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鄂尔多斯盆地西南缘奥陶纪火山活动记录:来自陕甘地区平凉组钾质斑脱岩地球化学和锆石年代学的信息*

2015-03-15王振涛周洪瑞王训练景秀春张永生

岩石学报 2015年9期
关键词:锆石鄂尔多斯盆地

王振涛 周洪瑞 王训练 景秀春 张永生

WANG ZhenTao1,2,3,ZHOU HongRui4,WANG XunLian4,JING XiuChun4 and ZHANG YongSheng1,2,3

1. 中国地质科学院矿产资源研究所,北京 100037

2. 中国地质科学院盐湖与热水资源研究发展中心,北京 100037

3. 国土资源部盐湖资源与环境重点实验室,北京 100037

4. 中国地质大学地球科学与资源学院,北京 100083

1. Institute of Mineral Resources,Chinese Academy of Geological Sciences,Beijing 100037,China

2. R & D Center for Saline Lakes and Epithermal Deposits,Chinese Academy of Geological Sciences,Beijing 100037,China

3. Key Laboratory of Saline Lake Resources and Environment,Ministry of Land and Resources,Beijing 100037,China

4. School of the Earth Sciences and Resources,China University of Geosciences,Beijing 100083,China

2014-08-15 收稿,2015-02-01 改回.

斑脱岩(Bentonite)是主要成分为蒙脱石的粘土岩,为火山碎屑岩在海相碱性环境下蚀变而来(Calarge et al.,2003;Christidis and Huff,2009),进一步的成岩和变质作用则使之转变为富含伊-蒙混层矿物和伊利石为主的钾质斑脱岩(Kbentonite)(Su et al.,2003;Huff,2008)。钾质斑脱岩还包含有锆石、黑云母、伊利石等(从斑脱岩中继承而来)可以用于同位素定年的矿物,常被作为地层对比的标志层(Huff et al.,1992,2000;Bergström et al.,1997;Kolata et al.,1998;Haaland et al.,2000;Desmares et al.,2007;Zhu et al.,2013),已成为确定地层时代和关键地质事件发生的重要依据(Dong et al.,1997;Foreman et al.,2007;Su et al.,2008,2010;胡艳华等,2008;万斌等,2013)。近年来,该类岩石在我国华北前寒武纪地层(苏文博等,2010;李怀坤等,2010;孙会一等,2013)、奥陶系(陈诚等,2012)和中生代地层(邱欣卫等,2009,2011;杨华等,2013;王亮亮等,2013;Qiu et al.,2014;张辉等,2014)中被识别出来并得到深入研究,不但为年代地层格架的重新厘定提供了重要依据,也为火山-岩浆活动事件及区域构造演化提供了客观证据。

鄂尔多斯盆地西南缘处于北祁连造山带、秦岭-大别造山带与华北板块交互作用的地区(Song et al.,2013),奥陶纪时发育了汇聚板块边缘的沟-弧-盆体系(许志琴等,2006;刘军锋等,2009;闫全人等,2009;Song et al.,2013),形成了北西-南东向展布的北祁连造山带走廊南山北缘早古生代岛弧及弧后盆地(冯益民等,1995)和北秦岭造山带早古生代岛弧及弧后盆地(闫全人等,2009),有证据表明二者是联系在一起的(Tseng et al.,2009)。目前,针对北祁连、秦岭地区的研究主要集中于岩石学、地球化学和同位素年代学(宋述光等,2004;宋述光,2009;Song et al.,2009,2013;闫全人等,2009;王涛等,2009;Wang et al.,2013a),而对它们向北俯冲过程中形成的弧火山喷发事件的年代学研究尚未受到足够的关注。值得注意的是,华北克拉通西南缘奥陶系发育多套凝灰岩(斑脱岩)夹层(贾振远,1988;贾振远等,1997;袁卫国,1995;李文厚等,1997;王学平,2002)。目前,鄂尔多斯盆地南缘东段奥陶系金粟山组的钾质斑脱岩地球化学和锆石SHRIMP U-Pb 年龄已有报道,但盆地南缘西段的相关研究尚未见有报道。

