柴北缘乌兰地区花岗岩锆石SHRIMP定年及其成因

吴才来, 雷 敏, 吴 迪, 李天啸

1)中国地质科学院地质研究所, 北京 100037; 2)中国地质大学(北京), 北京 100083

柴北缘乌兰地区花岗岩锆石SHRIMP定年及其成因

吴才来1), 雷 敏1), 吴 迪2), 李天啸2)

1)中国地质科学院地质研究所, 北京 100037; 2)中国地质大学(北京), 北京 100083

柴北缘乌兰地区花岗岩锆石SHRIMP U-Pb定年结果表明, 哈德森沟岩体的年龄为(413±3) Ma, 许给沟岩体的年龄为(254±3) Ma, 椅落山岩体的年龄为(251±1) Ma, 察汗诺岩体角闪闪长岩和花岗岩的年龄分别为(249±1) Ma和(248±2) Ma, 察汗河岩体年龄为(240±2) Ma, 晒勒克郭来岩体的花岗闪长岩和花岗岩年龄分别为(250±1) Ma和(244±3) Ma。从年龄上看, 这些花岗岩明显地分为两期: 早期属早泥盆世(年龄为413 Ma), 形成的岩石组合为: 石英二长岩+碱长花岗岩; 晚期属晚二叠世—早三叠世(年龄为254～240 Ma),又可进一步细分为254～251 Ma、250～248 Ma、244～240 Ma三次侵位, 对应的岩石组合为: 闪长岩+花岗闪长岩+花岗岩。岩石地球化学研究表明, 早期花岗岩类不仅富集大离子亲石元素, 而且还富集部分高场强元素(Zr、Y、Nb等), 属A型花岗岩; 晚期花岗岩类富集大离子亲石元素, 亏损高场强元素, 属I型花岗岩。早期花岗岩的87Sr/86Sr比值(0.710 8)和Nd模式年龄(T2DM=2.10 Ga)均高于晚期花岗岩(0.707 6～0.710 7, T2DM=1.41～1.58 Ga), 但晚期花岗岩的εNd(t)值(−11.6)低于早期花岗岩(−4.8 ～ −6.8), 表明早期A型花岗岩可能起源于古元古代的大陆地壳, 而晚期I型花岗岩起源于中元古代地壳。结合区域地质构造特征, 我们认为,早期A型花岗岩的形成与祁连岩石圈拆沉导致欧龙布鲁克陆块北缘减薄、拉伸有关, 也标志着宗雾隆裂陷的开始; 而晚期I型花岗岩类的形成与宗雾隆洋壳向南俯冲于欧龙布鲁克陆块之下有关。

花岗岩; 锆石SHRIMP定年; 欧龙布鲁克陆块; 宗雾隆构造带; 乌兰

柴北缘自1998年发现榴辉岩以来, 一直是地学界研究的热点地区之一(Yang et al., 1998; 杨经绥等, 2000), 许多学者对区内的超高压变质作用开展了深入的研究, 取得了许多重要成果(宋述光和杨经绥, 2001; Song et al., 2003a, b, 2005, 2006, 2009, 2011, 2014a, b; Yang et al., 2005; Zhang et al., 2005, 2008, 2009, 2010, 2014; Mattinson et al., 2006; 宋述光等, 2007, 2009, 2013; Chen et al., 2009; Yu et al., 2012, 2014)。在该区的榴辉岩及其围岩片麻岩中发现了许多超高压变质矿物如柯石英、金刚石和超高压微结构(Liu et al., 1996, 2002; 杨经绥等, 2001; 张建新等, 2002; Song et al., 2005), 因此, 该区成为中国境内继苏鲁—大别之后的又一条超高压变质带(杨经绥等, 2000; 陈丹玲等, 2005)。柴北缘地区呈北西向窄长带状从阿尔金山向东沿伸到鄂拉山, 约800 km(图1),其南北分别被柴达木北缘断裂和中祁连南缘断裂(宗务隆—青海南山断裂)所切, 北西端被阿尔金左行走滑断裂所切, 东南端被哇洪山断裂所切(陆松年等, 2002, 2004; 林慈銮等, 2006)。柴北缘地区以NW向的鱼卡—乌兰断裂为界分为南北两个构造单元,南部构造单元为超高压变质带, 北部构造单元是欧龙布鲁克陆块(青海省地质矿产局, 1991)。近些年,对南部构造单元上的花岗岩做了较多的研究工作,并取得了许多成果, 特别是花岗岩的年代学研究方面取得了长足进展(吴才来等, 2001, 2004, 2007, 2008, 2010, 2014; 袁桂邦等, 2002; Xiao et al., 2004;孟繁聪等, 2005; 卢欣祥等, 2007; Yu et al., 2012; Song et al., 2014a)。然而, 柴北缘北部构造单元上的花岗岩定年及研究工作仍不多见。尤其是和都兰超高压地体相邻的乌兰地区, 出露众多的花岗岩侵入体, 这些花岗岩体的时代及成因以及与都兰地区乃至与柴北缘南部构造单元上花岗岩有何成因联系、所反映的构造信息等问题仍不清楚。因此, 我们完成了乌兰地区花岗岩类的详细地球化学和年代学研究, 试图对上述问题作一探讨。

