岩石学报  2020, Vol. 36 Issue (3): 781-798, doi: 10.18654/1000-0569/2020.03.09   PDF    
内蒙古阿巴嘎旗中元古代片麻状花岗岩锆石U-Pb年龄、地球化学特征及大地构造意义
孙立新1,2, 张云1,2, 李艳锋1,2, 许凡3, 任邦方1,2     
1. 中国地质调查局天津地质调查中心, 天津 300170;
2. 华北地质科技创新中心, 天津 300170;
3. 河北省区域地质矿产调查研究所, 廊坊 065000
摘要: 在内蒙古阿巴嘎旗南部新发现一处面积约为9.2km2的片麻状正长花岗岩体,采用LA-MC-ICP-MS锆石U-Pb测年法获得该岩体的同位素年龄为1377±10Ma~1385±14Ma,属中元古代。它是继苏尼特左旗南部锡林浩特微地块的中元古代花岗岩发现后又一处中元古代片麻状正长花岗岩体。片麻状正长花岗岩具条纹状-弱片麻状构造,矿物组成为石英(25%~30%)、微斜长石(30%~40%)、条纹长石(15%~20%)、斜长石5%~10%以及少量黑云母(约5%)。其地球化学成分显示高SiO2(76.05%~78.16%)、高碱(K2O+Na2O=6.69%~8.04%)和较高的K2O/Na2O比值(1.01~2.48),低TiO2(0.17%~0.23%)、A12O3(10.99%~11.97%)、MgO(0.08%~0.44%)和CaO(0.15%~0.37%),铝饱和指数A/CNK值均大于1.1,里特曼指数主体小于1.8,属于过铝质和钙质系列;并且它们具有稀土元素总量高,轻稀土富集而重稀土亏损的特征,显示强烈负铕异常(δEu=0.44~0.53)和高的10000Ga/Al比值(均值为4.43)。上述岩相学、地球化学组成均显示A型花岗岩的特点,暗示其形成于伸展环境。片麻状正长花岗岩锆石εHft)值主体介于+0.5~+4.6,两阶段模式年龄t2DM为1.88~2.10Ga,表明岩浆可能来源于古元古代新增生地壳的部分熔融。阿巴嘎旗与苏尼特左旗中元古代岩体的发现为锡林浩特微地块的存在提供了直接证据。区域对比显示中亚造山带中的阿拉善微地块、中天山微地块、库鲁克塔格微地块和锡林浩特微地块上均具有~1.4Ga岩浆事件产物,可能是全球哥伦比亚超大陆裂解事件(1.5~1.3Ga)的岩浆响应。
关键词: 中元古代    A型花岗岩    锆石U-Pb年龄    阿巴嘎旗    中亚造山带    
Zircon U-Pb age and geochemistry of the Mesoproterozoic gneissic granite from Abaga Banner, Inner Mongolia and its tectonic significances
SUN LiXin1,2, ZHANG Yun1,2, LI YanFeng1,2, XU Fan3, REN BangFang1,2     
1. Tianjin Center of Geological Survey, CGS, Tianjin 300170, China;
2. North China Center for Geoscience Innovation, Tianjin 300170, China;
3. Institute Regional Geology and Mineral Resources of Hebei Province, Langfang 065000, China
Abstract: A gneissic syenogranite pluton with an outcropped area of about 9.2km2 is identified in the southern Abaga Banner, Inner Mongolia, and the LA-MC-ICP-MS zircon U-Pb dating results indicate that it was formed in the Mesoproterozoic period (1377±10Ma~1385±14Ma), which is consist with the identification of the Mesoproterozoic granites in the Xilinhaote Massif, Sonit Zuoqi. The syenogranites are characterized by banded and weaken-gneissic structures, and dominantly consist of quartz (25%~30%), microcline (30%~40%), perthite (15%~20%), plagioclase(5%~10%)and biotite (~5%). Geochemical data indicate they have high abundances of SiO2(76.05%~78.16%)and alkali (K2O+Na2O of 6.69%~8.04%) with K2O/Na2O ratios of 1.01~2.48, low TiO2 (0.17%~0.23%), A12O3(10.99%~11.97%), MgO (0.08%~0.44%) and CaO (0.15~0.37%) contents, and their A/CNK ratios are >1.1 with Ritman index mainly less than 1.8, i.e., they belong to peraluminous and calcium series. Furthermore, all the gneissic syenogranites are characterized by high total REE, LREE enrichments and HREE depletions, with obvious negative Eu anomalies and high 10000Ga/Al ratios (averaged at 4.43). The above petrographic and geochemical features are similar to those of A-type granites, most likely suggesting an extensional setting. Additionally, zircons in the syenogranites show dominantly slight positive εHf(t) values of +0.5~+4.6 and Paleoproterozoic t2DMages (1.88~2.10Ga), indicating their primary magma was probably generated by partial melting of Paleoproterozoic lower crustal materials. Discovery of the Mesoproterozoic granotoids in Abaga and Sunitezuoqi areas implies the presence of the Xilinhot Massif. Regional comparison indicates that~1.4Ga magma event widely occurred in the Alxa, Central Tianshan, Kuluktag microblocks and Xilinhot Massif of the Central Asian Orogenic Belt, representing a magmatic response to the breakup of the Columbia supercontinent (1.5~1.3Ga).
Key words: Mesoproterozoic    A-type granite    Zircon U-Pb age    Abaga Banner    Central Asian Orogenic Belt    