本文选取鄂尔多斯盆地南缘东段陕西泾阳地区西陵沟剖面和西段甘肃平凉地区的银洞官庄剖面平凉组出露的钾质斑脱岩为研究对象,对上述两个剖面平凉组不同层位钾质斑脱岩进行了采样,用ICP-MS 等离子质谱仪和激光剥蚀电感耦合等离子体质谱法(LA-MS-ICP)进行常量元素、微量元素、稀土元素和锆石U-Pb 年代学分析。迄今为止,鄂尔多斯盆地奥陶系地层划分和对比,其意见仍不甚统一(孙肇才和胡显穆,2002;郭彦如等,2012,2014;李文厚等,2012)。譬如,对研究区平凉组的地质年代归属尚存不同认识,有中奥陶世(甘肃省地质矿产局,1989;贾振远等,1997;孙肇才和胡显穆,2002;冯增昭等,1999)、中奥陶世-晚奥陶世早期(陕西省地质矿产局,1998;郭彦如等,2012)和晚奥陶世(曹金舟等,2011;李文厚等,2012)三种方案。王志浩等(2013)、Wang et al. (2013)通过对甘肃平凉银洞官庄剖面平凉组的牙形刺的详细研究,将平凉组厘定为上奥陶统,但尚无绝对年龄的约束。因此,亟待建立以精确可靠的锆石U-Pb 年龄为基础的时间框架,为区域地层对比、周缘火山-岩浆活动事件和区域构造演化提供必要的时限约束。

1 地质概况

鄂尔多斯盆地是位于华北克拉通西部的一个克拉通盆地(图1a)。在古生代早期盆地处于稳定的克拉通盆地发育阶段,沉积了一套以海相碳酸盐岩夹碎屑岩为主的地层(王鸿祯,1985),与盆地中东部的碳酸盐台地相沉积不同,晚奥陶世鄂尔多斯盆地整体抬升,大部分地区遭受剥蚀,而西缘和南缘沉降迅速,继续接受平凉组沉积,甚至发育深水海相沉积(杨华等,2010)。早古生代早期,鄂尔多斯盆地西缘和南缘为华北克拉通的西部和南部的大陆边缘(屈红军等,2010;陈诚等,2012),至中-晚奥陶世时盆地南缘已演化为古秦岭洋北侧具沟、弧、盆构造体系中的主动大陆边缘(冯益民和何世平,1995;袁卫国和赵一鸣,1996;贾振远等,1997;李文厚等,1997;钱锋和艾永峰,2002;Song et al.,2009;宋述光,2009)。在盆地西部大陆边缘,奥陶系总体上呈西厚东薄,南北向条带状展布(郭彦如等,2014)。平凉组沉积期(中和/或晚奥陶世)鄂尔多斯盆地的西缘和南缘为L 形的边缘海,盆地南缘为末端变陡的继承性碳酸盐缓坡沉积环境(图1b),相对于早、中奥陶世海平面相对加深(图1c)。

图1 鄂尔多斯盆地位置(a,据Zhao et al.,2005)、西南部中奥陶世岩相古地理(b,据吴胜和等,1994)、研究层位及本文研究剖面位置简图(c,据郭彦如等,2012)Fig.1 Location (a,after Zhao et al.,2005),lithofacies-paleogeographic map (b,after Wu et al.,1994)and the studied position (c,after Guo et al.,1994)of the Middle Ordovician in the western margin of Ordos Basin,with sample locations

本文的采样层位位于不同地区的平凉组的不同层位(图2)。陕西泾阳地区的采样位置位于鄂尔多斯盆地南缘陕西省泾阳县崔黄村小学北部的唐贞陵山沟中,地理坐标为34°42'30.04″N,108°37 '50.99″E;甘肃平凉地区的采样位置位于甘肃平凉西南约8 km 的银洞官庄桥边的小山上,地理坐标为35°29'29.48″N,106°36'27.73″E。

