岩石学报  2020, Vol. 36 Issue (8): 2521-2536, doi: 10.18654/1000-0569/2020.08.14   PDF    
中亚造山带中段古亚洲洋北向平板俯冲过程:来自埃达克岩的证据
李世超1,2, 王洪涛1, 李刚1, 王兴安3, 杨小鹏1, 赵哲仁1     
1. 吉林大学地球科学学院, 长春 130061;
2. 自然资源部东北亚矿产资源评价重点实验室, 长春 130061;
3. 东北师范大学地理科学学院, 长春 130024
摘要: 早三叠世是中亚造山带(CAOB)中部构造演化的关键时期,尽管该时期古亚洲洋在地表已经闭合,但残余的大洋板片仍在持续的挤压作用下继续俯冲,造山作用依然活跃。本文对中亚造山带中段林西地区的下三叠统幸福之路组火山岩地层进行了锆石U-Pb测年、岩石地球化学以及锆石原位Lu-Hf同位素研究。LA-ICP-MS锆石U-Pb定年结果显示了该地层的形成时代为247Ma,为早三叠世岩浆活动的产物。岩石学和地球化学研究表明,幸福之路组火山岩具有高SiO2(64.10%~68.90%)、Al2O3(13.47%~17.50%),低MgO(0.51%~1.42%),轻稀土富集、重稀土亏损,无Eu异常(δEu平均值为1.09)以及高Sr(384×10-6~956×10-6,平均616×10-6),低Y(5.66×10-6~7.63×10-6,平均6.51×10-6)的特点,表明其为大洋板片熔融产生的典型埃达克岩。锆石原位Lu-Hf同位素分析结果显示其εHft)为+10.8~+15.6,平均为+13.9。样品单阶段Hf地壳模式年龄(tDM1)为264~462Ma,表明其岩浆源区的亏损特征。结合区域资料,我们将研究区中晚二叠世至晚三叠世划分为四个构造演化阶段:1)中-晚二叠世时期,特征为钙碱性岩浆岩及碰撞杂岩的发育;2)早-中三叠世时期,特征为广泛分布的典型埃达克岩,是古亚洲洋地表闭合后大洋板片继续北向平板俯冲的产物;3)230Ma左右开始持续10Myr,该时期是岩浆活动宁静期;4)220Ma至晚三叠世末,研究区进入区域性伸展,A型花岗岩、富钾钙碱性花岗岩类和超基性岩大量侵位,变质核杂岩及韧性剪切带也在此时产生。前三个阶段代表了完整的古亚洲洋大洋板块平板俯冲过程,而最后一个阶段标志着研究区地壳进入了新的演化阶段。
关键词: 林西    幸福之路组    埃达克岩    平板俯冲    中亚造山带    
Northward plate subduction process of the Paleo-Asian Ocean in the middle part of the Central Asian Orogenic Belt: Evidence from adakites
LI ShiChao1,2, WANG HongTao1, LI Gang1, WANG XingAn3, YANG XiaoPeng1, ZHAO ZheRen1     
1. College of Earth Sciences, Jilin University, Changchun 130061, China;
2. Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Natural Resources, Changchun 130061, China;
3. School of Geographical Sciences, Northeast Normal University, Changchun 130024, China
Abstract: The Early Triassic was a key period of geological evolution in middle part of the Central Asian Orogenic Belt (CAOB). Although the Paleo-Asian Ocean was closed at the surface during this period, the residual oceanic slabs continued to subduct under continuous compression, i.e., the orogeny was still active. In this paper, zircon U-Pb dating, petrological geochemistry, and in-situ zircon Lu-Hf isotope studies have been conducted on the Lower Triassic Xingfuzhilu Formation volcanic strata in the Linxi area of the middle part of CAOB. LA-ICP-MS zircon U-Pb dating results show that the formation age of Xingfuzhilu Formation is 247Ma, which is the product of Early Triassic magmatism. Petrology and geochemical study have shown that the volcanic rocks in the Xingfuzhilu Formation have high SiO2(64.10%~68.90%), Al2O3(13.47%~17.50%) and Sr (384×10-6~956×10-6, averaged at 616×10-6), low MgO (0.51%~1.42%) and Y (5.66×10-6~ 7.63×10-6, averaged at 6.51×10-6) contents; with a LREE-enrichment and HREE deficit, without a Eu anomaly (averaged δEu is 1.09), showing that it has typical characteristics of adakites that derived from the melting of residual oceanic slabs. The εHf(t) of in-situ zircon Hf isotopic analysis is between +10.8~+15.6 (averaged at +13.9). The single-stage Hf crustal model ages (tDM1) of the zirons are within the range of 264~462Ma, indicating the magma is from a depleted mantle. Based on regional published data, we divided the tectonic evolution of the study area into four stages from the Middle-Late Permian to the Late Triassic: 1) The Middle-Late Permian period, characterized by the development of calcareous-alkali magmatic rocks and collisional complexes; 2) The Early-Middle Triassic period, characterized by widely distributed typical adakite, which is the product of the continuous northward subduction of the oceanic slab after the surface closure of the Paleo-Asian Ocean; 3) It's last 10My from 230Ma, a period without any magmatic activities; 4) From 220Ma to the end of the Late Triassic, a regional extension period, characterized with large amounts of A-type granites, massive potassium-rich calcium alkaline granites and ultrabasic rocks emplacement, accompanied with metamorphic core complexes and ductile shear zones development. The first three stages represent the complete plate subduction process of the Paleo-Asian Ocean oceanic plate, while the last stage standardizes that the crustal of the study area has entered a new evolutionary stage.
Key words: Linxi    Xingfuzhilu Formation    Adakite    Plate subduction    Central Asian Orogenic Belt    

