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扬子西北缘碧口块体花岗质岩体锆石U-Pb年龄、Hf同位素特征及其地质意义

2014-06-06平先权郑建平张志海

关键词:大安扬子源区

平先权,郑建平,熊 庆,张志海,夏 冰

中国地质大学地球科学学院,武汉 430074

扬子西北缘碧口块体花岗质岩体锆石U-Pb年龄、Hf同位素特征及其地质意义

平先权,郑建平,熊 庆,张志海,夏 冰

中国地质大学地球科学学院,武汉 430074

对扬子西北缘碧口块体中白雀寺和大安花岗质岩体进行了锆石U-Pb定年及Hf同位素研究。结果表明,白雀寺岩体的年龄分2组:(855±6)Ma(n=23)和(917±14)Ma(n=3)。前者εHf(t)为3.8~10.4,一阶段 Hf模式年龄为1.00~1.27Ga;后者εHf(t)为2.0~8.1,一阶段 Hf模式年龄为1.14~1.40 Ga。而大安岩体给出了6组不同的年龄:(854±10)Ma(n=3)、(801±7)Ma(n=3)、(702±10)Ma(n=3)、(565±6)Ma(n=1)、(246±7)Ma(n=1)和(207±2)Ma(n=1)。其εHf(t)为-12.6~11.4(t=854 Ma),一阶段和二阶段Hf模式年龄分别为0.95~1.90Ga和1.02~2.55Ga。白雀寺和大安岩体的侵位年龄分别为(855±6)Ma和(854±10)Ma,代表了扬子西北缘碧口块体内的一次重要岩浆事件,前者主体来源于中元古代新生陆壳源区,而后者主要来源于中元古代新生陆壳与晚太古代地壳物质的混合源区。此外,白雀寺岩体中(917±14)Ma的年龄代表新元古代早期的岩浆活动,而大安岩体中年轻的年龄可能记录着岩体后期所经历的复杂热改造事件。

碧口块体;花岗质岩体;锆石U-Pb年龄;Hf同位素;扬子西北缘;岩石学

0 前言

碧口块体位于扬子板块的西北缘,前人对其的研究工作主要限于碧口群火山岩,所得到的认识也非常 不 一 致,有 岛 弧[1-7]、弧 内 裂 谷[8]、大 陆 裂谷[9-12]、洋中脊和(或)大洋板内说[13-14]或元古宙古陆块[15]等观点。在前人的研究中,很少涉及该块体的深部地壳性质。花岗岩类是揭示深部地壳组成和地球动力学过程的有效途径[16]。近年来,有学者对碧口块体中生代花岗岩体进行了年代学和物质组成的研究[17-21],而对侵入其中的新元古代岩体研究较少[22-23],特别是有关花岗质岩体年代学及其源区性质的研究更少。在本文中,笔者选取了位于碧口块体内部偏北的白雀寺岩体和东南边部的大安岩体为研究对象,通过详细的岩石学、锆石内部结构和年代学与Hf同位素的原位激光分析,并结合前人的研究成果探讨它们的形成时代、岩石成因及其源区性质,进一步制约碧口块体的深部地壳组成和演化。

1 区域地质背景及岩石学特征

位于扬子板块西北缘的碧口块体(图1)呈长三角形状分别以勉略缝合带、青川—阳平关断裂、岷江断裂为界与西秦岭造山带、龙门山构造带及松潘—甘孜造山带相接[15,24-25]。块体内主体出露的岩石地层单位是碧口群和横丹群,均属浅变质火山-沉积岩,并以枫相院—铜钱坝断裂分隔。其中最古老的岩石单元为鱼洞子群,主要由片麻岩类和少量斜长角闪岩组成,形成年龄为(3 150±79)Ma[26]。碧口群主要由变中—基性火山熔岩和少量的火山碎屑岩组成[1-4,9-10],并 有 新 元 古 代 和 中 生 代 岩 体 侵 入 其中[18,22-23]。碧口群的变中—基性火山熔岩中锆石SHRIMP U-Pb确定的时代为846~776Ma[6,12];横丹群主要由巨厚的深海浊积岩系组成,显示为前弧盆地沉积特征[27-30],并且有中生代岩体侵入其中[18-19]。二 者 都 经 历 了 低 绿 片 岩 相 变 质 作用[2,4,9-10,31]。横 丹 群 中 的 深 海 浊 积 岩 碎 屑 锆 石SHRIMP U-Pb年龄为850~700Ma[7]。

