华北板块北缘中段早石炭世构造属性:察哈尔右翼后旗高分异Ⅰ型花岗岩地球化学的制约

2015-03-06王挽琼邱殿明白新会张金凤

吉林大学学报（地球科学版） 2015年4期

刘群，王挽琼，邱殿明，白新会，张金凤，王新

1.黑龙江省地球物理勘察院，哈尔滨 150036 2.西南石油大学地球科学与技术学院，成都 610500 3.吉林大学学报编辑部，长春 130026 4.吉林大学地球科学学院，长春 130061

刘群1，王挽琼2，邱殿明3，白新会4，张金凤4，王新4

察哈尔右翼后旗二长花岗岩岩体位于华北板块北缘中段。岩体富SiO2、富K、富碱、低Ca和P、贫Fe和Mg，w(P2O5)与w(SiO2)呈负相关，铝指数(A/CNK)为0.96～1.15，分异指数为90.36～92.96；主要造岩矿物为条纹长石、斜长石和石英，其铁镁矿物主要为黑云母，未出现碱性铁镁矿物和富铝矿物，副矿物为锆石、磁铁矿、磷灰石、榍石和钛铁矿；可见察哈尔右翼后旗二长花岗岩属高分异钙碱性I型花岗岩类。结合较低的w(∑REE)((46.8～94.4)×10-6)、w(Th)((1.74～2.39)×10-6)、w(U)((0.27～0.39)×10-6)和微量元素判别图解，二长花岗岩岩浆源区可能为下地壳，源岩可能为岩石圈地幔。岩石轻重稀土分馏较强 ((La/Yb)N= 6.52～28.39)，δEu以正异常为主(0.83～3.51)，富集大离子亲石元素(LILEs，Cs、Rb、Ba和K)，亏损高场强元素 (HFSEs，Nb、Ta、P和Ti)。地球化学特征反映了二长花岗岩具有火山弧岩石特征，且岩体侵入新元古界埃迪卡拉系什那干群，表明岩体形成于活动大陆边缘弧环境，其侵位反映了古亚洲洋的俯冲岩浆事件。

华北板块北缘；内蒙古；早石炭世；高分异I型花岗岩；活动大陆边缘弧；古亚洲洋

0 引言

中亚造山带从哈萨克斯坦延伸至中国天山及华北板块北缘[1]，其形成与古亚洲洋演化、相邻板块间相互作用密不可分[2-3]，即古亚洲洋闭合，西伯利亚板块与塔里木--华北板块碰撞造山，形成中亚造山带[4]，与华北板块北缘毗邻的中亚造山带即为兴蒙造山带[5]。中亚造山带是全球显生宙大陆增生最显著的地区，同时也是全球最大的增生造山带[6]，兴蒙造山带是中亚造山带重要的一部分[7]。造山带是岩浆活动最为显著的地带。在该造山带及华北板块北缘，发育大量近东西向的古生代岩浆岩带[8]，其中以晚古生代火成岩最为显著[9-14]。

华北板块北边界线为赤峰--白云鄂博断裂[1，4，15]。察哈尔右翼后旗二长花岗岩岩体位于该断裂以南，集宁--隆化断裂以北(图1)，主要由元山洼、察汗脑包和高木匠村岩体组成(图1)。张臣等[10]2007年报道了该岩体的锆石SHRIMP U-Pb 定年((342.5±4.9) Ma) 和其主量元素特征，但其痕量元素及岩浆源区特征等关键问题目前尚未被研究过。且目前报道的华北板块北缘中段晚古生代岩浆岩时代集中在晚石炭世--二叠纪[5,19-20]，而早石炭世岩浆岩的报道较少。鉴于此，本文在1∶25万集宁市幅*吉林大学地质调查研究院. 1∶25万集宁市幅地质图.长春：吉林大学，2012.区调修测基础上，结合察哈尔右翼后旗二长花岗岩岩体的野外产状、矿物特征、地球化学特征，进一步探讨了其成因及形成的构造环境。

