地球科学  2018, Vol. 43 Issue (4): 975-990.   PDF    
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西藏雅鲁藏布江缝合带西段达巴蛇绿岩年代学、地球化学特征及其构造意义
程晨1,2, 夏斌1,2, 郑浩1,2, 袁亚娟1,2, 殷征欣3, 陆野1,2, 徐迟1,2, 张霄1,2     
1. 中山大学海洋科学学院, 广东广州 510006;
2. 海洋石油勘探开发广东省高校重点实验室, 广东广州 510006;
3. 国家海洋局南海技术调查中心, 广东广州 510300
摘要:达巴蛇绿岩位于雅鲁藏布江蛇绿岩带西段南亚带,主要由地幔橄榄岩、基性岩脉和硅质岩组成,其形成时代和构造背景尚不清楚.首次报道了达巴蛇绿岩中基性岩脉的岩石学、锆石U-Pb年代学、全岩地球化学数据.达巴辉长岩和辉绿岩具有高的Al和Mg、低Ti、K和P,为低钾钙碱性玄武质岩石.岩石具有与N-MORB一致的稀土配分模式,但是N-MORB标准化蛛网图中显示Nb负异常,显示了受到俯冲板片流体的影响.辉绿岩的锆石U-Pb年龄为120.0±1.7 Ma,为早白垩世晚期,地球化学特征显示源自于低程度的尖晶石相地幔源区的部分熔融.结合前人研究成果,认为达巴蛇绿岩形成于与初始俯冲有关的弧前扩张中心,是受到俯冲板片流体交代的上覆地幔楔部分熔融的产物.
关键词雅鲁藏布江缝合带    达巴蛇绿岩    地球化学    锆石U-Pb定年    构造环境    
Chronology, Geochemistry and Tectonic Significance of Daba Ophiolites in Western Segment of Yarlung Zangbo Suture Zone, Tibet
Cheng Chen1,2 , Xia Bin1,2 , Zheng Hao1,2 , Yuan Yajuan1,2 , Yin Zhengxin3 , Lu Ye1,2 , Xu Chi1,2 , Zhang Xiao1,2     
1. School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006, China;
2. Key Laboratory of Offshore Oil Exploration and Development of Guangdong Higher Education Institutes, Guangzhou 510006, China;
3. South China Sea Marine Survey and Technology Center, State Oceanic Administration, Guangzhou 510300, China
Abstract: The Daba ophiolite is located in the southern sub-belt of the western segment of the Yarlung Zangbo suture zone (YZSZ) and mainly composed of mantle peridotite, mafic dikes and siliceous rocks. Its formation time and tectonic setting are not clear. This is the first report of zircon U-Pb age, petrologic and whole-rock geochemical data from the Daba ophiolite. The major elements of Daba gabbro and dolerite are characterized by high contents in Al, Mg and low contents in Ti, K and P, which belong to subalkaline basalt. The REE distribution patterns are very similar to those of the normal mid-ocean ridge basalt (N-MORB). However, the normalized spider diagram shows significant depletion of Nb, suggesting the influence of fluids from downgoing slab.The dolerites yielded zircon U-Pb ages of 120.0±1.7 Ma, and geochemical features show partial melting from the low degree spinel-bearing mantle source. Combining with previous studies, it is suggested that the intrusion of the mafic rocks into the Daba peridotites may be interpreted as a result of mixing process between MORB-like melts and arc-derived fluids, which may have occurred during the stage of subduction initiation in an extending forearc region.
Key Words: Yarlung Zangbo suture zone    Daba ophiolite    geochemistry    zircon U-Pb age    tectonic setting    