中亚造山带位于西伯利亚板块、塔里木板块和华北板块之间,是典型的显生宙增生造山带(Șengör et al., 1993吴福元等, 1999Jahn et al., 2000, 2004Badarch et al., 2002Xiao et al., 2003; 洪大卫等,2003),是古亚洲洋及其众多微陆块长期演化的结果(Șengör et al., 1993)。兴蒙造山带位于中亚造山带东南缘(图 1),分布着众多由前寒武纪变质岩系组成的古老块体(邵济安和张履桥,1990Gordienko,1996李述靖等,1998任纪舜等,1999刘正宏等,2000Kuzmichev, 2001Khain et al., 2002, 2003Pisarevsky and Natapov, 2003Li, 2006Rojas-Agramonte, 2011徐备等,2014),对造山带格局及其形成演化具有重要影响,是造山带研究的热点问题之一。

图 1 内蒙古中部地质简图(a, b, 据内蒙古自治区地质矿产局,1991修改)及采样位置(c) 图 1a, b中:Ce-新生界;K-白垩系陆相碎屑岩;J-K-侏罗-白垩系陆相火山岩夹碎屑岩;J-侏罗系碎屑岩;P-二叠系碎屑岩夹灰岩;C-P石炭-二叠系海相碎屑岩、灰岩夹火山岩;D3-C1s上泥盆统-下石炭统碎屑岩夹灰岩;D-泥盆系碎屑岩;S3-4-中-上志留统灰岩夹碎屑岩, O1-2-下-中奥陶统海相碎屑岩;Ch-Jx-长城-蓟县系碎屑岩夹含铁矿层;Pt1B-古元古代宝音图群石英岩、片岩、白云质大理岩;γJ-侏罗纪花岗岩;γT-三叠纪花岗岩;γP-二叠纪花岗岩;γPt2-中元古代花岗岩;δP-二叠纪闪长岩;δO-奥陶纪闪长岩;Σ-超基性岩.图 1c中:Qpa-新近系阿巴嘎玄武岩;D3-C1s-晚泥盆世-早石炭世碎屑岩夹灰岩;γC1-早石炭世花岗岩;MξγPt2-中元古代片麻状花岗岩;19TW5-样品号 Fig. 1 Geological sketch map of central Inner Mongolia (a, b, after BGMRN, 1991) and sampling location(c)

内蒙古阿巴嘎旗地区位于中亚造山带东南缘,介于二连-贺根山构造带和索伦山-西拉木伦构造带之间(图 1a),大地构造上隶属于兴蒙造山带东段(内蒙古自治区地质矿产局,1991)。该地区是艾力格庙-锡林浩特微地块的主要残留区,为中亚造山带众多残存的微地块之一(邵济安和张履桥,1990邵济安,1991程裕琪,1994李述靖等,1998刘正宏等,2000Li, 2006徐备等,2014)。该地块发育绿片岩相-角闪岩相变质作用的中级变质岩系,主要岩石类型是石英岩、云母石英片岩、大理岩、黑云斜长片麻岩、长英质片麻岩等构成的表壳岩系,曾被称为“锡林郭勒杂岩”、或“宝音图群”(内蒙古自治区地质矿产局,1991)、“昌特敖包岩群”。其次还发育温都尔庙群,为经历蓝片岩相-绿片岩相变质的中浅变质岩系(内蒙古自治区地质矿产局,1996),主要岩石类型包括绢云石英片岩、绿泥石英片岩、蓝片岩、石英岩、含铁石英岩、变玄武岩、辉长岩和辉绿岩等,原岩以变质陆源碎屑-火山岩建造为主,形成时代为早古生代早期(徐备等,2016)。宝音图群作为锡林浩特地块的基底岩系,原岩形成时代为古元古代,角闪岩相变质作用发生在早古生代奥陶纪末期(内蒙古自治区地质矿产局,1991李述靖和高德臻,1995肖荣阁等,1995刘敦一等,2003)。

