岩石学报  2017, Vol. 33 Issue (1): 93-103   PDF    
新疆准噶尔盆地西北部二叠纪玄武岩浆作用流体介质条件:流体化学组成及碳同位素制约
张铭杰1, 余明1, 汤庆艳1, 杨扬1, 陈思童1, 李中平2, 史基安2     
1. 兰州大学地质科学与矿产资源学院, 甘肃省西部矿产资源重点实验室, 兰州 730000;
2. 中国科学院地质与地球物理研究所, 甘肃省油气资源研究重点实验室, 兰州 730000
摘要: 准噶尔盆地西北部深层分布着大量的二叠纪玄武岩,其发育的流体介质条件缺乏系统研究。本文采用二叠纪玄武岩岩心样品,对基质中的流体化学组成和碳同位素组成进行分步加热质谱法分析,结果表明二叠纪玄武岩的流体组成中H2O的含量极高(平均13789mm3.STP/g),次要组分为CO2、O2、H2S和N2等,CO2和CH4δ13C值(分别为-28.9‰~-15.8‰和-36.8‰~-21.9‰)位于地壳与甲烷氧化来源组分的范围内;CH4、C2H6、C3H8和C4H10等甲烷同系物的碳同位素组成随碳数增高总体具有正序分布特征,400~800℃温度段释出CH4δ13C值(-25‰左右)明显重于200~400℃的(-35‰左右),部分样品400~800℃的CH4-C3H8存在碳同位素反序分布的特征,显示幔源流体挥发份特征,其中混入大量的沉积有机质热裂解成因烃类气体,应为俯冲蚀变大洋板片的沉积物脱出流体挥发份。大量的H2O指示岛弧俯冲的再循环流体的加入,甲烷氧化来源特征的CO2和CH4碳同位素组成揭示经历了长期的风化过程,表明相关岩浆作用后遭受长期的抬升。
关键词: 来源     碳同位素     流体挥发份     二叠纪玄武岩     准噶尔盆地西北部    
The volatile conditions of Permian basaltic magmatism in northwestern Junggar basin of Xinjiang, China: Constraints from chemical and carbon isotopic compositions of volatiles in basalts
ZHANG MingJie1, YU Ming1, TANG QingYan1, YANG Yang1, CHEN SiTong1, LI ZhongPing2, SHI JiAn2     
1. Key Laboratory of Mineral Resources in Western China (Gansu), School of Earth Sciences, Lanzhou University, Lanzhou 730000, China;
2. Gansu key Laboratory of Petroleum Resources, Institute of Geology and Geophysics, CAS, Lanzhou 730000, China
Abstract: The Carboniferous-Permian basalts are occurred in deep part of northwestern Junggar basin, Xinjiang, China. The volatile conditions of the Permian basaltic magmatism had not been studied. The chemical and carbon isotopic compositions of volatiles in the matrix of Permian basalt cores were measured by stepwise heating mass spectrometer. The results showed that the volatiles in Permian basalts from northwestern Junggar basin were composed of dominated H2O with an average of 13789mm3.STP/g (STP-standard tempeture and pressure), with minor CO2, O2, H2S and N2. The δ13C values of CO2 (-28.9‰ to -15.8‰) and δ13CCH4 (-36.8‰~-21.9‰) were plotted in the δ13C ranges of crust and methane oxidation. The CH4, C2H6, C3H8 and C4H10 etc. methane homologues showed the normal distribution pattern of carbon isotopic compositions among CH4 to C4H10, which are the carbon isotopic features of thermogenic hydrocarbon gases of organic materials in oceanic sedimentary. The δ13C values of CH4released at 400~800℃ (about -25‰) are heavier than those at 200~400℃ (about -35‰), and some samples showed partial reversal pattern in CH4 to C3H8 released at 400~800℃, suggested that volatiles originated from mantle could be mixed by hydrocarbons from thermally cracked by sedimentary organic materials, which should be volatiles degassed from alterated subducted oceanic sedimentary. Large quatity of H2O indicated the addition of recycled volatiles in arc. The chemical and carbon isotopic compositions of CO2 and CH4 reveal long time alteration of organic materials, and imply long time uplifting after basaltic magmatism.
Key words: Origin     Carbon isotope     Volatile     Permian basalt     Northwestern Junggar basin, China    
1 引言