蛇绿岩是古老大洋岩石圈的“化石”,是恢复洋-陆格局及构造演化历史的重要指示物,可以为大洋岩石圈的岩浆事件、变质事件、构造作用以及古洋盆演化的过程提供重要的信息(Coleman, 1977; Bezard et al., 2011吴福元等,2014).现代大洋洋壳和全球多处典型蛇绿岩的研究成果(Stern et al., 2012; Dilek and Furnes, 2014; Furnes and Dilek, 2017)显示,洋壳(蛇绿岩)的形成往往是一个持续的过程,岩石的地球化学成分会随之发生变迁.因此,目前所提出的蛇绿岩的成因及分类的经典模型存在着一定的局限,而其中一个突出的问题就是无法有力地解释在俯冲带中蛇绿岩往往兼具N-MORB和岛弧的特征,即所谓的蛇绿岩的“难题”(Moores et al., 2000).最新研究发现,在雅鲁藏布江缝合带广泛存在这种具有双重地化特征的蛇绿岩(刘飞等,2015a; Xiong et al., 2016; Zheng et al., 2017),使得雅鲁藏布江蛇绿岩与绝大多数新特提斯蛇绿岩系统有所区别,从而为其起源及动力学机制的深入研究提供了新的视角.此外,雅鲁藏布江蛇绿岩中大量地幔橄榄岩体不仅构成以铬铁矿为主体的雅鲁藏布江成矿带(许志琴等,2016),而且与之相关的蛇绿混杂岩、构造混杂岩和岛弧火山岩与北部的冈底斯成矿带和南部的特提斯喜马拉雅成矿带中的金属矿床(铜,金等)(冷秋锋等,2016张志等,2017)具有密切的联系,是研究青藏高原成矿作用的关键所在.达巴蛇绿岩位于雅鲁藏布江蛇绿岩带西段南亚带最西端,是研究新特提斯西部地区演化的重要区域.在本次工作中,笔者报道了达巴蛇绿岩的野外地质学、地球化学、地质年代学(锆石年龄)数据.结合雅鲁藏布江缝合带西段其他蛇绿岩的研究成果,探讨其形成的构造背景,为恢复和完善特提斯洋构造格局提供新的约束.

1 地质背景及岩石学特征 1.1 地质背景

雅鲁藏布江缝合带是青藏高原南部夹持于印度板块和欧亚板块之间的一条年轻的巨型构造分界线(图 1a),其中的蛇绿岩沿着缝合带不连续分布,代表一套与深海沉积物和混杂岩有关的新特提斯洋大洋岩石圈的残余(Nicolas et al., 1981; Yin and Harrison, 2000).

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图 1 研究区区域地质简图 Fig. 1 Geological sketch map of the research region a.雅鲁藏布江缝合带和班公湖-怒江缝合带蛇绿岩分布简图;b.达巴蛇绿岩区域地质简图.据张双增等(2005)编1:25万日新-札达县-姜叶马幅区域地质调查报告及地质图;Yin and Harrison(2000); Zhang et al.(2012, 2014)修改

雅鲁藏布江蛇绿岩带呈东西向分布,长约2 000 km,主体走向与雅鲁藏布江一致,分为西、中、东三段(图 1b):西段从萨嘎以西至中印边境与拉达克南侧的Indus蛇绿岩(包括Nidar及Spontang蛇绿岩等)相连,中段由昂仁至仁布,东段由曲水到墨脱(潘桂棠等,1997).其中西段自萨嘎分为南北两支(图 1b),即达机翁-萨嘎蛇绿岩带(北亚带)和达巴-休古嘎布蛇绿岩带(南亚带),其间由奥陶系-三叠系海相地层为主的仲巴-札达微地体分隔(刘强等,2017).

达巴蛇绿岩位于雅鲁藏布江蛇绿岩带西段南亚带西北段,位于西藏阿里地区达巴乡北20 km处(图 1b),蛇绿岩岩体呈东西向展布,东西向稍长,约1~2 km,南北向短,约0.7 km,面积约为1 km2.四周与托林组三段砂岩、砂质泥岩断层接触(张双增等, 2005, 1:25万日新-札达县-姜叶马幅区域地质调查报告及地质图),主要由橄榄岩、辉长岩和辉绿岩组成(图 2a),堆晶岩几乎不发育,上覆紫红色和灰绿色放射虫硅质岩,局部硅质岩与基性/超基性岩构造混杂(图 2b).区域上,该蛇绿岩岩体东南断续延伸至东波寺蛇绿岩,共同组成雅鲁藏布江西段蛇绿岩南亚带.

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图 2 达巴蛇绿岩剖面图(a)和野外照片(b, c, d) Fig. 2 The geological section (a) and field photographs (b, c, d) of Daba ophiolites
1.2 岩石学特征

室内显微镜鉴定达巴蛇绿岩橄榄岩为方辉橄榄岩,岩石普遍蛇纹石化,新鲜面为灰绿色,粒状结构,块状构造,主要由斜方辉石和橄榄石组成,含有少量的铬尖晶石(图 3a).

