2020, Vol. 36
塔里木克拉通是中国最重要的三大克拉通之一，但是由于沙漠覆盖和研究程度低，一直被认为是具有统一前寒武纪基底的克拉通(辛后田等, 2013)。然而，随着对塔里木克拉通前寒武纪基底研究的深入，越来越多的年代学及岩石学证据显示塔里木克拉通早前寒武纪基底是由两个或多个独立的块体组成。有学者通过对塔中钻孔岩芯研究认为塔里木克拉通前寒武纪基底分为南北两个块体(Yang et al., 2018)，且这两个块体最终沿中央缝合带于新元古代拼合在一起(Xu et al., 2013)。也有学者通过对比塔里木周缘古-中元古代变质事件和岩浆活动提出塔里木克拉通前寒武纪基底可能由库鲁克塔格-敦煌地体、阿克塔什塔格地体、塔里木西南地体和阿克苏地体组成(张健等, 2014; Ye et al., 2016; Zhang et al., 2014)。显然，这一认识目前还存在争议。
太古宙TTG岩石广泛分布于各个古老的克拉通(张旗和翟明国, 2012)，如西加拿大地盾(Sandeman et al., 2006)、苏必利尔克拉通(Henry et al., 2000)、波罗的地盾(Samsonov et al., 2005)、西格林兰克拉通(Polat et al., 2008)、华北克拉通(万渝生等, 2017)、南非和津巴布韦克拉通等(Kröner et al., 1999)。年代学研究表明，全球大多数TTG岩石形成于2.9~2.7Ga，其主要来源于基性岩石部分熔融，并有不同程度地幔橄榄岩的加入(Zhai and Santosh, 2011)。同时，这一时代也是大陆地壳增生的主要阶段(Condie, 1998; Zhai and Santosh, 2011)。因此，太古宙TTG岩石作为克拉通变质基底的重要组成部分(翟明国, 2017)。它不仅为地球早期提供了陆壳物质，还记录了地球早期构造环境和地壳演化的重要信息(Jahn et al., 1981; Moyen and Martin, 2012)。所以，对太古宙TTG岩石的成因研究具有非常重要的意义。另外，太古宙TTG岩石作为花岗岩类富含锆石等副矿物，由于锆石具有良好的稳定性和抗风化等特点，使其能够很好地记录和保存早期地质体经历的构造热事件(Wu and Zheng, 2004)。所以对太古宙TTG进行精细的年代学研究同样可以为区域构造演化提供非常重要的信息(Zong et al., 2013)。
本文通过对塔里木克拉通东南阿尔金北缘基底岩石(TTG片麻岩)进行锆石SHRIMP U-Pb定年、全岩主微量元素及同位素分析，结合前人研究资料，探讨阿尔金北缘TTG片麻岩的成因，揭示塔里木克拉通周缘块体的构造演化和亲缘性。
1 区域地质背景与样品采集塔里木克拉通位于中国西北地区，是中国最重要的三个克拉通之一，也是中国最主要的前寒武纪克拉通(图 1a, Zhao and Cawood, 2012; Zhang et al., 2013a, b ; Zhu et al., 2020)。其面积将近60万平方千米，大部分被沙漠覆盖(大于85%)(Xu et al., 2013; Zhu et al., 2013, 2014, 2017)。因此，塔里木前寒武纪基底主要出露于塔里木盆地周缘地区，主要有北缘的库鲁克塔格地区，西南的铁克里克地区以及东南的敦煌-阿尔金北缘地区(图 1b)。
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图 1 研究区地质简图 (a)中亚造山带构造图；(b)塔里木克拉通前寒武纪岩石分布图(据Lu et al., 2008修改)；(c)阿尔金北缘-敦煌地区地质简图(据梅华林等, 1997修改)；(d)阿尔金北缘地区地质简图 Fig. 1 Simplified geological maps of studied area (a) simplified tectonic map of Central Asian Orogenic Belt (CAOB) showing the location of the Tarim Craton; (b) geological map of the Tarim Craton showing the distribution of the Precambrian rocks (modified after Lu et al., 2008); (c) simplified geological map of the North Altyn Tagh-Dunhuang area (modified after Mei et al., 1997); (d) geological map of the North Altyn Tagh area |