新疆北部中亚造山带晚古生代(主要为石炭纪-二叠纪)基性岩浆作用形成了众多的镁铁-超镁铁质岩体,赋存了镍铜硫化物矿床,如喀拉通克、黄山等岩体(Fu et al., 2012; Li et al., 2012; Song et al., 2011; Zhang et al., 2009a, 2011; 张招崇等, 2003; 贾志永等, 2009; 傅飘儿等, 2009; 胡沛青等, 2010),相邻盆地发育了同期的玄武岩(Su et al., 2012; Gao et al., 2014; Zhang et al., 2015),被认为与造山带中同期镁铁-超镁铁质岩体具有相同的岩浆系统(Song et al., 2011, 2013),甚至为地幔柱体系的一部分(Xia et al., 2004; Li et al., 2014; Zhang et al., 2014; 杨梅珍等, 2006; 李涤等, 2012)。其中准噶尔盆地西北缘是该地区晚古生代镁铁-超镁铁质岩浆作用相对发育的地区,火山岩及岩浆演化研究主要集中于盆地周边的褶皱造山带,盆地腹部地区因被中、新生代连续沉积覆盖,相关研究较少(杨梅珍等, 2006; 吴小奇等, 2009; 方琳浩等, 2009; 毛翔等, 2012; Zhang et al., 2009c)。准噶尔盆地西北部二叠纪玄武岩的流体地球化学特征对认识该地区晚古生代镁铁质岩浆活动及构造演化具有重要意义。

准噶尔盆地西北部油气勘探钻孔揭示在盆地深部沉积盖层之下发育了巨厚的二叠纪火山岩,早二叠世多期火山作用的岩性从玄武岩、玄武安山岩、安山岩到英安岩。陆西地区玛东3井揭示二叠纪玄武岩厚50m,安山岩厚25m,其间夹数十米的凝灰岩,作为油气的有利储层进行了储集方面的研究(鲜本忠等, 2013; 匡立春等, 2008; 方琳浩等, 2009; Su et al., 2012)。二叠纪玄武岩通过岩浆分异演化为安山岩,微量元素具有LILE富集,Nb、Ta相对亏损的岛弧玄武岩地球化学特征(方琳浩等, 2009)。准噶尔盆地深层巨厚的二叠纪玄武岩是否与塔里木盆地内的玄武岩一样具有地幔柱岩浆属性,其流体组成研究可提供岩浆作用的性质及环境的信息。

本文采集油田勘探获得的准噶尔盆地内部新鲜的二叠纪玄武岩岩心样品,在岩石地球化学的基础上开展了流体地球化学分析,探讨了玄武岩的成因、演化及深部动力学意义。

2 区域地质背景

新疆北部准噶尔盆地(地体)大地构造位置上属于欧亚板块的组成部分,北邻西伯利亚板块,西接哈萨克斯坦板块,南依塔里木板块。盆地南宽北窄,三面通过古生代缝合线被古生代褶皱山系所夹持,东北为阿尔泰山、青格里底山和卡拉麦里山,西北为扎伊尔山和哈拉阿拉特山,南为伊连哈比尔尕山和博格达山(图 1)。盆地基底为早古生代大洋和岛弧杂岩体(Gao et al., 2014),西准噶尔地区寒武纪时已经处于古亚洲洋环境,早寒武世发育洋岛玄武岩(OIB型)-沉积建造(朱永峰等, 2007),在泥盆-石炭纪时准噶尔古洋盆处于俯冲、消减环境,发育岛弧岩浆作用(杨梅珍等, 2006; 柴凤梅等, 2012; Zhang et al., 2009b; Yin et al., 2010; Yang et al., 2013),大洋于晚古生代最后闭合。大洋闭合至今的多期构造运动使得周边演化形成褶皱造山带,中部为连续堆积的巨厚沉积盖层,形成多旋回大型陆内叠合盆地(Yang et al., 2013)。

