南秦岭迷魂阵岩体LA-ICP-MS锆石U-Pb年代学和Lu-Hf同位素特征*

阎明 刘树文 李秋根 杨朋涛 王伟 郭荣荣 白翔 邓正宾

北京大学地球与空间科学学院,造山带与地壳演化教育部重点实验室, 北京 100871

图1 迷魂阵岩体区域地质简图1-迷魂阵岩体；2-东江口岩体；3-柞水岩体；4-梨园堂岩体；5-上震旦统陡山沱组；6-上震旦统灯影组；7-寒武系；8-奥陶系；9-中泥盆统古道岭组；10-上泥盆统九里坪组；11-上泥盆统刘岭组；12-中石炭统铁厂铺组；13-断层；14-主要村镇；15-取样位置Fig.1 Regional geological map of the Mihunzhen pluton in southern Qinling tectonic belt, Central China

横亘中国中部的秦岭造山带是连接华北克拉通和华南克拉通的主要造山带(张国伟等，2001)，向东与大别-苏鲁超高压变质带相连，其西与祁连-昆仑造山带相接。秦岭造山带经历了多期复杂构造演化历史，记录了华北克拉通与华南克拉通的拼合，是典型的复合型大陆造山带(刘树文等，2011；孙卫东等，2000；张国伟等，2001；Dongetal., 2011a, b, c, 2012a, b; Lai and Zhang, 1996; Laietal., 2004a, b; Liuetal., 2011; Meng and Zhang, 1999, 2000; Ratschbacheretal., 2003; Sun and Li, 1998; Sunetal., 2002; Yangetal., 2011, 2012; Zhangetal., 1996, 2011)。

前人将秦岭造山带划分为6个近东西向分布的构造单元，从北向南依次为：华北克拉通南缘，北秦岭构造带，商州-丹凤断裂带，南秦岭构造带，勉县-略阳断裂带，扬子克拉通北缘(图1) (刘树文等，2011；秦江锋等，2005，2007；秦江峰和赖少聪，2011；张国伟等，2001；Dongetal., 2011a, b, c, 2012a, b; Jiangetal., 2010; Liuetal., 2011; Mattaueretal., 1985; Meng and Zhang, 1999, 2000; Qinetal., 2007, 2008a, b, 2010a, b; Zhangetal., 1996)。

北秦岭构造带位于洛南-栾川断裂和商州-丹凤断裂带之间(陈岳龙等，1995)，主要由前寒武地块、新元古代-早古生代蛇绿岩套、早古生代中级变质的沉积岩-火山岩、古生代-早中生代花岗质侵入体组成(刘树文等，2011；张国伟等，2001；Dongetal., 2011a, b; Lietal., 2009; Liuetal., 2011; Qinetal., 2010a; Yangetal., 2012)。南秦岭构造带位于商州-丹凤断裂带和勉县-略阳断裂带之间，发育大量晚古生代沉积岩系，少量上古生界-下三叠系沉积岩系及三叠纪-侏罗纪早期花岗质侵入体(刘树文等，2011；张国伟等，2001；Dongetal., 2011b; Liuetal., 2011; Mattaueretal., 1985; Qinetal., 2010a, b; Yangetal., 2012)。 扬子克拉通北缘主要由一套新元古代火山-沉积岩和大量的花岗质侵入岩组成(刘树文等，2011；Liuetal., 2011)。商州-丹凤断裂带作为分隔南、北秦岭构造带的边界断层，记录了新元古代和晚古生代北向的俯冲-碰撞事件(张国伟等，2001；王宗起等，2009)。勉县-略阳断裂带则被认为代表了古特提斯洋北支的闭合的主要构造带(张国伟等，2001; Laietal., 2004a, b; Lai and Zhang, 1996; Meng and Zhang, 1999, 2000)。

图2 迷魂阵岩体野外地质特征及手标本照片(a)-花岗闪长岩-二长花岗岩脉侵入到小磨岭杂岩中；(b)-石英闪长岩-花岗闪长岩脉侵入到闪长岩之中；(c)-闪长岩中有浸染状磁黄铁矿、黄铜矿Fig.2 Field and specimen photos of the Mihunzhen pluton