碧口块体的碧口群中有新元古代岩体侵入,如坪头山、刘家坪、关口垭、白雀寺、罗素河、大安和铜厂等岩体。本次主要对该块体内部偏北的白雀寺岩体和东南边部的大安岩体进行研究。白雀寺岩体出露于陕西省略阳县白雀寺—石瓮子一带,平面上呈南—北向的长透镜状产出,出露面积约60km2。该岩体除东北部与泥盆系呈断层接触外,主体侵位于元古宇碧口群火山岩系中,岩体主要由闪长岩和石英二长岩组成。采自其中的样品BQS01为角闪石英二长岩,呈浅红色,半自形粒状结构(图2A),块状构造。该样品主要由碱性长石(45%~50%)、斜长石(30%~35%)、石英(5%~10%)和少量角闪石(5%)等组成,粒径为0.3~3mm,副矿物主要为锆石、磷灰石、磁铁矿等。大安岩体出露于陕西省宁强县大安镇以南铁炉沟一带,紧邻青川—阳平关断裂带及其以东地区,出露面积约15km2。该岩体主要由斜长花岗岩组成,并侵位于碧口群千枚岩和砂岩中。采自该岩体中的样品DA03为斜长花岗岩,呈灰白色,中细粒花岗结构(图2B),块状构造。该样品主要由斜长石(60%~65%)、石英(25%~30%)和少量角闪石(5%)等组成,副矿物主要为锆石、磷灰石、榍石、磁铁矿等。另外,斜长石表面均较脏(图2B),且角闪石存在绿泥石化现象,表明岩石可能经历了后期的改造作用。

图1 碧口块体区域地质简图(据文献[18]修编)Fig.1 Regional geological map of Bikou terrane(modified after references[18])

图2 样品BSQ01(A)和DA03(B)正交偏光镜下照片Fig.2 Photos in thin section of BQS01(A)and DA03(B)

2 分析方法

锆石经过粉碎、淘洗、重液及电磁分选获得精尾,然后在双目镜下人工挑选出锆石。将挑纯的锆石颗粒置于环氧树脂制作的靶圈中,待固结后抛磨至粒径的1/2左右,使锆石内部充分暴露,然后进行锆石显微(反射光和透射光)照相。锆石的阴极发光(CL)图像(图3)在中国地质大学(武汉)地质过程与矿产资源国家重点实验室(GPMR)的JXA-8100电子探针上完成,工作使用的加速电压和电子束电流分别为15kV和20nA。锆石U-Pb年龄(表1)、微量元素(表2)及Lu-Hf同位素(表3)均在西北大学大陆动力学国家重点实验室的LA-ICPMS和LA-MC-ICPMS上分析获得。采用单点原位同时分析锆石U-Pb年龄、微量元素和Hf同位素的方法[32]。激光剥蚀系统(LA)为德国MicroLas公司生产的Geolas200M,该系统由德国Lambda Physik公司的Compex102Excime激光器(工作物质ArF,波长193nm)与 MicroLas公司的光学系统组成。ICPMS为Agilient公司最新一代带有Shield Torch的Agilient 7500a。锆石原位Lu-Hf同位素测定采用Nu Plasma HR(Wrexham,UK)多接收电感耦合等离子质谱仪完成(MC-ICP-MS)。激光剥蚀以氦气作为剥蚀物质的载气,氩气为补偿气,激光束直径为44μm,频率为10Hz,激光能量为90mJ,每个分析点的气体背景采集时间为20~30s,信号采集时间为40s。