图 1 准噶尔盆地东南缘二叠纪玄武岩地质分布图(据方琳浩等, 2009; Zhang et al., 2011) Fig. 1 Geological map of Permian basalts in SE margin of the Junggar basin, Xinjiang, China (after Fang et al., 2009; Zhang et al., 2011)

准噶尔盆地二叠系火山岩埋藏较深,不同地区的岩相有所差异,克百地区风城组以溢流相火山熔岩为主,为玄武粗安岩和碱玄岩,乌夏地区风城组以爆发相火山碎屑流亚相熔结火山碎屑岩或火山碎屑熔岩为主,为流纹岩、碱玄质响岩和粗安岩。西北腹部二叠系火山岩发育溢流相和爆发相,自下至上为上段玄武岩、下段安山岩,上覆夏子街组上段褐色泥质砂砾岩呈不整合接触,下伏佳木河组下段褐色凝灰岩呈整合接触。下二叠统火山岩以熔岩和火山碎屑岩,岩性从下向上由玄武岩演化成为玄武安山岩、安山岩和英安岩(鲜本忠等, 2013)。准噶尔盆地晚古生代火山岩岩心年代学数据相对较少,北部石炭系火山岩开展了锆石U-Pb定年(杨甫等, 2014; Su et al., 2012),区域地层对比确定本文所采样品层位之火山岩为早二叠世。

3 样品及测试方法 3.1 样品及前处理

本文研究样品主要为新疆准噶尔盆地西北缘玛东地区采集的二叠纪玄武岩岩心样品,采样位置见图 1。主要为杏仁状玄武岩和块状玄武岩。样品呈灰黑色,块状构造,密度较大,有杏仁状填充构造。经岩石化学数据投图确定岩石类型为玄武岩,相关岩相学特征、岩性柱状图见文献(方琳浩等, 2009)。

通过薄片显微岩石学观察,选择新鲜的样品去除表层部分,碎至40~60目颗粒样品,用蒸馏水清洗-烘干,在双目显微镜下挑选出纯净的火山岩基质。用0.3mol/L的HCl浸泡24h,用超声波清洗以除去杏仁、气孔构造中的次生碳酸盐或蚀变部分。用去离子水反复冲洗至中性后,再用分析纯CH2Cl2-超声波清洗以除去样品表面可能吸附的有机质,在110℃条件下烘干用于实验。详细的样品处理方法见汤庆艳等(2012)

3.2 流体组成测定方法

流体化学组成分析采用改进的真空分步加热提取流体组分,在线导入气体组成及同位素质谱计进行化学组成和稳定同位素组成分析。该方法克服了样品释出流体组分间相互反应的影响,并对高温岩浆条件下平衡的流体挥发份予以恢复(Zhang et al., 2004, 2009c; 张铭杰等, 2000)。分步加热装置由石英样品管和金属管真空-冷阱系统组成。

样品在100℃加热高真空去气2h以排除样品表面吸附的组分,确认样品没有气体排出后,从200℃开始升温,以100℃为升温段,最高加热温度为1100℃(之后样品开始熔融)。样品加热过程中采用液氮冷阱冷冻分离样品释出的低冰点流体挥发份,每个温度点加热1h后,样品释出的流体挥发份在线导入MAT271型质谱计测定化学组成,流体组分提取装置和详细的实验步骤见Zhang et al.(2004, 2007)描述。CO2和H2测量的相对误差小于1%,H2O的测量误差小于5%(Zhang et al., 2007)。

