大兴安岭北段诺敏大山地区早白垩世侵入岩地球化学特征及其构造意义

任永健,张成信,张明明,程烁,高立湧,于汪,赵寒,王畅,张伟胜

中化地质矿山总局地质研究院,北京,100013

内容提要: 笔者等选择大兴安岭北段诺敏大山地区早白垩世侵入岩进行了锆石U-Pb年代学和地球化学研究,探讨该区域侵入岩成因类型、岩浆来源及构造环境。该区侵入岩岩性主要为正长花岗岩、正长花岗斑岩和似斑状正长花岗岩,对其中正长花岗岩样品进行LA-ICP-MS锆石U-Pb测年表明,正长花岗岩侵位年龄为129.5±0.4 Ma,应为早白垩世岩浆活动的产物,结合区内侵入岩与地层相互接触关系,本区侵入岩形成时代为早白垩世。区内侵入岩具有富硅(SiO2= 67.36%~74.09%)、富碱(K2O+Na2O= 8.88%~9.34%)、高铝(Al2O3= 12.56%~16.15%),低MgO、TiO2、CaO的特点,属于高钾钙碱性岩石系列;铝饱和指数(A/CNK)为0.94~1.31,为准铝质—过铝质岩石。微量元素富集Rb、U、Th、K等大离子亲石元素,强烈亏损Ti、Nb、Sr、P等高场强元素,具有明显的Eu负异常,属于高分异I型花岗岩。岩石Rb/Sr为0.9~2.0,Sr/Y为4.2~7.2,显示出高Sr、低Y的特点,指示岩浆源区为地壳物质的部分熔融。结合区域研究成果,蒙古—鄂霍茨克构造域在早白垩世之前已结束碰撞,诺敏大山地区早白垩世岩浆活动可能发生在蒙古—鄂霍茨克造山后的伸展环境。

关键词:大兴安岭北段;诺敏大山地区;早白垩世;地球化学;构造环境

大兴安岭地区作为兴蒙造山带的重要组成部分,由北西向南东跨越了额尔古纳地块、兴安地块及松嫩地块三大地块,是研究造山带构造演化的重要地区(刘永江等,2010,2016;曾涛等,2012)。显生宙以来,受古亚洲洋构造体制、蒙古—鄂霍茨克洋构造体制及古太平洋构造体制等多期重要构造事件影响,区域内岩浆活动极其强烈。目前,关于大兴安岭北段中生代岩浆活动的构造背景还存在较大争议,主要存在受蒙古—鄂霍茨克洋闭合的影响、受古太平洋板块俯冲的影响、或受两者共同作用的影响等观点(高源等,2013;宋立忠等,2010;Tang Jie et al., 2015;Wang Tao et al., 2015;李锦轶等,2009)。

由此可见,还需要进一步深入研究大兴安岭北段中生代侵入岩构造背景,有助于对额尔古纳—兴安地块与松嫩地块构造演化提供新的依据,具有重要的研究意义。笔者等对大兴安岭北段诺敏大山地区早白垩世侵入岩体进行了系统采样,深入研究了早白垩世侵入岩地球化学特征、锆石U-Pb年龄和构造背景等,在前人研究的基础上,探讨了蒙古—鄂霍茨克洋构造域和太平洋构造域对于区域构造演化和构造岩浆活动的影响,以期为区域构造演化提供新的资料。

1 区域地质背景及岩体地质

研究区位于大兴安岭北段诺敏大山一带,大地构造位置处于鄂伦春早—中华力西褶皱带与伊尔施加里东褶皱带结合部位,以区域型断裂构造头道桥—鄂伦春深大断裂为界,北西侧处于陈巴尔虎旗—根河晚中生代火山盆地,南东侧处于博克图—鄂伦春晚古生代深成岩杂岩区(图1)。区域上出露的地层主要有新元古界青白口系佳疙疸组、中生界上侏罗统满克头鄂博组和玛尼吐组、下白垩统白音高老组及新生界第四系。区域上印支期—燕山期岩浆活动较为频繁,除大规模中生代火山岩外,还形成了广泛分布的侵入岩,主要由中酸性—酸性岩浆岩组成。岩石类型主要为正长花岗岩、正长花岗斑岩和似斑状正长花岗岩等,其中以正长花岗岩体出露面积最大。

