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南海盆地位于欧亚板块、菲律宾海板块及印-澳板块交汇部位,是西太平洋最大的边缘海盆地(Morley, 2013; Hall et al., 2015; Li Gang et al., 2022)。围绕南海盆地形成的动力学机制,迄今存在多种假说。早期研究认为,印度大陆与欧亚大陆碰撞导致印支地块软流圈沿哀牢山-红河断裂向南逃逸形成了南海盆地(Tapponnier et al., 1982; Briais et al., 1993);也有观点认为南海盆地的开启是古南海向南俯冲到沙巴/婆罗洲之下的拖曳力诱发早新生代古华南陆缘拉张所致(Holloway, 1982; Taylor et al., 1983);另一方面,新特提斯洋(孙卫东等,2018)、西太平洋板块(Karig, 1971)和古南海(Wu Chauron et al., 2016; Wu et al., 2017)俯冲后撤引发的弧后扩张和大陆岩石圈伸展也被认为是南海盆地形成重要机制;近年地球物理探测表明,南海盆地之下存在延伸至下地幔的蘑菇状地震低速异常区域(Lebedev et al., 2003; Montelli et al., 2004; Huang Zhouchuan et al., 2015),暗示南海盆地的开启可能与地幔柱活动有关(Wang Xuance et al., 2012; Xu Yigang et al., 2012; Zhang Guoliang et al., 2018)。
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南海盆地及周缘地区新生代玄武岩(28.5~＜0.1Ma)广泛分布在雷琼地区和中南半岛,其成因研究对约束南海形成和演化历史至关重要。研究表明,南海盆地的开启始于33Ma并在15Ma终止,随后新生代玄武岩广泛喷发(Li Chunfeng et al., 2014)。这些玄武岩大致可分为拉斑系列(~16Ma)和碱性系列(＜8Ma) (Li Naisheng et al., 2013), 然而它们的起源深度、地幔源区和动力学背景尚存争议(Fan Qicheng et al., 1991; Ho Kungsuan et al., 2000; Fedorov et al., 2005; 韩江伟等, 2009; Wang Xuance et al., 2012, 2013; Yan Quanshu et al., 2015; Fan Chaoyan et al., 2017; Zhang Guoliang et al., 2018)。值得注意的是,先前的研究主要聚焦玄武岩全岩主微量和同位素地球化学,而矿物原位微区成分却鲜有涉及(Wang Xuance et al., 2012, 2013),限制了对火山岩成因和南海盆地演化的认识。中上更新统多文组是南海北部陆缘最大的玄武岩露头(龙文国等, 2006a, 2006b)。本文选取临高县福山镇多文组火山岩开展全岩和矿物地球化学研究,以期对其成因和南海盆地的演化提供制约。
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1 地质背景
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南海盆地被西北的华南印支地块和东南的婆罗洲-菲律宾地块围限,为NE—SW向延伸的菱形盆地,面积约为3500000km2,是东海、黄海和渤海总面积的2.8倍(图1a; Morley, 2013; Hall et al., 2015)。构造上,南海盆地可分为北部陆缘、南海海盆和南部陆缘三个部分(Yan Quanshu et al., 2015)。海南岛地处南海盆地北部陆缘地区,经琼州海峡与华南大陆相连,该岛屿北部广泛出露第四纪玄武岩和火山碎屑岩(樊祺诚等, 2004;Wang Xuance et al.,2012)。这些玄武岩的喷发始于晚渐新世 (28.4Ma), 在中新世—上新世(23~1.8Ma)逐渐增多, 并在更新世达到峰期 (1.8~0.012Ma),随后逐渐降低并在全新世(＜0.012Ma)之前完全停止 (Flower et al., 1992; Ho Kungsuan et al., 2000, 2003; 樊祺诚等, 2004)。根据玄武岩玻璃的K-Ar和Ar-Ar定年、地层接触关系以及岩石学特征, 这些玄武岩从老到新大致可分为5期(图1b; 樊祺诚等, 2004; 龙文国等, 2006a, 2006b; Wang Xuance et al., 2012):① 中新世—上新世石马村组和石门沟组、② 早—中更新世多文组、③ 中更新世东营组、④ 晚更新世道堂组和 ⑤ 全新世石山组。
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多文组火山岩是海南岛北部分布最广泛的火山岩,最大厚度可达250m, 包括下段早—中更新世玄武岩和上段中更新世玄武岩和火山碎屑岩(龙文国等, 2006a, 2006b; Wang Xuance et al., 2012)。本文采样点位于多文组下段的临高县福山镇,共计采集7个玄武岩样品,采样点位置的火山地层、岩石类型和位置如图1b所示。
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2 岩相学特征
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本次获得福山镇多文组玄武岩代表性样品7件。样品较为新鲜,手标本呈黑色或灰黑色,斑状结构,块状和气孔-杏仁状构造发育,经常可见暗色的包体(图2c)。斑晶矿物以橄榄石(~10%)、斜长石(~30%)和单斜辉石(~25%)为主,斜方辉石、铬尖晶石和和铁钛氧化物含量较少,基质矿物杂乱分布在斑晶矿物周围,包括橄榄石、斜长石、辉石和不透明矿物等。橄榄石斑晶呈自形—半自形结构,粒径介于0.2~0.5mm之间, 橄榄石裂纹发育,少量颗粒蚀变为伊丁石(图2d);斜长石斑晶为半自形—自形板状,粒度为介于0.2~0.5mm之间,发育聚片双晶(图2d、e);单斜辉石斑晶呈自形粒状结构,最大粒径可达600 μm,常见明暗相间的成分环带(图2e)。不透明矿物主要分布在玄武岩基质中,以铁钛氧化物为主。铁钛氧化物呈半自形—他形粒状结构,并且切穿了其他硅酸盐矿物(图2e)。
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3 分析方法
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岩石的测试分析在武汉上谱分析科技有限责任公司完成,其中主量元素用熔片X-射线荧光光谱法(XRF)测定,仪器型号为日本理学PrimusⅡX射线荧光光谱仪,二价铁含量用重铬酸钾法测定;微量元素和稀土元素测试使用电感耦合等离子体质谱仪(Agilent 7700e)测定,稀土元素用ICP-MS法测定,测试标样为BHVO-2、BCR-2、RGM-2,含量大于10×10-6的元素的测试精度优于5%, 小于10×10-6的元素精度优于10%。
