内蒙古中部乌拉盖复式岩体古生代两期岩浆成因：古亚洲洋板块从前进式到后撤式俯冲-增生的岩浆响应

Genesis of Paleozoic Two-Stage Magmatism of Wulagai Pluton in Central Inner Mongolia:Response to Tectonic Switch from Advancing to Retreating Subduction of Paleo-Asian Ocean Plate

摘要

俯冲带是壳幔物质相互作用的重要场所，并伴随着不同时期和性质的岩浆活动.理解不同俯冲过程（例如前进式俯冲和后撤式俯冲）中的岩浆作用特征对深入认识造山带演化历史具有重要意义.对内蒙古中部东乌旗地区乌拉盖复式岩体内古生代侵入岩开展了锆石U-Pb年代学、全岩主-微量元素地球化学、全岩Sr-Nd同位素和锆石Hf同位素特征研究.LA-ICP-MS锆石 U-Pb定年结果显示，岩体内辉长闪长岩形成时代为早奥陶世（~480 Ma）；花岗质岩石年龄为348~344 Ma，为早石炭世岩浆活动的产物.早奥陶世辉长闪长岩（SiO₂=51.27%~53.39%）相对富集轻稀土元素和Rb、Th、U等大离子亲石元素，亏损Nb、Ta等高场强元素，具有正的全岩ε_Nd（t）值（+3.3~+3.5）和锆石ε_Hf（t）值（+7.3~+13.3）的特征，指示岩浆来源于受到俯冲板片流体交代的亏损岩石圈地幔部分熔融.早石炭世花岗质岩石根据地球化学性质可进一步分为两组.第1组主要为正长花岗岩，具有高的SiO₂含量（76.19%~77.52%），高稀土总量，强Eu负异常，高10 000 Ga/Al值（3.90~5.95）和锆饱和温度（平均965℃）的特征，显示出A型花岗岩的特征；第2组主要为花岗斑岩和黑云母二长花岗岩，发育少量角闪石，其SiO₂含量相对第1组偏低（70.67%~76.42%），且与P₂O₅呈负相关关系，铝饱和指数为0.97~1.1，显示出I型花岗岩的特征.早石炭世两组花岗质岩石具有相似的Nd-Hf同位素特征（第1组ε_Nd（t）=+5.8~+6.3，ε_Hf（t）=+11.8~+15.5；第2组ε_Nd（t）=+4.9~+5.1，ε_Hf（t） = +11.6~+15.4），表明其岩浆可能源自于相似的新生地壳的部分熔融，但是经历了不同的岩浆演化过程.结合区域岩浆性质和演化特征，认为早古生代期间内蒙古中部地区为古亚洲洋板块的前进式俯冲，其至少在晚寒武世已俯冲至南蒙古地块南缘，如发育乌拉盖复式岩体内辉长闪长质弧岩浆岩；该前进式俯冲过程造成一系列弧地体拼贴到南蒙古地块南缘，晚志留世与南蒙古地块发生弧-陆碰撞.在晚泥盆世-早石炭世随着古亚洲洋板片回撤诱发弧后伸展，导致贺根山洋打开并形成了研究区内早石炭世早期的I型和A型花岗质岩石.

Abstract

Subduction zone is an important place for the interaction between crust and mantle, and is accompanied by magmatism of different periods and nature.Understanding the characteristics of magmatism in different subduction processes (e.g., advancing subduction and retreating subduction) is of significance for an in-depth understanding of the evolution history of orogenic belts. In this paper, it presents zircon U-Pb ages, whole-rock major- and trace element data, whole-rock Sr-Nd isotope and zircon Hf isotope for Paleozoic intrusions of the Wulagai pluton from East Ujimqin of central Inner Mongolia. Zircon LA-ICP-MS dating for gabbro-diorite yields a weighted mean age of ~480 Ma, suggesting the crystallization age is Early Ordovician; the age of granitoids is 348-344 Ma, representing the products of magmatic activities during Early Carboniferous. Early Ordovician gabbro-diorites(SiO₂=51.27%-53.39%) are relatively enriched in light rare earth elements and large ion lithophile elements (e.g., Rb, Th and U), and depleted in high field strength elements (e.g., Nb and Ta), with positive whole-rock ε_Nd(t) values (+3.3 to +3.5) and zircon ε_Hf(t) values (+7.3 to +13.3). The gabbro-diorites were originated from partial melting of a depleted lithospheric mantle that was metasomatized by slab-derived fluids. The Early Carboniferous granitoids can be divided into two groups according to their geochemical compositions.The first group is mainly syenogranite with high SiO₂ content (76.19%-77.52%), high total rare earths content, strong Eu negative anomalies, high 10 000 Ga/Al values (3.90-5.95) and zircon saturation temperature (average 965 °C), displaying typical A-type granite affinity.The second group is mainly granite porphyry and biotite monzogranite with SiO₂ content (70.67%-76.42%) lower than the first group.They showing metaluminous to weakly peraluminous characteristics, combined with negative correlation between SiO₂ and P₂O₅, and hornblende can be observed, displaying I-type granite affinity.In terms of Nd-Hf isotopic compositions,these two groups of Early Carboniferous granitoids have similar isotopic compositions (first group: ε_Nd(t) = +5.8 to +6.3, ε_Hf(t) = +11.8 to +15.5；second group: ε_Nd(t) = +4.9 to +5.1, ε_Hf(t) = +11.6 to +15.4), indicating that they have similar magmatic source and both derived from partial melting of the juvenile crust, but underwent different evolution processes.Combined with regional magmatism, it proposes an advancing subduction of Paleo-Asian Ocean plate occurred during Early Paleozoic in central Inner Mongolia.Paleo-Asian Ocean plate subducted beneath the South Mongolian microcontinent before Late Cambrian and lead to the formation of gabbro-diorites reported in this study. The advancing subduction caused a series of arc accreted to the southern margin of the South Mongolian microcontinent.Then an arc-continent collision occurred during Late Silurian.The Early Carboniferous A-type and I-type granitoids were formed in a back-arc extensional environment which was resulted from the roll back of Paleo-Asian Ocean plate during Late Devonian to Early Carboniferous.

Graphical abstract

关键词

中亚造山带 / 东乌旗 / 早古生代 / 早石炭世 / 岩浆作用 / 板片回撤 / 岩石学.