碳同位素以CO2、CH4、C2H6、C3H8和C4H10含碳流体挥发份为测试对象,采用分步加热-氦载气-分子筛-GC-C-稳定同位素质谱计系统,在200~400℃、400~800℃和800~1100℃三个温度段测定碳同位素组成。含碳流体挥发份提取装置由石英样品管和分子筛吸附阱组成。分子筛预先在400℃加热、氦载气驱赶净化,样品在不同温度段释出的含碳流体挥发份用液氮-分子筛冷冻吸附收集,每一温度点恒温1h后,采用加热分子筛(~96℃)解吸含碳流体挥发份,用氦载气载入在线GC-C-MS系统,通过4m长的GDX-05不锈钢色谱填充柱分离含碳流体组分,He载气流速5mL/min,填充柱升温范围为-20~95℃,升温速率3℃/min。分离的组分经燃烧系统转化为CO2,进入Delta-plus XP稳定同位素质谱计测量碳同位素组成。δ13C值采用PDB标准,系统误差小于1.6‰。详细分析步骤见Zhang et al. (2007)汤庆艳等(2012)

4 结果 4.1 流体挥发份化学组成

准噶尔盆地西北部二叠纪玄武岩分步加热过程中流体挥发份相在200~400℃、400~800℃和800~1100℃分阶段释出,不同温度段释出流体组分化学组成见表 1

表 1 准噶尔盆地二叠纪玄武岩基质不同温度段释出流体组分的含量(mm3.STP/g) Table 1 Volatile contents (mm3.STP/g) at different temperature from matrix separates of the Permian basalts in Junggar Basin, China

玄武岩流体挥发化学组成主要以H2O为主,平均为13789mm3.STP/g (STP-标准温度压力条件,下同)。除水外的流体组分为CO2(57.2vol.%)及少量的O2、H2S、N2和H2等,分别14.5%、10.4%、7.3%和6.5%。大部分玄武岩样品以CO2为主要组分,平均58.1vol.%;2个粗玄岩样品(MD3-1, 3)以CO2和N2为主,平均分别为37.1vol.%和49.4vol.%。

准噶尔西北部二叠系玄武岩流体挥发分在200~400℃、400~800℃和800~1100℃三个温度段释出的流体的含量不同,分别为4133mm3/g、7900mm3/g和3956mm3/g,流体挥发份主要在400~800℃释出。部分样品中流体挥发份释出特征略有不同,如样品MD3-2流体主要在600℃释出,且含量较高(图 2a),而样品(MD3-5)主要在900℃释出流体,含量较低(图 2b)。主要流体挥发份相H2O和CO2的释出特征与总气体释出特征相似,而H2主要在400~700℃释出(图 2d)。

图 2 准噶尔盆地二叠纪玄武岩不同温度段释出流体组分的含量(mm3/g) Fig. 2 The contents of volatiles (mm3/g) released at different temperatures from the Permian basalts in Junggar basin, China

三个温度段释出流体中H2O和CO2为主要流体挥发份相,H2O释出量平均分别为3915mm3/g、7197mm3/g和3217mm3/g,CO2分别为81.0mm3/g、464.2mm3/g和179.8mm3/g。除H2O外各温度段流体的化学组成有所差异,200~400℃释出流体主要为CO2(37.2vol.%)、O2(16.9%)、H2S (15.4%)和N2(12.3%),400~800℃释出流体主要为CO2(66.0vol.%)和少量的H2S (9.7%)、O2(9.3%)和H2(8.4%),800~1100℃释出流体主要为CO2(52.4vol.%)、O2(21.1%)、N2(11.0%)和H2S (8.6%)。

4.2 流体挥发份碳同位素组成

准噶尔盆地西北部二叠纪玄武岩分步加热流体挥发份具有分步释出特征,据此在200~400℃、400~800℃和800~1100℃三个温度段分别测定了CO2和CH4、C2H6、C3H8和C4H10甲烷同系物的碳同位素组成(表 2)。

表 2 准噶尔盆地二叠纪玄武岩基质不同温度段释出流体组分的碳同位素组成(vPDB‰) Table 2 The carbon isotopes (vPDB‰) of carboniferous volatiles at different temperatures from matrix separates of the Permian basalts in Junggar Basin, China