二叠纪末-三叠纪初是全球大陆汇聚的时期,该时期潘基亚超大陆(Pangaea)面积的增大引起其山根变大、陆壳增厚,造成全球海平面下降,使得火山活动频繁且强烈爆发,并最终导致气候急剧变化,由此诱发了地球历史上最大的一次生物灭绝事件——PTB (Yin and Song, 2013)。该时期古亚洲洋在华北板块、西伯利亚板块及北美板块的联合作用下不断收缩,古亚洲洋构造域内的多个微陆块、岛弧最终拼贴、碰撞、造山,形成阿穆尔(Mongolia or Amuria)板块(Şengör and Natal'in, 1996; Fritzell et al., 2016)。在此过程中,阿穆尔板块和东亚陆块群汇聚后与潘基亚超大陆主体拼合(Unrug, 1992; Zhao et al., 2018)。晚三叠世潘基亚超大陆进入超大陆旋回的裂解阶段,区域性伸展作用强烈,由于处于潘基亚超大陆边部,东亚陆块群及其周边地块的这种由汇聚到离散的地质作用表现的尤为强烈。内蒙古林西地区位于东亚陆块群的核心部位——索伦-林西缝合带内(图 1a),该缝合带北与宝力道岛弧增生杂岩带相邻,南与温都尔庙俯冲增生杂岩带相接(Xiao et al., 2003)。近年来,越来越多的研究者达成共识,认为该缝合带为西伯利亚板块与华北板块碰撞拼合的最终缝合带(Xiao et al., 2003Chen et al., 2009李益龙等, 2009)。古亚洲洋闭合的时间和闭合后的造山带演化一直以来都是地质学家们的研究热点。古亚洲洋闭合过程中,林西地区广泛地接受了一套二叠-三叠系沉积,并伴随有大量的岩浆侵入和变质及构造变形作用,为揭示古亚洲洋闭合前后的地质演化提供了理想的研究对象。前人在该地区年代学及地球化学研究方面取得了丰硕的研究成果,然而对造山及俯冲的作用样式和过程却鲜有探究。

图 1 研究区构造位置(a, 据Xiao et al., 2003)及地质简图(b,据刘建峰等, 2013; 张海华等, 2015; 王璐等, 2016) Fig. 1 Tectonic location (a, after Xiao et al., 2003) and simplified geological map (b, after Liu et al., 2013; Zhang et al., 2015; Wang et al., 2016) of study area

本文对林西地区巴彦呼硕镇附近的幸福之路组火山岩进行了系统的野外地质调查和剖面测量(图 1b),对其中的英安岩样品进行了锆石U-Pb测年和锆石原位Lu-Hf同位素测试以及全岩地球化学分析。结合区域已有研究成果对研究区中晚二叠世-晚三叠世的区域地质演化历史进行了梳理,揭示古亚洲洋地表闭合后的板片俯冲作用过程和样式,为探究东亚陆块群汇聚到离散这一全球性地质事件提供新的地质素材。

