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中晚二叠世在我国西南部发育的大规模岩浆喷溢事件形成了我国最早被国际认可的峨眉山大火成岩省(Chung Sunlin et al.,1995; Courtillot et al.,1999;Xu Yigang et al.,2001; Zhang Zhaochong, 2009),它的形成直接影响了我国西南地区的海陆变迁(Chen Wenyi et al.,2003)、沉积(He Bin et al.,2003, 2005; Xu Yigang et al.,2013)及成矿作用(Ali et al.,2005; Zhang Zhaochong,2009),甚至可能导致了二叠纪全球性气候变化和生物大灭绝事件(Hu Ruizhong et al.,2005;Lai Xulong et al.,2009;Zhu Jiang et al.,2011; Bagherpour et al.,2018),因此成为国内外学者的关注热点。多年来对峨眉山大火成岩省(ELIP)的研究取得了很多的进展,其地幔柱成因的观点已基本得到了大多学者的认可,但在火山作用喷发及持续时限等方面还存在很多争议。
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近些年来的数据统计显示,峨眉山玄武岩的主相喷发时间在260Ma左右(Ali et al., 2002; Zhou Meifu et al., 2002; Xiao Long et al., 2003; Zhong Yuting et al., 2014; Huang Hu et al., 2016),持续时间在1~3Ma内。但也有学者提出,峨眉山玄武岩的喷发是分阶段的(Hou Zengqian et al.,1999;Chen Wenyi et al.,2003;Hu Ruizhong et al.,2005),持续时间应延续到了晚二叠世末期。Shellnutt et al.(2020)对越南北部流纹岩的高精度测年结果研究显示峨眉山熔岩至少经历了6Ma的爆发,持续到了吴家坪中期。部分学者于峨眉山玄武岩顶部凝灰岩获得了较为精准的锆石U-Pb年龄(Zhong Yuting,2013, 2014; Yang Jianghai et al., 2018),并用于限定峨眉山玄武岩的喷发结束时限。也有学者认为宣威组底部碎屑锆石来源于ELIP顶部的长英质组分,他们以宣威组底部碎屑锆石中获得的锆石U-Pb年龄来限定约束峨眉山玄武岩的喷发结束时间(He Bin et al., 2007; He Binghui,2015)。最新的研究成果则将研究对象延伸到了二叠系—三叠系界线(PTB)灰岩(Huang Hu et al.,2018; Zhang Han et al.,2020; Zhong Yuting et al.,2020)。华南二叠系乐平统长英质凝灰岩的成因争议较大,主要集中在峨眉山酸性火山作用成因(Huang Hu et al.,2016)、古特提斯大陆岩浆弧成因(Yang Jianghai et al.,2012; Cao Qiuling et al.,2013; Liao Zhiwei et al.,2016; Wang Man et al.,2018)及西伯利亚大火成岩省基性火山作用(Xu Lin et al.,2007)。Li Xia(2015)总结了前人晚二叠世不同性质Tonstein于地层中的分布规律,并认为这些火山灰是峨眉山玄武岩地幔柱消亡阶段或岩浆高度演化的产物。Zhang Han et al.(2020)总结了广元吴家坪组凝灰岩地球化学、锆石微量元素及Hf同位素特征,提出锆石具有较高εHf(t)值与较低Th/Nb值为特征的凝灰岩为峨眉山玄武岩后期酸性火山活动的产物,但其获得年龄的凝灰岩层位(260.1±2.8Ma)无疑是偏老的。
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图1 研究区大地构造位置(a)及地质简图(b, 据Hou Zengqian et al.,1999; He Bin et al.,2005修改)
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Fig.1 Tectonic outline (a) and geology map (b, after Hou Zengqian et al.,1999; He Bin et al.,2005) of study area
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本文通过永善宣威组剖面下部与上部发现的凝灰岩U-Pb锆石年龄与锆石微量元素、Hf同位素特征的讨论,判定凝灰岩性质及成因,约束研究区峨眉山大火成岩省火山活动的结束时间及古特提斯弧火山作用的启动时限。
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1 地质概况及岩石学特征