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选取新鲜的岩石样品磨制光薄片,进行岩相学和矿物学研究。单矿物电子探针成分测试在中国地质科学院自然资源部深地动力学重点实验室完成,使用仪器为日本电子公司JXA-8100、INCA能谱仪,探针束流20nA,加速电压15.0kV,电子束斑2 μm。
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单矿物微量测试和稀土元素测试在武汉上谱分析科技有限责任公司完成,仪器型号为安捷伦电感耦合等离子体质谱仪(Agilent 7900),分析用激光剥蚀系统为GeoLas HD,激光能量80mJ,频率5Hz,激光束斑直径44 μm。具体分析条件及流程详见文献Liu Yongsheng et al.(2008)。质控标样主量元素测试误差小于5%,微量元素元素测试误差小于10%,含量低的样品测试误差大于10%。
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图1 南海盆地及周边(a)和海南岛北部玄武岩分布和采样点(b)地质简图(据Wang Xuance et al., 2012)
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Fig.1 Sketch geological map of the South China Sea and surrounding areas (a) and distribution of late Cenozoic basalts and sample location on northern Hainan Island (b) (modified after Wang Xuance et al., 2012)
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4 矿物地球化学特征
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4.1 矿物主量元素
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橄榄石。本文共分析了28个橄榄石的化学成分,分析结果见表1。多文组玄武岩橄榄石均为镁橄榄石,橄榄石的CaO含量范围为0.16%~2.74%,MgO含量为26.36%~35.97%,NiO含量为0.06%~0.18%,MnO为0.23%~0.40%,Fo值介于55.5~71.1之间。橄榄石的Fo值和NiO含量表现较弱的正相关关系(图3a),与CaO含量无相关性。
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辉石。偏光显微镜下观察玄武岩中的辉石主要为单斜辉石,少量为斜方辉石。对5件玄武岩样品中的62颗辉石进行化学成分分析,斜方辉石和单斜辉石的测试结果分别列于表2和表3。其中5颗斜方辉石的En端元分布在59.06~76.25的范围内,Mg#值变化于63.9~79.6之间,Al2O3=0.63%~1.15%,Cr2O3 含量低于0.24%,NiO=0.02%~0.08%,CaO的含量2.09%~4.14%;57颗单斜辉石的Mg#值介于66.0~80.6之间,Al2O3的含量更高,0.63%~1.15%,NiO=0.08%~0.14%,CaO=13.9~20.0%(图4a)。
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图2 海南岛北部福山镇多文组火山岩野外及镜下照片
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Fig.2 Field occurrence and microphotographs of Duowen Formation in Fushan Town, northern Hainan Island
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(a)—多文组岩体;(b)—多文组火山岩露头;(c)—含捕掳体的火山岩;(d)—橄榄石斑晶和斜长石正交偏光照片; (e)—单斜辉石斑晶和铁钛氧化物BSE照片
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(a)—Duowen massif; (b)—Duowen Formation outcrop; (c)—volcanic rocks with xenolith; (d)—orthogonal polarimetric photograph of olivine porphyry and plagioclase; (e)—BSE photographs of clinopyroxene phenocryst and Fe-Ti oxides
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铬尖晶石。本次研究通过光学显微镜和扫描电镜选择40颗新鲜的铬尖晶石进行主量测试,分析结果列于表4。玄武岩尖晶石的Cr2O3含量为49.90%~62.57%,Al2O3含量为7.04%~13.86%,Cr#值在74.1~82.7的范围内变化,Mg#值分布在45.5~63.8区间内,TiO2含量小于0.58%,FeO含量为12.62%~31.25%,MnO含量为0.13%~0.44%,属于富铬尖晶石(Irvine, 1967),在图解中,铬尖晶石成分都投影原始尖晶石的成分区间内(图3b),说明蚀变对尖晶石的成分并无影响。
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斜长石。玄武岩中的长石均为斜长石,选取5件样品中43个斜长石的化学成分进行主量元素测试,分析结果见表5。斜长石(Ab36.56~52.78Or0.89~2.26An44.96~62.39)的Al2O3和CaO的含量分别为25.83%~29.64%和9.50%~12.77%,K2O的含量为0.15%~0.41%,TiO2为0.02%~0.06%,主要为拉长石及少量中长石(图4b)。
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注:Fo=100×Mg/(Mg+Fe2+);b.d.—低于检测下限。
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图3 福山镇多文组玄武岩的橄榄石和铬尖晶石成分图解
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Fig.3 Compositional variations of olivine and Cr-spinel from basalts of the Duowen Formation, Fushan Town
