岩石学报  2019, Vol. 35 Issue (10): 3048-3064, doi: 10.18654/1000-0569/2019.10.06   PDF    
班公湖-怒江洋早-中侏罗世洋内俯冲:来自洞错蛇绿岩的证据
范建军1,2, 张博川1, 刘海永3, 刘一鸣4, 于云鹏1, 郝宇杰2, 阿旺旦增5     
1. 吉林大学地球科学学院, 长春 130061;
2. 自然资源部东北亚矿产资源评价重点实验室, 长春 130061;
3. 成都理工大学地球科学学院, 成都 610059;
4. 海底科学与探测技术教育部重点实验室, 中国海洋大学海洋地球科学学院, 青岛 266100;
5. 西藏自治区地质矿产勘查开发局第二地质大队, 拉萨 850000
摘要: 本文在班公湖-怒江缝合带中段洞错蛇绿岩中新厘定一套洋内俯冲成因的岩石组合,岩性以橄榄岩、堆晶岩(包括堆晶辉长岩和斜长花岗岩)、辉长岩墙、枕状熔岩和辉绿岩脉等为主。堆晶辉长岩、辉长岩墙和辉绿岩脉锆石U-Pb测年显示,它们形成于中侏罗世(172~165Ma)。辉长岩墙和辉绿岩脉地球化学和锆石Lu-Hf同位素分析显示,它们兼具N-MORB和岛弧玄武岩地球化学特征,且均来自亏损地幔源区,形成过程中受到了俯冲流体的影响。结合区域上同时期的玻安岩、高镁安山岩和钙碱性岩浆岩等资料,我们得出班公湖-怒江缝合带内保存了一套相对完整的早-中侏罗世洋内弧岩石层序,记录了班公湖-怒江洋早-中侏罗世时期的洋内俯冲事件。早-中侏罗世是班公湖-怒江洋快速消减期,洋内俯冲和洋-陆俯冲同时存在。
关键词: 班公湖-怒江缝合带    洞错蛇绿岩    早-中侏罗世    洋内俯冲    
Early-Middle Jurassic intra-oceanic subduction of the Bangong-Nujiang oceanic lithosphere: Evidence of the Dong Co ophiolite
FAN JianJun1,2, ZHANG BoChuan1, LIU HaiYong3, LIU YiMing4, YU YunPeng1, HAO YuJie2, AWANG DanZeng5     
1. College of Earth Sciences, Jilin University, Changchun 130061, China;
2. MNR Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Changchun 130061, China;
3. College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China;
4. MOE Key Lab of Submarine Geosciences and Prospecting Techniques, College of Marine Geosciences, Ocean University of China, Qingdao 266100, China;
5. The Second Geological Brigade of the Tibet Autonomous Region Geology and Mineral Exploration and Development Bureau, Lhasa 850000, China
Abstract: In this paper, a set of intra-oceanic subduction-related rock assemblage was newly found, which consists of peridotite, cumulate rocks (comprising layered gabbro and plagiogranite), gabbro dyke, pillow lava and diabase veins, in the Dong Co ophiolite in the middle segment of the Bangong-Nujiang Suture Zone (BNSZ). Zircon U-Pb analysis of the layered gabbro, gabbro dyke and diabase vein show that they are formed during the Middle Jurassic (172~165Ma). Geochemical analysis of the gabbro dyke and diabase vein show that they have light rare earth element (LREE) depleted chondrite-normalized REE patterns that are similar to those of mid-ocean ridge basalt (MORB), and have enrichments in the light ion lithophile elements (LILE; Rb, Ba, U, Pb and Sr), and depletions in the high field strength elements (HFSE; Th, Nb and Ta) that are similar to those of island arc basalt (IAB). The gabbro dyke and diabase vein have negative δNb values (-0.2~-0.4 and -0.4~-1.0, respectively), high positive εHf(t) values (+16.5~+19.5 and +17.3~+22.3, respectively), high Sr/Nd ratios (52~114 and 34~42, respectively), and low Th/Yb ratios (0.04~0.06 and 0.05~0.11, respectively), indicating that they were all derived from the depleted mantle source, and were affected by the subduction fluids during their formation. Combined with the Middle Jurassic boninites, high-Mg andesites and calc-alkaline magmatic rocks within the BNSZ, a complete Early-Middle Jurassic intra-oceanic subduction-related rock sequence is identified, indicating that the intra-oceanic subduction of the Bangong-Nujiang oceanic lithosphere was ongoing during this period. Combined with large amounts of Early-Middle Jurassic calc-alkaline magmatic rocks on the South Qiangtang Terrane, north of the BNSZ, we conclude that the Bangong-Nujiang Ocean withered rapidly during the Early-Middle Jurassic, with the co-existence of intra-oceanic subduction and ocean-continent subduction.
Key words: Bangong-Nujiang Suture Zone    Dong Co ophiolite    Early-Middle Jurassic    Intra-oceanic subduction    

班公湖-怒江缝合带沿东西方向横贯于青藏高原中部,夹持于南羌塘地体和拉萨地体之间,是国内外长期关注的特提斯演化的重要地段(王希斌等,1987潘桂棠等,2006史仁灯,2007夏斌等,2008黄启帅等,2012Fan et al., 2014, 2018aLi et al., 2014a, b20152017Liu et al., 2014, 2017Wang et al., 2016Zhu et al., 2016Chen et al., 2017Ma et al., 2017Tang et al., 2018)。但由于缝合带本身复杂的构造演化历史和藏北高原极端恶劣的交通和气候条件,使得有关班公湖-怒江缝合带的许多重要地质问题尚处于争论之中,尤其是该条缝合带所代表洋盆的性质和构造演化时限等等。