近年来，在南秦岭构造带内陆续发现大量的中-新元古代的老地块，主要由变质火山-沉积岩系和古深成侵入体构成，主要分布于汉南一带, 以及沿着柞水-山阳断裂南部出露的陡岭岩群等，其中汉南一带的新元古代深成侵入体表现出较弱的变形和变质特征(陆松年等，2004)。然而，目前对南秦岭构造带中柞水-山阳地区出露的中-新元古代古深成侵入体的认识还欠深入，制约了对南秦岭构造带构造演化的理解和认识。小磨岭地区出露的前寒武纪岩块与邻区佛坪、武当等地的古老岩块共同构成了南秦岭构造带前寒武纪老陆块,其岩性组成较为复杂，主要出露有基性火山岩、辉长岩、辉绿岩和中酸性侵入岩体。本文对其中的迷魂阵岩体进行了系统的地质学、岩石学、锆石U-Pb年代学和Lu-Hf同位素的系统研究，目的在于确定其岩石成因和区域构造意义，为深入理解南秦岭构造带的形成与演化提供老地块方面的依据。

1 地质背景和岩体特征

迷魂阵岩体主要出露于柞水县南，在柞水-山阳断裂北部，西与东江口岩体相邻，东至柞水地区的磨沟峡和小岭镇地区。该岩体呈近东西向展布，呈长椭圆形，向东被断裂分割，面积约140km2(图1)。岩体的南缘侵入到中级变质的小磨岭杂岩中，其北部、西部、东部与泥盆系地层相邻，表现为挤压破碎或断层接触(图1)。中生代梨园堂似斑状二长花岗岩-钾长花岗岩岩体侵入其中。迷魂阵岩体的岩石类型包括闪长岩、石英闪长岩、花岗闪长岩和少量二长花岗岩，主要由两期岩浆活动形成，早期阶段的闪长岩分布面积较大，其中含细粒闪长岩包体。晚期阶段的岩石主要分布在岩体的中部及东部，以石英闪长岩-花岗闪长岩为主，伴有少量的二长花岗岩，三者之间为渐变过渡关系，侵入到早期阶段的闪长岩之中(图2a, b)。其中闪长岩通常发生磁黄铁矿化和铜矿化, 磁黄铁矿和黄铜矿以浸染状或微细脉状产出(图2c)。

迷魂阵岩体的南部围岩是小磨岭杂岩的火山-沉积岩系。该岩系主要由基性火山岩、陆源碎屑岩、辉长-辉绿岩，闪长岩和花岗岩，多遭受绿片岩相-低角闪岩相变质作用，之上被上震旦统陡山沱组和灯影组不整合覆盖。它们均被泥盆系和少量寒武系-奥陶系-石炭系沉积盖层覆盖，以断裂或断裂带接触(杨钊等，2008)。陡山沱组下部为砂砾岩，上部为石英砂岩、钙质砂岩与微粒灰岩互层。刘鹏举等(2009)对陡山沱组中部暴露间断面之下火山岩进行了锆石SHRIMP U-Pb年龄测定，获得了陡山沱组的沉积时代为614±8Ma；灯影组下部为白云质灰岩、白云岩夹少量薄层状含泥质微粒灰岩，上部为厚层硅质白云质灰岩及白云岩。显生宙盖层从底部到顶部依次为寒武系()炭质千枚岩、铝土质页岩、泥质白云质灰岩、硅质白云质灰岩；奥陶系(O)白云质结晶灰岩、燧石灰岩夹泥质灰岩条带；中泥盆统(D2)古道岭组黑云母石英片岩、结晶灰岩、石英岩、炭质硅质岩及大理岩；上泥盆统(D3)九里坪组砂质板岩、变质砂岩、钙质板岩与泥质灰岩；中石炭统(C2)铁厂铺群千枚岩、钙质千枚岩及灰岩。

