岩石学报  2020, Vol. 36 Issue (3): 799-819, doi: 10.18654/1000-0569/2020.03.10   PDF    
内蒙中部苏左旗早石炭世火山岩年代学与地球化学研究:对中亚造山带东部石炭纪构造演化和地壳属性的制约
李梦瞳1,2, 唐军1,2, 王志伟1,2,3, 徐备1,2, 张焱杰1,2, 田英杰1,2, 翟鹏1,2     
1. 河北地质大学区域地质与成矿作用重点实验室, 石家庄 050031;
2. 河北地质大学资源学院, 石家庄 050031;
3. 自然资源部东北亚矿产资源评价重点实验室, 长春 130061
摘要: 本文对苏尼特左旗北部沙尔塔拉地区原定早二叠世大石寨组火山岩进行了系统的岩相学、LA-ICP-MS锆石U-Pb定年、主微量元素和锆石Hf同位素研究。定年结果显示,该套地层中火山岩的形成时代可分为~346.5Ma和335.1Ma两期,其岩石组合分别为玄武安山岩-安山岩-英安岩-流纹岩和英安岩-流纹岩,其形成时代为早石炭世,而非前人认为的早二叠世。玄武安山岩具有高Al2O3(>17%),低MgO含量(2.43%~2.70%)和Cr、Co、Ni的特点,可能是高温低压和含少量水条件下母岩浆经历了橄榄石和辉石(早)以及斜长石(晚)等分离结晶作用的产物;相对富集轻稀土元素、大离子亲石元素(如Rb、Ba、Sr)以及Pb、Zr和Hf,亏损重稀土元素和Nb、Ta和Ti,具有较高的正εHft)值(8.92~13.79),同时存在大量434~490Ma的捕获锆石,表明早石炭世早期玄武安山岩的原始岩浆应起源于陆内伸展机制下受早期俯冲流体交代的亏损岩石圈地幔部分熔融,并经历了一定程度的地壳混染。英安岩和流纹岩的主微量元素特征(富硅、铝,贫铁、镁,富集LREEs和LILEs,亏损HREEs和HSFEs等)和锆石Hf同位素组成(εHft)值分别为7.97~12.14和8.35~14.71,tDM2分别为838~577Ma和812~407Ma),暗示它们的原始岩浆主体来源于新元古代末期和早古生代新增生的地壳部分熔融,同时,335Ma英安岩和流纹岩显示出高温(平均值为876℃)和A型花岗岩的特征,表明它们形成于陆壳伸展环境。安山岩的地球化学特征显示其岩浆可能是上述中基性和酸性岩浆混合的产物。另外,研究区石炭纪以新增生地壳的部分熔融为主,而且酸性火山岩锆石Hf tDM2整体随岩浆活动时代变新而变年轻。综上所述,早石炭世早期玄武安山岩表现出板内成因特点,安山岩具有岩浆混合成因,玄武安山岩与同时代英安岩-流纹岩共同构成了双峰式火山岩组合,从而表明研究区早石炭世早期已经处于陆内伸展环境;而早石炭世中期酸性火山岩的形成也进一步揭示了伸展作用的持续进行。结合前人沉积、变质作用等方面的研究成果,本文认为研究区石炭纪所记录的拉张环境可能与古亚洲洋在泥盆纪闭合后的伸展环境有关。
关键词: 内蒙古苏左旗    早石炭世    火山岩    年代学与地球化学    中亚造山带    
Geochronology and geochemistry of the Early Carboniferous volcanic rocks in Sonid Zuoqi, Inner Mongolia: Implication for the Carboniferous tectonic evolution and crustal nature of the eastern Central Asia Orogenic Belt
LI MengTong1,2, TANG Jun1,2, WANG ZhiWei1,2,3, XU Bei1,2, ZHANG YanJie1,2, TIAN YingJie1,2, ZHAI Peng1,2     
1. Key Laboratory of Regional Geology and Mineralization, Hebei GEO University, Shijiazhuang 050031, China;
2. College of Resources, Hebei GEO University, Shijiazhuang 050031, China;
3. Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Nature and Resources, Changchun 130061, China
Abstract: The newly identified Early Carboniferous magmatism in Sonid Zuoqi, Inner Mongolia will provide new constraints on the Late Paleozoic evolution and crustal accretion history of the eastern Central Asia Orogenic Belt (CAOB). We conducted systematic researches on petrography, LA-ICP-MS zircon U-Pb dating, major and trace element and zircon Hf isotopes upon the volcanic rocks that was previously assigned to the Early Permian Dashizhai Formation in the Shaertala area, north of the Sonid Zuoqi, Inner Mongolia. Zircon U-Pb dating results show that the formation ages of the volcanic rocks can be divided into two stages:~346.5Ma and 335.1Ma, i.e., Early Carboniferous rather than Early Permian. Their rock associations are basaltic andesite-andesite-dacite-rhyolite and dacite-rhyolite, respectively. Geochemical studies indicate that the basaltic andesites have relatively high Al2O3 (>17%), low MgO (2.43%~2.70%) and Cr, Co and Ni, suggesting that their primary magma could undergo fractional crystallization of olivine and pyroxene (early stage) and plagioclase (late stage) at a hydrous condition with a high temperature and low pressure. These rocks are characterized by relatively enrichment in light rare earth elements, large ion lithophilic elements (such as Rb, Ba, Sr), Pb, Zr and Hf, depletion in heavy rare earth elements and Nb, Ta and Ti, and furthermore, they have high positive zircon εHf(t) values (8.92~13.79) and a large number of 434~490Ma captured zircons. The above features indicate that the primary magma of the basaltic andesites was probably originated from partial melting of the depleted lithospheric mantle which was previously modified by subduction-related fluids under the intracontinental extension mechanism, and experienced a certain degree of crust contamination. While the dacites and rhyolites are characterized by high SiO2 and Al2O3, low Fe2O3T and MgO, enrichment in LREEs and LILEs, depeletion in HREEs and HSFEs, as well as depleted zircon Hf isotopic compositions (εHf(t) values of 7.97~12.14 and 8.35~14.71, tDM2 of 838~577Ma and 812~407Ma), suggesting that their primary magma was originated from the partial melting of the Late Neoproterozoic and Early Paleozoic juvenile crust. Furthermore, the 335Ma dacites and rhyolites exihibit high temperature (average value of 876℃) and A-type granite signatures, indicating that they were formed in an extensional environment. Besides, the geochemical features of andesites imply that their source magma could be formed by the mixing between basaltic and felsic magma. Additionally, the Carboniferous crustal evolution was dominated by partial melting of juvenile crust, and the zircon Hf tDM2 for the felsic volcanic rocks gradually decrease with their formation ages. In summary, it can be inferred that the early Early Carboniferous in studied area was already under an extensional setting since the magma mixing origin of the andesites, the within-plate origin basaltic andesites and the coeval dacites and rhyolites constitute a bimodal volcanic rock association. The formation of middle Early Carboniferous A-type felsic volcanic rocks further revealed that the extension continued in this area. Hence, together with the previous researches on sedimentology and metamorphism, we propose that the Carboniferous extension could be related to the post-collisional process after the closure of Paleo-Asian Ocean during the Devonian.
Key words: Sonid Zuoqi, Inner Mongolia    Early Carboniferous    Volcanic rocks    Geochronology and Geochemistry    Central Asia Orogenic Belt (CAOB)    

