岩石学报  2013, Vol. 29 Issue (11): 3659-3670   PDF    
为什么要提出西藏东南部早白垩世措美大火成岩省
朱弟成1, 夏瑛1, 裘碧波2, 王青1, 赵志丹1     
1. 地质过程与矿产资源国家重点实验室,中国地质大学地球科学与资源学院,北京 100083;
2. 中国海洋大学,青岛 266003
摘要: 近年在西藏东南部特提斯喜马拉雅带东段大规模白垩纪火成岩受到了很多学者的关注。这里的火成岩岩石类型包括玄武岩、镁铁质岩墙/岩床、辉长岩侵入体以及少量层状超镁铁质岩和酸性火山岩。锆石U-Pb定年结果指示现今覆盖面积约50000km2的岩浆活动发生在130~136Ma(峰期约132Ma)之间。镁铁质岩显示OIB型(高Ti)、N-MORB型(低Ti)和过渡型(介于二者之间)三种地球化学类型,其中未受地壳混染的镁铁质岩的Sr-Nd同位素和锆石Hf同位素成分类似于Kerguelen地幔柱产物。在扣除堆晶橄榄石之后,通过橄榄石-熔体平衡计算,苦橄玢岩母岩浆的MgO含量约20%,对应的地幔潜温>1560℃。西藏东南部白垩纪火成岩浆活动这种覆盖范围大、持续时间短和地幔潜温高等特征,非常类似于世界上其它地幔柱成因的大火成岩省或热点,因而将其描述和命名为措美(Comei)大火成岩省是合理的。年代学、地球化学和古地理重建资料显示藏南措美大火成岩省和南西澳大利亚同期的Bunbury玄武岩可能代表了同一个大火成岩省(即Comei-Bunbury大火成岩省)。Comei-Bunbury大火成岩省很可能记录了Kerguelen地幔柱在132Ma左右的早期岩浆作用,拉开了大印度从澳大利亚分离出来的序幕,影响了同期Weissert大洋缺氧事件的形成。
关键词: 锆石U-Pb年龄     早白垩世措美大火成岩省     Kerguelen地幔柱     地幔潜温     东冈瓦纳大陆裂解     Weissert大洋缺氧事件     特提斯喜马拉雅带     藏东南    
Why do we need to propose the Early Cretaceous Comei large igneous province in southeastern Tibet?
ZHU DiCheng1, XIA Ying1, QIU BiBo2, WANG Qing1, ZHAO ZhiDan1     
1. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China;
2. Ocean University of China, Qingdao 266003, China
Abstract: Growing attention has been focused on the extensive igneous rocks in eastern Tethyan Himalaya, southeastern Tibet. The rock types here include basalts, mafic dikes/sills, gabbroic intrusions, and minor layered ultramafic and silicic volcanic rocks. Zircon U-Pb dating indicates that these rocks were emplaced between 130~136Ma (peaked at ca. 132Ma), covering an areal of ca. 50000km2 in the present-day southeastern Tibet. The mafic rocks show OIB-type (high-Ti), N-MORB-type (low-Ti), and transitional (between the first two types) geochemical types. Whole-rock Sr-Nd and zircon Hf isotopic compositions of the uncontaminated mafic rocks resemble the products of the Kerguelen mantle plume. After removing the effect of cumulated olivine, about 20% MgO content is obtained for the parental magmas of the picritic porphyrites through olivine-liquid equilibrium calculation, corresponding to mantle potential temperature at about 1560℃. The large areal extent, short duration of magmatism, and high mantle potential temperature of the extensive igneous rocks in southeastern Tibet are well comparable with mantle plume-related large igneous province (LIP) worldwide and thus can be best described as a large igneous province, i.e. the Comei LIP. Geochronological, geochemical, and paleogeographical data suggest that the Comei LIP in SE Tibet and the coeval Bunbury basalts in SW Australia may represent a single LIP (i.e. the Comei-Bunbury LIP). The Comei-Bunbury LIP documents the early stage magmatism of the Kerguelen mantle plume at about 132Ma, resulting in the separation of the Great India from Australia and the development of the Weissert Oceanic anoxic events.
Key words: Zircon U-Pb age     Early Cretaceous Comei large igneous province     Kerguelen mantle plume     Mantle potential temperature     Breakup of Eastern Gondwana     Weissert Oceanic anoxic events     Tethyan Himalaya     Southeastern Tibet    
1 引言

