Discovery and Sedimentary Sequences of a Special Paleoseamount within the Mazongshan Subduction Accretionary Complex in Beishan Orogen, Gansu Province
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摘要: 古海山是缝合带的关键组成部分,中亚造山带西段的天山造山带内已发现多处古海山,而北山地区却鲜有报道,古海山的发现可以弥补该地区海山研究的不足.通过在北山中部野马泉开展地质调查、测制剖面,发现了一套原始层序完整、以玄武岩、玄武质凝灰岩和大理岩为主的地层,具有火山岩基座和碳酸盐岩顶盖的结构,二者原始接触关系为整合接触,符合海山的沉积特征.依据地层中的岩石组合和沉积构造,确定其形成于海山斜坡相.野马泉古海山残骸呈NWW-SEE向延伸,东南部更接近海山顶.该海山中的玄武岩富集大离子亲石元素、亏损高场强元素,具有岛弧玄武岩的特征.该海山为洋内弧型海山,野马泉一带位于海山斜坡,其被构造肢解后呈NWW-SEE向分布.Abstract: Paleoseamounts are key components of suture zones. In the Tianshan orogen located in the southwestern segment of the Central Asian Orogenic Belt, many paleoseamounts have been reported, while few were discovered in the Beishan orogen. Discovery of a paleoseamount can compensate for the lack of studies on seamounts in the Beishan orogen. After geological survey on the Yemaquan area in central Beishan orogen and measuring a profile, strata composed of basalts, basaltic tuffites and marbles with primitive sequences were discovered. The strata contain volcanic basement and carbonate cap, and the two segments primitively conformably contacted, indicating that the strata are fragments of a paleoseamount. The sedimentary structures and rock associations of the paleoseamount in the Yemaquan area indicate that they were deposited in a seamount slope. The seamount fragment in this study trends NWW-SEE and the southeastern part approaches the seamount top. The basalts in the seamount are enriched in large iron lithophile elements and depleted in high field strength elements, showing characteristics of island arc basalts. The seamount in this study is an oceanic arc type seamount. The rocks in the Yemaquan area were deposited in seamount slope facies and trended NWW-SEE after they were dismembered.
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0. 引言
海山是大洋中重要的地貌单元,现代洋底高度大于1 km的潜在海山数量有8 458座,高度大于0.1 km的潜在海山数量有24 643座(Kim and Wessel, 2011).海山由大洋板内环境中地幔深部岩浆上涌并贯穿洋壳喷发形成的玄武岩和上覆碳酸盐岩、远洋沉积物等组成(王永标,2005;Feng et al., 2008;Safonova,2008;Safonova 2014a,Safonova and Santosh, 2014a,2014b;杨高学等,2015),其形成与地幔热点活动密切相关,对研究深部地幔活动和岩浆作用具有重要的指示意义(Hart et al., 2004;Safonova and Santosh, 2014b).作为为数不多的几种可以接触到的地幔物质之一,海山(或海底高原、洋岛等)是研究壳幔相互作用、板块运动的动力机制和地壳增生作用的重要窗口.
作为洋壳内凸起的地质体,海山在洋壳俯冲过程中会改造俯冲通道,常被破坏并保存在板块缝合带中(张继恩等,2018),是缝合带的重要组成部分.增生楔是缝合带的重要标志,主要由海沟浊积岩、洋壳残片、洋内弧和海山等构造侵位形成(潘桂棠等,2019),识别和解析造山带中的古海山有助于确定大洋板块地层的发育时代,更好地研究俯冲带附近的增生作用.
中亚造山带位于西伯利亚板块和华北、塔里木板块之间,是世界上显生宙以来最大的增生造山带(Sengör et al., 1993;Xiao et al., 2010).中亚造山带由诸多微地块、岛弧、蛇绿混杂岩带、海山、陆缘沉积物等组成,其演化历程与古亚洲洋息息相关(Sengör et al., 1993),是研究增生造山作用的良好天然实验室(Xiao et al., 2010).北山造山带位于中亚造山带南缘,是连接西部的天山造山带和东部的兴蒙造山带的枢纽(图 1a)和研究中亚造山带的重要区域(Xiao et al., 2010).