中亚造山带位于西伯利亚板块与中朝板块之间,是古亚洲洋消减而形成的巨型缝合带,也是全球显生宙时期增生最为强烈的地区之一,记录了显生宙期间巨量的新增生地壳的形成(Jahn et al., 2000a, bWu et al., 2000Hong et al., 2004; Jahn,2004)。越来越多的研究也显示在中亚造山带古生代的地壳演化过程中古老地壳物质的再造也发挥了重要的作用(Demoux et al., 2009Kröner et al., 2014Wang et al., 2016b, 2017)。

中亚造山带东部称为兴蒙造山带,在中国境内主要分布于东北和内蒙古中东部地区,前人根据前寒武纪地质体和其间的缝合带将其划分为额尔古纳、兴安-爱力格庙、松辽-浑善达克、佳木斯等地块(徐备等,2014图 1a)。兴蒙造山带经历了古亚洲洋构造体系作用,前人对古亚洲洋在古生代时期的演化有不同认识。一种观点认为古亚洲洋在古生代发生陆陆碰撞,之后古亚洲洋闭合并处于造山后的伸展环境(Tang,1990Xu et al., 2013徐备等, 2014, 2018Zhang et al., 2015庞崇进等,2018张晋瑞等,2018)。具体来说,这一过程又分为两个阶段:第一阶段为早古生代俯冲碰撞造山过程,即早-中古生代古亚洲洋沿苏尼特左旗-红格尔一带发育向北俯冲,沿温都尔庙-西拉木伦河一带发育向南俯冲,形成南、北两条造山带,且在380Ma闭合;第二阶段表现为晚古生代伸展和裂谷发育过程,形成了晚古生代双峰式火成岩和碱性岩、低压高温变质岩、上叠盆地和伸展盆地等,对早期构造格局产生了强烈的改造。另一种观点则认为:华北克拉通和南蒙古微陆块之间的古亚洲洋南部在整个古生代均处于南北双向俯冲加积的过程中,即从晚寒武世开始大洋板片向南、向北分别俯冲于华北克拉通和南蒙古微陆块下,这一俯冲作用一直持续到晚古生代末,直到中-晚二叠世至中三叠世古亚洲洋才沿索伦-西拉木伦-长春-延吉缝合带最终闭合(Chen et al., 2000, 2009Xiao et al., 2003, 2009, 2015Li,2006Wu et al., 2007Eizenhöfer et al., 2014Song et al., 2015Liu et al., 2017)。

图 1 兴蒙造山带构造单元划分(a,据徐备等,2014修改)和苏左旗沙尔塔拉地区地质简图(b) Fig. 1 Simplified geological map of Xing'an-Mongolia Orogenic Belt (a, modified after Xu et al., 2014) and geological map of Shaertala, Suonid Zuoqi, central Inner Mongolia (b)

苏左旗地区位于兴安-爱力格庙地块西南部,是北造山带的典型出露地区(图 1a),该区域分布有大面积晚古生代岩浆岩,是揭示古亚洲洋构造体系晚古生代演化和地壳增生历史的关键区域。前人对该区晚古生代演化历史存在不同认识:一种观点认为,古亚洲洋板块在石炭纪期间向北俯冲形成宝力道岛弧带,并产出一系列中基性深成岩和酸性侵入岩(Chen et al., 2000, 2009);另一种观点认为该区晚古生代处于古亚洲洋闭合后的伸展环境,从早石炭世开始在大陆基础上拉张形成二连-贺根山蛇绿岩带、二叠纪双峰式火山岩带和碱性岩带(徐备等, 2014, 2018Zhang et al., 2015)。

苏左旗北部沙尔塔拉地区出露有一套较大面积的晚古生代火山-沉积地层,蒋干清等(1995)高德臻和蒋干清(1998)通过岩石组合、地质剖面、沉积环境和Rb-Sr定年研究,将其确定为早二叠世大石寨组,并认为形成于活动大陆边缘环境。笔者通过对其中火山岩的LA-ICP-MS锆石U-Pb定年,将其时代厘定为早石炭世,并对其进行系统的岩石组合、年代学、主微量元素和锆石Hf同位素研究,试图查明该期岩浆活动的岩石组合、时空分布及形成环境,从而制约苏左旗早石炭世岩浆作用的性质,进而探讨内蒙中部早石炭世岩浆作用时空分布和构造背景以及地壳增生过程。

1 区域地质概况和样品描述

研究区位于内蒙古中部苏尼特左旗东北部,大地构造位置属于兴安-爱力格庙地块西部(图 1a)。研究区出露最古老的地质体是中元古代片麻状花岗岩(~1.3Ga)和昌特敖包组石英岩与变质砂岩(沉积时限为1245~926Ma,贺跃等,2018)。前人认为该期花岗岩具有A型地球化学特征,并将其与哥伦比亚超大陆的裂解相联系(Han et al., 2017)。研究区内晚古生代-早中生代建造包括早二叠世大石寨组、哲斯组、晚石炭世和三叠纪(310Ma、222Ma)基性和酸性侵入岩。前人将大石寨组分为三段:下段以砾岩、含砾砂岩和砂岩、粉砂岩为主;中段分布于昌特敖包到达尔罕敖包一带,以中基性-酸性火山岩及其凝灰岩为主;上段主要分布在包尔敖包附近,岩石组合以英安岩、流纹岩及其熔结凝灰岩为特征。根据中段安山岩的Rb-Sr年龄(281Ma),有研究者认为其时代为早二叠世(蒋干清等,1995高德臻和蒋干清,1998);近年来,Zhang et al.(2017)通过系统的火山岩锆石U-Pb年代学研究将其时代厘定为292~279Ma。哲斯组出露面积较大,主要由一系列砾岩、含砾砂岩、长石石英砂岩组成,产腕足、双壳和腹足类化石,与下伏大石寨组呈平行不整合接触(内蒙古自治区地质矿产局,1991蒋干清等,1995高德臻和蒋干清,1998)。晚石炭世(310Ma)辉长岩-闪长岩侵入于新元古代昌特敖包组内,三叠纪(222Ma)A-型花岗岩大面积出露在研究区东南部(石玉若等,2007)。另外,在苏左旗南部也分布有大量的晚古生代-早中生代侵入岩,例如宝力道地区出露有322~316Ma高钾钙碱性系列的花岗闪长岩和二长花岗岩、310Ma闪长岩、234Ma二长花岗岩-花岗闪长岩-淡色花岗岩组合和222~204Ma A-型花岗岩(Chen et al., 2000, 2009石玉若等,2004Hu et al., 2015)。