CO2δ13C介于-28.9‰~-15.8‰,在地壳和有机成因CO2之间。CH4δ13C为-36.8‰~-21.9‰,C2H6δ13C为-29.8‰~-20.7‰,C3H8δ13C为-25.7‰~-15.1‰,C4H10δ13C为-23.2‰~-18.5‰。位于世界范围内岩浆来源CO2δ13C值(-10‰-0‰)和CH4δ13C值(-52‰~-9‰)的范围内(Sherwood Lollar et al., 1993; Giggenbach, 1997; Birkle et al., 2001)。玄武岩CO2δ13C值大部分位于-9‰~-4‰,少部分位于-30‰~-25‰(Deines, 1992)。

玄武岩含碳流体组分碳同位素组成随释出温度段有所不同,200~400℃、400~800℃和800~1100℃释出CO2δ13CCO2值分别为-28.9‰~-24.5‰、-26.8‰~-19.8‰和-22.4‰~-15.8‰,平均值(-26.1‰、-23.4‰和-19.0‰)逐渐变重。200~400℃释出CH4、C2H6、C3H8和C4H10δ13C (平均分别为-34.8‰、-28.3‰、-22.7‰和-20.6‰)轻于400~800℃温度段的(平均分别为-25.0‰、-23.3‰、-21.5‰和-19.5‰)。

200~400℃温度段释出甲烷同系物碳同位素组成随碳数增加具有正序分布特征,CH4δ13C为-36.8‰~-33.0‰,平均-34.8‰,为沉积有机质来源组分的特征。400~800℃温度段释出甲烷同系物碳同位素组成随碳数出现局部反序分布特征,CH4δ13C较重,为-28.2‰~-21.9‰,平均-25.0‰,为地幔无机来源组分特征。

5 讨论

玄武岩中的流体挥发份主要保存在斑晶和基质中,斑晶存在同源岩浆结晶及捕虏晶等不同成因,而大量的基质快速冷却过程中密封了不同来源的大量组分,包括岩浆结晶排出的流体组分,及岩浆上升演化过程的所有流体组分(Zhang et al., 2009c),赋存于基质气泡及孔隙,可指示玄武岩浆形成演化的流体介质环境。

5.1 准噶尔二叠纪玄武岩浆作用流体组成

准噶尔盆地西北部二叠纪玄武岩中流体组分以H2O为主(平均92%),其次为CO2、O2、H2S和N2等。H2O的总含量(平均13788mm3/g)远远高于西秦岭和腾冲等地大陆环境新生代玄武岩的(分别为7825mm3/g和1290mm3/g),CO2含量(745mm3/g)也高于两地新生代玄武岩(分别为265.6mm3/g和28mm3/g)(汤庆艳等, 2012; 余明等, 2014)。水含量接近于岛弧玄武岩(13263mm3/g)、弧后盆地玄武岩(12731mm3/g)和Samon洋岛玄武岩(11455mm3/g, Zhang et al., 2009c),低于西伯利亚二叠纪地幔柱成因苦橄岩中基质和辉石斑晶的水含量(分别为27578mm3/g和28832mm3/g),西伯利亚二叠纪苦橄岩中存在俯冲再循环物质的贡献(Tang et al., 2013),表明准噶尔二叠纪玄武岩喷发环境可能为水下环境。另外,准噶尔盆地二叠纪玄武岩中CO2和O2等组分的含量较高,表明玄武岩浆演化环境具有较高的氧逸度。

准噶尔盆地西北部二叠纪玄武岩中高含量H2O、O2和N2可能与水下玄武岩浆作用(毛翔等, 2012)、俯冲环境板片脱挥发份或后期蚀变有关。玄武岩中大量的H2O在较高温度段释出(图 2e),因此可排除蚀变作用带入组分的贡献。另外,玄武岩中O2/N2值(平均0.52)与大气值(3.17)明显不同,CO2δ13CCO2值随含量的变化特征表明玄武岩没有明显的脱气作用(图 3),可排除后期蚀变的影响,O2和N2等在较高温度段释出也支持这一推论(图 2e)。玄武岩高含量H2O和高氧逸度挥发份组成特征应该为岩浆冷却包裹的组分及熔体溶解流体挥发份,是玄武岩浆形成演化的流体介质环境的特征,是不同来源流体挥发份的混合。