大火成岩省(Large Igneous Province,LIP)一般用来表示侵位于较短时间(1~5Myr)内、覆盖面积超过50000km2(Sheth, 2007)或100000km2(Coffin and Eldholm, 1994; Bryan and Ernst, 2008)的各种以镁铁质岩石为主的火成岩岩石组合,其形成常与地幔柱上升和岩石圈伸展所引起的大规模减压熔融有关(Richards et al., 1989; White and McKenzie, 1989)。这种异常强烈的大规模岩浆活动可引起大陆裂解和洋盆开启(Storey, 1995; Segev, 2002),造成环境改变并导致生物绝灭(White and Saunders, 2005; Bryan and Ferrari, 2013),因此,大火成岩省的识别及其在超大陆重建中的意义是国内外学术界近年来关注的热点科学问题(见: Ernst et al., 2013)。在青藏高原南部,很早就有学者注意到作为特提斯喜马拉雅带沉积序列一部分的大规模白垩纪岩浆作用记录(图 1a)(西藏自治区地质矿产局, 1993)。随着区域地质调查的完成和相关研究工作的深入,这期岩浆作用受到了很多学者的关注,一些学者将其与雅鲁藏布特提斯洋的大规模扩张联系起来(钟华明等, 2005; Jiang et al., 2006; 江思宏等, 2007; 童劲松等, 2007)。在国内外学者近十年工作积累基础上,Zhu et al. (2009) 首次将主要分布于措美地区(Comei)的大规模白垩纪火成岩作为一个大火成岩省来考虑,并将其与澳大利亚南西部同期Bunbury玄武岩联系起来命名为Comei-Bunbury大火成岩省。这一大火成岩省提出以后,在国际上受到了很多学者的关注(见: Pirajno and Hoatson, 2012; Ernst et al., 2013; Sensarma et al., 2013),被认为是过去25年来国际大火成岩省研究中取得的重要新进展之一(Bryan and Ferrari, 2013)。本文目的是,通过总结已有地质和地球化学资料,首先介绍作者为什么要提出地幔柱成因的大火成岩省而不是雅鲁藏布特提斯洋大规模扩张的结果来解释措美地区早白垩世大规模火成岩的成因,然后在此基础上讨论了措美大火成岩省的全球构造和环境意义。

图 1 西藏东南部特提斯喜马拉雅带中东部地质简图(据Zhu et al., 2008, 2009; 裘碧波等, 2010修改) (a)-Kerguelen地幔柱在印度洋及其周边大陆相关产物的时空分布图;(b)-藏东南构造纲要图;(c)-措美大火成岩省地质简图,其中蓝色虚线圈为最初提出的措美大火成岩省范围(Zhu et al., 2009),灰色虚线圈为后来识别出的措美大火成岩省西延范围(裘碧波等, 2010) Fig. 1 Simplified geological map of the east-central Tethyan Himalaya, southeastern Tibet (after Zhu et al., 2008, 2009; Qiu et al., 2010)
2 西藏东南部白垩纪火成岩的分布和组成

西藏东南部白垩纪火成岩主要出露于特提斯喜马拉雅带东段。特提斯喜马拉雅位于雅鲁藏布缝合带以南、高喜马拉雅结晶岩系以北(图 1b),被认为是大印度北缘晚三叠世以来被动大陆边缘的典型代表(余光明和王成善, 1990; Hu et al., 2008, 2010; Wang et al., 2011)。特提斯喜马拉雅带东段的白垩纪火成岩主要由玄武岩、镁铁质岩墙、岩床、辉长岩侵入体和少量层状超镁铁质岩和酸性火山岩组成(Zhu et al., 2009, 2013)。火山岩主要以夹层的形式赋存于下白垩统桑秀组(图 2a)和拉康组(图 2b)中,主体为玄武岩,在桑秀组中还有少量的英安岩。基性侵入岩(包括辉绿岩床/岩墙和辉长岩侵入体)广泛散布于遭受强烈构造变形的下二叠统-上侏罗统沉积地层和变形相对较弱的下白垩统沉积地层中。这些镁铁质岩床或岩墙的厚度一般介于20~30m,大多顺地层走向近东西向展布,正地形(图 2c),延伸数百米到数十千米,总体与喜马拉雅带构造线平行(图 1c)。在哲古错和错那东部谷觉一带的侏罗系-下白垩统地层中,还出露有层状超镁铁质岩(图 2d-g)。综合区域地质调查资料发现,这些白垩纪火成岩主要分布在特提斯喜马拉雅带东部江孜、康马及其以东地区,尤其是由浪卡子-拉康村-错那-隆子-琼结等围限的区域(图 1a),在该带中西部地区很少或没有出露同期火成岩(朱弟成等, 2004)。