该地区自中元古代至新生代岩浆活动频繁,发育大量花岗质岩石和火山岩。野外调查发现,其中一部分以片麻状花岗质岩石为主,经受区域变质作用改造,具有与宝音图群近于一致的片麻理构造,此类岩石变形、变质均较强烈,为地块结晶基底的组成之一。另一部分以变形弱、未变质变形的花岗岩、中酸性火山岩为主,多为早古生代岛弧和同碰撞岩浆岩、晚古生代岩浆岩和新生代大陆溢流玄武岩,其研究程度相对较高。

本区元古代岩浆活动鲜有发现,研究程度较低。近年来我们在苏尼特左旗地区片麻状花岗岩中获得了中元古代年龄(1.39~1.51Ga),反映锡林浩特地块存在中元古代裂解事件的岩浆记录(孙立新等, 2013, 2018)。笔者最新的野外调查发现,阿巴嘎旗南部同样分布着与苏尼特左旗一带相同的元古代片麻状正长花岗岩体,为了查明其时代和地球化学特征,笔者进行了LA-ICP-MS锆石U-Pb年代学、岩石学、地球化学和锆石Hf同位素研究,并结合兴蒙造山带及前寒武纪研究的区域资料,探讨其岩石成因以及大地构造意义。

1 岩体地质特征

阿巴嘎旗南部片麻状花岗岩体,位于阿巴嘎旗南部约35km处,中心地理坐标为N43°46′、E115°59′,出露面积约9.2km2;北东侧、西南侧与晚泥盆-早石炭世色日巴彦敖包组呈断层接触,周围被早石炭世花岗岩侵入(图 2e),其局部被第四系阿巴嘎玄武岩覆盖(图 1b)。

图 2 片麻状花岗岩野外(a、c、e)和正交显微镜下照片(b、d、f) (a)片麻状正长花岗岩露头照片;(b、d)片麻状正长花岗岩显微照片;(c)正长花岗岩不均一条带片麻状构造;(e)早石炭世花岗岩侵入中元古代片麻状正长花岗岩;(f)片麻状正长花岗岩中的条纹长石.Q-石英;Bi-黑云母;Mi-微斜长石;Per-条纹长石;Pl-斜长石 Fig. 2 Photographs of the outcrops (a, c, e) and photomicrographs of the samples (b, d, f) from gneissic granitiods

片麻状花岗岩由浅红色弱片麻状中细粒正长花岗岩、混合花岗质片麻岩以及弱条带状片麻状中粗正长花岗岩等组成。露头上条带或片麻状正长花岗岩具有弱的混合岩化,普遍发育条带状片麻理构造(图 2a),由长英质石英和长石浅色条带与黑云母组成的暗色条带相间排列构成,长英质条带宽0.3~0.5cm,长10~30cm,边界模糊不清;暗色条带宽0.5~1.5cm, 长约30~50cm,具有变质分异条带特点(图 2a, c)。片麻理总体近东西向,产状近于直立。弱片麻状中粗粒二长花岗岩岩石相对均匀,局部发育弱的片麻状构造,不具有变质分异条带特征,岩石成分单一,主要为长英质成分,少量黑云母。

片麻状正长花岗岩:岩石新鲜面为浅红色,中粒-中粗粒半自形粒状结构(图 2b),局部可见似斑状结构,条纹状-弱片麻状构造。主要矿物为石英(25%~30%)、钾长石(微斜长石)(30%~40%)、条纹长石(15%~20%)、斜长石5%~10%,以及少量黑云母(约5%)。长石类矿物多为半自形板状,粒径2~3mm,其中,微斜长石则具有格子双晶特征(图 2d),微斜长石内部多发生了轻微的绢云母化蚀变。条纹长石具细条带状、叶脉状条纹结构(图 2f)。斜长石为半自形板状,发育聚片双晶和卡钠复合双晶结构,粒径多为1~2mm。石英多为他形粒状。黑云母为半自形片状,呈集合体。副矿物可见磷灰石、锆石、榍石等。

2 样品及分析方法 2.1 主量、微量元素

本文对采自阿巴嘎旗(图 1)新鲜样品,进行了主量和微量元素测定,测试在天津地质调查中心实验室完成。主量元素采用PW4400/40X-射线荧光光谱法(XRF)测试,Fe2O3、FeO应用氢氟酸硫酸溶样、重铬酸钾滴定容量法测定,分析精度优于2%,微量元素使用Thermo Fisher X series-Ⅱ等离子体质谱仪(MC-ICP)测试,分析精度优于5%。