图 3 准噶尔二叠纪玄武岩CO2碳同位素值(δ13C‰ vPDB)与CO2含量(mm3/g)关系图 Rayleigh-瑞利脱气分馏曲线,碳同位素初始值为-7‰,1000lnα=-4.6‰ Fig. 3 The variations of δ13C (‰ vPDB) of CO2 vs. the CO2 contents (mm3/g) in Junggar Permian basalts Rayleigh-Rayleigh degassing pattern of carbon isotope (initial value of δ13C is -7‰, 1000lnα=-4.6‰)

另一方面,样品(MD3-8)具有高含量的H2,而样品释出的CO等组分的含量较低,可以排除含碳组分与H2O发生反应(即费-拓反应)形成H2的可能性,高含量的H2与地幔柱岩浆的流体组成类似(Tang et al., 2013)。水含量(13789mm3/g)高于Samon洋岛玄武岩(11455mm3/g, Zhang et al., 2009c),低于西伯利亚二叠纪地幔柱成因苦橄岩中基质的水含量(27578mm3/g, Tang et al., 2013)。CH4δ13C平均值(-23.6‰)为幔源CH4δ13C值。

5.2 流体挥发分的来源

准噶尔盆地西北部二叠纪玄武岩中流体挥发份来源有地幔部分熔融时源岩中的流体挥发份、源区加入的挥发份及岩浆上升演化过程中混入的流体挥发份。理论模拟和实验观测结果表明,玄武质岩浆中广泛存在CO2和CH4等含碳流体挥发份(Duan et al., 1992; Zhang et al., 2007; 张铭杰等, 2000; 汤庆艳等, 2012),其碳同位素组成可制约其来源。幔源CO2δ13C值范围为-8‰~0‰,CH4δ13C值范围为-25‰~-15‰(Carapezza and Federicob, 2000; Taran et al., 2001, 2002)。热催化成因的δ13CCH4=-38‰~-25‰,生物成因的δ13CCH4=-93‰~-50‰(Schoell, 1980)。

准噶尔盆地西北部二叠纪玄武岩的δ13CCO2值(-28.9‰~-15.8‰)和CH4δ13C值(-36.8‰~-21.9‰)普遍轻于幔源CO2和CH4δ13C值(Carapezza and Federicob, 2000; Taran et al., 2001, 2002)。在CO2和CH4δ13C图解(Ueno et al., 2006)中,200~400℃和400~800℃温度段释出的CO2和CH4δ13C均位于地壳及甲烷氧化区域内(图 4),表明CO2和CH4等可能来源于有机质氧化,揭示玄武岩经受了长期风化作用或氧化性流体的注入,但二者有所不同。

图 4 准噶尔二叠纪玄武岩CO2和CH4碳同位素组成 Mantle-地幔来源;Crust-地壳来源;Thermogenic-有机质热裂解成因;Microbial-微生物成因;Methane Oxidation-甲烷氧化成因 Fig. 4 The plot of carbon isotope of CO2 vs. CH4 from Permian basalts in northwestern Junggar basin, Xinjiang, China

200~400℃释出CO2和CH4的含量较低,δ13C值比较集中,可能为有机质风化作用的产物;同时200~400℃释出CH4同系物的碳同位素组成随碳数增加呈正序分布的特征,CH4δ13C值较轻,位于-35‰左右(图 5a),指示有机质热裂解成因来源。这部分流体主要来源于样品裂缝等孔隙或次生包裹体,为后期作用捕获的流体组分,表明可能与油气充填(鲜本忠等, 2013; 匡立春等, 2008; 方琳浩等, 2009; Su et al., 2012)相关的沉积有机质组分,经历了长期蚀变作用的氧化过程。

图 5 准噶尔二叠纪玄武岩200~400℃(a)和400~800℃(b)温度段释出烷烃气体碳同位素δ13C值特征 Fig. 5 The δ13C values (‰ vs. PDB) of CH4, C2H6, C3H8 and C4H10 alkane hydrocarbons released from Permian basalts in Junggar basin, Xinjiang, China