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研究区位于上扬子西缘威宁-昭通褶-冲带,北面四川盆地,西临小江断裂。区内出露震旦系—第四系盖层,未见中元古代—新元古代早期褶皱基底(图1)。晚古生代—中二叠世,由于古特提斯洋的存在,华北板块与扬子板块扩张成谷,上扬子为在勉-略洋盆南侧发育的被动大陆边缘盆地。晚二叠世—早三叠世,古特提斯洋向华南与印支板块俯冲,伴随着古特提斯洋的闭合,板块、地块碰撞频繁,发育了大量的长英质火山作用(Hou Zengqian et al.,2003;Zhang Wanping et al.,2011)。如腾冲地块与保山地块碰撞形成的249Ma的花岗岩(Hu Peiyuan et al.,2014);拉萨地块与羌塘地块碰撞发育的弧火山作用(Zhu Dicheng et al.,2009; Yang Tiannan et al.,2011);印支板块与华南板块俯冲碰撞形成于宋马-哀牢山-金沙江构造带内的流纹岩(Zi Jianwei et al.,2012)、花岗岩(Liu Huichuan et al.,2015)。同时,该时期还存在巨量的玄武质火山作用:峨眉山大火成岩省和西伯利亚大火成岩省。峨眉山玄武岩具有OIB亲属性(Xiao Long et al.,2004),为典型的地幔柱形成的大陆溢流玄武岩(Xu Yigang et al.,2004; He Bin et al.,2005),而西伯利亚玄武岩则表现出具有弧相关的微量元素特征(Puffer et al.,2001)。这些火山活动都或多或少于地层中留下了凝灰岩沉积记录,引起了学者们的广泛关注。研究区二叠系自下而上发育梁山组(P1l)、栖霞组(P1q)、茅口组(P1m)、峨眉山玄武岩(Pe)、宣威组(P3x)。杂色细碎屑岩为主的宣威组(P3x)中发育多层凝灰岩,与下伏峨眉山玄武岩(Pe)为平行不整合接触。
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图2 永善地区晚二叠世宣威组柱状剖面简图及野外照片特征
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Fig.2 Columnar profile and feature of field photos of Late Permian Xuanwei Formation in Yongshan area
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(a)、(b)—宣威组下部凝灰岩(PM003-5TW); (c)、(d)—宣威组上部凝灰岩(PM003-56TW)
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(a), (b)—Tuffs in the lower part of Xuanwei Formation (PM003-5TW); (c), (d)—tuffs in the upper part of Xuanwei Formation (PM003-56TW)
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本文样品采自于永善县莲峰镇老鹰岩露头较好且连续的宣威组实测剖面,主要岩性为含煤碎屑岩夹凝灰岩、凝灰质黏土岩。样品分别为宣威组下部最后一层凝灰岩PM003-5TW、顶部最后一层可采凝灰岩PM003-56TW(图2)。据野外观察(图2a、b):PM003-5TW风化面浅灰绿色,新鲜面灰色,凝灰结构,质密,中层状构造,单层厚43cm,主要成分为火山灰。镜下观察(图3a):凝灰结构,成分主要由晶屑、玻屑和填隙物及微量的外生碎屑等组成。晶屑:＜0.20mm,他形、粒状,成分主要为石英和少量长石,含量约占全部的15%。玻屑:＜0.60mm,不规则状,多数发生脱玻化作用,而呈霏细脱玻结构,含量约占全部的5%。填隙物:颗粒细小,主要为火山灰和铁质,局部见火山灰发生脱玻化和泥化现象;铁质胶结物局部发生强烈氧化,而呈褐红色,含量约占全部的80%。外生碎屑:＜0.25mm,棱角状,主要为粉砂、含微量细砂,成分主要为石英,量微。野外观察(图2c、d):PM003-56TW风化面褐灰绿色,新鲜面灰绿色,凝灰结构,中—厚层状构造,凝灰岩层厚55cm,岩石风化较为破碎,主要成分为火山灰及少量外生碎屑。镜下观察(图3b):变余凝灰结构,岩石主要由外生碎屑和凝灰质等组成。外生碎屑:＜0.15mm,棱角状,主要为陆源砂屑,粒级主要为粉砂、含微量细砂。成分主要为石英和糜粒状隐晶燧石岩屑,含微量海绿石。含量约占全部的3%;凝灰质:颗粒细小,主要为火山灰,含微量的晶屑及玻屑。火山灰含较多铁质,并发生强烈的氧化作用;火山灰中混有较多陆源泥质,难以分辨。晶屑成分主要为斜长石,发生强烈的碳酸盐化;玻屑含量很少,发生强烈的脱玻化和黏土化作用。火山灰总体上发生强烈的碳酸盐化,碳酸盐矿物类型主要为隐晶—微晶方解石,含量约占全部的97%。
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取样时将明显风化层面剔除,以保证样品新鲜、分析结果有效。采集样品重15kg,由北京锆年领航科技有限公司进行锆石分选、制靶及阴极发光(CL)成像,锆石U-Pb定年使用LA-MC-ICP-MS分析完成。在开始锆石U-Pb分析前,先进行阴极发光(CL)分析,以确定锆石颗粒内部结构。
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图3 永善地区晚二叠世宣威组下部凝灰岩样品PM003-5TW (a)和上部凝灰岩样品PM003-56TW (b)镜下特征(单偏光)
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Fig.3 Microscopic characteristics of samples PM003-5TW (a) and PM003-56TW (b) of Late Permian tuff in Yongshan area (single polarized light)
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2 分析测试