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图 3 达巴蛇绿岩显微镜下照片 Fig. 3 Microstructure photographs of Daba ophiolites a.蛇纹石化方辉橄榄岩;b.辉长岩;c.辉绿岩;d.放射虫硅质岩;Opx.斜方辉石;Cpx.单斜辉石;Pl.斜长石;Rad.放射虫

辉长岩呈透镜状侵入蛇纹石方辉橄榄岩中,新鲜面为灰褐色,辉长结构,块状构造,主要由辉石和斜长石组成(图 3b),受后期构造事件影响辉石普遍破碎,斜长石葡萄石化强烈.

辉绿岩呈脉状侵入于蛇纹石方辉橄榄岩中,宽约1~3 m,新鲜面为灰绿色,局部辉石具含长特征,但也因碎裂作用而错位,有些地方“镶嵌含长”表现不明显,辉石角闪石化,斜长石强烈泥化,部分异剥钙榴岩化(图 3c).

紫红色和灰绿色放射虫硅质岩覆盖在橄榄岩之上,局部硅质岩与基性/超基性岩构造混杂(图 2b),呈隐晶-微粒结构,可见大小不一的亮白色放射虫发育于硅质岩中,充填硅质脉(图 3d).

2 样品分析方法 2.1 锆石U-Pb定年

本文所用定年样品取自橄榄岩中的辉绿岩脉,样品号为DBN-23,采样点的GPS坐标为31°19′18.24″ N,79°54′16.80″E.为挑选到足量、大小合适、晶形良好的锆石,辉绿岩取样为±20 kg,人工破碎为直径约2 cm的碎块,放入直径为20 cm的不锈钢钵中,置于XZW100型振动磨样机(1.1/0.75 kW)研磨30 s后过0.4 mm孔径筛,如此反复至样品均通过0.4 mm孔径筛,经铝制淘沙盘淘洗富集重矿物,后以磁选和电磁选获非电磁性矿物,再淘洗富集锆石,最后在双目镜下手工挑选锆石.整个分选流程所使用装置均彻底清洗,以避免混染.将挑选好的待测锆石以环氧树脂固定,抛光至暴露出锆石中心面,用阴极发光(CL)照相以确定其内部结构,在中国科学院广州地球化学研究所同位素地球化学国家重点实验室利用LA-ICP-MS进行单颗粒锆石U-Pb定年分析(测试流程详见Compston et al., 1992; Yuan et al., 2004).

测试时,激光剥蚀所用斑束直径为33 μm,频率为8 Hz,能量恒定为80 MJ/pluse.He作为载气,使用Agilent 7500a ICP-MS进行测试.应用NIST SRM 610玻璃和标准锆石Temora(Pearce et al., 1997; Black et al., 2003)作为外标,标准锆石Qinhu作为内标,进行元素间的分馏校正,测试结果见表 1.时间偏移校正和定量标定及计算,采用软件ICPMS Data Cal 7.2(Liu et al., 2010)进行,谐和曲线和加权平均年龄使用Isoplot 3(Ludwing, 2003)计算.由于样品比较年轻,加权平均年龄采用206Pb/238U年龄,置信度95%(锆石普通铅校正方法参见Andersen, 2002).

表 1 达巴辉绿岩锆石U-Pb同位素分析结果 Table 1 Analytical results of zircon U-Pb isotopes from Daba dolerite
2.2 主量和微量元素分析

本文主要对侵入于橄榄岩中的辉长岩和辉绿岩(3件辉长岩和6件辉绿岩)进行主量和微量元素分析.通过野外地质观察和镜下薄片检查,选取较为新鲜的样品进行测试.将样品碎成小块之后,用无离子水浸泡,超声波振荡清洗1 min,然后用1 mol/L的HCl溶液浸泡2 h,除去可能有的晚期碳酸盐岩矿物和浮尘.最后用去离子水清洗烘干后,在不锈钢研钵粉碎至200目以下.

样品的主量元素分析在澳实(广州)分析测试有限公司测定.粉末样硝酸锂溶解后采用WD-XRF26进行测试,分析精度优于2%.