锆石U-Pb年龄数据处理时以标准锆石91500为外标进行同位素比值分馏校正,标样GJ-1监控91500数据质量。锆石微量元素含量采用NIST610作外标、Si作内标。锆石测定点的U-Pb同位素比值、表面年龄和U-Th-Pb含量计算采用GLITTER 4.0程序进行处理。普通Pb校正利用Andersen等[33]的方法,并采用Isoplot程序(ver.3.0)[34]进行加权平均年龄的计算及谐和图的绘制。锆石Lu-Hf同位素测定用176Lu/175Lu=0.026 69和176Yb/172Yb=0.588 6[35]进行同量异位干扰校正,计算测定样品的176Lu/177Hf和176Hf/177Hf值。在样品测定期间,对标样91500和GJ-1进行分析,以进行仪器状态监控和对样品进行校正。εHf的计算采用的176Lu衰变常数为1.865×10-11a-1[36],球粒陨石现今的176Hf/177Hf=0.282 772,176Lu/177Hf=0.033 2[37],现今亏损地幔176Hf/177Hf=0.283 25,176Lu/177Hf=0.003 84[38],Hf亏损地幔二阶段模式年龄(tDM2)的计算采用平均地壳176Lu/177Hf=0.015[39]。

3 分析结果

3.1 锆石 U-Pb年龄

白雀寺角闪石英二长岩(BQS01)中锆石为淡黄色透明,颗粒普遍较大,粒径一般为100~200μm,呈自形—半自形柱状。锆石阴极发光(CL)图像(图3A,B,C)显示大部分颗粒内部结构清楚,具有岩浆生长振荡环带。所有26个U-Pb数据都位于谐和线上或谐和线附近(图4),并可分为2组年龄:23个谐和点的206Pb/238U加权平均年龄为(855±6)Ma(2σ,MSWD=2.1);剩余3个谐和点(BQS01-1、BQS01-12-1、BQS01-13-1)给出206Pb/238U 加权平均年龄为(917±14)Ma(2σ,MSWD=1.3)。

大安岩体的斜长花岗岩(DA03)中锆石为淡黄色透明,粒径一般为80~150μm,多数颗粒呈自形—半自形柱状。锆石阴极发光(CL)图像(图3)显示颗粒内部结构比较复杂,大部分具振荡环带(图3E、G、H),常见变质重结晶结构(图3F、H),少数颗粒具核-幔结构(图3I),个别具很薄的变质边(图3G)。17个点分析中有12个分析点在谐和线上(图4),分别给出了(854±10)Ma(n=3,2σ,MSWD=1.8)、(801±7)Ma(n=3,2σ,MSWD=0.41)、(702±10)Ma(n=3,2σ,MSWD=1.12)、(565±6)Ma(DA03-7)、(246±5)Ma(DA03-11)和(207±2)Ma(DA03-15)非常分散的年龄值;其余5个分析点(DA03-1、-3、-4、-10和-17)偏离谐和线但构成不一致线,并给出(850±65)Ma和(164±140)Ma(MSWD=2.6)的上、下交点年龄。

3.2 锆石微量元素特征

对白雀寺角闪石英二长岩(BQS01)中26颗锆石同时进行了微量元素分析,结果表明:锆石的Th/U值较大且分布集中(图5),第一组锆石(855Ma)Th/U 值的变化范围为0.30~0.62,平均值为0.45,第二组锆石(917Ma)Th/U 值变化范围与第一组锆石相似(0.30~0.68),它们也具有相似的稀土配分模式(图6),都表现为轻稀土亏损、重稀土富集的右倾型,并具明显的正Ce异常(δCe=13.2~109.4)和负Eu异常(δEu=0.12~0.20)。

所分析的17颗大安斜长花岗岩(DA03)锆石中,主体年龄为854Ma颗粒的Th/U值较大且分布集中(图5),为0.78~0.92,其他年龄锆石的Th/U值变化范围大(0.28~1.44),并有随年龄变新Th/U值变小的趋势。这些锆石均表现为轻稀土亏损、重稀土富集的右倾型,并具明显的正Ce异常(δCe=1.77~229.8)。除 DA03-1和 DA03-15的负Eu异常特征不明显外,其他颗粒具负Eu异常(δEu=0.20~0.55)(图6)。