本文主要对苏左旗北部沙尔塔拉地区原定早二叠世大石寨组中段(达尔罕敖包)和上段(包尔敖包)火山岩进行系统的岩相学、年代学和地球化学研究(图 1)。野外地质剖面显示这套火山岩岩石组合为玄武安山岩、安山岩、英安岩和流纹岩(图 2)。

图 2 沙尔塔拉地区早石炭世火山岩路线剖面和野外特征 (a)达尔罕敖包剖面;(b)包尔敖包剖面;(c)野外特征 Fig. 2 Geological sections and field outcrops for Early Carboniferous volcanic rocks in Shaertala, Inner Mongolia (a) Daerhanaobao; (b) Baoeraobao; (c) field outcrops

达尔罕敖包早石炭世火山岩地层剖面(图 2a),该剖面总厚大于157m,层序如下:

中二叠统哲斯组(P2z)砾岩厚度大于10m

——平行不整合——

早石炭世火山岩地层

6.灰白色流纹岩20m

5.黑色玄武安山岩,具有块状构造和隐晶质结构11m

4.灰绿色安山岩夹少量英安岩28m

3.灰白色流纹岩25m

2.暗红色玄武安山岩7m

1.灰白色流纹岩夹少量英安岩66m

——未见底——

包尔敖包早石炭世火山岩地层剖面(图 2b),该剖面总厚大于51.5m,层序如下:

花岗岩侵入

7.青灰色砂岩7m

6.灰绿色安山岩5m

5.灰白色流纹岩,斑晶主要为石英、斜长石、碱性长石为主6m

4.凝灰质砾岩3.5m

3.灰白色流纹岩,具有流纹构造和斑状结构19m

2.灰白色英安岩,发育块状构造和斑状结构4m

1.灰绿色安山岩,斑晶以斜长石为主7m

辉长岩侵入,发育冷凝边和围岩捕掳体。

各类火山岩的岩相学特征如下:

玄武安山岩 样品(DXT1)新鲜面为黑色,具有块状构造和隐晶质结构,主要由细粒且发生暗化的柱状暗色矿物(~20%)和斜长石(~75%)以及少量粒状不透明矿物和副矿物(~5%)组成;斜长石呈板条状,局部可见聚片双晶,呈交织排列,后期发生较强烈的绢云母化蚀变(图 3a)。样品(DXT3)新鲜面为暗红色,发育块状构造、斑状结构和安山结构;斑晶以斜长石(~15%)和暗化的暗色矿物(~10%)为主,斜长石遭受了绢云母化蚀变;基质为隐晶质结构,主要矿物有细粒的斜长石和少量暗色矿物(图 3b)。安山岩均具有斑状结构和块状构造,斑晶为斜长石(~5%),基质主要为细粒斜长石交织排列,同时还存在少量暗色矿物,发生绿帘石化(图 3c)。

图 3 沙尔塔拉地区早石炭世火山岩岩相学特征 (a、b)玄武安山岩;(c)安山岩;(d)英安岩;(e、f)流纹岩. Af-碱性长石;Pl-斜长石;Q-石英 Fig. 3 Microphotographs for Early Carboniferous volcanic rocks in Shaertala, Inner Mongolia (a, b) basaltic andesite; (c) andesite; (d) dacite; (e, f) rhyolite. Af-alkali feldspar; Pl-plagioclase; Q-quartz

英安岩 样品(DXT2-2、17SZ23-9)新鲜面为灰白色,发育块状构造和斑状结构,斑晶以石英(8%~10%)和斜长石(10~15%)为主,斜长石发生绢云母化蚀变,基质为隐晶质结构,由长英质矿物组成(图 3d)。

流纹岩 样品(DXT6,17SZ23-1、3、5)新鲜面为灰白色,具有流纹构造和斑状结构,基质呈隐晶质结构;斑晶主要由石英(~8%)、斜长石(~10%)和碱性长石(~5%)组成,斜长石发育聚片双晶和绢云母化蚀变,碱性长石主要为自形的长条状透长石,卡式双晶发育,石英发育波状消光,具有港湾状熔蚀结构;基质主要为隐晶质的长英质矿物,斜长石可见聚片双晶(图 3ef)。

2 分析方法

锆石挑选由河北省廊坊市拓轩岩矿检测服务有限公司完成。锆石U-Pb同位素和Hf同位素测试在中国地质调查局西安地质调查中心国土资源部岩浆作用成矿与找矿重点实验室利用7700x型四级杆等离子体质谱仪、Neptune Plus型多接收等离子体质谱仪同时与Geolas Pro型激光剥蚀系统联机完成。激光束斑为32μm,采用锆石国际标样91500作外标。氦气作为载气,为了调节和提高仪器灵敏度,气路中间引入了氩气和少量氮气,气溶胶匀化之后被分别输送到等离子体质谱仪和多接收等离子体质谱仪中同时进行锆石微量元素和U-Pb同位素年龄测试和Hf同位素测试。每分析6个点样品分析一次标准样品NIST610、91500和GJ-1,GJ-1同时作为U-Pb年龄和Hf同位素测试监控样品。实验数据运用GLITTER 4.4软件进行处理(Griffin et al., 2008)。普通Pb校正采用Andersen(2002)的方法,锆石年龄谐和图及加权平均年龄和频谱图均通过ISOPLOT宏程序获得(Ludwig,2003),单个测试点的同位素比值和年龄误差均为1σ。本文计算εHf(0)和εHf(t)所采用的现代球粒陨石176Hf/177Hf比值为0.282772,176Lu/177Hf比值为0.0332(Blichert-Toft and Albarède,1997),用以计算tDM1176Lu/177Hf和176Hf/177Hf比值分别为0.0384和0.282325,而用来计算tDM2176Lu/177Hf比值为0.015(Griffin et al., 2000, 2002)。

主量元素测试分别在北京大学造山带与地壳演化教育部重点实验室、武汉上谱分析科技有限公司采用X光荧光光谱(XRF)分析完成,元素分析的准确度优于3%;微量元素分析分别在中国科学院地质与地球物理研究所岩石圈演化国家重点实验室、河北地质大学区域地质与成矿作用重点实验室和武汉上谱分析科技有限公司利用电感耦合等离子质谱仪(ICP-MS)完成,微量元素的分析精度及准确度一般优于10%。

3 分析结果 3.1 锆石年代学

本文主要对沙尔塔拉地区原定早二叠世大石寨组内玄武安山岩(DXT1-1)、流纹岩(17SZ23)进行了锆石U-Pb年代学研究,部分锆石的CL图像见图 4,定年结果见表 1