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(a)—橄榄石CaO-Fo图解(据Herzberg, 2011);(b)—铬尖晶石MnO-Cr# 图解(据Khedr et al., 2017)
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(a)—Plot of CaO versus Fo in olivine (modified after Herzberg, 2011); (b)—Plot of MnO versus Cr# in Cr-spinel (modified after Khedr et al., 2017)
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注:Mg#=100×Mg/(Mg+Fe2+);b.d.—低于检测下限; En—顽火辉石;Fs—斜方辉石;Wo—硅辉石。
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注:Mg#=100×Mg/(Mg+Fe2+);b.d.—低于检测下限;En—顽火辉石;Fs—斜方辉石;Wo—硅辉石。
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注: b.d.—低于检测下限;Mg#=100×Mg/(Mg+Fe2+);Cr#=100×Cr/(Mg+Al);Fe3#=100×Fe3+/(Cr+Al+ Fe3+)。
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铁钛氧化物。利用BSE和电子探针对玄武岩中的15个铁钛氧化物进行分析,结果见表6。FeO含量范围为46.79%~47.74%,TiO2含量范围为50.19%~51.46%,MgO=0.61%~1.41%。在FeO-Fe2O3-TiO2三元体系中分别位于“磁铁矿-钛铁尖晶石”和“钛铁矿-赤铁矿”两个固溶体系列(图4c)。
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4.2 矿物微量元素
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橄榄石。本文共分析了28个橄榄石的化学成分,分析结果见表7。多文组玄武岩橄榄石的Al含量范围为60×10-6~158×10-6,Ti含量为86×10-6~150×10-6,Li含量为2×10-6~7×10-6,V的含量为11×10-6~16×10-6,Cr为8×10-6~144×10-6,Co为209×10-6~227×10-6, Ni为999×10-6~1663×10-6,Cu为0.5×10-6~5.5×10-6,Zn为240×10-6~404×10-6,而REE元素含量通常不足0.5×10-6。在Ni与其他元素协变图解中,随着橄榄石的不断结晶,Ni的含量降低,Al和Cr的含量明显下降,而Ti、Sc、Li和Zn的含量逐渐升高(图5)。
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注: b.d.—低于检测下限;Ab—钠长石;An—钙长石;Or—钾长石。
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铬尖晶石。选取多文组玄武岩11个铬尖晶石进行原位微量元素测试,分析结果见表8。铬尖晶石的Sc、Ti和V含量变化较大,分别为4.28×10-6~7.12×10-6、0.08%~0.10%和663×10-6~839×10-6。Mn和Co的含量分别为0.13%~0.26%和189×10-6~389×10-6。Ni和Zn的含量分别为443×10-6~760×10-6和435×10-6~1796×10-6,Ga为20.23×10-6~30.18×110-6。在尖晶石的MORB标准化蛛网图中,多文组玄武岩富集Zn,亏损Al、Ni、Ti和Sc,与玻安岩的配分曲线相似,它们低的Ti含量不同于夏威夷玄武岩、层状铬铁矿以及科马提岩的铬尖晶石(图6a)。
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图4 福山镇多文组玄武岩的辉石 (a)、斜长石 (b)和铁钛氧化物 (c)成分判别图解
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Fig.4 Compositional discrimination diagram of pyroxene, plagioclase and Fe-Ti oxides from basalts of the Duowen Formation, Fushan Town
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(a)—福山镇多文组玄武岩辉石Wo-En-Fs三元图(改自Morimoto et al., 1988); (b)—福山镇多文组玄武岩斜长石Or-Ab-An三元图(改自O'Connor, 1965); (c) —福山镇多文组玄武岩铁钛氧化物在FeO-Fe2O3-TiO2三元体系判别图(改自Yavuz, 2021)
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(a)—Wo-En-Fs compositional discrimination diagram of pyroxene (modified after Morimoto et al., 1988); (b)—Or-Ab-An compositional discrimination diagram of plagioclase (modified after O'Connor, 1965); (c)—FeO-Fe2O3-TiO2 compositional discrimination diagram of Fe-Ti oxides (modified after Yavuz, 2021)
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图5 福山镇多文组玄武岩橄榄石微量元素成分协变图解
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Fig.5 Compositional variations of olivines from basalts of the Duowen Formation, Fushan Town
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(a)—Ni-Al协变图解;(b)—Cr-Ni协变图解;(c)—Ti-Ni协变图解;(d)—Sc-Ni协变图解;(e)—Li-Ni协变图解;(f)—Zn-Ni协变图解