图 1 (a) 北山地区构造位置示意图;(b)北山造山带构造单元划分图a.据Yu et al.(2016);b.据Xiao et al. (2010); 张克信(2015)修改,蛇绿岩年龄据周国庆等(2000); 任秉琛等(2001); 于福生等(2006); 张元元和郭召杰(2008); Ao et al. (2012); 武鹏等(2012);Tian et al. (2014); 胡新茁等(2015); Wang et al. (2018)Fig. 1. (a)Tectonic map show the locations of the Beishan orogen; (b) Map showing the tectonic units of the Beishan orogen近些年来,在中亚造山带西段的天山造山带内已有多处蛇绿混杂岩中发现古海山(或古洋岛).然而,在北山地区却很少有古海山的报道,古海山的研究还相对缺乏.本次在北山造山带中部的马鬃山俯冲增生杂岩中发现了具有海山沉积特征的岩石组合(即玄武岩、玄武质沉凝灰岩及碳酸盐岩),并对该岩石组合的沉积序列进行了详细解析.沉积序列、不同岩石之间的接触关系表明该处共生的玄武岩、玄武质凝灰岩和大理岩为古海山的残片.在识别出古海山沉积序列的基础上,划分出了该处古海山所处的相带,并通过大比例尺填图填绘了其空间展布状态,通过对玄武岩的地球化学研究确定了该海山的形成机制.本次研究可以很好地弥补北山地区缺乏古海山研究的不足,为进一步研究古海山形成过程、地幔活动及丰富北山地区的洋板块地层奠定基础.
1. 区域地质概况
北山造山带由多个岛弧、蛇绿混杂岩带、微陆块、大陆边缘沉积物等构造单元组成,是古亚洲洋南缘多期俯冲增生、碰撞拼合的结果(Xiao et al., 2010).北山地区的构造单元由北至南可划分为雀儿山岛弧、红石山蛇绿混杂岩带、明水-旱山地块、小黄山蛇绿混杂岩带、公婆泉岛弧、红柳河-牛圈子-洗肠井蛇绿混杂岩带、花牛山岛弧、柳园蛇绿混杂岩带、石板山岛弧和敦煌地块(图 1b,Xiao et al., 2010;张克信,2015).有不同观点认为柳园地区的基性-超基性岩并非蛇绿混杂岩带,而是在早二叠世形成于后碰撞环境的裂谷(Zhang et al., 2011;Chen et al., 2016).
图 2 野马泉地区地质图(据Wang et al., 2018修改)Fig. 2. Geological map of the Yemaquan area (modified after Wang et al., 2018)红柳河-牛圈子-洗肠井蛇绿混杂岩带位于北山造山带的中部,是古亚洲洋在红柳河至洗肠井一带的分支洋盆消失的残迹. 任秉琛等(2001)和武鹏等(2012)分别对牛圈子蛇绿混杂岩中的玄武岩和辉长岩进行了全岩Rb-Sr同位素和锆石U-Pb同位素测年,发现其年龄为463±18 Ma和446.5±4.0 Ma. Tian et al.(2014)对火石山-牛圈子一带蛇绿混杂岩内的辉长岩和斜长花岗岩进行了锆石U-Pb同位素测年,确定其年龄为435.0±1.9 Ma,410.5±3.7 Ma和444.3±1.9 Ma,结合其岩石地球化学特征,认为火石山-牛圈子蛇绿混杂岩带代表了志留纪-石炭纪发育的弧间盆地闭合的位置,其闭合时间可能在晚石炭世.在牛圈子蛇绿混杂岩中最年轻的辉长岩年龄为354.0±3.3 Ma(Wang et al., 2018).牛圈子东侧的洗肠井地区有时代为寒武纪(Ao et al., 2012;胡新茁等,2015)和奥陶纪(周国庆等,2000)的蛇绿岩.西侧的红柳河地区曾发现时代为寒武纪(张元元和郭召杰,2008)和志留纪(于福生等,2006)的蛇绿岩;侵入到红柳河蛇绿岩中未变形的黑云母花岗岩年龄为404.8±5.2 Ma,表明红柳河地区的洋盆在早泥盆世已闭合(张元元和郭召杰,2008).当前的研究表明红柳河-牛圈子-洗肠井洋在寒武纪已经形成并至少存在至石炭纪,是北山地区时代最老、时间跨度最大的分支洋盆.