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锆石U-Pb定年使用激光剥蚀-电感耦合等离子体质谱仪(LA-ICP-MS)在北京锆年领航科技有限公司完成。激光剥蚀平台为Resolution SE型193nm深紫外激光剥蚀进样系统(Applied Spectra,美国),配备S155型双体积样品池。质谱仪采用Agilent 7900型电感耦合等离子体质谱仪(Agilent,美国),详细调谐参数见Thompson et al.(2018)。锆石样品固定在环氧树脂靶上,抛光后在超纯水中超声清洗,分析前用分析级甲醇擦拭样品表面。采用5个激光脉冲对每个剥蚀区域进行预剥蚀(剥蚀深度~0.3 μm),以去除样品表面可能的污染。本次实验在束斑直径30 μm、剥蚀频率5Hz、能量密度2J/cm2的激光条件下分析样品。
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数据处理采用Iolite程序,锆石91500作为校正标样,GJ-1作为监测标样,每隔10~12个样品点分析2个91500标样及一个GJ-1标样。采集20s的气体空白,35~40s的信号区间进行数据处理,按指数方程进行深度分馏校正(Paton et al., 2010)。以NIST610作为外标,91Zr作为内标计算微量元素含量。实验过程中测定的91500(1061.5±3.2Ma,2σ)、GJ-1(604±6Ma,2σ)年龄在不确定范围内与推荐值一致。Plesovice标样作为未知样品的分析结果为337.5±1.5Ma(n=11,2σ),对应的年龄推荐值(337.13±0.37Ma,2σ)在不确定范围内与推荐值一致。
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锆石Hf同位素分析利用激光剥蚀多接收等离子体质谱(LA-MC-ICP-MS)在中国地质调查局成都地质调查中心完成。激光剥蚀系统为Geolas (Coherent), MC-ICP-MS为Neptune Plus(Thermo Fisher Scientific)。激光输出能量密度为~7.0J/cm2。采用单点斑束44 μm剥蚀模式,操作条件及分析方法见Hu Zhaochu et al.(2012)。分析采用的两个国际锆石标准与实际样品同时分析,其测试值与推荐值在误差范围内一致。后期数据分析处理采用ICPMSDataCal(Liu Yongsheng et al., 2010)软件完成。
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3 测试结果
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两个样品的阴极发光图片显示(图4),锆石多呈透明—半透明,晶体呈柱状,近等轴状,且具有岩浆环带,少数具有暗色增生边,部分锆石有一定的磨蚀现象,大部具有较好的晶形,岩浆锆石特征明显。
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在样品PM003-5TW中挑选锆石40颗,剔除谐和度小于90%的数据2个,其余38个数据谐和度在92%~108%间,206Pb/238U锆石年龄分析值在264.3±4.9~254.7±3.3Ma之间(表1),年龄加权平均值为258.9±0.7Ma(MSWD=0.98)(图5a、b)。锆石微量元素分析结果见表2。Y含量为730×10-6~5030×10-6,Hf含量为7100×10-6~9630×10-6,Th含量为26.1×10-6~533×10-6,U含量为41.4×10-6~522×10-6,Th/U比值为0.42~0.94,Th/Nb比值为2.53~13.05。从38颗锆石中挑选9颗进行微区Lu-Hf同位素分析(表3)。εHf(t)值为3.0~5.0,二阶段Hf模式年龄为988~871Ma。
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在样品PM003-56TW中挑选锆石40颗,剔除谐和度小于90%的数据9个,其余31个数据谐和度在94%~105%之间, 206Pb/238U锆石年龄分析值在257.6±5.2~250.7±3.4Ma之间(表1),年龄加权平均值为254.9±0.6Ma(MSWD=1.00)(图5c、d)。锆石微量元素分析结果见表2。Y含量为645×10-6~3014×10-6,Hf含量为7950×10-6~11610×10-6,Th含量为47.8×10-6~366×10-6,U含量为82.9×10-6~966×10-6,Th/U比值为0.54~1.06,Th/Nb比值为36.50~311.85。从31颗锆石中挑选6颗进行微区Lu-Hf同位素分析(表3)。εHf(t)值为-7.4~-6.9,二阶段Hf模式年龄为1573~1529Ma。
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图4 永善地区晚二叠世宣威组凝灰岩样品PM003-5TW(a)和PM003-56TW(b)部分锆石阴极发光图形
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Fig.4 Cathodoluminescence images of some zircons from samples PM003-5TW (a) and PM003-56TW (b) of Late Permian tuff in Yongshan area
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实圈为锆石U-Pb年龄分析点,虚圈为锆石Hf同位素分析点
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The solid circle is the zircon U-Pb age analysis point, and the imaginary circle is the zircon Hf isotope analysis point