1 地质背景及岩体地质特征

柴北缘前寒武纪基底由一套中-高级片麻岩、角闪岩和片岩组成, 其上被早古生代奥陶纪火山岩和砂岩、灰岩覆盖, 这是一套大陆活动边缘的海相沉积。晚古生代地层由晚泥盆世陆相碎屑岩、火山岩和石炭纪浅海相沉积岩组成。侏罗纪和白垩纪含煤沉积地层覆盖在晚古生代地层之上(青海省地质矿产局, 1991)。柴北缘北部构造单元沿乌兰—德令哈—欧龙布鲁克—全吉山—达肯大坂山一线分布, 由德令哈杂岩、达肯大坂群和新元古代岩群组成结晶基底, 其上被寒武—奥陶纪稳定的海相沉积岩所覆盖(陆松年等, 2002, 2004; 辛后田等, 2002; 王惠初等, 2006); 南部构造单元沿都兰北部的沙柳河—野马滩—锡铁山—绿梁山—鱼卡—赛什腾山一线分布,是一条早古生代俯冲-碰撞杂岩带, 由含榴辉岩的花岗质片麻岩和早古生代滩间山群岛弧火山岩组成(陆松年等, 2002, 2004; 辛后田等, 2002; 王惠初等, 2006), 其上被泥盆纪磨拉石沉积岩不整合覆盖。柴北缘北构造单元欧龙布鲁克陆块与中南祁连陆块之间是土尔根大坂—宗雾隆—青海南山构造带(简称宗雾隆构造带)(彭渊等, 2016)(图1)。宗务隆构造裂谷带由石炭纪宗务隆群和早—中三叠世郡子河群组成, 两者呈断层接触。宗务隆群包括土尔根大坂组和果可山组, 是一套半深海相复理石沉积和中酸性-基性火山岩系。郡子河群属于浅海相碎屑岩和碳酸盐岩沉积建造(青海省地质矿产局, 1991)。宗务隆构造带的沉积建造向东可延至青海橡皮山一线, 被认为属于华北陆块南缘斜坡相沉积, 并与秦岭—昆仑海盆相邻(郭安林等, 2007, 2009)。

本文研究的花岗岩体主要分布在柴北缘北部构造单元欧龙布鲁克微陆块东部乌兰地区(图1), 与宗雾隆构造带相邻。从东到西分别为许给沟岩体、椅落山岩体、察汗诺岩体、察汗河岩体、哈德森沟岩体和晒勒克郭来岩体(图2)。这些岩体长轴方向为NW向, 部分岩体由若干个NW向延伸的岩体复合而成, 形成不规则状近EW向的岩基, 如椅落山岩体和晒勒克郭来岩体。岩体的岩石类型主要为角闪闪长岩、石英闪长岩、石英二长岩、花岗闪长岩和花岗岩。岩体的围岩主要为晚古生代(C-T2)的灰色片岩、片麻岩、灰白色大理岩、变粒岩及绿色中基性火山岩, 其次有少量的早古生代(∈-O)凝灰岩、安山岩夹片岩、薄层大理岩、白云岩、石英岩、阳起透辉石岩等。

图2 乌兰地区花岗岩分布示意地质图(据1/200 000乌兰幅和天峻幅地质图修编)Fig. 2 Geological sketch map showing distribution of granites in Wulan area (modified after 1/200 000 Geological Map of Wulan Sheet and Tianjun Sheet)

各岩体主要地质特征见表1, 主要岩石类型的岩石学特征见表2。

表1 各岩体地质特征Table 1 Geological characteristics of rock mass

表2 岩石学特征Table 2 Petrologic characteristics of rock mass

图3 锆石CL图像Fig. 3 Cathodoluminescence (CL) images of zircons of the granites from Wulan area

2 锆石SHRIMP定年

8个样品锆石SHRIMP U-Pb定年数据列于表3, 定年方法见吴才来等(2014)。

样品CL05-82取自椅落山岩体花岗闪长岩。该样品的锆石为柱状, 长宽比为1.5∶1～2∶1。CL图像显示, 大多数锆石具有明显的振荡环带, 反映了岩浆结晶锆石的特点(Pidgeon et al., 1998; Corfu et al., 2003; Hoskin and Schaltegger, 2003)。少量的锆石具有老的继承性核, 如锆石6、7号测点(图3)。测定的9颗锆石14个测点得出, U的含量变化于172×10-6到1 190×10-6之间, Th的含量变化于74×10-6到820×10-6之间, Th/U比值除老的继承性锆石核外,其余的均大于0.3(0.33～0.71)(表3)。该样品除老的继承性锆石核(6、7号测点)的年龄为(430.2±3.4) Ma、(405.9±3.2) Ma外, 其余具有环带结构的锆石U-Pb年龄变化于(244.4±1.7) Ma到(267.6±2.9) Ma之间,计算平均年龄为(251.3±1.5) Ma(图4), 代表锆石的结晶年龄(图4)。

续表3

图4 柴北缘东段花岗岩锆石207Pb/206Pb-238U/206Pb谐和图及平均年龄Fig. 4207Pb/206Pb-238U/206Pb concordia diagram and average age of the granites from Wulan area

样品CL05-84取自察汗诺岩体的角闪闪长岩,样品中的锆石大小不一, 但都呈柱状, 长宽比为2:1～3:1。CL图像显示, 锆石内部具有宽条带状、扇状结构, 部分锆石外围具有环带结构(图3)。该样品12颗锆石测得的U、Th含量分别为176×10-6～520×10-6和160×10-6～960×10-6, Th/U比值变化于0.87到1.92之间(表3)。12个分析点得出U-Pb年龄变化于(244.8±2.9) Ma到(251.7±2.2) Ma之间, 得出平均年龄为(249.1±1.3) Ma, 与207Pb/206Pb-238U/206Pb谐和图产生的交点年龄为(248.1±2.4) Ma, 在误差范围内与平均年龄一致(图4)。