本次发现的古海山残片位于马鬃山俯冲增生杂岩内,其中马鬃山俯冲增生杂岩紧邻红柳河-牛圈子-洗肠井蛇绿混杂岩带.代文军和谈松(2008)依据马鬃山俯冲增生杂岩中发现的藻类化石和洗肠井地区斜长花岗岩年龄将其时代置于震旦纪-寒武纪.红柳河-牛圈子-洗肠井蛇绿混杂岩带内同样存在大量寒武纪蛇绿岩,马鬃山俯冲增生杂岩很可能为红柳河-牛圈子-洗肠井蛇绿混杂岩的一部分.
马鬃山俯冲增生杂岩带北部为发育在明水-旱山地块上的公婆泉岛弧,岛弧成因火成岩种类包括玄武岩、花岗岩、二长花岗岩等,形成时代主要集中于奥陶纪至石炭纪(王立社等,2009;Zhang et al., 2012;Song et al., 2013;Yu et al., 2016),在东七一山和骆驼圈地区的岩浆岩具有埃达克岩的特征(Zhang et al., 2012;Yu et al., 2016).
马鬃山俯冲增生杂岩带以南为双鹰山-花牛山岛弧,其发育时间为奥陶纪-石炭纪(Xiao et al., 2010),岛弧岩浆岩的年龄主要集中在368~453 Ma之间(Mao et al., 2012;Guo et al., 2014;丁嘉鑫等,2015).双鹰山-花牛山岛弧上存在前寒武纪基底,主要包括长城纪古硐井群石英岩、石英砂岩夹大理岩,蓟县纪平头山组大理岩、灰岩夹变质砂岩、千枚岩,青白口纪大豁落山组大理岩.该岛弧上寒武纪-奥陶纪主要为大陆边缘沉积,寒武系包括双鹰山组和西双鹰山组硅质岩、灰岩;奥陶系包括罗雅楚山组砂岩、板岩,锡林柯博组硅质岩和白云山组砂岩、板岩,代表了红柳河-牛圈子-洗肠井洋盆开裂初期的沉积建造.泥盆系主要包括三个井组和墩墩山组,其中3个井组为海陆交互相-滨海相碎屑岩夹中酸性火山岩和碳酸盐岩,墩墩山组为中酸性火山岩(杨合群等,2009).
北山地区多期次、不同性质的构造运动为成矿作用提供了有利条件,寒武纪浅海-半深海沉积物富含磷、钒、铀、锰等矿产,洋盆发育期的海相火山-沉积岩系为铜矿的形成提供了良好条件,岛弧岩浆作用是形成斑岩型铜矿的有利背景,古陆壳重熔形成的花岗岩有利于形成钨锡矿等(杨合群等,2012).
2. 剖面沉积序列及古海山的确定
在野马泉一带的马鬃山俯冲增生杂岩的岩性组合主要是板岩、凝灰质板岩、片岩等基质包裹玄武岩、辉长岩、辉石岩、砾岩岩块.本次测制的剖面位于野马泉南部的马鬃山俯冲增生杂岩带中,其岩性包括玄武岩、玄武质沉凝灰岩、大理岩、砾岩及少量砂岩(图 3).