早-中侏罗世,是班公湖-怒江洋构造演化的重要时期,也是争论较大的时期。传统认为,班公湖-怒江洋初始打开于晚三叠世-早侏罗世或早侏罗世,早-中侏罗世是洋盆大洋化的主体阶段(王冠民和钟建华,2002谢国刚等,2009曲晓明等,2010宋扬等,2019),即班公湖-怒江洋在早-中侏罗世处于威尔逊旋回的幼年期-青年期。但有的学者提出班公湖-怒江洋至少在晚二叠世之前已经存在(Pan et al., 2012Zhu et al., 2013王保弟等,2015Chen et al., 2017Zhang et al., 2017),且在早-中三叠世已经发育成熟(Fan et al., 2018b);在早-中侏罗世时期,甚至更早时期(如晚三叠世或晚二叠世),班公湖-怒江洋已经开始了大洋岩石圈的俯冲,即大洋在早-中侏罗世应处于威尔逊旋回的消亡期(史仁灯,2007Zhu et al., 2013Li et al., 2014a, bLiu et al., 2014, 2017; Zeng et al., 2016aHuang et al., 2017aTang et al., 2018)。也有的学者提出班公湖-怒江在早-中侏罗世已经处于洋盆演化的最晚期,且至少在中侏罗世已经闭合消亡(王建平等,2002Wang et al., 2008李奋其等,2014Ma et al., 2017Sun et al., 2019),即处于威尔逊旋回的遗迹期。甚至有的学者提出,班公湖-怒江洋是一个局限、短命的小洋盆,仅在侏罗纪时期,就完成了洋盆打开和闭合等过程(卢书炜等,2003赵文津等,2004Wang et al., 2008曲晓明等,2010)。

想要准确限定班公湖-怒江洋早-中侏罗世构造演化历程,需要更多证据的精确制约。在这篇文章里,我们在班公湖-怒江缝合带中段洞错蛇绿岩中新厘定出一套早-中侏罗世洋内俯冲成因的岩石组合,并对其开展了详细的岩石学、地球化学、年代学和锆石Lu-Hf同位素等方面的研究,最终结合区域地质资料,我们探讨了班公湖-怒江洋早-中侏罗世的构造演化。

1 区域地质概况

研究区位于青藏高原中部改则县洞错一带(图 1),大地构造位置处于班公湖-怒江缝合带中段。区内地质情况极其复杂,不同时代、不同类型的沉积岩、岩浆岩、洋岛型岩石组合和蛇绿岩等均有不同程度的出露,是恢复和反演班公湖-怒江洋构造演化的重要窗口之一(范建军等,2018)。

图 1 西藏改则洞错地区地质简图(底图据曾庆高等,2010修改) 1-全新统冲积物;2-下白垩统去申拉组;3-上侏罗-下白垩统地层;4-下-中侏罗统地层;5-木嘎岗日岩群;6-蛇绿岩;7-洋岛型岩石组合;8-中酸性侵入岩;9-区域主干断裂;10-角度不整合 Fig. 1 Geological map of the Dong Co area, Gerze County, Tibet (modified after Zeng et al., 2010)

研究区内的蛇绿岩,前人统称为洞错蛇绿岩。洞错蛇绿岩总体走向北西西,东西延长约50km,西段最宽处约5km,呈一系列大小不等的透镜体混杂于复理石沉积之中。通过前人详细的地质调查和专题研究,初步明确洞错蛇绿岩的岩石组合是比较齐全的,其底部至顶部依次由地幔橄榄岩(Oph1)、镁铁-超镁铁质堆晶杂岩(Oph2)、基性岩墙(Oph3)、枕状熔岩(Oph4)和硅质岩(Oph5)等五部分组成,可恢复的洋壳总厚度大于5km(鲍佩声等,2007)。

普遍认为,洞错蛇绿岩形成于侏罗纪(邱瑞照等,2004曾庆高等,2010),主要依据在于以下三点:(1)洞错蛇绿岩被早白垩世末期去申拉组(K1q)角度不整合覆盖;(2)洞错蛇绿岩的层状堆晶辉长岩中获得Sm-Nd法同位素年龄为191±22Ma,时代为早侏罗世;(3)洞错一带少量硅质岩中放射虫时代为侏罗纪。然而,鲍佩声等(2007)在洞错蛇绿岩橄榄辉长岩中获得132Ma的SHRIMP锆石U-Pb谐和年龄,在玄武岩中获得137Ma和141Ma的全岩40Ar/39Ar年龄,时代均为早白垩世;Wang et al.(2016)在洞错蛇绿岩辉长岩中获得LA-ICP-MS锆石U-Pb谐和年龄为167Ma,时代为中侏罗世。武勇等(2018)在洞错蛇绿岩辉长岩中获得225Ma的LA-ICP-MS锆石U-Pb谐和年龄,时代为晚三叠世。陈志(2016)在洞错以北的才隆拉地区灰岩块体中采获了较多的三叠纪牙形石化石,并将其解释为仲岗洋岛型岩石组合中的生物化石,但详细的对比发现,这些含三叠世牙形石化石的灰岩块体,全部出露于洞错蛇绿岩之内,且其围岩均为洞错蛇绿岩的变质橄榄岩。根据大洋板块地层理论和洋底核杂岩理论,在蛇绿岩形成的洋中脊环境,也有灰岩的沉积,且灰岩可与橄榄岩等直接接触,并在后续的俯冲增生和造山过程中,保存于缝合带内。因此,洞错以北才隆拉地区灰岩块体内的三叠纪牙形石化石,可能反映的是洞错蛇绿岩的年龄信息。