图1 东亚(a)和中国东北地区(b)大地构造简图(据任永健,2019 修改)
Fig. 1 The tectonic sketch map of eastern Asia(a) and northeastern China(b)(modified after Ren Yongjian,2019&)

似斑状正长花岗岩主要分布于研究区72 km幅莫果吉大山附近,总体呈北东向展布,侵入上侏罗统满克头鄂博组中。岩石具斑状结构,块状构造,由斑晶和基质两部分组成,矿物成分主要有斜长石、钾长石,次要矿物黑云母、石英,副矿物有磷灰石、磁铁矿等,蚀变矿物主要有绢云母、绿泥石、高岭石、帘石类等。

正长花岗斑岩主要分布于研究区哈达汗萤石矿北侧、十四支线两侧,总体呈北东向展布。岩石呈肉红色,斑状结构,块状构造。主要矿物成分由斑晶和基质组成,其中斑晶以钾长石为主,含量占斑晶的80%~95%,粒径变化较大,中细粒在0.4~2.6 mm、中粗粒在2~3.7 mm、粗粒最大可达7.6 mm左右,有的可见卡尔斯巴双晶,部分可见条纹结构,其次见少量斜长石或石英等斑晶。基质主要也由他形钾长石组成,其次为少量的石英、斜长石或绢云母等,多为显微微粒结构,部分为细粒结构,半自形—他形。副矿物多为磁铁矿、磷灰石等,多数见绢云母化、高岭石化等蚀变。

中(细)粒正长花岗岩,在区内分布广泛,出露面积最大。分别侵入青白口系佳疙疸组、上侏罗统满克头鄂博组地层之中。岩石类型有中细粒正长花岗岩、细粒正长花岗岩。岩石呈浅肉红色,中细粒花岗结构,块状构造。主要由石英、碱性长石、斜长石组成,斜长石呈自形—半自形粒状,粒径多为0.45~3 mm,粒内聚片双晶发育,发育轻微土化;碱性长石呈半自形粒状,粒径0.3~3 mm,发育轻微土化、绢云母化,表面污浊,主要为钾长石和少量微斜长石;微斜长石粒内见细密而宽窄不一的格子双晶;石英呈他形粒状。

图2 大兴安岭北段诺敏大山地区地质简图
Fig. 2 Geological sketch map of the Mount Nuomin area, northern Great Hinggan Mountains

2 样品采集与分析方法

本文所涉及的部分岩石地球化学样品数据来源于笔者等在该区进行1∶5万矿调工作的资料,用于岩石地球化学分析的样品包括2件正长花岗岩和2件似斑状正长花岗岩,用于同位素测年分析样品采集于正长花岗岩体,其他样品数据来源于1∶25万额尔古纳左旗幅区域地质调查报告

主量、微量和稀土元素分析在中化地质矿山总局中心实验室完成。野外采集新鲜无蚀变的岩石样品,首先用水将样品表面冲洗干净并晾干,机械破碎至200目后送实验室分析。主量元素在样品制成熔片后通过X射线荧光光谱法(XRF)测试,相对误差在元素丰度>1.0%时为±1%,元素丰度<1.0%时为±10%;FeO采用氢氟酸、硫酸溶样、重铬酸钾滴定容量法,分析相对误差小于2%,微量元素使用ICP-MS测试,样品测定值和推荐值的相对误差小于10%,且所有值均在5%以内。

图3 研究区侵入岩野外及显微照片: (a)、(b)—中细粒正长花岗岩;(c)、(d)—似斑状正长花岗岩;
(e)、(f)—正长花岗斑岩
Fig. 3 Geological feature and photomicrographs of granite in the study area:(a), (b)— medium—fine-grained syenite
granite;(c), (d)— porphyritic syenite granite;(e), (f)— syenite granite porphyry
Pl—斜长石;Kfs-钾长石;Mc—微斜长石;Pth—条纹长石;Qtz—石英
Pl— plagioclase;Kfs- potassium feldspar;Mc— micro plagioclase;Pth— perthite;Qtz— quartz

