区部分熔融形成,且经历了较低程度的部分熔融。综上,我们认为石山组橄榄玄武岩由含石榴子石辉石岩经历较低程度的部分熔融(约为5%)而形成,其源区深度应该大于80km。
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图10 海南岛石山组橄榄玄武岩构造判别图
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Fig.10 Tectonic discrimination diagrams of the Shishan Formation olivine basalts from Hainan Island
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(a)—Ti/100-Zr-Y×3图解(据Pearce et al., 1973);(b)—Nb×2-Zr/4-Y图解(据Meschede,1986);(c)—Hf/3-Th-Ta图解(据Wood, 1980)
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(a)—Ti/100-Zr-Y×3diagram (after Pearce et al., 1973); (b)—Nb×2-Zr/4-Y diagram (after Meschede, 1986); (c)—Hf/3-Th-Ta diagram (after Wood, 1980)
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4.3 辉石岩源区的形成原因
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大量的研究表明,包括中国东部、南海海盆及南海周边在内的新生代玄武岩,大多具有OIB型微量元素组成,其源区物质组成都为辉石岩源区或者有辉石岩的贡献(徐峥,2012;Yan Quanshu et al., 2018; 赵智华等,2019),那么这种大范围的可生成OIB型熔体的辉石岩源区是如何形成的呢?
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众所周知,整个中国东部地区自早中生代以来,都经历了古太平洋板块的俯冲作用,海南岛四周发育多个俯冲带,因此辉石岩源区的形成可能同板块俯冲而导致的壳幔相互作用有关。由于大陆地壳具有弧形微量元素组成,与OIB源区组成不同,因此加入地幔源区的富集组分很可能来自于俯冲洋壳及其上覆沉积物,俯冲洋壳在浅部(50~100km)析出流体会导致易溶于水的活动性元素(如Cs、Rb、Ba、K、Pb和LREE等)流失,由于此时金红石为稳定的残留相,因此流体中亏损高场强元素(Nb和Ta),受到交代的地幔也亏损高场强元素。随着俯冲深度的不断加大(100~200km),洋壳上部的变质玄武岩(榴辉岩)和变质沉积物会发生部分熔融,此时金红石不再稳定而发生分解,导致产生的熔体不亏损高场强元素(Ringwood, 1990; Zheng Yongfei, 2012),再循环洋壳和沉积物变质成硅饱和的榴辉岩,其固相线相对于围岩橄榄岩低很多,在较高压力下首先熔融,产生的富硅熔体交代围岩橄榄岩将其转化为不含橄榄石的斜方辉石,在橄榄石消耗殆尽后,熔体会与斜方辉石进一步反应形成单斜辉石,从而形成相对富硅的地幔辉石岩(徐峥,2012)。辉石岩的熔点低于橄榄岩,因此在地幔上升过程中先于橄榄岩发生熔融(徐义刚等,2015),形成玄武岩。因此,俯冲洋壳和沉积物熔/流体交代作用可以很好地解释辉石岩源区的形成过程以及碱性橄榄玄武岩富集或者不亏损高场强元素的特征。同时中国东部中生代镁铁质岩浆岩多呈现弧形微量元素特征而新生代玄武岩大多为OIB型,可能同大洋板片俯冲深度有关,当俯冲深度在50~100km时,洋壳析出流体交代上覆岩石圈地幔形成弧形镁铁质岩浆岩,板片深俯冲达到100km以上,洋壳和俯冲沉积物熔融形成的熔体不再亏损高场强元素,同时仍然一定程度富集大离子亲石元素和LREE,这种熔体交代过的地幔源区便可形成OIB型镁铁质岩浆岩。
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图11 海南岛石山组橄榄玄武岩源区物质组成
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Fig.11 Mantle sources of the Shishan Formation olivine basalts from Hainan Island