关于洞错蛇绿岩的构造背景,争议也较大。张玉修(2007)通过地幔橄榄岩和堆晶岩的地球化学分析和尖晶石矿物化学分析后得出,洞错蛇绿岩兼具MORB和IAT特征,应形成于不成熟的弧后盆地环境。Wang et al.(2008)通过角闪岩的研究得出洞错蛇绿岩形成于一个短命的弧后盆地环境。曾庆高等(2010)在该地区开展区域地质调查时,认为洞错蛇绿岩应形成于与初始拉张洋盆有关的洋中脊环境。鲍佩声等(2007)通过洞错蛇绿岩的岩石学、玄武岩地球化学和Sr-Nd同位素学的研究后认为,洞错蛇绿岩形成于有大量富集地幔物质上涌的洋岛(OIB)环境。Zhang et al.(2014)得出了类似的结论,认为洞错蛇绿岩可能是班公湖-怒江洋洋底高原的一部分。李建峰等(2013)对洞错蛇绿岩地幔橄榄岩和均质辉长岩中的尖晶石和辉石进行了详细的矿物化学成分测定和构造环境判别,得出洞错蛇绿岩形成于正常洋中脊环境。Wang et al.(2016)通过洞错蛇绿岩堆晶辉长岩、辉绿岩岩墙和玄武岩的地球化学分析后得出洞错蛇绿岩可能形成于洋内俯冲背景下,并受到了后期OIB物质的影响。

综上所述,关于洞错蛇绿岩的时代,存在着晚三叠世、侏罗纪和早白垩世等多种观点。关于洞错蛇绿岩的构造背景,也存在着弧后盆地、洋中脊、洋岛、洋底高原和洋内俯冲等多种模型。我们更倾向于认为洞错蛇绿岩是多期次构造混杂组成的混杂体,洞错蛇绿岩不同地区的蛇绿岩残块可能代表了不同时期,不同构造背景的洋壳残余(范建军等,2018),但基于大部分测年和化石均集中在侏罗纪,我们推测洞错蛇绿岩主体应形成于侏罗纪。对洞错蛇绿岩开展深入研究,是解决班公湖-怒江洋侏罗纪构造演化争论的关键之一。

为精确制约班公湖-怒江洋侏罗纪构造演化,我们选取洞错蛇绿岩出露情况较好,岩石组合较为复杂的那热村以南一带开展研究。详细的路线踏勘表明(图 1),那热村以南的洞错蛇绿岩残块主体由堆晶岩(包括堆晶辉长岩和斜长花岗岩)、辉长岩墙和枕状熔岩等组成,其中堆晶岩在野外和镜下均表现出明显的堆晶结构(图 2a, b)。辉长岩墙野外呈灰色(图 2c),镜下表现为辉长结构(图 2d)。在那热村以南蛇绿岩残块附近的舍拉玛沟及拉他沟一带,前人报道了大规模席状岩墙群的存在(曾庆高等,2010)。本文研究的辉长岩墙与堆晶岩断层接触,其是否是区域上席状岩墙群的一部分,有待进一步研究和确认。枕状熔岩岩性主要为玄武岩和玄武安山岩(图 2e),单个岩枕直径约20~50cm。

图 2 西藏改则那热村以南洞错蛇绿岩残块的野外及镜下照片 堆晶岩野外(a)和镜下(b)照片;辉长岩墙野外(c)和镜下(d)照片;(e)枕状熔岩野外照片;辉绿岩脉野外(f)和镜下(g)照片;(h)橄榄岩近景照片.Px-辉石;Pl-斜长石 Fig. 2 Photographs and photomicrographs of the Dong Co ophiolite within the Nare Village of Gerze County, Tibet

在那热村以南蛇绿岩残块的堆晶岩中见有较多的辉绿岩脉侵入(图 2f);在区域上舍拉玛沟及拉他沟席状岩墙群中,前人也报道了这类辉绿岩脉的侵入(曾庆高等,2010)。辉绿岩脉单个岩脉宽度变化较大,最宽可达100cm,最窄不足5cm(图 2f)。在显微镜下,辉绿岩脉呈现典型的辉绿结构(图 2g)。此外,在那热村以北,甚至在那热村以南的蛇绿岩残块中,前人也报道了较多的橄榄岩等超镁铁质岩的出露(图 2h武勇等,2018)。

2 分析方法 2.1 主微量元素测试方法

为确定那热村以南洞错蛇绿岩残块的构造属性,我们采集12件辉长岩墙样品和3件辉绿岩脉样品进行全岩主微量元素测试分析。测试样品均在河北省地质调查研究院实验室经过无污染碎样至200目,测试工作在中国地质大学(北京)地学实验中心完成。主量元素采用X-射线荧光光谱仪(XRF-1500)分析。微量和稀土元素的化学预处理采用两酸(硝酸和氢氟酸)高压反应釜溶样方法,分析仪器为Agilent 7500a型等离子质谱仪,分析方法详见于红(2011)