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4 讨论
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4.1 凝灰岩成因
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关于二叠系—三叠系之交的火山灰,前人已做了大量研究,Li Xia (2015)总结前人成果得出长英质火山灰产于宣威组上部、碱性火山灰产于宣威组下部,而镁铁质火山灰则产于晚二叠世不同地区的不同层位。而现有争议较大的是关于火山作用的构造环境及背景的讨论(Yang Jianghai et al., 2012; He Bin et al.,2014; Liao Zhiwei et al.,2016; Hong Hanlie et al.,2017),锆石的地球化学特征为其母岩浆成分提供了灵敏的潜在信息,对构造岩浆物源的指示是有效的(Grimes et al.,2015)。
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对PM003-5TW、PM003-56TW 2个样品69个锆石微量元素分析显示(表2),所有锆石颗粒均具有亏损轻稀土元素,逐步富集重稀土元素且显示Ce正异常、Eu负异常,为典型的未变质岩浆锆石特征(图6)。Th/U比值分别为0.42~0.94、0.54~1.06,相对较低的Th/U比值反映出地幔锆石特征。Y、Hf含量表现出下部低、上部高的特点,PM003-56TW样品Hf含量普遍大于9000×10-6,反应出长英质岩浆高度演化的特点(Claiborne et al., 2010)。在Y-Hf岩性判别图中两个样品均落入含石英中酸性岩中,但PM003-5TW具有偏碱性交代岩的亲属性(图7a);在Y2O3-HfO2 图解中这种特征更为明显,显示出PM003-5TW偏碱性、PM003-56TW偏亚碱性的特征(图7b)。
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锆石中不相容元素地球化学特征可为岩浆源区提供潜在信息。岩浆弧锆石的Nb含量相对于板块内环境亏损,因此岩浆弧锆石的Nb/Hf含量较低,Th/Nb比值较高(Yang Jianghai et al., 2012)。板内岩浆则由于陆壳混染会增加Th/Nb比值、降低Hf/Th比值,岩浆弧背景下含Th矿物先结晶,导致岩浆具有低的Th/Nb比值及高Hf/Th比值(Yang Jianghai et al., 2012)。Th/Nb-Hf/Th及Th/U-Nb/Hf双变量图解均较好地显示了PM003-5TW落于板内(后碰撞)背景、PM003-5TW落于弧相关环境(图8)。此外,在Nb/Yb-U/Yb图解中也较好显示出PM003-5TW具有与峨眉山玄武岩相似的洋岛型(OIB)特征,PM003-56TW则显示出岩浆弧相关特征(图9)。
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续表1
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图5 永善地区晚二叠世宣威组凝灰岩PM003-5TW(a、b)、PM003-56TW(c、d)锆石U-Pb年龄谐和图
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Fig.5 Zircon U-Pb age concordance diagram of samples PM003-5TW (a, b) and PM003-56TW (c, d) of Late Permian tuff in Yongshan area
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续表2
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注:εHf(0)为现今εHf值;εHf(t)为年龄t对应的εHf值;fLu/Hf为Hf富集系数。
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Huang Hu et al.(2018)对川东广元市上寺剖面、朝天剖面,大理宾川剖面长英质火山灰进行了详细的地球化学、锆石微量元素及Hf同位素的对比研究,认为具有较高的εHf(t)值与较低的锆石Th/Nb值为特征的火山灰代表了峨眉山玄武岩后期酸性火山作用的产物,而较低的εHf(t)值与较高的锆石Th/Nb值为特征的火山灰则代表了古特提斯岩浆弧作用产物。Zhong Yuting et al.(2020)也得出了相似的结论,并认为峨眉山长英质火山作用的消退持续到约257.4Ma。
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本文报道的两层凝灰岩在锆石Th/Nb值及Hf同位素上也反应出了明显的差异特征(图10a、b)。下部凝灰岩(PM003-5TW)显示较高的εHf(t)值(3.0~5.0)和较低的Th/Nb比值(2.53~13.05),二阶段Hf模式年龄988~871Ma。表明岩浆来源于地幔源的熔融。此外,本次研究于峨眉山玄武岩内获得的两层凝灰岩(PM027-2TW、PM033-22TW)也表现出了较高的εHf(t)值(6.4~8.9、3.0~8.8),较低的Th/Nb比值(4.53~31.63、3.45~40.08),二阶段Hf模式年龄分别为660~799Ma、665~986Ma(未发表)❶。相似的锆石微量元素特征、Hf同位素组成及模式年龄特征暗示了宣威组下部凝灰岩为峨眉山玄武岩后期酸性火山活动的成因(图10)。