Key words

central asian orogenic belt / east Ujimqin / Early Paleozoic / Early Carboniferous / magmatism / roll back / petrology

0 引言

岩浆弧是俯冲带重要的岩石组成单元，是理解俯冲作用过程的钥匙.俯冲作用又进一步分为前进式和后撤式两种方式（Collins，2002；Cawood et al.，2009）.前者的特征是俯冲板块向上覆板块推进，伴随着地壳增厚，使上覆板块遭受强烈挤压，且通过沉积物俯冲和俯冲侵蚀作用加强了（古老）地壳物质向地幔的再循环作用；而后者代表了俯冲板块远离上覆板块，通过板片回撤导致地壳变薄以及弧后盆地的打开，在岩浆形成过程中古老地壳物质逐渐减少（Cawood et al.，2009；Kemp et al.，2009；Collins et al.，2011；Spencer et al.，2017）.在造山过程中，前进式和后撤式的俯冲往往交替发生（Beltrando et al.，2007；Cawood et al.，2009；Lister and Forster，2009）.很多现代包括中生代汇聚板块边缘均鉴别出前进式俯冲及后撤式俯冲过程，如安第斯中新生代岩浆作用和西太平洋的中生代岩浆作用（Li et al.，2019；Jara et al.，2021）.

中亚增生造山带是全球最大的显生宙增生型造山带，经历了多期次、多块体的俯冲、增生到碰撞过程，总体显示了向南东（古亚洲洋方向）的依次增生拼贴（如李舢等，2023）.近些年来在中亚造山带内西天山、东天山、阿拉善等地都识别出从早期前进式俯冲到后期后撤式俯冲的转变（Zhang et al.，2019；Wang et al.，2021；Zheng et al.，2022）.晚古生代至三叠纪期间，由于俯冲带大规模后撤过程，最终促成了中亚蒙古造山拼贴体系的山弯构造格局（Xiao et al.，2018）.可见，揭示俯冲带从早期前进式到晚期后撤式俯冲的岩浆响应机制研究对认识古亚洲洋的俯冲-增生造山过程具有重要意义.

在中亚造山带东南缘内蒙古中部地区，目前大多数的研究表明索伦缝合带为古亚洲洋的最终闭合位置并且闭合时间为晚二叠世-中三叠世（Windley et al.，2007；Jian et al.，2008；Chen et al.，2009；Xiao et al.，2015，2018；李舢等，2023）.而在索伦缝合带以北，还存在着二连浩特-贺根山蛇绿岩带，并且在其南北两侧均发育有大量古生代岩浆岩.南北两个近平行的古生代岩浆岩带具有长期的岩浆活动历史，并且持续时间相似.南侧的宝力道岛弧带由晚寒武世-志留纪和晚石炭世-早二叠世两个时期的弧岩浆岩组成，被认为与古亚洲洋主洋盆俯冲有关（Chen et al.，2000，2009；Jian et al.，2008；Liu et al.，2013）.关于北侧的南蒙古活动大陆边缘之上的早古生代岩浆岩，部分研究者认为其形成于弧后环境且与二连浩特-贺根山蛇绿岩带南部的早古生代弧岩浆岩共同构成了弧盆体系（李红英等，2016；那福超等，2022；胡飞等，2023），另一部分研究者认为其与南部的早古生代岩浆岩均为古亚洲洋俯冲形成的岩浆弧，是在晚古生代由于贺根山洋的打开将两者分隔开来（杨泽黎等，2017，2018，2020）.在晚古生代期间，南蒙古活动大陆边缘和北部造山带内岩浆岩时代主要集中在晚石炭世之后，早石炭世的岩浆岩少有研究，因此早石炭世的构造演化尚不明确.整体上来看，区域内古生代俯冲-增生过程并不十分明确.

Lu et al.（2020b）通过对区域内古生代沉积岩的研究提出早石炭世区域内经历了古亚洲洋板块从前进式到后撤式俯冲的转变，Yuan et al.（2022）同样提出早石炭世内蒙古中部地区发生了板片回撤.该过程导致了弧后伸展的发生，并且其对应构造体制从挤压转变为伸展环境，可能会产生如A型花岗岩等伸展环境下岩浆岩.李梦瞳等（2020）报道了北部造山带内早石炭世的双峰式火山岩，可以看作是该过程导致的伸展环境下的产物，然而李梦瞳等（2020）认为其是古亚洲洋在泥盆纪最终闭合后伸展环境下的产物而非与俯冲相关伸展环境下的产物；此外，在二连浩特-贺根山蛇绿岩带以北的南蒙古活动大陆边缘上还未有早石炭世伸展环境下岩浆岩的报道.可见，区域内有关从前进式到后撤式俯冲的转变过程的岩浆响应仍需进一步研究.

针对以上问题，本文对内蒙古中部东乌旗地区乌拉盖复式岩体内早奥陶世辉长闪长岩和早石炭世花岗质岩石开展了锆石U-Pb年代学、全岩元素地球化学和Sr-Nd同位素以及锆石Hf同位素特征研究，并结合区域地质资料探讨其岩浆成因和大地构造背景，进一步完善了区域内岩浆岩时空分布并探讨了内蒙古中部地区晚寒武世-早石炭世俯冲-增生过程.