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(a)—橄榄玄武岩MgO-Al2O3图解(据刘建强等,2013);(b)—橄榄石斑晶Fo-Ni图解(底图据Straub et al., 2008; 罗清晨等,2020; 夏威夷Koolau洋岛玄武岩中橄榄石斑晶成分据Sobolev et al., 2005);(c)—橄榄玄武岩MgO-CaO图解(橄榄岩熔体与辉石岩熔体之间的分界线据Herzberg (2006))和(d)—橄榄玄武岩全岩La-La/Sm图解(底图据Yan Quanshu et al., 2018)
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(a)—Whole-rock MgO versus Al2O3 diagram (after Liu Jianqiang et al., 2013); (b)—Fo versus Ni diagram for olivine porphyry (after Straub et al., 2008; Luo Qingchen et al., 2020; data for Koolau ocean island basalt from Hawaii after Sobolev et al., 2005); (c)—whole-rock MgO versus CaO diagram (the boundary between peridotite melt and pyroxene melt is after Herzberg, 2006); (d)—whole-rock La versus La/Sm diagram (after Yan Quanshu et al., 2018)
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南海及其周边(包括雷州半岛、海南岛、印支半岛)广泛分布新生代以来的板内玄武岩,海南岛和印支半岛的新生代玄武岩整体具有低于亏损橄榄岩地幔的Mg同位素组成,表明南海及周缘地区的地幔源区中有俯冲板块带入的沉积碳酸盐混入(Li Shuguang, 2017)。俯冲深海沉积物脱碳酸盐化释放的富CO2流体具有高Th/U和Pb/U的特点。少量碳酸盐熔体(约0.1%)可有效萃取岩石体系中30%~60%的不相容微量元素(Klemme et al., 1995; Sweeney et al., 1995)。这些富含不相容微量元素的碳酸盐熔体在向上迁移的过程中与地幔橄榄岩发生反应,致使地幔愈发富集、饱满。碳酸盐熔体交代作用最主要的特征是生成单斜辉石,从而增大橄榄岩中辉石的比例,将难熔的方辉橄榄岩或二辉橄榄岩转变为二辉橄榄岩、富单斜辉石二辉橄榄岩到异剥橄榄岩。因此,碳酸盐交代也可能是促成源区变成辉石岩的原因之一,但由于碳酸盐熔体具有亏损高场强元素的特征,因此该区碳酸盐交代作用应该比较有限。
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4.4 地幔源区形成过程中的壳幔相互作用
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石山组橄榄玄武岩Sr-Nd-Pb同位素组成都较为均一,说明其源区具有均一的地幔物质组成。其较低的87Sr/86Sr比值(0.704371~0.704508)和正的εNd(t)值(3.8~4.2),表现出亏损地幔源区特征,但与正常的洋中脊玄武岩(MORB)相比,87Sr/86Sr比值相对较高,而εNd(t)相对较低,说明其地幔源区不是简单的MORB源区,而是两种或者两种以上物质端元的混合。在87Sr/86Sr-143Nd/144Nd同位素图解中(图8),石山组橄榄玄武岩落入DM和EMII二端元混合区域,说明其源区为亏损地幔DM和富集端元EMII的混合。在206Pb/204Pb-207Pb/204Pb(图9a)和 206Pb/204Pb-208Pb/204Pb(图9b)图解中,石山组橄榄玄武岩同南海以及北部边缘新生代玄武岩具有相似的同位素组成,同华南地区新生代玄武岩相比,更接近EMII端元组成。对于亏损端元,大部分学者认为来自于软流圈地幔,但对于富集端元EMII的来源还存在较大争议。