2 沉积层序及钾质斑脱岩特征

平凉组分布于鄂尔多斯地区的西缘和南缘,岩性以页岩、砂岩为主,其次为灰岩和白云岩(吴胜和等,1994)。岩性变化大是平凉组地层的一大特点,在不同地区表现出不同的岩性组合特征(倪春华等,2011)。改组的标准剖面位于甘肃平凉市西南约8km 的银洞官庄桥边的小山上,由一套不等厚互层的灰岩、泥岩和页岩组成,顶部为二叠系山西组覆盖,呈假整合接触(王志浩等,2013)。该剖面发育多层厚度不等的斑脱岩夹层,野外呈浅绿色,较软而疏松(图3)。该剖面牙形石生物地层研究程度颇高(陈均远等,1984;安太庠和郑昭昌,1990;Finney et al.,1999;王志浩,2001;王志浩等,2013;Wang et al.,2013b)。近年,Wang et al.(2013b)在该剖面新发现了产于北美中大陆区的Plectodina aculeate,Erismodus quadridactylus 和Belodina compressa 等牙形刺动物群,为该剖面钾质斑脱岩锆石年代学提供了严格的生物地层约束。本文在该剖面不同层位采集新鲜钾质斑脱岩样品6 件,每件重量约10kg,样 品 编 号 分 别 为YDGZ-2、YDGZ-5、YDGZ-6、YDGZ-7、YDGZ-8 和YDGZ-9。

陕西泾阳地区平凉组与下伏马家沟组呈连续沉积,岩性以浅黑、黄绿色薄层页岩及灰岩为主,夹砾状灰岩、砂屑灰岩、钾质斑脱岩、凝灰质砂岩和生物碎屑灰岩及黑色硅质条带、硅质透镜体,沉积构造以水平纹层和正递变层理为主。同时,该组还发育多层斑脱岩夹层,粗略统计大概有26 层,其厚度不一,最薄层不到1cm,最厚者达30cm,夹于薄层灰岩之中,风化面上呈现黄色至浅灰绿色,在以灰岩为主的地层中格外醒目(图4)。湿的钾质斑脱岩可以呈现不同的颜色,但风化后的颜色一般是黄色。外观貌似黄泥,有柔软和光滑感,符合钾质斑脱岩的野外鉴别标准(Kolata et al.,1998;Marker and Huff,2005)。在样品采集过程中首先挖掘至钾质斑脱岩的新鲜位置,而后由下到上采集新鲜钾质斑脱岩样品4 件,每件重量约10kg,样品编号分别为XL-1、XL-2、XL-3 和XL-4。其中XL-1-D-4 采自最接近平凉组与下伏马家沟组界线处易采集、厚度较大且稳定的斑脱岩层位。

图2 西陵沟剖面和银洞官庄剖面柱状图和采样层位Fig.2 Lithological column and sampling horizon of Xilinggou Section and Yindongguanzhuang Section

3 测试方法及流程

钾质斑脱岩样品主量、微量、稀土元素测试和单颗锆石的挑选在河北省区域地质矿产调查研究所实验室完成。其中,主量成分、Zr、Sr 采用GB/T 14506.25—2010 方法,Axios max X 射线荧光光谱仪测定,微量元素、稀土元素采用GB/T 14506.30—2010 方法,用X Serise2 等离子体质谱仪测定分析。主量元素的分析精度好于5%,微量元素的分析精度优于10%,其分析流程参阅刘颖等(1996)。

锆石分选大致流程如下:将新鲜的样品破碎到适当大小(一般为80 ~100 目),淘洗后通过人工重砂、电磁分选出锆石,然后在双目镜下随机挑选出锆石颗粒。将挑出的锆石随机选取一定数量在玻璃板上排列粘牢,使用环氧树脂固定凝结,经打磨抛光处理制成靶样,再进行透射、反射光和阴极发光(CL)照相以观察锆石的表面及内部结构。锆石晶体的透射、反射和CL 图像拍照由北京锆年领航科技有限公司完成。根据锆石反射、透射图像和CL 图像所揭示的锆石晶体裂隙、包体、环带等内部结构特征,反复对比以选择合适的激光剥蚀点位进行U-Pb 同位素年龄测定。定年分析尽量选择岩浆成因特征明显的锆石区域,以期较好地限定岩浆喷发时限。