研究区北部、西部、东部主要为泥盆纪刘岭群沉积岩系(图1)，由上泥盆统桐峪寺组和下东沟组、中泥盆统牛耳川组、池沟组和青石垭组组成。桐峪寺组为一套浅海相沉积建造，主要由石英砂岩、石英长石砂岩、钙质砂岩和石灰岩组成。下东沟组、牛耳川组、池沟组和青石垭组为潮坪相组合，下东沟组主要由泥质板岩、钙质粉砂岩和泥质粉砂岩组成；牛耳川组主要由细粒砂岩、粉砂岩、泥-砂质板岩和少量白云石灰岩组成；池沟组主要由细粒砂岩、泥质粉砂岩、钙质泥岩和石灰岩组成；青石垭组由粉砂岩、泥岩和石灰岩组成(杜定汉，1986；闫臻等，2007；Yanetal., 2006, 2012)。

图3 迷魂阵岩体样品显微照片(正交偏光)Ser-绢云母；Bt-黑云母；Amp-角闪石；Pl-斜长石；Kfs-钾长石；Q-石英；Mt-磁铁矿Fig.3 Micrographs of samples from the Mihunzhen pluton (cross polarized light)Ser-sericite; Bt-biotite; Amp-amphibole; Pl-plagioclase; Kfs-K-feldspar; Q-quartz; Mt-magnetite

2 样品特征及分析方法

2.1 样品特征描述

本文选择3件代表性样品进行了LA-ICP-MS锆石U-Pb同位素年代学分析，其中样品08LY2-8采于姜家沟，为迷魂阵岩体早期阶段的闪长岩，地理坐标为N33°36′24″，E109°04′10″；样品08LY6-1取自磨沟峡，为晚期阶段的石英闪长岩，地理坐标为N33°35′52″，E109°09′55″；样品08LY8-3取自小岭镇附近，为晚期阶段的花岗闪长岩，地理坐标为N33°34′33″，E109°15′45″。

早期阶段侵位的闪长岩为中-细粒半自形粒状结构，块状构造，主要矿物为斜长石(50%～55%)、钾长石(2%～5%)、角闪石(20%～25%)、辉石(5%～10%)、石英(1%～3%)，副矿物为锆石、磷灰石(图3a)。晚期阶段侵位的石英闪长岩的结构为中粒，半自形粒状结构，块状构造，其中斜长石(55%～60%)、钾长石(～5%)、角闪石(15%～20%)、黑云母(5%)、石英(10%～15%)，副矿物为磁铁矿、锆石、磷灰石(图3b)；花岗闪长岩为中-粗粒不等粒花岗结构，块状构造，主要矿物为斜长石(45%～50%)、石英(15%～20%)、钾长石(10%)，暗色矿物为黑云母(10%)和角闪石(5%)，副矿物为磁铁矿、锆石、磷灰石(图3c)。

2.2 分析方法

各样品的锆石单矿物分离是在河北区测队(廊坊)完成，将大约5kg重的样品破碎到60～80目，经常规浮选和磁选方法分选后，在双目镜下挑纯。将挑纯的锆石颗粒粘在双面胶上，然后用无色透明的环氧树脂固定，待环氧树脂充分固化后抛磨至粒径的大约二分之一，使锆石内部结构充分暴露，然后进行锆石显微(反射光、透射光和CL图像)照相。锆石的显微照相在北京大学电子学系及物理学系扫描电镜上完成。

锆石的U-Th-Pb及Lu-Hf同位素测试均在西北大学大陆动力学国家重点实验室完成。锆石定年分析所用的ICP-MS是Agilient公司最新一代的带有Shield Torch的Agilient 7500a。原位锆石Lu-Hf同位素测定采用Nu Plasma HR (Wrexham, UK)多接收电感耦合等离子体质谱仪完成(MC-ICP-MS)。采用的激光剥蚀系统为德国MicroLas公司生产的GeoLas200M, 该系统由德国Lambda Physik公司的ComPex102 Excimer激光器(波长193nm)与MicroLas公司的光学系统组成(第五春荣等，2008，2010)。

图4 闪长岩(08LY2-8)的锆石特征和U-Pb年龄图Fig.4 The zircon U-Pb isotopic concordia diagram and CL images of representative zircon grains from early diorite sample (08LY2-8)

3 分析结果

3.1 锆石U-Pb年龄

3.1.1 样品08LY2-8(闪长岩)