1 区域地质背景与样品描述

内蒙古中部地区在大地构造位置上处于中亚造山带东段.从北往南依次为南蒙古活动大陆边缘、二连浩特-贺根山蛇绿岩带、北部造山带、索伦缝合带和南部造山带（图1；Xiao et al.，2003；Jian et al.，2008）.南蒙古活动大陆边缘主要包括元古代片麻岩、片岩、石英岩和寒武纪灰岩、粉砂岩（Xiao et al.，2003）以及古生代-中生代侵入岩以及相关的火山-沉积地层.奥陶纪出现的钙碱性岩浆岩表明该区域开始转变为活动大陆边缘（Xiao et al.，2003），晚古生代火山岩中伴有大量钙碱性花岗质岩体，代表了长期持续活动的大陆边缘弧；在晚石炭世到二叠纪期间南蒙古活动大陆边缘发育大量碰撞后环境下的A型花岗岩（Tong et al.，2015）.二连浩特-贺根山蛇绿岩带由一系列晚古生代蛇绿岩残片组成，整体呈北北东向展布，沿二连浩特、贺根山、小坝梁、乌斯尼黑一带分布，岩石组合包括二辉橄榄岩、方辉橄榄岩、纯橄榄岩、橄长岩、辉长岩、玄武岩、辉绿岩、斜长花岗岩和放射虫硅质岩，其形成时间为359~295 Ma，就位时代不晚于早二叠世（Miao et al.，2008；Jian et al.，2012；Zhang et al.，2015），其代表的洋盆具有一定规模.北部造山带主要由晚寒武世-晚志留世（498~420 Ma）和晚石炭世-早二叠世的宝力道弧-增生杂岩（Chen et al.，2000，2009；Jian et al.，2008；Liu et al.，2013）、中泥盆世蓝片岩（徐备等，2001）和锡林郭勒杂岩体（徐备等，1996）组成.其中宝力道弧-增生杂岩为古亚洲洋往北俯冲-增生的产物.宝力道弧主要由准铝质到弱过铝质的辉长闪长岩、石英闪长岩、英云闪长岩和花岗闪长岩组成，在其北部沉积的同时代火山-沉积岩形成于岛弧和弧后环境（Xiao et al.，2003；Jian et al.，2012；Liu et al.，2013）.俯冲-增生杂岩由岩块和非均质变形的基质组成，其中蓝片岩中钠角闪石⁴⁰Ar/³⁹Ar年龄为383±13 Ma（徐备等，2001）.锡林郭勒杂岩主要为片麻岩、片岩和透镜状或似层状角闪岩（薛怀民等，2009），以往认为代表了前寒武纪基底岩石.然而近年锆石年代学研究显示，锡林郭勒杂岩中存在许多古生代（452~382 Ma）的岩浆锆石（施光海等，2003；薛怀民等，2009；Li et al.，2011，2017b），说明锡林郭勒杂岩内部分正变质岩形成于古生代造山过程（肖文交等，2019）.索伦缝合带被认为记录了古亚洲洋最终碰撞缝合的过程，其中镁铁-超镁铁质岩石、白云岩、石灰岩和蓝片岩岩块不均匀散乱分布于由绢云母-绿泥石-石英片岩、硅质岩、含铁石英岩和少量大理岩组成的基质中（Xiao et al.，2003），沿线还发育一些晚二叠世-中三叠世碰撞相关的岩浆岩（Li et al.，2016b，2017a）.研究表明索伦缝合带内蛇绿岩年龄大多集中在280~250 Ma，表明该缝合带最终形成于三叠纪期间（Xiao et al.，2015）.南部造山带以发育晚寒武世-早志留世温都尔庙俯冲增生杂岩和中奥陶世-晚志留世白乃庙弧为特征（Xiao et al.，2003；Jian et al.，2008；Zhang et al.，2014；Zhou et al.，2018a；Chen et al.，2020）.温都尔庙俯冲增生杂岩在乌兰谷地区出露较好，包含蛇绿岩、高压变质岩和糜棱质岩石等，被认为是早-中古生代期间古亚洲洋南向俯冲-增生的产物（Xiao et al.，2003）；白乃庙弧形成于古亚洲洋往南俯冲的活动大陆边缘环境（Xiao et al.，2003）或为白乃庙洋北向俯冲的产物（Zhang et al.，2014）.

研究区所在的东乌旗乌拉盖复式岩体位于南蒙古活动大陆边缘，经历多阶段的岩浆侵位过程，岩体面积约60~70 km²（图2）.其中早古生代侵入岩以奥陶纪石英闪长岩和晚寒武世-奥陶纪的辉长闪长岩为主，被后期花岗岩、花岗斑岩及闪长玢岩脉岩侵入.晚古生代侵入岩以石炭纪花岗岩和花岗斑岩为主，并侵入到奥陶纪石英闪长岩和奥陶纪铜山组砂岩中，且多被晚石炭世-早二叠世宝力高庙组覆盖.中生代侵入岩以晚侏罗世的花岗岩为主，局部可观察到其侵入到晚石炭世-早二叠世宝力高庙组中.

本文选取乌拉盖复式岩体内古生代侵入岩开展了样品采集与研究工作.镜下研究显示，辉长闪长岩样品呈中粒半自形粒状结构、块状构造，矿物组成包括斜长石（50%~55%）、角闪石（30%~35%）、单斜辉石（15%~20%）以及磷灰石等副矿物，斜长石发育聚片双晶（图3a）.正长花岗岩样品具有似斑状结构，块状构造，由石英（25%~30%）、碱性长石（40%~50%）、斜长石（20%~25%） 以及锆石等副矿物组成（图3b）.花岗斑岩样品呈斑状结构、块状构造，斑晶含量为25%~30%，包括石英、斜长石、碱性长石、角闪石和黑云母，基质含量为70%~75%，主要由长石和石英组成（图3c）.黑云母二长花岗岩样品呈中细粒半自形粒状结构，块状构造，主要矿物组成包括石英（25%~30%）、斜长石（30%~35%）、碱性长石（35%~40%）、黑云母（2%~3%）以及锆石、磷灰石等副矿物（图3d）.

2 分析方法

2.1　锆石U-Pb定年分析

锆石颗粒采用常规的重液法和磁法进行初步分选，然后在双目显微镜下进一步手工挑选.将选出的锆石固定在环氧树脂靶上并抛光表面至一半左右厚度.锆石制靶完成后，对其进行透射光、反射光和阴极发光（CL）图像的采集工作，用于锆石内部结构分析，以便选择适宜的分析测试点位置.锆石制靶和透反射光、阴极发光图像的采集工作在南京宏创地质勘探技术服务有限公司完成.

两件样品（19HLH-1和19HLH-14）的锆石U-Pb同位素分析在北京燕都中实测试技术有限公司利用LA-ICP-MS完成.ICP-MS为德国耶拿PlasmaQuant MS，激光剥蚀系统为NWR193（Elemental Scientific Lasers LLC）.激光剥蚀过程中采用氦气作载气、氩气为补偿气以调节灵敏度，每个时间分辨分析数据包括大约20~30 s的空白信号和50 s的样品信号.测试过程中，激光剥蚀直径为25 μm.

5件样品（22WLG-1.1、22WLG-6.3、22WLG-6.4、22WLG-6.6和22WLG-6.7）的锆石U-Pb同位素分析在中国地质科学院矿产资源研究所自然资源部成矿作用与资源评价重点实验室完成.测试仪器为德国Finnigan公司生产的Neptune型激光多接收等离子体质谱（LA-MC-ICP-MS），激光剥蚀系统采用美国NEW Wave公司生产的UP213nm，频率为10 Hz，能量密度约为2.5 J/cm²，以He为载气.在具体测试过程中，4件样品（22WLG-6.3、22WLG-6.4、22WLG-6.6和22WLG-6.7）的激光斑束直径为24 μm，1件样品（22WLG-1.1）的激光斑束直径为20 μm.采用锆石标样M127（U=923×10^-6；Th/U=0.475）为外标进行U和Th含量校正.每测定10个样品点前后重复测量两次锆石标样GJ-1和一次锆石标样Plesovice，观察仪器状态以保证测试的精确度.数据处理采用ICPMSDataCal程序（Liu et al.，2010），锆石U-Pb年龄谐和图采用Isoplot程序完成.

2.2　全岩主量、微量元素分析

将测试样品去除风化面后压碎，然后用玛瑙研钵将小岩屑磨成粒度小于200目的粉末.全岩主量、微量元素分析在贵州同微测试科技有限公司完成.主量元素使用X射线荧光光谱仪测定，详细方法过程见于Norrish and Hutton（1969）.微量元素及稀土元素采用（Thermo Fisher）iCAP RQ电感耦合等离子质谱仪测定.测定过程中，主量元素分析的不确定度一般在1%以内，大多数微量元素的分析精度优于5%.

