地球科学  2018, Vol. 43 Issue (4): 1164-1182.   PDF    
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东昆仑古特提斯构造带中的原特提斯记录:来自苦海镁铁质岩块的证据
张航1,2, 王宗起1, 马昌前2,3, 熊富浩4, 蒋红安5, 郭宇衡6     
1. 中国地质科学院矿产资源研究所, 北京 100037;
2. 中国地质大学地球科学学院, 湖北武汉 430074;
3. 中国地质大学地质过程与矿产资源国家重点实验室, 湖北武汉 430074;
4. 成都理工大学地球科学学院, 四川成都 610059;
5. 核工业二三〇研究所, 湖南长沙 410011;
6. 四川省国土勘测规划研究院, 四川成都 610000
摘要:研究苦海地区是否发育原特提斯洋盆有助于揭示阿尼玛卿构造带和秦岭勉略构造带是否具有相似的演化过程,确定"秦-祁-昆"原特提斯洋的南界.产出于苦海地区的镁铁质岩构造就位于上古生界强片理化浊积岩中,显示出混杂特征.LA-ICP-MS锆石年代学研究显示该地区雪穷辉长岩块结晶年龄为610~630 Ma,雅日玄武安山岩结晶年龄约为504 Ma.微量元素和同位素地球化学分析表明,雪穷镁铁质岩浆起源于大陆岩石圈地幔,未受到地壳混染,形成于板内环境,雅日镁铁质岩浆起源于受俯冲沉积物产生的流体交代的地幔楔,源区受到一定地壳混染,形成于俯冲环境,反映秦昆结合地区存在早古生代大洋的俯冲-增生作用.结合区域资料,阿尼玛卿构造带和勉略构造带都有新元古代晚期至早古生代的大洋物质记录,表明这一时期阿尼玛卿和勉略构造带有可对比性,原特提斯洋的南界达到阿尼玛卿和勉略构造带.
关键词苦海群    混杂带    锆石U-Pb年代学    地球化学    
Proto-Tethys Record in Paleo-Tethys Belt of East Kunlun: Evidence from Kuhai Mafic Blocks
Zhang Hang1,2 , Wang Zongqi1 , Ma Changqian2,3 , Xiong Fuhao4 , Jiang Hong'an5 , Guo Yuheng6     
1. Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China;
3. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;
4. College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China;
5. Changsha Uranium Geology Research Institute No. 230, CNNC, Changsha 410011, China;
6. Sichuan Institute of Land Planning and Survey, Chengdu 610000, China
Abstract: The junction of the East Kunlun orogen and the West Qinling orogen is a key area for contrasting the tectonic evolution of the East Kunlun orogen with West Qinling orogen and determining the south margin of "Qinling-Qilian-Kunlun" proto-Tethys ocean. Kuhai mafic rocks, situated in the junction, were structurally placed into a Late Paleozoic foliated turbidites, indicating typical characteristics of mélange. The LA-ICP-MS zircon U-Pb dating indicates that the gabbros in Xueqiong were crystallized at 610-630 Ma, and the mafic rocks in Yari were crystallized with an age of about 504 Ma. Geochemical and isotopic studies show that Xueqiong gabbros were derived from subcontinent lithosphere mantle in a rift setting. Yari mafic rocks were derived from enrichment mantle wedge modified by subduction metasomatic fluids in subduction, representing the existence of the ocean. According to regional data analysis, the situation that proto-tethys materials mixed in paleo-tethys mélange belt may demonstrates a subduction-accretion process occurred in the junction of the East Kunlun orogen and the West Qinling orogen. From Late Neoproterozoic to the Early Palaeozoic, A'nyemaqen ocean had similar tectonic evolution with Mianlüe ocean. The margin of proto-Tethys ocean reached A'nyemaqen and Mianlüe mélange.
Key Words: Kuhai Group    mélange    zircon U-Pb chronology    geochemistry    

0 引言

混杂带(mélange)通常用于描述一套层序破坏、结构混乱的地质单元,具有“岩块嵌入富泥、砂质或蛇纹岩质基质(blocks-in-matrix)”结构(Cowan, 1985),广泛出露于世界各增生造山带和碰撞造山带中,保留有造山带构造演化的第一手记录(Wakabayashi, 2012).组成岩块既有俯冲形成的变质岩和俯冲板片地壳等俯冲带物质(Bailey et al., 1964; Moore and Karig, 1980; Hamilton, 1988),也有受构造、重力作用混入的大洋物质(如深海硅质岩、海山岩石等)(Bailey et al., 1964; Hsü, 1971),岩块的形成时代可以远远早于基质(Shervais et al., 2011).正确地识别和划分混杂带及其单元,是认识造山带的重要途径.