图 4 部分锆石阴极发光图像 圆圈代表U-Pb同位素年龄和Hf同位素联机测试位置 Fig. 4 Cathodoluminescence images of selected zircon grains The cycle represents the U-Pb dating and Hf isotope test point at the same time

表 1 沙尔塔拉地区火山岩锆石年代学LA-ICP-MS测试结果 Table 1 Zircon LA-ICP-MS dating results for the volcanic rocks in Shaertala

玄武安山岩(DXT1-1):采于前人原定的早二叠世大石寨组地层中段。通过锆石CL图像可以看出玄武安山岩中岩浆锆石均呈自形-半自形晶,显示出清晰的内部结构,但具有两类特征:第一类岩浆锆石呈半自形板状,具有条痕状吸收,与中基性岩中锆石特征一致(图 4a);另一类锆石多呈长柱状,发育密集的岩浆振荡生长环带,与酸性岩中锆石特征基本吻合。结合其高的Th/U比值(0.30~1.45)表明它们都具有岩浆成因(Pupin,1980Koschek,1993)。该样品中所有锆石测点均位于谐和线上及其附近,其中10个锆石测点给出了一组最年轻的谐和年龄,其206Pb/238U加权平均年龄为349.0±5.2Ma(MSWD=2.3,n=10),剩余13个测点给出的206Pb/238U年龄分别介于363~382Ma和439~489Ma(图 5a, b)。最年轻的一组岩浆锆石加权平均年龄(349Ma)代表了该火成岩的形成时代,而相对较老的年龄代表了岩浆中捕获锆石的年龄。该形成年龄与贺跃等(2018)在该段地层中获得的英安岩年龄(345.7±3.9Ma;图 5d)基本一致。

图 5 沙尔塔拉地区早石炭世火山岩锆石U-Pb年龄谐图与频谱图 DXT6数据来源于贺跃等(2018) Fig. 5 Zircon U-Pb concordia diagrams and relative probability plots for Early Carboniferous volcanic rocks in Shaertala, Inner Mongolia Data for DXT6 from He et al. (2018)

流纹岩(17SZ23):采于该套地层上段,样品中锆石颗粒均呈自形-半自形短柱状,CL图像特征均一,发育密集的岩浆振荡生长环带,不发育变质边(图 4b)。结合测试的24个锆石U-Pb年龄中的Th/U比值(0.48~0.84)表明其岩浆成因(Pupin,1980Koschek,1993)。22个锆石测点均位于谐和线上及其附近,给出的206Pb/238U年龄介于330~343Ma之间,其206Pb/238U加权平均年龄为335.1±2.0Ma(MSWD=0.62,n=22;图 5c),该年龄代表了英安岩的形成时代,即早石炭世中期,而非早二叠世。

3.2 地球化学 3.2.1 主微量元素

本文主要对苏尼特左旗北部沙尔塔拉地区上述早石炭世玄武安山岩、安山岩、英安岩和流纹岩进行了系统的全岩主微量元素地球化学研究,分析结果见表 2

表 2 沙尔塔拉地区早石炭世火山岩主量(wt%)和微量(×10-6)元素分析结果 Table 2 Major (wt%) and trace (×10-6) element data for Early Carboniferous volcanic rocks in Shaertala, Inner Mongolia

玄武安山岩和安山岩的SiO2=54.23%~61.54%,TiO2=0.76%~1.22%,全Fe2O3(Fe2O3T)= 6.57%~9.92%,MgO=2.17%~2.70%,CaO=2.91%~3.44%,Na2O=1.54%~4.14%,K2O= 1.52%~4.51%,P2O5=0.06%~0.25%,Na2O+K2O=5.33%~6.07%,Mg# [100Mg2+/(Mg2++TFe2+)]值为33~40。在硅碱图(TAS)中均落入亚碱性系列区域和玄武安山岩-玄武粗安岩和安山岩区域(图 6a),FeOT/MgO-FeOT图解显示玄武安山岩属于拉斑系列,而安山岩主要落入拉斑和钙碱性系列过渡区域(图 6b)。在球粒陨石标准化稀土元素配分图解中,玄武安山岩和安山岩具有较高的稀土元素总量(95.32×10-6~185.7×10-6),轻重稀土元素分馏较弱,(La/Yb)N比值介于4.34~4.76,同时存在较弱的Eu负异常(Eu/Eu*=[2×EuN/(SmN+GdN)]=0.68~0.97,图 7a)。原始地幔标准化微量元素蛛网图显示玄武安山岩和安山岩相对富集Rb、Ba、Th、U和Pb等元素,亏损Nb、Ta、P和Ti等元素(图 7b)。

图 6 沙尔塔拉地区早石炭世火山岩TAS图解(a, 底图据Irvine and Baragar, 1971)和FeOT/MgO-FeOT图解(b, 底图据Miyashiro, 1974) 灰色数据来源于Zhang et al., 2015; Yang et al., 2017 Fig. 6 TAS (a, base map after Irvine and Baragar, 1971) and FeOT/MgO vs. FeOT (b, base map after Miyashiro, 1974) diagrams for Early Carboniferous volcanic rocks in Shaertala, Inner Mongolia Data for the grey samples come from Zhang et al., 2015; Yang et al., 2017

图 7 沙尔塔拉地区早石炭世火山岩球粒陨石标准化稀土元素配分图(a、c, 标准化值据Boynton, 1984)和原始地幔标准化微量元素蛛网图(b、d, 标准化值据Sun and McDonough, 1989) 阴影数据来源于Zhang et al., 2015 Fig. 7 Chondrite-normalized REE patterns (a, c, normalization values after Boynton, 1984) and primitive mantle-normalized trace element spider diagrams (b, d, normalization values after Sun and McDonough, 1989) for Early Carboniferous volcanic rocks in Shaertala, Inner Mongolia Data for shadow areas from Zhang et al., 2015

英安岩和流纹岩的SiO2=62.58%~79.23%,TiO2=0.23%~1.21%,Fe2O3T=1.72%~7.69%,MgO=0.26%~2.10%,CaO=0.37%~3.56%,Na2O=1.85%~6.78%,K2O=0.22%~6.07%,P2O5=0.03%~0.45%。它们在TAS图解中主要落入亚碱性系列和英安岩-流纹岩区域(图 6a)。球粒陨石标准化稀土元素配分图解显示,英安岩和流纹岩也具有较高的稀土元素总量(107.6×10-6~215.5×10-6),轻重稀土元素分馏较弱,(La/Yb)N比值介于4.20~4.97,相对富集轻稀土元素、亏损重稀土元素(图 7c),同时表现出较为明显的Eu负异常(Eu/Eu*=0.48~0.76)。原始地幔标准化微量元素蛛网图显示早石炭世酸性火山岩相对富集大离子亲石元素(如Rb、Ba、Th、U等)和Pb元素,亏损Nb、Ta、Ti、Sr和P元素(图 7d)。