2.2 锆石U-Pb年代学测试方法

为确定那热村以南洞错蛇绿岩残块的形成时代,我们采集了1件堆晶辉长岩、2件辉长岩墙样品和1件辉绿岩脉进行锆石U-Pb测年。测试样品均在河北省区域地质调查所实验室采用常规方法进行锆石单矿物分选。样品靶在中国地质科学院地质研究所制备。锆石的阴极荧光图像分析(CL)在中国地质科学院大陆动力学实验室完成,并在中国地质大学(北京)地学实验中心进行了透射光和反射光显微照相。

1件堆晶辉长岩样品(编号为DT19)和1件辉长岩墙样品(编号为DT22)的锆石U-Pb测试工作在中国地质大学(北京)地学实验中心完成,所使用的ICP-MS为美国Agilent科技有限公司的7500A ICP-MS,激光剥蚀系统为美国New Wave贸易有限公司UP193SS型,深紫外(DUV)193nm、ArF准分子激光剥蚀系统。激光束斑直径为36μm,剥蚀采样时间为45s,具体分析流程见Yuan et al.(2004)。另外1件辉长岩墙样品(编号为DT21)和1件辉绿岩脉(编号为DT20)的锆石U-Pb测年工作在自然资源部东北亚矿产资源评价重点实验完成。所使用的ICP-MS为美国Agilent科技有限公司的7900 ICP-MS,激光剥蚀系统为美国New Wave贸易有限公司COMPEx Pro型,深紫外(DUV)193nm、ArF准分子激光剥蚀系统。激光束斑直径为32μm,剥蚀采样时间为45s,具体分析流程见Yuan et al.(2004)

2.3 锆石Lu-Hf同位素测试方法

为进一步探讨那热村以南洞错蛇绿岩残块的成因,我们在锆石U-Pb测年结果的基础上,对1件辉长岩墙和1件辉绿岩脉样品进行了锆石Lu-Hf原位同位素分析。辉长岩墙锆石Lu-Hf同位素分析在南京大学内生金属矿床成矿机制研究国家重点实验室完成,使用仪器为Thermo Neptune Plus多接收等离子质谱和New Wave UP193激光剥蚀系统(MC-ICP-MS)。测试分析的激光束斑直径为44μm,脉冲频率为6Hz。测试时所用锆石标样为91500和锆石MT。

辉绿岩脉锆石Lu-Hf同位素分析在北京科荟测试技术有限公司完成,采用的激光剥蚀多接收器电感耦合等离子体质谱仪(MC-IPC-MS)及213nm的激光剥蚀取样系统。测试分析的激光束斑直径为44μm,脉冲频率为6Hz。测试时所用锆石标样为GJ-1。

锆石Lu-Hf同位素测试详细的测试流程和方法见Hu et al.(2012)

3 测试结果 3.1 全岩主微量元素测试结果

辉长岩墙和辉绿岩脉全岩主微量测试结果见表 1

表 1 辉长岩墙和辉绿岩脉主量元素(wt%)与微量元素(×10-6)分析结果 Table 1 Major (wt%) and trace (×10-6) elements data for the gabbro dyke and diabase vein
3.1.1 辉长岩墙

辉长岩墙全岩主微量元素测试结果显示,它们具有较高的SiO2(51.0%~56.8%)、MgO(6.78%~8.32%)和CaO(9.39%~12.7%)含量以及高的Mg#(63.2~67.2),中等的Al2O3(12.3%~14.9%)和Fe2O3(8.28%~9.62%)含量,低的TiO2(0.63%~0.79%)、Na2O(0.91%~2.84%)、K2O(0.25%~1.11%)和P2O5(0.05%~0.07%)含量。在Nb/Y-Zr/TiO2不活动元素分类图解上,辉长岩墙样品落入了亚碱性玄武岩和安山玄武岩的交界区域(图 3a)。在AFM图解中,大部分样品落入拉斑玄武岩区域,少部分样品落入拉斑玄武岩和钙碱性玄武岩的交界区域(图 3b)。

图 3 辉长岩墙和辉绿岩脉分类图解 (a) Nb/Y-Zr/TiO2图解(Winchester and Floyd, 1976);(b) AFM图解(Irvine and Baragar, 1971) Fig. 3 Classification diagrams for the gabbro dyke and diabase vein

辉长岩墙稀土元素含量较低,总量变化于21.1×10-6~26.9×10-6之间,平均为24.0×10-6。在球粒陨石标准化稀土元素配分曲线中(图 4a),所有分析样品均表现为平坦型,轻稀土弱亏损,(La/Yb)N介于0.56~0.73,无Eu异常(Eu/Eu*值介于0.75~1.04,平均0.97),与N-MORB曲线类似。在原始地幔标准化微量元素蛛网图中(图 4b),辉长岩墙富集Rb、Ba、U、Pb和Sr等大离子亲石元素,亏损Th、Nb和Ta等高场强元素,类似于岛弧玄武岩特征。

图 4 辉长岩墙和辉绿岩脉的球粒陨石标准化稀土元素配分图和原始地幔标准化微量元素蛛网图(标准化值据Sun and McDonough, 1989) Fig. 4 Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element spider diagrams (b) of the gabbro dyke and diabase vein (normalization values after Sun and McDonough, 1989)
3.1.2 辉绿岩脉