敦煌-阿尔金北缘地区位于星星峡-且末断裂、北山造山带和阿尔金断裂所夹持的区域(图 1b)，即前人所说的阿北地块(陆松年和袁桂邦, 2003;辛后田等, 2012)或者敦煌地块(甘肃省地质矿产局, 1989;梅华林等, 1997, 1998;辛后田等, 2013;许志琴等, 1999;张建新等, 2011)。前寒武纪岩石主要分布在敦煌地区(东巴兔山、三危山、红柳峡以及多坝沟)和阿尔金北缘地区(喀腊大湾和阿克塔什塔格)(Zhao et al., 2015;甘肃省地质矿产局, 1989;李志琛, 1994)(图 1c)。该区早前寒武纪基底主要由TTG片麻岩、基性斜长角闪岩、基性麻粒岩、具有“孔兹岩系”特征的敦煌群和米兰群组成(甘肃省地质矿产局, 1989;刘永顺等, 2009;梅华林等, 1997;于海峰等, 1998)。
敦煌地区TTG片麻岩的原岩的形成时代主要集中在2.5~2.7Ga(Wang et al., 2014; Yu et al., 2014; Zhang et al., 2012a, 2013b; Zhao et al., 2015; Zong et al., 2013;梅华林等, 1997, 1998;王忠梅等, 2013;张建新等, 2011;赵燕等, 2013, 2015)，并且在TTG片麻岩中普遍记1.85~1.80Ga的变质年龄(Zhang et al., 2012a, 2013b; Zhao et al., 2015)。另外，该地区的基性麻粒岩也记录了~1.84Ga的高压变质事件(Zhang et al., 2012a)。此外，原岩年龄为~1.83Ga岛弧拉斑玄武岩遭受了低麻粒岩-高角闪岩相变质形成含石榴石斜长角闪岩(Wang et al., 2014;王忠梅, 2013)。基性麻粒岩相对较高的变质压力以及顺时针P-T演化轨迹都指示了敦煌地区在1.84~1.83Ga时处于碰撞造山环境(Zhang et al., 2012b; Wang et al., 2014)。直到1.77Ga出现A型花岗岩可能发生了由碰撞挤压向伸展拉张的构造转换(Yu et al., 2014)。之后，约1.61Ga时板内岩浆活动开始发育并形成了具有OIB特征的玄武岩(现为角闪岩)，此时敦煌地区已处于板内稳定环境(Wang et al., 2014;王忠梅, 2013)。
阿尔金北缘地区的早前寒武纪研究相对较早，积累了一系列重要的年代学资料。研究显示阿尔金北缘地区TTG片麻岩的形成于2.5~2.8Ga(Long et al., 2014; Lu et al., 2008; Zhang et al., 2014)，并在奥长花岗片麻岩中发现了~3.6Ga的继承锆石。同时TTG片麻岩的Hf同位素证据也都显示该区存在太古代地壳可能性(Long et al., 2014; Lu, 2001; Lu et al., 2008;李惠民等, 2001)。值得一提的是，近来有学者在阿克塔克塔格地区发现了~3.7Ga的英云闪长片麻岩(Ge et al., 2018, 2020)，这是塔里木克拉通迄今发现的最老的岩石。阿尔金北缘地区发育一套2.03~2.01Ga具有的岛弧特征的片麻状花岗岩和片麻状辉长岩，证实该地区古元古代中晚期仍处于俯冲构造环境(Zhang et al., 2014;辛后田等, 2011)。此后，该地区普遍遭受了~2.0Ga变质事件(Long et al., 2014; Zhang et al., 2014;辛后田等, 2012)。且这一变质事件同样被石榴角闪岩(Zhang et al., 2019)和麻粒岩所记录(Wu et al., 2019)。这都表明~2.0Ga左右阿尔金北缘地区已进入碰撞造山阶段(Zhang et al., 2014;辛后田等, 2012)。随后，~1.85Ga具有OIB地球化学特征的基性岩墙侵入到TTG片麻岩和古元古代片麻状花岗岩中，初步研究显示他们可能与地幔柱岩浆活动或者板内伸展环境有关(Zhang et al., 2014)。