图 2 西藏东南部措美大火成岩省的野外出露情况 Fig. 2 Field occurrences of the Comei LIP in southeastern Tibet
3 西藏东南部早白垩世措美大火成岩省的提出

为可靠约束西藏东南部白垩纪火成岩的侵位时代和岩浆源区特点,很多学者近年来已对出露于泽当-隆子-错那、泽当-琼结-措美、浪卡子-打隆镇-日莫瓦村-洞加村三条路线和江孜地区的白垩纪火成岩开展了大量高质量锆石U-Pb定年(图 1c表 1)和全岩地球化学分析。

表 1 西藏东南部措美大火成岩省的锆石U-Pb年龄数据 Table 1 Zircon U-Pb age data of the Comei LIP in southeastern Tibet
3.1 岩浆活动时代

火山岩 火山岩呈夹层出露于北部的下白垩统桑秀组和南部的下白垩统拉康组地层中(图 2a, b),显示双峰式火山岩特征(Zhu et al., 2007)。其中的玄武岩厚度变化于几米到数百米(Zhu et al., 2008),在日莫瓦村出露有一套厚约130m的流纹英安岩和流纹岩(图 2b)(Zhu et al., 2007),这里1件英安岩样品的锆石SHRIMP U-Pb年龄为133±3Ma(朱弟成等, 2005b),另1件流纹英安岩样品的锆石LA-ICPMS U-Pb年龄为131±5Ma(Zhu et al., 2009)(图 2b)。

中基性侵入岩 在江孜南尼寺,侵位于下侏罗统地层的1件辉绿岩墙样品的锆石Cameca U-Pb年龄为135.8±1.5Ma(笔者未刊数据),向东到羊卓雍错西岸白地村附近,2件侵位于上三叠统朗杰学群中的辉绿岩墙样品的锆石SHRIMP U-Pb年龄分别为134.1±2.0Ma和133.4±1.6Ma(Jiang et al., 2006)。向南到打隆镇附近,辉绿岩墙侵位于下-中三叠统地层中,1件样品的锆石SHRIMP U-Pb年龄为129.5±1.3Ma,另1件样品的锆石LA-ICPMS U-Pb年龄为133±2Ma(Zhu et al., 2009)。在洞加村机布淌,侵位于中侏罗统地层中辉长岩的锆石SHRIMP U-Pb年龄为129.7±1.4Ma,锆石LA-ICPMS U-Pb年龄为132±3Ma。在洞加村南部,辉绿岩墙的锆石LA-ICPMS U-Pb年龄为132±2Ma(Zhu et al., 2009)。在哲古错西岸,笔者最近在侵位于中侏罗统地层的2件闪长岩样品中获得了132.4±1Ma和133.7±1.4Ma的锆石LA-ICPMS U-Pb年龄(图 1a)。

超镁铁质岩 在哲古错南西岸,1件侵位于侏罗系地层中的层状橄榄辉石岩(图 2d, e)样品的锆石SHRIMP U-Pb年龄为130±2.0Ma(Zhu et al., 2009)。在错那地区,1件顺层侵位于下白垩统拉康组地层中的层状苦橄玢岩(图 2f, g)的全岩K-Ar年龄为125±11Ma (Xia et al., 2013)。

①Xia Y, Wang Q, Zhu D C, Zhao ZD, Liu SA, Wang LQ and Mo XX. 2013. Picritic porphyrites and associated basalts from the Comei Large Igneous Province in SE Tibet: Records of a mantle plume activity (in revising)