3.3 锆石Hf同位素特征

样品BQS01中:年龄为855Ma颗粒的初始值(176Hf/177Hf)i为0.282 345~0.282 532,εHf(t)为3.8~10.4,一阶段 Hf模式年龄(tDM1)和二阶段 Hf模式年龄(tDM2)分别为1.00~1.27Ga(平均1.14 Ga)和1.09~1.51Ga(平均1.32Ga);年龄为917 Ma颗粒的初始值(176Hf/177Hf)i为0.282 256~0.282 427,εHf(t)也都为正值,为2.0~8.1,锆石的tDM1为 1.14~1.40Ga(平 均 1.27Ga),tDM2为1.29~1.67Ga(平均1.48Ga)。

图3 样品BQS01(A,B,C)和DA03(D,E,F,G,H,I)中代表性锆石CL图像Fig.3 Zircon CL images of BQS01(A,B,C)and DA03(D,E,F,G,H,I)

图4 白雀寺岩体BQS01及大安岩体DA03锆石U-Pb年龄谐和图Fig.4 Zircon U-Pb concordant diagram of Baiquesi(BQS01)and Daan pluton(DA03)

图5 锆石206Pb/238 U年龄-Th/U值相关图Fig.5 Zircon 206Pb/238 U age-Th/U ratio diagram

锆石中Hf同位素的组成很稳定,即使在岩浆过程或者高级变质作用中也不会被改造[41],因此,大安岩体中经历后期热事件的锆石(见后述)仍将保存其形成时的Hf同位素组成。大安岩体斜长花岗岩(DA03)中的锆石的176Lu/177Hf值为0.000 314~0.006 306,176Hf/177Hf值为0.281 895~0.282 573,初始值(176Hf/177Hf)i为0.282 093~0.282 562,其εHf(854Ma)变化范围大(-12.6~11.4)。tDM1和tDM2Hf模式年龄分别为0.95~1.90Ga和1.02~2.55Ga。

图6 样品BQS01和DA03锆石稀土配分模式Fig.6 Zircon REE distribution patterns of BQS01and DA03

4 讨论

4.1 岩体侵位年龄的确定

研究显示:利用锆石的形态结构、Th/U值,以及稀土元素配分模式,可以对锆石的成因类型进行较有效的判断[42-43]。本次研究中,白雀寺岩体样品BQS01中锆石具振荡环带结构,Th/U值较大且集中,具轻稀土亏损、重稀土富集、正Ce异常和负Eu异常明显的左倾型稀土配分模式,指示该样品2组年龄的锆石均为岩浆成因,它们的年龄分别是(855±6)Ma和(917±14)Ma。大安岩体样品DA03中获得的锆石年龄数据可分为6组(图4),包括近谐和的(854±10)Ma(n=3,MSWD=1.8),以及谐和的(801±7)Ma(n=3,MSWD=0.41)、(702±10)Ma(n=3,MSWD=1.12)和(565±6)Ma(n=1),及近谐和的(246±5)Ma(n=1)和(207±2)Ma(n=1)。其中5个不谐和的点拟合的不一致线所给出的上交点年龄(850±65)Ma(MSWD=2.6),与第一组年龄一致。854Ma年龄锆石打点位置多在振荡环带上或面状分带中心,并且轻稀土亏损、重稀土富集,正Ce异常和负Eu异常明显(图6),为岩浆成因,其年龄与锆石(850±65)Ma的上交点年龄在误差范围内一致,而且也与白雀寺岩体较年轻一组的年龄(855±6)Ma相当。笔者解释855Ma代表着研究区这2个岩体的侵位结晶年龄,其中白雀寺岩体中的(917±14)Ma可能代表所捕获较早岩浆事件的年龄,而大安岩体中的5组年龄较年轻的锆石形态复杂、Th/U值变化大,可能是岩体后期改造的结果。