昆仑-秦岭-大别山中央造山带近东西向横亘于中国中部,是一条重要的地理、地质分界线,也是一个典型的“复合造山带”.苦海地区位于东昆仑南缘,是昆仑、秦岭衔接的重要地区(图 1a),区域内地质单元组成复杂.1956年,张文佑、王鸿祯等人将区域内中深变质岩命名为“苦海群”,其组成成分为片麻岩与砂泥质片岩(青海省地质调查院, 2001, 兴海幅1:25万区域地质调查报告).许多学者将苦海群作为一个结晶基底,争议在于苦海群与昆北金水口群是同一基底(孙崇仁, 1997; Liu et al., 2005; 李瑞保等, 2016),还是不同基底(姜春发等,1992; 许志琴等,1996; 王国灿等,2004; 刘强等,2016).然而,杜光辉等(1982)认为砂泥质片岩与片麻岩属于两个不同的构造层位,“下伏”砂泥质片岩新于“上覆”片麻岩,砂泥质片岩为下二叠系.近年来,许多学者认为原苦海群是一套晚古生代的非史密斯地层,命名为苦海-兴海构造混杂带(王秉璋等,2000a; 祁生胜等,2001; 祁生胜和宋泰钟,2002).原苦海群中发育的镁铁-超镁铁质岩石研究也较弱,早期工作将这些镁铁质岩石分别划为岩脉和地层单元,而王秉章等(2000b)认为其是泥盆纪至二叠纪蛇绿岩.对苦海地区不同的认识及年代学问题制约着研究者对“秦-昆”造山系地质构造格局与演化的认识.

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图 1 秦昆结合部地质简图 Fig. 1 Geological sketch map of the junctional zone of Qinling and Kunlun 闫臻等(2012)修改.新元古代晚期-早古生代岩浆年龄数据: 1.亿可哈拉尔沟花岗-英云闪长岩(边千韬等,2007); 2.得力斯坦蛇绿岩(刘战庆等, 2011a, 2011b); 3.白日切特流纹岩、花岗闪长岩(刘战庆等,2011b); 4.玛积雪山辉长岩(李王晔,2008); 5.德尔尼闪长岩(李王晔等,2007); 6.苦海辉长岩(李王晔等,2007); 7.龙通英云闪长岩(张智勇等,2005); 8.呼若合石英闪长岩(张智勇等,2005); 9.雅日花岗闪长岩(张智勇等,2005); 10.扎那和二长花岗岩(张智勇等,2005)

苦海、雅日地区出露一套辉长岩和玄武安山岩为主的镁铁质岩石, 裹夹于由强片理化砂泥质碎屑岩组成的基质中,具有“block-in-matrix”结构,显示出混杂带特征.本文选取雪穷辉长岩、雅日玄武安山岩为研究对象,开展岩石学、锆石U-Pb年代学和岩石地球化学等工作,分析其岩石成因,为进一步探讨东昆仑南缘构造演化提供新的资料和认识.

1 区域地质背景和镁铁质岩地质特征

研究区位于青海省兴海县西侧苦海地区(图 1b),西侧为东昆仑造山带,东侧为西秦岭造山带.该地区主要出露一套遭受低绿片岩相韧性剪切变形的泥砂质浊积岩,形成于石炭纪至二叠纪(王秉璋等,2000b).其次为片麻岩,主要为苦海片麻杂岩和沙乃亥副片麻岩,前者为变质的中酸性杂岩体,在平面上呈不规则断块状; 后者组合的原岩建造以富铝泥质、杂砂质沉积碎屑岩为主体,夹中基性火山岩、碳酸盐岩建造,均被认为形成于元古代(Liu et al., 2005).本文研究的镁铁质岩石均出露于上古生界强片理化长石杂砂岩中(图 2).

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图 2 苦海地区雪穷(a)和雅日(b)附近地质简图 Fig. 2 Geological sketch map of Xueqiong (a) and Yari (b) in Kuhai area 据青海省地质调查院(2001)编兴海幅1:25万区域地质调查报告

苦海西岸雪穷山附近,有多处辉长岩出露于变长石杂砂岩中,具有混杂特征(图 3),矿物有弱定向性(图 4a),两者接触界限附近均为劈理化带(图 4b).辉长岩主体呈透镜状,局部片理化,产状大约为150°∠30°.从北向南,辉长岩岩块规模依次增大,粒度依次递增,倾角主体为30°~40°,南侧两中粒辉长岩岩块近乎垂直.本文分别对6处透镜辉长岩进行采样,并按矿物颗粒大小分为雪穷细粒辉长岩和中粒辉长岩.

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图 3 雪穷野外剖面 Fig. 3 Geological section of Xueqiong 据青海省地质调查院(2001)编兴海幅1:25万区域地质调查报告
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图 4 苦海地区雪穷辉长岩及雅日玄武安山岩野外照片(a~d)和正交偏光镜下照片(e~h) Fig. 4 Field photos (a-d) and microphotographs (e-h) of Xueqiong gabbros and Yari basaltic andesites a.雪穷中粒辉长岩,辉石堆晶明显,有一定的矿物定向; b.雪穷细粒辉长岩,边部片理化发育,与强变形长石杂砂岩呈构造接触; c.雅日细斑晶玄武安山岩,夹有层状凝灰岩; d.雅日粗斑晶玄武安山岩,块状构造,可见斜长石斑晶; e.雪穷细粒辉长岩,蚀变强烈,斜长石、辉石分别被绢云母、阳起石取代; f.雪穷中粒辉长岩,暗色矿物颗粒明显,有堆晶结构; g.雅日细斑晶玄武安山岩,斑晶多为斜长石,自形程度较好,基质间隐结构,有流动定向; h.雅日粗斑晶玄武安山岩,风化强烈,但可以看到斑晶自形程度较好,基质具有间粒结构.Act.阳起石; Pl.斜长石; Spn.榍石; Ep.绿帘石; Chl.绿泥石

在雅日山附近,出露岩石主体为玄武安山岩,整体为块状构造,局部夹有层状含火山角砾凝灰岩(图 4c),以及硅泥质板岩; 主体玄武安山岩颗粒极细,仅可见少量斑晶,以斜长石为主.在雅日山顶附近,玄武安山岩斑晶含量增多,粒度变粗,暗色矿物含量增多(图 4d),结晶程度略高于其他部位.本文按斑晶特征,将雅日玄武安山岩分为细斑晶单元和粗斑晶单元,并分别采样.岩石整体脆性破碎发育强烈.据野外观察和地质资料(青海省地质调查院, 2001, 兴海幅1:25万区域地质调查报告),雅日玄武安山岩整体与围岩呈构造接触,呈近北东-南西弧形展布的逆冲型叠瓦状构造岩片产出(王秉璋等,2000b).因此,苦海地区这些镁铁质岩石均为构造岩块,而非侵入体.

