阿拉善地块前中生代构造归属的新认识

杨振宇,袁 伟 ,仝亚博,刘 静,王 艳

1)中国地质科学院地质力学研究所,国土资源部古地磁与古构造重建重点实验室,北京 100081;2)中国石化石油勘探开发研究院无锡石油地质研究所,江苏无锡 214126;3)同济大学海洋与地球科学学院,上海 200092;4)江苏省有色金属华东地质勘查局,江苏南京 210007

通常地质学家认为横贯中国中部的昆仑—秦岭—大别造山带(或中央造山带)将中国大陆分为中国北方地块和南方地块(群),南、北方地块展示了不同的地层组合(王泽九等,2014)。这些地块群组成一个复合大陆(任纪舜等,1994,1995),北方地块又通常被认为由中朝(—阿拉善)—塔里木板块构成(李春昱等,1982;王鸿祯,1985),阿拉善地块作为“地腰”连接着华北和塔里木地块。许效松等(1996)通过对中国主要陆块的古地理研究,则将界于古亚洲洋以南与冈瓦纳陆块群之间的特提斯洋盆系统中具特提斯暖水动物群和华夏植物群的地块称之为“泛华夏陆块群”,该群又被分之为“北方陆块群”包括华北、朝鲜、阿拉善等地块,而南方型“泛扬子陆块群”包括扬子、华夏、塔里木、柴达木等地块,也有称之为“西域地块”(葛肖虹等,1996),这些地块的显著特点是具有晋宁构造旋回和扬子型南华—震旦系盖层。与前人的认识基本一致,他们认为阿拉善地块一直是华北的一部分,早古生代至早泥盆世初塔里木陆块和柴达木陆块分别与华北(—阿拉善)陆块及其所围绕的微陆块聚合,形成“北方陆块群”;Xiao等(2009)总结了阿拉善地块与祁连山造山带的构造演化,也提出晚泥盆世阿拉善地块与祁连山造山带拼合,构成柴达木—祁连山—阿拉善构造体。似乎晚泥盆世华北通过柴达木—祁连山—阿拉善构造体与塔里木连接构成北方陆块群。显然,泥盆纪后阿拉善地块似乎构成了塔里木—柴达木地块的一部分,这些地质方面的认识还存在差异,使我们对古生代时期阿拉善地块究竟是华北地块的一部分？还是与塔里木地块亲缘产生疑问,因为古地磁的研究表明,晚二叠世时塔里木地块已与西伯利亚—哈萨克斯坦板块碰撞、拼合(Zhao et al.,1990;Enkin et al.,1992),而华北与蒙古碰撞并拼合形成了华北—蒙古联合地块,两者则远离西伯利亚板块,直至晚侏罗世华北—蒙古联合地块和西伯利亚板块的古地磁极才趋向一致,形成统一整体(朱日祥等,1998)。

阿拉善地块前寒武纪地质研究表明,其基底由新太古代迭布斯格岩群下部依肯乌苏岩组(2700 Ma)和古元古代的巴彦乌拉山岩群(2500—2264 Ma)组成(耿元生等,2007;Dan et al.,2012),并经历了早古生代(480—446 Ma)和晚古生代(281—266 Ma)两次构造热事件。近年来,在阿拉善北部发现的多条蛇绿岩带,恩格尔乌苏蛇绿岩带的发现表明塔里木陆块和华北陆块在早古生代也是分离的(王廷印等,1992),详细的年代学研究表明,蛇绿岩的年龄可晚至晚二 叠 世 ((274±3) Ma (MSWD=0.35)和(262±5) Ma(MSWD=1.2),Feng et al.,2013)。阿拉善地块与华北及塔里木地块的构造关系长期受到关注,但由于腾格里沙漠的覆盖,阿拉善地块的构造属性和构造演化一直悬而未决,对其深入研究应是理解中国北方大陆构造演化的一个重要课题。