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图6 永善地区晚二叠世宣威组凝灰岩样品锆石球粒陨石标准化REE配分曲线图(球粒陨石据Sun et al.,1989)
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Fig.6 Chondrite normalized REE patterns of Late Permian tuff in Yongshan area (chondrite data from Sun et al.,1989)
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上部凝灰岩(PM003-56TW)具有较低的εHf(t)值(-7.4~-6.9)和较高的Th/Nb比值(36.50~311.85),二阶段Hf模式年龄为1573~1529Ma,暗示岩浆的地壳物质来源。宋马-哀牢山-金沙江构造带保存了大量247~246Ma的流纹岩(Zi Jianwei et al.,2012)、251.6~247.5Ma的花岗岩(Liu Huichuan et al.,2015),为印支板块与华南板块之间的俯冲碰撞的岩浆弧产物。Liu Huichuan et al.(2015)于新安寨岩体、通天阁岩体获得的锆石εHf(t)值分别为-11.1~-3.1、-11.9~-3.4,二阶段Hf模式年龄分别为2000~1470Ma、2030~1490Ma。相同的锆石微量元素特征、Hf同位素组成及模式年龄特征,反映了宣威组上部凝灰岩来源于古特提斯岩浆弧,位置可能为宋马-哀牢山-金沙江构造带(图10)。
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4.2 峨眉山火山作用结束时间及古特提斯岩浆弧作用开始时间的约束
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峨眉山玄武岩的喷发、持续时限一直是个悬而未决的问题,目前,大部分学者认为峨眉山玄武岩主喷发时间应为~260Ma(Ali et al., 2002; Zhou Meifu et al., 2002; Xiao Long et al., 2003; Zhong Yuting et al., 2014; Huang Hu et al., 2016),生物地层及磁性地层的证据均显示其持续时间应该在1~3Ma范围内(Sun Yadong et al., 2010; Liu Chengying et al., 2012)。Zhu Jiang et al.(2021)提出ELIP大部分玄武岩喷发在约260.1~259.5Ma之间;其间的海底火山活动导致了海洋酸化,其后在约257.9Ma至约256.9Ma之间发生了凝灰岩的爆发。Zhong Yuting et al.(2014)在宾川剖面峨眉山玄武岩顶部酸性喷出岩中获得了CA-TIMS年龄259.1±0.5Ma。Yang Jianghai et al.(2018)于贵州普安峨眉山玄武岩顶部凝灰岩获得年龄259.51±0.21Ma。这些高精度年龄无疑对峨眉山地幔柱火山活动的结束时间起到了很好的限定作用。然而,峨眉山玄武岩序列上部的酸性火山岩可能被剥蚀并沉积在相邻的晚二叠世地层中(He Bin et al.,2007; Yang Jianghai et al.,2015;Dai Shifeng et al.,2016),这暗示了现有的峨眉山玄武岩顶部凝灰岩年龄数据可能并不能代表峨眉山地幔柱火山活动的结束时间。
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图7 永善地区晚二叠世宣威组凝灰岩锆石微量元素判别火山岩岩性
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Fig.7 Discrimination of volcanic rock lithology by zircon trace elements of Late Permian tuff in Yongshan area
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(a)—Y-Hf图解(据Shnukov et al.,1997; Belousova et al.,2002):Ⅰ—金伯利岩;Ⅱ—超镁铁质、镁铁质和中性岩;Ⅲ—含石英的中酸性岩;Ⅳ—具有高SiO2含量的酸性岩;Ⅴ—云英岩;Ⅵ—碱性杂岩中的碱性岩和碱性交代岩;Ⅶ—碳酸盐;(b)—HfO2-Y2O3图解(据Pupin, 2000):1a—拉斑质斜长花岗岩;1b-c-d-e—固溶线上碱性花岗岩/流纹岩;1c-d-e—碱性/过碱性正长岩/粗面岩;1c—夏威夷岩和碱性玄武岩;1e、2、3a-b-c、4a-b-c—固溶线下碱性花岗岩/流纹岩;4a-b-c、5a-b-c、6a-b—中基性钙碱性岩石;5a-b-c—钙碱性花岗岩/流纹岩;4a-b、5a-b-c—高钾钙碱性或Mg-K花岗岩/流纹岩;4c、5a-b-c—亚碱性或Fe-K花岗岩/流纹岩;3b-c、4b-c、5b-c、6a-b—过铝质斑状花岗岩/流纹岩;3c、4c、5c、6a—过铝质淡色花岗岩;3c、4c、5c、6a—原地产生的过铝质淡色花岗岩;红色箭头指示从碱性—亚碱性—铝质/过铝质的变化趋势