目前未能获得桑秀组和拉康组玄武岩的可靠年代学数据,但根据野外地质观察,可以推测桑秀组玄武岩可能仅仅稍早于约131~133Ma的英安岩。如在日莫瓦村,桑秀组玄武岩和上覆英安岩之间的碎屑沉积岩(粗粒石英砂岩和板岩夹砂岩)厚约70m(图 2b),这在伸展背景下可由碎屑沉积物在较短时间间隔内(如小于1~3Myr)快速堆积而成(Zhu et al., 2009)。因此,在给定分析误差条件下,英安岩的锆石U-Pb年龄(约131~133Ma)可用来代表桑秀组玄武岩年龄。

上述19件可靠的锆石U-Pb年龄数据(表 1),指示西藏东南部大规模白垩纪火成岩浆活动主要发生在130~136Ma,并且无论是根据19件样品共247个锆石测点(图 3a),还是根据19件样品的加权平均年龄计算(图 3b),均给出了约132Ma的加权平均年龄。这些数据表明,西藏东南部大规模白垩纪火成岩浆活动的峰期活动时间约132Ma。

图 3 西藏东南部措美大火成岩省的锆石U-Pb年龄柱状图 (a)-单个锆石测点的206Pb/238U年龄柱状图;(b)-不同样品的锆石U-Pb谐和年龄柱状图.数据来源同表 1 Fig. 3 Histograms of zircon U-Pb age dates for the Comei LIP
3.2 岩浆活动覆盖范围

早期报道的锆石U-Pb年龄样品来自西部羊卓雍错西岸、东部卡达村、中部措美和北部琼结等地(Zhu et al., 2009),现今覆盖面积约40000km2(图 1a)。随后对江孜和康马等地镁铁质岩石的地球化学对比(裘碧波等, 2010)和笔者最近在江孜龙马地区获得的约131Ma辉绿岩岩墙锆石U-Pb年龄数据(表 1)表明,江孜和康马地区白垩纪火成岩很可能与东部措美周边地区的火成岩同期并同成因,由此限定出西藏东南部白垩纪火成岩浆活动的现今覆盖面积约50000km2。由于特提斯喜马拉雅在新生代时期经历了强烈构造活动,地壳发生了明显缩短(Hodges, 2000),前新生代不同时代地层(包括火山岩)不可避免地发生了强烈的深位侵蚀。换句话说,现今在西藏南东部保存下来的大规模白垩纪火成岩,仅仅是约132Ma前发生的岩浆活动记录经历强烈变形、深位侵蚀和构造运动改造后的残余部分。因此,有理由相信现今西藏南东部约132Ma岩浆活动的最初覆盖范围远大于50000km2,很可能超过了大火成岩省的最低面积要求(50000km2或100000km2; Coffin and Eldholm, 1994; Sheth, 2007; Bryan and Ernst, 2008)。

3.3 全岩地球化学特征

大量地球化学数据表明,西藏南东部约132Ma的镁铁质岩显示OIB型(高Ti)、N-MORB型(低Ti)和过渡型(介于二者之间)等三种地球化学类型(图 4a, b),类似于峨眉山玄武岩(Xiao et al., 2004)。微量元素模拟和Sr-Nd同位素组成显示OIB型镁铁质岩可能与含石榴石的陆下岩石圈地幔与OIB型地幔源区物质的相互作用有关,而N-MORB型镁铁质岩可能来源于含尖晶石的岩石圈地幔与亏损软流圈地幔的相互作用,过渡型明显受到了地壳混染(Zhu et al., 2008)。羊卓雍错南部日莫瓦村的流纹英安岩和流纹岩显示A型花岗岩地球化学特征,其Sr-Nd同位素组成明显不同于镁铁质岩,很可能是长英质大陆地壳重熔的产物(朱弟成等, 2005b; Zhu et al., 2007)。

图 4 西藏东南部措美大火成岩省镁铁质岩的地球化学类型图 (a)-MgO-TiO2图;(b)-球粒陨石标准化稀土元素配分模式图.数据来源:球粒陨石数据(Sun and McDonough, 1989);措美大火成岩省镁铁质岩地球化学数据(朱弟成等, 2004, 2005a; Zhu et al., 2007, 2008; 裘碧波等, 2010; 夏瑛等, 2012) Fig. 4 Geochemical types of the mafic rocks from the Comei LIP in southeastern Tibet
3.4 苦橄玢岩和OIB型镁铁质岩的成因联系