图1 中国大陆(A)及阿拉善地块东南缘(B)地质简图Fig.1 Simplified geological map of China’s mainland(A) and southeastern Alxa Block(B)

1 中—晚泥盆世砂岩碎屑锆石 U-Pb年龄及Hf同位素结果

本次研究对河西走廊带—阿拉善地块东部牛首山地区中—晚泥盆世砂岩中碎屑锆石开展了 U-Pb年代学与Hf同位素的详细研究(Yuan et al.,2014a)(图1),锆石U-Pb年龄谱主要集中在0.4—0.7 Ga、1.0—1.3 Ga、1.5—1.8 Ga、2.4—2.7 Ga 和＞3.0 Ga五个年龄区间(图2d,g)。研究表明华北板块西部前寒武纪—晚中生代地层中碎屑锆石 U-Pb年龄谱,主要集中在1.8—2.1 Ga和2.4—2.7 Ga(图2e)。而本次研究中获得牛首山地区明显有别于华北的锆石年龄谱,其年龄谱中并未出现华北板块典型的 1.85—2.1 Ga峰值。鄂尔多斯盆地西北部桌子山地区的新元古代—早古生代沉积岩中碎屑锆石 U-Pb年龄谱存在1.85—2.1 Ga和2.4—2.6 Ga两个明显峰值(图2f),它具有典型的华北板块特征。桌子山(华北板块西缘)与牛首山(阿拉善地块南缘)相距很近,却具有截然不同的碎屑物质来源,这也暗示着二者的古地理位置可能是彼此分开。

目前 Gondwana大陆各主要板块均有碎屑锆石U-Pb年龄峰值为～2.5 Ga的报道,但是单从U-Pb年龄的峰值已经很难判断碎屑物质的来源。而锆石Hf同位素数据则被认为是判断碎屑物源的一个很好的辅助工具(Hoskin et al.,2000;Fedo et al.,2003)。阿拉善地块东部牛首山地区中—晚泥盆世砂岩中的～2.5 Ga碎屑锆石Hf同位素研究结果表明,其εHf(t)值在+4.9 ～ –23.7之间,对应的地壳模式年龄分布在2689—4501 Ma,其峰值为3010 Ma。然而,目前报道的华北板块峰值在～2.5 Ga的锆石对应的Hf同位素模式年龄峰值为2.75 Ga(Li et al.,2007;Jiang et al.,2010;Diwu et al.,2011;Wan et al.,2011a,b;Wang et al.,2011),二者存在明显的差别。此外,牛首山地区＞3.0 Ga碎屑锆石的 Hf同位素特征也明显区别于华北板块同时代碎屑锆石,这些特征说明牛首山地区中—上泥盆统的碎屑物源很可能不是来自华北地块,这些结果暗示河西走廊—阿拉善地块与华北板块的拼合时间可能是在晚泥盆世以后。本次研究中牛首山中—上泥盆统出现的典型的格林威尔期碎屑物质,其U-Pb峰值和Hf同位素特征与华夏地块古生代地层中碎屑锆石十分相似(图2c,d)。古生物研究表明祁连山地区寒武纪地层中所产的三叶虫Galahetes,Sanduspis,Huzhuia,Jialaopsis,Szechuanella,Acrocephalina等均为塔里木扬子生物地理区的特有属种(段吉业等,2005),与我国湘西、黔东过渡相区及澳大利亚昆士南西北的三叶虫动物群有比较密切的关系(周志强等,1996)。牛首山地区中寒武统碎屑物质来源与澳大利亚也有一定的亲缘性(袁伟等,2012b)。而在本次采样的牛首山地区,上泥盆统中宁组中发现的淡水鱼类胴甲鱼,在华南以及澳大利亚的新南威尔士同时期地层中均有出现(潘江等,1987;Jia et al.,2010)。并且,中宁组下段地层中发现了中华豆石介族(介形类),该化石以下垂“V”字型肌痕为特征与世界其它地区地层同期的豆石介族相区别,其主要产于华南的泥盆纪地层中(王尚启等,1995)。这些古生物学证据表明寒武纪—泥盆纪期间阿拉善地块与华南同属一个生物地理区,它们的地理位置应该比较接近。本研究还在牛首山上泥盆系地层中发现了一颗 U-Pb年龄为(4022±17) Ma的碎屑锆石,它与西澳的Jack Hills、华南、西藏等地冥古代锆石沉积物源区可能具有成因联系(袁伟等,2012a)。另外,0.4～0.7 Ga碎屑锆石的Hf同位素研究显示,对于＞550 Ma锆石的εHf(t)在0.5 ～ –26.0之间(只有一个正值),代表地壳物质的再循环,而＜550 Ma的锆石εHf(t)开始出现了正值,表明开始出现新生地壳物质,标志北祁连洋洋壳在 550 Ma开始形成,与北祁连造山带中发现的蛇绿岩的年代分布(490—550 Ma)相一致(袁伟等,2012b)。