图3 甘肃平凉地区银洞官庄剖面平凉组钾质斑脱岩的野外特征及采样层位(a)第4 层钾质斑脱岩(样品YD-4);(b)第7 层钾质斑脱岩(样品YD-7);(c)第8 层钾质斑脱岩(样品YD-8);(d)第9 层钾质斑脱岩(样品YD-9)Fig.3 The field rock characteristics of K-bentonite and its sample lacations(a)Layer 4 K-bentonite (Sample YD-4);(b)Layer 7 K-bentonite (Sample YD-7);(c)Layer 8 K-bentonite (Sample YD-8);(d)Layer 9 Kbentonite (Sample YD-9)

图4 陕西泾阳地区西陵沟剖面平凉组底部钾质斑脱岩的野外特征及采样层位Fig.4 The field characteristics of K-bentonite in the bottom of Pingliang Formation,Xilinggou Section,and its sample locations

样靶制作完成后,在天津地质矿产研究所同位素实验室利用激光烧蚀多接收器等离子体质谱仪(LA-MC-ICP MS)进行微区原位U-Pb 同位素测定,仪器配置和测试流程参见相关文献(李怀坤等,2009a,2009b,2010,2011;耿建珍等,2012)。采用208Pb 校正法对普通铅进行校正(Anderson,2002)。利用NIST612 玻璃标样作为外标计算锆石样品的Pb、U、Th 含量。在测试前用纯酒精对锆石靶进行清洗,以降低普通铅的污染。每个样品在开始和结束时分别测试2 次NIST612 标样,每完成8 个测点再加测2 次NIST612 标样。采用GJ-1 作为外部锆石年龄标准进行U、Pb 同位素分馏校正(Jackson et al.,2004)。采用ICPMSDataCal 程序(Liu et al.,2009)和Isoplot 程序(版本3.41)(Ludwig,2003)进行数据处理,分析结果在Isoplot 软件(版本3.41)完成加权平均年龄计算和谐和图的绘制。

4 结果

4.1 钾质斑脱岩主微量元素特征

主量元素、微量元素和稀土元素的地球化学特征,对于钾质斑脱岩的区分和鉴定具有重要意义,同时也是其原岩恢复和构造环境判别的基础(万斌等,2013)。本研究所采样品的主量元素、微量元素和稀土元素含量示于表1。测试结果表明,所有样品具有较高的SiO2(41.42% ~70.5%),平均含量54.19%,表明钾质斑脱岩的源岩为中酸性岩浆。K2O 含量为2.07% ~6.81%,平均值为5.41%,与Huff et al.(1997)研究的欧洲Osmundsberg 斑脱岩11 个样品的平均值(4.40%)接近,与陈诚等(2012)报道的鄂尔多斯盆地南缘的7 个斑脱岩的K2O 平均含量(4.24%)亦具有相似特征。所有样品均为K2O >Na2O,显示了其钾质特征。

微量元素原始地幔标准化蛛网图(图5)表明本次研究的样品Th、U 明显富集,且均具有明显的Pb、Sr 和Eu 亏损,两个地区的样品微量元素配分模式基本相似,唯一的不同之处在于甘肃平凉地区的6 个样品具有明显的Yb、Y 亏损,暗示它们可能为不同期次火山活动的产物,也可能指示它们分别来自不同的喷发区域。

稀土元素球粒陨石标准化模式图(图6)显示,10 个样品均表现为轻稀土元素相对富集的右倾型特征,δEu 轻度亏损,“谷”状明显。整体上曲线展布特征、规律一致,各样品之间的差别不甚明显。这种轻稀土富集,具有Eu 负异常而无Ce 负异常,呈“V”型右倾曲线的配分模式,与花岗岩的稀土元素配分模式,较为相似,而不同于NASC、PAAS 和CCR 的配分模式(万斌等,2013)。理论上,Eu 负异常形成的原因不外乎有二,一是继承了母岩浆Eu 负异常,另外一种可能是钾质斑脱岩沉降后成岩作用的结果(胡艳华等,2009)。从他们稀土元素的配分模式来看,前一种可能性较大,同时也暗示了研究样品的火山灰属性。所有样品均无Ce 的负异常,说明钾质斑脱岩虽然在海相环境中蚀变而来,但Ce 受海水影响不甚明显。