1 区域地质背景与样品介绍

林西地区处于古亚洲洋构造域的中-东段,因叠加了古生代的东西向构造和中生代的北东向构造,使区内地质情况变得复杂(Xiao et al., 2003; Wilde, 2015; Liu et al., 2017; 刘永江等, 2019; Wang et al., 2020)。林西地区处于索伦-林西缝合带内(图 1a),在晚古生代-早中生代经历了俯冲和碰撞演化过程,发育了一套海相沉积地层和火山沉积地层,形成下古生界二叠系寿山沟组、大石寨组、哲斯组和林西组;早中生代该区域处于古亚洲洋构造域、蒙古-鄂霍茨克俯冲体系(Li et al., 2015; Fritzell et al., 2016)和环太平洋俯冲体系共同作用阶段,地壳以早期挤压、晚期伸展变形为主,在区内形成了中生界火山-沉积建造、并伴有酸性侵入岩就位。形成了以三叠系幸福之路组,侏罗系新民组、满克头鄂博组、玛尼吐组、白音高老组为代表的火山-沉积岩组合。其中下三叠统幸福之路组广泛分布于大兴安岭南部地区, 岩性为砾岩、杂砂岩为主,部分地区夹火山熔岩、火山碎屑岩。地层中化石具有晚二叠世-早三叠世生物相混生的特点。但从区域地层对比来看,该组在岩性及构造变形方面,与晚二叠世地层存在明显差异,其时代应为早三叠世。

幸福之路组主要分布于幸福之路村和巴彦呼硕镇附近(图 1b)。其中,幸福之路村附近为一套夹有红层的杂色碎屑岩(郑月娟等, 2014),巴彦呼硕镇附近以火山岩为主。未见两套岩石组合间的接触关系,根据地层产状可知火山岩为上部层位,杂色碎屑岩为下部层位。在巴彦呼硕镇东山的实测剖面中,幸福之路组与上二叠统林西组以逆断层的形式接触,断层面产状325°~340°∠47°~43°。该处幸福之路组厚134m,岩性以晶屑凝灰岩、安山质火山角砾岩、英安岩为主,未见底(图 2)。

图 2 幸福之路组实测地质剖面 1-变质粗砂岩; 2-变质粉砂岩; 3-变质含砾粗砂岩; 4-变质粗粒长石砂岩; 5-沉凝灰岩; 6-安山岩; 7-晶屑沉凝灰岩; 8-流纹质火山角砾岩; 9-流纹质沉凝灰岩; 10-英安岩; 11-安山质晶屑凝灰岩; 12-年龄样品; 13-地球化学样品; 14-断层 Fig. 2 Geological section of Xingfuzhilu Formation 1-metamorphic gritstone; 2-metamorphic siltstone; 3-metamorphic gravel gritstone; 4-metamorphic coarse arkose; 5-sedimentary tuff; 6-andesite; 7-crystallized sedimentary tuff; 8-rhyolitic volcanic breccias; 9-rhyolitic sedimentary tuff; 10-dacite; 11-andesitic crystal tuff; 12-age sample; 13-geochemical sample; 14-fault

锆石U-Pb测年、Hf同位素分析样品(WN1402,取样坐标为118°37′31.5″E, 43°53′4.6″N)及地球化学样品(WN1402-1~WN1402-5)采集自剖面的英安岩段的新鲜岩石(图 3),具体采样位置见图 2。英安岩的风化面呈浅灰色,新鲜面呈灰白色,斑状结构,块状构造。斑晶大小2~5mm,斑晶中斜长石占10%,自形-半自形板柱状,多见碳酸盐化但有残留的聚片双晶,有碎斑结构;石英约占5%,半自形粒状,表面干净,多有熔蚀结构;黑云母占5%;普通角闪石:柱状,多析铁但形态残留清楚,其横断面为菱形;基质为霏细结构,多为细小粒状或板条状斜长石显微晶体及细小柱状普通角闪石晶体。

图 3 幸福之路组英安岩野外照片和显微镜下照片 Pl-斜长石;Hb-角闪石;Qz-石英.硬币直径为19mm Fig. 3 Photograph (a) and microphotograph (b) for dacite in Xingfuzhilu Fm. Pl-plagioclase; Hb-hornblende; Qz-quartz. The diameter of the coin is 19mm
2 分析方法 2.1 锆石LA-ICP-MS U-Pb定年