1~10GPa压力下的相平衡实验表明,苦橄岩和科马提岩在岩石圈中分离结晶出橄榄石和/或辉石后,将转变成玄武岩(Herzberg and O'Hara, 1998)。错那地区苦橄玢岩的堆晶结构(Xia et al., 2013)和镁铁质岩中斜长石的堆晶作用(Zhu et al., 2008),均指示西藏南东部岩石圈层次存在深位岩浆房。全岩MgO-FeO模拟结果显示,西藏南东部白垩纪OIB型镁铁质岩可以由苦橄质或麦美奇质的母岩浆先分离结晶橄榄石(主要的),再经历单斜辉石和斜长石结晶分异而形成(图 5a, b)。微量元素模拟结果表明,OIB型镁铁质岩可以由苦橄玢岩的母岩浆通过50%~70%的橄榄石分离结晶形成(图 5d)。苦橄玢岩的εNd(t)值(=2.1~2.7)、共生玄武岩(=1.7)和错那OIB型镁铁质岩的εNd(t)值(1.8~4.7; Zhu et al., 2008)接近,表明苦橄玢岩和OIB型镁铁质岩有一个共同的岩浆源区。这些结果表明,错那地区深部来源的原始岩浆在岩浆房经历橄榄石堆晶形成了错那苦橄玢岩,残余熔体进一步通过辉石和斜长石分离结晶形成了错那OIB型镁铁质岩。

图 5 西藏东南部措美大火成岩省的原始岩浆成分和地幔潜温 (a)-全岩MgO-FeO图:斜线上的数字为橄榄石的Fo值,虚线代表与该Fo值平衡的熔体成分,分配系数是用Toplis (2005) 的模型于1atm下计算得出.小圆圈代表了移除1%橄榄石后的组成成分,移除橄榄石的方法来自Herzberg and Asimow (2008) .红色五角星代表了样品移除堆晶橄榄石后与该橄榄石中最高Fo值相平衡的熔体成分,即母岩浆的成分.黑色箭头代表了单斜辉石和斜长石分离结晶的趋势;(b)-全岩XFe-XMg图:斜线、小圆圈和五角星所代表的含义与图a一致.底图和地幔潜温的计算取自Putirka (2005) ,其中XFe+XMg=0.68取自Fo值最高的橄榄石,Mg-Fe分配系数KD=0.35;(c)-富集地幔橄榄岩原生岩浆绝热熔融温度-压力图解(据Herzberg and O'Hara, 2002修改):固相线左边黑线上的数字为与橄榄石相平衡的熔体的MgO含量,黑线代表了橄榄石/熔体P-T饱和路径.计算地幔潜温(TP=1463+12.74MgO-2924/MgO)所用的峨眉山大火成岩省原始岩浆的MgO含量来自Zhang et al. (2006) ;(d)-原始地幔标准化蛛网图:原始地幔数据(Sun and McDonough, 1989),黑线为模拟CN15-9-3橄榄石分离结晶的结果,线上数字代表分离橄榄石晶体的百分含量.数据来源:微量元素的橄榄石/熔体分配系数(Fujimaki et al., 1894),桑秀组玄武岩数据(Zhu et al., 2007),错那OIB型镁铁质岩数据(Zhu et al., 2008),苦橄玢岩和共生玄武岩数据(Xia et al., 2013) Fig. 5 Primitive magma compositions and mantle potential temperature calculated for the Comei LIP
3.5 苦橄玢岩恢复的地幔潜温

对错那苦橄玢岩脉进行的地球化学研究表明,苦橄玢岩高Mg#(80.6~83.1)和高Ni(1252×10-6~1718×10-6)特征(Xia et al., 2013)指示了大量橄榄石的堆晶作用(图 2g)。运用PRIMELT2(Herzberg and Asimow, 2008),在扣除36%的堆晶橄榄石后,得到了与最高Fo值(89.8)平衡的母岩浆成分(图 5a, b): SiO2≈46.6%、FeO≈13%、MgO≈20%、TiO2≈2.7%、Al2O3≈8%、Ni≈700×10-6、Cr≈1230×10-6(Xia et al., 2013)。该母岩浆成分为苦橄质(Le Maitre et al., 1989)或者麦美奇质岩石(Le Bas, 2000)。进一步运用 Herzberg et al. (2007) 模型,计算获得了错那苦橄玢岩母岩浆形成时的地幔潜温为1560~1570℃(图 5c)。利用Putirka (2005) 提出的模型也得到了同样的结果(图 5b)。这种高的地幔潜温接近于由夏威夷热点和峨眉山大火成岩省苦橄岩复原的地幔潜温(图 5b, c)(Herzberg and O'Hara, 2002; Zhang et al., 2006)。