叶霖等[23]获得了该块体东部碧口群中铜厂闪长岩体的锆石SHRIMP U-Pb年龄((842±6.5)Ma)。同时Yan等[6]通过对碧口群中火山岩锆石的研究,获得了该群最大的SHRIMP年龄((846±19)Ma)。位于碧口块体西部的松潘—甘孜造山带中同样也发育相似年龄的侵入岩体,如格宗岩体((864±8)Ma[44])。同时期的岩浆活动在扬子板块西缘和北缘也有产出,西缘的有刀关山岩体和桥头岩体,二者的年龄分别为(857±13)Ma[45]和(853±42)Ma[46],北缘的有汉南侵入杂岩中天平河岩体和西乡群火山岩,它们的年龄分别为(863±10)Ma[47]和(845±17)Ma[48]。上述岩体的形成年龄在误差范围内基本一致,这些年龄结果均表明860~850 Ma是碧口块体及其周缘地区的一次重要的岩浆事件。

4.2 花岗质岩体揭示碧口块体存在复杂的演化历史

白雀寺岩体石英二长岩(BQS01)中还存在年龄为(917±14)Ma的岩浆结晶锆石,这一年龄与前面所分析的岩体形成年龄(850~860Ma)相比稍微偏老,记录着该区新元古代早期的一次岩浆事件。这样年龄的结果在该块体其他岩石中也有产出,如:位于该块体西部的南一里花岗闪长岩以及白雀寺岩体围岩碧口火山岩中的捕获晶锆石的年龄为896~954Ma(平均值为925Ma[6,19]);该块体西边的汉南地区西乡群孙家河组火山岩中所捕获岩浆锆石的峰值年龄为925Ma[48];扬子西缘也有相似年龄的岩浆活动(921Ma[49-50])。它们可能反映碧口块体中存在未被剥蚀的新元古代早期的岩浆活动记录。

大安斜长花岗岩侵位结晶后,还可能经历了新元古代中期((801±7)Ma、(702±10)Ma),新元古代晚期((565±6)Ma),早中生代印支期((246±5)Ma、(207±2)Ma)等复杂事件的影响。

新元古代中期(约800Ma)的岩浆热事件在碧口块体及周缘很发育,如白雀寺岩体中存在(810.8±6.9)Ma的角闪石40Ar/39Ar坪年龄[51],其周边围岩碧口群中也存在(790±15)Ma[6]和(811±12)Ma[12]的火山喷发年龄。分隔碧口块体与扬子板块的龙门造山带中也发育有(793±11)Ma的轿子顶岩体[52]。同时期的岩浆活动在扬子板块的西缘也发育,如:Zhou等[44]通过对川西3个康定杂岩样品的研究,确定其形成年龄分别为(797±10)Ma、(795±13)Ma和(796±14)Ma。扬子北缘汉中地区的侵入岩也有对该期热事件的响应,如汉南侵入杂岩中城固斜长花岗岩也存在(796±20)Ma的黑云母40Ar/39Ar年龄[53]。因此,大安岩体中(801±7)Ma可能是对上述岩浆热事件的响应。

Li等[54]认为作为Rodinia超大陆一部分的扬子板块于700Ma左右从超大陆裂解过程中分离出来。该裂解事件在碧口块体也应该有响应,如碧口群中(689±24)Ma侵位的基性岩墙群[6],碧口群北侧的横丹群中大量年龄为700Ma左右的碎屑锆石[7]。此外,在南秦岭造山带东部有701~711Ma

的甘沟岩体[55]、色河岩体[56]和吐雾山岩体[57]的侵位。看来Rodinia超大陆裂解对该块体及其中的花岗岩体(如大安岩体)存在影响。新元古代晚期的年龄在本区及周缘报道相对较少。在全球范围内,600~550Ma是导致东、西冈瓦纳拼合的泛非造山事件的主要活动时期[58-59]。在北秦岭秦岭岩群中确定了(520±23)Ma的晚泛非期构造热事件[60],同时在南秦岭武当群中的基性侵入岩中也存在(579±3)Ma的后期热事件年龄[61]。大安岩体中(565±6)Ma的谐和年龄是否为这一事件晚期记录,仍需进一步的验证。