锆石分选在河北省区域地质矿产调查研究所完成,流程如下:将样品均匀粉碎至80~100目,淘洗后用电磁法使重矿物分离,将分离出来的锆石置于双目镜下挑选,挑选过程中挑选晶形完好且无包裹体或裂痕的锆石。锆石制靶和反射光、透射光以及阴极发光图像的采集均在北京锆年领航科技有限公司完成,流程如下:将锆石嵌于树脂表面,待干燥定型打磨抛光后,拍摄透射光、反射光和阴极发光图像。锆石U-Pb测年和锆石原位Lu-Hf同位素分析在中国地质科学院矿产资源研究所完成。U-Pb同位素测年采用LA-ICP-MS原位分析方法,所用仪器为Neptune型MC-ICP-MS和Newwave UP 213激光剥蚀系统,实验中采用He气作为剥蚀物质的载气,激光频率为6Hz,激光强度为50mJ,激光斑束为30μm,使用人工合成硅酸盐玻璃标准物质NIST 610进行仪器状态调整参考,使用国际标准锆石91500作为同位素组成的外标,具体测试过程参见侯可军等(2009)。获得的分析数据使用ICPMSDataCal (Liu et al., 2010)处理,之后用Andersen (2002)的方法对所得数据进行普通铅同位素比值校正,最后使用Isoplot (Ludwig, 2008)计算和绘制锆石U-Pb谐和图,年龄误差为1σ, 锆石年龄较为年轻(<1000Ma)因此采用206Pb/238U年龄值。

2.2 锆石Hf同位素分析

Lu-Hf同位素分析同样在Neptune型多接收等离子质谱仪和Newwave UP213紫外激光剥蚀系统(LA-MC-ICP-MS)上进行,所选Hf测试点编号与U-Pb测年点序号相对应,实验过程中采用He作为剥蚀物质的载气,根据锆石的大小选择剥蚀直径为40μm,测定时使用国际标准锆石GJ-1作为外标,分析过程中锆石标准GJ1的176Hf/177Hf测试加权平均值为0.281993±0.000026 (2σ, n = 28),与文献报道值(侯可军等,2007; Morel et al., 2008)在误差范围内完全一致。εHf(t)计算采用的176Lu衰变常数为1.865×10-11y-1 (Scherer et al., 2001),球粒陨石现今的176Hf/177Hf = 0.282772, 176Lu/177Hf = 0.0332。亏损地幔Hf模式年龄(tDM1)计算采用的现今亏损地幔176Hf/177Hf = 0.28325和176Lu/177Hf = 0.0384进行计算;二阶段Hf模式年龄(tDM2)采用平均大陆壳176Lu/177Hf = 0.015进行计算(Griffin et al., 2000)。更进一步详细的实验流程参见侯可军等(2007)

2.3 主量和微量元素分析

本文选取代表性样品进行主量元素、微量元素分析。分析测试由澳实分析检测(广州)有限公司完成,主量元素分析采用X射线荧光法(XRF),精度优于1%;微量元素分析采用电感耦合等离子体质谱法(ICP-MS), 精度优于5%,含量极少(<10-8)的元素精度优于10%。

3 分析结果 3.1 锆石U-Pb测年结果

样品WN1402的LA-ICP-MS锆石U-Pb测年结果见表 1。样品中的锆石多数呈长柱状,其次为短柱状,还有少量不规则形状,自形程度较高,锆石长度80~150μm不等,长短轴比集中在2:1左右,最大可达3:1。CL图(图 4a)显示锆石内部结构清晰,多具震荡环带,表明其为岩浆锆石。Th/U比值较为集中,从0.25到0.42不等,均值为0.33 (表 1),同样指示锆石为岩浆成因。27个测试点中有23个落在了谐和线上,另外4个测试点偏离谐和线(已剔除)。谐和图中23颗锆石的206Pb /238U年龄为243~250Ma之间,加权平均年龄为247.3±0.55Ma (MSWD = 5.7) (图 4b),表明岩浆活动于早三叠世。

表 1 幸福之路组英安岩LA-ICP-MS锆石U-Pb测年数据 Table 1 LA-ICP-MS zircon U-Pb dating data of Xingfuzhilu Fm. dacite

图 4 幸福之路组英安岩(样品WN1402)部分锆石阴极发光图及协和图 实线圆圈、虚线圆圈和数字分别代表U-Pb分析点、Lu-Hf分析点及测点号,测点号同表 1表 3 Fig. 4 Cathodoluminescence (CL) images and concordia U-Pb diagram of selected zircons from the dacite of Xingfuzhilu Fm. Solid circle, dotted circle and numbered spots indicate the locations of ICP-MS analysis, the number is the same as in Table 1 and Table 3
3.2 主量和微量元素

幸福之路组英安岩的主量、微量元素分析结果见表 2,其SiO2含量为64.10%~68.90%,MgO含量为0.51%~1.42%,TiO2含量为0.40%~0.45%,Al2O3含量为13.47%~17.50%,FeOT含量为2.80%~3.21%,CaO含量为1.92%~4.23%,K2O含量为1.63%~2.09%,Na2O+K2O为5.81%~6.89%、Na2O/K2O为1.78~3.61,Mg#为24~44,A/CNK为0.84~1.18,里特曼指数δ为1.44~2.88,岩石属于偏铝质-过铝质钙碱性系列岩石(图 5)。

表 2 幸福之路组英安岩主量元素(wt%)、稀土及微量元素(×10-6)分析结果 Table 2 The analysis results of major elements (wt%) and trace elements (×10-6) of dacites from the Xingfuzhilu Fm.