样品08LY2-8为迷魂阵岩体早期阶段侵位的闪长岩，共对其进行了25颗锆石25个点的LA-ICP-MS U-Pb同位素分析。CL图像显示大多数锆石颗粒为半自形-自形短柱状晶形，长度在100～200μm之间，长宽比在1:1到2:1之间，具有震荡环带或扇形带，应属于岩浆成因锆石(图4)。其Th/U比为0.45～0.96，206Pb/238U表观年龄为876～892Ma(表1)。在207Pb/235U-206Pb/238U的谐和图(图4)上，所有分析点全部落在谐和线上或者附近，其206Pb/238U加权平均年龄为885±4Ma(图4)，MSWD=0.14，代表了早期阶段闪长质岩浆侵位年龄。

3.1.2 样品08LY6-1(石英闪长岩)

样品08LY6-1为迷魂阵岩体晚期阶段侵位的石英闪长岩，共对其进行了18颗锆石18个点的LA-ICP-MS U-Pb同位素分析。在CL图像中这些被分析的锆石表现为灰黑色，不具备明显的振荡环带，多数为长柱状，长度在80～300μm之间，颗粒的长宽比变化较大，为3:2～7:2，其Th/U比0.91～1.21，为岩浆锆石。其206Pb/238U表观年龄比较集中，为734～749Ma(表1)。在207Pb/235U-206Pb/238U构建的谐和图中(图5)，分析点均落于谐和线上，获得的206Pb/238U加权平均年龄为740±4Ma(图5)，MSWD=0.20，代表晚期阶段岩浆侵位年龄。

表1锆石U-Th-Pb同位素分析数据和年龄值

Table 1 Analytical data of zircon U-Th-Pb isotopes and age values

测点号Th/U207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/235U年龄(Ma)1σ206Pb/238U年龄(Ma)1σ闪长岩08LY2⁃8⁃01051006457000096132212003258014627000182847148801008LY2⁃8⁃02048006521000087132015003066014682000178855138831008LY2⁃8⁃0309600654200007613259800275701459000172853128781008LY2⁃8⁃0407100669000089134975003121014632000178867138801008LY2⁃8⁃050490067940000913815900319014748000179881148871008LY2⁃8⁃060710064710000913318400312401470200018851148841008LY2⁃8⁃07079006352000085132655002992014689000178840138841008LY2⁃8⁃0804500701200008214220800302014708000175898138851008LY2⁃8⁃09078006562000083134294002994014787000178862138891008LY2⁃8⁃10077006413000092132941003175014695000181845148841008LY2⁃8⁃1105100665000098134185003337014634000181864148801008LY2⁃8⁃12052006838000106138597003588014698000185883158841008LY2⁃8⁃13067006675000104136009003525014777000186872158881008LY2⁃8⁃14062006611000099134501003396014754000184865158871008LY2⁃8⁃1508006789000103138427003509014787000184882158891008LY2⁃8⁃16083006679000119136634003899014835000193875178921108LY2⁃8⁃17053006792000118138737003893014814000191884178911108LY2⁃8⁃18046006677000121136008003933014772000193872178881108LY2⁃8⁃1904500682900011813907200387701476900019885168881108LY2⁃8⁃20076007077000127143825004151014737000193905178861108LY2⁃8⁃21069006726000126136473004049014715000194874178851108LY2⁃8⁃22051006806000302136516008255014547000262874358761508LY2⁃8⁃230730066850001351365030042880148090002874188901108LY2⁃8⁃2408300668500012913567004127014718000196870188851108LY2⁃8⁃25047007010001361430460043770147980001989021889011石英闪长岩08LY6⁃1⁃01114005859000139105319002301217100014769612737808LY6⁃1⁃0211100585600015710512200258601215400015169513739908LY6⁃1⁃0310600592700014210651400235101216900014670212737808LY6⁃1⁃0411500582700013110518300217401222300014469511743808LY6⁃1⁃0512100588100013510611500224501221900014570011743808LY6⁃1⁃06110057740001410341200229101213300014468712738808LY6⁃1⁃07116006180001710993800275101205300014971814734908LY6⁃1⁃0811800586900013210552200216401218300014169711741808LY6⁃1⁃0912600583600013610431900220901211600014169111737808LY6⁃1⁃1009100607500017711033000291801231200015472015749908LY6⁃1⁃111180059220001510719800245601227500014670512746808LY6⁃1⁃1211400601700016310835700264801221200014871013743808LY6⁃1⁃131190058670001541044860024670120800014469213735808LY6⁃1⁃1412100603300016810751500265501210100014670613736808LY6⁃1⁃151180058200016410407200260801214400014669013739808LY6⁃1⁃161190060500016410813000258401214100014470913739808LY6⁃1⁃171150058500018210458700288601214700015169215739908LY6⁃1⁃1811800603000171108287002688012204000146710147428花岗闪长岩08LY8⁃3⁃011540061020001510018300362801190600016470518725908LY8⁃3⁃021300688000213115301005113012153000191779247391108LY8⁃3⁃0308800661100007811223200235701230900014276411748808LY8⁃3⁃0414300626300016710558004101012223000175732207431008LY8⁃3⁃051470126770002082081370057260119040001621143197259