综上所述,主要分布在特提斯喜马拉雅带东段的西藏南东部白垩纪火成岩,其最初覆盖面积很可能远大于50000km2,岩浆活动主要发生在130~134Ma(峰期约132Ma),地幔潜温(>1560℃)高出MORB地幔源区160~270℃。西藏东南部白垩纪火成岩浆活动这种覆盖范围大、持续时间短和地幔潜温高等特征,非常类似于世界上其它地幔柱成因的大火成岩省或热点。因此,根据措美县城位于这些大规模白垩纪火成岩分布区的中心位置,将其描述并命名为措美(Comei)大火成岩省是合理的。

本文注意到,对江孜、康马等地的白垩纪镁铁质岩石,目前还存在与雅鲁藏布特提斯洋在晚侏罗世-早白垩世大规模扩张有关的被动大陆边缘伸展背景(Jiang et al., 2006; 江思宏等, 2007; 童劲松等, 2007)和与大印度从南极洲和澳大利亚超大陆裂离出来有关的深位断裂背景(Hu et al., 2010)等不同解释。本文强调,如果雅鲁藏布特提斯洋在晚侏罗世-早白垩世时期发生了大规模扩张,那么由于洋脊扩张产生的推动力和洋脊地势上的隆起,就将促使扩张脊两侧的洋壳岩石圈发生相向运动,从而在活动大陆边缘一侧发生洋壳岩石圈俯冲而形成岩浆弧,在被动大陆边缘一侧则造成一种挤压而不是伸展背景。同时,特提斯喜马拉雅带白垩纪火成岩主要分布在江孜、康马及其以东地区,在更西部地区记录到的同期镁铁质岩浆活动规模非常有限(Jiang et al., 2006; 江思宏等, 2007; 童劲松等, 2007; Zhu et al., 2007, 2008, 2009),并且在这一地区靠近特提斯洋一侧还可能存在现今已经俯冲消减的宽度未知的大印度(图 6a)。这种地理分布特征,很难用被动大陆边缘伸展或深位断裂机制来解释。

图 6 东冈瓦纳大陆约132Ma构造重建图(据Heine and Müller, 2005; Zhu et al., 2008修改) (a)-早白垩世Comei-Bunbury大火成岩省在东冈瓦纳的位置;(b)-东冈瓦纳大陆早白垩世初始裂解示意图 Fig. 6 Reconstructions of Eastern Gondwana at ca. 132Ma (after Heine and Müller, 2005; Zhu et al., 2008)
4 西藏东南部措美大火成岩省的全球构造和环境意义 4.1 东冈瓦纳大陆Comei-Bunbury大火成岩省

目前已有锆石U-Pb年代学数据表明,西藏东南部措美大火成岩省岩浆作用主要发生在约132Ma(图 3a, b)。在约132 Ma的构造重建图上(Schettino and Scotese, 2001; Coffin et al., 2002),措美大火成岩省靠近现今澳大利亚南西部(图 6a, b)。非常重要的是,在澳大利亚南西部出露的覆盖面积约10000~100000km2(Macdougall, 1988)的Bunbury玄武岩,也侵位于约132Ma(Frey et al., 1996; Coffin et al., 2002)。考虑到这些约132Ma大规模岩浆活动的地理分布,Zhu et al. (2009) 将措美大火成岩省和澳大利亚Bunbury玄武岩联系起来作为一个单一的大火成岩省,并将其命名为Comei-Bunbury大火成岩省。与现今散布于非洲和南美的Paraná-Etendeka大火成岩省一样(图 6a),Comei-Bunbury大火成岩省的最初记录也受随后东冈瓦纳大陆裂解的影响而散布在现今的藏东南(印度陆块)、澳大利亚、南极洲和印度洋。