样品CL05-85取自察汗诺岩体的花岗岩, 锆石呈柱状, 长宽比为1.5∶1～2∶1。CL图像颜色较深, 锆石具有环带结构和板状结构, 部分含有大小不同的矿物包裹体(图3)。锆石的U、Th含量较高, 变化较大, 分别为309×10-6～1 907×10-6和169×10-6～3 257×10-6, Th/U比值通常大于0.5, 变化于0.52～1.76之间(表3)。12颗锆石分析得出U-Pb年龄变化于(236.4±3.3)～(425.9±3.1) Ma, 其中12号锆石测点为老的继承性核(年龄为(425.9±3.1) Ma), 其余锆石测点得出平均年龄为(247.9±2.1) Ma, 解释为锆石的结晶年龄(图4)。

图5 锆石CL图像Fig. 5 Cathodoluminescence (CL) images of zircons of the granites from Wulan area

图6 柴北缘东段乌兰花岗岩锆石207Pb/206Pb-238U/206Pb谐和图及平均年龄Fig. 6207Pb/206Pb-238U/206Pb concordia diagram and average age of the granites from Wulan area

表4 柴北缘东段乌兰花岗岩类化学成分Table 4 Chemical composition of the granites from Wulan area, eastern segment of North Qaidam

续表4

图7 SiO2-(Na2O+K2O)图Fig. 7 Diagram of SiO2-(Na2O+K2O) for the granites from Wulan area

样品CL05-88取自晒勒克郭来岩体花岗闪长岩,样品中的锆石呈柱状, 长宽比为1∶1～2∶1。CL图像显示出明显的振荡环带(图3)。锆石的U、Th含量分别为231×10-6～554×10-6、120×10-6～438×10-6, Th/U比值大于0.5(变化于0.52～0.95之间)。14颗锆石测年产生206Pb/238U年龄变化于(245.0±2.1) Ma到(254.4±1.9) Ma之间, 平均年龄为(249.9±1.4) Ma(图4)。207Pb/206Pb-238U/206Pb谐和图得出交点年龄为(250.2±1.3) Ma, 与平均年龄在误差范围内一致(图4)。

样品CL05-90取自哈德森沟花岗岩, 锆石为柱状, 长宽比为1∶1到2∶1之间。CL图像颜色较深, 少量的锆石可见同心环带, 多数锆石含有不规则的黑色团块, 可能反映了后期流体沿裂隙的改造(图 5)(Cherniak and Watson, 2000)。16颗锆石分析结果表明, U、Th的含量分别为546×10-6～4 060×10-6、195×10-6～2 329×10-6, Th/U比值变化于0.32到1.89之间,分析得出平均年龄为(412.6±3.2) Ma(图6),207Pb/206Pb-238U/206Pb谐和图得出交点年龄为(412.4±3.5) Ma(图6), 两者非常一致, 代表岩体结晶年龄。

样品CL05-92取自察汗河花岗闪长岩, 锆石呈自形的短柱状, 长宽比为1∶1～1.5∶1。CL图像显示出较好的振荡环带(图5)。U、Th含量分别为131×10-6～644×10-6、108×10-6～530×10-6, Th/U比值大于0.5(变化于0.53到0.91之间)(表3)。除11号锆石为老的捕获锆石(年龄为(2 474.4±26.8) Ma)外,其余12颗锆石测年得出较为一致的结果, U-Pb年龄为(234.9±1.9)～(247.0±1.7) Ma, 平均为(240.3±2.3) Ma (图6),207Pb/206Pb-238U/206Pb谐和图年龄为(242.5±3.2) Ma(图6), 在误差范围内与平均年龄一致, 代表岩体结晶年龄。

样品CL05-93取自晒勒克郭来岩体的花岗岩,样品中锆石为柱状, 锆石长宽比为1.5∶1～2∶1。尽管锆石的CL图像颜色较深, 但部分锆石仍具有明显的振荡环带, 部分锆石核部结构很复杂, 显示出受到流体的改造(图5)。测定结果表明, 锆石的U、Th含量变化较大, 分别为184×10-6～5 733×10-6和102×10-6～16 707×10-6, Th/U比值变化于0.49和3.01之间(表3)。17颗锆石测年得出206Pb/238U年龄变化于(209.6±1.1) Ma到(249.8±0.7) Ma之间, 除去受流体改造的锆石, 得出平均年龄为(243.9±2.9) Ma(图6),207Pb/206Pb-238U/206Pb谐和图得出交点年龄为(236.2±7.8) Ma(图6), 与平均年龄在误差范围内基本一致, 可解释为岩体结晶的年龄(图6)。

图8 SiO2-Fe＊和MALI图(据Frost et al., 2001; Frost and Frost, 2008; 图例同图7 )Fig. 8 Diagrams of SiO2-Fe＊ and SiO2-MALI for the granites from Wulan area (after Frost et al., 2001; Frost and Frost, 2008; symbols as for Fig. 7) Fe*=FeOT/(FeOT+MgO); MALI=Na2O+K2O-CaO