本次研究的样品主要采集于阿尔金北缘西部喀腊大湾地区，采样位置见图 1d(GPS: 39°11′02.6″N、91°40′41.2″E)。所采岩石在该区域分布较广，露头上可见被未变形和未变质的基性岩墙侵入(图 2a, b)。本文针对TTG片麻岩选取1个年龄样品及5个地球化学样品。TTG岩石呈灰白色，具有典型的片麻状构造(图 2c)，露头局部发育左形剪切韧性断层(图 2d)。岩石主要由斜长石(45%~55%)、石英(15%~25%)、钾长石(5%~10%)、黑云母(10%~15%)和角闪石(5%~10%)组成，其中角闪石和黑云母遭受了不同程度的蚀变作用(图 2e-g)。此外，部分矿物还可见明显的增生边(图 2h)。
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图 2 阿尔金北缘喀腊大湾地区典型的野外照片和镜下照片 (a、b)新太古代TTG片麻岩被未变形基性岩墙侵入；(c、d)新太古代TTG片麻岩的露头特征；(e-h)新太古代TTG片麻岩的镜下照片.Amp-角闪石；Bi-黑云母；Kfs-钾长石；Pl-斜长石；Q-石英 Fig. 2 Representative field photos and photomicrographs showing the Precambrian rocks in Kaladawan, North Altyn Tagh area (a, b) Neoarchean TTG gneiss intruded by undeformed mafic dykes; (c, d) Neoarchean TTG gneiss show typical gneissic structure; (e-h) representative photomicrographs of the Neoarchean tonalitic gneisses. Amp-amphibole; Bi-biotite; Kfs-K-feldspar; Pl-plagioclase; Q-quartz |
本研究用于SHRIMP U-Pb定年的锆石颗粒来源于17ALT06样品。首先对该岩石样品进行粗碎，再通过重液和磁选的方法分选出锆石颗粒，然后在双目镜下将具代表性的锆石颗粒和锆石标样(TEMORA)一起黏贴在环氧树脂表面并抛光至露出锆石颗粒中心。通过对锆石靶进行透射光、反射光显微照相和阴极发光(CL)照相检查锆石的外部和内部结构。锆石U-Pb定年在北京离子探针中心SHRIMP Ⅱ上完成，详细的分析测试方法参见Williams (1998)。分析过程中仪器使用的一次流(O2-)强度为3~5nA，束斑直径为25μm。在锆石样品分析之前先对其扫描120秒，以去除表面杂质，然后对分析点进行5组扫描和数据采集。在分析过程中所用标样为M257(U=840×10-6, Nasdala et al., 2008)和TEMORA(417Ma, Black et al., 2003)，分别用于锆石U含量和U-Pb年龄校正。每测试3~4个样品点测试1次标样TEMORA，检验U-Pb定年数据质量。SHRIMP U-Pb年龄数据采用SQIUD和ISOPLOT程序(Ludwig, 2001, 2003)进行处理和作图。同位素比值和单点的年龄误差均为1σ，加权平均年龄误差的置信度为95%。
2.2 全岩主微量元素分析全岩主量元素采用X荧光光谱分析方法在中国科学院地球化学研究所矿床地球化学国家重点实验室完成，测试仪器为Rigaku ZSK 100e型荧光光谱仪。样品处理流程类似于Li et al. (2000)，将0.5g样品和4g Li2B4O7混合均匀后倒入铂金坩埚并加入适量脱模剂溴化锂和氧化剂硝酸锂，置于1200℃高温熔融。待熔融完成后取出倒入铂金磨具中冷却成玻璃片以后进行XRF测试。样品的烧失量(LOI)为将干燥的样品在1000℃下灼烧1小时所损失的重量百分率。分析精度优于5%。全岩微量元素分析在中国科学院地球化学研究所矿床地球化学国家重点实验室采用电感耦合等离子质谱仪(ICP-MS)完成，分析仪器为Perkin-Elmer Sciex ELAN DRC-e ICP-MS，分析测试过程参见Qi et al. (2000)。样品前处理流程为：准确称取200目或以下的样品粉末50mg加入特氟龙闷罐中，加入1mL HF，在电热板上蒸干以赶去SiO2，然后加入1mL HF和0.5mL HNO3，加盖并放入不锈钢外套中密封置于烘箱中于200℃下消解48小时。取出待冷却后于电热板上蒸干，加入1mL HNO3蒸干并重复一次。加入2mL HNO3和5mL蒸馏水重新置于烘箱中130℃溶解残渣8小时。完成后取出冷却，加入500ng Rh内标溶液并转移至50mL离心管中待测。测试中采用国际标样GBPG-1、OU-6和国家标样GSR-1和GSR-3进行分析质量控制，分析精度优于10%。