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(a)—Plot of Ni versus Al; (b)—plot of Ni versus Cr; (c)—plot of Ni versus Ti; (d)—plot of Ni versus Sc; (e)—plot of Ni versus Li; (f)—plot of Ni versus Zn
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单斜辉石。单斜辉石稀土和微量元素分析结果见表9。多文组玄武岩单斜辉石含Cr 2235×10-6~5224×10-6、Ni 268×10-6~426×10-6、Sc 52.2×10-6~69.7×10-6和Sr 20.8×10-6~33.7×10-6。在原始地幔标准化图解显示(图6b),多文组玄武岩单斜辉石富含Th,亏损Nb和Zr,与峨眉山和塔里木大火成岩省玄武岩单斜辉石成分相近。它们的(La/Yb)N介于0.51~1.31之间, Ce/Yb)N为0.81~1.50,(La/Sm)N为0.22~0.50,(Gd/Yb)N为2.42~3.56,稀土配分曲线富集MREE,亏损LREE和HREE,呈拱形分布,也可与峨眉山和塔里木等大火成岩省玄武岩的单斜辉石成分相比较,而不同于重HREE平坦的N-MORB单斜辉石(图6c)。
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注: b.d.—低于检测下限。
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注: b.d.—低于检测下限。
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图6 福山镇多文组玄武岩单矿物微量元素标准化配分模式图
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Fig.6 Normalized trace elements patterns of minerals from basalts of the Duowen Formation, Fushan Town
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(a)— 福山镇多文组玄武岩铬尖晶石微量元素MORB标准化配分模式图, 玻安岩、MORB、Hawaiian(夏威夷)拉斑玄武岩、科马提岩、Stillwater铬铁矿数据引自Pagé et al.(2011); (b)—福山镇多文组玄武岩单斜辉石微量元素原始地幔标准化配分模式图(据Wei Xun et al., 2015; Yang Shengbiao et al., 2022); (c)—福山镇多文组玄武岩单斜辉石稀土元素球粒陨石标准化配分模式图(据Wei Xun et al., 2015; Yang Shengbiao et al., 2022); (d)—福山镇多文组玄武岩斜长石稀土元素球粒陨石标准化配分模式图,标准值引自Sun et al.(1989)
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(a)—MORB normalized trace elements patterns of Cr-spinel from basalts of the Duowen Formation, Fushan Town, data source of boninite, MORB, Hawaiian tholeiite, komatiite, Stillwater chromite values are from Pagé et al.(2011); (b)—MORB normalized trace elements patterns of clinopyroxene from basalts of the Duowen Formation, Fushan Town (modified after Wei Xun et al., 2015; Yang Shengbiao et al., 2022); (c)—chondrite-normalized REE patterns of clinopyroxene from basalts of the Duowen Formation, Fushan Town (modified after Wei Xun et al., 2015;Yang Shengbiao et al., 2022); (d)—chondrite-normalized REE patterns of plagioclase from basalts of the Duowen Formation, Fushan Town Normalization values are from Sun et al.(1989)
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斜长石。斜长石稀土和微量元素分析结果见表10。多文组玄武岩斜长石Sr含量变化于691×10-6~728×10-6之间,Ti的含量变化于355×10-6~443×10-6之间,Ba的含量为47.1×10-6~68.6×10-6。在球粒陨石标准化图解中(图6d),多文组玄武岩斜长石明显富集LREE和Eu,亏损MREE。
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5 全岩地球化学特征
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对7件多文组火山岩样品进行全岩主量元素和微量元素测试,详细结果见表11。样品均较为新鲜,烧失量(LOI)介于-0.04%~0.60%之间。SiO2含量为52.24%~53.12%,全碱含量(Na2O+K2O)为4.08%~4.47%,Al2O3含量为14.12%~14.75%,TiO2变化范围1.85%~2.08%; TFe2O3含量介于11.30%~11.58%之间。 Mg#值[Mg/(Mg+ Fe2+)]的范围为49.56~52.73,扣除烧失量归一化处理后,样品均投影在硅碱图的玄武安山岩区内,属亚碱性基性岩范畴(图7)。
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图7 福山镇多文组火山岩全碱(Na2O+K2O)-SiO2 图解(据Middlemost, 1994)
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Fig.7 SiO2 vs.(Na2O+K2O) diagram (modified after Middlemost, 1994) of the basalts from the Duowen Formation, Fushan Town