2.3　全岩Sr-Nd同位素分析

全岩Sr-Nd同位素分析在贵州同微测试科技有限公司完成.Sr同位素组成利用Nu Plasma多接收等离子质谱仪（MC-ICP-MS）测定，Nd同位素组成利用Neptune Plus多接收等离子质谱仪（MC-ICP-MS）测定.样品测试过程中采用BCR-2和BHVO-2国际标样作为Sr-Nd标准溶液，测得BCR-2和BHVO-2的⁸⁷Sr/⁸⁶Sr比值分别为0.705 007±0.000 009（2σ）和0.703 477±0.000 009（2σ），测得BCR-2和BHVO-2的¹⁴³Nd/¹⁴⁴Nd比值分别为0.512 640±0.000 006（2σ）和0.512 989±0.000 005（2σ）.

2.4　锆石Hf同位素分析

锆石Hf同位素测试在中国地质科学院矿产资源研究所自然资源部成矿作用与资源评价重点实验室完成.测试仪器为 Neptune 多接收等离子质谱和Newwave UP213 紫外激光剥蚀系统.仪器的运行条件及详细分析过程可参见侯可军等（2007）.本次实验中剥蚀直径采用44 μm.测定时使用锆石标样GJ1作为参考物质，GJ1的¹⁷⁶Hf/¹⁷⁷Hf 测试加权平均值为0.282 000 ± 0.000 030 （2σ），与文献报道值（侯可军等，2007）在误差范围内一致.

3 分析结果

3.1　锆石U-Pb年龄

7件样品的锆石U-Pb年龄分析测试结果见图4和附表1.锆石CL图像显示，样品中锆石均显示清晰的岩浆振荡环带，且锆石Th/U比值均大于0.1（0.26~1.24），指示岩浆成因.

辉长闪长岩样品（22WLG-6.6和22WLG-6.7）锆石自形程度好，多呈长柱状，长轴为100~200 μm，长宽比为1∶1~2∶1.²⁰⁶Pb/²³⁸U加权平均年龄分别为480±5（MSWD=2.9，n=24）和480±3（MSWD=1.7，n=24）（图4a，4b）.正长花岗岩样品（22WLG-1.1）锆石自形程度好，多呈短柱状，长轴为60~120 μm，长宽比为1∶1~1.5∶1.²⁰⁶Pb/²³⁸U加权平均年龄为347±3（MSWD=0.62，n=16）（图4c）.花岗斑岩样品（22WLG-6.3和22WLG-6.4）锆石自形程度好，多呈短柱状，长轴为60~100 μm，长宽比为1∶1~1.5∶1.²⁰⁶Pb/²³⁸U加权平均年龄分别为348±6（MSWD=0.86，n=8）和344±4（MSWD=1.15，n=18）（图4d， 4e）.黑云母二长花岗岩样品（19HLH-1和19HLH-14）锆石自形程度好，多呈长柱状，长轴为100~200 μm，长宽比为1∶1~2∶1.

19HLH-14样品²⁰⁶Pb/²³⁸U加权平均年龄为348±2（MSWD=0.34，n=24）（图4f）.19HLH-1样品显示出两组加权平均年龄（图4g， 4h），较年轻的一组为348.3±3.1 Ma，代表了其侵位年龄，较老的一组年龄（357.5±1.6 Ma）可能为继承锆石年龄，与二连浩特-贺根山蛇绿岩内早石炭世年龄（354 Ma）（Jian et al.，2012）年龄相近，反映了区域内早石炭世多期次岩浆活动.

3.2　全岩主量、微量地球化学特征

全岩主量元素以及微量元素和稀土元素分析测试结果见图5，图6和附表2.

乌拉盖复式岩体内早奥陶世辉长闪长岩SiO₂含量为51.27%~53.39%.岩石具有较高的Al₂O₃（17.06%~18.30%）和CaO（7.27%~10.03%），较低的TiO₂（0.62%~1.97%）和P₂O₅（0.08%~0.11%）.全碱含量（Na₂O+K₂O）介于3.93%~5.46%，K₂O/Na₂O为0.11~0.24，整体显示钙碱性特征，少数样品呈拉斑系列特征（图5）.MgO含量为4.99%~6.44%，Mg^#为52.53~64.90.稀土总含量较低（∑REE=44.69×10^-6~53.23×10^-6），相对富集LREE，轻微亏损HREE，轻重稀土分馏程度较弱（（La/Yb）_N=2.67~3.26），无明显Eu异常（δEu=0.94~1.15）（图6a），可能与样品中富集斜长石有关.此外，样品富集Rb、Th、K、Sr等大离子亲石元素，亏损Nb、Ta等高场强元素（图6b）.

依据样品主量、微量地球化学特征，乌拉盖复式岩体内早石炭世花岗质岩石可进一步分为两组：

第1组样品为正长花岗岩.SiO₂含量介于76.19%~77.52%.Al₂O₃、CaO、TiO₂含量分别为10.44%~11.52%、0.07%~0.14%和0.12%~0.27%.铝饱和指数ASI为1.02~1.16，属于过铝质系列.全碱含量（Na₂O+K₂O）为6.93%~7.88%之间.属于高钾钙碱性系列.MgO的含量为0.03%~0.11%，Mg^#为2.06~7.12，所有样品均落在铁质花岗岩区域（图5）.在稀土元素方面，总稀土含量较高（∑REE=188.39×10^-6~411.33×10^-6），轻重稀土分馏程度较弱（LREE/HREE=3.89~5.42、（La/Yb）_N=2.49~4.44），富集LREE，轻微亏损HREE，Eu负异常明显（δEu=0.14~0.29）（图6c）.微量元素方面，样品富集Rb、Th、K、Zr、Hf等元素，亏损Ba、Nb、Ta、Sr等元素（图6d）.

第2组样品为花岗斑岩和黑云母二长花岗岩.与第1组正长花岗岩相比，其SiO₂含量较低（70.67%~76.42%），Al₂O₃（12.49%~15.12%）、CaO（0.48%~1.32%）、TiO₂（0.16%~0.40%）含量较高.样品铝饱和指数ASI介于0.97~1.10，属于准铝质-弱过铝质系列.全碱含量（Na₂O+K₂O）为8.00%~9.70%，属于高钾钙碱性系列.MgO的含量为0.32%~0.57%，Mg^#为24.88~43.60，绝大多数样品落在镁质花岗岩区域，少数样品落在铁质花岗岩区域（图5）.在稀土元素方面，总稀土含量较第1组样品偏低（∑REE=84.31×10^-6~177.41×10^-6），轻重稀土分馏程度中等偏低（（La/Yb）_N=5.52~8.04），相对富集LREE，亏损HREE，具有中等偏弱的Eu负异常（δEu=0.63~0.73）（图6e）.在微量元素方面，富集Rb、Ba、K等大离子亲石元素，亏损Nb、Ta、Ti等高场强元素（图6f）.