样品CL9956取自许给沟花岗岩, 该样品的锆石呈长柱状, 长宽比在2∶1到3∶1之间。CL图像表明, 大多数锆石具有明显的振荡环带, 少数锆石具有板状结构(图5), 这也是典型的岩浆锆石(Pidgeon et al., 1998)。测试分析表明, 锆石的U、Th含量分别为613×10-6～1 030×10-6、305×10-6～811×10-6, Th/U比值为0.44～0.94(表3)。该样品9颗锆石测年得出的年龄变化于(244.0±6.5)～(270.0±12.0) Ma, 计算平均年龄为(254.2±3.5) Ma, 代表锆石的结晶年龄(图6)。

图9 A/CNK-A/NK图(据Maniar and Piccoli, 1989; 图例同图7 )Fig. 9 Diagram of A/CNK-A/NK for the granites from Wulan area(after Maniar and Piccoli, 1989; symbols as for Fig. 7)

图10 哈克图解(图10 g据Peccerillo and Taylor, 1976; 图例同图7 )Fig. 10 Harker diagrams for the granites from Wulan (Fig. 10g after Peccerillo and Taylor, 1976; symbols as for Fig. 7)

根据上面锆石SHRIMP U-Pb定年结果, 乌兰地区花岗岩类除哈德森沟岩体为(412.6±3.2) Ma(属早泥盆世)外, 其余各岩体的年龄变化于254～240 Ma之间, 属晚二叠—中三叠世。早期岩石组合: 石英二长岩+花岗岩; 晚期花岗岩进一步可划分出三个侵入次序, 即(1)晚二叠(254～251 Ma), 主要为椅落山岩体和许给沟岩体; (2)早三叠(250～248 Ma), 主要为察汗诺岩体和晒勒克郭来西岩体; (3)中三叠(244～240 Ma), 主要为察汗河岩体和晒勒克郭来东岩体。岩石组合分别为: (1)石英闪长岩+花岗闪长岩+花岗岩; (2)角闪闪长岩+花岗闪长岩+花岗岩; (3)花岗闪长岩+花岗岩。

3 岩石地球化学

16个样品分析了主量和微量元素, 定年样品分析了Sr、Nd同位素, 数据分别列于表4、5。全岩及同位素分析方法见吴才来等(2014)。

3.1 主量和微量元素

早期花岗岩类石英二长岩、花岗岩具有60.72～73.37 wt％的SiO2, 13.23～18.07 wt％ Al2O3, 2.08～3.07 wt％ TFeO, 0.37～0.43 wt％ MgO, 0.52～2.2 wt％ CaO, 3.67～6.58 wt％ Na2O, 5.11～5.97 wt％ K2O; 晚期花岗岩类除样品CL05-82-2(角闪闪长岩)外, 其余的具有64.97～77.69 (wt％)的SiO2, 12.30～16.40 wt％ Al2O3, 0.82～4.61 wt％ TFeO, 0.15～2.24 wt％ MgO, 0.56～5.04 wt％ CaO, 2.98～3.65 wt％ Na2O, 1.62～4.79 wt％ K2O。在硅碱图上,早期花岗岩类样品分别落入正长岩区和花岗岩区,晚期的石英闪长岩和花岗闪长岩主要投入花岗闪长岩区, 二长花岗岩和正长花岗岩主要投入花岗岩区,而样品CL05-82-2落入辉长岩区(图7)(Irvine and Baragar, 1971; Middlemost, 1994)。由图7可见, 乌兰地区侵入岩类主要为亚碱性系列岩石, 早期花岗岩类的全碱含量明显高于晚期岩石(图7), 且主要为碱钙性-碱性的Fe质类型(图8)。晚期岩石主要为钙性-钙碱性的Mg质类型, 少量的为Fe质类型(图8)(Frost et al., 2001; Frost and Frost, 2008), 且随着SiO2含量的增加, 岩石由准铝质到弱过铝质(图9)(Maniar and Piccoli, 1989)(表4), 其中, 晚期第一、二次侵位的花岗岩类为准铝质, 第三次侵位的岩石为弱过铝质(图9)。同时, 早、晚两期花岗岩类随硅的增加, K2O由中钾(钙碱性)到高钾(高钾钙碱性)地增加, TiO2、Al2O3、TFeO (FeO+Fe2O3)、MgO、CaO和P2O5表现出规律性地减少, 但Na2O变化规律不同(图10)。早期的Na2O变小, 晚期第一次花岗岩类几乎不变, 其余各次岩石的Na2O增加。

图11 花岗岩类微量元素蛛网图(原始地幔值据McDonough and Sun, 1985; 样品号同表1)Fig. 11 Primitive mantle-normalized trace-element spider diagrams(normalized values after McDonough et al., 1985; sample numbers as for Table 1)

早期的花岗岩类不仅富集大离子亲石元素(LILE)如K、Rb、La和Th, 而且也富集高场强元素(HFS)如Sc、Y、Zr、Hf和Nb(表4), 晚期花岗岩类则富集大离子亲石元素, 相对亏损高场强元素(表 4)。在微量元素蛛网图上, 早期花岗岩类具有非常明显的Ba、Nb、Sr、P和Ti负异常(图11); 晚期花岗岩类具有相似的微量元素原始地幔标准化模型,即Nb、P、Ti具有负异常(图11), 而Sr具有弱的正异常到弱的负异常, 与早期花岗岩具有明显的Sr负异常不同(图11)(McDonough and Sun, 1985)。

图13 花岗岩类稀土元素球粒陨石标准化图(稀土球粒陨石值据Boynton, 1984; 样品号同表3)Fig. 13 Chondrite-normalized REE patterns for the granites (normalized values after Boynton, 1984; sample numbers as for Table 3)