续表1

Continued Table 1

测点号Th/U207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/235U年龄(Ma)1σ206Pb/238U年龄(Ma)1σ08LY8⁃3⁃061300616700014109768600334601148300015669217701908LY8⁃3⁃070760061200010110255100274301214800015171714739908LY8⁃3⁃080900633000091066330025701221300014873713743908LY8⁃3⁃09159006129000123102376003201210900016171616737908LY8⁃3⁃1013700607700016810076200405301202000178708207321008LY8⁃3⁃1112800609100013510277500347012233000169718177441008LY8⁃3⁃12102005911000163099791003935011994000179693207301008LY8⁃3⁃132130060140001251020190031901196300016370016728908LY8⁃3⁃1417500657700017110212004255012149000183754217391108LY8⁃3⁃1510600708100016116662004159012254000177799197451008LY8⁃3⁃1615400611900016096873003798011483000177688207011008LY8⁃3⁃1711400593000165097055003997011871000187689217231108LY8⁃3⁃18149005832000152103944003825012182000189693207411108LY8⁃3⁃19094006144000161103154004059012181000191720207411108LY8⁃3⁃2015900591000162099749004051012123000195697217381108LY8⁃3⁃2111005965000162101092004038012054000195699217341108LY8⁃3⁃221090056660002101384004788012089000218675257361308LY8⁃3⁃2317200593100019310221004711012140002137002473912

图5 石英闪长岩(08LY06-1)的锆石特征和U-Pb年龄图Fig.5 The zircon U-Pb isotopic concordia diagram and CL images of representative zircon grains from late quartz diorite sample (08LY06-1)

3.1.3 样品08LY8-3(花岗闪长岩)

图6 花岗闪长岩(08LY8-3)的锆石特征和U-Pb年龄图Fig.6 The zircon U-Pb isotopic concordia diagram and CL images of representative zircon grains from late granodiorite sample (08LY8-3)

样品08LY8-3为迷魂阵岩体晚期侵位的花岗闪长岩，共对其进行了23颗锆石23个点的LA-ICP-MS U-Pb同位素分析。锆石在CL图像中表现为短柱状，具备明显的振荡环带，长度在100～200μm之间，颗粒的长宽比变化不大，为1:1～3:2，其Th/U比0.76～2.13，为岩浆锆石。其中分析点5远离谐和线，分析点6和16给出较小的206Pb/238U表观年龄701Ma，并表现为不谐和，故年龄计算中没有使用。其它20个分析点的206Pb/238U表观年龄为723～748Ma(表1)，在207Pb/235U-206Pb/238U谐和图(图6)中均落于谐和线上及其附近，获得了737±4Ma的206Pb/238U加权平均年龄(图6)，MSWD=0.50，代表该样品岩浆侵位年龄，与石英闪长岩的侵位年龄在误差范围内一致，因此迷魂阵岩体中石英闪长岩-花岗闪长岩的形成时代为740±4Ma。