2.3 全岩Sr-Nd同位素分析样品的Sr-Nd同位素的化学分离和测试均在天津地质矿产研究所实验室完成。分析流程为：称取200目或以下的样品粉末约100mg于特氟龙闷罐中，分别加人纯化的2mL HF、0.3mL HClO4和1mL HNO3于120℃熔样一周，蒸干，加入6mL 6N HCl再蒸干，然后加入纯化的1mL 2.5N HCL并转移至离心管中静置过夜，离心后取清液置于Rb-Sr阳离子交换柱(AG50w×12)分离出Sr和稀土元素，然后将接收的稀土溶液加入到Sm-Nd交换柱(P507)中分离Sm和Nd，蒸干后点样待测。测试仪器为Thermo Fisher公司生产的Triton型的热电离质谱仪(TIMS)。Nd同位素的质量分馏用146Nd/144Nd=0.7219进行校正。本次测试中实测的USGS标样BCR-2的143Nd/144Nd平均值为0.512641±0.000004(2σ)，87Sr/86Sr平均值为0.704985±0.000006 (2σ)。
3 分析结果 3.1 锆石SHRIMP U-Pb年龄英云闪长片麻岩中的锆石颗粒多数呈自形，长度达100~150μm，长宽比为2:1至3:1(图 3)。多数锆石颗粒呈粉红色、棱柱状，阴极发光显示几乎全部锆石具有清晰的核-边结构。CL图像显示锆石核部为灰色，且具有典型岩浆锆石的振荡环带；边部白色，为典型的变质增生边。对15粒锆石的19个点进行SHRIMP U-Pb同位素分析，其中核部13个点，边部6个点(表 1)。这些核部锆石的U含量变化较大，为75×10-6~1142×10-6，Th含量为77×10-6~603×10-6，Th/U比值为0.22~1.06；其中9个点的206Pb/238U和207Pb/235U年龄在误差范围内谐和度超过90%(表 1)；另外4个点由于Pb的丢失或获得分别落在了谐和线的下方和上方(图 4a)，这一现象在太古宙TTG片麻岩中很常见(Corfu, 2013)。这13个点构成了一条不一致线，其交点年龄为2733±19Ma(MSWD=5.5)，除谐和度最差的8.1和12.1外，其余11个点的207Pb/206Pb年龄的加权平均值为2740±19Ma(MSWD=10.2, n=11，图 4b)。因此，~2750Ma可以作为该英云闪长片麻岩的形成年龄。变质增生边6个点的U与Th的含量很低，分别为8×10-6~15×10-6和6×10-6~20×10-6，Th/U比值为0.66~1.32(表 1)。这些边部年龄可以分为两组：年龄较大的一组共有4个点，它们的206Pb/238U和207Pb/235U年龄在误差范围内谐和(图 4a)，其交点年龄为2512±110Ma(MSWD = 0.67)，207Pb/206Pb加权平均年龄为2494±53Ma(MSWD=0.52, n=4，图 4c)；年龄较小的一组共有2个点，其206Pb/238U和207Pb/235U年龄均在谐和线附近，交点年龄和207Pb/206Pb加权平均年龄分别为1964±82Ma和1962±78Ma。
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图 3 典型锆石CL图像圆圈代表分析点位和离子探针束斑大小，对应的年龄(Ma)已标出 Fig. 3 CL images of representative zircons Analytical spots and ages in Ma are shown |
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表 1 阿尔金北缘新太古代TTG片麻岩(17ALT06)锆石SHRIMP U-Pb年龄结果 Table 1 Zircon SHRIMP U-Pb isotopic analyses for Neoarchean TTG gneiss (17ALT06) from the North Altyn Tagh area |
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图 4 阿尔金北缘新太古代TTG片麻岩锆石U-Pb谐和图 Fig. 4 Concordia plots of U-Pb zircon data for zircons from the Neoarchean gneiss in the North Altyn Tagh area |