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多文组玄武岩稀土元素总量(ΣREE)介于79.69×10-6~91.01×10-6之间,其中,(La/Yb)N和(Ce/Yb)N变化于6.21~7.06和5.07~5.66之间,(La/Sm)N在1.69~2.02之间,(Gd/Yb)N为2.74~3.01之间,样品δEu分布在0.96~1.03的范围内,无明显Eu异常。在球粒陨石标准化图解中(图8a),多文组玄武岩的配分曲线趋势与其他地区OIB型基性岩一致,为轻稀土(LREE)和中稀土(MREE)富集,重稀土(HREE)亏损的右倾型曲线,接近洋岛玄武岩的成分。原始地幔标准化图解中(图8b),多文组玄武岩样品相较于N-MORB均显示富集大离子亲石元素(LILEs, 如Ba、Pb和U)和(HFSEs,Nb、Ta、Zr、Hf和Ti)的特征, 与典型的OIB一致, 明显区别于N-MORB和E-MORB (图8b)。
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6 讨论
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6.1 蚀变混染与结晶分异的影响
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玄武质岩浆自地幔源区侵位到浅表,不可避免地会受热液交代、岩浆同化混染和结晶分异 (AFC)和熔体地幔相互作用影响,导致原岩元素含量和比值发生改变(Rudnick et al., 2003)。因此,讨论玄武岩成因之前需要评估这些过程对地球化学成分的影响。镜下观察显示福山镇多文组玄武岩原始矿物成分和结构保留完好,表明样品蚀变程度低,这一特征与全岩低的LOI(烧失量)含量一致,说明全岩地球化学成分不受热液蚀变和风化作用影响;陆壳混染可以通过全岩同位素和微量元素进行鉴别(Rudnick et al., 2003; Stracke et al., 2003)。来自同位素方面的证据显示海南玄武岩Sr-Nd-Hf-Pb同位素比值不随MgO的含量变化而改变,表明陆壳混染对样品同位素的影响可以忽略不计(Liu Jianqiang et al., 2015; Wang Xuance et al., 2012, 2013)。多文组火山岩Nb/U (36.15~39.22)、Ce/Pb (12.70~16.24)比值明显高于陆壳(Nb/U=6.15、 Ce/Pb=3.91; Rudnick et al., 2003; Stracke et al., 2003), 也指示陆壳混染对玄武岩微量元素含量的影响相对有限。陆壳混染通常导致玄武质岩石在原始地幔标准化蛛网图中富集LILEs、 LREEs、Zr和Hf,亏损Nb、Ta、Ti和Eu,但多文组火山岩并未显现Zr和Hf的异常,Nb和Ta显著富集(图8b),表明它们形成过程中基本没有大陆地壳的贡献;与原始的玄武质岩浆 (MgO=10%~15%,Mg# ＞70, Ni=400~500×10-6; Irving et al., 1978) 相比, 多文组火山岩具有低的过渡元素(如Ni＜100×10-6)含量和Mg#值(＜55),同时它们的SiO2和Al2O3含量较高,这些特征指示它们经历了一定程度的橄榄石和单斜辉石的结晶分异作用,这与橄榄石斑晶低的Fo值和Ni含量一致(图3a、图5)。此外,这些样品均没有显示Eu的负异常(δEu为0.96~1.04),反映斜长石的分离结晶作用不明显。因此,多文组玄武岩的矿物结晶顺序可能为橄榄石+铬尖晶石→单斜辉石+斜长石→铁钛氧化物。
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图8 海南岛多文组玄武岩球粒陨石标准化稀土元素配分图解(a)和原始地幔标准化微量元素配分图解 (b)
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Fig.8 Chondrite-normalized REE patterns (a) and primitive-mantle normalized spider diagram (b) of the basalts from the Duowen Formation, Fushan Town
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标准化值、OIB、E-MORB和N-MORB值据Sun et al.(1989);文献数据引自Liu Jianqiang et al.(2015) 和Wang Xuance et al.(2012); OIB—洋岛玄武岩;E-MORB—富集型洋中脊玄武岩;N-MORB—正常大洋中脊玄武岩
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Normalization values and OIB, E-MORB and N-MORB are from Sun et al.(1989); literature data on Hainan basalts cited from Liu Jianqiang et al.(2015) and Wang Xuance et al.(2012); OIB—ocean island basalt; E-MORB—E-type mid-ocean ridge basalt; N-MORB—N-type mid-ocean ridge basalt
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6.2 平衡熔体
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多文组玄武岩的结晶分异以橄榄石为主,玄武岩原始岩浆的成分可由与瞬时熔体平衡的橄榄石重建。利用MEGAPRIMELT3软件将多文组玄武岩校正到橄榄石Fo=90(Herzberg et al., 2015),获得原始熔体如表12所示。多文组玄武岩原始岩浆SiO2(48.64%~49.17%)、TiO2(1.24%~1.37%)、Al2O3(9.24%~9.65%)、FeO(9.22%~9.37%)、 MnO(0.13%~0.14%)、MgO(20.67%~21.37%)、CaO(5.35%~5.57%)和Na2O(2.10%~2.21%)含量与先前的研究结果一致(Wang Xuance et al., 2012)。多文组原始岩浆富镁、贫硅和低钙的特征可与夏威夷玄武岩对比(Jackson et al., 2012)。利用Norman et al.(2005)给定的分配系数,计算多文组玄武岩单斜辉石和斜长石平衡熔体微量元素的组成如图9,结果显示,多文组单斜辉石平衡熔体与全岩成分相似,它们富集LREE的洋岛型微量元素组成可与峨眉山以及塔里木大火成岩省的平衡熔体相比较(图9a)。除了Pb、Nd和Eu元素含量变化较大,多文组玄武岩斜长石的平衡熔体也显示出右倾型配分曲线(图9b),与全岩结果一致。以上结果表明,多文组玄武岩原始岩浆的微量元素组成与洋岛玄武岩相似。