1 地质背景及岩体特征

察哈尔右翼后旗二长花岗岩岩体出露面积约109.54 km2，整体呈北东向产出，侵入到新元古界埃迪卡拉系什那干群和早泥盆世石英二长岩之中，后被大面积新生界新近系上新统宝格达乌拉组砂砾黏土及少量中新统汉诺坝组玄武岩覆盖，早白垩世花岗斑岩侵入其中(图1)。见少量斜长角闪片麻岩捕掳体。

角图据文献[1]修改。图1 研究区地质简图Fig.1 Geological sketch map of research area

察哈尔右翼后旗二长花岗岩风化面灰色，新鲜面肉红色，大部分为斑状结构(图2a、b)，少量为含斑结构和细粒花岗结构，块状构造为主，局部石英拉长、黑云母不连续定向形成片麻理构造(图2c、d)。片麻理走向近东西向，可能受后期岩浆作用及区域挤压作用形成。斑晶体积分数为2%～10%，主要为条纹长石。基质为中细粒结构，主要矿物(体积分数)为：条纹长石(40%～45%，弱高岭土化)、斜长石(30%～35%，绢云母化)、石英(25%)和黑云母(2%～5%，褪色析铁)；副矿物为磁铁矿，细双锥状、粒状锆石，柱状、粒状磷灰石，榍石和钛铁矿。高木匠村南1 km处为该岩体的中心相，中心相面积不大，岩性为浅肉红色细粒含斑黑云母钾长花岗岩，主要矿物为条纹长石(体积分数小于60%)，石英(体积分数25%)，斜长石(体积分数大于10%)，黑云母(体积分数5%)。张臣等[10]2007年报道的定年样品采自该岩体的中心相。

2 样品采集及实验方法

在高木匠村附近采集了5块新鲜二长花岗岩样品进行全岩主量和痕量元素分析。岩石化学分析在国家地质实验测试中心完成，主量元素采用ICP-AES(原子发射光谱)，烧失量为1 000 ℃烧失；稀土元素中La、Ce采用ICP-AES，其他元素采用ICP-MS；微量元素中Sc、Sr、Ba采用ICP-AES，Nb、Rb、Zr、Cr、V、Co、Th、Hf、Ta、U采用ICP-MS。

3 地球化学特征

3.1 主量元素

察哈尔右翼后旗二长花岗岩的主量元素和稀土微量元素分析结果见表1。从表1易见，花岗岩高SiO2(w(SiO2)=68.96%～71.25%)、富K、富碱(w(K2O)=4.31%～5.24%，w(K2O+Na2O)=9.13%～10.65%)，低Ca(w(CaO)=0.73%～1.12%)和P(w(P2O5)=0.028%～0.044%)，贫Fe(w(TFeO)=1.14%～2.02%)和Mg(w(MgO)=0.08%～0.16%)。在TAS侵入岩分类图(图3a)中主要落在花岗岩范围内，少量落在石英二长岩范围内；铝指数(A/CNK)为0.96～1.15，属准铝质--弱过铝质花岗岩类(图3b)。

样品编号wB/%SiO2Al2O3Fe2O3FeOCaOMgOK2ONa2OTiO2P2O5MnO烧失量K2O+Na2OP32b6-171.0016.111.410.251.030.165.084.050.130.0440.0140.69.13P32b10-171.0916.031.390.120.730.135.244.560.120.0360.0260.429.80P32b15-168.9616.302.200.041.040.084.406.250.200.0430.0260.3610.65P32b16-169.6017.271.430.161.120.094.485.380.160.0340.0480.139.86P32b27-271.2516.211.200.060.950.084.315.280.120.0280.0310.389.59样品编号Na2O/K2OA/CNKMg#wB/10-6CrNiCoRbCsSrBaVScNbP32b6-10.801.1515.813.022.541.7076.50.92632165024.00.331.60P32b10-10.871.1114.462.742.071.5267.30.99649120013.35.524.90P32b15-11.420.966.603.384.061.1455.90.4855488513.47.117.17P32b16-11.201.109.485.043.391.0686.61.96887159018.06.009.42P32b27-21.231.0811.003.433.921.1268.40.17683111012.04.724.64