3.3　全岩Sr-Nd同位素特征

乌拉盖复式岩体样品的Sr-Nd同位素比值根据样品测得的加权平均年龄计算，结果见图7和附表3.其中，早奥陶世辉长闪长岩样品Rb/Sr比值为0.029 5~0.073 5，初始⁸⁷Sr/⁸⁶Sr比值（⁸⁷Sr/⁸⁶Sr）_i为0.704 418~0.705 029，ε_Nd（t）值为+3.3~+3.5（图7a），单阶段Nd模式年龄t_DM为1 264~1 364 Ma.早石炭世第1组花岗质岩石样品Rb/Sr比值为5.557 5~15.129 5，ε_Nd（t）值为+5.8~+6.3（图7a），单阶段Nd模式年龄t_DM为625~683 Ma.早石炭世第2组花岗质岩石样品Rb/Sr比值为0.498 3~0.711 9，初始⁸⁷Sr/⁸⁶Sr比值（⁸⁷Sr/⁸⁶Sr）_i为0.702 466~0.703 301，ε_Nd（t）值为+4.9~+5.1（图7a），单阶段Nd模式年龄t_DM为666~711 Ma.

3.4　锆石Hf同位素特征

锆石原位Hf同位素测试分析结果见图7和附表4.其中，早奥陶世辉长闪长岩样品22WLG-6.7的¹⁷⁶Hf/¹⁷⁷Hf值介于0.282 696~0.282 865，ε_Hf（t）值为+7.3~+13.3（图7b），Hf同位素单阶段模式年龄t_DM1为559~805 Ma，Hf同位素二阶段模式年龄t_DM2为603~988 Ma.早石炭世正长花岗岩样品22WLG-1.1的¹⁷⁶Hf/¹⁷⁷Hf值介于0.282 915~0.283 021，ε_Hf（t）值为+11.8~+15.5（图7b），Hf同位素单阶段模式年龄t_DM1为357~522 Ma，Hf同位素二阶段模式年龄t_DM2为361~596 Ma；花岗斑岩样品22WLG-6.4的¹⁷⁶Hf/¹⁷⁷Hf值介于0.282 902~0.283 010，ε_Hf（t）值为+11.6~+15.4（图7b），Hf同位素单阶段模式年龄t_DM1为357~518 Ma，Hf同位素二阶段模式年龄t_DM2为364~608 Ma.

4 讨论

4.1　乌拉盖复式岩体形成时代及区域内古生代两期岩浆岩时空分布特征

前人曾报道乌拉盖复式岩体内辉长闪长岩锆石U-Pb年龄为500 Ma（杨泽黎等，2018），本研究测得其锆石U-Pb年龄为480 Ma，表明辉长闪长岩的形成时代为晚寒武世-早奥陶世.在区域上，早古生代岩浆岩分布主要集中在内蒙古中部的阿巴嘎旗和东乌旗等地，早古生代岩浆岩年龄范围为500~430 Ma，峰期年龄为500~479 Ma、461~450 Ma和~430 Ma（图8，图9）.此外，在兴安增生地体东部以及北部造山带同样存在一些早古生代岩浆岩.兴安增生地体东部早古生代岩浆岩主要分布在多宝山地区，嫩江、扎兰屯和蘑菇气等地也有发育，年龄范围为506~426 Ma，峰期年龄为478~474 Ma、448~435 Ma（图8，图9）.北部造山带早古生代岩浆岩主要分布在苏尼特左旗和锡林浩特地区，在大石寨地区也有少量相关报道，年龄范围在496~421 Ma之间，峰期年龄为481~471 Ma、449~441 Ma和~424 Ma（图8，图9）.整体上，3个区域内早古生代岩浆活动持续时间相近，并且南蒙古活动大陆边缘和北部造山带内早古生代岩浆岩呈近平行分布.

本次研究获得的乌拉盖复式岩体内花岗质岩石年龄在348~344 Ma之间，时代为早石炭世早期，区域内鲜有报道.根据区域岩浆岩资料，南蒙古活动大陆边缘晚古生代岩浆岩时代主要集中在早石炭世晚期-早二叠世（336~272 Ma）（韩宝福等，2010；程银行等，2014；Cheng et al.，2014b；Fu et al.，2016；Tian et al.，2018；Wei et al.，2018；Wang et al.，2022；刘博等，2023），同期（早石炭世早期）岩浆活动仅在阿巴嘎旗地区见有发现（348 Ma，Zhou et al.，2018b）.北部造山带内早石炭世早期岩浆岩同样较少，主要集中在苏尼特左旗东北部地区（349 Ma，李梦瞳等，2020；347 Ma，贺跃等，2018）.然而在兴安增生地体东部，早石炭世早期岩浆岩报道相对较多（图8，图9）.如扎兰屯地区的花岗质岩石和辉长闪长岩的锆石U-Pb年龄分别为359~354 Ma和350 Ma（Zhang et al.，2018；Li et al.，2020），位于牙克石地区的辉长岩年龄为344~341 Ma（Zhang et al.，2018），嫩江地区二长花岗岩和流纹岩年龄分别为352 Ma和354~353 Ma（赵芝等，2010；Li et al.，2020），蘑菇气地区的辉长岩、花岗闪长岩和英安岩的锆石U-Pb年龄分别为353 Ma、345 Ma和349 Ma（Feng et al.，2018；Ma et al.，2019）.可见，本文发现的东乌旗地区乌拉盖复式岩体内早石炭世早期的花岗质岩石对进一步完善区域内古生代岩浆岩时空分布及演化关联具有重要意义.