图5 研究样品钾质斑脱岩球粒陨石标准化多元素蛛网图(标准化值据Sun and McDonough,1989)Fig.5 Chondrite-normalized trace element spider diagram of the K-bantonite (normalization values after Sun and McDonough,1989)

图6 钾质斑脱岩样品的稀土元素球粒陨石标准化模式图(标准化值据Sun and McDonough,1989)Fig.6 Chondrite-normalized REE patterns for K-bentonite samples (normalization values after Sun and McDonough,1989)

4.2 锆石特征及其年龄

阴极发光图像(图7)分析表明,6 个样品中的锆石形态和内部特征各异,内部岩浆生长振荡环带的发育程度也各有差别。整体上,锆石颗粒普遍为自形-半自形晶体,生长振荡环带发育,具有明显的岩浆锆石的特点。上述样品中锆石Th/U 比值绝大多数大于岩浆成因0.4 的临界值(Rubatto and Gebauer,2000;Moller et al.,2003),也显示了岩浆型锆石的特征。测试分析之前进行了锆石阴极发光结构观察,激光剥蚀过程中尽量回避了包体和裂隙,加之锆石大多具有自形-半自形晶形特征,岩浆环带清楚,Th/U 比值大于0.1,年龄结果谐和度高,表明这些年龄结果反映了钾质斑脱岩的就位(沉积)年龄。

每个锆石颗粒进行1 个分析点的测试。分别对样品XL-1 ~XL-4 和YDGZ-8、YDGZ-9 进行了37、32、40、32 和24、24个锆石颗粒U-Pb 同位素年龄分析(表2、图8),具体结果分述如下。

表1 钾质斑脱岩样品的主量(wt%)和微量元素(×10 -6)分析结果Table 1 The result of major element composition (wt%)and trace element composition (×10 -6)of K-bentonite

样品XL-1:在进行206Pb/238U 年龄加权平均计算时,删除了5 个不谐和分析点的数据,其中分析点1.27 的年龄(2496±16Ma)明显偏老,后分析表明该锆石颗粒为碎屑锆石,其年龄与华北克拉通新太古代晚期到古元古代早期的构造岩浆热事件(~2.5Ga)(Zhao et al.,2001;彭澎和翟明国,2002;耿元生等,2010;董春艳等,2012;Zhang et al.,2012;胡波等,2013)相对应,推测其可能源于该时期的岩浆岩。在207Pb/235U-206Pb/238U 图解上,多数投点落在谐和线上(图8),少量不在谐和线上的分析点表明了可能的放射成因Pb 丢失。36 颗锆石颗粒中的31 个用于加权平均计算的206Pb/238U 年龄介于431 ±7Ma ~487 ±4Ma 之间,它们的加权平均年龄为454 ±4.3Ma(MSWD=4.3);

样品XL-2:在207Pb/235U-206Pb/238U 图解上,数据投点落在谐和线上或其附近,33 颗锆石颗粒的22 个用于加权平均计算的206Pb/238U 年龄介于430 ±5Ma ~486 ±7Ma 之间,其加权平均年龄为452 ±5Ma(MSWD=3.6);

样品XL-3:在207Pb/235U-206Pb/238U 图解上,数据投点落在谐和线上或其附近,40 颗锆石颗粒的34 个用于加权平均计算的206Pb/238U 年龄介于435 ±4Ma ~461 ±4Ma 之间,它们的加权平均年龄为449 ±3Ma(MSWD=2.4);

图8 钾质斑脱岩锆石U-Pb 谐和年龄图Fig.8 Concordia diagrams of zircon U-Pb datings for the K-bentonites