3.2 稀土元素

花岗岩类稀土总量变化于73.35×10-6到740.89×10-6之间(表4), 早期花岗岩类具有最高的稀土总量(480.57×10-6～740.89×10-6), 且随着SiO2含量的增加, 稀土总量降低(表4); 晚期各次花岗岩类稀土总量变化范围分别为: 73.35×10-6～130.94×10-6、87.6×10-6～214.59×10-6、82.01×10-6～203.54×10-6(表4), 第一、二次花岗岩类稀土总量随SiO2增加而升高, 但第三次花岗岩类降低(图12a, 表4)。两期花岗岩类的稀土总量与Zr的含量成正相关, 表明锆石可能是稀土元素的主要载体矿物(图12b, 表4)。所有样品均富集轻稀土元素, LREE/HREE比值变化于5.25～25.27之间(表4)。相对而言, 晚期各次花岗岩类各样品的轻重稀土比值变化较大, 分别为5.25～18.02、6.34～25.27、6.14～14.55, 早期花岗岩类为10.00～12.03。球粒陨石标准化模型表明, 早期花岗岩类具有明显的负Eu异常(δEu=0.11～0.15), 晚期花岗岩类大多数不具有Eu负异常和少数具有弱的负Eu异常, δEu分别为0.63～1.15、0.35～0.95、0.30～0.80。各期次花岗岩类样品具有相似的稀土配分曲线, 且基本平行(Boynton, 1984)(图13)。相比较而言, 各期次岩石样品的轻稀土元素分异明显, 重稀土元素分异不明显,即早期岩石的(La/Sm)N为4.68～5.61, (Gd/Yb)N为1.76～2.62; 晚期各次岩石的(La/Sm)N为2.48～7.21、3.50～9.06、3.75～8.17, (Gd/Yb)N为1.19～2.08、1.19～2.65、0.98～2.01。

3.3 Sr、Nd同位素

选择部分定年样品做全岩Sr、Nd同位素分析,结果见表5。

图14 柴北缘乌兰花岗岩类(87Sr/86Sr)i-εNd(t)图解Fig. 14 Isotopic (87Sr/86Sr)i-εNd(t) diagram of the granites from Wulan area

表5 柴北缘东段乌兰花岗岩类Sr-Nd同位素分析Table 5 Sr-Nd isotopic analyses of the granites from Wulan area in eastern section of Northern Qaidam

图15 花岗岩成因类型判别图解(据Whalen et al., 1987; 图例同图7 )Fig. 15 Discrimination of granite genetic-types (after Whalen et al., 1987; symbols as for Fig. 7)A-A型花岗岩; I, S & M-I型、S型和M型花岗岩; FG-分异的I型花岗岩; OGT-世界I型、S型和M型花岗岩A-A-type granite; I, S & M-I-type, S type and M type granite; FG-fractionated I-type granite; OGT-world I-type, S-type and M-type granite

由表5可见, 乌兰早期花岗岩类比晚期花岗岩类具有较高的(87Sr/86Sr)i、较大的T2DM和较低的εNd(t)值, 早期花岗岩的(87Sr/86Sr)i、T2DM、εNd(t)值分别为0.710 99、2.10 Ga、–11.6, 晚期花岗岩的分别为: 0.707 567～0.710 691、1.41～1.58 Ga、–4.8 ～ –6.8(表5)。图14中, 早期花岗岩落入澳大利亚拉克兰(Lachlan)I型和S型花岗岩区域之下(Keay et al., 1997; Serhat and Goncuoglu, 2007), 但晚期花岗岩类样品落入I型花岗岩区(图14)。

4 讨论

4.1 花岗岩成因类型

研究表明, 乌兰早期花岗岩类(413 Ma)岩石组合为石英二长岩+花岗岩, 这些岩石不仅富集大离子亲石元素, 而且还富集部分高场强元素(Zr、Y、Nb等), 稀土元素配分曲线以明显的负Eu异常为特征(图13a), 具有A型花岗岩的地球化学特征(图15a, b, c, d)。按张旗等(2010)的划分方案, 这组花岗岩类似于华南A型花岗岩, 以低Sr高Yb为特征(图16)。同时, 本期花岗岩类具有较高的10 000×Ga/Al (＞2.6, 2.7～3.6, 平均为3.15)、Zr(277×10-6～380×10-6)和Zr+Nb+Ce+Y(＞350×10-6, 586×10-6～815×10-6, 平均为700.5×10-6)值和较低的MgO(0.37～0.43 wt％)、Ba(151×10-6～250×10-6)和Sr(59.8×10-6～63.0×10-6)的含量, 还具有明显的Eu负异常(0.11～0.15)(Whalen et al., 1987)和明显的Ba、Sr、P、Eu、Ti亏损(表4,图11, 13), 这些都是A型花岗岩的特征。岩石中没有碱性暗色矿物和岩石的A/CNK=0.85～1.06, 表明其属准铝质-铝弱饱和的A型花岗岩(King et al., 1997)。负的Ti、P、Eu异常可能与含Ti矿物相(如钛铁矿和金红石)、磷灰石、斜长石和/或钾长石的分异有关。钾长石的分馏还可能产生Eu、Ba的同时负异常(Wu et al., 2003)。实验岩石学和锆石饱和温度证明(Clemens et al., 1986), A型花岗岩不可能由I型花岗岩分异产生, 因为A型花岗岩需要非常高的温度(Wu et al., 2003)。从花岗岩的Sr、Yb含量来看, 乌兰早期A型花岗岩类似于华南南岭A型花岗岩(图16)。