2.2 锆石U-Pb测年

阿巴嘎旗片麻状花岗岩样品锆石挑选由河北省区域地质调查研究所实验室采用常规方法选取,制靶、锆石透射、反射、阴极发光照相天津地质调查中心实验室完成,锆石U-Pb年龄以及Lu-Hf同位素分析采用LA-MC-ICP-MS仪器,由New Wave的193nm激光器剥蚀系统和Thermo Fisher的Neptune型多接收等离子质谱仪完成,采用的激光剥蚀的束斑直径为35μm,进行锆石U-Pb同位素测定,采用TEMORA作为外部锆石年龄标准。利用NIST612玻璃标样作为外标计算锆石样品的Pb、U、Th含量,详细的实验原理和流程参见文献(Liu Y S et al., 2010)。采用ICP-MS DataCal程序和Isoplot(ver3.0)程序(Ludwig, 2003)进行数据处理。

2.3 锆石Hf同位素分析

锆石Lu-Hf同位素测试在天津地质矿产研究所实验室完成,采用配有193nm的LA-MC-ICP-MS仪器上进行,分析时采用8~10Hz的激光频率、100mJ的激光强度和50μm的激光束斑直径。激光剥蚀物质以He为载气送入Neptune,采用GJ-1作为监控标样,具体测试过程见耿建珍等(2011)。为使Hf同位素分析与锆石U-Pb年龄分析相对应,锆石Hf分析点与锆石U-Pb年龄分析点位于同一颗粒相同锆石晶域内,计算Hf同位素的相关参数时,采用同一颗粒锆石测得的U-Pb年龄。在计算176Lu的衰变常数采用1.865×10-11/y(Scherer et al., 2001; Li et al., 2007)。球粒陨石的176Lu/177Hf和176Hf/177Hf的比值分别为0.0332和0.282772(Blichert-Toft and Albarède, 1997;吴福元等,2007),亏损地幔的176Lu/177Hf和176Hf/177Hf的比值分别为0.0384和0.28325(Griffin et al., 2000),二阶段模式年龄分别采用平均地壳的Lu/Hf=-0.55, (176Lu/177Hf)平均地壳为0.015(Griffin et al., 2002; Söderlund et al., 2004)。

3 测试结果 3.1 锆石U-Pb年龄

本文对阿巴嘎旗南部片麻状花岗岩体中的3件样品进行了LA-ICP-MS锆石U-Pb测年,采样位置(图 1c)分别为19TW5(N43°45.87′、E114°59.24′)、19TW07(N43°46.86′、E114°59.12′)和19TW9(N43°46.93′、E114°58.39′),测年分析结果见表 1

表 1 片麻状花岗岩LA- ICP-MS锆石U-Pb测年数据 Table 1 Zircon LA-ICP-MS U-Pb data of gneissic granites

片麻状正长花岗岩3件样品(19TW5、19TW7、19TW9)的锆石特征基本一致,多为无色或浅桔黄色的透明至半透明的短柱状、长柱状晶体,粒径多在150~200μm之间,晶面光滑,晶棱平直,长宽比2:1,锆石类型相对单一。阴极发光图像显示出清晰的环带结构(图 3),且锆石的Th/U为0.49~0.98,均大于0.4(表 1),指示它们为典型的岩浆结晶锆石。

图 3 片麻状花岗岩样品锆石阴极发光(CL)图像和锆石U-Pb同位素年龄谐和图 锆石CL图中实圆圈为锆石测年点、白点为锆石Hf同位素测点 Fig. 3 Zircon CL images and U-Pb concordia plots of the gneissic granites

样品19TW5锆石共测得25个有效测试点数据(表 1),在206Pb/238U-207Pb/235U谐和图上(图 3b),绝大多数测点皆位于在谐和线上,25个测点的207Pb/206U年龄变化于1327~1414Ma之间,个别2个点偏离谐和线构成不一致线,计算获得207Pb/206Pb的统计权重平均值为1380±21Ma(MSWD=0.15),代表片麻状正长花岗岩形成的年龄。样品19TW7锆石共测得23个有效测试点数据(表 1),23个测点的207Pb/206U年龄变化于1352~1407Ma之间,在206Pb/238U-207Pb/235U谐和图上(图 3d),21个测点皆位于谐和线上,2个点偏离谐和线铅丢较为失严重,计算获得207Pb/206Pb的统计权重平均值为1385±14Ma(MSWD=0.46),可代表片麻状正长花岗岩形成的年龄。样品19TW9共测得锆石24个有效测试点数据(表 1),与19TW7年龄特征近于一致,锆石微区记录的207Pb/206Pb年龄变化于1349~1396Ma之间。在206Pb/238U-207Pb/235U谐和图上(图 3f),除1个点偏离谐和线外,其他点皆位于谐和线上或谐和线附近,去除谐和度不理想的1个点,23个分析点计算获得207Pb/206Pb的统计权重平均值为1377±10Ma(MSWD=0.32)(图 3),代表片麻状正长花岗岩的形成时代。