表1(续)

图3 研究区二长花岗岩TAS图解(a)和铝饱指数图解(b)Fig.3 TAS (a) and Shand's index (b) diagrams of the monzonite ranites in the study area

3.2 痕量元素

察哈尔右翼后旗二长花岗岩具相对富集轻稀土(w(∑LREE)=(42.70～82.80)×10-6)、亏损重稀土元素(w(∑HREE)=(2.31～11.61)×10-6)的右倾型稀土元素配分曲线(图4a)。稀土元素总量低(w(∑REE)=(46.8～ 94.4)×10-6)，轻重稀土分馏程度强(∑LREE/∑HREE=7.13～22.16，(La/Yb)N=6.52～28.39)。轻稀土分馏强((La/Sm)N=2.77～13.24)，重稀土分馏较弱((Gd/Lu)N=0.98～1.56)。Eu以正异常为主(δEu=0.83～3.51)。相对位于岩体边部的样品P32b6-1(图1)正Eu异常显著(δEu=3.51)，重稀土总量低(w(∑HREE)=2.31×10-6)，轻稀土分馏强((La/Sm)N=13.24)，可能是受到了围岩(早泥盆世石英二长岩)的混染。

微量元素原始地幔标准化蛛网图(图 4b)中，察哈尔右翼后旗二长花岗岩富集大离子亲石元素(LILEs，Cs、Rb、Ba和K)，亏损高场强元素(HFSEs，Nb、Ta、P和Ti)，较明显的Sr正异常。

球粒陨石标准化值据文献[20]；原始地幔标准化值据文献[21]。图4 研究区二长花岗岩稀土元素球粒陨石标准化配分图(a)和微量元素原始地幔标准化蛛网图(b)Fig.4 Chondrite-normalized REE distribution patterns (a) and primitive mantle-normalized trace element patterns (b) of the monzonite granites in the study area

4 讨论

4.1 成因类型

岩浆分异指数为90.36～92.96，固结指数为0.62～1.47，亲铁元素质量分数低(w(V)=(12.0～24.0)×10-6，w(Co)=(1.06～1.70)×10-6)，P2O5质量分数很低(w(P2O5)=0.028%～0.044%)，富硅贫铁、镁、钙和钛，反映了岩体经历了高度分异演化作用[22]。岩浆经历了高分异作用，为岩浆演化的晚期产物，矿物组成和化学成分都趋于低共结的花岗岩，因此很难区分花岗岩是I型、S型或A型，相对P2O5、Th和Rb等元素是判断I、S型花岗岩较为可靠的标志[23-24]。察哈尔右翼后旗二长花岗岩w(P2O5)随w(SiO2)增高而降低(图5a)，w(Th)随w(Rb)升高而升高(图5b)，显示出I型花岗岩岩浆的演化趋势；而S型花岗岩的w(P2O5)与w(SiO2)呈正相关，w(Th)与w(Rb)呈负相关[24-25]。在(Na2O+K2O)/CaO-w(Zr+Nb+Ce+Y)图解(图6a)中，样品位于分异型花岗岩范围内；在主量元素判别图解(图6b)中，样品落在高分异钙碱性岩范围内。二长花岗岩不发育铁镁质碱性矿物，较低质量分数的Zr、Nb、Y、La和Ce也反映了其为I型花岗岩并非A型[28]。