剖面岩性组合如图 4所示,下部(剖面1~3层)为玄武质沉凝灰岩与大理岩互层,劈理化强烈;中部(4~13层)为玄武岩与大理岩、大理岩化灰岩互层,局部夹玄武质沉凝灰岩;上部(14~21层)为玄武质沉凝灰岩夹大理岩、角砾大理岩、砾岩、粗砂岩,大理岩中夹薄层硅质岩或硅质岩透镜体.
剖面中多见玄武岩(玄武质碎屑岩)与大理岩以逆断层接触,接触带附近大理岩和凝灰岩发生强烈劈理化(图 3a,3b).原始沉积层序未被完全破坏部分中玄武岩(玄武质碎屑岩)与大理岩之间原始接触关系为整合接触.例如,在剖面南侧,在玄武岩之上依次沉积薄层具水平层理玄武质沉凝灰岩和灰岩,体现了火山熔岩-碎屑岩向碳酸盐岩逐渐过渡的沉积过程,而后又依次沉积沉凝灰岩和灰岩,界线清晰可见,产状完全一致,中间无断层(图 5a).在第17层与16层界线处,大理岩整合沉积于具有水平层理的沉凝灰岩之上,产状一致(图 5b).部分沉积序列中玄武岩和凝灰岩具有从(灰)黑色渐变为灰黄色的现象(图 5e),(灰)黑色处于还原状态,为火山岩下部,灰黄色处于相对氧化状态,为火山岩上部.
随着现代海山逐渐被研究和世界各地缝合带中的古海山不断地被发现,古海山的沉积序列也逐渐被解析出来. Phinney et al.(2004)将太平洋中Ontong Java洋底高原的沉积序列总结为4段:(1)枕状玄武岩夹灰岩和凝灰岩;(2)泥岩、粉砂岩和灰岩;(3)硅质岩和灰岩;(4)富微体化石的软泥. 王永标(2005)总结出巴颜喀拉和阿尼玛卿地区的古海山由火山岩基座和碳酸盐岩顶盖组成,有3种结构类型:(1)火山熔岩-泥岩-碳酸盐岩;(2)火山熔岩-角砾岩-层状熔岩-泥岩-碳酸盐岩;(3)枕状熔岩-碳酸盐岩.泰国清迈古海山的沉积序列为玄武岩-玄武质凝灰岩-碳酸盐岩,部分古海山中没有凝灰岩(Feng et al., 2008).巴拿马西部古近纪海山的沉积序列为玄武质熔岩夹半深海沉积物-玄武质角砾岩-碳酸盐岩-玄武质熔岩(Buchs et al., 2011).中亚造山带西段中古海山残片的岩石类型可分3类:(1)火山岩类,主要为玄武岩夹灰岩、硅质岩透镜体;(2)火山-沉积岩类,主要为灰岩、泥岩、硅质岩、枕状玄武岩、火山角砾岩、砾岩夹泥岩及凝灰质砂岩等;(3)沉积岩类,主要为碳酸盐岩及硅质岩(Safonova,2008).虽然海山由于地理位置和演化历程的不同而有所区别,但其共同特征是具有玄武质熔岩(碎屑岩)火山基座和碳酸盐岩顶盖,由火山岩、火山-沉积岩、沉积岩组成(Safonova,2008).
此外,碳酸盐岩与火山岩之间的关系也是判断其是否为海山的重要标志.在海山中,碳酸盐岩与火山岩之间的接触关系为整合接触或者沉积不整合接触(王永标,2005).而在俯冲增生杂岩中,由构造作用造成洋壳残片内玄武岩或岛弧玄武岩与碳酸盐岩通过断层接触在一起的“共生”则不能判断其为古海山.