3.2.2 锆石Hf同位素

本文主要对沙尔塔拉地区早石炭世早期玄武安山岩、英安岩和早石炭世中期流纹岩进行了锆石Lu-Hf同位素研究,分析结果见表 3图 8

表 3 沙尔塔拉地区早石炭世火山岩锆石Hf同位素组成 Table 3 Zircon Hf isotopic data for Early Carboniferous volcanic rocks in Shaertala, Inner Mongolia

图 8 沙尔塔拉地区早石炭世火山岩锆石εHf(t)-t图解 XMOB-兴蒙造山带; YFTB-燕山褶皱带(据Yang et al., 2006).虚线和实线区域数据分别来源于张焱杰等(2018)Hu et al. (2015) Fig. 8 Diagrams of εHf(t) vs. t for Early Carboniferous volcanic rocks in Shaertala, Inner Mongolia Data of XMOB (Xing'an-Mongolia Orogenic Belt) and YFTB (Yanshan Fold and Thrust Belt) after Yang et al., 2006; Data for the imaginary and full lines come from Zhang et al. (2018); Hu et al. (2015)

玄武安山岩样品(DXT1-1)中代表形成年龄的10个岩浆锆石(349Ma)测点的176Hf/177Hf比值介于0.282818~0.282956之间,εHf(t)值介于8.92~13.79之间,Hf同位素一阶段模式年龄(tDM1)介于431~622Ma之间。其余捕获锆石(363~489Ma)的176Hf/177Hf比值介于0.282419~0.282987之间,εHf(t)值介于-1.95~15.03之间,tDM1介于397~1169Ma,tDM2介于410~1583Ma。

早石炭世早期英安岩(DXT6)中岩浆锆石(345.7Ma)的176Hf/177Hf比值介于0.282788~0.282910之间,εHf(t)值介于7.97~12.14之间,对应的tDM2介于577~838Ma之间。该样品中2个捕获锆石(418Ma和1790Ma)测点的176Hf/177Hf比值分别为0.281958和0.281735,εHf(t)值为-19.63和2.29,对应的Hf同位素二阶段模式年龄分别为2642Ma和2313Ma。早石炭世中期流纹岩(17SZ23)中代表形成年龄的岩浆锆石(335.1Ma)的176Hf/177Hf比值介于0.282806~ 0.282990之间,εHf(t)值主要介于8.35~14.71之间,相应的Hf同位素二阶段模式年龄介于407~812Ma之间。

4 讨论 4.1 早石炭世火成岩形成时代与时空分布

已有研究表明,苏左旗北部大面积分布的原大石寨组火山岩地层可能属于不同时代。前人主要获得安山岩的Rb-Sr年龄为281Ma,将其时代确定为早二叠世晚期(蒋干清等,1995高德臻和蒋干清,1998)。然而,由于Rb-Sr同位素体系容易受后期岩浆热事件的改造,该年龄可能不足以准确限定该套火山岩的形成时代。Zhang et al.(2017)对研究区附近的火山岩进行LA-ICP-MS锆石U-Pb定年研究,确定早二叠世(292~279Ma)火山岩地层的存在。而贺跃等(2018)在该地区对原定大石寨组火山岩进行锆石年代学研究,获得了英安岩的形成时代为346Ma,即早石炭世。因此,目前对该套地层的时代分布仍需要更多的年代学约束。本文对该套地层中段(达尔罕敖包)和上段(包尔敖包)火山岩进行了系统的LA-ICP-MS锆石U-Pb年代学研究,分别厘定出早石炭世早期和早石炭世中期的形成年龄,结合区域上最新的年代学资料,可以进一步揭示研究区及邻区石炭纪岩浆作用的时空分布。

玄武安山岩和英安岩中岩浆锆石呈自形-半自形板状和柱状,具有条痕状吸收以及岩浆振荡生长环带。另外上述样品中的锆石均具有较高的Th/U比值(0.30~1.45),表明它们均是岩浆结晶作用的产物。定年结果显示,玄武安山岩中最年轻的一组206Pb/238U加权平均年龄为349.0±5.2Ma,与贺跃等(2018)报道的英安岩年龄(346Ma)在误差范围内基本一致,即地层中部玄武安山岩和英安岩的形成时代应为早石炭世早期。相比之下,地层上部的流纹岩给出了最年轻的一组谐和年龄为335.1±2.0Ma,代表了上部火山岩的形成时代,即早石炭世中期。综上所述,前人原定早二叠世大石寨组中上部的火山岩形成时代为早石炭世,而非早二叠世。另外,玄武安山岩中还存在少量363~381Ma和439~489Ma的捕获锆石,暗示研究区可能存在上述岩浆作用。早古生代年龄与苏左旗南部弧岩浆作用时代基本一致,但晚泥盆世岩浆作用目前尚未有报道。

通过本文的年代学研究,结合前人的研究成果可以将兴安-爱力格庙地块西南部苏左旗地区石炭纪岩浆作用分为四期:349~345Ma、335Ma、322~316Ma和310~309Ma。如前所述,前两期主要分布于苏左旗北部的沙尔塔拉地区;322~309Ma岩浆岩主要分布在苏左旗南部宝力道地区,其中322~316Ma是一套高钾钙碱性系列的花岗闪长岩和二长花岗岩,表现出碰撞后花岗岩的地球化学特征(Hu et al., 2015);310~309Ma岩浆作用的产物以闪长岩为主(Chen et al., 2000, 2009)。另外,在研究区以北的二连-贺根山蛇绿岩带内还识别出了少量同期次的岩浆活动,包括贺根山蛇绿岩中的354Ma辉长岩和333Ma斜长花岗岩(Jian et al., 2012),二连蛇绿岩中的360~348Ma流纹岩和斜长花岗岩、354~345Ma辉长岩和313Ma闪长玢岩(Zhang et al., 2015Yang et al., 2017)。一些研究者还在二连北部以及白音乌拉地区发现了少量317~310Ma花岗岩(云飞等,2011许立权等,2012李可等,2015),在苏右旗以及锡林浩特地区还出露有晚石炭世(310~300Ma)玄武岩和角闪辉长岩(潘世语等,2012Pang et al., 2016庞崇进等,2018),兴安-爱力格庙地块西延部分——蒙古Zamyn Uud地区也存在少量晚石炭世与区域伸展有关的岩浆活动,例如辉绿岩和花岗斑岩脉(306Ma)以及石英二长岩(300Ma;Hu et al., 2017)。