样品XL-4:在207Pb/235U-206Pb/238U 图解上,数据投点落在谐和线上或其附近,32 颗锆石颗粒的15 个用于加权平均计算的206Pb/238U 年龄介于446 ±6Ma ~468 ±5Ma 之间,它们的加权平均年龄为457 ±3Ma(MSWD =1.08),这一年龄代表该层钾质斑脱岩的就位(沉积)年龄;

样品YDGZ-8:在207Pb/235U-206Pb/238U 图解上,数据投点

落在谐和线上或其附近,24 颗锆石颗粒的10 个用于加权平均计算的206Pb/238U 年龄介于181 ~454Ma 之间,它们的加权平均年龄为450 ±2Ma(MSWD =0.9),这一年龄代表该层钾质斑脱岩的就位(沉积)年龄;

表2 锆石LA-ICP-MS U-Pb 同位素分析结果Table 2 LA-MC-ICP MS U-Pb isotopic data of zircons from this studied samples

续表2Continued Table 2

续表2Continued Table 2

续表2Continued Table 2

样品YDGZ-9:在207Pb/235U-206Pb/238U 图解上,数据投点落在谐和线上或其附近,24 颗锆石颗粒的9 个用于加权平均计算的206Pb/238U 年龄介于181 ~1214Ma 之间,它们的加权平均年龄为451 ±1Ma(MSWD =0.1),这一年龄代表该层钾质斑脱岩的就位(沉积)年龄。

5 讨论

5.1 平凉组地质年代和区域地层划分方案

陕西泾阳西陵沟剖面平凉组4 个钾质斑脱岩样品的206Pb/238U 年龄(LA-ICP-MS)介于449 ±3Ma ~457 ±3Ma 之间,属于晚奥陶世早期的桑比阶-凯特阶(Sandbian-Katian)(458 ~445Ma)。由于样品采自平凉组底部,但离平凉组的底界尚有2m,考虑到灰岩的沉积速率较大,故而据此推断其沉积时代始于桑比阶(Sandbian)。陈诚等(2012)报道了西陵沟剖面和赵老峪剖面金粟山组(与平凉组可对比)的钾质斑脱岩SHRIMP U-Pb 年龄,分别为451 ±4.9Ma ~452.1 ±5.1Ma,457 ±5.1Ma,465.8 ±8.3Ma,与本文的结果在误差范围内一致。甘肃平凉银洞官庄剖面平凉组其中的两个样品(YDGZ-8 和YDGZ-9)的206Pb/238U 年龄(LA-ICP-MS)分别为450 ±2Ma 和451 ±2Ma,表明其沉积期为凯特阶(Katian)。但由于这两个样品靠近顶部,未能限定该组的起始年龄,参考泾阳地区的测年结果,认为平凉组的主体沉积时代应为晚奥陶世早期的桑比阶-凯特阶(Sandbian-Katian)。

5.2 原岩恢复及形成构造环境分析

不活泼的的微量元素和稀土元素如TiO2和高场强元素如Nb、Ta、Zr、Hf 被认为在成岩作用和低级变质作用中是稳定的(胡艳华等,2009),常被用于判断其源岩的物质组成和原岩形成的构造环境(Huff et al.,1997;苏文博等,2006)。在Zr/TiO2-Nb/Y 图解(图9a)中,本次研究的样品(1 个样品落在粗面岩的范围内)均落在流纹英安岩和粗面安山岩的区域内,显示了中酸性的原始岩浆亲缘关系。而在Nb-Zr 图解(图9b)中,所有样品(1 个除外)都落在高钾、亚碱性岩浆范围内。综上所述,笔者认为本次研究样品的原始岩浆为高钾亚碱性流纹英安岩和粗面安山岩。

钾质斑脱岩中稳定的微量元素和稀土元素除了能够反映原始岩浆的亲缘性外,还可以有效的指示原始火山的构造环境(Teale and Spears,1986;Roberts and Merriman,1990)。钾质斑脱岩研究中应用最多的为Nb-Y、Nb-(Y +Nb)和Zr-TiO2图解(万斌等,2013)。分别将本次研究的样品投入上述判别图中,在Nb-Y、Nb-(Y+Nb)图解(图9c,d)中均落在火山弧花岗岩和板内花岗岩的范围内且主要在火山弧花岗岩一侧,在Zr-TiO2图解(图9e)中均落在岛弧熔岩区域内,判别结果出现了少量的不一致,但都说明火山弧花岗岩是其主要的构造环境。按照万斌等(2013)的研究,参考选择了Th/Yb-Nb/Yb 图解(图9f)对本次研究的样品进行判别,结果都落入了大陆弧岩浆的区域,这和Zr-TiO2图解的结果比较一致。