乌兰晚期花岗岩类岩石组合为闪长岩+花岗闪长岩+二长花岗岩, 其中, 第一次侵入的花岗岩类ASI变化于0.97～1.04之间, 平均为1.0, 第二次的为0.81～1.00, 平均为0.95, 第三次的为1.01～1.08, 平均为10.5, CIPW标准矿物计算结果, 前两次的花岗岩几乎不出现刚玉, 但第三次花岗岩出现0.6％～1.21％的刚玉。可见, 第一、二次花岗岩属准铝质, 而第三次花岗岩属铝弱过饱和型。三次岩石随SiO2含量的增加, P2O5含量明显地呈线性减少,反映了I型花岗质岩浆的演化特点。三次岩石的元素地球化学以富集大离子亲石元素, 亏损高场强元素为特征(图11), 稀土元素以富集轻稀土、且轻稀土分异明显重稀土分异不明显、不具有或具有弱的负Eu异常为特征(图13), 表现出岛弧I型花岗岩类地球化学属性。另外, 三次花岗岩类的微量元素原始地幔标准化曲线(图11)和稀土元素球粒陨石标准化曲线(图13)相同或相似, 表明它们具有相同或相似的物质来源和岩浆演化过程。在图16中, 三次花岗岩样品的Sr、Yb投点位于张旗等(2010)划分的埃达克型、闽浙型、喜马拉雅型和华南型花岗岩的过渡区域, 表现出由埃达克型到闽浙型/喜马拉雅型向华南型过渡的I型花岗岩特征(图16)。此外, 晚期花岗岩类Sr、Nd同位素特征与澳大利亚拉克兰褶皱带I型花岗岩相似(图14), 也表明其具有I型花岗岩的地球化学属性。

图16 花岗岩Yb-Sr图解(据张旗等, 2010; 图例同图7 )Fig. 16 Yb-Sr diagram of granites (after ZHANG et al., 2010; symbols as for Fig. 7)

4.2 花岗质岩浆起源

实验岩石学证明, 在非常宽的温度、压力条件下, 多种源岩的部分熔融均可以产生花岗质熔体(Rapp et al., 1991; Wolf and Wyllie, 1994; Rapp and Watson, 1995; Patino Douce and Johnston, 1996, 1998; Winther, 1996; Skjerlie and Patino Douce, 2002), 熔体成分的变化取决于初始熔融物质的成分、熔融的温度和压力、初始物质的含水量(Jogvan et al., 2006),如泥质的沉积岩部分熔融可以产生强烈富铝和富钾的熔体, 硬砂岩的部分熔融可以产生中等到强烈富铝的花岗闪长岩/花岗岩熔体, 玄武质岩石的部分熔融可以产生云英质-奥长-花岗闪长质熔体(Rapp et al., 1991; Sen and Dunn, 1994; Wolf and Wyllie, 1994; Rapp and Watson, 1995; Winther, 1996)。可见,只要源岩含水或存在含水相的矿物, 部分熔融就可以产生花岗质熔体(Patino Douce and Johnston, 1996, 1998)。研究表明, 柴北缘北部构造单元欧龙布鲁克地块出露的基底变质表壳岩可以划分为2个类型:第I类变质表壳岩的T2DM=2.57～2.83 Ga, εNd(t)=–1.18～2.08, 应属有幔源物质混入的变质陆源沉积岩; 第II类变质表壳岩的T2DM=1.61～2.17 Ga, εNd(t)为高的正值(7.23～15.12) (陈能松等, 2007a, b)。乌兰早期花岗岩类的(87Sr/86Sr)St为0.710 80, (143Nd/144Nd)st为0.511 514, εNd(t)为–11.6, T2DM2.10 Ga, 晚期各次侵位的花岗岩类Sr、Nd同位素比值相似, 即(87Sr/86Sr)St变化于0.707 57～0.710 69,(143Nd/144Nd)st为0.511 965～0.551 207 4, εNd(t)为–4.8～ –0.68, T2DM为1.41～1.58 Ga, 可见, 它们与欧龙布鲁克基底两类表壳岩的Sr、Nd同位素特征不同, 表明它们不可能来自暴露地表的前寒武纪变质岩系的部分熔融。通常认为A型花岗岩在地壳伸展期间,伴随着地幔源岩浆为地壳深熔作用提供热源, 壳源物质部分熔融形成的(Clemens et al., 1986; Ostendorf et al., 2014)。因此, King(1997)认为准铝质A型花岗岩是壳内部分熔融形成的。根据(87Sr/86Sr)St、(143Nd/144Nd)St同位素值和T2DM年龄, 结合它们的岩石地球化学特征, 我们认为, 乌兰早期的花岗岩类可能起源于古元古代的陆壳物质, 而晚期的花岗岩类起源于中元古代的陆壳物质, 并可能混合了幔源成分。