4.2 岩石成因 4.2.1 中基性岩

苏左旗沙尔塔拉地区早石炭世早期玄武安山岩具有较低的SiO2(54.23%~56.35%)、高的Al2O3(>17%)、Fe2O3T(9.14%~9.92%)和K2O(3.29%~4.51%),低的MgO(2.43%~2.70%)、Mg#(33~37)以及Cr(25.58×10-6~41.79×10-6)、Co(16.05×10-6~18.41×10-6)、Ni(25.47×10-6~33.31×10-6)含量,暗示其岩浆来源于地幔物质的部分熔融。与幔源原始岩浆相比,其SiO2含量偏高,MgO和Cr、Co、Ni含量偏低,表明研究区玄武安山岩岩浆不具有幔源原始岩浆的特点(Frey and Prinz, 1978),其可能与分离结晶作用有关。首先,该火山岩中的Cr、Ni和CaO与MgO均呈正相关关系(图略),表明其岩浆可能经历了以橄榄石和单斜辉石为主的分离结晶作用;P和Ti的负异常暗示了岩浆经历了磷灰石和铁钛氧化物的分离结晶;较明显的负Eu异常,暗示了其母岩浆经历了明显的斜长石分离结晶(Rollinson,1993)。其次,该套拉斑系列玄武安山岩中存在少量暗化了的含水暗色矿物,而且具有高Al2O3和低MgO的特征,与前人研究的低MgO高铝玄武岩特征相似,它们可以通过高温低压和含少量水条件下母岩浆发生分离结晶作用形成(Sisson and Grove, 1993ab),橄榄石和辉石等矿物先结晶,同时含水会抑制斜长石的成核作用,从而导致岩浆中的MgO和CaO含量降低,Al2O3含量升高(Beard and Lofgren, 1992Sisson and Grove, 1993abAriskin,1999; Grove et al., 2012)。上述条件的存在也可以得到位于研究区西侧80km本巴图地区出露的早石炭世蛇绿岩(360~333Ma)形成的佐证,其中的玄武岩和含角闪石的辉长岩具有MORB的地化特征,暗示了母岩浆是含水的,形成于拉张环境下(Zhang et al., 2015Yang et al., 2017)。而且近年来对锡林浩特及邻区晚石炭世玄武岩和角闪辉长岩的研究显示,其母岩浆含水量高达4.4%,可能是伸展背景下富水地幔源区部分熔融的产物(Pang et al., 2016Wang et al., 2016a庞崇进等,2018)。

另外,玄武安山岩相对富集轻稀土元素(LREEs)和大离子亲石元素(LILEs,如Rb、Ba和Sr等),亏损重稀土元素(HREEs)和高场强元素(HFSEs,如Nb、Ta和Ti;图 7ab),而且具有高的La/Nb(2.26~2.93)、Ba/La(13.70~31.17)和Ba/Nb(40.10~70.37)比值,显示出类似弧火山岩的地球化学特征。然而玄武安山岩具有高的K2O、Th、U和Pb含量,Zr和Hf的富集(图 7b),同时还存在大量439~489Ma的捕获锆石(图 5a),与早古生代弧岩浆作用时代相一致,暗示了其上升过程中可能遭受了地壳物质的混染。研究表明,Zr和Y在蚀变或低级变质作用过程是相对不活动的,Zr-Zr/Y图解可以相对有效地区分岛弧玄武岩和板内玄武岩,在该图解中研究区早石炭世早期玄武安山岩主要落入到了板内区域(图 9)。另外,在苏左旗南部宝力道地区目前尚未发现晚古生代增生杂岩以及早石炭世弧岩浆岩(徐备等,2018)。综上所述,结合玄武安山岩中岩浆锆石较高的正εHf(t)值(8.92~13.79),本文认为早石炭世早期玄武安山岩的原始岩浆应起源于陆内伸展机制下受早期俯冲流体交代的亏损岩石圈地幔部分熔融,并经历了一定程度的矿物分离结晶和地壳混染作用。

图 9 沙尔塔拉地区早石炭世中基性岩Zr-Zr/Y图解(底图据Pearce and Norry, 1979) Fig. 9 Zr vs. Zr/Y diagram for Early Carboniferous volcanic rocks in Shaertala, Inner Mongolia (base map after Pearce and Norry, 1979)

相比之下,安山岩具有中等的SiO2(61.54%)、Al2O3(17.18%)含量、低的MgO(2.17%)和P2O5(0.06%)含量。早石炭世早期岩石组合为玄武安山岩-安山岩-英安岩和流纹岩。那么,该套安山岩的岩浆可以由玄武安山质岩浆结晶分异形成,也可以通过中基性和酸性岩浆混合形成。首先,野外地质调查显示该期火山岩以中酸性岩为主,如果安山岩岩浆是中基性岩浆分异的产物,意味着应存在大量的中基性岩。此外,地幔熔融形成的基性岩浆一般具有低的P2O5(< 0.2%),而P含量主要受磷灰石等副矿物的控制,基性岩浆开始结晶时P并不饱和,矿物不断从岩浆中分离出去,残余岩浆的P将会逐渐升高,随着岩浆的继续冷却和结晶,磷灰石开始结晶,导致岩浆中的P逐渐降低(图 10Lee and Bachmann, 2014)。玄武安山岩和安山岩P2O5的变化并不符合结晶分异的趋势。因此,可以排除安山岩是中基性岩浆结晶分异的产物。另一方面,安山岩的SiO2、TiO2、Al2O3、Fe2O3T、MgO和P2O5含量均介于中基性岩和酸性岩之间,同时基性和酸性岩浆混合形成的中性岩具有低的P含量以及单一的变化趋势,这与本文的两种岩石类型显示的含量变化以及趋势吻合(图 10)。综上所述,本文认为研究区早石炭世早期安山岩岩浆是中基性和酸性岩浆混合的产物,而Eu、P和Ti的负异常表明该岩浆经历了一定程度的斜长石、磷灰石和铁钛氧化物等的分离结晶。

图 10 沙尔塔拉地区早石炭世火山岩P2O5-SiO2图解(底图据Lee and Bachmann, 2014) Fig. 10 P2O5 vs. SiO2 diagrams for Early Carboniferous volcanic rocks in Shaertala, Inner Mongolia (base map after Lee and Bachmann, 2014)
4.2.2 酸性岩

年代学研究显示,研究区早石炭世酸性岩可分为346.5Ma和335.1Ma两期,岩石组合均为英安岩和流纹岩。它们都具有高的SiO2(68.91%~79.23%、62.60%~72.54%)和Al2O3(10.01%~16.62%、14.29%~15.80%)含量、低的Fe2O3T(1.72%~3.97%、1.94%~7.69%)和MgO(0.64%~1.41%、0.26%~1.05%)含量,轻稀土元素(LREEs)和大离子亲石元素(LILEs,如Rb、Ba和Sr等)富集以及重稀土元素(HREEs)和高场强元素(HFSEs,如Nb、Ta和Ti)亏损,表明它们具有壳源成因(Zen,1986Barbarin,1999Nabelek et al., 2001Koepke et al., 2007Xu et al., 2009)。