图 5 幸福之组英安岩岩石类型与系列划分图解 (a) TAS图解(据Le Bas et al., 1986);(b) SiO2-K2O图解(据Maniar and Piccoli, 1989).灰色圆圈为211个O型埃达克岩,数据来自GEOROC数据库(http://georoc.mpch-mainz.gwdg.de/georoc/),图 6 Fig. 5 Major elements plots for identifying rock type and series of the Xingfuzhilu Fm. dacite (a) TAS diagram (after Le Bas et al., 1986); (b) SiO2 vs. K2O diagram (after Maniar and Piccoli, 1989). The grey circles are 211 adakite samples that is O-type from the GEOROC database (http://georoc.mpch-mainz.gwdg.de/georoc/), also in Fig. 6

幸福之组英安岩稀土元素总量(∑REE)为61.87×10-6~80.06×10-6,平均值72.67×10-6,(La/Yb)N = 13.30~19.75,为轻稀土富集型;δEu值为1.00~1.18,平均1.08,无明显异常。在球粒陨石标准化稀土元素配分图中(图 6a),曲线向右缓倾,轻稀土富集,重稀土亏损,在Eu处无明显异常,大部分相互平行或重合叠加。在原始地幔标准化微量元素蛛网图中(图 6b),样品表现出强烈亏损高场强元素Nb(Ta),轻微亏损P和Ti,部分样品轻微亏损Th,而明显富集大离子亲石元素K、Sr。

图 6 幸福之路组英安岩球粒陨石标准化稀土元素配分图解(a, 标准化值据Boynton, 1984)和原始地幔标准化微量元素蛛网图(b, 标准化值据Sun and McDonough, 1989) Fig. 6 Chondrite-normalized REE patterns diagram (a, normalization values after Boynton, 1984) and primitive mantle-normalized trace element spidergrams (b, normalization values after Sun and McDonough, 1989) of Xingfuzhilu Fm. dacite
3.3 锆石Hf同位素分析结果

在样品WN1402的定年锆石中挑选协和度好、且足够大的锆石进行Hf同位素测试,取得了18个有效的Hf同位素数据点(表 3)。锆石测点的176Lu/177Hf值为0.000807~0.001841,比值均小于0.002,表明锆石形成后,放射成因Hf的积累非常少,所测定的176Hf/177Hf值为岩石形成时体系的Hf同位素组成,可以有效地被用来进行示踪(吴福元等, 2007)。初始176Hf/177Hf值为0.282932~0.283066,平均为0.283017,εHf(t)为+10.8~+15.6,平均为+13.9。样品亏损地幔模式年龄tDM1为263.7~461.6Ma, 平均为335.3Ma。锆石Hf模式年龄略大于结晶年龄(247.3±0.55Ma),表明岩浆源区源于亏损地幔。t-εHf(t)图解(图 7)显示所有的数据点都落在球粒陨石与亏损地幔之间。

表 3 幸福之路组英安岩锆石Lu-Hf同位素数据 Table 3 Zircon Lu-Hf isotope data of dacite in Xingfuzhilu Fm.

图 7 幸福之路组英安岩锆石t-εHf(t)图解 地壳演化趋势线中176Lu/177Hf为0.011 (after Wedepohl, 1995).兴蒙造山带东段的锆石εHf(t)值的范围参考自Yang et al. (2006) Fig. 7 t vs. εHf(t) plot for the zircons of the dacite in Xingfuzhilu Fm. The crustal evolution path assuming a crustal 176Lu/177Hf ratio of 0.011 (after Wedepohl, 1995). The range of zircon εHf(t) values of the eastern section of the Xingmeng orogenic belt is referenced from Yang et al. (2006)
4 讨论 4.1 岩石成因类型