微量元素在中国科学院贵阳地球化学研究所矿床学国家重点实验室进行.粉末样在190 ℃下用HF和HNO3溶解36 h,然后使用Perkin-Elmer ELAN-DRC-e计数的ICP-MS进行测试.为确保测试的准确性,采用BCR-1、AVG-2和GBPG-1作为标样,测试结果相对精度优于±5%~±10%(分析流程详见Qi and Grégoire, 2000).

3 分析结果 3.1 LA-ICP-MS锆石U-Pb定年

达巴蛇绿岩辉绿岩样品的锆石较大,粒径主要为70~150 μm,长宽比值为1~2,半自形粒状,自形程度好.在CL图片上,锆石多为无色,形态清晰,内部结构均一,个别锆石存在裂隙,基本不发育韵律环带(图 4),为岩浆结晶的产物(Simon and Nigel, 2007).对辉绿岩样品DBN-23中的锆石进行原位分析,获得17组数据(表 1).

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图 4 达巴辉绿岩锆石阴极发光图 Fig. 4 Cathodoluminescence (CL) images of zircons for Daba dolerite

表 1图 5可以看出,达巴辉绿岩中锆石各测试点的U和Th含量较低,且变化不大,U和Th含量分别介于102×10-6~551×10-6和45×10-6~306×10-6,平均值分别为237×10-6和141×10-6.Th/U值比较高,为0.44~1.25,属于岩浆成因.样品17个测点的LA-ICP-MS锆石206Pb/238U年龄介于114.2~125.6 Ma,加权平均年龄为120.0±1.7 Ma(1σ,MSWD=1.8,n=17,95%置信度,图 4a, 4b),属早白垩世晚期,结合锆石CL特征和Th/U比值,该年龄为达巴辉绿岩的结晶年龄.

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图 5 达巴辉绿岩锆石U-Pb年龄谐和图(a)和加权平均年龄(b) Fig. 5 U-Pb age concordia plots (a) and weight average results (b) for Daba dolerite
3.2 岩石地球化学特征

本次分析的基性岩样品共9件,包括3件辉长岩和6件辉绿岩,各元素含量如表 2所示.

表 2 达巴辉长岩、辉绿岩主量元素(%)和微量元素(10-6)含量 Table 2 Major (%) and trace element (10-6) contents for gabbro and dolerite from the Daba ophiolites
3.2.1 主量元素

达巴辉长岩SiO2含量为45.27%~47.30%;TiO2含量较低,为0.37%~0.85%;Al2O3含量为12.91%~15.82%;全铁Fe2O3T为4.58%~8.02%;MgO为6.38%~10.03%,Mg#值介于72~79;CaO为17.75%~18.83%.达巴辉长岩具有较低的K2O(平均0.02%)、Na2O(平均0.17%)和P2O5(平均0.06%).

6件辉绿岩样品SiO2含量中,除1件异剥钙榴岩化辉绿岩为29.7%外,其余为45.88%~49.77%,平均为47.51%,全部落入玄武岩范围;TiO2为0.53%~1.20%,平均0.75%,低于洋脊玄武岩TiO2的平均值1.15%(Wilkinson, 1982),但部分落入岛弧玄武岩0.58%~0.85%的范围(Pearce and Cann, 1973);Al2O3含量为12.77%~14.56%,平均13.68%,具有俯冲带高铝玄武岩的特征(张旗和周国庆,2001);全铁Fe2O3T为6.45%~10.83%,平均7.96%;MgO为5.75%~11.25%,平均9.06%,Mg#值介于63~76,平均为72,具有初始岩浆Mg#值(68~75)(Wilkinson, 1982)的特点;CaO为15.35%~23.94%,平均18.83%;K2O为0.01%~0.04%,平均0.03%;Na2O为0.01%~2.20%,平均0.51%,K2O<Na2O,具有洋内岩石的特征;P2O5为0.03%~0.11%,平均0.06%,小于洋脊玄武岩P2O5的平均值0.14%(Gribble et al., 1998).总体上,达巴辉长岩和辉绿岩样品都具有富Al、Mg,低Ti、K、P的特点.