雪穷细粒辉长岩共4处,蚀变严重,主要矿物为斜长石(50%~55%)、辉石(40%~45%),副矿物有榍石(3%~5%); 辉石粒度大约为1 mm,多被阳起石取代,呈假象; 斜长石粒度约0.2~0.5 mm,几乎全部蚀变为钠长石、绢云母、绿帘石,仅保留晶形.局部石英脉穿入(图 4e).中粒辉长岩共2处,主要矿物为斜长石(60%~65%)、辉石(30%~35%),副矿物有榍石(5%)、磁铁矿(3%),斜长石0.3 mm×1.0 mm左右,辉石粒度约2~3 mm; 蚀变强烈,辉石多阳起石化,斜长石几乎全部发生绢云母化、黝帘石化(图 4f).

雅日地区细斑晶玄武安山岩具有斑状结构,斑晶/基质大约为35%,斑晶主体几乎全部为斜长石,斜长石斑晶0.1 mm左右,自形程度较好,绢云母化强烈; 基质为间隐结构,粒度小于0.05 mm; 有较明显的流动定向(图 4g); 暗色矿物绿泥石化强烈.粗斑晶玄武安山岩单元具有斑状结构,斑晶含量约20%~25%,斑晶粒度大约在0.2~0.3 mm,大者可达0.5 mm,主要矿物为斜长石(80%~85%)、角闪石(15%~20%),晶形较好; 基质为间粒结构,粒度约0.1 mm左右,主要为斜长石(70%~75%)、角闪石(20%~25%)、黑云母(5%左右),整体蚀变强烈(图 4h).

2 样品处理与测试方法

锆石年代学分析的样品均采自新鲜露头,去除表面风化层,经粗碎、淘洗、磁选等分离方法分选锆石,在双目镜下仔细选择不同晶形、不同颜色的锆石颗粒,粘在双面胶上并用环氧树脂固定,待环氧树脂充分固化后,将锆石靶表面抛光,然后进行锆石颗粒内部结构分析(反射光和透射光、阴极发光显微图像研究),最后进行锆石LA-ICP-MS分析.LA-ICP-MS锆石U-Pb年代学分析在中国地质大学(武汉)地质过程与矿产资源国家重点实验室(GPMR)完成.系统为GeoLas 2005,采用Agilent 7500a ICP-MS仪器与装配有193 nm气体激光的GeoLas 2005激光剥蚀系统联机进行,使用He作为剥蚀物质的载气,氩气作为补偿气进行灵敏度的调节,激光束斑为32 μm.利用锆石标样GJ-1为外标对同位素分馏进行校正,采用国际标准锆石91500外部校正法进行锆石分析.微量元素含量采用国际标样NIST 610作为外标、29Si作为内标元素进行校正.数据处理利用ICPMSDataCal(Liu et al., 2008, 2010)完成,U-Pb年龄谐和图及加权平均年龄的计算与绘制采用3.23版本的Isoplot程序计算,详细的分析程序见文献(Zong et al., 2010).原位微区锆石Hf同位素比值测试在GPMR利用LA-MC-ICP-MS完成.MC-ICP-MS为Neptune Plus,激光剥蚀系统为GeoLas 2005的193 nm激光,采用单点剥蚀模式,束斑固定为44 μm.详细仪器操作条件和分析方法可参照Hu et al.(2012)分析数据的离线处理采用软件ICPMSDataCal(Liu et al., 2009)完成.

全岩主量、微量元素分析的样品均为200目以下的新鲜粉末样.有5件主微量样品(13MD08一件、13MD09两件、13XH05两件)在GPMR完成,12件主微量样品(14MD03两件、14MD04两件、14MD05两件、14MD06两件、14XH04四件)在澳实分析检测(广州)有限公司进行.全岩主量元素测试利用X射线荧光光谱仪(XRF)对熔融玻璃片进行分析.全岩烧失量由干燥岩石粉末样品的测试获得.全岩微量元素含量利用Agilent 7500a ICP-MS分析完成.用于ICP-MS分析的样品处理如下: (1)称取粉碎至大约200目的岩石粉末50 mg置于Teflon溶样器中; (2)采用Teflon溶样弹将样品用HF + HNO3在195 ℃条件下消解48 h; (3)将在120 ℃条件下蒸干除Si后的样品用2% HNO3稀释2 000倍,定容于干净的聚酯瓶中,详细的样品消解处理过程、分析精度同Liu et al.(2008).

3 锆石U-Pb年代学

本次分别针对雪穷细粒和中粒辉长岩以及雅日粗斑晶玄武安山岩开展锆石U-Pb定年工作.