样品的主微量元素分析结果见表 2。TTG片麻岩具有富SiO2(62.96%~66.99%)、中等K2O(1.18%~2.88%)和高Na2O(3.99%~4.40%)的特点(表 2)。在TAS图中，所有样品均落在花岗闪长岩靠近英云闪长岩的区域内(图 5a, LeMaitre, 1989)，属于钙碱性岩石系列(图 5b)。与晚太古代TTG平均成分(Condie, 2005)相比，这些样品的Al2O3(14.82%~15.71%)、CaO(2.97%~4.03%)和MgO(1.33%~3.08%)更高(图 5c)。在标准化An-Ab-Or图中，所有样品都投在英云闪长岩的区域内，与前人的样品一起构成典型的TTG岩石组合(图 5d)。
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表 2 阿尔金北缘新太古代TTG片麻岩主量元素(wt%)与微量元素(×10-6)地球化学数据 Table 2 Major (wt%) and trace (×10-6) elements of the Neoarchean TTG gneisses from the North Altyn Tagh aera |
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图 5 阿尔金北缘新太古代TTG片麻岩的地球化学判别图 (a)硅碱图(Middlemost, 1994)，碱性与亚碱性的分界线来源于Irvine and Baragar (1971)；(b) SiO2-K2O图(据Le Maitre, 1989修改)；(c) SiO2-MgO图(据Xiong et al., 2014修改)；(d)标准矿物分类图(据O’Connor, 1956修改).阿尔金北缘TTG数据来源于Long et al. (2014)和Zhang et al. (2014);敦煌TTG数据来源于Zhang et al. (2013b)和Zong et al. (2013);库鲁克塔格TTG数据来源于Long et al. (2010)和Zhang et al. (2012a); 图 6、图 8数据来源同此图 Fig. 5 Geochemical discrimination diagrams for the Neoarchean TTG gneisses in the North Altyn Tagh area (a) SiO2 vs. total alkali (Na2O + K2O) content diagram (Middlemost, 1994), and alkaline and subalkaline division is after Irvine and Baragar (1971); (b) SiO2 vs. K2O diagram (after LeMaitre, 1989); (c) SiO2 vs. MgO diagram (modified after Xiong et al., 2014); (d) normative feldspar classification (after O'Connor, 1965). Data for North Altyn Tagh TTG from Long et al. (2014) and Zhang et al. (2014); data for Dunhuang TTG from Zhang et al. (2013b) and Zong et al. (2013); data for Kuluketage TTG from Long et al. (2010) and Zhang et al. (2012a); also in Fig. 6 and Fig. 8 |
上述TTG片麻岩具较低的REE总量(85×10-6~272×10-6)，且在球粒陨石标准化的稀土元素图解上表现为轻稀土强烈富集((La/Yb)N=25~49)的配分型式，除17ALT06为Eu正异常(Eu/Eu*=1.75)外，其余样品均表现出不明显的Eu异常(Eu/Eu*=0.77~0.99)(图 6a)。此外，这些岩石样品具有高Sr (469×10-6~764×10-6)、低Y (4.72×10-6~13.5×10-6)和Yb(0.369×10-6~0.989×10-6)含量，Sr/Y比值为41.03~99.36，与现今的埃达克岩的特点非常相似(Kay, 1978; Defant and Drummond, 1990; Martin et al., 2005)。在原始地幔标准化的微量元素蛛网图上，它们显示出强烈的Nb-Ta亏损和Ti负异常的特征(图 6b)。