林西地区与本文报道的巴彦呼硕镇附近的幸福之路组火山岩外,紧邻的东梁岩体(王璐等, 2016)和建设岩体(刘建峰等, 2013)与幸福之路组火山岩的年龄在误差内一致, 地球化学成分相似,他们应为同源岩浆活动。幸福之路组英安岩的SiO2的含量介于64.10%~68.90%,平均为66.6%, Al2O3含量介于13.47%~17.50%之间,平均为15.4%, 而MgO含量介于0.51%~1.42%, 平均为0.98%,Sr含量较高(介于384×10-6~956×10-6之间,平均616×10-6)、而Y和Yb含量则较低(分别介于5.66×10-6~ 7.63×10-6之间,平均6.51×10-6和介于0.54×10-6 ~ 0.73×10-6之间,平均0.62×10-6)。这些地球化学特征与典型的埃达克岩特征一致。与此同时,除了主量元素、微量元素具有埃达克岩地球化学特征外,样品具有较高的Sr/Y比(53.3~162.3,均值96.2)和La/Yb比(19.7~29.3,均值24.7),在典型的埃达克岩判别图解Sr/Y-Y图(图 8a)和(La/Yb)N-YbN图(图 8b)上,样品均落入埃达克岩区域。再者,幸福之路组英安岩的TAS图解(图 5a)、SiO2-K2O图解(图 5b)、球粒陨石标准化稀土元素配分图解(图 6a)和原始地幔标准化微量元素蛛网图(图 6b)也基本与来自GEOROC数据库中典型的211个O型埃达克岩数据吻合(Zhang et al., 2019)。因此,我们认为幸福之路组英安岩应为埃达克岩。

图 8 幸福之路组英安岩的埃达克岩Sr/Y-Y (a)和(La/Yb)N-YbN (b)判别图解(底图据Defant and Drummond, 1990; Martin, 1999) Fig. 8 Sr/Y vs. Y(a) and (La/Yb)N vs. YbN (b) discrimination diagrams for the dacite of Xingfuzhilu Fm. (after Defant and Drummond, 1990; Martin, 1999)

埃达克岩(Kay et al., 1978; Defant and Drummond, 1990)拥有高的Sr/Y和La/Yb,被认为是俯冲环境中板片熔融的产物,作为具有特殊大陆动力学意义的一种岩石类型,建立了岩性对应构造环境的桥梁,这就增加了我们对汇聚板块边缘地壳物质循环和岩浆作用过程的理解。目前,常见的埃达克岩分类有以下几种方案:(1)张旗等(2001)依据岩石的形成背景将埃达克岩分为Ocean型(O型)和Continental型(C型)两类,O型埃达克岩与板块的俯冲作用或玄武岩底侵作用有关,它的形成需要较高的压力,其成分富Na;C型埃达克岩则是加厚的地壳底部中-基性岩部分熔融的产物,其成分富K。与此相似的分类方法是由王强等(2001)提出的Ⅰ类和Ⅱ类埃达克岩。(2)Martin et al.(2005)依据地球化学特征将埃达克岩分为高硅埃达克岩(High-SiO2 Adakites, 缩写HSA)和低硅埃达克岩(Low-SiO2 Adakites,缩写LSA)两种主要类型。HSA的主要来源是洋壳俯冲,但所产生的熔体在上升期间也与地幔楔橄榄岩相互作用。LSA的来源是地幔橄榄岩,其经过与俯冲洋壳埃达克质熔体反应,通常具有相对高的MgO含量。(3)除此之外,受地幔污染而使Mg#提高的一类埃达克岩被单独列为高镁埃达克岩(HMA) (Martin et al., 2005; Liu et al., 2012), 其地球化学性质与LSA近似; 张旗等(2009)又根据Na2O和K2O含量及其比值的不同,将埃达克岩分为低钾、中钾、高钾和超钾质型四类。

以上几种分类方案可以看出,HSA (Martin et al., 2005)与O-type (张旗等, 2001)、Ⅰ型(王强等, 2001)基本内容一致,均为钠质的洋壳重熔成因的埃达克岩,也与埃达克岩原始定义相一致,我们可以称之为“典型埃达克岩”。

林西地区幸福之路组英安岩的SiO2平均含量为67%、MgO平均含量为0.98%, 、CaO+Na2O平均含量为6.79%和Sr平均含量为616×10-6,Na2O/K2O介于1.78~3.61之间,均值为2.78,这些特征也都表明其归类应为典型埃达克岩。筛选自GEOROC数据库中的979个埃达克岩样本的验证显示,基于硅镁的分类图解的重叠区较大、区分度低(Zhang et al., 2019),因此本文使用K-Rb (图 9a)、Sr-(CaO+Na2O) (图 9b)、K/Rb-SiO2/MgO (图 9c)和Cr/Ni-TiO2 (图 9d)经验图解来进行HSA、LSA分类。分类结果显示巴彦呼硕镇附近的幸福之路组火山岩、东梁岩体和建设岩体均为HSA,也就是典型埃达克岩。