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我们将海南岛新生代玄武岩同华南新生代玄武岩进行对比发现,TAS图解中(图5)两者都属于碱性玄武岩,只有个别华南新生代玄武岩为钙碱性玄武岩,在稀土元素球粒陨石标准化图解和微量元素蛛网图中(图7),两者具有大致相同的变化趋势,但是大部分华南新生代玄武岩具有比石山组橄榄玄武岩更高的LREE以及更低的HREE,这种特征可以用不同程度的部分熔融作用来解释(图11d),华南新生代玄武岩具有更低的部分熔融程度(1%~5%)。Zou Haibo et al.(2000) 通过对华南新生代玄武岩的研究认为,这些玄武岩的源区也是由亏损地幔和EMII两个端元通过不同比例混合形成,而且自台湾海峡向北EMII端元所占比例逐渐降低,在206Pb/204Pb-207Pb/204Pb(图9a)和 206Pb/204Pb-208Pb/204Pb(图9b)图解中,台湾海峡牛头山地区新生代玄武岩的Pb同位素组成同南海、南海北缘包括海南岛新生代玄武岩的Pb同位素组成相似,由此可知,中国东部从华南到南海及周边地区,自北向南EMII组分的比例逐渐加大,在87Sr/86Sr-143Nd/144Nd同位素图解中同样具有这种趋势。Zou Haibo et al.(2000) 和Tu Kan et al.(1992) 结合古地磁证据(Lin Jinlu et al., 1985),认为华南地区包括南海及其周边地区都曾是具有EMII组成的冈瓦纳大陆的一部分,即形成玄武岩的EMII端元组成来自大陆岩石圈地幔。
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然而对南海及其北部边缘新生代玄武岩的大量研究认为EMII端元不是来自大陆岩石圈地幔,主要原因如下:① 大陆岩石圈地幔熔融形成的玄武岩通常亏损Nb和Ta(An et al., 2017),而起源于地幔柱的玄武岩的微量元素在原始地幔标准化图解中不存在Nb和Ta的亏损,且具有低的La/Ta比值,一般在8~15之间,受岩石圈地幔混染后La/Ta值增加到25以上(Lassiter et al., 1997;王智琳等,2013)。石山组橄榄玄武岩La/Ta比值在9.8~10.2之间,同地幔柱源区较为一致,因此推测其源区可能为地幔柱,同时基本没有岩石圈地幔物质的加入;② 全球代表性大陆岩石圈地幔的地幔包体的Nd-Hf同位素具有明显的解耦现象,不呈现线性正相关趋势,而南海及其周缘晚新生代玄武岩的Nd-Hf同位素呈现明显的正相关趋势(Yan Quanshu et al., 2018),也指示EMII端元不可能来源于岩石圈地幔;基于以上认识,更多的学者认为南海及周边新生代玄武岩源区EMII端元组成来源于地幔柱。首先俯冲板片和沉积物被认为是地幔柱源区物质的重要组成部分(Hofmann et al., 1982),OIB中LILE亏损和HFSE不亏损甚至富集是继承自脱水的俯冲洋壳,这些俯冲带循环物质的加入与前述辉石岩源区的形成过程相吻合,是理解俯冲带壳幔物质相互作用的关键。此外,熔岩中Pb同位素体系的变化也主要是受到再循环物质中238U/204Pb、232Th/204Pb组成以及再循环年龄的影响,随着时间的演化再循环洋壳和一定量俯冲沉积物的加入会形成富集型地幔(EMI或EMII)的Pb同位素特征(Hofmann et al., 1982;Jackson et al., 2007; Willbold et al., 2010; 梅盛旺等,2019);同时洋壳上层的海底沉积物可以含有古老的陆源物质,因此Sr-Nd同位素组成相对富集(Plank,2014),这也同石山组橄榄玄武岩以及南海和周边地区的Sr-Nd同位素特征一致。因此基于地幔柱中再循环组分为俯冲洋壳及沉积物的认识,我们认为地幔柱成因可以解释石山组橄榄玄武岩源区物质组成以及同位素地球化学特征。
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4.5 岩浆形成及演化的动力学机制
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OIB是深部对流地幔部分熔融的产物,其形成过程和化学组成受到地幔源区组成和性质、部分熔融程度、地幔潜能温度和岩石圈厚度等多个因素的控制(徐义刚等,2012)。其中地幔潜能温度是检验地幔柱假说的一个重要手段,鄢全树等(2008a)利用南海新生代玄武岩中呈弱成分分带的橄榄石斑晶的核、幔部Fo组分与寄主岩浆的X Fe值估算出南海的潜在地幔温度平均值为1661℃,高出正常地幔温度360℃,而Wang Xuance et al.