雪穷中粒辉长岩样品(13MD08-5)共测12个有效点,锆石透明半自形柱状晶形,无色至淡黄色,锆石晶体约50~100 μm,宽/长比为1:1~1:2之间,在阴极发光图像中有较宽的结晶环带(图 5a).得到的数据结果见附表1和附表2.所测锆石具有较高的Th/U值,大约为1.11~2.07,锆石REE含量为2 348.00×10-6~6 679.93×10-6,HREE明显富集,有明显的Ce正异常和Eu负异常(图 5c),具有岩浆锆石的特点,明显不同于变质锆石特征(吴元保和郑永飞,2004),因此此次挑选的锆石均为原生岩浆锆石.12个锆石206Pb/238U年龄结果变化于615±10 Ma至632±5 Ma之间,较为集中,所有分析点均位于谐和曲线上,加权平均年龄为628±3 Ma(图 5b),表明雪穷中粒辉长岩形成于新元古代晚期.

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图 5 雪穷中粒辉长岩(13MD08-5)锆石CL图像(a),U-Pb谐和图、加权平均年龄(b)和锆石REE分布(c) Fig. 5 CL images (a), zircon U-Pb concordia, weighted average age diagram (b) and zircon REE patterns (c) from the Xueqiong medium-grain gabbros (13MD08-5) 图c标准化值据Sun and McDonough(1989)

雪穷细粒辉长岩(14MD06-1)选出锆石很少,共测试6个有效点,锆石呈透明半自形柱状晶形,无色至淡黄色,锆石晶体较小,约为50 μm左右,宽/长比为1:1左右.在阴极发光图像中岩浆振荡环带不明显(图 6a).得到的数据结果见附表3和附表4.所测锆石具有较高的Th/U值,大约为1.1~1.7,锆石REE含量为2 023.68×10-6~7 284.59×10-6,HREE明显富集,有明显的Ce正异常和Eu负异常(图 6c),具有岩浆锆石的特点(吴元保和郑永飞,2004),因此此次挑选的锆石均为原生岩浆锆石,代表原岩结晶时代.6颗锆石206Pb/238U年龄结果变化于600±4 Ma至619±6 Ma之间,相对集中,所有分析点均位于谐和曲线上,加权平均年龄为609±9 Ma(图 6b),代表细粒辉长岩形成时代为新元古代晚期,雪穷辉长岩应是同时期形成.

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图 6 雪穷细粒辉长岩(14MD06-1)锆石CL图像(a)、U-Pb谐和图、加权平均年龄(b)、锆石REE分布图(c) Fig. 6 CL images (a), zircon U-Pb concordia, weighted average age diagram (b) and zircon REE patterns(c) from the Xueqiong fine-grain gabbros(14MD06-1) 图c标准化值据Sun and McDonough(1989)

雅日粗斑晶单元玄武安山岩(13XH05-4)共测试15个有效点,锆石呈透明半自形,无色至淡黄色,锆石晶体约为100 μm,宽/长比为1:2~1:3之间.在阴极发光图像中岩浆振荡环带明显(图 7a).得到的数据结果见附表5、附表6.所分析的锆石Th/U值大约为0.31~0.54,锆石REE含量为368.51×10-6~1 264.39×10-6,HREE相对富集,有明显的Ce正异常,Eu负异常较弱(图 7c),大部分锆石整体振荡环带清楚,Th/U>0.1,应该是岩浆锆石(吴元保和郑永飞,2004).15颗锆石206Pb/238U年龄结果变化于497±9 Ma至516±6 Ma之间,较为集中,所有分析点均位于谐和曲线上,加权平均年龄为504±3 Ma(图 7b),代表雅日玄武安山岩形成时代为寒武纪.

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图 7 雅日粗斑晶玄武安山岩(13XH05-4)锆石CL图像(a)、U-Pb谐和图、加权平均年龄(b)、锆石REE分布图(c) Fig. 7 CL images (a), zircon U-Pb concordia, weighted average age diagram (b) and zircon REE patterns (c) from the Yari basaltic andesites (13XH05-4) 图c标准化值据Sun and McDonough(1989)
4 元素地球化学特征

岩石地球化学分析结果见附表7.可以看出,雪穷辉长岩有5个样品的烧失量小于4.0%,6个样品的烧失量大于4.0%,雅日地区镁铁质岩石6个样品均大于4.0%,说明样品苦海地区各镁铁质岩石整体受到了较强的改造(Lechler and Desilets, 1987),与镜下观察相同.因此,仅有稳定的主量和大部分高场强元素可以用来分析岩石成因.所测镁铁质岩样品各单元的主要氧化物含量相对稳定,但是K2O、Na2O、CaO含量变化较大.