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(a)—Y-Hf diagram (after Shnukov et al., 1997; Belousova et al., 2002);Ⅰ—kimberlite;Ⅱ—ultramafic, mafic and neutral rocks;Ⅲ—quartz bearing intermediate acid rocks;Ⅳ—acid rocks with high SiO2 content;Ⅴ—greisen;Ⅵ—alkaline rocks and alkaline metasomatic rocks in alkaline complexes;Ⅶ—carbonate.(b)—HfO2-Y2O3 diagram (after Pupin, 2000): 1a—tholeiitic plagiogranite;1b-c-d-e—above solution line alkaline granite/rhyolite;1c-d-e—alkaline/peralkaline syenite/trachyte;1c—hawaiite and alkaline basalts;1e, 2, 3a-b-c, 4a-b-c—below solution line alkaline granite/rhyolite;4a-b-c, 5a-b-c, 6a-b—medium basic calc alkaline rocks;5a-b-c—calc alkaline granite/rhyolite;4a-b, 5a-b-c—high K calc alkaline or Mg-K granite/rhyolite;4c, 5a-b-c—subalkaline or Fe-K granite/rhyolite;3b-c, 4b-c, 5b-c, 6a-b—peraluminous porphyry granite/rhyolite;3c, 4c, 5c, 6a—peraluminous leucogranite;3c, 4c, 5c, 6a—in situ peraluminous leucogranite; the red arrow indicates the change trend from alkaline to subalkaline to aluminous/peraluminous
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图8 永善地区晚二叠世宣威组凝灰岩锆石微量元素判别构造背景
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Fig.8 Discrimination of tectonic setting by zircon trace elements of Late Permian tuff in Yongshan area
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(a)—Th/Nb-Hf/Th图解;(b)—Th/U-Nb/Hf图解(底图据Yang Jianghai et al., 2012); 猫耳山、朝天数据据Zhong Yuting et al., 2020;上寺数据据Huang Hu et al., 2018;煤山数据据Wang Xiangdong et al., 2019
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(a)—Th/Nb-Hf/Th diagram; (b)—Th/U-Nb/Hf diagram (background image from Yang Jianghai et al., 2012); data of Maoershan and Chaotian after Zhong Yuting et al., 2020; data of Shangsi after Huang Hu et al., 2018; data of Meishan after Wang Xiangdong et al., 2019
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图9 永善地区晚二叠世宣威组凝灰岩锆石微量元素Nb/Yb-U/Yb判别构造背景(底图据Grimes et al.,2015)
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Fig.9 Discrimination of tectonic setting by zircon trace elements Nb/Yb-U/Yb of Late Permian tuff in Yongshan area (background image from Grimes et al.,2015)
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He Bin et al.(2007)认为宣威组底部碎屑锆石来源于ELIP顶部的长英质组分,两者及中晚二叠世边界的黏土岩(不是空降酸性凝灰岩,而是ELIP风化剥蚀沉积的碎屑岩)在一个等时线上,并分别于贵州威宁、四川峨眉宣威组底部碎屑岩中获得257±4Ma、260±5Ma年龄,用以限定峨眉山玄武岩的喷发结束时间;He Binghui (2015)在会泽县者海镇宣威组下部砂岩中获得了252.4±4.14Ma的最新年龄。显然,这些数据的差距较大,且精度也是不够的,一方面可能是受地质记录出露情况所致,并未采集到记录宣威组最底部沉积层位,另一方面可能是由于所获锆石为捕获成因,不一定来自峨眉山玄武岩,此外,样品本身出现的混样、锆石数目不足、数据处理欠佳等也可能导致数据的偏差。
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图10 永善地区晚二叠世凝灰岩锆石εHf(t)值(a)与微量元素Th/Nb值(b)分布特征