5.3 钾质斑脱岩的物源及其对周围造山带的约束

鄂尔多斯盆地南缘奥陶纪钾质斑脱岩分布广泛。由于这些钾质斑脱岩为火山成因,因此它们的物源成为一个必须要解答的问题。从理论上讲,每次火山喷发事件形成的大量的钾质斑脱岩都可以在周围地区的地层中有所保存。陈诚等(2012)认为鄂尔多斯盆地南缘的斑脱岩可能源自沿商丹洋盆北缘展布的火山弧喷发,而张进等(2012)认为鄂尔多斯西南缘平凉地区的中奥陶统沉积中含有的大量凝灰质组分可能来自早古生代北祁连造山带的火山喷发。但由于火山灰可以在空中搬运较远的距离,目前盆地南缘的钾质斑脱岩尚不能排除有一些是远源来源的。

钾质斑脱岩的微量元素特征和原岩构造环境判别表明这些斑脱岩的原岩主要产于岛弧环境。而在出露钾质斑脱岩的鄂尔多斯盆地南缘地区,并未有同时代的火山喷发事件。然而,鄂尔多斯盆地南缘周围满足这种构造背景的地区不外乎两个,一个是其南部的北秦岭地区发育的广泛的岛弧岩浆活动(Huang and Wu,1992;Xue et al.,1996;Li et al.,2001;Sun et al.,2002;马昌前等,2004;Shi et al.,2013)和西南部北祁连岛弧带的岩浆活动(夏林圻等,1995;张建新等,1997;Mao et al.,1999,2000;Xia et al.,2003;Wang et al.,2005;Wu et al.,2005;陈化奇,2007;Enkelmann et al.,2007),前者时限集中在464 ~442Ma,后者时限集中在480 ~445Ma。一般认为早古生代秦岭洋和北祁连洋是相通的,本文的测年结果和秦岭-祁连洋板块俯冲导致的火山活动的时限是一致的。

另一个可能是火山灰来自北部的古亚洲洋俯冲带。早古生代中亚造山带存在的较强烈的与俯冲作用相关的岩浆活动(刘敦一等,2003;施光海等,2003;石玉若等,2004;葛文春等,2005;裴福萍等,2006;苗来成等,2007;Jian et al.,2008;郭锋等,2009)。内蒙古中部地区早-中奥陶世时洋盆非常活跃(施光海等,2003;石玉若等,2004),内蒙古图林凯地区曾发生过467 ~451Ma 的洋壳消减事件,导致了459 ±8Ma 的英安岩喷发(施光海等,2003);紧挨华北克拉通北缘的内蒙古贝特(Bater)地区存在488 ~444Ma 的弧后火山喷发事件,形成了高钾的钙碱性岩石(闪长岩和石英闪长岩)(Jian et al.,2008)。上述年龄与研究区钾质斑脱岩的就位的时间一致,因此,虽然古亚洲洋俯冲带离研究区较远,但还不能排除来自同时期北部古亚洲洋俯冲带火山喷发的可能。

图9 本次研究样品的原始岩浆和构造环境判别图(a)Zr/TiO2-Nb/Y 图解(Winchester and Floyd,1977);(b)Zr-Nb 图解(Leat et al. ,1986);(c、d)Nb-Y 和Rb-(Y +Nb)图解(Pearce et al. ,1984);(e)Zr-TiO2 图解(Pearce and Norry,1979);(f)Th/Yb-Nb/Yb 图解(Pearce and Peate,1995)Fig.9 The protolith and tectonic environment discrimination diagrams of this study samples(a)Zr/TiO2-Nb/Y diagram (Winchester and Floyd,1977);(b)Zr-Nb diagram (Leat et al. ,1986);(c,d)Nb-Y diagram and Rb-(Y +Nb)diagram (Pearce et al. ,1984);(e)Zr-TiO2 diagram (Pearce and Norry,1979);(f)Th/Yb-Nb/Yb diagram (Pearce and Peate,1995)