苦海雪穷辉长岩样品具有相对较低的SiO2(43.80%~48.90%),MgO(5.60%~7.83%),MnO(0.15%~0.18%)和Mg#(45.95~53.02),而Al2O3(10.94%~16.30%)含量接近洋岛拉斑玄武岩的Al2O3平均含量(13.45%, Bas, 2000),TiO2含量(1.92%~3.45%)较高,接近板内拉斑玄武岩的TiO2含量(2.23%,Pearce, 1982),整体具有低镁、高钛的特征.雅日玄武安山岩样品的SiO2(48.4%~56.6%)含量分布范围较广,具有较低的MgO(3.64%~4.89%)、MnO(0.12%~0.26%)和Mg#(47.65~53.06)含量,Al2O3(14.88%~17.95%)含量较高,整体具有低镁、高铝的特点.在Nb/Y-Zr/TiO2×0.000 1图解中(图 8a),雪穷辉长岩样品均落入亚碱性玄武岩区域,雅日镁铁质岩样品大部分落入亚碱性玄武岩和玄武岩/安山岩区域边界附近,结合镜下观察及主量元素,斜长石含量较多,多呈板状,SiO2含量偏高,判断雅日镁铁质岩石应属于玄武安山岩系列.在SiO2-FeOT/MgO图中(图 8b),雪穷样品落入拉斑玄武岩系区域,雅日镁铁质岩石整体在钙碱性系列和拉斑玄武岩系列分界附近.

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图 8 苦海镁铁质岩岩石分类命名(a)和SiO2-FeOT/MgO图解(b) Fig. 8 Zr/TiO2×0.000 1-Nb/Y diagram (a) and SiO2-FeOT/MgO diagram (b) of mafic rocks in Kuhai 图a据Winchester and Floyd(1977); 图b据Miyashiro(1975)

雪穷各辉长岩具有相似的稀土元素、微量元素特征.稀土元素总量(∑REE)变化于54.13×10-6~99.16×10-6之间,细粒辉长岩稀土元素总量较低,(∑REE)变化于70.44×10-6~119.83×10-6之间.所有岩石样品的轻重稀土分异明显,样品的LREE/HREE为4.72~5.32,(La/Yb)N为4.75~6.25,轻稀土富集,重稀土亏损,大部分雪穷辉长岩样品具有轻微的Eu正异常(δEu为1.00~1.13),个别样品具有较强的Eu正异常(1.24~1.44),与斜长石堆晶作用有关.原始地幔标准化微量元素蛛网图和稀土配分模式图(图 9a, 9b)表明,雪穷辉长岩样品,Nb、Ta、Zr、Hf等高场强元素(HFSE)具有不同程度的亏损,而大离子亲石元素(LILE)的Ba、U富集.高场强元素Ti呈现较强的正异常,可能与榍石的堆晶作用有关.Pb同时呈现较强的正异常和负异常,可能与它们较强的活动性有关.雅日玄武安山岩稀土元素总量较低,(∑REE)变化于25.85×10-6~48.29×10-6之间.细斑晶玄武安山岩样品的LREE/HREE和(La/Yb)N分别为4.22~5.70和3.81~5.00,粗斑晶玄武安山岩样品的LREE/HREE和(La/Yb)N分别为9.48~10.44和11.02~12.72,显示所有样品轻重稀土分异明显,轻稀土富集,重稀土亏损,Eu异常(δEu为0.86~1.03)不明显.原始地幔标准化微量元素蛛网图和稀土配分模式图(图 9c, 9d)表明,雅日样品Nb、Ta、Zr、Hf和Ti等高场强元素(HFSE)具有明显亏损,富集Rb、Ba、Th、U和Pb等大离子亲石元素(LILE),LREE富集,HREE相对平坦.

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图 9 原始地幔标准化微量元素蛛网图(a, c)和球粒陨石标准化稀土元素配分模式图(b, d) Fig. 9 Primitive mantle-normalized trace element abundances (a, c) and chondrite-normalized REE abundances (b, d) a, b.雪穷辉长岩; c, d.雅日镁铁质岩石辉长岩.图a, c据标准化值据Sun and McDonough(1989); 图b, d据标准化值据Sun and McDonough(1989); Ethiopian裂谷数据来自Shinjo et al.(2011)
5 Hf同位素地球化学特征

本文对雪穷中粒辉长岩除4号锆石点之外的其余共11个已进行年代学测试的样品进行了原位微区Lu-Hf同位素测试,测试结果见附表8及图 10.相关参数以628 Ma计算.雪穷中粒辉长岩11个分析点的176Hf/177Hf变化于0.282 706~0.282 846,所有分析点εHf(t)比值均较亏损,εHf(t)值变化于+10.39~+13.91之间,其一阶段模式年龄约为0.67~0.82 Ga,与成岩年龄相近,反映源区可能较年轻.

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图 10 雪穷中粒辉长岩锆石Hf同位素组成 Fig. 10 Hf isotopic compositions of zircons from Xueqiong medium-grain gabbros
6 成因讨论 6.1 分离结晶作用

苦海雪穷和雅日地区镁铁质岩石的MgO含量均低于8.0%,Mg#低于64,Ni含量小于200×10-6,暗示这些镁铁质岩石演化程度较高.以MgO为横坐标(图 11)的哈克图解能较好地解释这些结晶分离过程.在哈克图解中,雪穷辉长岩的Cr、Ni含量均与MgO含量呈正相关(图 11a, 11b)暗示有橄榄石分离结晶.Sc/Y比值与单斜辉石、角闪石具有良好的相关性,受橄榄石、斜长石分离结晶影响较小(Naumann and Geist, 1999),因此可以用来判断是否发生了单斜辉石或角闪石的分离结晶,雪穷样品的TiO2含量、Sc/Y比值均与MgO含量有良好的正相关性(图 11c, 11d),证明有单斜辉石的分离结晶作用; 而Al2O3/TiO2比值随MgO含量无明显改变(图 11e)说明斜长石分离结晶作用较弱,然而13MD08-4、14MD04-1、14MD06-13等样品的稀土元素球粒陨石标准化图解显示有明显的Eu正异常,代表个别样品有明显的斜长石堆晶作用.对于雅日玄武安山岩,Sc/Y比值与MgO含量有良好的正相关性(图 11d),Al2O3/TiO2比值与MgO含量有良好的正相关性(图 11e),暗示雅日镁铁质岩有暗色矿物和斜长石的结晶分离作用.以上元素特征表明,雪穷镁铁质岩浆经历了单斜辉石为主要矿物相的分离结晶作用,个别样品有明显的斜长石的堆晶作用; 雅日镁铁质岩浆经历了斜长石、辉石为主要矿物相的分离结晶作用,与镜下观察一致.