4.2 Comei-Bunbury大火成岩省与印度洋Kerguelen地幔柱早期活动的联系

早期研究提出,大约在130Ma时,Kerguelen地幔柱位于澳大利亚、南极洲和大印度三联点的下面(Davies et al., 1989)。在措美大火成岩省中,未遭受蚀变和地壳混染影响的OIB型镁铁质岩,无论是Sr-Nd同位素(图 7a)还是锆石Hf同位素(图 7b),均与澳大利亚南西部Bunbury玄武岩、印度北东部Rajmahal玄武岩(图 7a)和代表白垩纪印度洋Kerguelen地幔柱头部(Site 1138)的玄武岩非常相似(夏瑛等, 2012)。更重要的是,新识别出来的措美大火成岩省纯的地幔柱头部物质成分((87Sr/86Sr)t=0.7047,εNd(t)=+1.5,εHf(t)=+2.1~+5.7)(夏瑛等, 2012)与代表白垩纪Kerguelen地幔柱头部物质的Site 1138和Bunbury Casuarina组玄武岩非常相似(图 7a)。因此,如果这种相似性指示Comei-Bunbury大火成岩省与Kerguelen地幔柱具有共同的岩浆源区(Frey et al., 1996; Ingle et al., 2004; Zhu et al., 2007, 2008),那么板块构造重建就暗示Comei-Bunbury大火成岩省很可能与Kerguelen地幔柱的早期活动有成因联系。

图 7 西藏东南部措美大火成岩省镁铁质岩与印度洋Kerguelen地幔柱产物的同位素地球化学对比 数据来源: BC=Bunbury Casuarina玄武岩(未受到地壳混染),BG=Bunbury Gosselin玄武岩(受到地壳混染;Frey et al., 1996; Ingle et al., 2004);RGI=Rajmahal Group I玄武岩(未受到地壳混染),RGII=Rajmahal Group II玄武岩(受到地壳混染;Baksi, 1995; Kent et al., 1997; Ingle et al., 2004);CKPH=白垩纪Kerguelen地幔柱头部物质(Ingle et al., 2004);STPH=Sylhet Traps地幔柱头部物质(Ghatak and Basu, 2011).Site 738、747C、1136、1137、1138、1139、1141、1142玄武岩(Mahoney et al., 1995; Frey et al., 2002; Neal et al., 2002; Ingle et al., 2003),印度洋MORB(Chauvel and Blichert-Toft, 2001),陆壳(CC)(Ingle et al., 2002, 2003, 2004),Juvenile rock array(Vervoort and Blichert-Toft, 1999).措美大火成岩省镁铁质岩和超镁铁质岩地球化学数据(朱弟成等, 2005a; Zhu et al., 2007, 2008; 裘碧波等, 2010; 夏瑛等, 2012) Fig. 7 Isotopic geochemical comparison between the mafic rocks from the Comei LIP and the products of Kerguelen plume in Indian Ocean
4.3 Comei-Bunbury大火成岩省揭示的东冈瓦纳大陆裂解过程

对Kerguelen地幔柱活动与东冈瓦纳大陆裂解的关系,国际上存在两种不同看法:一种观点认为印度北东部Rajmahal暗色岩(约118Ma)代表了Kerguelen地幔柱活动的开始,因而Kerguelen地幔柱不是东冈瓦纳大陆在130Ma左右发生裂解的主要因素;另外一种观点认为Kerguelen地幔柱活动起始于132Ma左右,东冈瓦纳大陆裂解与该地幔柱活动有关(见: Zhu et al., 2008; 朱弟成等, 2009)。新近识别出的Comei-Bunbury大火成岩省,结合南Kerguelen高原底部有可能存在老于120Ma的玄武岩(Duncan, 2002; Kieffer et al., 2002),很可能表明Kerguelen地幔柱在约132Ma发生了一次强烈的岩浆作用。非常重要的是,Comei-Bunbury大火成岩省的大规模岩浆作用(约132Ma)与位于大印度北东缘和澳大利亚西缘之间沿Cape山破碎带(CRFz)和Wallaby-Zenith破碎带(WEFz)(图 6b)最老的海底磁异常(M10N,约130.9Ma; Heine and Müller, 2005)基本同期。这种一致性很可能表明导致形成Comei-Bunbury大火成岩省的Kerguelen地幔柱活动在东冈瓦纳大陆裂解过程中发挥了重要作用,即Kerguelen地幔柱在约132Ma的首次大规模岩浆活动导致了大印度从澳大利亚分离出来,东印度洋开启(图 6b)。随后,大印度和东印度可能以打开拉链的形式由北东向南从澳大利亚分离开来(Ingle et al., 2002),并在约120Ma时,由于Kerguelen地幔柱再次发生大规模岩浆作用,形成印度东部的Rajmahal暗色岩和南印度洋Kerguelen高原玄武岩,最终在约100Ma导致印度陆块从南极洲完全裂离出来(Coffin et al., 2002; Ingle et al., 2002; Hu et al., 2010)。