利用蚀变过程中不活泼元素进行判别,在Zr/TiO2×10-4-Nb/Y图解上(Winchester and Floyd, 1977)(图 6a),所有样品落入亚碱性玄武岩区域;在P2O5-Zr图中(Winchester and Floyd, 1976)(图 6b),样品全部落入拉斑玄武岩区域.

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图 6 达巴蛇绿岩中基性岩Zr/TiO2×10-4-Nb/Y(a)和P2O5-Zr(b)图解 Fig. 6 Zr/TiO2×10-4-Nb/Y (a) and P2O5-Zr (b) diagrams from mafic rocks in the Daba ophiolites 图a据Winchester and Floyd(1977);图b据Winchester and Floyd(1976).Rhyolite+Dacite.流纹岩+英安岩;Alkali Rhyolite.碱性流纹岩;Phonolite.响岩;Trachyte.粗面岩;And/BasAnd.安山岩/玄武安山岩;Trachy/And.粗面安山岩;Tephri-Phonolite.碱玄岩+响岩;Subalkaline Basalt.亚碱性玄武岩;Alkaline basalt=Alk-Bas.碱性玄武岩;Foidite.副长岩;Tholeiitic basalt.拉斑玄武岩
3.2.2 微量元素

达巴辉长岩和辉绿岩稀土元素标准化配分曲线(图 7a),具有与N-MORB一致的总体呈左微倾右平直的趋势.两者的LREE/HREE平均比值分别为1.35和1.41,(La/Yb)N平均比值分别为0.64和0.65,均小于1;两者的(Gd/Yb)N平均比值分别为1.00和1.03,轻重稀土分馏不明显.δEu平均值均为0.95,显示弱的负异常,表明斜长石结晶分异程度较低.辉长岩和辉绿岩稀土元素总量(∑REE)平均值分别为21.10×10-6和24.94×10-6,均低于洋脊玄武岩的平均值39.11×10-6(Sun and McDonough, 1989),这些特征表明达巴基性岩可能来源于早期亏损的地幔源区.

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图 7 达巴基性岩稀土(a)和微量元素(b)特征曲线 Fig. 7 Chondrite-normalized REE (a) and N-MORB-normalized trace element (b) diagrams for the mafic rocks in Daba ophiolites 标准化值据Sun and McDonough(1989);Barren岛弧玄武岩数据来源于Luhr and Haldar(2006);IBM弧前玄武岩源于Reagan et al.(2010);日喀则弧前玄武岩源于Dai et al.(2013)

在N-MORB标准化微量元素蛛网图中(图 7b),达巴辉长岩和辉绿岩均呈左高右低的特征.曲线前半段显示富集Ba等大离子亲石元素(LILE)和Th、U等放射性热元素,明显亏损Nb等非活动性元素,后半段Nd-Lu段为近平坦的分布型式,总体显示出具有岛弧火山岩的特点,其中辉长岩和辉绿岩Nb(平均值分别为0.85×10-6和1×10-6)、Yb(平均值分别为1.61×10-6和1.75×10-6)、Ti(平均值分别为3 616×10-6和4 495×10-6)、La(平均值分别为1.40×10-6和1.63×10-6)、Hf(平均值分别为1.11×10-6和1.26×10-6)、Y(平均值分别为15.7×10-6和17.2×10-6)等不相容元素丰度均低于正常洋中脊玄武岩(Sun and McDonough, 1989).部分样品Nb和Ta出现脱耦现象,其中Ta含量的升高可能是使用钨钢碎样钵处理样品所导致的.

总体上,稀土和微量元素特征(图 7a, 7b)与典型的大洋中脊玄武岩(N-MORB)和岛弧玄武岩(IAT)(Luhr and Haldar, 2006)各有不同,而与日喀则弧前玄武岩(Dai et al., 2013)和Mariana弧前玄武岩(Reagan et al., 2010)具有较好的一致性.

4 讨论 4.1 达巴蛇绿岩的形成时代

蛇绿岩中的辉长(绿)岩脉是洋脊扩张的产物,是研究蛇绿岩形成时代和构造环境的“岩石探针”.本次研究在达巴蛇绿岩体中获得辉绿岩LA-ICP-MS锆石U-Pb年龄为120.0±1.7 Ma(MSWD=1.8,n=17),为早白垩世晚期.依据本次测试的辉绿岩样品锆石的形态及地球化学特征,该年龄代表了洋壳的形成年龄.