4.2 源区性质

在微量元素蛛网图(图4b)上，察哈尔右翼后旗二长花岗岩明显亏损高场强元素Nb、Ta、P和Ti，富集Eu，w(Th)和w(U)较低。若岩浆源区残留金红石，会致使岩浆亏损Nb、Ta、Ti、Zr、Hf等高场强元素[29]，但二长花岗岩只亏损Nb、Ta和Ti并不亏损Zr、Hf，因此二长花岗岩亏损Nb、Ta、Ti与岩浆源区是否残留金红石无关。岩浆源区或者岩浆演化中陆壳物质的加入，也会使岩浆亏损Nb、Ta。二长花岗岩低的稀土总量和w(Th)、w(U)表明，岩浆演化早期可能不存在大规模的陆壳物质混染。那么该岩体Nb、Ta亏损应该是俯冲流体或者熔体交代作用所致[30]。Ti负异常主要是钛铁矿和角闪石分异所致。二长花岗岩经历了高程度的结晶分异作用，磷灰石一般在岩浆作用的早期结晶，磷灰石的结晶分异使熔体Eu富集，P亏损[31]。磷灰石富集REE、Th和U[25]，其结晶分异，可能也导致了二长花岗岩w(∑REE)、w(Th)和w(U)偏低。斜长石结晶分异会导致Sr、Eu亏损，钾长石结晶分异致使Eu、Ba亏损[28]，但二长花岗岩的Sr、Eu和Ba富集，显然岩浆演化过程中斜长石和钾长石结晶分异不显著。

图5 研究区二长花岗岩哈克图解Fig.5 Harker diagrams of the monzonite granites in the study area

A.A型花岗岩；FG.分异型M+I+S型花岗岩；OGT.非分异M+I+S型花岗岩。a底图据文献[26]，b底图据文献[27]。图6 研究区二长花岗岩(Na2O+K2O)/CaO-w(Zr+Nb+Ce+Y)图解(a)和主量元素判别图解(b)Fig.6 (Na2O+K2O)/CaO-w(Zr+Nb+Ce+Y) diagram (a) and discrimanation diagram by major elements(b) of the monzonite granites in the study area

岩浆分异作用的晚期，稀土元素易保留在残余液相中[31]。二长花岗岩经历了强烈的分异结晶，那么其稀土总量应该比源区岩石的稀土总量更高。而二长花岗岩的w(∑REE)=(46.8～94.4)×10-6，部分样品的稀土总量比下地壳(56.55×10-6[32])的还要低；w(Th)=(1.74～2.39)×10-6、w(U)=(0.27～0.39)×10-6，接近下地壳(w(Th)=1.2×10-6，w(U)=0.2×10-6[32])，暗示二长花岗岩源区可能为下地壳，源岩可能为岩石圈地幔，这与承德一带晚石炭世花岗岩类具有类似的源区[11]。

4.3 地质意义

在w(Nb)-w(Y)图解(图7a)中，察哈尔右翼后旗二长花岗岩落在火山弧+同碰撞范围内；w(Rb)-w(Y+Nb)图解(图7b)中，样品落在火山弧范围内，据Pearce[33]对花岗岩源区和成岩途径的分析，二长花岗岩源区可能是富集地幔和俯冲带流体的混合；Rb/30-Hf-3Ta图解(图7c)中，样品落在火山弧花岗岩范围内，反映了二长花岗岩形成于岛弧或者活动大陆边缘弧环境。二长花岗岩相对富集大离子亲石元素(LILEs，Cs、Rb、Ba和K)，亏损高场强元素(HFSEs，Nb、Ta、P和Ti)，也反映了其类似俯冲带岩浆岩的特征[34]。结合察哈尔右翼后旗二长花岗岩位于赤峰--白云鄂博断裂以南(图1)，侵入新元古界埃迪卡拉系什那干群中这种特征，表明二长花岗岩形成于活动大陆边缘弧环境。

图7 研究区二长花岗岩w(Nb)-w(Y)图解(a), w(Rb)-w(Y+Nb)图解(b)和Rb/30-Hf-3Ta图解(c)Fig.7 w(Nb)-w(Y) (a), w(Rb)-w(Y+Nb) (b) and Rb/30-Hf-3Ta(c) diagrams of the monzonite granites in the study area