海山在海沟附近向洋一侧常发生破裂、瓦解(潘桂棠等,2019),规模较大时则很难完全肢解(张克信等,2020),判断其中玄武岩与碳酸盐岩初始接触关系时需从这些破碎块体中找到沉积序列未被破坏的部分.本次发现的共生的玄武质岩、玄武质碎屑岩和碳酸盐岩具备火山岩基座和碳酸盐岩顶盖的结构,未经破坏的碳酸盐岩与玄武岩原始接触关系为整合接触(图 5a,5b),符合海山的形成规律,因此可以确定其为混入到俯冲增生杂岩内的古海山.
3. 海山沉积相分析及空间展布
3.1 沉积相
Safonova(2008)将海山划分为3个相带:海山顶相、海山斜坡相和海山脚相:海山顶相主体为枕状或块状熔岩,熔岩中很少含有角砾灰岩、白云岩、硅质岩和凝灰质砂岩的夹层或透镜体,下部为玄武质熔岩,上部为鲕粒灰岩或块状灰岩;海山斜坡相的岩性包括层状或块状灰岩、钙质泥岩、硅质岩、枕状熔岩、火山角砾岩以及互层的砾岩与凝灰质砂岩,以具有滑塌构造、角砾岩和Z型褶皱为特征;海山脚相主要为深海沉积和滑塌远端沉积,岩性上包括硅质岩(夹灰岩透镜体)、泥岩和硅质凝灰岩(Safonova,2008).
野马泉一带的古海山剖面中存在大量的玄武质沉凝灰岩,沉凝灰岩中发育水平层理,为火山-沉积成因.此外,该处古海山中有较多滑塌沉积,部分大理岩中发育Z型褶皱(图 5f).在剖面16层中有磨圆度较差的角砾岩,其砾石的主要岩性为玄武岩,另有少量大理岩(图 3e),砾石直径在0.5 cm左右;凝灰质粗砂岩中发育正粒序层理和平行层理.在剖面附近的部分大理岩中存在角砾,角砾成分为重结晶程度更高的碳酸盐岩(图 5c);在剖面西北侧有角砾岩沉积,其砾石成分以大理岩为主,另有少量玄武岩,磨圆度极差;剖面东南侧的部分砾岩中砾石主要为大理岩,磨圆度较好,粒径在8~10 cm(图 5d).砾岩的成分表明,其物源主要来自海山上部的玄武岩和碳酸盐岩滑塌.野马街古海山中深海泥质和硅质沉积岩较少,仅部分大理岩中夹少量薄层硅质岩或硅质岩透镜体,与海山脚相岩性有明显区别.通过野马泉一带古海山中识别出的大量滑塌沉积物及Z型褶皱(图 5e,5f),并将其岩石组合与海山不同相带的沉积特点相比较,可以确定其形成于海山斜坡相.
3.2 空间展布
野马泉古海山的地层产状稳定,空间上呈NWW-SEE向展布,厚度近600 m,沿走向伸展超过2 km(图 6).其向南部以出现凝灰质板岩包裹玄武岩岩块为界线(图 7d),北部出现大量劈理化的凝灰质板岩且无碳酸盐岩出现,可能不属于古海山沉积序列或因较严重的第四系覆盖而无法判断.
图 7 (a) 野马泉海山西北侧角砾岩;(b)海山北侧玄武岩-凝灰岩-大理岩的沉积序列;(c)海山西南侧角砾大理岩;(d)海山的边界;(e)海山东南侧凝灰岩-大理岩-砾岩的沉积序列Fig. 7. (a) Breccia in the northwestern seamount; (b) Sedimentary sequences composed of basalts, tuffites and marbles in the northern seamounts; (c) Calcirudytes in the southwestern seamount; (d) Boundary of the seamount; (e) Sedimentary sequences composed of tuffites, marbles and conglomerates在路线地质调查的基础上,我们发现野马泉古海山东南部有更多厚层砂砾岩且厚度更大(图 7e),东南部砾岩中砾石的直径明显大于西北部(东南部砾岩见图 5d,西北部角砾岩见图 7a),可见向东南部更加接近滑塌沉积的上部和古海山顶.在野马泉古海山西部的厚层大理岩中局部夹角砾大理岩(图 7c),砾石成分也是大理岩,具有一定磨圆度,是古海山中滑塌沉积的重要标志.