本文测年样品的锆石分选、制靶及阴极发光照相均由河北省廊坊市科大岩石矿物分选技术服务有限公司完成,先用常规方法将样品粉碎至80~100目,双目镜下挑选晶形和透明度较好、无裂痕、无包裹体、颗粒较大的锆石颗粒,用无色透明的环氧树脂浇灌固定,待环氧树脂固化后进行打磨、抛光,然后对其进行透射光、反射光和阴极发光(CL)图像的采集。LA-ICP-MS测试在中国地质科学院矿产资源研究所国土资源部成矿作用与资源评价重点实验室完成。激光剥蚀系统为GeoLas Pro,ICP-MS为Agilent 7500,激光束斑直径为32 μm。实验中采用高纯He作为剥蚀物质载气,用美国国家标准技术研究院研制的人工合成硅酸盐剥离标准参考物质NIST SRM610进行仪器最佳化,样品测定时用哈佛大学标准锆石91500作为外部校正,以保证标准和样品的仪器条件完全一致。详细的实验原理和流程参见Liu Yongsheng等(2010),采用ICP-MS DataCal程序和Isoplot(ver3.0)程序进行数据处理(Ludwig,2003),给定的同位素比值和年龄误差均为1σ

3 分析结果

3.1 形成时代

样品LA-ICP-MS锆石U-Pb测年结果见表1。

样品TK24-TW为中细粒正长花岗岩,从锆石阴极发光图像可以看出锆石全部为半自形—自形粒状,显示典型的岩浆振荡生长环带,锆石Th/U值介于0.7~1.56之间,均反映其锆石岩浆成因特点。20颗锆石校正后有效数据18个,所测定的18颗锆石n(206Pb)/n(238U)年龄值介于126.3±5.5~130.7±1.7 Ma,测定数据谐和度较高,n(206Pb)/ n(238U)加权平均年龄为129.5±0.4 Ma(MSWD=0.69),代表了中细粒正长花岗岩体岩浆结晶年龄,其形成时代为早白垩世(图4),结合野外各岩体与地层间接触关系及前人研究成果,研究区侵入岩形成时代均为早白垩世。

图4 大兴安岭北段诺敏大山地区正长花岗岩锆石U-Pb同位素年龄和部分锆石阴极发光(CL)图像
Fig. 4 Zircon U-Pb isotopic age and partial CL image of syenite in the Mount Nuomin area, northern Great Hinggan Mountains

3.2 岩石地球化学特征

诺敏大山地区早白垩世侵入岩的主量元素、稀土元素及微量元素分析数据见表2。

表2 诺敏大山地区早白垩世侵入岩主量元素(%)分析结果和CIPW标准矿物参数及稀土元素(×10-6)、
微量元素(×10-6)分析结果
Table 2 Analysis results of major elements (%), CIPW standard mineral parameters, rare earth elements (×10-6)
and trace elements (×10-6) of Early Cretaceous intrusive rocks in the Mount Nuomin area

注:带*号数据来源于1∶25万额尔古纳左旗幅区域地质调查报告,其他数据来源于内蒙古呼伦贝尔市诺敏大山等三幅1∶5万区域矿产地质调查报告。TFeO=FeO+(Fe2O3×0.8998); K/Na为w(K2O)/w(Na2O)的值; R1=4n(Si)-11[n(Na)+n(K)]-2[n(Fe)+n(Ti)]; R2=6n(Ca)+2n(Mg)+n(Al); 里特曼指数(见邓晋福等,2015)。

由表2可以看出,研究区侵入岩样品的主量元素特征基本一致,其SiO2含量相对较高(67.36%~74.09%),MgO含量较低(0.2%~1.03%),TiO2含量较低(0.17%~0.4%),Al2O3含量普遍较高(12.56%~16.15%),碱质含量高(K2O+Na2O=8.88%~9.34%),且相对富K2O,属于高钾钙碱性岩石系列(图5a)。其K2O/Na2O平均为1.18,里特曼指数σ为2.61~3.23,分异指数DI为86.62~95.84,铝饱和指数(A/CNK)在0.94~1.31,区内侵入岩呈现出准铝质—过铝质特征(图5b)。