中亚造山带作为古生代以来大陆地壳生长最重要的地带[35]，80年代至今，众多学者对其进行了大量研究，最初的研究对象主要为古生物学、地层学和古地磁，近几年报道了大量的侵入岩及地震剖面相关的证据。关于两大板块最终碰撞形成索伦缝合带的时间目前主要有以下几种观点：晚志留世--泥盆纪[36]；中--晚泥盆世[3，37]；晚泥盆世--晚石炭世[38-40]；二叠纪--早三叠世[1-2，9，15-16，19，41-45]。察哈尔右翼后旗二长花岗岩的侵位表明华北板块北缘中段在早石炭世为活动大陆边缘弧环境，古亚洲洋处于俯冲状态，显然其闭合时间肯定在早石炭世之后。满都拉南部推喇嘛庙一带发育327、330 Ma的花岗闪长岩，其形成与洋壳俯冲有关[9]；固阳--满都拉地区晚石炭世本巴图组火山岩显示岛弧或活动大陆边缘弧的特征[16]；四子王旗活佛滩闪长岩侵位年龄为(331±4)Ma，是古亚洲洋俯冲的产物[16]；苏尼特右旗晚石炭世本巴图组安山岩成岩时代为(300.9±1.6) Ma，其形成与古亚洲洋的俯冲作用有关[18]；Zhang[11]报道了大量滦平--承德--隆化地区晚石炭世闪长岩，形成于安第斯型活动大陆边缘弧环境，侵位时代集中为(324±6)～(302±4) Ma；吉林省安图县海沟岩体于322～328 Ma侵位，其成因与古亚洲洋俯冲作用有关[46]；这些火成岩构造构成了华北板块北缘早石炭世--晚石炭世火山弧。察哈尔右翼中旗具活动大陆边缘弧特征的辉长岩于(297.2±1.7) Ma侵位(待发表数据)；察哈尔右翼后旗北至温都尔庙南地区，发育大面积早--中二叠世石英闪长岩和花岗闪长岩岩基，都具活动大陆边缘弧特征[13]。童英[19]和张拴宏[5]总结了华北板块北缘晚古生代弧岩浆岩时空展布等特征，其中岩浆活动的高峰期为二叠纪，分布在靠近索伦缝合带的南侧；石炭纪岩浆活动相对较弱，多分布在靠近华北板块北缘内陆一侧。上述表明：早石炭世--晚石炭世古亚洲洋向华北板块北缘俯冲，在靠近板块内侧形成了早石炭世--晚石炭世火山弧；二叠纪的俯冲作用形成了靠近索伦缝合带的二叠纪弧岩浆岩带。

5 结论

本文通过对察哈尔右翼后旗二长花岗岩的岩相学、主量、痕量元素研究，得出以下几点结论：

1)二长花岗岩主要造岩矿物为条纹长石、斜长石和石英，铁镁矿物主要为黑云母，副矿物为锆石、磁铁矿、磷灰石、榍石和钛铁矿；富SiO2、K和(K2O+Na2O)，贫Ca、P、TFeO和Mg，铝指数(A/CNK)为0.96～1.15，分异指数为90.36～92.96，属高分异钙碱性I型花岗岩。

2)经历高分异作用的二长花岗岩w(∑REE)、w(Th)和w(U)低，结合w(Rb)-w(Y+Nb)图解，表明岩浆源区可能为下地壳，源岩为岩石圈地幔，俯冲带流体作用显著。Eu以正异常为主，Sr和Ba富集，表明岩浆结晶分异过程中斜长石和钾长石分异不显著。

3)岩体富集大离子亲石元素(LILEs，Cs、Rb、Ba和K)，亏损高场强元素(HFSEs，Nb、Ta、P和Ti)，显示火山弧岩浆岩特征；样品在花岗岩构造环境判别图解中均落入火山弧范围内，且察哈尔右翼后旗二长花岗岩位于赤峰--集宁断裂以南，侵入新元古界埃迪卡拉系什那干群,这种地质特征表明岩体形成于活动大陆边缘弧环境。