(2012)根据海南晚新生代玄武岩中橄榄石斑晶较高的Fo含量(高达90.7%)估算的地幔潜能温度为1541±10℃,高出正常地幔260℃,这种地幔热异常表明南海及周边地区下的地幔存在地幔柱;同时层析成像技术也显示海南岛下方存在低速体,而且低速异常可以延伸到地表以下1900km(即中地幔深度)(Lebedev et al., 2003; Montelli et al., 2004)。因此地幔柱的存在已基本成为共识,那么华南地区没有地幔柱源区的加入,为何两者在地球化学特征上如此相似呢?我们认为,究其根本原因就是两者源区组成均有“俯冲再循环”组分的加入,此再循环地幔组分可能与该区长期俯冲滞留板块的重熔有关。但是两者源区发生部分熔融并上升的动力学机制不同,华南新生代玄武岩的形成可能同太平洋板块东向后撤导致的中国东部大陆岩石圈伸展拉张作用有关,而南海及其周边地区新生代玄武岩的形成可能主要受控于海南地幔柱。两者在同位素组成上的差异指示不同端元组分混合比例不同,整个华南包括南海及其周边地区大体显示自北往南EMII端元比例逐渐增大的趋势,也即俯冲洋壳和沉积物的影响逐渐加大。华南地区206Pb/204Pb与207Pb/204Pb以及208Pb/204Pb具有明显的正相关关系,而南海及周边地区正相关趋势相对不明显,具有明显的不均一性,可能同地幔柱上升机制有关,当地幔柱头达到地幔过渡带时几乎不受浮力影响而发生横向扩展形成一个化学成分和温度结构不均一的热化学堆积层(Ballmer et al., 2013)。Hofmann et al.(1982) 最先提出古洋壳深俯冲形成地幔柱的模型,Wang Xuance et al.(2013) 通过对海南新生代玄武岩Sr-Nd-Pb-Os同位素分析识别出地幔柱源区存在4.5~4.4Ga古老地幔及少量0.5~0.2Ga俯冲再循环物质,梅盛旺等(2019)通过Pb-Nd-Hf同位素演化模型认为海南岛玄武岩中再循环物质年龄应小于1.0Ga。综合上述分析,我们认为海南岛晚新生代玄武岩形成同古大洋板块俯冲并滞留在地幔过渡带,随后随地幔柱上升并与软流圈地幔发生反应有关,具体的岩石成因为:古大洋地壳俯冲过程中再循环洋壳和沉积物变质形成硅饱和的榴辉岩,因密度差不断下沉并聚集存储在核-幔边界处,放射性热能以及地核传导的热能使这些俯冲物质温度变高以至于石榴子石不稳定,随后底劈上升至上部地幔,形成海南地幔柱。上升过程中发生熔融并与周围地幔橄榄岩(石榴子石二辉橄榄岩)反应形成含石榴子石辉石岩,随着地幔柱的不断上升,辉石岩首先发生部分熔融,形成的岩浆经历了橄榄石和微弱的单斜辉石的分离结晶作用后形成了海南岛玄武岩。
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5 结论
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(1)海南岛石山组玄武岩为碱性橄榄玄武岩,同南海及周边区域新生代玄武岩相似,具有OIB型微量元素及同位素组成特征;岩浆演化过程中没有经历明显的地壳混染,经历了橄榄石和较弱的单斜辉石的分离结晶作用。
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(2)橄榄石化学成分和橄榄玄武岩主微量元素特征指示其源区为含石榴子石辉石岩,经历了较低程度的部分熔融作用(约为5%)。辉石岩源区的形成同洋壳深俯冲相关,脱水后的再循环洋壳和沉积物熔体交代橄榄岩,形成相对富硅的辉石岩源区,随后在地幔上升过程中先于橄榄岩熔融形成了研究区玄武岩。
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(3)石山组橄榄玄武岩的Sr-Nd-Pb同位素组成显示亏损地幔DM和富集地幔端元EMII混合的地幔源区特征。其中亏损地幔端元为软流圈地幔,而EMII端元为地幔柱来源,地幔柱特征为再循环古洋壳及沉积物深俯冲至地幔后长期演化的结果。
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(4)海南新生代玄武岩同华南新生代玄武岩具有相似的地幔源区物质组成,两者源区组成均有俯冲再循环组分加入软流圈地幔,但两者岩浆演化上升的动力学机制不同,同时不同地幔端元混合的比例及部分熔融程度也有差异。
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致谢:海南省地质局梁定勇高级工程师,武汉地调中心周岱高级工程师和中国地质科学院地质研究所李观龙博士在野外地质考察中给予了重要帮助,两位审稿人对本文进行了仔细审阅并提出了宝贵意见和建议,在此表示诚挚感谢!