图 9 林西地区幸福之路组英安岩的埃达克岩分类图解 (a) K-Rb; (b) Sr-(CaO+Na2O); (c) K/Rb-SiO2/MgO; (d) Cr/Ni-TiO2.其中HSA和LSA范围引自Martin et al. (2005) Fig. 9 Discrimination diagrams of adakites for the Xingfuzhilu Fm. dacite (a) K vs. Rb; (b) Sr vs. (CaO+Na2O); (c) K/Rb vs. SiO2/MgO; (d) Cr/Ni vs. TiO2. The fields of HSA and LSA in the diagrams are derived from Martin et al. (2005)
4.2 构造环境

林西地区处于中亚造山带的中段,晚古生代-早中生代时期该区发生了古亚洲洋闭合、洋壳消亡、造山事件。近年来,随着精确测试技术的发展,西拉木伦河以北的林西至东乌旗一带识别出大量的晚二叠世-中三叠世的火成岩,其中多数为石英闪长岩、花岗闪长岩以及花岗岩,也见少量火山岩报道。通过分析我们认为,其中很大一部分具有高SiO2、Al2O3、Sr,低MgO、NaO、Y和HREE的钠质典型埃达克岩特征(表 4)。对于埃达克岩而言,成因即代表了构造环境。因此,典型埃达克岩的存在为我们了解板块汇聚过程中洋壳消亡提供了确着的证据。这为我们利用其时空分布来了解古亚洲洋俯冲的过程提供了可靠的媒介。尽管林西-东乌旗地区晚二叠世沉积地层已由海相逐渐转变为陆相,但这些典型埃达克岩的存在表明晚二叠世-中三叠世时期汇聚活动仍在继续,该区陆壳之下残余洋壳仍处于持续俯冲作用之中。这些埃达克岩的形成时间跨度大,就位空间的展布长(沿俯冲方向)(图 10)。这种时空分布特征与常见的马里亚纳型大洋陡俯冲(Uyeda and Kanamori, 1979; Uyeda, 1983)所产生的狭窄的钙碱性火山弧分布相比,有更为宽阔的面状埃达克岩带、更为持久的活动时间。这些特点是秘鲁-智利型平板俯冲产生的岩浆活动特征(Uyeda, 1983)。

表 4 林西-东乌旗地区晚二叠世-晚三叠世火成岩列表 Table 4 List of Late Permian-Late Triassic igneous rocks in Linxi-Dongwuqi area

图 10 林西-东乌旗地区典型埃达克岩分布图(详细信息列于表 4) Fig. 10 Distribution of typical adakites in Linxi-Dongwuqi area (details are shown in Table 4)

平板俯冲是大洋板块以小于10°的低角度俯冲至大陆板块之下的俯冲样式。现今全球俯冲带中只有约10%的俯冲为平板俯冲。相对于常见的陡俯冲, 平板俯冲对上覆板块(尤其是板块内部)的作用距离更远、时间更持久(Gutscher et al., 2000; 颜智勇等, 2020)。平板俯冲发展的第一阶段与陡俯冲相似,都会以高角度启动俯冲,在地幔楔上方形成增生杂岩及狭窄的钙碱性岩浆弧;第二阶段,随着洋壳浮力增大, 俯冲角逐渐变缓, 俯冲洋壳会沿着大陆岩石圈底部进行近水平滑移, 来自洋壳前缘的软流楔及大陆岩石圈底部的热会持续不断地为洋壳提供热源,从而使洋壳熔融,在大陆内部形成一个宽达数百千米的埃达克岩带(Gutscher et al., 2000);第三阶段,随着系统性热能消耗及俯冲动力的下降,整个俯冲系统的岩浆活动趋于宁静。在整个平板俯冲作用过程中随着重力均衡和挤压作用的持续,上覆大陆岩石圈会持续隆升,在第三阶段后由于应力的松弛,会使得大陆岩石圈进入伸展阶段,由此诱发A型花岗岩、基性岩和富钾钙碱性花岗岩(Bonin et al., 1998)的侵入和区域性韧性剪切带、变质核杂岩的产生(Gutscher et al., 2000)。