4.2　乌拉盖早奥陶世辉长闪长岩成因

乌拉盖复式岩体早奥陶世辉长闪长岩具有较低的烧失量（1.27%~2.02%），在镜下并未观察到蚀变现象，此外一些活动元素（如Ba、Sr）和过渡元素（如Co、Ni）与烧失量之间缺乏明显的相关关系，表明蚀变影响十分有限.辉长闪长岩具有较低的SiO₂含量（51.27%~53.39%），高的MgO含量（4.99%~6.44%）和Mg^#（52.53~64.90），指示岩浆来源于地幔.来源于软流圈地幔的熔体一般具有类似于OIB特征的微量元素配分模式，即富集大离子亲石元素和高场强元素（如Nb、Ta）.但是，乌拉盖辉长闪长岩富集大离子亲石元素、亏损Nb、Ta等高场强元素，具有高的La/Nb值（2.03~3.22）和La/Ta值（25.22~41.23），表明其岩浆来源于受到地壳物质（流体）混染或交代作用的岩石圈地幔的部分熔融（Thompson and Morrison，1988）.由于地壳中相对富集Zr和Hf，因此受到地壳混染作用的样品会显示Zr-Hf的正异常.在微量元素蛛网图上乌拉盖辉长闪长岩并没有显示明显的Zr-Hf的正异常；样品具有较高的Nb/Th（1.07~1.77）和较低的Th/Yb （ 1.06~1.38）比值，与上地壳物质明显的贡献不吻合；辉长闪长岩具有相对均一的初始⁸⁷Sr/⁸⁶Sr比值和ε_Nd（t）值 （（⁸⁷Sr/⁸⁶Sr）_i =0.704 418~0.705 029，ε_Nd（t）=+3.3~+3.5），表明岩浆在就位过程中地壳物质的混染不明显；此外，样品锆石年龄集中，缺乏古老的继承锆石，以上特征均暗示了岩浆没有受到明显的地壳混染作用.在Rb/Y-Nb/Y图解和Nb/Zr-Th/Zr图解中，辉长闪长岩表现出受到流体交代作用的趋势（图10a， 10b），进一步说明其岩浆源于受到流体交代的岩石圈地幔的部分熔融.

乌拉盖辉长闪长岩具有较低的（⁸⁷Sr/⁸⁶Sr）_i值（0.704 418~0.705 029）、正的ε_Nd（t）值（+3.3~+3.5）和ε_Hf（t）值（+7.3~+13.3）（图7），指示岩浆源区为亏损的岩石圈地幔.在Sm/Yb-Sm图解和Sm/Yb-La/Sm图解中，绝大部分样品落在石榴子石-尖晶石相（1∶1）二辉橄榄岩部分熔融曲线附近（图10c， 10d），进一步表明岩浆可能来源于石榴子石-尖晶石相二辉橄榄岩部分熔融.MgO和Fe₂O₃^T与SiO₂呈负相关（图11）以及低的Cr、Co、Ni含量代表岩浆演化过程中可能发生过橄榄石的分离结晶.另外，无显著的负Eu异常，表明斜长石的分离结晶程度较弱.综上所述，本文认为乌拉盖辉长闪长岩为受流体交代作用的亏损岩石圈地幔部分熔融的产物.

4.3　乌拉盖早石炭世花岗岩成因

乌拉盖复式岩体早石炭世正长花岗岩（第1组）具有高SiO₂（76.19%~77.52%）、FeO^T（1.97%~4.06%）、全碱含量（Na₂O+K₂O=6.93%~7.88%）和10 000Ga/Al值（3.90~5.95），低Al₂O₃（10.44%~11.52%）和Rb（96.7×10^-6~146×10^-6）含量.绝大多数样品显示弱过铝质特征（A/CNK=1.02~1.08，仅两件样品大于1.1），并且镜下未观察到白云母等富铝矿物.上述特征与A型花岗岩类似（Whalen et al.，1987），而不同于S型花岗岩（Chappell，1999）和高分异I型花岗岩的特征（王强等，2000）.并且在花岗岩类型判别图中落在A型花岗岩区域（图12）.在微量元素和稀土元素方面，轻重稀土分馏弱（（La/Yb）_N=2.49~4.44），具有明显的负Eu异常（δEu=0.14~0.29），相对富集Rb、Th、U、Pb、Zr和Hf，亏损Ba、Sr、P、Nb、Ta和Ti，也显示出A型花岗岩的特征（Collins et al.，1982；Whalen et al.，1987）.最重要的是，A型花岗岩最大的特点是高温（King et al.，1997；吴福元等，2007），其形成时的温度高于I型和S型花岗岩.根据Watson and Harrison（1983）提出的锆饱和温度计算公式，得出该组样品平均锆饱和温度为965 ℃，符合A型花岗岩高温的特征.Eby（1992）通过研究后将A型花岗岩进一步分为A₁型和A₂型花岗岩，在Nb-Y-Ce图解和Rb/Nb-Y/Nb图解中（图12e，12f），正长花岗岩落入A₂型花岗岩区域.综上所述，第1组样品为A₂型花岗岩.

目前关于A型花岗岩的成因主要包括：（1）幔源玄武质岩浆发生强烈的结晶分异（Turner et al.，1992；Bonin，2007）；（2）幔源岩浆和壳源岩浆混合（Yang et al.，2006）；（3）地壳物质部分熔融（Collins et al.，1982；Clemens et al.，1986；King et al.，1997）.由幔源基性岩浆高度结晶分异而来的A型花岗岩，在区域上会相伴一定规模的同期次中基性岩，然而在乌拉盖正长花岗岩周边并未发现相关中基性岩，因此排除这种成因的可能.其次，正长花岗岩体中缺乏铁镁质包体，并且锆石Hf同位素特征相对一致（ε_Hf（t）=+11.8~+15.5），也排除壳源和幔源岩浆混合成因.此外，该组样品具有高的Rb/Sr（5.56~15.13）、低的Ti/Y（13.29~24.65）和Ti/Zr（0.91~2.55）比值，与地壳物质部分熔融产生的岩浆（Rb/Sr>0.5，Ti/Y<100，Ti/Zr<20）相似（Pearce，1982）.综上所述，乌拉盖复式岩体内早石炭世A型花岗岩最可能的成因为地壳物质部分熔融.该组样品具有正的全岩ε_Nd（t）值（+5.8~+6.3）、锆石ε_Hf（t）值（+11.8~+15.5）（图7）和年轻的二阶段Hf模式年龄（t_DM2=361~596 Ma），指示岩浆可能来源于新生地壳的部分熔融.这些A型花岗岩样品相对富集重稀土元素Y和Yb，具有低的Sr/Y比值（0.08~0.34），显示出平坦的重稀土配分模式，可能反映了其源区熔融后的残留相中缺乏石榴子石；另外样品表现出强的Ba、Sr和Eu的负异常，表明源区有斜长石残留，这些特征均暗示了源岩部分熔融时的压力较低（Patiño Douce，1997）.综上所述，本文认为早石炭世第1组A型花岗岩是相对低压环境下新生地壳部分熔融的产物.

乌拉盖复式岩体早石炭世花岗斑岩和黑云母二长花岗岩（第2组）属于准铝质到弱过铝质系列，SiO₂含量和P₂O₅含量呈负相关（图11），具有相对较低的10 000Ga/Al值（1.91~2.67）和FeO^T/MgO值（2.33~5.43），并且镜下可见角闪石矿物，上述特征与I型花岗岩类似（Chappell and White，1992）.此外，在花岗岩类型判别图中（图12），它们横跨分异的花岗岩和未分异的花岗岩区域，具有过渡性特征，表明样品可能为具有一定分异演化程度的I型花岗岩.