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图 11 苦海镁铁质岩哈克图解(a~f)和P2O5-Mg#、Nb/La-Mg#图解(g~h) Fig. 11 Hark diagrams of Kuhai mafic blocks (a-f) and diagrams of P2O5-Mg#、Na/La-Mg# (g-h)
6.2 地壳混染与源区混染

地壳混染不仅可以造成LILE和LREE的富集,也可能造成高场强元素的亏损(Ryerson and Watson, 1987),因此有必要对地壳混染程度进行评价.地壳混染通常有两种模式,一种是镁铁质岩浆在上升过程中经历地壳混染作用,通常与结晶分离作用伴生出现(即AFC过程)(Depaolo, 1981; Halama, 2004),另一种是最原始的玄武质岩浆就已受到强烈的混染.

在野外和镜下观察中,雪穷辉长岩中未看到捕掳体,反映雪穷辉长岩受地壳混染影响可能较弱.从微量元素特征看,雪穷辉长岩Zr/Hf比值为35.75~40.69,平均值为38.95,Nb/Ta比值为15.30~16.88,平均值为16.18,与原始地幔的Zr/Hf比值(36.10)和Nb/Ta比值(17.65)相近(Rudnick, 1995),而远高于大陆地壳的Zr/Hf比值(约33)和Nb/Ta比值(约11)(Taylor and McLennan, 1985),没有表现出地壳组分加入的特征.在岩浆上升过程中地壳混染造成HFSE亏损时,会明显的改变Nb/La的比值(Huang et al., 2000),以及富集LREE(Lassiter and DePaolo, 1997),导致相应的Nb/La和Sm/La比值有较大变化.雪穷辉长岩的La/Nb比值、La/Sm比值变化很小,与MgO、Mg#含量变化相关性差(图 11f, 11h),也证明雪穷辉长岩上升过程中未受到地壳混染.锆石εHf(t)值变化于+10.39~+13.91之间,相对集中,也进一步证明雪穷辉长岩未受到明显的地壳混染.

雅日镁铁质岩石在野外和镜下观察中,也未见捕掳体以及壳源碎屑组分,锆石U-Pb年龄测试中有1颗年龄在1.8 Ga左右的捕获锆石,整体显示受地壳混染影响较弱.然而,雅日细斑晶玄武安山岩Zr/Hf比值为30.83~35.00,平均值为32.35,Nb/Ta比值为9.00~16.84,平均值为12.94,与地壳Zr/Hf比值和Nb/Ta比值相近; 粗斑晶玄武安山岩Zr/Hf比值为36.54~37.37,平均值为37.04,Nb/Ta比值为13.67~19.33,平均值为16.83,与原始地幔相似,反映部分雅日镁铁质岩可能受到了地壳混染作用.在La/Sm-MgO、Nb/La-Mg#图解中,雅日镁铁质岩石La/Sm比值相对集中,Nb/La比值与Mg#含量表现出弱的正相关性(图 11f, 11h),也暗示雅日镁铁质岩石可能受到了弱的地壳混染.然而,雪穷辉长岩和雅日镁铁质岩石的P2O5与Mg#含量呈负相关(图 11g),则与上升过程中地壳混染的总体特征不符(Mir et al., 2010).因此,雅日镁铁质岩石的地壳混染更可能发生在源区.

6.3 地幔源区性质

国内外的研究通常认为,镁铁质岩石的原生岩浆应来自岩石圈或软流圈地幔橄榄岩或辉石岩的部分熔融(Zhao and Zhou, 2009).而地幔熔融有多种形式,如洋中脊减压熔融、热点升温熔融和俯冲带流体交代熔融(Wilson, 1989).不相容元素因具有相似的分配系数,受分离结晶作用影响较小,且在地幔部分熔融过程中只有微小变化,可以用来分析源区特征(Taylor and McLennan, 1985).雪穷辉长岩样品具有较低的MgO含量、Mg#,高场强元素Nb、Ta也呈弱亏损,不同于典型的OIB型特征; 根据其球粒陨石标准化稀土元素配分模式图(图 9b),除Eu元素外,整体处于OIB、E-MORB型曲线之间,原始地幔标准化微量元素蛛网图(图 9a)主体也具有与E-MORB相似的特征,显示岩浆源区含有富集组分; 然而其较高的εHf(t)值,及锆石Hf同位素模式年龄与其结晶年龄相近,又都显示亏损地幔特征(吴福元等,2007).这一情况可在大陆下岩石圈地幔(subcontinental lithospheric mantle)产生的岩浆中观察到,如北Tanzania地区火山岩(Paslick et al., 1995)等.在Sm/Yb-La/Yb图解(图 12a)中,雪穷辉长岩样品偏离尖晶石二辉橄榄岩的熔融曲线,并且位于石榴子石-尖晶石二辉橄榄岩(50:50)下方,指示雪穷辉长岩可能来自含尖晶石和少量石榴子石的二辉橄榄岩4%~5%的部分熔融,而大陆下岩石圈地幔演化产生的火山岩中也多含尖晶石相橄榄岩捕掳体(Bedini et al., 1997),也表明源区可能是含尖晶石的二辉橄榄岩.因此,雪穷辉长岩样品显示雪穷辉长岩的源区可能是大陆下岩石圈地幔,在形成过程中经历斜长石、单斜辉石的分离结晶,地壳混染不明显.