图5 研究区早白垩世侵入岩SiO2—K2O图解(a,据Maitre, 1989)和A/CNK—ANK图解(b,据Maniar and Piccli, 1989)
Fig. 5 SiO2—K2O diagramming (a, after Maitre, 1989) and A/CNK—ANK diagramming (b, after Maniar
and Piccli, 1989) of Early Cretaceous intrusive rocks in the study area

早白垩世侵入岩稀土元素分析结果显示,其∑REE介于229.58×10-6~335.5×10-6,LREE/HREE介于14.75~21.38,岩石稀土总量较高,轻稀土元素相对富集。(La/Yb)N =13.19~34.85,轻重稀土分馏明显,铕负异常明显(δEu=0.49~0.91),表明源区有斜长石的残留或斜长石结晶。各稀土配分曲线近一致,表现为同源岩浆演化特征,稀土元素球粒陨石标准化曲线表现为明显右倾的海鸥型(图6a),表明轻稀土元素(LREE)富集和重稀土元素(HREE)亏损特征。在微量元素原始地幔标准化蛛网图(图6b)中,大离子亲石元素(LILE) Rb、U、Th、K富集,Ba和高场强元素(HSFE)Ti、Nb、Sr、P等则明显亏损。

图6 研究区早白垩世侵入岩稀土元素球粒陨石标准化配分曲线(a)和微量元素原始地幔标准化蛛网图(b)
(球粒陨石和原始地幔标准化值据Sun and McDonough, 1989)
Fig. 6 Chondrite-normalized REE patterns diagrams(a) and Primitive mantle-normalized traceelement spider diagrams(b) of Early Cretaceous intrusive rocks in the study area (chondrites and mantle normalized values according to Sun and McDonough, 1989)

4 讨论

4.1 岩石成因

诺敏大山地区早白垩世侵入岩主要矿物成分为石英、钾长石(微斜长石为主),暗色矿物较少,矿物组合特征与高分异花岗岩较一致。主量元素分析结果显示,岩石总体富硅、富碱,MgO、CaO、TiO2含量较低,微量元素富集Rb、U、Th、K等大离子亲石元素,强烈亏损Ti、Nb、Sr、P等高场强元素,具有明显的Eu负异常,这些特征表明侵入岩浆经历了高分异演化。前人对大兴安岭北段中生代花岗岩进行了较多的研究,表明区域上中生代花岗岩主要为A型和I型(林强等,2004;刘宇崴,2018;周传芳等,2020)。研究区早白垩世侵入岩矿物中未见到石榴子石、堇青石、白云母等富铝S型花岗岩的特征矿物,结合岩石P2O5含量明显较低,可以排除该区侵入岩为S型花岗岩的可能。在P2O5—SiO2相关图上表现为明显的负相关关系(图7a),符合磷灰石饱和规则,同时在Th—Rb图解上表现为正相关趋势(图7b),这些都与I型花岗岩演化趋势一致。在FeOT/MgO—Zr+Ce+Nb+Y和(K2O+Na2O)/CaO—Zr+Ce+Nb+Y图解中(图7c、图7d),样品多数落入或靠近高分异花岗岩区域。综上所述,诺敏大山地区早白垩世侵入岩属于高分异I型花岗岩。

图7 诺敏大山地区侵入岩岩石分类判别图(a和b,据徐立明等,2021;c和d,据Whalen et al., 1987)
Fig. 7 Classification discriminant map of intrusive rocks in the Mount Nuomin area (a and b, after Xu Liming et al., 2021&;
c and d,after Whalen et al., 1987)
FG—分异的I或S型花岗岩;OGT—未分异的I或S型花岗岩;A— A型花岗岩
FG—differentiated I or S-type granite;OGT— undifferentiated I or S-type granite;A—A-type granite