对于雅鲁藏布江西段蛇绿岩,前人对不同地区蛇绿岩体的年代学进行了大量的研究(表 3).可以看出,达巴蛇绿岩的形成时间与东波、拉昂错、休古嘎布等南亚带蛇绿岩以及巴尔、错不扎和加纳崩等北亚带蛇绿岩的形成时间均集中在约120~130 Ma,并且与雅鲁藏布江中段日喀则蛇绿岩(Dai et al., 2013)和东段泽当蛇绿岩(Xiong et al., 2016)具有一致的形成时代.

表 3 雅鲁藏布江缝合带西段蛇绿岩同位素年龄 Table 3 Summary of ages and tectonic settings of the Yarlung Zangbo ophiolite

然而,根据放射虫化石定年确定雅鲁藏布江缝合带蛇绿岩的形成年代为晚三叠世到古近纪之间(张双增等, 2005, 1:25万日新-札达县-姜叶马幅区域地质调查报告及地质图;王保弟等, 2009, 1:5万仁布县、恩马幅、亚德幅、卡扎幅区域地质调查报告及地质图;黄圭成等,2010;尼玛次仁, 2013, 西藏1:5万噶尔地区6幅区域地质调查报告及地质图;普琼等, 2014, 西藏1:5万拉孜地区6幅区域地质调查报告及地质图),而锆石定年方法主要集中在侏罗纪-白垩纪之间,120~130 Ma这一时期的锆石年代学数据,占了很大比例,并且这些蛇绿岩的构造环境判别多为MOR兼具SSZ特征,这一方面说明了特提斯洋是一个长期演化的大洋,另一方面表明在早白垩世晚期,雅鲁藏布江新特提斯洋开启了大规模的活动.

4.2 达巴蛇绿岩的岩石成因和构造环境

辉长岩作为蛇绿岩中的深成杂岩单元,成岩过程中会受到不同程度结晶分异作用的影响,因此对研究区岩石成因和构造环境的讨论主要由辉绿岩进行展开.另外,研究区辉绿岩样品存在不同程度的蚀变作用(LOI=2.83%~9.55%),在对岩石成因及构造环境的解释中,主要对高场强元素、稀土元素和过渡族元素进行讨论.

4.2.1 岩石成因

达巴蛇绿岩中辉绿岩Mg#值(平均为72),具有初始岩浆Mg#值(68~75)(Wilkinson, 1982)的特点,主要元素(Al2O3平均为13.68%、TiO2平均为0.75%、MgO平均为9.06%)含量,与N-NORB(Al2O3为15.60%、TiO2为0.82%、MgO为8.83%)(Schilling et al., 1983)相比具有相似的特征,总体为低钾拉斑钙碱性玄武质岩石(表 2图 6).稀土元素具有与N-MORB一致的水平配分特征,稀土总量(∑REE)(表 2图 7)低于洋脊玄武岩的平均值,表明达巴辉绿岩可能源于比N-MORB更为亏损的地幔源区,这也与邻区东波蛇绿岩中辉绿岩锆石具有高的εHf(t)值(+16~+19)(未刊)和巴尔蛇绿岩辉绿岩具有高的εNd(t)值(+8.2~+9.1)(Zheng et al., 2017)相一致.

通常情况下,岩浆源自低程度尖晶石相地幔源区的部分熔融,地幔残留体和熔体具有一致的Sm/Yb比值,而La/Sm比值将随着部分熔融程度的增高而降低,稀土元素的配分曲线表现为较为平缓的特征(Saccani et al., 2008);而低程度的石榴子石相地幔源区的部分熔融具有显著增高的Sm/Yb比值,显示重稀土元素富集的稀土配分曲线特征(M/HREE)N<0.1(Hellebrand, 2002).在La-La/Sm和La/Sm-Sm/Yb图解中(图 8a, 8b)显示,达巴辉绿岩源自5%~15%的尖晶石相地幔源区较低程度的部分熔融.