两期酸性火山岩具有较高的重稀土元素含量(如Yb分别为3.02×10-6~4.01×10-6和4.10×10-6~5.06×10-6)和较平坦的配分形式,较强的Eu和Sr负异常,表明岩浆源区可能存在角闪石+斜长石、而非石榴石的残留(Rapp et al., 1991Martin,1999)。此外,锆石原位Hf同位素研究显示,早石炭世两期酸性火山岩都表现出高的εHf(t)值(7.97~12.14和8.35~14.71)和相对年轻的tDM2(838~577Ma和812~407Ma),其整体变化范围较大,但主体具有高的正εHf(t)值,暗示它们的原始岩浆主要来源于新元古代末期和早古生代新增生地壳的部分熔融,可能有少量相对古老的地壳物质的涉入。这也可以得到元古代捕获锆石存在的佐证。

其次,上述两期酸性火山岩地球化学特征也存在一定的差异,例如,与早石炭世早期的酸性岩相比,335Ma英安岩和流纹岩具有更高的重稀土元素含量和更加平坦的稀土配分模式、较强的Eu、Sr、P和Ti负异常,和较高的Zr、Nb、Y、Ce含量和Ga/Al比值,上述特征与A型花岗岩一致(Whalen et al., 1987King et al., 1997邱检生等,2000)。这也与中期的酸性火山岩给出了比早期岩石更高的Zr饱和温度(早期和中期岩石的平均值分别为830℃和876℃;表 2Watson et al., 2006)相吻合。上述源区性质和高温等特征表明早石炭世酸性火山岩形成于陆壳伸展环境,与同时代板内成因基性岩的出现相一致。

4.3 地壳属性与增生历史

锆石是酸性火成岩中大量存在的一种副矿物,不易被后期的地质过程改造,其Lu-Hf同位素体系能可靠地记录新增生地壳形成的时间、大陆地壳的性质及演化(新增生地壳或是古老地壳再造)和地壳物质组成的不均一性(Yang et al., 2007吴福元等,2007; Vervoort and Kemp, 2016Wang et al., 2016b)。本文通过对苏左旗北部沙尔塔拉地区早石炭世英安岩和流纹岩进行了锆石原位Hf同位素分析,并总结了研究区附近晚石炭世花岗岩和酸性凝灰岩的锆石Hf同位素特征,进一步揭示兴安-爱力格庙地块西部石炭纪地壳属性与增生历史。

首先,早石炭世早期英安岩(DXT6)中岩浆锆石(~346.5Ma)具有正的εHf(t)值(7.97~12.14)和相对年轻的tDM2(838~577Ma),相比之下,晚期的流纹岩(17SZ23,335.1Ma)中岩浆锆石具有更加亏损的Hf同位素组成(εHf(t)=8.35~14.71,tDM2=812~407Ma),与二连蛇绿岩中早石炭世流纹岩和斜长花岗岩(348Ma)全岩Nd同位素特征一致(εNd(t)平均值分别为6.9和10.2;Yang et al., 2017)。上述同位素数据表明,苏左旗-二连浩特地区早石炭世整体以新元古代-早古生代新增生地壳的部分熔融为主,而不是以古老地壳物质的再造为主。随着岩浆活动时代变新,tDM2整体随之变年轻,暗示了重熔的新增生地壳也具有逐渐变年轻的趋势。

其次,苏左旗南部和二连浩特地区晚石炭世酸性火成岩岩浆锆石显示出更加年轻的tDM2,以早古生代年龄为主,同时也存在少量新元古代甚至中元古代的模式年龄(图 11),暗示了晚石炭世以早古生代新增生地壳的部分熔融为主,同时也发生了相对古老地壳物质的再造。古老地壳的再造作用在兴安-爱力格庙地块西北部同样存在,例如,二连-贺根山蛇绿岩带以北的阿拉坦地区晚石炭世流纹岩具有负的εNd(t) (-11)和古老的tDM(1.9~2.1Ga),与之共生的安山岩也显示出负的εNd(t) (-2.4~-2.2)和古老的模式年龄(Fu et al., 2016)。此外,Deng and Macdougall(1992)对阿巴嘎新生代玄武岩中地幔包体的研究也发现深部存在中-古元古代(~1.6Ga)地幔源区,而且苏左旗地区也被证实存在中元古代(~1.4Ga)陆壳(孙立新等,2013Han et al., 2017)。综合上述研究,我们可以看出苏左旗及邻区存在古老的前寒武纪结晶基底和岩石圈地幔,地壳增生的时代主要集中在古元古代(2.1~1.9Ga)、新元古代(0.8~0.6Ga)和早古生代(0.5~0.4Ga)。早古生代期间古亚洲洋板片持续俯冲于该地块之下,可能同时存在地壳的横向和垂向增生。横向上有增生楔的侧向加积,而垂向上大量玄武质岩浆底侵至下地壳下部形成新的地壳,同时也可以导致古老的地壳物质重熔。研究区早古生代酸性火成岩的研究显示,从445Ma到439Ma,锆石εHf(t)从负值逐渐上升至较高的正值(Chen et al., 2016),暗示了随着俯冲作用的持续进行,苏左旗地区之下古老的下地壳物质可能不断被消耗,更多新增生地壳物质发生部分熔融。苏左旗-二连一带在石炭纪伸展背景下主要以新元古代和早古生代新增生地壳的部分熔融为主,而北部的阿拉坦地区以古元古代地壳的再造为主。此外,兴安-爱力格庙地块东北部扎兰屯-多宝山一带石炭纪酸性岩岩浆锆石主体显示出较高的正εHf(t)和新元古代的二阶段模式年龄(张彦龙等,2010Zhang et al., 2018),表明地块东北部石炭纪以新元古代地壳物质的部分熔融为主,这与该区尚未发现前寒武纪结晶基底相一致。由此可见该地块西部和东部的深部地壳组成在横向上是不均一的。

图 11 苏左旗及邻区石炭纪酸性岩浆岩锆石Hf二阶段模式年龄频谱图 (a)早石炭世英安岩和流纹岩;(b)晚石炭世花岗岩和熔结凝灰岩.数据来源于Hu et al., 2017张焱杰等,2018 Fig. 11 Relative probability plots of magmatic zircon Hf tDM2 for Carboniferous felsic rocks in Sonid Zuoqi and nearby regions, Inner Mongolia (a) Early Carboniferous dacite and rhyolite; (b) Late Carboniferous granitoids and tuffs. Data from Hu et al., 2017; Zhang et al., 2018
4.4 构造背景与动力学过程

如前所述,前人对该区古亚洲洋早古生代演化的认识相对统一,而对晚古生代的演化存在较大争议:即古亚洲洋俯冲作用是一直持续至早中生代,还是在晚古生代经历了碰撞闭合后的伸展直到最终闭合?上述争论主要归结于中亚造山带东部晚古生代是否存在古大洋、深海沉积和俯冲增生杂岩等洋壳存在的证据(徐备等, 2014, 2018Xiao et al., 2015),另一方面,这也受到了本区泥盆纪-早石炭世岩浆岩的出露情况及相关研究不足的制约。因此,本文对兴安-爱力格庙地块西南部苏左旗地区早石炭世火山岩的研究将为该区石炭纪早期深部动力学过程研究提供重要依据。