4.2 岩浆源区

诺敏大山地区早白垩世侵入岩富集大离子亲石元素和轻稀土元素,强烈亏损高场强元素,与俯冲带岩浆岩特征一致,暗示岩浆为地壳来源或受到地壳物质的混染。前人通过研究认为,Rb和K有相似的地球化学性质,富集于成熟度高的地壳中;Sr和Ca有相似的地球化学行为,在成熟度低、演化不充分的地壳中富集(隋振民,2007;张彦龙等,2008;尹志刚等,2018)。Rb/Sr 值能较好地记录物质的性质,Rb/Sr值高,一方面说明岩浆演化程度很高,另一方面说明源岩可能主要来自地壳。研究区早白垩世侵入岩Rb/Sr=0.9~2.0,符合陆壳岩石的范围(>0.5);Sr/Y=4.2~7.2,显示出高Sr、低Y的特点,以上特征均反映了岩浆的壳源特征。区域上大兴安岭北段阿里河地区早白垩二长花岗岩和正长花岗岩(133~140 Ma)岩浆起源于新增生基性下地壳的部分熔融(刘宇崴,2018);大兴安岭北段新林镇地区侵入岩εHf(t)值均为正值,显示其岩浆源区存在新增生的年轻地壳物质加入,Hf同位素特征显示其岩浆源区为新元古代底侵的地壳物质(周传芳等,2020);大兴安岭伊勒呼里山早白垩世碱长花岗岩(138~140 Ma)岩浆源区可能为地壳物质的部分熔融(尹志刚等,2018)。结合区域资料和研究区侵入岩岩石地球化学特征显示研究区早白垩世侵入岩岩浆源区为地壳物质的部分熔融。

4.3 构造环境

研究区处于天山—兴蒙造山带大兴安岭岩浆岩带北段,靠近额尔古纳地块和兴安地块的缝合带。显生宙以来,大兴安岭北段受到古生代亚洲洋构造域、中—新生代蒙古—鄂霍茨克洋构造域和太平洋构造域影响,其中生代特别是燕山期岩浆演化问题一直存在争议,尤其是与蒙古—鄂霍茨克洋构造域的关系。在空间上,大兴安岭北段距蒙古—鄂霍茨克构造带较近,并且与大兴安岭北段火山岩带组合起来的展布方向与蒙古—鄂霍茨克构造带的展布方向一致,且自西向东火山岩年龄逐渐年轻化。前人大量研究表明,大兴安岭北段火山岩时代主要集中在晚侏罗世—早白垩世早期,岩石地球化学特征也显示它们源于被俯冲流体交代富集的岩石圈地幔,暗示蒙古—鄂霍茨克构造域在早白垩世之前已结束碰撞,之后进入碰撞造山后伸展构造环境(张连昌等,2007;Donskaya et al., 2013;孟凡超等,2014;Li Yu et al., 2015)。从时空关系上看,大兴安岭北段早白垩世时期岩浆活动应受蒙古—鄂霍茨克洋闭合作用的影响。本文诺敏大山地区早白垩世侵入岩样品在微量元素Rb—(Y+Yb)构造环境判别图中均落入后造山花岗岩区域(图8a),在花岗岩R2—R1构造环境判别图中落入同碰撞—后造山环境附近(图8b)。

图8 诺敏大山地区早白垩世侵入岩构造判别图解(a,据Maniar and Piccoli,1989;b,据Batchelor and Bowden, 1985)
Fig. 8 The tectonic distinguishing diagram of the Early Cretaceous intrusive rocks in the Mount Nuomin area
(a,after Maniar and Piccoli,1989;b,after Batchelor and Bowden, 1985)

样品微量元素强烈亏损Ti、Nb、Sr、P等高场强元素,同时具有高Sr、低Y和Yb特征,也显示该侵入岩形成可能与地壳快速缩短增厚的碰撞后期相对松弛阶段相关,推测为地幔底侵岩浆熔融下地壳物质,与前人提出的“后碰撞”构造环境形成的花岗岩特征极相似,相当于碰撞造山晚期阶段。综上所述,诺敏大山地区早白垩世侵入岩形成的构造环境可能为蒙古—鄂霍茨克造山后的伸展环境。

5 结论

(1)大兴安岭北段诺敏大山地区正长花岗岩U-Pb年龄为129.5±0.4 Ma,结合野外各侵入岩与地层接触关系,表明岩石侵入时代应为早白垩世,为燕山期岩浆活动的产物。