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图 8 达巴辉绿岩的La-La/Sm (a)和La/Sm-Sm/Yb (b)图解 Fig. 8 The La-La/Sm (a) and La/Sm-Sm/Yb (b) diagrams for dolerites in Daba ophiolites Aldanmaz et al.(2000)

达巴辉绿岩的N-MORB标准化微量元素蛛网图中,明显亏损Nb等非活动性元素,Yb(平均1.75×10-6)、Ti(平均4 495×10-6)、La(1.63×10-6)、Hf(1.26×10-6)、Y(17.2×10-6)等不相容元素丰度均低于正常洋中脊玄武岩(Sun and McDonough, 1989),反映了岩浆形成过程中有俯冲带流体的参与.在Nb/Yb-Th/Yb图解中(图 9a)随着Th/Yb值的增加而Nb/Yb值相对稳定,显示由正常洋中脊向岛弧火山岩递进的趋势,具有超俯冲带环境的痕迹,在Nb/Yb-TiO2/Yb图解中(图 9b)样品大都落入正常洋中脊区域并存在良好的线性关系,显示其受到过后期流体改造的作用.

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图 9 达巴辉绿岩的Th/Yb vs. Nb/Yb (a) and TiO2/Yb vs. Nb/Yb (b)图解 Fig. 9 The Th/Yb vs. Nb/Y (a) and TiO2/Yb vs. Nb/Yb (b) diagrams for dolerites in Daba ophiolites Pearce(2008)修改

Ba在遭受变质作用及海水蚀变过程中相对稳定,而在流体中相对活泼;Th在源于沉积物-板块的熔体中化学性质相对活泼,而在低温流体中相对稳定(李曙光,1993; Tian et al., 2008).因此,这些元素可以很好地用来限定岩浆源区中的俯冲组分的叠加.在Th/Nb vs. U/Th和Th/Nb vs. Th/Ce图解(图 10a, 10b)中岩浆组分的贡献主要来自俯冲板片的流体.

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图 10 达巴辉绿岩的Th/Nb vs. U/Th和Th/Nb vs. Th/Ce图解 Fig. 10 The Th/Nb vs. U/Th and Th/Nb vs. Th/Ce diagrams for dolerites in Daba ophiolites Singer et al.(2008)修改

总之,达巴蛇绿岩中的辉绿岩具有洋中脊玄武岩向岛弧火山作用过渡的特征,主要为俯冲板片脱水流体交代上覆地幔楔,使亏损地幔再部分熔融的产物.

4.2.2 构造环境分析

基性岩中Nb、Zr、Th、Y、HREE等高场强元素一般不受热液蚀变和变质作用的影响,对判别不同构造环境具有指示意义(Dilek and Furnes, 2011; Pearce, 2014).

在与日喀则弧前玄武岩和马里亚纳弧前玄武岩对比的多元素构造环境判别图显示,在Nb×2-Zr/4-Y判别图(图 11a)中,达巴辉绿岩与日喀则弧前和马里亚纳弧前玄武岩均位于洋脊玄武岩和火山弧玄武岩区域;在Ti/1000-V图中,样品落入洋中脊玄武岩和岛弧玄武岩之间的区域(图 11b);在Y/15-La/10-Nb/8判别图上样品落入洋中脊玄武岩和岛弧玄武岩的重合区域(图 11c);以上图解总体反映了达巴辉绿岩叠加了岛弧和洋中脊两种构造环境特征,与稀土元素具有N-MORB特征而微量元素显示部分岛弧玄武岩特征相一致.在NbN-ThN图解(图 11d)上达巴辉绿岩位于洋内俯冲的弧前和N-MORB区域.

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图 11 达巴辉绿岩构造环境判别图解 Fig. 11 Discrimination diagrams for the dolerites in Daba ophiolites a.Nb×2-Zr/4-Y图解,据Meschede(1986);b.Ti/1000-V图解,据Shervais(1982);c.Y/15-La/10-Nb/8图解,据Cabanis and Lecolle(1989);d.NbN-ThN图解,据Saccani(2015).IBM弧前玄武岩数据源自Reagan et al.(2010);日喀则弧前玄武岩源自Dai et al.(2013).N-MORB.正常大洋中脊玄武岩;E-MORB.富集大洋中脊玄武岩;OIB.洋岛玄武岩;IAT.岛弧拉斑玄武岩;BABB.弧后盆地玄武岩;FAB.弧前玄武岩;WPB.板内玄武岩;VAB.火山弧玄武岩;AB.碱性洋岛玄武岩