400~800℃温度段释出CO2和CH4的含量较高,CH4δ13C值明显偏重(>-25‰左右),CH4同系物碳同位素组成随碳数整体呈正序分布特征,部分样品的CH4-C3H8间碳同位素组成反序分布(图 5b),表明玄武岩浆流体组分中保存了地幔来源特征的流体组分,其中混入了沉积有机质热裂解成因来源的烷烃气体。结合δ13C指示CO2和CH4为地壳及甲烷氧化来源,以及较高的O2含量,表明混入的流体应该为氧化性的沉积有机质热裂解成因流体。这部分氧化性流体来源于玄武质熔体固结之前,可排除岩浆喷发后蚀变作用带入的流体,可能为俯冲的蚀变洋壳板片脱出的流体组分,或岩浆上升喷发过程中混入的围岩蚀变流体。

H2O、N2、O2和含碳流体挥发份具有相似的释出特征(图 2),应该来自相同的赋存状态,可能具有上述相同的来源。

5.3 流体挥发份组成的构造环境意义

不同构造环境基性岩浆作用过程中有不同来源的流体混合,导致具有不同的流体组成特征(Zhang et al., 2004, 2007, 2013; Fu et al., 2012; Xing et al., 2012; Tang et al., 2013)。准噶尔盆地西北部二叠纪玄武岩中极高含量的H2O、CO2和O2组分,与西秦岭和腾冲等地造山带环境俯冲相关流体有所不同,可能与玄武岩发育的准噶尔盆地构造演化条件有关。

准噶尔盆地晚石炭世至早-中二叠世处于后碰撞环境下,中晚二叠世处于板内断陷-裂陷盆地张性的大地构造背景(方世虎等, 2006)。岩石地球化学研究表明板片断离导致大规模的软流圈地幔物质上涌,玄武质岩浆起源于亏损的岩石圈地幔,在地幔源区被俯冲板片来源的流体或熔体交代(Zhang et al., 2014)。准噶尔盆地二叠纪玄武岩流体挥发份中大量的H2O表明玄武岩发育于俯冲相关的岛弧环境(李永军等, 2016; Zhang et al., 2009c)。岩浆源区为被流体或沉积物熔体交代改造的地幔楔或软流圈地幔(柴凤梅等, 2012),被流体交代的亏损地幔部分熔融形成玄武质岩浆(Li et al., 2012; Zhang et al., 2011, 2012; Gao et al., 2014)。

含碳流体挥发份碳同位素揭示幔源流体挥发份中混入大量的沉积有机质热解成因流体,并具有较高的氧逸度,这种氧化性流体最可能为蚀变沉积物脱出的流体组分。由于玄武岩快速上升,及喷发于水下快速冷却,可排除岩浆上升喷发过程中混入大量的蚀变围岩流体。因此俯冲洋壳板片脱出的流体组分是最可能的来源,且为蚀变的洋壳,因此推断此时准噶尔的构造环境可能为岛弧环境(李永军等, 2016)。200~400℃释出CO2和CH4δ13C指示为有机质甲烷氧化的产物,表明相关岩浆作用后遭受长期的抬升和蚀变作用。

6 结论

准噶尔盆地西北缘二叠纪火山岩基质分步加热释出流体挥发份化学组成及碳同位素组成特征表明:

(1)准噶尔盆地西北缘二叠纪玄武岩中流体组成以高含量的H2O (平均为13789mm3/g)为特征,含有较高的CO2、O2、H2S和N2等组分。

(2)碳同位素组成揭示玄武岩中幔源流体组分的混入了大量的蚀变的沉积有机质热解来源的烷烃气体,CO2和CH4为地壳及有机质甲烷氧化的产物。

(3)玄武岩流体挥发份组成及碳同位素表明准噶尔盆地二叠纪玄武岩可能发育于俯冲相关的岛弧环境,相关岩浆作用后遭受长期的抬升和蚀变。

致谢 张顺存、汪扬和杜丽等参与了野外考察和实验分析;郭正府研究员及两位匿名评议人在论文撰写中给予了指导和帮助,对论文修改提供了建设性的意见;在此一并表示衷心的感谢。
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