扬子板块和秦岭微陆块及华北陆块发生大规模碰撞的时代为晚二叠世—早三叠世[62-64],伴随该过程引发了碧口块体及勉略带北侧大量的220~200 Ma的后碰撞花岗岩[17-19,65]。同时大安岩体中存在(221.0±0.5)Ma的黑云母40Ar/39Ar坪年龄[51]。样品DA03中近谐和下交点的年龄((246±5)Ma和(207±2)Ma)可能为这一碰撞事件的年代学记录。

4.3 岩浆源区性质

锆石具有极强的稳定性,使其Hf同位素组成较少受到后期地质事件的影响,并且其具有高的Hf同位素封闭温度和低的Lu/Hf值,使其成为目前探讨地壳演化和示踪岩石源区的重要工具[66]。碧口块体中白雀寺岩体和大安岩体锆石的Hf同位素组成存在明显的差异(图7),可能反映二者源区物质不同。

图7 白雀寺岩体和大安岩体中锆石Hf同位素特征Fig.7 Zircon Hf isotopic composition of Baiquesi and Daan plutons

前人对白雀寺岩体的Nd同位素开展过研究,其中6个样品的εNd(855Ma)较为集中,分布范围为2.5~4.3,平均值为3.5[51];本文获得的该岩体中锆石原位 Hf同位素组成(图7)也为正值,εHf(855 Ma)为3.8~10.4,tDM1为1.00~1.27Ga,平均值为1.14Ga。这样的Hf同位素组成反映的源区性质有2种可能:1)主体来自中元古代的亏损地幔;2)主体来自中元古代的新生陆壳物质。本次研究样品BQS01为石英二长岩,属花岗质岩石,直接来自亏损地幔熔融的可能性很小。对扬子板块西北缘大量新元古代花岗质岩体锆石Hf同位素的研究发现,在该区地壳中存在大量1.0~1.3Ga的新生陆壳[67-74],因此笔者认为该岩体的源区可能与中元古代的新生陆壳有关。而新元古代早期岩浆记录(917 Ma)的εHf(t)为2.0~8.1,tDM1为1.14~1.40Ga,反映该源区最早可能形成于中元古代早期。

大安岩体中报道的Nd同位素研究较少,仅存在1个样品的εNd(854Ma)值为2.7[51]。本次研究所获得该岩体锆石原位Hf同位素组成变化大(图7),反映其来源比较复杂。该样品锆石的εHf(854 Ma)为-12.6~11.4,tDM2为1.02~2.55Ga。上述Hf同位素组成反映源区性质存在2种可能性:1)主体来自中元古代亏损地幔物质与晚太古代地壳物质混合;2)主体来自中元古代新生陆壳物质与晚太古代地壳物质混合。如果考虑扬子西北缘及碧口块体都存在1.0~1.3Ga的新生陆壳物质,那么大安岩体中正的εHf(t)值可能反映源区存在与白雀寺岩体相似的新生陆壳源区,而非常低的Hf同位素比值则可能与其含非常古老的陆壳物质有关。碧口块体中最古老的岩石单元太古代鱼洞子群位于该岩体北侧且相距较近(约100km),该岩石单元中还产出约2.7Ga的鱼洞子花岗岩侵入体[26],同时该岩石单元东侧铜厂岩体中的斜长花岗岩εNd(842Ma)为-10.5~-5.5,其tDM2为2.0~2.4Ga[50],暗示该岩体源区存在早元古代或更老的地壳物质。碧口块体中还存在早元古代的年龄记录,该块体碧口群火山岩中存在1个2.4Ga左右的锆石捕获年龄[12],同时该群西边的横丹群中也存在2.0~2.5Ga(n=7)的碎屑锆石年龄[7],看来碧口块体存在新太古代的古老物质是无疑的。