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注: 熔融压力(P①)计算公式为Albarède (1992)的公式2和3;Tol②计算公式为Putirka (2008)的公式22;Tol ③计算公式为Putirka (2008) 公式14; Tol④计算公式为Herzberg (2015) 的公式12和13;Tol ⑤计算公式为Albarède (1992)的公式2;TP⑥计算公式为Herzberg et al.(2009);TP⑦计算公式为Kelley et al.(2006) 的公式12。
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6.3 温度压力和氧逸度
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玄武质岩浆原始熔体的成分被广泛应用于揭示地幔源区的热状态(McKenzie et al., 1988; Putirka, 2008; Wang Xuance et al., 2012)。多文组原始岩浆中橄榄石初始结晶的温压条件可由多种温压计计算获得(Putirka, 2008; Herzberg et al., 2015),计算结果列于表12。不同方法计算获得多文组玄武岩熔融压力略有差异,但它们主要分布在1.8~2.3GPa的区间内,与先前的结果一致(1.8~3.3GPa; Wang Xuance et al., 2012)。温度计取平均值得到多文组玄武岩橄榄石初始结晶温度(1566~1592℃)和地幔潜在温度(1546~1556℃),尽管不同温度计的结果稍有不同,但它们都投影到了洋岛玄武岩的范围内(图10a),明显高于洋中脊玄武(1260~1280℃ 和1410~1475℃;McKenzie et al., 1988; Kinzler et al., 1992; Asimow et al., 2001; Presnall et al., 2002)。
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岩相学分析表明,铁钛氧化物与其他硅酸盐矿物呈镶嵌结构(图3e),是岩浆结晶晚期的产物,因此它们的成分可以反映岩浆结晶分异过程中的温度和氧逸度。使用WinMIgob软件对钛铁矿-磁铁矿矿物计算两相平衡时的氧逸度(-logfo 2)和温度(Yavuz, 2021)。分别为-15.9~-10.9和743.5~896.5℃(表13)。磁铁矿与钛铁矿平衡时所记录的多文组玄武岩氧逸度ΔNNO为-0.47~+1.32,要高于处在FMQ缓冲剂范围内的MORB玄武岩(如Bézos et al., 2005; Lee et al., 2005),指示了一个更为氧化的结晶环境。
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6.4 地幔源区和构造背景
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火山岩橄榄石斑晶的成分可用于识别其地幔源区和演化过程(Sobolev et al., 2005, 2007)。相较于橄榄岩源区熔体,辉石岩源区熔体的橄榄石往往具有更高的Fe/Mn比值,更低的Ni、Ca和Mn含量,这是因为Ni在橄榄石的相容性要强于辉石,而Ca和Mn则是橄榄石的强不相容元素和辉石的相容元素(Sobolev et al., 2007; Le et al., 2010)。多文组玄武岩橄榄石斑晶含有较低的Ni含量(999×10-6~1663×10-6) 和较高的Fe/Mn比值(图11a、b), 指示它们的源区可能有辉石岩的贡献。多文组玄武岩全岩CaO的含量较低,而1000Zn/Fe比值较高(图11c、d),也指示熔体源区中辉石岩的存在。这一结果也被同位素方面的研究证实(Liu Jianqiang et al., 2015; An et al., 2017)。
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图9 多文组玄武岩单斜辉石(a)和斜长石(b)平衡熔体微量元素蛛网图
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Fig.9 Normalized trace elements patterns of equilibrium primary melt from clinopyroxene (a) and plagioclase (b) from basalts of the Duowen Formation, Fushan Town
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标准值引自Sun et al.(1989);峨眉山和塔里木火成岩省单斜辉石平衡熔体数据引自Wei Xun et al.(2015)
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Normalization values are from Sun et al.(1989);value of equilibrium primary melt are from Wei Xun et al.(2015)
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图10 海南玄武岩橄榄石计算的温度压力(a) (改自Wang Xuance et al.2012)和磁铁矿-钛铁矿矿物对计算温度-氧逸度图解(b)(改自陈艳虹等, 2015)
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Fig.10 Temperatures and pressures calculated by olivine from the Hainan basalts (a) (modified after Wang Xuance et al.2012) and oxygen fugacity and temperature determined for the Hainan basalts based on hematite-magnetite equilibration (b) (modified after Chen Yanhong et al.,2015)
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所有氧逸度反应线均基于压力为0.3GPa; F —熔融比例; MORB—洋中脊玄武岩; LAB—岩石圈-软流圈边界; HM—赤铁矿-磁铁矿缓冲剂;NNO—Ni-NiO缓冲剂;QFM—石英-铁橄榄石-磁铁矿缓冲剂;WM—方铁矿-磁铁矿
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All trajectories are calculated at 0.3GPa; F —the fraction of melting; MORB—mid-ocean ridge basalt; LAB—lithosphere-asthenosphere boundary; HM—hematite-magnetite buffer; NNO—nickel-nickel-oxide buffer; QFM—quartz-fayalitemagnetite buffer; WM—wüstite-magnetite