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摘要
南海北部边缘新生代玄武岩广泛分布,分布于海南岛北部的全新世早期石山组玄武岩,岩石组成为碱性橄榄玄武岩,其中可见较大颗粒的橄榄石斑晶,橄榄石斑晶具有较高的Fo值(82.8~83.5)和Ni含量(0.14%~0.20%)。橄榄玄武岩的微量元素及同位素组成与洋岛玄武岩(OIB)高度吻合,富集大离子亲石元素Rb、Ba和轻稀土元素,同时富集高场强元素Nb、Ta、Zr和Hf。矿物组成及主微量元素特征指示其源区为含石榴子石辉石岩源区,经历了较低程度的部分熔融(约5%)。此外,石山组橄榄玄武岩具有基本一致的Sr-Nd-Pb同位素组成,表明岩浆在上升过程中没有明显的地壳物质的混染,但是相比正常洋中脊玄武岩(MORB),石山组橄榄玄武岩具有较高的87Sr/86Sr比值,较低的εNd(t )值,以及较高的207Pb/204Pb(15.639~15.643)和208Pb/204Pb(38.977~38.996)比值,说明其源区为亏损地幔(DM)和富集地幔端元(EMII)的混合。结合地球物理学证据,我们认为亏损地幔来源于软流圈,而EMII端元为含再循环古洋壳及俯冲沉积物的地幔柱,随着海南地幔柱上升,再循环物质熔融并与周围软流圈地幔橄榄岩反应形成含石榴子石辉石岩,随着地幔柱的不断上升,辉石岩首先发生部分熔融,形成的岩浆经历了橄榄石和微弱单斜辉石的分离结晶作用后形成了石山组橄榄玄武岩。华南新生代玄武岩同石山组橄榄玄武岩具有相似的地球化学特征,反映二者具有相同的物质端元组成,但两者岩浆演化上升的动力学机制不同,同时不同端元混合的比例及部分熔融程度也有差异。
Abstract
The Cenozoic basalts are widely distributed in the northern margin of the South China Sea. The Early Holocene Shishan Formation basalts crop out in the north of the Hainan Island. These rocks are mainly consisted of olivine basalts. The olivine basalts contain large olivine phenocrysts characterized by high Fo (82.8~83.5) and Ni (0.14%~0.20%). The studied olivine basalts are enriched in large ion lithophile elements Rb and Ba, and light rare earth elements, as well as high field strength elements Nb, Ta, Zr and Hf, similar to OIB-type rocks. Their mineral compositions and signatures of major and trace elements indicate that the Shishan Formation olivine basalts were generated by low degree partial melting of garnet pyroxenites. The olivine basalts are characterized by homogeneous Sr-Nd-Pb isotopic compositions, which exclude the possibility of significant crustal contamination. Compare with the MORB, the Shishan Formation rocks have higher 87Sr/86Sr ratios, lower εNd(t ), and higher 207Pb/204Pb (15.639~15.643) and 208Pb/204Pb (38.977~38.996) ratios, which define a mixing trend between a depleted MORB mantle end-member and an enriched mantle component (EMII). Combined with geophysical evidences, we suggest that the depleted mantle originated from the asthenosphere, while the EMII end-member originated from the Hainan mantle plume which contains subducted oceanic crust and sediments. During the ascent of the Hannan plume, the recycled material melted and reacted with the surrounding mantle peridotites in the asthenosphere to form garnet pyroxene. The Shishan Formation olivine basalts were formed after partial melting of garnet pyroxene and crystallization of olivine and minor clinopyroxene. The similarities and differences between the Cenozoic basalts from South China and the Shishan Formation basalts indicate that they had the same end member compositions but were different in the dynamic mechanism, the ratios of different end member and the degrees of partial melting.