与辉长岩墙相比,辉绿岩脉具有相似的SiO2(54.2%~54.6%)、Al2O3(13.7%~13.8%)、CaO(9.97%~10.4%)和Fe2O3(7.07%~7.12%)含量,但具有更高的MgO含量(9.36%~9.60%)和Mg#(75.4~75.9),更低的TiO2(0.45%~0.48%)和K2O(0.15%~0.22%)含量。在Nb/Y-Zr/TiO2不活动元素分类图解上,辉绿岩脉落入了安山玄武岩区域(图 3a)。在AFM图解中,落入拉斑玄武岩区域(图 3b)。

与辉长岩墙相比,辉绿岩脉具有较低的稀土总量(16.9×10-6~21.5×10-6)。在球粒陨石标准化稀土元素配分曲线中(图 4a),表现为平坦型,与N-MORB曲线类似。辉绿岩脉具有更弱的轻稀土亏损,(La/Yb)N相对较高(0.67~0.94),Eu异常不明显(Eu/Eu*值为0.90~0.98)。与辉长岩墙相似,在原始地幔标准化微量元素蛛网图中(图 4b),辉绿岩脉富集Rb、Ba、U、Pb和Sr等大离子亲石元素,亏损Th、Nb和Ta等高场强元素。

3.2 LA-ICP-MS锆石U-Pb测年结果

堆晶辉长岩锆石U-Pb测年数据见表 2,辉长岩墙锆石U-Pb测年数据见表 3,辉绿岩脉锆石U-Pb测年数据见表 4

表 2 堆晶辉长岩锆石LA-ICP-MS U-Pb同位素分析结果 Table 2 Zircon LA-ICP-MS U-Pb data of the layered gabbro

表 3 辉长岩墙锆石LA-ICP-MS U-Pb同位素分析结果 Table 3 Zircon LA-ICP-MS U-Pb data of the gabbro dyke

表 4 辉绿岩脉锆石LA-ICP-MS U-Pb同位素分析结果 Table 4 Zircon LA-ICP-MS U-Pb data of the diabase vein
3.2.1 堆晶辉长岩

堆晶辉长岩中的锆石较少,颗粒直径通常介于100~200μm(图 5a)。锆石多呈他形短柱状或粒状,长宽比变化于1:1到2:1之间。在CL图像上,锆石多呈半透明,浅灰色-深灰色,发育宽缓的岩浆震荡环带(图 5a),显示了基性岩浆锆石的特点(Hoskin and Black, 2000)。9颗锆石获得206Pb/238U谐和年龄为172.4±0.8Ma(MSWD=1.10;图 5b)。

图 5 堆晶辉长岩、辉长岩墙和辉绿岩脉LA-ICP-MS锆石U-Pb年龄谐和图 Fig. 5 Results of LA-ICP-MS zircon U-Pb dating for the layered gabbro, gabbro dyke and diabase vein
3.2.2 辉长岩墙

辉长岩墙中含有的锆石较多,锆石颗粒也较大,颗粒直径普遍在200μm(图 5b, c)左右。锆石多呈板条状,长宽比变化于2:1到3:1之间。在CL图像上,锆石多呈半透明,无色或者浅灰色,发育宽缓的岩浆震荡环带(图 5b, c),显示了基性岩浆锆石的特点(Hoskin and Black, 2000)。2件辉长岩墙样品获得的206Pb/238U谐和年龄分别为169.2±1.1Ma(MSWD=0.94;图 5b)和167.6±0.55Ma(MSWD=0.00017;图 5c)。

3.2.3 辉绿岩脉

与堆晶辉长岩相似,辉绿岩脉含有锆石较少,但单颗锆石颗粒粒径较大,颗粒直径介于100~200μm(图 5d)。锆石多呈它形短柱状或粒状,少量呈板条状,长宽比变化于1:1到3:1之间。在CL图像上,锆石多呈半透明,无色或者浅灰色,发育宽缓的岩浆震荡环带(图 5d),显示了基性岩浆锆石的特点(Hoskin and Black, 2000)。12颗锆石获得206Pb/238U谐和年龄为165.3±1.2Ma(MSWD=0.0005;图 5d)。

3.3 锆石Lu-Hf同位素测试结果

辉长岩墙和辉绿岩脉锆石Lu-Hf同位素分析结果见表 5

表 5 辉绿岩脉和辉长岩墙锆石Lu-Hf同位素分析结果 Table 5 Zircon Lu-Hf isotopic composition of the diabase vein and gabbro dyke

辉长岩墙锆石初始的176Hf/177Hf比值介于0.283148~0.283229之间,获得的εHf(t)值介于+16.5~+19.5之间,平均为+18.5;辉绿岩脉锆石初始的176Hf/177Hf比值介于0.283164~0.283305之间,获得的εHf(t)值介于+17.3~+22.3之间,平均为+19.0。