(2)诺敏大山地区侵入岩岩石地球化学特征表现为富硅、富碱,MgO、CaO、TiO2含量较低,微量元素富集Rb、U、Th、K等大离子亲石元素,强烈亏损Ti、Nb、Sr、P等高场强元素,具有明显的Eu负异常特征,表明侵入岩浆经历了高分异演化,属于高分异I型花岗岩。

(3)诺敏大山地区早白垩世岩浆活动可能发生在蒙古—鄂霍茨克造山后的伸展环境。

注 释 / Notes

❶ 中化地质矿山总局内蒙古地质勘查院.2020.内蒙古呼伦贝尔市诺敏大山等三幅1∶5万区域矿产地质调查报告.

❷ 黑龙江省地质调查总院.2003.1∶25万额尔古纳左旗幅区域地质调查报告.

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Geochemical characteristics and tectonic significance of Early Cretaceous intrusions in the Mount Nuomin area, northern Great Hinggan Mountains

REN Yongjian, ZHANG Chengxin, ZHANG Mingming, CHENG Shuo, GAO Liyong, YU Wang, ZHAO Han, WANG Chang, ZHANG Weisheng

Geological Institute of China Chemical Geology and Mine Bureau, Beijing, 100013

AbstactThis paper selects the Early Cretaceous intrusive rocks in the Mount Nuomin area in the northern Great Hinggan Mountains to study the zircon U-Pb geochronology and geochemistry, and discuss the genetic types, magma sources and tectonic environment of the intrusive rocks in this area. The lithology of intrusive rocks in this area is mainly syenite, syenite granite porphyry and porphyritic syenite. LA-ICP-MS zircon U-Pb dating of syenite samples shows that the emplacement age of syenite is 129.5±0.4 Ma, which should be the product of early Cretaceous magmatic activity, and combined with the contact relationship between intrusive rocks and strata in the area, the formation age of intrusive rocks in this area is Early Cretaceous. The Intrusive rocks are characterized by high silica(SiO2=67.36%~74.09%), alkali(K2O+Na2O=8.88%~9.34%) and alumina(Al2O3= 12.56%~16.15%), with low MgO, TiO2 and CaO. They are of high potassium calcium alkaline rock series and Quasialuminum—peraluminum as indicated by A/CNK ratio of 0.94~1.31. The trace elements are enriched in large ion lithophile elements such as Rb, U, Th, K, and strongly depleted of high field strength elements such as Ti, Nb, Sr, P, and have obvious negative Eu anomalies, which belong to highly differentiated type I granites. The rock Rb/Sr is 0.9~2.0, Sr/Y is 4.2~7.2, showing the characteristics of high Sr and low Y, indicating that the magma source area is the partial melting of crustal materials. Based on regional research results, the Mongolia—Okhotsk tectonic domain has ended its collision before the Early Cretaceous, and the early Cretaceous magmatic activity in the Mount Nuomin area may have occurred in the extensional environment after the Mongolia—Okhotsk orogen.

KeywordsNorthern Great Hinggan Mountains; Mount Nuomin area; Early Cretaceous; geochemistry; tectonic environment

注:本文为内蒙古地质勘查基金项目“内蒙古呼伦贝尔市诺敏大山等三幅1∶5万区域矿产地质调查”(编号:NMKD2015-18)的成果。

收稿日期:2022-01-04;改回日期:2022-04-24;网络首发:2022-05-20;责任编辑:章雨旭。Doi: 10.16509/j.georeview.2022.05.055

作者简介:任永健,男,1986年生,高级工程师,矿产普查与勘探专业,主要从事区域地质矿产调查工作;Email: mysky0315@126.com。

Acknowledgements: This research was funded by Inner Mongolia Geological Exploration Fund(No. NMKD2015-18). We are thankful to the reviewers who gave us a lot of suggestion to improve this article

First author: REN Yongjian, male, born in 1986, senior engineer, mineral prospecting and exploration major, Mainly engaged in regional geological and mineral investigation work; Email: mysky0315@126.com

Manuscript received on: 2022-01-04;Accepted on: 2022-04-24; Network published on: 2022-05-20

Doi: 10. 16509/j. georeview. 2022. 05. 055

Edited by: ZHANG Yuxu