表2 LA-MC-ICPMS锆石Lu-Hf同位素分析结果

Table 2 LA-MC-ICPMS zircon Lu-Hf isotopic compositions

测点号Age(Ma)176Yb/177Hf176Lu/177Hf176Hf/177Hf2s176Hf/177HfiεHf(0)εHf(t)tDM(Ma)fLu/Hf闪长岩08LY2⁃8⁃108LY2⁃8⁃208LY2⁃8⁃308LY2⁃8⁃408LY2⁃8⁃508LY2⁃8⁃608LY2⁃8⁃708LY2⁃8⁃808LY2⁃8⁃908LY2⁃8⁃1008LY2⁃8⁃1108LY2⁃8⁃1208LY2⁃8⁃1308LY2⁃8⁃1408LY2⁃8⁃1508LY2⁃8⁃1608LY2⁃8⁃1708LY2⁃8⁃1808LY2⁃8⁃1908LY2⁃8⁃2008LY2⁃8⁃2108LY2⁃8⁃2208LY2⁃8⁃2308LY2⁃8⁃2408LY2⁃8⁃2588500133150016358004230300200430017149001585000260650018466002562100245490021516001807800196670022915002990400296070015575001664700236860021313002049300130090019696003422300159330000535000067100016070000766000068300006250000959000073200009360000895000081700006820000738000086800011300001072000059300006390000937000076700007520000487000070400011930000606028253102824170282514028251002825710282513028248902824190282502028247702825740282514028247202825340282507028251802824730282477028252002825480282499028256702824920282538028252500000310000034000003200000260000027000003300000260000023000002400000280000021000002700000210000024000002400000250000020000002200000250000024000001900000210000026000002600000220282521899028240581902824869240282497423028255976602825021990282472655028240691202824868260282462233028256084028250238302824593560282519808028248853402825006102824627910282466737028250456602825349930282486602028255903102824799770282517957028251513-85-126-91-93-71-92-100-125-95-104-70-91-106-84-94-90-106-104-89-79-96-72-99-83-8710766959912110090679586121100851079610086881011129512193106105100811701061104395510351078116910581093954103510961012105710401090108510339901058956106710161017-098-098-095-098-098-098-097-098-097-097-098-098-098-097-097-097-098-098-097-098-098-099-098-096-098石英闪长岩08LY6⁃1⁃208LY6⁃1⁃308LY6⁃1⁃408LY6⁃1⁃508LY6⁃1⁃608LY6⁃1⁃708LY6⁃1⁃808LY6⁃1⁃908LY6⁃1⁃1008LY6⁃1⁃1108LY6⁃1⁃1208LY6⁃1⁃1308LY6⁃1⁃1408LY6⁃1⁃1508LY6⁃1⁃1608LY6⁃1⁃1708LY6⁃1⁃1874000355970001292028246800000240282450347-107501117-09600360740001293028244200000300282423919-117401154-09600430240001518028237200000340282350704-142141260-09500442680001599028250400000320282481604-95611075-09500434230001577028244500000320282422665-116401159-09500406360001468028241200000380282392043-127291201-09600413350001486028258600000290282565047-6690955-09600454320001660028253600000300282512672-84721031-09500565410002013028256900000310282540518-7282994-09400264120000965028244700000250282433398-115441137-09700495860001782028243700000330282412409-118361176-09500424460001522028243800000300282416852-118381167-09500444950001590028255500000350282532639-77791002-09500396240001426028251700000310282496909-90661052-09600464460001666028249800000240282474691-97581086-09500396660001439028245600000280282435681-112441139-09600474520001689028245500000300282431456-112431148-095

3.2 锆石Hf同位素

根据样品地质和锆石U-Pb年代学特征，对分别代表先后两个不同阶段的样品08LY2-8和08LY6-1进行了锆石原位Lu-Hf 同位素的测试，数据列于表2，并表示于图7。

图7 迷魂阵岩体Hf同位素特征(a)-闪长岩(08LY2-8)；(b)-石英闪长岩(08LY6-1)Fig.7 In-situ zircon Hf isotopic data of Mihunzhen pluton(a)-diorite (08LY2-8); (b)-quartz diorite (08LY6-1)