4 讨论 4.1 岩石成因

通常高场强元素(如Zr、Nb和Yb)被用来判别岩浆源区,它们的比值(如Nb/Yb)可作为判断地幔富集程度的指示剂(Pearce and Stern, 2006)。辉长岩墙Nb/Yb比值介于0.27~0.43之间,与辉绿岩脉的Nb/Yb比值(0.11~0.35)相似,表明二者来自相似的地幔源区。为确定二者的地幔源区,我们计算了辉长岩墙和辉绿岩脉的δNb(δNb=1.74+log(Nb/Y)-1.92log(Zr/Y)值(Fitton et al., 1997)。结果显示,无论是辉长岩墙(δNb=-0.2~-0.4),还是辉绿岩脉(δNb=-0.4~-1.0),δNb值均小于0,说明他们来自于亏损地幔源区(Fitton et al., 1997)。该点推论也得到了辉长岩墙和辉绿岩脉极高的正的εHf(t)值(分别为+16.5~+19.5和+17.3~+22.3)的支持。

辉长岩墙和辉绿岩脉均富集大离子亲石元素Rb、Ba、U、Pb和Sr,亏损高场强元素Nb、Ta等(图 4b),表明在他们的形成过程中,具有俯冲物质的加入。俯冲物质也具有多样性的特点,包括俯冲洋壳、俯冲沉积物、以及俯冲洋壳和沉积物脱水过程中产生的流体等(Pearce and Peate, 1995)。一般而言,俯冲洋壳熔融物质的加入会导致岩石中Nb含量的增高(Kepezhinskas et al., 1997),但辉长岩墙和辉绿岩脉含有较低的Nb含量(分别为0.44×10-6~0.81×10-6和0.13×10-6~0.48×10-6),表明在他们形成过程中,并没有俯冲洋壳熔融物质的加入。

通常认为高Sr/Nd比值归因于板片流体,而高Th/Yb比值则归因于俯冲沉积物的加入(Davidson, 1987)。辉长岩墙和辉绿岩脉具有高的Sr/Nd比值(分别为52~114和34~42),明显高于N-MORB的相应值(12.33,Sun and McDonough, 1989)和上地壳的相应值(11.85,Rudnick and Gao, 2003)。Th/Yb比值分别为0.04~0.06和0.05~0.11,类似于N-MORB(0.04,Sun and McDonough, 1989)和E-MORB(0.25,Sun and McDonough, 1989)。这些暗示了辉长岩墙和辉绿岩脉在形成过程中受到了较为强烈的俯冲流体的影响,而缺少俯冲沉积物的影响。

综上所述,我们得出那热村以南洞错蛇绿岩残块中的辉长岩墙和辉绿岩脉均来自亏损的地幔源区,形成过程中受到了俯冲流体的影响。

4.2 构造背景

那热村以南洞错蛇绿岩残块中的辉长岩墙和辉绿岩脉均微弱亏损LREE,具有与N-MORB相似的球粒陨石标准化配分曲线(图 4a);它们具有非常低的Th/Yb比值(0.04~0.06和0.05~0.11)和较高的Zr/Nb比值(48.4~67.5和67.7~223),也与N-MORB相似(Th/Yb=0.04;Zr/Nb>30;Sun and McDonough, 1989)。在Nb×2-Zr/4-Y图解(图 6a)中,所有样品落入了正常洋中脊区域。然而,辉长岩墙和辉绿岩脉的Nb/U和Ta/U等比值分别为6.33~9.00和1.44~3.20、0.33~0.67和0.11~0.27,低于N-MORB的相应值(分别为15.95和1.22;Hofmann,1997Sun and MeDonough, 1989),表明虽然辉长岩墙和辉绿岩脉具有N-MORB的亲缘性,但并非典型的N-MORB。辉长岩墙和辉绿岩脉的TiO2的含量(分别为0.63%~0.79%和0.45%~0.48%)明显低于洋中脊玄武岩TiO2的平均含量(1%~1.5%),而类似于岛弧玄武岩TiO2的平均含量(0.8%;Irvine and Baragar, 1971Winchester and Floyd, 1976Pearce, 1983Crawford et al., 1989)。在原始地幔标准化微量元素蛛网图中(图 4b),它们表现了Nb和Ta等高场强元素的亏损,且在La/Nb-La图解中(图 6b),辉长岩墙落入了岛弧玄武岩和洋中脊玄武岩的交界区域,辉绿岩脉落入了岛弧玄武岩区域。在La/Nb-La图解中(图 6c),二者均落入岛弧玄武岩区域。以上特征表明,辉长岩墙和辉绿岩脉兼具N-MORB和岛弧玄武岩地球化学特征,这样的地球化学特征与前人总结的洋内弧环境中的前弧玄武岩(FAB或MORB-like玄武岩)比较相似(肖庆辉等,2016)。在V-Ti/1000图解中(图 6d),辉长岩墙样品落入了Izu-Bonin-Mariana(IBM)前弧玄武岩附近,而辉绿岩脉样品则落入了IBM玻安岩区域,支持它们形成于洋内弧的前弧环境。

图 6 辉长岩墙和辉绿岩脉构造环境判别图解 (a) Nb×2-Zr/4-Y图解(Meschede, 1986);(b) La/Nb-La图解;(c) Nb/Th-Nb图解(b, c, 李曙光, 1993);(d) V-Ti/1000图解(Shervais, 1982; Zhong et al., 2017). AI+AII-板内碱性玄武岩; AII+C-板内拉班玄武岩; B-E型洋中脊玄武岩;D-正常洋中脊玄武岩;OIB-洋岛玄武岩;MORB-洋中脊玄武岩;BABB-弧后盆地玄武岩;IAT-岛弧拉斑玄武岩;IBM FAB-西太平洋伊豆-小笠原-马里亚纳前弧玄武岩 Fig. 6 Discrimination diagrams for the gabbro dyke and diabase vein