林西地区在西拉木伦缝合带附近的中-晚二叠世时期的钙碱性岩浆岩(Chen et al., 2009; Zhang et al., 2009; 李益龙等, 2009; 刘建峰等, 2009)及碰撞杂岩(李锦轶等, 2007)代表了平板俯冲第一阶段的产物。及至晚二叠世-早三叠世时期,西拉木伦河两侧沉积已由中二叠世时期的哲斯组海相沉积转变为林西组湖相沉积(和政军等, 1997; 周阳, 2019),这代表古亚洲洋地表已闭合,但岩石圈尺度的俯冲仍在继续(图 11a);广泛分布的典型埃达克岩(表 4图 10)则是平板俯冲第二阶段的岩浆活动表现,其时间为约236~255Ma,其空间分布范围从林西向北西方向至少延伸至东乌旗附近,现今距离约为300km。表明该时期的造山作用显著,林西地区双井岩体(李锦轶等, 2007)、新林岩体中黑云母的地球化学特征显示了其形成于碰撞造山的强烈接触碰撞阶段(王亮等, 2017)。与此同时,西拉木伦河缝合带以南地区也发生了同步快速隆升(邵济安等, 2000; Liu et al., 2019)。区域上三叠纪砂岩的物源分析显示,砂岩成熟度低、含有大量代表岩浆弧物源环境的岩浆岩单晶石英和火山岩岩屑。砂岩碎屑锆石的U-Pb同位素年龄峰值集中在247Ma, 表明岩浆侵位到剥蚀沉积的旋回周期很短, 这些都反映了源区的快速隆升(申亮, 2016)。在该阶段的作用过程中,中二叠世时期就位的房框子沟花岗质片麻岩岩体中锆石的增生变质边记录了231~262Ma之间的持续挤压变形(李益龙等, 2009)。野外的构造变形观测也显示了古生代末-中三叠世该区域的NWW-SEE向挤压作用(张欲清等, 2019)。这种长时间的持续挤压作用也是平板俯冲的一个显著特点,这对应了平板俯冲环境下洋板片沿上覆板块底部持续俯冲的过程(图 11b);230Ma左右开始,研究区进入短暂的岩浆活动宁静期后(图 11c),于220Ma前后开始进入区域性伸展,火成岩类型也变的复杂,除了代表非造山环境的A型花岗岩外(石玉若等, 2007; 李红英等, 2015; 杨俊泉等, 2016; 王兴安等, 2020)外,也发育有富钾钙碱性花岗岩类(Liu et al., 2012; 杨俊泉等, 2012)、地幔来源的超基性岩(田伟等, 2007)。研究区西侧的苏尼特左旗地区发育有晚三叠世至侏罗纪最早期的变质核杂岩(Davis et al., 2004),西拉木伦河缝合带两侧还发育有近EW向、NEE展布的正断式韧性剪切带(李锦轶等, 2007; 王亮等, 2017; 张晓飞等, 2019; 王兴安等, 2020),这都标志着研究区晚三叠世地壳伸展演化阶段的开始。

图 11 林西-东乌旗地区中二叠世至晚三叠世早期构造演化模式图(据颜智勇等, 2020修改) Fig. 11 A cartoon of geological evolution model from Middle Permian to early Late Triassic in Linxi-Dongwuqi area (modified after Yan et al., 2020)
5 结论

(1) 林西地区巴彦呼硕镇附近的幸福之路组火山岩的锆石U-Pb年龄为247Ma, 表明其形成时代为早三叠世。

(2) 幸福之路组火山岩具有高的SiO2、Al2O3含量,低MgO和高Sr低Y、低Yb含量。与相邻的建设屯、东梁岩体具有一致的地球化学特征和形成时代,为同源岩浆演化的产物,是大洋板片熔融产生的典型埃达克岩; 锆石原位Lu-Hf同位素分析结果显示, 幸福之路组火山岩锆石测年样品的εHf(t)为+10.8~+15.6,平均为+13.9,源区具有亏损地幔性质。

(3) 结合区域地质资料可知,研究区中晚二叠世至晚三叠世划分为四个构造演化阶段:ⅰ)中-晚二叠世时期, 特征为钙碱性岩浆岩及碰撞杂岩的发育;ⅱ)早-中三叠世时期,特征为广泛分布的典型埃达克岩,是古亚洲洋地表闭合后大洋板片继续北向平板俯冲的产物;ⅲ)230Ma左右开始持续10Myr,该时期是岩浆活动宁静期;ⅳ)220Ma至晚三叠世末,研究区进入区域性伸展,A型花岗岩、富钾钙碱性花岗岩类和超基性岩大量侵位,变质核杂岩及韧性剪切带也在此时产生。前三个阶段代表了完整的古亚洲洋大洋板块平板俯冲过程,该阶段仍然属于古亚洲洋构造域演化阶段,而最后一个阶段的区域性伸展标志着研究区地壳进入了新的演化阶段。因此,我们认为古亚洲洋构造域在晚三叠世220Ma左右停止活动。

致谢      两位匿名审稿专家对本文提出了宝贵的修改意见,在此表述衷心的感谢。

谨以此文祝贺杨振升先生90华诞暨从事地质事业70周年。

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