虽然野马泉古海山在洋壳俯冲过程中已被破坏,但其局部地层层序仍然完整,并且在大比例尺填图过程中依然可以识别出其地层的相变规律.后续工作中如能继续通过大比例尺填图追索出该古海山的海山顶和海山脚相的沉积序列则可对其进行完整地恢复.
4. 玄武岩地球化学特征
本次采集了3件玄武岩样品进行主量元素和微量元素地球化学分析以确定野马泉古海山中玄武岩岩浆的源区性质(采样位置见图 4).主量元素分析采用ZSXPrimusⅡ波长色散X射线荧光光谱仪,微量元素分析采用Agilent 7700e电感耦合等离子体质谱仪,测试过程在武汉上谱分析科技有限责任公司完成.
样品的烧失量(LOI)在1.35%~1.94%,整体较低,故氧化物含量可不重新换算(氧化物含量见表 1).野马泉玄武岩具有较高SiO2含量(52.35%~53.44%)、低TiO2含量(0.80%~0.87%)和低全碱(K2O+Na2O)含量(3.03%~3.98%).在SiO2 vs.(K2O+Na2O)判别图中,3件样品均投影于亚碱性玄武安山岩区域(图 8a).
表 1 玄武岩主量、微量元素含量表Table Supplementary Table Major and trace elements concentrations of basalts in this study样品号 SiO2 TiO2 Al2O3 TFe2O3 MnO MgO CaO Na2O K2O P2O5 LOI PM21-7-1H 52.35 0.87 16.54 7.35 0.56 6.10 9.57 2.36 1.62 0.22 1.94 PM21-8-1H 53.80 0.80 15.90 9.61 0.23 2.88 12.09 1.14 1.88 0.15 1.35 PM21-9-1H 54.22 0.84 17.78 8.60 0.12 2.69 9.73 2.46 1.25 0.23 1.88 样品号 Rb Ba Th U Nb Ta Pb Sr Zr Hf Cs Pb Y PM21-7-1H 51.6 809 2.46 0.82 3.31 0.23 7.13 341 98.8 2.66 1.41 7.13 23.0 PM21-8-1H 42.5 625 3.61 1.15 6.64 0.47 10.40 239 108.0 2.84 4.33 10.40 33.6 PM21-9-1H 28.8 419 2.16 0.65 2.21 0.15 9.33 268 79.1 2.12 2.41 9.33 19.5 样品号 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu PM21-7-1H 13.1 31.9 4.45 19.1 4.56 1.46 4.40 0.69 4.04 0.77 2.16 0.32 2.05 0.31 PM21-8-1H 10.6 25.5 3.34 14.6 3.88 1.09 4.90 0.86 5.54 1.10 2.98 0.43 2.64 0.38 PM21-9-1H 11.7 25.9 3.64 15.8 3.69 1.19 3.88 0.57 3.27 0.71 1.91 0.28 1.78 0.26 注:主量元素单位为%, 微量元素单位为10-6. 图 8 (a) 火山岩SiO2-(K2O+Na2O)分类命名图解; (b)原始地幔标准化微量元素蛛网图; (c)球粒陨石标准化稀土元素配分图a.据Peccerillo and Taylor(1976); c.球粒陨石、原始地幔、OIB、E-MORB、N-MORB值据Sun and McDonough(1989); IAB值据杨婧等(2016)Fig. 8. (a) SiO2 versus (K2O+Na2O) discrimination diagram; (b) Primitive mantle normalized trace element diagrams; (c) Chondrite normalized REE diagrams在原始地幔标准化微量元素蛛网图中,玄武岩的Rb、Ba、K等大离子亲石元素明显富集,高场强元素相对亏损(其中Nb、Ti元素强烈亏损),具有岛弧岩浆岩的特征(图 8b).总稀土含量低(∑REE在74.65×10-6~89.34×10-6),轻稀土元素(LREE)相对重稀土元素(HREE)表现出明显富集.球粒陨石标准化稀土元素配分曲线呈明显右倾(LaN/YbN=2.87~4.73),Eu呈弱负异常(δEu=0.76~0.99)(图 8c).整体来看,玄武岩样品的微量元素和稀土元素配分型式与岛弧玄武岩(IAB)非常相似,而与洋中脊玄武岩(N-MORB和E-MORB)和洋岛玄武岩(OIB)有较大区别(图 8a,8b).