早石炭世第2组花岗质岩石样品具有较高的SiO₂（70.67%~76.42%）和全碱含量（Na₂O+K₂O=8.0%~9.7%）以及低的Cr、Ni含量.富集Rb、Ba、Th、U、K等大离子亲石元素，亏损Nb、Ta、Ti、P等高场强元素，与大陆地壳成分组成十分相似（Rudnick and Gao，2003）.此外其Mg^#（25~44，平均33）、Nb/Ta值（7.68~13.74，平均12.5）接近壳源岩浆值（Mg^#=40；Nb/Ta=11.4）（Taylor and McLennan，1985）而不同于幔源岩浆（Nb/Ta=17.8）（McDonough and Sun，1995），以上特征均暗示其可能为大陆地壳部分熔融的产物.该组样品具有较低的（⁸⁷Sr/⁸⁶Sr）_i值（0.702 466~0.703 301）、较高的ε_Nd（t）值（+4.9~+5.1）和锆石ε_Hf（t）值（+11.6~+15.4）（图7），并且具有年轻的二阶段Hf模式年龄（t_DM2=364~608 Ma），说明岩浆源区为新生地壳.在哈克图解上（图11），可以观察到主量元素（如Fe₂O₃^T、TiO₂、MgO、P₂O₅、Al₂O₃、CaO）与SiO₂含量呈负相关关系，可能与岩浆演化过程中某些矿物的结晶分异有关.结合样品具有中等偏弱的Eu负异常（δEu=0.63~0.73），以及Sr的负异常，指示存在斜长石的分离结晶；P负异常可归因于磷灰石的分离结晶；Ti的负异常可能与金红石或钛铁矿等含Ti矿物分离结晶有关.综上所述，本文认为早石炭世第2组I型花岗岩为幔源基性岩浆底侵作用导致的新生地壳部分熔融，并进一步发生一定程度结晶分异的产物.

值得注意的是，A型花岗岩样品表现出强的Sr、Ba、Eu的负异常（δEu=0.14~0.29），说明其可能经历较强的结晶分异作用，并且该组样品普遍具有较高的分异指数DI（94.18~95.71，仅一个样品偏低，为91.8），其高硅的特点（ SiO₂含量>75%）也说明其岩浆演化程度相对较高.而I型花岗岩样品SiO₂含量相对A型花岗岩偏低，其Sr、Ba、Eu负异常偏弱，在一定程度上能说明两者经历的岩浆演化过程的差异.早石炭世两种类型的花岗岩具有亏损且相似的Nd-Hf同位素特征（A型花岗岩：ε_Nd（t） = +5.8~+6.3，ε_Hf（t） = +11.8~+15.5；I型花岗岩：ε_Nd（t） = +4.9~+5.1，ε_Hf（t） = +11.6~+15.4），说明两者都来源于新生地壳源区，只是上述的岩浆演化过程可能不同.

4.4　古亚洲洋板块从前进式到后撤式俯冲-增生的岩浆响应及构造意义

乌拉盖复式岩体内辉长闪长岩具有亏损Nb、Ta等高场强元素，以及富集Rb、Ba、Th、U、K、Sr等大离子亲石元素的特征，显示出弧岩浆岩的地球化学特征.在Th/Yb-Nb/Yb图解和Hf-Th-Ta图解中，乌拉盖复式岩体内辉长闪长岩落入火山弧区域（图10e，10f）.此外，辉长闪长岩主要表现为钙碱性系列特征，这与阿巴嘎旗吉尔嘎郎图岩体内近同期花岗闪长岩（496~479 Ma）具有高钾钙碱性的特征类似，表明乌拉盖复式岩体内辉长闪长岩可能形成于大陆边缘弧构造环境（杨泽黎等，2018；本研究）.林敏等（2019）在贺根山-黑河缝合带中段的科尔沁右前旗海勒斯台地区厘定出一套早古生代俯冲增生杂岩，并获得其中火山弧岩块的年龄为540~464 Ma，认为在早寒武世俯冲作用就已经开始并形成了大陆边缘弧.对东乌旗瓦窑地区奥陶纪铜山组碎屑岩的研究也表明此时处于活动大陆边缘环境（于洋等，2017）.

在二连浩特-贺根山蛇绿岩带南北两侧均发育有早古生代岩浆岩.以往的研究普遍认为其南北两侧的早古生代岩浆岩的形成存在一定的关联.在北侧的南蒙古活动大陆边缘之上，杨泽黎等（2017）认为阿巴嘎旗北部的吉尔嘎郎图岩体内高钾钙碱性花岗质岩石（496~455 Ma）是形成于活动大陆边缘的岩浆弧，属于南侧苏尼特左旗-锡林浩特一带早古生代岩浆弧的一部分.而位于东乌旗巴润布尔嘎斯台及朝不楞辉长岩（461~450 Ma）以及巴润布尔嘎斯台变辉石闪长岩（465 Ma）和片麻状角闪辉长岩（456 Ma）兼具岛弧玄武岩和N-MORB的特征，被认为可能是弧后盆地的产物（李红英等，2016；Li et al.，2016a；那福超等，2022），与南侧苏尼特左旗-锡林浩特一带早古生代弧岩浆岩共同构成了弧盆体系（那福超等，2022）.在东乌旗东北部额仁高壁苏木南侧的多宝山组内发现具有弧岩浆岩特征的中基性火山岩（452 Ma）和板内特征的酸性火山岩（430 Ma）（杨泽黎等，2020）.综合区域地质背景，南蒙古地块南缘在早古生代期间存在一个岩浆弧，东乌旗东北部中奥陶世的拉斑系列的中基性岩应该是早期弧后的产物.