图10 鄂尔多斯南缘奥陶纪斑脱岩夹层出露位置图(据袁卫国,1995;陈孟晋等,2007;陈诚等,2012 修编)1-钾质斑脱岩出露的剖面;2-见有钾质斑脱岩的钻井;3-钾质斑脱岩层数/厚度不详;4-钾质斑脱岩总厚度(m)/层数Fig.10 The outcrop lacations of the Ordovician bentonite beds in the southern margin of Ordos Basin (modified after Yuan et al.,1995;Chen et al.,2007,2012)1-the section of bentonite outcrops;2-the well drillings of containing bentonites;3-the number/thickness of bentonitesis unknow;4-total thickness(m)/number of bentonites

为了进一步确定鄂尔多斯南缘陕西泾阳地区钾质斑脱岩的物源,统计了钾质斑脱岩的区域分布及其厚度变化(图10)。结果表明钾质斑脱岩的厚度和层数大致呈现由南向北或由南西向北东变小的趋势,表明鄂尔多斯盆地南缘奥陶纪钾质斑脱岩来自其南部北秦岭地区或西南部北祁连地区的火山弧喷发。

事实上,北秦岭地区的火山喷发事件记录不止仅限于北秦岭造山带以北的区域,胡艳华等(2009)研究了扬子地台内湖北宜昌地区和贵州桐梓地区奥陶-志留系界限剖面的钾质斑脱岩,认为它们很可能与北面早古生代秦岭洋的闭合过程有关,也可能来自华南板块东南缘外侧存在的一古老洋壳向华南板块的俯冲。杨颖(2011)报道了湖北宜昌黄花场剖面临湘组-五峰组间的钾质斑脱岩地球化学、锆石U-Pb 年代学和锆石Hf 同位素特征,认为其中的第一层和第八层斑脱岩更可能来自秦岭地区。结合区域地质演化和火山发育的情况,认为推测秦岭地区加里东期的火山活动是华南奥陶-志留系界线附近斑脱岩最有可能的物源。

6 结论

(1)钾质斑脱岩锆石的CL 图像及Th/U 比值显示其成因为岩浆锆石,且所有样品均为K2O >Na2O,显示了其钾质特征,野外和室内特征都符合钾质斑脱岩的定义。

(2)钾质斑脱岩地球化学特征表明其源岩为同碰撞火山弧构造环境下形成的中酸性火山岩。利用在风化过程中不活动元素对钾质斑脱岩的原岩进行了恢复,结果表明其原始岩浆为高钾亚碱性流纹英安岩和粗面安山岩。通过钾质斑脱岩的时代、区域分布和周缘造山带的对比分析,认为这些钾质斑脱岩来源于北祁连或北秦岭岛弧火山喷发。

(3)分别对陕西泾阳和甘肃平凉地区两个剖面的4 个和2 个钾质斑脱岩样品进行了LA-ICP-MS 锆石U-Pb 定年,获得了457 ±3Ma、454 ±4Ma、452 ±5Ma、449 ±3Ma、450 ±2Ma和451 ±2Ma 6 组谐和年龄,结合锆石的岩浆成因特征,这些定年结果代表了平凉组同沉积喷发的火山凝灰岩的年龄。根据钾质斑脱岩发育的层位,将平凉组的主体沉积年龄厘定为晚奥陶世早中期。

致谢 房强、传婷婷、袁路朋一起参加了野外工作;河北省廊坊区域地质调查研究所实验室帮助完成了锆石单矿物分选工作;锆石U-Pb 测年得到天津地质矿产研究所李怀坤研究员与耿建珍博士的热情帮助;方小敏研究员和胡修棉教授对本文提出了富有建设性的意见;在此一并致谢!

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