与IBM典型的前弧玄武岩相比,辉长岩墙具有高的SiO2含量(51.0%~56.8%),低的TiO2(0.63%~0.79%)、Nb(0.44×10-6~0.86×10-6)和Ta(0.03×10-6~0.06×10-6)等高场强元素含量,且更加富集Rb、Ba、U和Pb等大离子亲石元素(图 4b),显示了更强的俯冲流体的影响,类似于IBM前弧玄武岩上部向玻安岩过渡的过渡型前弧玄武岩(Reagan et al., 2010)。

依据野外接触关系和锆石U-Pb测年结果,辉绿岩脉的形成时代略晚于辉长岩墙(图 5)。相比辉长岩墙,辉绿岩脉具有更高的Cr(474×10-6~597×10-6)、Ni(168×10-6~200×10-6)和MgO(9.36%~9.60%)含量,更高的Mg#值(75.4~75.9),更低的TiO2含量(0.45%~0.48%)、更弱的LREE的亏损(图 4a)和更低的稀土总量,且在V-Ti/1000图解中(图 6d)中,落入了IBM玻安岩区域,显示了玻安岩的亲缘性。但它并不具有玻安岩所特有的“U”型球粒陨石标准化稀土元素配分曲线(图 4a),表明虽然辉绿岩脉具有玻安岩的亲缘性,但并非典型的玻安岩。鉴于此,本文暂将其称为玻安质岩石。从早期的辉长岩墙,到后期的辉绿岩脉,在地球化学特征上显示出从过渡型前弧玄武岩向玻安质岩石渐变的演化趋势,这样的地球化学演化趋势与前人总结的洋内弧前弧环境下的岩浆岩演化趋势也是可以对比的(Reagan et al., 2010肖庆辉等,2016)。

为进一步确定那热村以南洞错蛇绿岩残块是否形成于洋内弧的前弧区域,我们将它们按照形成时代建立一个岩石柱状图(包括那热村附近的橄榄岩和枕状玄武岩等),并与现今IBM前弧岩石柱状图、古造山带的特鲁多斯和赛迈尔等洋内弧成因的岩石柱状图进行对比(Ishizuka et al., 2014)。结果表明,那热村以南洞错蛇绿岩残块与典型洋内弧岩石层序底部的蛇绿岩组合是可以对比的(图 7),仅缺乏玻安岩、高镁安山岩和岛弧钙碱性岩浆岩等洋内弧岩石层序的上部端元。但在班公湖-怒江缝合带中西段,与那热村以南洞错蛇绿岩残块同时期的玻安岩、高镁安山岩和岛弧钙碱性岩浆岩是存在的,如缝合带西段班公湖一带存在玻安岩(167Ma;史仁灯,2007Shi et al., 2008)和岛弧钙碱性岩浆岩(164Ma;周涛等,2014),缝合带中段达如错地区存在高镁安山岩(164~162Ma;李小波等,2015Zeng et al., 2016b)。如果我们将这些玻安岩、高镁安山岩、岛弧钙碱性岩浆岩和本文的玻安质岩石等一起放到对比图中(图 7),可以看到,它们与那热村以南洞错蛇绿岩残块一起组成了相对完整的洋内弧岩石层序,共同记录了班公湖-怒江洋洋内俯冲事件。

图 7 班公湖-怒江缝合带洋内弧岩石层序与典型地区洋内弧岩石层序的对比 特鲁多斯、赛迈尔和IBM前弧岩石层序引自Ishizuka et al.(2014) Fig. 7 Stratigraphic column of the intra-oceanic subduction-related rock assemblage in the Bangong-Nujiang suture zone and other typical areas

从班公湖-怒江缝合带洋内弧岩石层序上看(图 7),底部的堆晶岩形成于~172Ma,属于中侏罗世最早期,最底部的橄榄岩和班公湖-怒江洋洋内俯冲的初始启动时间应略早于堆晶岩的形成时代。我们推测,班公湖-怒江洋洋内俯冲可能初始启动于早侏罗世晚期,主体活动于中侏罗世。

如果班公湖-怒江洋洋内俯冲初始启动于早侏罗世晚期,如何解释前人在洞错一带报道的晚三叠世洋内俯冲成因的辉长岩呢?详细对比发现,前人报道的晚三叠世洋内俯冲成因的辉长岩具有极高的MgO(平均16.2%)和Mg#值(平均84),极低的TiO2(平均0.11%)和显著的Eu、Sr正异常,且野外照片也显示了堆晶结构(武勇等,2018)。我们推测,前人报道的晚三叠世辉长岩可能经历了较强的堆晶作用,已经不能代表原始岩浆。晚三叠世辉长岩的成因及班公湖-怒江洋晚三叠世是否存在洋内俯冲,有待更多资料的深入研究。

综上所述,我们得出那热村以南洞错蛇绿岩残块形成于洋内俯冲环境,其成因模式概述如下:在早侏罗世晚期,班公湖-怒江洋发生初始洋内俯冲,俯冲大洋板块下沉且快速后退,于中侏罗世早期(172~168Ma)引起弧前拉张,软流圈上涌形成橄榄岩、层状堆晶岩和辉长岩墙等蛇绿岩组分(Stern et al., 1992Reagan et al., 2010肖庆辉等,2016)。在这个过程中,下伏俯冲板片脱水并参与蛇绿岩的形成,造成了辉长岩墙兼具N-MORB和岛弧玄武岩的地球化学特征。随着俯冲作用的继续,俯冲板片继续脱水并交代难熔软流圈,使其继续熔融(Shervais,2001肖庆辉等,2016),至中侏罗世晚期(167~162Ma),洋内弧发育成熟,形成玻安岩、高镁安山岩、岛弧钙碱性岩浆岩及本文的玻安质辉绿岩脉等。