4.3 花岗岩形成的构造环境

宗雾隆构造带北边以青海南山断裂为界与中南祁连地块相隔, 南边以宗雾隆南缘断裂为界与柴北缘欧龙布鲁克地块相邻, 向西延至阿尔金断裂,向东分离西秦岭与南祁连造山带(图1)。宗雾隆构造带与鄂拉山构造带地质特征及演化过程十分相似,可能是由于西秦岭沿共和坳拉谷强烈斜向碰撞柴达木—欧龙布鲁克地块, 造成了印支期宗务隆构造带东段造山隆升及强烈的岩浆活动(彭渊等, 2016)。柴达木东缘花岗岩浆-火山活动带称为鄂拉山构造带,该构造带上苦海—赛什塘蛇绿构造混杂岩带中玄武岩的40Ar/39Ar年龄为(368.6±1.4) Ma(张智勇等, 2004), 说明洋盆从晚泥盆世—早石炭世开始打开,到中石炭世—早二叠世形成了有限的洋盆, 乌兰地区欧龙布鲁克陆块北部边缘泥盆纪A型花岗岩的出现, 是对这一构造事件的响应。受西秦岭向西挤出以及华南地块向北俯冲碰撞的共同影响, 晚二叠世—中三叠世洋盆向西斜向俯冲, 洋盆闭合收缩(孙延贵, 2004), 形成了鄂拉山构造带上年龄为220～200 Ma(Rb-Sr、K-Ar、U-Pb)的岩体(孙延贵等, 2001; 孙延贵, 2004; 李玉晔, 2008; 李永祥等, 2011)。鄂拉山构造岩浆带岩浆岩主体属于高钾钙碱性花岗闪长岩, 形成于板块碰撞及碰撞后阶段, 是西秦岭地块沿共和坳拉谷向柴达木地块下斜向强烈俯冲碰撞的产物(张森琦等, 2000; 孙延贵等, 2001;孙延贵, 2004; 李玉晔, 2008)。对比宗务隆构造带与鄂拉山构造带内侵入岩特征, 两者具有相同的岩浆岩类型, 相似的构造成因, 均为印支期构造岩浆活动的产物。

除本文报道的两期花岗岩外, 前人也报道过宗雾隆构造带上天峻南山花岗岩、青海湖南花岗岩、二郎洞二长花岗岩的锆石U-Pb年龄均为印支期,加上天峻南山果可山组超镁铁质-镁铁质蛇绿岩地体(Rb-Sr年龄(318±3) Ma)的发现(王毅智等, 2001)以及天峻南山等岛弧型高钾钙碱性I型花岗岩的产出(郭安林等, 2009), 认为宗雾隆构造带是一条具有完整构造旋回的印支期造山带(王毅智等, 2001; 郭安林等, 2009)。

图17 花岗岩类La/Yb-Th/Yb构造环境判别图解(据Condie, 1989; 图例同图7 )Fig. 17 Geochemical compositions of two episodes of the granites from Wulan area plotted in the tectonic setting discrimination diagrams (after Condie, 1989; symbols as for Fig. 7)

Gorton和Schandl(2000)收集了世界上26个不同地方的花岗岩和中酸性火山岩的地球化学资料,利用不相容元素Ta、Th和Yb的丰度和比值, 有效地区分出大洋岛弧、活动大陆边缘和板内火山岩带三种不同的构造环境。其中板内火山岩带的资料来自冰岛、埃塞俄比亚和新墨西哥的瓦勒斯火山, 大陆活动边缘的有希腊、智利、阿根廷、日本、墨西哥、阿拉斯加和汤加—克马德克及伊豆小笠原弧(Tonga–Kermadec and Izu–Bonin arcs), 大洋岛弧的有吕宋岛(Luzon arc)。三种构造环境中火成岩的Th逐步富集主要归因于弧的成分增加, Th/Ta比值1～6是板内火山岩带, 6～20是活动大陆边缘, ＞20～90的是大洋岛弧(Gorton and Schandl, 2000)。乌兰地区早期花岗岩类的Th/Ta比值为27.1～29.4, 平均为28.25,晚期的变化较大, 为7.5～51.4, 平均15.17, 可见,本区早期A型花岗岩类Th/Ta比值高于活动大陆边缘火成岩, 而和大洋岛弧区火成岩的相似, 这可能与花岗岩产出的位置和源岩有关。从图2可以看出,该A型花岗岩产在欧龙布鲁克陆块的北部边缘, 哇洪山左行走滑断裂穿过岩体, 可能是该断裂的走滑拉分作用, 导致混入了洋壳成分的古元古代陆壳发生部分熔融, 形成了类似大洋岛弧火成岩Th/Ta比值的A型花岗岩。晚期花岗岩类除个别样品外, 所有样品的Th/Ta比值落入活动大陆边缘区的范围内。从Th/Yb-La/Yb图解(Condie, 1989)来看, 本区两期花岗岩类样品投点主要落在大陆边缘弧的范围内(图17), 也说明两期花岗质岩浆活动发生在活动大陆边缘, 在Muller和Groves(1994)的图解上, 本区早期花岗岩类落入板内区(图18a), 晚期的花岗岩类落入与弧相关的区域内; 在图18b上, 两期次花岗岩类的样品投点均落入与弧相关的大陆和碰撞后区域(图18b); 在Gorton和Schandl(2000)图解中,两期花岗岩投点均落入大洋弧和活动大陆边缘区域(图18c), 反映了两期岩浆作用的构造环境与活动大陆边缘相关。这与两期花岗岩体分布在欧龙布鲁克微陆块北部边缘的地质事实相吻合。

图18 Y-Zr (a)、Zr/Al2O3-TiO2/Al2O3 (b)和Th/Yb-Ta/Yb (c)构造判别图解(据Muller and Groves, 1994; Gorton and Schandl, 2000; 图例同图7 )Fig. 18 (a) Y-Zr, (b) Zr/Al2O3-TiO2/Al2O3and (c) Th/Yb-Ta/Yb geotectonic discrimination diagrams (after Muller and Groves, 1994; Gorton and Schandl, 2000; symbols as for Fig. 7)