4)具有活动大陆边缘弧特征的二长花岗岩的侵位，表明早石炭世((342.5±4.9) Ma)存在古亚洲洋向华北板块北缘的俯冲事件。

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Early Carboniferous Tectonic Attribute of the Central-Northern Margin of North China Craton: Constraints from Geochemistry of Highly Fractionated Ⅰ-Type Granites in Cahayouhouqi Area

Liu Qun1，Wang Wanqiong2，Qiu Dianming3，Bai Xinhui4，Zhang Jinfeng4，Wang Xin4

1.HeilongjiangInstituteofGeophysicalExploration，Harbin150036，China2.SchoolofGeoscienceandTechnology,SouthwestPetroleumUniversity,Chengdu610500,China3.EditorialDepartmentofJournal,JilinUniversity,Changchun130026,China4.CollegeofEarthScience,JilinUniversity,Changchun130061,China

This study focuses on the petrology and geochemistry of the monzonitic granites from the Cahayouhouqi area in Inner Mongolia. Tectonically it is located in the central-northern margin of the North China craton. These rocks have high content of silica, kalium and alkalis, low abundance of calcium, phosphorus, iron, magnesium;w(P2O5) decreases with the increasing ofw(SiO2); A/CNK equals 0.96-1.15; and differentiation index DI is between 90.36-92.96. The granites consist mainly of perthite, plagioclase, quartz, minor biotite, no mafic alkaline minerals and Al-rich minerals; their accessory minerals are zircon, magnetite, apatite, sphene and ilmenite. Combined with the petrological and geochemical characteristics, the granites can be classified as a highly fractionated I-type granite. The low content of ∑REE ((46.8-94.4)×10-6), Th ((1.74-2.39)×10-6), and U ((0.27-0.39)×10-6), as well as their trace element discrimination diagrams indicate that the magma source derived from the lower crust， and their source-rock was lithosphere mantle. The chondrite normalized patterns are right declined with a serious positive Eu anomaly (δEu=0.83-3.51). They are enriched in the large ion lithophile elements (LILEs, e.g., Cs, Rb, Ba, and K), and depleted of the high field-strength elements (HFSEs, e.g., Nb, Ta, P, and Ti). Their geochemical characteristics indicates that the monzonitic granites emplaced in an active continental margin as a result of the Early Carboniferous subduction of Paleo-Asian Ocean towards the northern margin of the North China craton.

northern margin North China craton; Inner Mongolia; Early Carboniferous; highly fractionated I-type granites; active continental margin arc; Paleo-Asian Ocean

10.13278/j.cnki.jjuese.201504115.

2014-10-21

中国地质调查局区域地质大调查项目(1212011120709，1212011220459)

刘群(1962--)，男，高级工程师，主要从事地质勘查工作和基础地质方面研究，E-mail:hrb-liuqun@sohu.com

王挽琼(1986--)，女，讲师，主要从事火成岩与区域地质调查方面研究，E-mail:ww_q2006@126.com。

10.13278/j.cnki.jjuese.201504115

P588.12

刘群，王挽琼，邱殿明，等.华北板块北缘中段早石炭世构造属性: 察哈尔右翼后旗高分异Ⅰ型花岗岩地球化学的制约.吉林大学学报:地球科学版,2015,45(4):1121-1131.

Liu Qun, Wang Wanqiong, Qiu Dianming, et al. Early Carboniferous Tectonic Attribute of the Central-Northern Margin of North China Craton: Constraints from Geochemistry of Highly Fractionated Ⅰ-Type Granites in Cahayouhouqi Area.Journal of Jilin University:Earth Science Edition,2015,45(4):1121-1131.doi:10.13278/j.cnki.jjuese.201504115.