4.3 班公湖-怒江洋早-中侏罗世构造演化

本文新识别的洋内俯冲成因的蛇绿岩岩石组合,及该岩石组合与班公湖-怒江缝合带内同时期的玻安岩、高镁安山岩和岛弧钙碱性岩浆岩等组成的相对完整的洋内弧岩石层序,表明班公湖-怒江洋在早侏罗世晚期-中侏罗世正在进行洋内俯冲。该推论也得到了缝合带内已报道的中侏罗世蛇绿岩残块普遍具有前弧玄武岩或洋内弧弧后盆地玄武岩地球化学特征的支持(Liu et al., 2014Wang et al., 2016Huang et al., 2017aTang et al., 2018)。

在班公湖-怒江缝合带以北的南羌塘地体之上,前人也报道了较多的早-中侏罗世的钙碱性岩浆岩(Li et al., 2014a, bLiu et al., 2014, 2017Wu et al., 2016),是班公湖-怒江洋北向发生洋-陆俯冲的岩浆岩记录。

基于上述分析,我们初步得出在早-中侏罗世时期,班公湖-怒江洋可能同时存在洋内俯冲和洋-陆俯冲,大洋处于快速消减期。

早-中侏罗世洋内俯冲和洋-陆俯冲同时存在,说明在早-中侏罗世之前,班公湖-怒江洋已经进行了较长时间的大洋化过程,且洋盆已经具有一定规模,否则不可能同时存在洋内俯冲和洋-陆俯冲,该点认识不支持班公湖-怒江洋在早-中侏罗世尚处于威尔逊旋回幼年期-青年期的推论,也不支持班公湖-怒江洋是一个局限、短命的小洋盆,仅在侏罗纪就完成打开和闭合等过程的推论。班公湖-怒江缝合带内洞错榴辉岩晚二叠世末期-早三叠世的原岩年龄(原岩为MORB或OIB型基性岩,王保弟等,2015Zhang et al., 2017)、早-中三叠世那热洋岛残片(Fan et al., 2018b)和晚三叠世孤峰洋岛残片(Fan et al., 2017)等的发现表明至少在晚二叠世末期-早三叠世,班公湖-怒江洋已经存在,且在早-中三叠世时期已经发育成熟(Fan et al., 2018b)。班公湖-怒江洋的“幼年期”阶段应早于晚二叠世末期-早三叠世。

班公湖-怒江洋洋内弧成熟于中侏罗世最晚期(图 7),且极有可能在晚侏罗世仍在活动,不支持该大洋在中侏罗世已经闭合消亡的推论。在班公湖-怒江洋演化的晚期阶段,即晚侏罗世-早白垩世早期,于缝合带及北侧报道了较多碰撞造山的物质记录,如晚侏罗世-早白垩世沙木罗组与蛇绿岩等的不整合(文世宣,1979陈国荣等,2004孙立新,2005)、南羌塘地体上晚侏罗世以来大规模的海陆变迁(Fan et al., 2018a)和早白垩世早期(140~130Ma)的岩浆岩间歇(Li et al., 2014aZhu et al., 2016)。针对这期碰撞造山的构造事件,大多数学者将它们解释为班公湖-怒江洋最终的闭合消亡(陈国荣等,2004孙立新,2005Kapp et al., 2007Zhu et al., 2016Huang et al., 2017bLi et al., 2017黄童童,2017),但基于本文早-中侏罗世洋内弧的厘定,及该洋内弧在晚侏罗世仍可能活动的推论,我们推测班公湖-怒江缝合带及北侧晚侏罗世-早白垩世早期碰撞造山的物质记录可能对应于该期洋内弧与南羌塘地体的碰撞拼贴,但该推论尚需大量后期工作的证实。

5 结论

(1) 那热村以南洞错蛇绿岩残块主要由橄榄岩、堆晶岩、辉长岩墙、枕状熔岩和辉绿岩脉等组成,形成于中侏罗世(172~165Ma)。

(2) 辉长岩墙和辉绿岩脉兼具N-MORB和岛弧玄武岩的地球化学特征。它们均来自于亏损的地幔源区,在形成过程中,受到了俯冲流体的影响。

(3) 那热村以南洞错蛇绿岩残块形成于洋内俯冲背景,它们与缝合带内同时期的玻安岩、高镁安山岩和岛弧岩浆岩一起组成了相对完整的洋内弧岩石层序,共同记录了班公湖-怒江洋早-中侏罗世洋内俯冲事件。

(4) 在早-中侏罗世时期,班公湖-怒江洋同时存在洋内俯冲和洋-陆俯冲,大洋处于快速消减期,即威尔逊旋回的“消亡期”。

致谢      野外工作得到了张天羽博士、吴浩博士、曾孝文硕士和李航硕士等的帮助和支持;两位审稿人对本文提出了建设性的修改意见;在此一并表示感谢。

恰逢肖序常先生90华诞,谨以此文表达对他的衷心祝愿。

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