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Fig.10 Distribution characteristics of εHf(t) (a) and Th/Nb (b) values of zircons from Late Permian tuff in Yongshan area
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底图据Zhong Yuting et al., 2020;猫耳山、朝天数据据Zhong Yuting et al., 2020;上寺数据据Huang Hu et al., 2018;煤山数据据Wang Xiangdong et al., 2019;峨眉山大火成岩省数据据Xu Yigang et al., 2008;新安寨岩体、通天阁岩体数据据Liu Huichuan et al., 2015
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Background image from Zhong Yuting et al., 2020;data of Maoershan and Chaotian after Zhong Yuting et al., 2020;data of Shangsi after Huang Hu et al., 2018;data of Meishan after Wang Xiangdong et al., 2019;data of Emeishan Large Igneous Province after Xu Yigang et al., 2008;data of Xin'anzhai pluton and Tongtiange pluton after Liu Huichuan et al., 2015
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晚二叠世凝灰岩的成因争议主要集中在峨眉山酸性火山作用成因(Huang Hu et al.,2016)与古特提斯大陆岩浆弧成因(Yang Jianghai et al.,2012; Cao Qiuling et al.,2013; Liao Zhiwei et al.,2016; Wang Man et al.,2018)。西伯利亚大火成岩省基性火山作用一度也被认为是华南二叠系—三叠系之交长英质凝灰岩的成因(Xu Lin et al.,2007)。然而,西伯利亚大火成岩省长英质火山作用较少,全球P—Tr剖面统计显示火山灰普遍分布于古特提斯区域(Wang Xiangdong et al.,2019)。此外,目前发现的西伯利亚长英质火山灰年龄晚于华南凝灰岩沉积时间(Burgess et al.,2015)。这些,都暗示了西伯利亚大火成岩省不可能是造成华南二叠系乐平统大量长英质凝灰岩的成因。Huang Hu et al.(2018)、Zhang Han et al.(2020)从Hf同位素及锆石微量元素特征的角度,认为峨眉山玄武岩后期酸性火山作用持续到约257.4Ma、260.1±2.8Ma。但他们都未对古特提斯弧火山作用起始时间做出限定。
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在本次研究的永善莲峰剖面上,第2层为溢流相致密状玄武岩后连续沉降的凝灰岩,代表了峨眉山玄武岩主喷期空落相产物。3层的铝土岩暗示了峨眉山玄武岩顶部熔结凝灰岩去顶而诱发的风化、搬运和改造作用的开始(Meng Changzhong et al.,2015),平行不整合接触的证据还包括研究区南部可见的宣威组底砾岩。PM003-5TW(5层)为宣威组下部最后一层火山灰,锆石微量元素显示其原岩为板内背景下具有OIB型亲缘性的中酸性岩,结合锆石微量元素、Hf同位素及模式年龄特征对比,认为其代表了峨眉山玄武岩后期酸性火山作用的产物,约束了研究区峨眉山玄武岩火山活动持续到了258.9±0.7Ma。PM003-56TW(56层)凝灰岩锆石微量元素则反映出弧相关背景下长英质火山作用,相似的锆石微量元素、Hf同位素组成和模式年龄分布特征暗示了其古特提斯弧火山作用产物的来源,位置可能为宋马-哀牢山-金沙江构造带。考虑到研究剖面在56层之前还存在小规模的凝灰岩层,笔者认为古特提斯相关弧火山作用至少于254.9±0.6Ma就已对研究区产生影响。
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5 结论
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(1)研究区晚二叠世宣威组下部凝灰岩锆石表现出较高的εHf(t)值与较低的Th/Nb值;锆石微量元素及Hf同位素特征暗示其为峨眉山玄武岩后期酸性火山作用成因;研究区峨眉山火山活动至少持续到了258.9±0.7Ma。
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(2)晚二叠世宣威组上部凝灰岩锆石表现出较低的εHf(t)值与较高的Th/Nb值;锆石微量元素及Hf同位素特征暗示其为古特提斯弧火山作用成因,可能来源于宋马-哀牢山-金沙江构造带;认为古特提斯相关弧火山作用至少于254.9±0.6Ma就已启动。
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注释
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❶ 吴亮, 等.乌蒙山岩溶石山区昭通—毕节地区区域地质调查.未发表.