3.2 地球化学特征 3.2.1 主量元素

阿巴嘎旗南部好来呼都格片麻状正长花岗岩7件样品的主元素和微量元素分析结果见表 2,其主量元素特征为:高SiO2(76.05%~78.16%)、低TiO2 (0.17%~0.23%)、低A12O3(10.99%~11.97%)和低MgO(0.08%~0.44%);FeO+Fe2O3为1.53%~2.82%;FeOT/MgO比值较高,为4.51~26.64,平均14.06;贫CaO(0.15%~0.37%)、Na2O(2.25%~3.49%)、K2O(3.36%~5.59%);高碱(K2O+Na2O=6.69%~8.04%),较高的K2O/Na2O (1.01~2.48),Na2O < K2O,岩石富钾,里特曼指数σ=1.28~1.88,大多数小于1.8,为钙性系列,碱性程度中等到强。片麻状花岗岩总体显示出高硅、富碱、贫钙、低镁的特征。在岩石分类TAS图(图 4a)中样品均落入花岗岩区,呈亚碱性。片麻花岗质岩石的铝饱和指数A/CNK为1.10~1.17,均大于1.1,为过铝质系列。在A/CNK-A/NK图中(图 4b)样品均落入过铝质区。

表 2 片麻状花岗岩主量(wt%)、微量和稀土元素(×10-6)含量 Table 2 Major (wt%) and trace element (×10-6) contents of gneissic granites

图 4 片麻状花岗岩的化学分类TAS图(a, 据Middlemost, 1994)及A/CNK-A/NK图解(b, 据Chappell and White, 1974) Fig. 4 Chemical classification of gneissic granites (a, after Middlemost, 1994; b, after Chappell and White, 1974)
3.2.2 微量和稀土元素

片麻状正长花岗岩微量元素(表 2)中的大离子亲石元素Rb、Ba含量较高,Rb为136×10-6~223×10-6、Ba含量为402×10-6~816×10-6、Rb/Sr=2.70~5.50,相对均一,变化不大。原始地幔标准化微量元素蛛网图中(图 5a)片麻状正长花岗岩表现出富集Rb、Th、U、La、Ce、Zr、Hf等高场强元素,亏损Ba、Sr、P、Eu和Ti的负异常特征(图 5a),其微量元素标准化图与铝质A型花岗岩较为一致,暗示了长石、磷灰石以及榍石、角闪石、黑云母等矿物的分离结晶作用。

图 5 片麻状花岗岩原始地幔标准化微量元素蛛网图(a)和球粒陨石标准化稀土元素配分图(b)(标准化值据Sun and McDonough, 1989) Fig. 5 Primitive mantle-normalized trace element spider diagrams (a) and chondrite-normalized REE patterns (b) (normalization values after Sun and McDonough, 1989) of gneissic granites

片麻状正长花岗岩的稀土总量较高(表 2),∑REE(127.7×10-6~346.2×10-6),稀土分布曲线形状相似,都具有相近的轻稀土富集趋势(图 5b),LREE/HREE (5.35~9.40)为分异作用明显轻稀土富集型;δEu为(0.44~0.53)显示明显负异常。(La/Sm)N介于3.22~4.43,(La/Yb)N介于4.10~10.96,反映轻稀土分馏程度明显高于重稀土分馏程度。岩石稀土元素球粒陨石标准化配分图中呈相似的右倾稀土配分曲线(图 5b)。

3.2.3 锆石Lu-Hf同位素

在LA-ICP-MS锆石定年基础上,选择19TW5、19TW9样品在每个测年点的锆石上进行了锆石微区Lu-Hf同位素分析,其结果列于表 3表 3中绝大部分锆石的176Lu/177Hf值小于0.002,表明锆石没有放射性成因Hf的积累,所测样品的176Lu/177Hf值可以代表其形成时体系的Hf同位素组成(吴福元等,2007)。