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多文组玄武岩全岩富集LILE和HFSEs并显示出与夏威夷玄武岩和峨眉山、塔里木等大火成岩省玄武岩相似的地球化学特征(图8),它们的地幔潜在温度和氧逸度范围均高于N-MORB(图10),此外,它们在形成和就位过程中很少受到陆壳成分的混染,指示玄武岩形成于洋岛环境。不同矿物的平衡熔体富集LREE和HFSEs,显示出与全岩一致的特征,表明多文组玄武岩是被动大陆边缘地幔柱活动的喷发产物。
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6.5 对南海盆地演化的启示
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地球物理观测显示海南岛和雷州半岛之下上地幔存在指示岩浆上涌的低波速异常区,被称为海南地幔柱(Lei Jianshe et al., 2009;Li Naisheng et al., 2013; Huang Zhouchuan et al., 2015; Hall et al., 2015; Xia Shaohong et al., 2018; Zhang Yu et al., 2020)。来自海南岛、雷州半岛、中南半岛和南海盆地的玄武岩具有明显的洋岛玄武岩地球化学特征,其规模与典型的大火成岩省相近,暗示它们的形成与海南地幔柱有关(Zou Haibo et al., 2010; Yan Quanshu et al., 2008, 2014, 2015; Fan Chaoyan et al., 2017; Yu Mengming et al., 2018; Zhang Guoliang et al., 2018),然而,南海盆地的开启是否受到地幔柱活动的影响至今仍然存在争议。南海北部陆缘发现了45~33Ma的玄武质火山岩(Yan Pin et al., 2006),此外,南海盆地钻孔中频繁出现E-MORB玄武岩,因而有观点认为在33Ma之前和16Ma之后南海均有板内玄武岩喷发,指示南海盆地在32Ma的打开可能与南海地幔柱活动有关(Zhang Guoliang et al., 2018)。但一些研究认为海南地幔柱与岩石圈相互作用发生在中渐新世,到达洋脊之下的时间为25Ma甚至更晚(Yu Mengming et al., 2018; Tian Zhixian et al., 2020)。也有学者通过橄榄石斑晶计算获得南海洋盆U1500B钻孔玄武岩地幔潜在温度为(TP约为1380℃),暗示南海的打开与地幔柱活动可能并无关联 (Yu Xun et al., 2020)。从年代学角度,南海北部陆缘广泛发育的玄武质岩浆主要形成于洋盆扩张裂谷期之后(Li Gang et al., 2022)。Yu Mengming et al.(2018) 对比了中国台湾南部25Ma的MORB、IODP 349航次16Ma的MORB和拖网获取的9Ma洋岛玄武岩的地球化学特征,发现南海地幔柱对玄武岩的影响主要发生在16Ma之后,与此相反,南海洋盆扩张时期的板内洋岛玄武岩却很少出露 (Yan Quanshu et al., 2014)。岩石和矿物地球化学研究表明,下—中更新统多文组玄武岩与夏威夷、峨眉山或塔里木等与地幔柱活动相关的玄武岩相似,指示了海南地幔柱的存在,但它们的形成时代却明显晚于南海洋盆的扩张时期,暗示南海地幔柱和南海盆地的扩张并无直接关联。更可能的情形是,32~23.6Ma期间,南海盆地的伸展和MORB的形成是印支地块软流圈地幔向东南方向逃逸的结果 (Tapponnier et al., 1982;Briais et al., 1993; Li Gang et al., 2022),而以多文组火山岩为代表的南海地幔柱活动则可能是对南海盆地扩张的被动响应(Zhao Fang et al.2016; Deng Peng et al.2019)。
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注:apfu—每一配方单位的原子; X'usp钛铁矿—钛铁矿中钛尖晶石比例; X'ilm—钛铁矿的比例; —氧逸度;ΔNNO—Ni-NiO缓冲区氧逸度。
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图11 海南玄武岩橄榄石矿物成分和全岩组分图解(改自Liu Jianqiang et al., 2015)
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Fig.11 Compositions of olivine phenocrysts and whole-rock samples suggesting contribution of pyroxenites in the mantle source of the Hainan basalts (modified after Liu Jianqiang et al., 2015)
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(a)—橄榄石Ni-Fo值协变图解;(b)—橄榄石Fe/Mn-Fo值协变图解;(c)—CaO-全岩MgO成分协变图解; (d)—Zn/TFe-全岩MgO协变图解;L—液相;Ol—橄榄石;Cpx—单斜辉石;Pl—斜长石;MORB—洋中脊型玄武岩
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(a)—Ni contents in olivine phenocrysts plotted versus Fo; (b)—Fe/Mn ratios of olivine phenocrysts versus Fo; (c)—CaO against MgO; (d)—Zn/TFeplotted against whole-rock MgO contents;L—liquid; Ol—olivine; Cpx—clinopyroxene; Pl—plagioclase; MORB—mid-ocean ridge basalt
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7 结论
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(1)海南岛北部多文组玄武岩斑晶和基质矿物由橄榄石、含钛单斜辉石、斜方辉石、斜长石和铁钛氧化物组成。
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(2)海南岛北部多文组玄武岩橄榄石Fo值变化于55.5~71.1之间,Ni的含量较低, Fe/Mn比值较高。铬尖晶石Cr#值为74.1~82.7,Mg#值为45.5~63.8, Ti的含量较高。斜方辉石Mg#值为63.9~79.6,单斜辉石为66.0~80.6。单斜辉石稀土配分曲线富集MREE,亏损LREE和HREE,呈拱形分布。斜长石以中-拉长石为主(Ab36.56~52.78), 富集LREE、Ba、Sr和Eu。铁钛氧化物TiO2的含量为50.19%~51.46%。