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图 12 苦海地区镁铁质岩Sm/Yb-La/Yb图解(a)和Th/Ta-La/Yb图解(b) Fig. 12 Sm/Yb-La/Yb diagram (a) and Th/Ta-La/Yb diagram (b) of mafic rocks in Kuhai 图a据Su et al.(2012); 图b据Condie(1997).Ethiopian裂谷数据来自Shinjo et al.(2011).DM.亏损地幔; PM.原始地幔; PSML.后太古宙大陆下岩石圈; HIMU.高U/Pb地幔源区; EM1和EM2.富集地幔; LC.下地壳; UC.上地壳; MORB.洋中脊玄武岩; OIB.洋岛玄武岩

雅日镁铁质岩石具有相对高的Th/Yb比值(0.99~5.43),在Th/Yb-Nb/Yb图解中(图 13a),明显偏离了MORB-OIB地幔演化趋势,表现出富集特征,暗示其可能来源于一个流体改造的地幔源区.Th/Ta-La/Yb图解可以提供镁铁质侵入岩的源区和岩浆演化信息(Condie, 1997),在图中(图 12b)可以看到雅日镁铁质岩石在岛弧区和上地壳附近,而上地壳很难熔融出玄武质岩浆,因此源区为岛弧概率更大.地幔交代作用主要有:深部软流圈地幔流体交代(Meen et al., 1989); 俯冲板片中海洋沉积物、蚀变洋壳在地幔深部脱水形成的流体交代(Maury et al., 1992; Ishikawa et al., 1999); 以及俯冲玄武质板片熔融产生的熔体交代(Hawkesworth et al., 1993; Elliott et al., 1997).来自深部地幔软流圈的熔体常常具有OIB的地球化学特征,而雅日样品Mg#较低,相对富集LREE和LREE,亏损Nb、Ta,相对低的Nb/U(1.59~4.56)以及高的Zr/Nb(14.06~28.46),与OIB明显不同(OIB分别为47.06和5.83,数据据Sun and McDonough(1989)),因此,交代的流体不应是深部软流圈上升的流体.俯冲板片形成的熔体与地幔交代形成高K、富HFSE系列岩浆,如富Nb玄武岩(Nb>7×10-6)(Polat and Kerrich, 2001),雅日镁铁质岩石相对富K,Nb含量高于一般玄武岩(2×10-6~3×10-6),然而由于后期蚀变强烈,K含量难以确信,Nb含量也并未达到富Nb玄武岩标准,实验岩石学显示玄武质板片发生熔融需要在60 km深度达到700 ℃和2 GPa,该情况下往往会产生埃达克质岩浆(Ni et al., 2016),区域内尚未有埃达克质岩石报道,因此,俯冲板片部分熔融的流体可能不占主要地位.俯冲板片中海洋沉积物在地幔深部脱水形成的流体往往富LILE、亏损HFSE,有较高的Sr、Pb含量.雅日镁铁质岩高铝的特征与俯冲带地幔部分熔融产生的熔体特征相似(Crawford et al., 1987),同时具有相对富集U、Th、La、Ce以及亏损Nb、Ta、Zr、Hf的特征,与俯冲沉积物脱水形成的流体特征相符,说明雅日镁铁质岩石源区受俯冲板片携带的沉积物脱水产生的流体影响较明显,是由受俯冲沉积物产生的流体交代的地幔楔部分熔融形成.

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图 13 微量元素构造环境判别图解 Fig. 13 Trace element tectonic discriminative diagrams 图a据Pearce(2008); 图b据Wood et al.(1979),Ethiopian裂谷数据来自Shinjo et al.(2011)
7 岩块构造意义

通过区域资料及野外考察,苦海地区具有典型的岩块裹夹于基质“block-in-matrix”结构,岩块有多种成因的辉长岩、火山岩以及硅质岩、灰岩等,基质主要为晚古生代强片理化长石杂砂岩,见粒序层理,局部见砂岩底面有重荷模,是一套深海浊积岩,其地球化学特征也显示其沉积环境与大陆岛弧密切相关(祁生胜等,2001),是典型的混杂带.雪穷辉长岩和雅日镁铁质岩石均为构造就位,较难保留原有的岩石组合特征,仅雅日镁铁质岩保留有部分原火山-沉积旋回信息,为构造环境分析带来了较大的难度.