表 3 样品Lu-Hf同位素分析数据和相关的特征参数 Table 3 Lu-Hf isotopic data and related parameters

样品19TW5共分析了23个Hf同位素测点,所测锆石的176Yb/177Hf和176Lu/177Hf比值范围分别为0.0337~0.0747、0.0011~0.0021(表 3)。图 6a显示锆石点集中于球粒陨石演化线上方,样品19TW5所测23颗锆石的Hf同位素相对比较均一,其中176Hf/177Hf比值变化于0.281984~0.282058,加权平均值为0.2820198±0.000007;Hf同位素初始比值176Hf/177Hfi分布在0.281952~0.282028之间,平均值为0.281978;εHf(t)变化于1.4~4.6,平均为2.42,εHf(t)均为正值,指示来源于新生的年轻地壳。单阶段模式年龄tDM变化范围为1.69~1.81Ga,平均为1.76Ga。两阶段模式年龄t2DM变化范围为1.88~2.07Ga,t2DM远大于其形成年龄。样品19TW9共分析了20个点,所测锆石的176Yb/177Hf和176Lu/177Hf比值范围分别为0.0254~0.0721,0.0009~0.0026(表 3)。图 6b显示锆石点也集中于球粒陨石演化线上方,具有与19TW5样品相同的Lu-Hf同位素特征,Hf同位素初始比值176Hf/177Hfi分布在0.281938~0.282161之间,平均值为0.281975;除5号点外,其他锆石测点εHf(t)变化于0.5~8.9,平均为2.77,εHf(t)均为正值,指示岩浆可能来源于古元古代新增生地壳的部分熔融。单阶段模式年龄tDM变化范围为1.51~1.82Ga,平均为1.75Ga。两阶段模式年龄变化范围为1.58~2.08Ga。t2DM远大于其形成年龄,其中εHf(t)为最大正值8.9时,单阶段模式年龄和二阶段模式年龄与岩石形成年龄近于一致,表明在1.40Ga前后存在一次地壳增生事件。

图 6 片麻状花岗岩锆石U-Pb年龄与εHf(t)相关图 Fig. 6 Plots of zircon U-Pb age vs. εHf(t) for the gneissic granite
4 讨论 4.1 岩石成因类型

阿巴嘎旗南部中元古代片麻状正长花岗岩类为钙性过铝质岩石,矿物组合以石英、钾长石(条纹长石、微斜长石)和黑云母为主,其较高的SiO2和K2O+Na2O、低CaO、MgO、P2O5、TiO2和MnO,高场强元素(HFSE)如Th、U、Zr、Hf相对于大离子亲石元素(LILE)明显富集,富集Rb和REE等,明显亏损Ba、Sr、Ti、P、Eu等元素,显示A型花岗岩特征。铝饱和指数A/CNK值(1.10~1.17),显示为过铝质花岗岩;具有较高的Na2O+K2O=6.69%~8.04%、FeOT/(FeOT+MgO)比值和高Zr+Nb+Ce+Y含量、10000Ga/Al= 3.17~6.13,平均4.43高于A型花岗岩的平均值2.6,与King et al. (1997)定义的铝质A型花岗岩相似。这些地球化学特征显示其具有A型花岗岩特征。在Zr+Nb+Ce+Y和10000×Ga/Al的图解中(图 7),它们的投点均落入A型花岗岩区(Whalen et al., 1987Frost and Frost, 2011)。同样在SiO2与FeOT/(FeOT+MgO)和Na2O+K2O-CaO图(Frost and Frost, 2011)以及CaO/(FeOT+MgO+TiO2)与(Al2O3+CaO)和Al2O3构建的判别图解中(Dall’Agnol和De Oliveira,2007),绝大部分投点也落入A型花岗岩区(图 8)。

图 7 片麻状花岗岩的A型花岗岩判别图解(据Whalen et al., 1987) 图 7-图 9样品图例同图 4.OGT代表未分异的I型,S型,M型,A型花岗岩; FG代表高分异的I型; A代表A型花岗岩 Fig. 7 Discrimination diagrams of A-type granite for gneissic granites (after Whalen et al., 1987)

图 8 片麻状花岗岩全岩SiO2与FeOT/(FeOT+MgO) (a)和SiO2与Na2O+K2O-CaO图解(b)(据Frost and Frost, 2011)及CaO/(FeOT+MgO+TiO2)与(Al2O3+CaO)(c)和Al2O3图解(d)(据Dall’Agnol and De Oliveira,2007) Fig. 8 Whole-rock SiO2 vs. FeOT/(FeOT+MgO) (a) and SiO2 vs. Na2O+K2O-CaO (b) (after Frost and Frost, 2011), CaO/(FeOT+MgO+TiO2) vs.(Al2O3+CaO)(c) and CaO/(FeOT+MgO+TiO2) vs. Al2O3 (d) (after Dall'Agnol and De Oliveira, 2007)

在成岩温度方面,A型花岗岩比I型和S型具有较高的成岩温度,锆石饱和温度计算出的结晶温度大于800℃(刘昌实等,2003),而岩石学实验表明其形成温度可能超过900℃。A型花岗岩具有高温属性,采用Watson and Harrison (1983)的方法估算了中元古代片麻状花岗岩的形成温度在826~890℃之间,平均为865℃(表 2)。