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(3)海南岛北部多文组玄武岩的原始岩浆组成与夏威夷、峨眉山、塔里木等玄武岩组成一致,地幔源区包含了辉石岩的成分,地幔潜在温度(＞1400℃)和氧逸度(ΔNNO)要高于大洋中脊玄武岩(N-MORB),表明玄武岩的喷发与海南地幔柱活动有关。
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(4)多文组以及其他与海南地幔柱有关的玄武岩时代晚于南海盆地的扩张时代,认为南海盆地的伸展可能是印支地块软流圈地幔向东南方向逃逸的结果,海南地幔柱的形成可能是对南海盆地扩张事件的被动响应。
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致谢:电子探针测试在中国地质科学院地质研究所自然资源部深地动力学重点实验室毛小红博士的帮助下完成;论文撰写过程中,与中国地质科学院地质研究所马绪宣和赵忠宝博士进行了有益讨论;两名匿名审稿人为本文提供了宝贵的建议,在此致以诚挚的谢意!
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摘要
南海盆地及周缘地区新生代玄武岩对揭示南海盆地的演化历史至关重要,然而这些玄武岩的成因还存在争议。本文研究了位于南海北部陆缘的海南岛临高县多文组玄武岩岩石地球化学和矿物地球化学特征,并探讨其成因和构造背景。多文组玄武岩主要由橄榄石、单斜辉石、斜长石、斜方辉石、铬尖晶石和铁钛氧化物等组成。橄榄石Fo值变化于55.5~71.1之间,Ni的含量较低, Fe/Mn比值较高。铬尖晶石Cr#值为74.1~82.7,Mg#值为45.5~63.8, Ti的含量较高。斜方辉石Mg#值为63.9~79.6,单斜辉石为66.0~80.6。单斜辉石稀土配分曲线富集MREE,亏损LREE和HREE,呈拱形分布。斜长石以中-拉长石为主(Ab36.56~52.78), 富集LREE、Ba、Sr和Eu。铁钛氧化物的TiO2含量为50.19%~51.46%。多文组玄武岩原始岩浆的主量和微量元素组成与夏威夷、峨眉山、塔里木等玄武岩组成一致,地幔源区包含了辉石岩的成分,而且其地幔潜在温度(＞1400℃)和氧逸度(ΔNNO)要高于大洋中脊玄武岩(N-MORB),表明多文组玄武岩的形成与海南地幔柱活动有关。由于海南地幔柱形成时代明显晚于南海盆地的扩张时代,认为南海地幔柱是南海盆地伸展的被动响应。
Abstract
The Cenozoic basalts in the South China Sea basin and surrounding areas are crucial to revealing the evolutionary history of the South China Sea basin. Yet, the genesis of these basalts is still debated. This paperdetermined the mineralogy petrology and geochemistry of the Cenozoic basalt from the Duowen Formation in Fushan Town, Lingao County, northern Hainan Island to discuss their genesis and tectonic setting. The basalts from the Duowen Formation are mainly composed of olivine, clinopyroxene, plagioclase, orthopyroxene, Cr-spinel and Fe-Ti oxides, etc. The Fo values of olivine vary from 55.5 to 71.1, with a low Ni content and a high Fe/Mn ratio. The Cr# values of Cr-spinel range from 74.1 to 82.7, Mg# values from 45.5 to 63.8, with a high Ti content. The Mg# values of orthopyroxene range from 63.9 to 79.6, and the clinopyroxene values between 66.0 to 80.6. Chondrite-normalized REE patterns of clinopyroxene are enriched in MREE and depleted in LREE and HREE, with an arch-shaped distribution. The plagioclase is dominated by andesine and labradorite (Ab 36.56~52.78), enriched in LREE, Ba, Sr and Eu. The TiO2 content of Fe-Ti oxides is 50.19%~51.46%. The main and trace element compositions of the primary magma of the basalts from the Duowen Formation are consistent with those of the basalts of Hawaii, Emeishan and Tarim, while their mantle source contains pyroxenite compositions, and its potential mantle temperature (＞1400℃) and oxygen fugacity (ΔNNO) are higher than those of the mid-ocean ridge basalts (N-MORB), suggesting that the formation of the basalts of the Duowen Formation is related to the activity of the Hainan mantle plume. As the formation age of the Hainan mantle plume is significantly later than the expansion age of the South China Sea basin, it is considered that the South China Sea mantle plume is a passive response to the extension of the South China Sea basin.