5. 讨论
5.1 野马泉海山的形成机制
海山通常由大洋板内地幔热点岩浆活动形成洋底火山基座和上部沉积的碳酸盐岩组成.地幔热点形成的玄武岩通常为OIB型,其在成因和特征方面与普通洋壳中的玄武岩有很大区别(Safonova 2014a,Safonova and Santosh, 2014a,2014b).在中亚造山带内已发现了大量由地幔热点活动形成的古海山,这些古海山在古亚洲洋闭合过程中拼贴到俯冲增生杂岩中(Safonova,2008;杨高学等,2015).
野马泉海山中的玄武岩具有岛弧玄武岩的特征而不是洋岛玄武岩的特征,表明其源区可能不是来自地幔热点的岩浆活动,而是来自洋壳俯冲形成的地幔楔.研究表明,红柳河-牛圈子-洗肠井洋盆内存在奥陶纪-志留纪洋内弧,洋盆俯冲消亡后保存在马鬃山俯冲增生杂岩中(Wang et al., 2020).野马泉古海山形成机制可能为:红柳河-牛圈子-洗肠井洋盆内发生洋-洋俯冲形成洋内弧,岛弧火山形成的玄武岩和火山碎屑岩在洋底组成了海山的基座,当基座的高度到达碳酸盐岩补偿深度以上时沉积了碳酸盐岩.此类“海山”与常规海山的地层序列和相带分布极为相似,是一类特殊的“海山”建造(可称为洋内弧型海山)(张克信等,2020),但其形成机制和物质源区与常规海山却截然不同.
5.2 构造意义
野马泉地区洋内弧型海山的发现表明红柳河-牛圈子-洗肠井洋盆中存在洋内俯冲,该洋内弧残片与Wang et al.(2020)和张克信等(2020)报道的马鬃山俯冲增生杂岩东部贾布泉口子地区的洋内弧很可能属于同期洋内俯冲的产物.识别洋出内弧带可以更准确地解析北山地区的增生造山作用.当存在洋内俯冲作用时很可能存在洋内弧型海山,研究造山带中的古海山时还应特别注意对此类特殊海山的区分以免对地幔的活动过程造成错误理解.
6. 结论
马鬃山俯冲增生杂岩中野马泉一带共生的玄武质熔岩、碎屑岩和大理岩具有火山岩基座和碳酸盐岩顶盖的结构且原始接触关系为整合接触,为古海山的残骸.该古海山中的岩性组合主要为玄武质沉凝灰岩、玄武岩、大理岩及角砾岩,普遍发育Z型褶皱,表明其形成于海山斜坡相.野马泉古海山在后期构造运动中被改造为NWW-SEE向延伸,向南东方向更接近海山顶,向北西方向更接近海山脚.野马泉古海山中玄武岩具有岛弧玄武岩的特征,是在洋内岛弧火山活动和碳酸盐岩沉积作用下形成的,是一类特殊的“海山”建造,其沉积序列与常规海山类似.对洋内弧型古海山的识别、沉积序列解析、沉积相分析以及空间展布填绘对精确解析北山造山带的增生造山过程具有重要意义.