4.4 Comei-Bunbury大火成岩省对全球海洋环境的影响

一般认为,巨量的玄武岩浆喷发将释放出大量岩浆热液、酸性气体及还原性气体,骤然输入海洋环境会引发温室效应、大洋缺氧、生物快速更替和绝灭等异常事件(Kaiho and Saito, 1994; Sinton and Duncan, 1997; Wignall et al., 2001; 胡修棉, 2005; Hu et al., 2012)。据估算,现今措美大火成岩省玄武岩体积为约7000km3(裘碧波等, 2010),加上澳大利亚南西部Bunbury玄武岩体积(约1000km3; Coffin et al., 2002),可以得出原始Comei-Bunbury LIP的玄武岩体积至少大于8000km3。结合江孜和错那地区早白垩世黑色页岩沉积(胡修棉等, 2006; 陈曦等, 2008)中的碳同位素正偏(Wang et al., 2005, 2011)和早白垩世Valanginian期Weissert大洋缺氧事件(OAE)中全球海洋生态系统(包括微型浮游生物和放射虫)发生的明显扰动和大洋碳酸盐的碳同位素正偏(Lini et al., 1992; Erba et al., 2004; Weissert and Erba, 2004),很可能指示Comei-Bunbury大火成岩省岩浆活动对区域内和全球同期Weissert大洋缺氧事件的形成发挥了重要作用。

5 结束语

西藏东南部特提斯喜马拉雅带东部大规模白垩纪火成岩浆活动主要发生于130~136Ma(峰期约132Ma),由目前年龄样品约束的现今面覆盖积约50000km2。措美大火成岩省中的镁铁质岩显示OIB型(高Ti)、N-MORB型(低Ti)和过渡型(介于二者之间)等三种地球化学类型,其中未受地壳混染的镁铁质岩的Sr-Nd同位素和锆石Hf同位素成分类似于Kerguelen地幔柱产物。在扣除堆晶橄榄石之后,通过橄榄石-熔体平衡计算,措美大火成岩省中苦橄玢岩的母岩浆的MgO含量约20%,对应的地幔潜温>1560℃。这些特征非常类似于其它地幔柱成因的大火成岩省或热点,指示西藏东南部大规模白垩纪火成岩的地幔柱成因,因而将其描述和命名为措美(Comei)大火成岩省是合理的。年代学、地球化学和古地理重建资料显示藏东南措美(Comei)大火成岩省和澳大利亚南西部的Bunbury玄武岩可能代表了同一个大火成岩省(即Comei-Bunbury大火成岩省)。Comei-Bunbury大火成岩省很可能是Kerguelen地幔柱在132Ma左右的早期岩浆作用记录,拉开了大印度从澳大利亚分离出来的序幕。这一地幔柱型岩浆作用可能对同期全球Weissert大洋缺氧事件造成了重要影响。

尽管目前对西藏东南部大规模白垩纪火成岩研究取得了一些进展,但仍存在一些尚需深入研究和思考的科学问题,如西藏东南部近东西向镁铁质岩墙群、印度北部近南北向岩墙群(Srivastava and Sinha, 2004)和澳大利亚南西部可能存在的北西向镁铁质岩墙(与李正祥教授私人交流)是否代表了与Comei-Bunbury大火成岩省岩浆活动有关的放射状岩墙群?措美大火成岩省中不同类型镁铁质岩的地幔源区性质及其与南印度洋Kerguelen地幔柱产物的地球化学对比和成因联系?措美大火成岩省岩浆活动与特提斯喜马拉雅带东段多金属成矿作用聚集的关系?等等。

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