4.2 岩石源区

花岗岩锆石原位Hf同位素组成,由于锆石极高的稳定性和封闭温度,使得其Lu-Hf同位素体系较少受后期构造热事件的影响,即便在麻粒岩相等高级变质条件下,所测锆石的176Hf/177Hf比值仍能很好反映其形成时体系的Hf同位素组成,甚至可记录岩浆源区不同源岩类型的特征(Scherer et al., 2001Griffin et al., 2002吴福元等,2007)。正的εHf(t)值代表来自亏损幔源物质或新生地壳的部分熔融,负的εHf(t)值指示来自于古老陆壳岩石源区,如果存在较大变化范围的εHf(t),可揭示其源区不同性质源岩物质存在的信息(Kröner et al., 2014)。本文片麻状正长花岗岩锆石具有较高的176Hf/177Hf比值(0.281987~0.282258),依形成年龄(1380Ma)计算获得的εHf(t)值均为正值(+0.5~+8.9),在t-εHf(t)图解上所有成分点落在球粒陨石演化线以上靠近亏损地幔演化线的区域(图 6ab),其二阶段模式年龄tDM2 =1.5~2.08Ga,指示它们主要来自古元古代末期新生地壳物质的部分熔融。当εHf(t)为最大+8.9时,单阶段模式年龄t1DMt2DM年龄近于一致,与岩石形成年龄相近,表明在~1.4Ga时存在一次地壳增生事件。

4.3 大地构造意义

A-型花岗岩作为花岗岩的一种重要成因类型,其成因分类、岩浆起源、大地构造背景及其地球动力学意义备受关注(Loiselle et al., 1979Collins et al., 1982Whalen et al., 1987Eby, 1990, 1992胡受奚等,1991洪大卫等,1995Hong et al., 1996刘昌实等,2003)。A型花岗岩为“非造山、碱性和贫水的花岗岩”,具有富碱、高Fe/(Fe+Mg)、Rb/Sr、HFSE,低Ca、Fe、Mg,强烈亏损Eu、Sr、Ba、P、Ti的特点(King et al., 1997Collins et al., 1982);按成分特点分为铝质和过碱性两类,其成因观点多样(Collins et al., 1982Bonin, 2007)。虽然两类A型花岗岩在岩石学和地球化学上各具特色,但其形成环境均与伸展作用有关(Frost and Frost, 19972011; Frost et al., 1999),常用于陆壳伸展环境的岩石学判据。其形成环境分非造山型和后造山型,前者主要包括地幔柱和板内裂谷等,形成于板内,其源岩接近于洋岛玄武岩;后者包括弧后伸展和造山后伸展等,形成于陆-陆碰撞的后造山环境,源岩接近于大陆壳或底侵垫托的下地壳侵入岩。

Eby (1992)将A型花岗岩划分为非造山与裂谷环境有关的A1型花岗岩和造山后构造环境形成的A2型花岗岩。在A1-A2分类判别图中(图 9),中元古代片麻状花岗岩类具有较高的Ce/Nb、Y/Nb和Yb/Ta比值,均落入A2区域,指示中元古代片麻状花岗岩类的形成构造背景与伸展作用密切相关。在Pearce et al. (1984)花岗岩Nb-Y和Rb-(Y+Nb)判别图解中落入与后碰撞花岗岩环境有关的区域(图 9)。

图 9 中元古代片麻状花岗岩的Rb-(Nb+Y)和Nb-Y (a, b, 据Pearce et al., 1984)及A1-A2分类(c, d, 据Eby, 1992)判别 WPG-板内花岗岩; ORG-洋脊花岗岩; VAG-火山弧花岗岩; syn-COLG and post-COLG-同碰撞/碰撞后花岗岩. A1型代表来源于幔源熔体的分离结晶; A2型代表来源于地壳部分熔源区 Fig. 9 Diagrams of Rb vs. (Nb+Y) and Nb vs. Y (a, b, after Pearce et al., 1984) and A1-A2 classification discriminant (c, d, after Eby, 1992) for the Mesoproterozoic gneissic granite