致谢: 本文得到物化探所中央级公益性科研院所基本科研业务费专项资金资助项目(No. AS2019Y01)、中国地质调查局项目(No.DD20190370)和国家自然科学基金项目(No. 41772107)资助.陈红芳在岩石地球化学测试过程中给予了大力支持.审稿专家提出了宝贵的修改意见,对提高本文质量发挥了巨大作用.在此一并表示感谢! -
图 1 (a) 北山地区构造位置示意图;(b)北山造山带构造单元划分图
a.据Yu et al.(2016);b.据Xiao et al. (2010); 张克信(2015)修改,蛇绿岩年龄据周国庆等(2000); 任秉琛等(2001); 于福生等(2006); 张元元和郭召杰(2008); Ao et al. (2012); 武鹏等(2012);Tian et al. (2014); 胡新茁等(2015); Wang et al. (2018)
Fig. 1. (a)Tectonic map show the locations of the Beishan orogen; (b) Map showing the tectonic units of the Beishan orogen
图 2 野马泉地区地质图(据Wang et al., 2018修改)
Fig. 2. Geological map of the Yemaquan area (modified after Wang et al., 2018)
图 7 (a) 野马泉海山西北侧角砾岩;(b)海山北侧玄武岩-凝灰岩-大理岩的沉积序列;(c)海山西南侧角砾大理岩;(d)海山的边界;(e)海山东南侧凝灰岩-大理岩-砾岩的沉积序列
Fig. 7. (a) Breccia in the northwestern seamount; (b) Sedimentary sequences composed of basalts, tuffites and marbles in the northern seamounts; (c) Calcirudytes in the southwestern seamount; (d) Boundary of the seamount; (e) Sedimentary sequences composed of tuffites, marbles and conglomerates
图 8 (a) 火山岩SiO2-(K2O+Na2O)分类命名图解; (b)原始地幔标准化微量元素蛛网图; (c)球粒陨石标准化稀土元素配分图
a.据Peccerillo and Taylor(1976); c.球粒陨石、原始地幔、OIB、E-MORB、N-MORB值据Sun and McDonough(1989); IAB值据杨婧等(2016)
Fig. 8. (a) SiO2 versus (K2O+Na2O) discrimination diagram; (b) Primitive mantle normalized trace element diagrams; (c) Chondrite normalized REE diagrams
表 1 玄武岩主量、微量元素含量表
Table 1. Major and trace elements concentrations of basalts in this study
样品号 SiO2 TiO2 Al2O3 TFe2O3 MnO MgO CaO Na2O K2O P2O5 LOI PM21-7-1H 52.35 0.87 16.54 7.35 0.56 6.10 9.57 2.36 1.62 0.22 1.94 PM21-8-1H 53.80 0.80 15.90 9.61 0.23 2.88 12.09 1.14 1.88 0.15 1.35 PM21-9-1H 54.22 0.84 17.78 8.60 0.12 2.69 9.73 2.46 1.25 0.23 1.88 样品号 Rb Ba Th U Nb Ta Pb Sr Zr Hf Cs Pb Y PM21-7-1H 51.6 809 2.46 0.82 3.31 0.23 7.13 341 98.8 2.66 1.41 7.13 23.0 PM21-8-1H 42.5 625 3.61 1.15 6.64 0.47 10.40 239 108.0 2.84 4.33 10.40 33.6 PM21-9-1H 28.8 419 2.16 0.65 2.21 0.15 9.33 268 79.1 2.12 2.41 9.33 19.5 样品号 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu PM21-7-1H 13.1 31.9 4.45 19.1 4.56 1.46 4.40 0.69 4.04 0.77 2.16 0.32 2.05 0.31 PM21-8-1H 10.6 25.5 3.34 14.6 3.88 1.09 4.90 0.86 5.54 1.10 2.98 0.43 2.64 0.38 PM21-9-1H 11.7 25.9 3.64 15.8 3.69 1.19 3.88 0.57 3.27 0.71 1.91 0.28 1.78 0.26 注:主量元素单位为%, 微量元素单位为10-6. -
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