前已述及,古生物资料显示在雅鲁藏布江缝合带中发育晚三叠世到古近纪之间的放射虫硅质岩(王保弟等, 2009, 1:5万仁布县、恩马幅、亚德幅、卡扎幅区域地质调查报告及地质图;尼玛次仁, 2013, 西藏1:5万噶尔地区6幅区域地质调查报告及地质图;普琼等, 2014, 西藏1:5万拉孜地区6幅区域地质调查报告及地质图),另在仲巴西部的纳久发现晚泥盆世OIB型碱性辉长岩(Dai et al., 2011),可能指示雅鲁藏布江缝合带是一个有古特提斯残余的复杂俯冲系统.而雅鲁藏布江蛇绿岩中广泛发育120~130 Ma之间的兼具IAT和MORB双重特征的基性岩脉,这些基性岩脉所侵入的蛇绿岩体均具有以下特点:(1)具有很薄的堆晶岩(吴福元等,2014);(2)二辉橄榄岩与先前的方辉橄榄岩互层,新鲜的基性岩脉广泛侵入于高度变形和蛇纹石化的橄榄岩中(Xiong et al., 2016);(3)在地幔岩体中发现金刚石等超高压矿物(Yang et al., 2007).这些特点指示雅鲁藏布江蛇绿岩的形成与演化具有特殊的动力学机制.

Hébert et al.(2012)认为雅鲁藏布江蛇绿岩的形成至少经历了5次洋内俯冲体系;Xiong et al.(2016)对雅鲁藏布江东段泽当地幔橄榄岩以及基性岩脉进行研究,认为雅鲁藏布江特提斯洋盆存在两次弧前俯冲体系,而120~130 Ma之间的基性岩脉主要是由于受到早白垩世早期拉萨地体和羌塘地体陆-陆碰撞的加剧所引起的二次俯冲阶段的产物;Butler and Beaumont(2017)对雅鲁藏布江中段日喀则蛇绿岩所代表的活动大陆边缘俯冲板片与板块之间的应力机制(主要为俯冲板片的拖拽力)进行了数值模拟,认为雅鲁藏布江特提斯洋的俯冲存在着初次俯冲(160~150 Ma)、俯冲板片的断离(150~130 Ma)以及俯冲板片的后撤、停滞和再次俯冲(130~120 Ma)多个阶段,这一复杂的俯冲体系虽然对雅鲁藏布江蛇绿岩的形成和演化机制进行了较为合理的解释,但仍需更为精细的野外地质、岩石学和地球化学数据予以支撑.

综合以上研究成果,结合达巴蛇绿岩的地球化学特征和形成时代,笔者认为雅鲁藏布江西段达巴蛇绿岩形成于俯冲初始阶段的弧前扩张中心,主要是受俯冲板片脱水流体影响的上覆地幔楔部分熔融的产物.

5 结论

(1) 达巴蛇绿岩位于雅鲁藏布江缝合带西段,主要由方辉橄榄岩、变质橄榄岩,辉长岩、辉绿岩和放射虫硅质岩组成.达巴辉长岩和辉绿岩具有高Al、Mg,低Ti、K和P的特征,为低钾钙碱性玄武质岩石,岩石富集大离子亲石元素Ba和放射性热元素Th、U,明显亏损高场强元素Nb,REE具有与N-MORB一致的平坦型配分模式.

(2) 达巴蛇绿岩辉绿岩LA-ICP-MS锆石U-Pb年龄为120.0±1.7 Ma(MSWD=1.8,n=17),为早白垩世晚期,地球化学特征显示达巴辉绿岩源自于低程度的尖晶石相地幔源区的部分熔融.结合前人研究成果,达巴蛇绿岩应形成于俯冲初始阶段的弧前扩张中心,是受俯冲板片脱水流体交代的上覆地幔楔部分熔融的产物.

致谢 野外地质调查过程中得到了西藏区调队曾庆高总工、西藏驱龙铜矿蒋光武总工的指导和帮助,岩石薄片分析得到了南京大学周国庆教授的悉心指导,两位审稿专家提出了宝贵的评审意见,在此一并感谢!

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