5 结论

1)碧口块体的白雀寺岩体和大安岩体的形成年龄分别为(855±6)Ma和(854±10)Ma,共同反映碧口块体及周缘经历了860~850Ma一期重要的岩浆活动。

2)碧口块体存在新元古代早期((917±14)Ma)的岩浆记录,还可能经历了新元古代中期((801±7)Ma、(702±10)Ma),新元古代晚期((565±6)Ma),印支期((246±5)Ma、(207±2)Ma)等多期事件的改造。

3)白雀寺岩体和大安岩体的源区性质存在明显的差异,前者来源于中元古代的新生陆壳源区,后者来源于中元古代新生陆壳与晚太古代地壳物质的混合。

4)碧口块体的大陆属性可能最老追溯到新太古代(>2.5Ga),并存在新元古代早期—中元古代的增生改造。

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Zircon U-Pb Ages and Hf Isotope Characteristics of the Granitic Plutons in Bikou Terrane,Northwestern Yangtze Block,and Their Geological Significance

Ping Xianquan,Zheng Jianping,Xiong Qing,Zhang Zhihai,Xia Bing

FacultyofEarthSciences,ChinaUniversityofGeosciences,Wuhan430074,China

The Baiquesi and Daan granitic plutons on the northwestern margin of the Yangtze block have been investigated by zircon U-Pb dating and Hf isotope analysis.Research results indicate that UPb ages of zircons in the Baiquesi pluton can be divided into two groups:(855±6)Ma(n=23)and(917±14)Ma(n=3).TheεHf(t)values of the former are 3.8-10.4,and the one-stage Hf isotope model age varies from 1.00to 1.27Ga;TheεHf(t)values of the latter are 2.0-8.1,and the one-stage Hf isotope model are 1.14-1.40Ga.However,the ages of the Daan pluton can be divided into six groups:(854±10)Ma(n=3),(801±7)Ma(n=3),(702±10)Ma(n=3),(565±6)Ma(n=1),(246±7)Ma(n=1)and(207±2)Ma(n=1).TheεHf(t)values of the Daan pluton are-12.6-11.4(t=854Ma),and the one-stage Hf isotope model ages and the two-stage Hf isotope model ages are 0.95-1.90Ga and 1.02-2.55Ga.The Baiquesi and Daan plutons were emplaced at(855±6)Ma and (854±10)Ma,respectively,which reflect a significant magmatic event in Bikou terrane,northwestern margin of Yangtze block,and the former was mainly derived from the Mesoproterozoic juvenile crustal source,while the latter was products of two end-member mixing magmas respectively sourced from the Mesoproterozoic juvenile crustal and the Neoarchean old crustal.In addition,the(917±14)Ma which is larger than the intruding age of the Baiquesi pluton represents the record of the Early Neoproterozoic magmatism.Ages which are smaller than the intruding age of Daan pluton probably record the subsequent modifications of other thermal events.

Bikou terrane;granitic plutons;zircon U-Pb age;Hf isotope;northwestern of Yangtze block;lithology

10.13278/j.cnki.jjuese.201404113

P588.1

A

平先权,郑建平,熊庆,等.扬子西北缘碧口块体花岗质岩体锆石U-Pb年龄、Hf同位素特征及其地质意义.吉林大学学报:地球科学版,2014,44(4):1200-1218.

10.13278/j.cnki.jjuese.201404113.

Ping Xianquan,Zheng Jianping,Xiong Qing,et al.Zircon U-Pb Ages and Hf Isotope Characteristics of the Granitic Plutons in Bikou Terrane,Northwestern Yangtze Block,and Their Geological Significance.Journal of Jilin University:Earth Science Edition,2014,44(4):1200-1218.doi:10.13278/j.cnki.jjuese.201404113.

2013-09-12

国家自然科学基金项目(91214204)

平先权(1986—,男,博士研究生,主要从事岩石学研究,E-mail:pxqzdx@163.com

郑建平(1964—,男,教授,主要从事岩石学研究,E-mail:jpzheng@cug.edu.cn。

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