中元古代的岩浆活动在全球范围分布十分广泛,由斜长岩-辉长岩及纹长二长岩与花岗岩类组成的“环斑花岗岩-斜长岩岩套”,从东端的西伯利亚及华北板块(1.70~1.60Ga左右)向西延至东欧板块和波罗的海地盾(1.65~1.54Ga),穿过格陵兰南部,直至北美大陆的拉布拉多地区及美国中西部(1.49~1.2Ga),在北半球组成一个近东西向的巨型岩带(Dickin et al., 1992; Åhäll and Connelly, 1998; Anderson and Cullers, 1999; Haapala and Rämö, 1999Corrigan et al., 2000; Blein et al., 2003),包括被称之为1.4Ga的岩浆活动带,它沿着劳伦古陆(Laurentia)太古宙克拉通的外侧边缘地带分布(Nyman et al., 1994胡霭琴等,2006)。在中亚造山带中多个微陆块内也存在1.4Ga岩浆事件,如哈萨克斯坦板块(Konopelko et al., 2012)、中天山地块(胡霭琴等,2006施文翔等,2010; He et al., 2015贺振宇等,2015)、阿拉善地块(史兴俊等,2016王毛毛等,2019)、锡林浩特地块(孙立新等, 2013, 2018);在中亚造山带周缘克拉通中,东欧板块也具有该时期构造岩浆记录,如Sveco-Fennian, Trans-Scandinavian和Gothian等构造带(Cawood et al., 2007; Kuznetsov et al., 2010)。华北板块南北缘同样也存在该时期构造热事件的记录(邵济安等, 2002, 2005), 例如在高于庄组(李怀坤等,2009)和铁岭组斑脱岩夹层(苏文博等,2010)及下马岭组辉绿岩床(李怀坤等,2009Zhang et al., 2012),都与中元古代燕辽裂陷槽形成的构造背景有关,也是Columbia超大陆中元古代裂解事件的响应(Lu et al., 2002; 翟明国和彭澎,2007; 翟明国等, 2014)。综上所述,~1.4Ga构造岩浆记录对揭示造山带微地块的来源及其形成的构造环境具有重要的意义。

目前,中元古代1.4Ga岩浆活动形成于两者截然不同的构造环境,一种形成于板缘挤压背景,具有造山作用的岩浆活动和变质作用特点(Dickin et al., 1992; Nyman et al., 1994);另一种形成于非造山构造背景,由大规模的地幔上涌导致拉斑质下地壳的部分熔融形成(Frost et al., 1997, 1999; Barnes et al., 2002; Vigneresse, 2005)。研究表明Columbia超大陆的汇聚可能是在2.1~1.8Ga发生的全球性碰撞造山事件中完成,而裂解可能开始于1.7Ga,结束于1.3~1.2Ga(Zhao et al., 2002, 2003a, b2004, 2005),但全球不同块体的裂解历史有较大的差异,其间发生了大量中元古代造山后-非造山阶段的非造山岩浆活动及镁铁质岩墙群的侵入(Hou et al., 2008; Goldberg, 2010)。华北板块南、北缘已发现的辉绿岩岩床(1.35Ga; Zhang et al., 2009)、钾质斑脱岩(1.37Ga; Su et al., 2008)、正长岩(1.35Ga)及~1.31Ga的A型花岗岩和1.3~1.2Ga基性岩墙群(Hou et al., 2008)等,说明中元古代中期整个华北克拉通处于大陆伸展构造环境。而锡林浩特微地块苏尼特左旗、阿巴嘎旗一带中元古代(~1.4Ga前后) A型花岗岩的发现,进一步说明中元古代中期锡林浩特地块也存在伸展作用的岩浆记录,与Columbia超大陆裂解事件在时代上基本一致,岩石为中元古代时期大陆裂解过程中的产物,可能与Columbia超大陆裂解(1.5~1.3Ga)有关。

5 结论

(1) 锆石U-Pb年代学结果表明本文报道的阿巴嘎旗中元古代片麻状花岗岩形成于1377~1385Ma.;Hf同位素结果表明其岩浆来源于古元古代新增生地壳的部分熔融。

(2) 中元古代片麻状花岗岩类具有高硅、高碱(K2O+Na2O)、富铁、贫镁、钙和富集高场强元素等特征,并具有较高的铝过饱和指数(ACNK=1.1~1.28)、较高的10000Ga/Al比值和较明显的δEu负异常等特征,表明岩石属于铝质A型花岗岩,岩石形成温度为826~890℃,平均865℃。

(3) 阿巴嘎旗一带中元古代A型花岗岩与中亚造山带中的中天山、阿拉善等微地块具有相同或基本一致的构造岩浆热事件,可能记录了哥伦比亚超大陆裂解事件的岩浆响应。

致谢      感谢中国地质科学院地质研究所任纪舜院士的支持和鼓励。同位素分析测试得到天津地质调查中心实验室周红英主任、郝爽、肖志斌高级工程师的大力帮助;匿名审稿人提出了宝贵的修改意见;在此一并致以诚挚的谢意!

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