综上所述, 乌兰地区泥盆纪A型花岗岩的出现,标志着欧龙布鲁克北缘宗雾隆裂谷作用的开始, 到晚石炭世(Rb-Sr年龄(318±3) Ma)开始出现宗雾隆洋盆(王毅智等, 2001), 晚二叠世—中三叠世洋壳向南俯冲, 形成一系列中酸性火山岩和青海湖南山及天峻南山花岗岩为代表的岛弧地体, 晚三叠世洋壳闭合进入陆内碰撞造山期(郭安林等, 2009; 彭渊等, 2016)。

5 结论

(1)柴北缘东段乌兰地区早期的哈德森沟花岗岩锆石SHRIMP U-Pb年龄为(413±3) Ma, 乌兰晚期的许给沟岩体的年龄为(254±3) Ma、椅落山岩体为(251±1) Ma、察汗诺岩体为(249±1) Ma、(248±2) Ma,晒勒克郭来岩体为(250±1) Ma、(244±3) Ma, 察汗河岩体为(240±2) Ma。

(2)乌兰地区早期花岗岩类(413 Ma)岩石不仅富集大离子亲石元素, 而且还富集部分高场强元素(Zr、Y、Nb等), 稀土元素配分曲线以明显的负Eu异常为特征, 同时, 岩石具有较高的10 000×Ga/Al比值和较低的MgO、Ba和Sr的含量, 属A型花岗岩。晚期花岗岩类以富集大离子亲石元素, 亏损高场强元素为特征, 稀土元素以富集轻稀土、且轻稀土分异明显重稀土分异不明显、不具有或具有弱的负Eu异常为特征, 属岛弧I型花岗岩。

(3)同位素研究表明, 乌兰早期A型花岗岩类起源于古元古代陆壳物质的部分熔融, 与祁连岩石圈拆沉导致欧龙布鲁克陆块北缘减薄、拉伸有关, 它的产出标志着欧龙布鲁克北缘裂陷的开始; 而晚期具有大陆活动边缘I型花岗岩类起源于中元古代陆壳物质的部分熔融, 并可能有幔源物质的加入, 其成因与宗雾隆洋壳俯冲于欧龙布鲁克陆块之下有关。

Acknowledgements:

This study was supported by China Geological Survey (Nos. 121201102000150005-06, 12120115027001 and 12120114079901), National Natural Science Foundation of China (Nos. 41472063, 40921001, 40472034 and 40672049), and the Science and Technology Project (No. Sino Probe 05-05).

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Zircon SHRIMP Dating and Genesis of Granites in Wulan Area of Northern Qaidam

WU Cai-lai1), LEI Min1), WU Di2), LI Tian-xiao2)
1) Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037; 2) China University of Geosciences (Beijing), Beijing 100083

Zircon SHRIMP U-Pb dating of granites in Wulan area of northern Qaidam indicates that Hadesengou rock mass was formed at (413±3) Ma, Xugeigou rock mass at (254±3) Ma, Yiluoshan rock mass at (251±1) Ma, hornblende diorite and granite of Chahanruo rock mass at (249±1) Ma and (248±2) Ma, Chahanhe rock mass at (240±2) Ma, granodiorite and granite of Shailekeguo rock mass at (250±1) Ma and (244±3) Ma respectively. These granites have two formation periods: 1) the early period belongs to Early Devonian (413 Ma) with the rock association being adamellite+alkali-feldspar granite; 2) the late period belongs to Late Permian–Early Triassic (254～240 Ma) which can be further divided into three emplacements (254～251 Ma, 250～248 Ma and 244～240 Ma) with the rock association being diorite+granodiorite+granite. Geochemical study indicates that the early granitoids are not only enriched in large ion lithophile elements but also enriched in some high field-strength elements (Zr, Y, Nb etc.), thus belonging to A-type granite; the late granitoids are enriched in large ion lithophile elements and depleted in high field-strength elements, thus belonging to I-type granite.87Sr/86Sr ratio (0.710 8) and Nd model age (T2DM=2.10 Ga) of the early granite are both higher than those of the late granite(0.707 6～0.710 7, T2DM=1.41～1.58 Ga). Nevertheless, εNd(t) of the late granite (−11.6) is lower than that of the early granite (−4.8 ～ −6.8). These data show that the early A-type granite might have originated from Paleoproterozoic continental crust, whereas the late I-type granite originated from Mesoproterozoic crust. Combined with regional geological structural characteristics, the authors consider that the formation of early A-type granite was related to the thinning and stretching of north Oulongbuluke block caused by Qilian lithosphere delamination which also marked the beginning of Zongwulong rift, while the formation of late I-type granite was related to the southward subduction of Zongwulong oceanic crust beneath Oulongbuluke block.

granite; zircon SHRIMP dating; Oulongbuluke block; Zongwulong tectonic zone; Wulan

P588.121; P597.1

A

10.3975/cagsb.2016.04.11

本文由中国地质调查局项目(编号: 121201102000150005-06; 12120115027001; 12120114079901)、国家自然科学基金项目(编号: 41472063; 40921001; 40472034; 40672049)和国家专项(编号: Sino Probe 05-05)联合资助。

2016-05-06; 改回日期: 2016-06-26。责任编辑: 闫立娟。

吴才来, 男, 1960年生。博士, 研究员, 博士生导师。主要从事火成岩岩石学及其成矿研究。E-mail: wucailai@126.com。