表1 河西走廊—阿拉善地块与华北板块晚古生代以来古地磁极Table 1 Late Paleozoic paleomagnetic data from the Hexi Corridor–Alxa Block and North China Block

2 石炭纪—二叠纪古地磁学研究结果

本次研究对河西走廊—阿拉善地块东部的早石炭世、晚二叠世和早中三叠世地层开展详细的古地磁研究(Yuan et al.,2014b)。岩石磁学研究结果表明,早石炭世灰岩的载磁矿物主要为磁铁矿和赤铁矿,可能含有钛磁铁矿,磁性矿物多为假单畴颗粒;晚二叠世和早—中三叠世紫红色长石石英砂岩主要载磁矿物为赤铁矿。

早石炭世样品系统热退磁共设12～17步,300℃以下以 50℃为间隔,340～420℃以 30～40℃为间隔,420℃以上以10～20℃为间隔。热退磁过程中出现两个分量,第一个分量在 200～300℃时解阻,为低温分量;第二个分量到达 460℃时剩磁达到最小,但是该分量未趋向于原点,应为中温分量。460℃以后样品剩磁过于微弱,超出了仪器测试范围,未能获得高温分量。运用主向量分析法对所有样品退磁结果进行分析(Kirschvink,1980),采点平均采用通用的 Fisher统计平均法(Fisher,1953),获得低温分量A和中温分量M。其中,低温分量未能通过褶皱检验(McFadden,1990),其方向与现代地磁场一致,很可能是现代地磁场的粘滞剩磁。中温分量的形成可能与造山热液流体活动有关(Addison et al.,1985)。研究区紧邻祁连造山带,并且上泥盆世—中石炭世的岩层间断裂中赋存有热液型金、铜矿,中石炭世地层局部受到热液活动影响,经受不同程度的变质作用,变质热液的温度在 250～300℃(邵洁涟等,1988;宋新华等,2009)。因此,该中温分量M很可能是热液流体作用后重磁化的结果。中温分量M在 95%置信度下通过了 Mcfadden褶皱检验(McFadden,1990),表明该分量为中侏罗世褶皱前获得。但是其极位置又明显有别于河西走廊—阿拉善地区二叠纪以后的古地磁极,因此我们推测早石炭世中温 M 分量重磁化时间可能在中—晚石炭世,其对应的古地磁极为 λ=12.1°N,Ф=10.7°E,A95=4.2°,古纬度在 3.6±4.2°N。

晚二叠世和早—中三叠世样品系统热退磁共设 14～16步,500℃以下以 80～100℃为间隔,500～600℃以 20～30℃为间隔,600℃以上以 5～20℃为间隔。运用主向量分析法对所有样品退磁结果进行分析(Kirschvink,1980),采点平均采用通用的Fisher统计平均法(Fisher,1953)。两个时代的样品均获得了低温和高温分量,其中低温分量为现代地磁场的粘滞剩磁;高温分量均趋向于原点,并且通过了褶皱检验和区域一致性检验,代表了原生特征剩磁。晚二叠世对应的古地磁极为λ=26.2°N,Ф=18.0°E,A95=7.5°,古纬度为 17.4±7.5°N;早—中三叠世古地磁极为λ=37.5°N,Ф=14.4°E,A95=3.7°,古纬度为 21.6°N(Liu et al.,2010)。