在二连浩特-贺根山蛇绿岩带南侧的北部造山带内在早古生代期间存在一个同时期的岛弧.苏尼特左旗地区的484~469 Ma的辉长岩-石英闪长岩-英云闪长岩具有低钾、低钛的特征，共同组成了新生的弧岩浆，被认为是俯冲带地幔楔部分熔融的产物（Chen et al.，2000；Jian et al.，2008），类似于IBM洋内弧，并且与区域内早古生代SSZ型蛇绿岩残片（482 Ma）在时间上具有很好的一致性，暗示了区域内早古生代洋内俯冲的存在.位于艾力格庙地区的晚奥陶世角闪闪长岩（451 Ma）具有富Nb玄武岩的地球化学特征，为板片熔体交代亏损的岩石圈地幔楔部分熔融的产物，表明此时由流体交代转变为熔体交代岩石圈地幔，动力学背景由正常的大洋板片俯冲转变为洋中脊俯冲（Yuan et al.，2022）；此外，苏尼特左旗地区464 Ma近海沟的埃达克质岩体（Jian et al.，2008）以及锡林郭勒杂岩中437 Ma的低P/T变质的黑云片麻斜长岩（施光海等，2003）均证明了洋中脊俯冲的发生.研究表明，洋中脊俯冲可能会导致板块俯冲角度变低，并且进一步造成弧后挤压收缩（Gutscher et al.，2000），导致之后弧-陆发生碰撞并在苏尼特左旗地区形成了同碰撞构造环境下的富钾花岗岩（427~423 Ma）（石玉若等，2004，2007；Jian et al.，2008）.在苏尼特左旗东北部识别出的晚泥盆世弧背前陆磨拉石盆地（Xu et al.，2013；贺跃等，2018）很可能也是这次弧-陆碰撞后的产物.此外，低角度或平板俯冲的发生也会使得岩浆活动急剧减弱或停止（Gerya et al.，2009；Finzel et al.，2011），也能较好地解释南蒙古活动大陆边缘和北部造山带在泥盆纪岩浆活动近乎停滞的现象（Lu et al.，2020a，2020b；Wang et al.，2022）（图8）.古亚洲洋沿苏尼特左旗-锡林郭勒一带俯冲造成了上覆地壳包括锡林郭勒杂岩的隆升、增厚以及在452 Ma左右发生的部分熔融表明内蒙古中部地区早古生代期间为前进式俯冲（Li et al.，2011）.此外，内蒙古中部地区早古生代中酸性岩浆岩锆石ε_Hf（t）值随年龄变小呈下降的趋势（图7b），也暗示了早古生代期间内蒙古中部地区为古亚洲洋板块的前进式俯冲.

晚古生代期间，北部造山带内的石炭纪弧岩浆岩普遍被认为是古亚洲洋持续往北俯冲的产物（Chen et al.，2000，2009；Liu et al.，2013；Shi et al.，2016）.而在二连浩特-贺根山蛇绿岩带北侧的南蒙古活动大陆边缘内同样出露有晚古生代岩浆岩，并且与二连浩特-贺根山蛇绿岩带呈近平行展布，暗示其可能与贺根山洋的演化有关.

前人通过对二连浩特-贺根山蛇绿岩的研究，表明在晚古生代贺根山洋的存在.目前的研究表明二连浩特-贺根山蛇绿岩的形成时间为359~295 Ma（Miao et al.，2008；Jian et al.，2012；Zhang et al.，2015；黄波等，2016），表明在此时可能存在着一个洋盆.然而在晚志留世，北部造山带内的早古生代岛弧和其北部的南蒙古地块已经发生碰撞，形成了一个统一的大陆，因此贺根山洋可能是晚古生代重新打开的一个洋盆.Xu et al.（2017）对位于二连浩特-贺根山蛇绿岩带以北的查干敖包地区内古生代碎屑岩研究表明从奥陶纪-泥盆纪到石炭纪-早二叠世碎屑岩中碎屑锆石的年龄峰期和ε_Hf（t）值的转变暗示在早石炭世北部造山带不再是区域内碎屑岩的物源，可能是由于贺根山洋在石炭纪打开导致的.

本研究中东乌旗地区乌拉盖复式岩体内早石炭世早期花岗质岩石类型包括I型和A₂型花岗岩.目前研究认为I型花岗岩可以形成于多种构造环境；而A型花岗岩形成时的温度较高，因此往往形成于伸展的构造环境中.出露于苏尼特左旗北部沙尔塔拉地区的早石炭世早期玄武安山岩（349 Ma）和安山岩-英安岩-流纹岩（347 Ma）共同组成了一套双峰式火山岩组合以及早石炭世中期的英安岩-流纹岩（335 Ma）显示出A型花岗岩的特征（李梦瞳等，2020），同样表明了早石炭世区域内处于伸展环境.A₂型花岗岩通常被认为是形成于碰撞后伸展环境，然而近些年研究表明A₂型花岗岩也可以形成于与板块俯冲相关的伸展环境当中（周红升等，2008；Jiang et al.，2017）.研究表明，古亚洲洋俯冲一直持续到晚二叠世-早中三叠世（Xiao et al.，2015，2018）.通过对内蒙古中部地区古生代中酸性岩浆岩Hf同位素统计研究后发现，晚泥盆世-早石炭世中酸性岩浆岩锆石ε_Hf（t）值随年龄变小呈现上升的趋势（图7b），说明新生物质的比例越来越多，暗示了板片回撤的发生.

综上所述，本研究认为早古生代期间内蒙古中部地区为古亚洲洋板块的前进式俯冲.至少在晚寒武世古亚洲洋已俯冲至南蒙古地块南缘并在其上形成大陆边缘弧（图13a）.而在南蒙古地块南侧，存在一个同时期的洋内弧，并在中奥陶世发生了洋脊俯冲（图13b），导致板块俯冲角度变低，随后造成该岛弧在晚志留世与南蒙古地块发生碰撞，增生至南蒙古地块南缘（图13c），并且造成南蒙古活动大陆边缘和北部造山带两个构造单元在之后一段时间内岩浆活动停滞.在晚泥盆世-早石炭世古亚洲洋俯冲板块发生了回撤（图13d），导致软流圈地幔上涌和岩石圈伸展，形成了二连浩特-贺根山蛇绿岩以及本研究中的早石炭世同期次的I型和A型花岗岩.

本文的研究成果揭示了内蒙古中部地区古生代时期古亚洲洋板块从早期前进式俯冲到后期板片回撤过程中伸展背景下的岩浆响应，为理解中亚造山带东南缘构造演化以及大陆地壳生长提供了新的制约.

5 结论

（1）锆石U-Pb定年结果显示本研究中乌拉盖复式岩体内古生代辉长闪长岩年龄为~480 Ma，表明其形成时代为早奥陶世.岩浆来源于受到俯冲板片流体交代的亏损岩石圈地幔部分熔融.

（2）乌拉盖复式岩体内古生代花岗质岩石形成于早石炭世早期（348~344 Ma）.岩石成因类型包括A型花岗岩和I型花岗岩，为新生地壳部分熔融的产物.

（3）乌拉盖复式岩体内晚寒武世-早奥陶世辉长闪长岩为古亚洲洋板块前进式俯冲-增生的岩浆响应，该前进式俯冲过程造成一系列弧地体拼贴到南蒙古地块南缘，在晚志留世发生弧-陆碰撞.早石炭世早期的I型和A型花岗岩为古亚洲洋俯冲板片回撤诱发弧后伸展过程的岩浆响应.

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基金资助

国家自然科学基金项目(42161144007)

中科院人才项目择优支持(E3ER0401A2)