岩石成因分析表明,雪穷镁铁质岩石的微量元素和同位素地球化学特征暗示其可能起源于大陆下岩石圈地幔.为进一步认识其产出的构造背景,笔者运用了多种构造环境判别图.在进行堆晶含量高的岩石样品分析时,辉石堆晶作用对Ti、Zr、Y影响较大,但是对一些三角图解中投影的相对比例以及一些高场强元素的比值影响较小(Pearce and Cann, 1973).本文将雪穷辉长岩数据与成熟裂谷环境的Ethiopian裂谷拉斑玄武岩(Shinjo et al., 2011)进行对比.通过微量元素蛛网图与稀土元素配分模式图对比(图 9),可以看出雪穷辉长岩与Ethiopian裂谷拉斑玄武岩走势大致相同,都具有Zr的轻微负异常.通过构造判别图解来看,在Th/Yb-Nb/Yb图解(图 13a)上,雪穷辉长岩与Ethiopian裂谷拉斑玄武岩样品均落入相同区域,在OIB区域附近; 在Hf/3-Th-Nb/16图解(图 13b)中,二者也落在相同区域,落入E-MORB和板内玄武岩区域.这些特征暗示雪穷辉长岩形成于裂谷环境, 可能与原特提斯洋打开有关.

雅日镁铁质岩相对富集LREE和Th,明显亏损HFSE,指示其源区受到了俯冲作用的影响.在Th/Yb-Nb/Yb图解中(图 13a),样品均偏离MORB-OIB地幔演化趋势,粗斑晶玄武安山岩落入活动大陆边缘区域,而细斑晶玄武安山岩则落入活动大陆边缘区域和大洋岛弧重叠区域; 在Hf/3-Th-Nb/16图解(图 13b, Wood et al., 1979)中,也均落入岛弧钙碱性玄武岩区域.同时,对雅日镁铁质岩石的岩石成因分析也显示俯冲带沉积物脱水流体对其源区地幔有交代作用.综合这些因素,可以认为雅日镁铁质岩石应形成于俯冲环境,有可能是活动大陆边缘,代表有俯冲洋盆存在.

通过区域填图资料对比,苦海混杂带与阿尼玛卿蛇绿混杂带具有相似的地层组合,应是东昆仑南缘混杂带的一部分(张克信等, 1999a, 1999b; 张智勇等,2004).

早期研究认为昆南构造带代表古特提斯洋演化(姜春发等,1992; 殷鸿福和张克信,1998; 杨经绥等,2010),昆中构造带代表原特提斯洋演化(高延林等,1988; 李怀坤等,2006; 刘彬等,2013; 陈加杰等, 2016; 赵菲菲等, 2017; Li et al., 2017).然而,在东昆仑南缘也发现大量新元古代晚期至泥盆纪岩浆活动记录(附表9).布青山地区保存有寒武纪蛇绿岩(刘战庆,2011a)及与俯冲相关的中酸性岩记录(边千韬等, 1999, 2007; 刘战庆,2011c),玛积雪山存在早寒武纪辉长岩(李王晔,2008),德尔尼地区保留有晚寒武纪闪长岩(李王晔等,2007); 此外,在布青山得力斯坦蛇绿岩顶部夹硅质岩的砂板岩中也发现了奥陶纪疑源类化石,也被认为是早古生代洋壳存在的证据(边千韬等,2001).苦海地区也存在元古代-早古生代记录,在花石峡附近出露有新元古代晚期(555±9 Ma)辉长岩(李王晔等,2007); 在苦海-赛什塘周边有一系列志留纪至泥盆纪花岗岩,呈构造岩片就位于上古生界,也有呈小岩株侵入元古代片麻岩、片岩(张智勇等,2005).本次在雅日发现的寒武纪玄武安山岩也证明在东昆仑南缘早古生代洋盆的存在.

这种年轻的构造带保留古老洋壳物质,多条蛇绿岩出现的现象是增生造山带的主要特征之一(李继亮, 2004),如现代日本就是由多条混杂带组成,较早的有二叠纪的Akiyoshi带, 含石炭纪岩块(Kanmera et al., 1990),较晚的有现在仍在发育的Shimanto带,含早白垩世岩块(Saito et al., 2014).东昆仑南缘混杂带内也发育多套不同时期的蛇绿岩,在早古生代及晚古生代-中生代都存在洋壳俯冲作用,暗示东昆仑南缘可能发育增生造山作用,具有极为复杂的构造历史.此外,在勉略构造混杂带中也保留有原特提斯时期的记录(杜远生等,1998; 闫全人等,2007; 徐通等,2017),说明原特提斯洋范围可能到达东昆仑和西秦岭构造带南缘.

8 结论

(1) 利用LA-ICP-MS U-Pb定年方法,获得苦海地区雪穷辉长岩的的结晶年龄为624±7 Ma和609±13 Ma,雅日玄武安山岩的年龄为504±3 Ma,镁铁质岩石均构造就位于强片理化长石杂砂岩之中,表现出混杂特征.

(2) 雪穷辉长岩起源于大陆下岩石圈地幔,岩浆上升过程中几乎没有受到地壳混染作用,有辉石、斜长石的堆晶; 雅日玄武安山质起源于俯冲板片携带沉积物脱水形成的流体交代的地幔楔,岩浆上升过程中局部受到弱的地壳混染,经历了斜长石的分离结晶作用.

(3) 雪穷辉长岩形成于裂谷环境,可能与洋盆的打开有关; 雅日玄武安山岩形成于俯冲带,代表有俯冲洋盆的存在,这些原特提斯洋物质均混入古特提斯构造带.结合区域资料,东昆仑、西秦岭构造带南缘应存在早古生代的俯冲-增生作用.

致谢 李艳青、付长垒博士后对本研究提供了帮助,审稿老师提出了宝贵的修改意见,在此一并表示感谢!

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