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摘要
扬子西缘二叠系—三叠系之交凝灰岩的研究对探讨峨眉山大火成岩省及古特提斯周缘岩浆弧火山活动的时限约束具有重要意义。本次工作在上扬子西缘永善地区晚二叠世宣威组中发现多层凝灰岩。对其底部和顶部凝灰岩进行LA-MC-ICP-MS锆石U-Pb测年和Hf同位素研究显示:底部凝灰岩加权平均年龄为258.9±0.7 Ma,锆石Th/Nb比值为2.53~13.05,εHf(t )值为3.0~5.0,TDM2为988~871 Ma;锆石微量元素、Hf同位素及二阶模式年龄显示其具有与峨眉山玄武岩相关的特征,为峨眉山大火成岩省后期产物。顶部凝灰岩加权平均年龄为254.9±0.6 Ma,锆石Th/Nb比值为36.50~311.85,εHf(t )值为-7.4~-6.9,TDM2为1573~1529 Ma;锆石微量元素、Hf同位素及二阶模式年龄特征表明其形成于宋马-哀牢山-金沙江构造带弧火山活动。研究表明:宣威组下部凝灰岩为峨眉山大火成岩省后期酸性火山活动的产物,研究区峨眉山火山活动至少持续到了258.9±0.7 Ma。古特提斯弧火山作用至少于254.9±0.6 Ma就已启动。
Abstract
The tuff at the intersection of Permian-Triassic in the western margin of Yangtze is of great significance in constraining the chronology of volcanic activity in the Emeishan Large Igneous Province and the magmatic arc around the Paleotethys. In this work, multi-layer tuff was found in the Late Permian Xuanwei Formation in the Yongshan area, western margin of Upper Yangtze. LA-MC-ICP-MS zircon U-Pb dating and Hf isotope study of the tuff at the bottom and top show that the weighted average age of the tuff at the bottom is 258.9±0.7 Ma, the zircon Th/Nb ratio is 2.53~13.05, the εHf(t ) value is 3.0~5.0, and the TDM2 value is 988~871 Ma. Zircon trace elements, Hf isotope and two-stage model ages (TDM2) shows that its characteristics are related to the Emeishan basalt, and that it is a late product of the ELIP. LA-MC-ICP-MS zircon U-Pb dating and Hf isotope study of the tuff at the bottom and top show that the weighted average age of the tuff at the top is 254.9±0.6 Ma, the zircon Th/Nb ratio is 36.50~311.85, the εHf(t ) value is -7.4~-6.9, and the TDM2 value is 1573~1529 Ma. Zircon trace elements, Hf isotope and two-stage model ages (TDM2) indicate that it was formed during arc volcanic activity in the Songma-Ailaoshan-Jinshajiang suture zone. The results show that the tuff at the bottom of the Xuanwei Formation is a product of acid volcanic activity in the later stage of the Emeishan Igneous Province. The volcanic activity of the Emeishan in the study area lasted until 258.9±0.7 Ma. Paleotethyan arc volcanism started at 254.9±0.6 Ma or earlier.
Keywords
Emeishan basalt ； Xuanwei Formation ； tuff ； zircon U-Pb dating ； Hf isotope