图3 阿拉善地块(蓝色)与华北板块(红色)石炭纪—三叠纪视极移曲线(据Yuan et al.,2014b修改)Fig.3 Equal-area projection of Carboniferous-Triassic apparent polar wander paths for the Alxa Block (blue) and North China Block (red) (modified after Yuan et al.,2014b)

图4 晚二叠世亚洲主要板块古地理重建图(据Yuan et al.,2014b修改)Fig.4 Paleomagnetic-based paleogeographic reconstruction of major Asian blocks in the Late Permian(modified after Yuan et al.,2014b)

通过系统的古地磁学研究,我们获得了阿拉善地块东部中—晚石炭世、晚二叠世和早—中三叠世可靠的古地磁极,这些数据与华北地块同期的古地磁极有明显差别(图3,表1)。河西走廊—阿拉善地块中东部新获得的中—晚石炭世、晚二叠世和早—中三叠世构成的视极移曲线可以与华北地块同时期极移曲线可以进行拟合对比,若以44°N,84°E为参考欧拉极,将阿拉善地块的视极移曲线旋转 32°,该曲线与华北地块同时期极移曲线重合(图 3),这说明了在中三叠世后,特别是印支运动使阿拉善地块相对于华北发生了 32°的逆时针旋转,正是这一旋转运动使河西走廊—阿拉善地块最终与华北地块拼合形成了统一地块,该边界很可能位于贺兰山与桌子山之间(图4;中国地质科学院,2014)。最新的野外地质研究表明巴彦乌拉山—狼山存在一条中生代早期线理近水平的韧性剪切带,很可能代表华北和阿拉善地块的拼合界线(Zhang et al.,2013)。

3 总结

阿拉善南缘牛首山地区的中—晚泥盆世沉积岩中碎屑锆石的U-Pb年代学和Hf同位素研究结果表明,其碎屑物质来源明显区别于华北板块,表明在晚泥盆世时二者并未相连,彼此的古地理位置还相距较远。此时,阿拉善地块与华南、澳大利亚以及特提斯—喜马拉雅在碎屑物质来源上更具亲缘性(图2a,b,c,d)。同时,对阿拉善地块南缘石炭纪—三叠纪沉积岩开展系统的古地磁研究结果表明,阿拉善地块与华北板块拼合时间是在早—中三叠世之后,二者的分界线可能位于贺兰山与桌子山之间的南北走向的延长线之上。华北地块在晚三叠世—中侏罗世发生了～42°的逆时针旋转,而扬子地块则发生了～35°顺时针旋转;这些相对旋转作用使得扬子地块与华北地块完成拼合(Yang et al.,1992)。秦岭—大别造山带南缘的勉略蛇绿混杂岩带,很可能代表了华南地块与华北地块最后拼合的结合带,勉略洋盆泥盆纪时打开,石炭纪—二叠纪是洋盆主体扩张期,至三叠纪逐渐闭合(赖绍聪等,2003;Lai et al.,2004);晚三叠世由于华北地块与扬子地块发生碰撞造山,形成大量印支期钙碱性同碰撞花岗岩(张国伟等,2001)。至晚侏罗世,西伯利亚、华北—蒙古联合地块、扬子地块完全拼合,形成了统一的大陆(Enkin et al.,1992;朱日祥等,1998)。

致谢:本研究是《中国西部大陆新生代构造格局与演化》(编号:SinoProbe08-01;201011074)的部分成果,吉林大学葛肖虹教授审阅了初稿并提出宝贵的修改意见;野外工作得到南京大学李军鹏、秦永鹏和张海峰等大力协助,在此深表感谢！

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