本文确定了苏左旗北部早石炭世火山岩可分为~347Ma和335Ma两期。其中早石炭世早期玄武安山岩属于拉斑系列,表现出板内成因特点,而安山岩则具有岩浆混合成因特点,因此玄武安山岩和同时代英安岩-流纹岩构成了一套双峰式火山岩组合。这些特征共同表明研究区处于陆内伸展环境。其次,早石炭世中期英安岩-流纹岩具有更高的Zr饱和温度(平均值为876℃)以及A型花岗岩的地化特征,共同揭示了伸展作用的持续进行,而且随着时代变新伸展作用可能有逐渐增强的趋势。这也可以得到上述两期酸性岩从老到新Sr/Y比值降低、重稀土元素含量和岩浆结晶温度升高的支持(图 12)。

图 12 沙尔塔拉地区早石炭世酸性火山岩年龄与TZr (a)和Sr/Y (b)协变图解 Fig. 12 Age vs. TZr(a) and Age vs. Sr/Y (b) plots for Early Carboniferous volcanic rocks in Shaertala, Inner Mongolia

从时空分布来看早石炭世岩浆活动分布范围较小,除了苏左旗地区,主要局限分布于二连-贺根山蛇绿岩带内(图 13)。Jian et al.(2012)从贺根山蛇绿岩中识别出早石炭世辉长岩(354~333Ma),认为其形成于岩石圈伸展的构造背景。黄竺等(2015)在贺根山蛇绿岩铬铁矿体中证实有金刚石、SiC等深部地幔矿物,也暗示了强烈的伸展作用伴随有深部地幔物质的上涌。另外,二连浩特蛇绿岩中~354Ma辉长岩和玄武岩表现出N-MORB地球化学特征,而且出现了Nd同位素强烈亏损的同时代斜长花岗岩,表明该带内可能已经拉张形成洋壳(徐备等,2014Zhang et al., 2015Yang et al., 2017)。沉积学研究显示,二连浩特本巴图和西乌旗迪彦庙地区晚石炭世碎屑岩-碳酸盐沉积与下伏蛇绿岩的中厚层硅质岩夹灰岩为连续沉积关系,说明从早石炭世的裂谷沉积作用过渡为晚石炭世的陆表海沉积(张焱杰等,2018)。上述研究,结合二连-贺根山蛇绿岩带南北两侧前石炭纪地质体的可对比性(Xu et al., 2017徐备等,2018张焱杰等,2018),本文认为二连-贺根山蛇绿岩带很可能代表了一个从早石炭世早期开始打开的陆间小洋盆。相对而言,晚石炭世(322~300Ma)岩浆作用较为强烈,也形成了一系列板内玄武岩、辉长岩、辉绿岩和铝质A型花岗岩,基本上呈面状分布于锡林浩特-苏左旗-二连浩特一带以及蒙古东南部,延续了早期的伸展环境(邵济安等, 2014, 2015徐备等,2014; Pang et al., 2016Hu et al., 2017Zhu et al., 2017; 庞崇进等,2018)。另一方面,石炭纪(345~309Ma)发育低压高温变质岩、广泛的混合岩化和基性岩脉的侵入,经历了顺时针P-T演化,可能也指示了造山后的陆内伸展过程(张晋瑞等,2018)。

图 13 苏左旗及邻区石炭纪岩浆岩时空分布图(据李可等,2015Song et al., 2015张焱杰等,2018Wang et al., 2019) Fig. 13 Spatial and temporal distributions for Carboniferous igneous rocks in Sonid Zuoqi and nearby regions (after Li et al., 2015; Song et al., 2015; Zhang et al., 2018; Wang et al., 2019)

综上所述,苏左旗及二连-贺根山带早石炭世岩浆作用可能形成于陆内伸展和有限洋盆环境,而非俯冲环境。其中二连-贺根山蛇绿岩带位于伸展的轴部,而苏左旗地区位于该带的南部边缘,主要表现为陆壳的局部伸展作用,该作用从早石炭世早期至晚期有增强的趋势。一部分研究显示苏左旗南部发育晚志留世同碰撞花岗岩带和泥盆纪前陆盆地磨拉斯建造,而且内蒙中部地区泥盆纪总体处于剥蚀状态,发育陆相和海陆交互相沉积;石炭纪以陆表海滨浅海相沉积建造为特点(张兴洲等,2012Zhao et al., 2017徐备等,2018)。另外,松辽-浑善达克地块北缘和南缘也存在与伸展作用有关的早石炭世碱性花岗岩和酸性火山岩(Li et al., 2014Wang et al., 2015)。因此,本文认为研究区石炭纪记录的拉张环境可能与古亚洲洋在泥盆纪闭合后的伸展环境有关。

5 结论

(1) LA-ICP-MS锆石U-Pb定年结果显示,苏尼特左旗北部沙尔塔拉地区原定大石寨组中、上段的玄武安山岩和流纹岩的年龄分别为349.0±5.2Ma和335.1±2.0Ma,表明其形成时代应为早石炭世,而非早二叠世。

(2) 早石炭世早期玄武安山岩的原始岩浆起源于陆内伸展机制下受早期俯冲流体交代的亏损岩石圈地幔部分熔融,并经历了高温低压和含水条件下矿物分离结晶作用和一定程度的地壳混染。而同时期的安山岩岩浆则是中基性和酸性岩浆混合的产物。早石炭世英安岩和流纹岩的原始岩浆主要来源于新元古代末期和早古生代新增生地壳的部分熔融。研究区石炭纪以新增生地壳的部分熔融为主,而且酸性火山岩锆石Hf tDM2随岩浆活动时代变新而变年轻。

(3) 研究区早石炭世早期板内成因的玄武安山岩与英安岩-流纹岩构成双峰式火山岩组合,暗示早石炭世早期研究区已经处于陆内伸展环境,早石炭世中期A型酸性火山岩的形成进一步揭示了伸展作用的持续进行,这一过程可能与古亚洲洋在泥盆纪闭合后的伸展作用有关。

致谢      感谢任云生教授、周建波教授和郝宇杰博士在稿件处理过程中的帮助;感谢中国地质调查局西安地质调查中心国土资源部岩浆作用成矿与找矿重点实验室李艳广工程师、河北地质大学区域地质与成矿作用重点实验室尹露老师、北京大学造山带与地壳演化教育部重点实验室和中国科学院地质与地球物理研究所岩石圈演化国家重点实验室、武汉上谱分析科技有限公司工作人员对本文测试提供的帮助。同时衷心感谢张志诚教授和匿名审稿人以及俞良军老师对本文提出的宝贵意见和建议。

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