早期阶段侵位的闪长岩样品08LY2-8共进行了25个点的Lu-Hf同位素分析，用样品的结晶年龄(885Ma)对176Hf/177Hf和εHf(t)值进行校正，得出闪长岩的初始176Hf/177Hf值为0.0282406～0.0282561，εHf(t)值为+6.6～+12.1，平均值为+9.8，Hf同位素模式年龄(tDM)为955～1170Ma，平均值为1047Ma，与岩浆结晶年龄接近(图7a)，揭示了闪长质岩浆主要源于亏损地幔，岩浆侵位结晶过程中可能受到了地壳物质的混染。

晚期阶段侵位的石英闪长岩样品08LY6-1共进行了18个点的Lu-Hf 同位素分析，用740Ma的结晶年龄校正后的初始176Hf/177Hf值为0.0282351～0.0282565，εHf(t)值为+1.4～+9.0，平均值为+5.0，tDM为955～1260Ma，平均值为1119Ma(图7b)，表现出以亏损地幔源岩浆为主受到了元古代地壳物质的混染。

4 讨论

3个样品的LA-ICP-MS锆石U-Pb定年结果表明迷魂阵岩体形成于两个岩浆阶段，早期岩浆阶段形成闪长岩，其年龄为885±4Ma；晚期岩浆阶段形成石英闪长岩-花岗闪长岩，时代为～740±4Ma。早期阶段侵入的闪长岩的年龄与出露于扬子克拉通北缘的铜厂闪长岩体侵位时代(879±7Ma，王伟等，2010；Wangetal., 2012)基本一致，位于扬子陆块核部的三斗坪英云闪长岩具有795±8Ma的结晶年龄，五堵门岩体英云闪长岩侵位于789±10Ma(凌文黎，2006)。这些年代学证据说明南秦岭构造带内的一些老地块与杨子克拉通北缘及内部新元古代时期有相似的岩浆活动历史，因此南秦岭构造带内一些老地块或者属于杨子克拉通新元古代结晶基底的一部分，或者是从扬子克拉通北缘分离出来的老地块。Lietal.(2008)指出Rodinia超大陆拼合于1100～900Ma，之后约40Myr (860～750Ma)，地幔物质上涌，诱发相关的的大陆裂谷作用和地幔柱活动。迷魂阵岩体的形成时间与Rodinia超大陆的主要裂解时期不一致，可能形成于新元古代与俯冲相关的伸展背景 (耿元生等, 2008; 刘树文等, 2009a, b; Zhouetal., 2002, 2006; Zhao and Zhou, 2009; Zhaoetal., 2010, 2011)。

在εHf(t)-年龄图解中(图7)，早期阶段的闪长岩样品(08LY2-8)的εHf(t)值范围+6.6～+12.1，平均值为+9.8，接近亏损地幔演化线，说明早期阶段岩浆主要来自于亏损地幔物质的部分熔融。而晚期阶段石英闪长岩样品(08LY06-1)的εHf(t)范围为+1.4～+9.0，仍说明亏损地幔是岩浆的主要来源之一，而壳源物质可能是制约岩浆作用的另一个重要因素，或者形成于早期阶段闪长质岩浆的结晶分异，之后又经地壳物质的混染，或者形成于闪长质结晶分异的岩浆与壳源岩浆的混合作用。

5 结论

通过对迷魂阵岩体LA-MC-ICPMS锆石U-Pb同位素年代学、Lu-Hf同位素分析，本文得出以下认识：

(1)迷魂阵岩体主要由闪长岩、石英闪长岩和花岗闪长岩组成，形成于两个岩浆作用阶段，早期阶段的闪长岩的形成时代为885±4Ma，晚期阶段的石英闪长岩-花岗闪长岩的形成时代为～740±4Ma。

(2)早期闪长质岩浆主要来源于亏损地幔的部分熔融，晚期石英闪长岩-花岗闪长岩岩浆主要形成于早期闪长质岩浆的结晶分异，并经历了地壳物质的混染或者来源于壳源岩浆的混合作用。

致谢样品的LA-ICPMS锆石U-Pb和Lu-Hf同位素测试得到了西北大学大陆动力学国家重点实验室袁洪林教授的支持和帮助；主微量元素测试获得了北京大学造山带与地壳演化教育部重点实验室杨斌老师、古丽冰老师的帮助；在此一并表示衷心的感谢。

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