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图 6 阿尔金北缘新太古代TTG片麻岩球粒陨石标准化稀土元素模式图(a, 标准化值据Boynton, 1984)和原始地幔标准化的微量元素蛛网图(b, 标准化值据Sun and McDonough, 1989) Fig. 6 Chondrite-normalized REE patterns (a, normalization values after Boynton, 1984) and primitive mantle-normalized spider diagrams (b, normalization values after Sun and McDonough, 1989) for the Neoarchean TTG gneisses in the North Altyn Tagh area |
上述TTG片麻岩样品的Rb/Sr比值为0.11和0.07，87Sr/86Sr比值变化较大(0.718598和0.714104)，对应的初始87Sr/86Sr比值为0.705583和0.705956(表 3)。它们的143Nd/144Nd比值变化较小(0.510768和0.510837)，对应的εNd(t)值分别为3.6和0.2(图 7)，两阶段亏损地幔的Nd模式年龄(tDM2)为3.70~3.62Ga(表 3)。
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表 3 阿尔金北缘地区新太古代TTG片麻岩Sr-Nd同位素组成 Table 3 Sr-Nd isotopic compositions of the Neoarchean TTG gneisses in the North Altyn Tagh area |
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图 7 岩石结晶年龄与εNd(t)图解(底图据Hu et al., 2000修改)敦煌地区TTG数据来源于梅华林等(1998)和Zong et al. (2013)；库鲁克塔格TTG数据来源于Zhang et al. (2012a) Fig. 7 Crystallization ages vs. εNd(t) diagram (modified after Hu et al., 2000) Data for Dunhuang TTG from Mei et al. (1998) and Zong et al. (2013); data for Kuluketage TTG from Zhang et al. (2012a) |
TTG或TTG岩套是由英云闪长岩(Tonalite)、奥长花岗岩(Trondhejmite)和花岗闪长岩(Granodiorite)三种岩石形成的岩石组合(Martin and Arndt, 2015)，其为了解地球早期大陆地壳演化和板块构造提供非常重要的信息(Condie, 2005)。前人基于Al2O3的含量，将TTG岩石分为高铝和低铝两种类型(Barker and Arth, 1976; Barker et al., 1976; Barker, 1979; Halla et al., 2009)。根据这一分类，全球大多数太古宙TTG岩石都属于高铝类型(万渝生等, 2017)。因此，也有学者根据岩浆起源的深度，将TTG岩石进一步划分为低压、中压和高压TTG，对应的压力条件分别为小于10kbar、10~25kbar和大于25kbar(Moyen, 2011)。一般来说，高压TTG岩石的化学成分具有更高的Na2O含量(>5%)和Sr/Y比值(50~500)以及更低的重稀土含量(Yb < 1×10-6)，岩石类型通常为奥长花岗岩；而中-低压TTG岩石相对贫硅(65%~72%)、低钠(4%~6%)和较高的重稀土含量(Yb < 1.5×10-6)、Sr/Y比值(10~200)，岩石类型通常为英云闪长岩和花岗闪长岩(Moyen and Martin, 2012)。高压TTG的形成通常认为与板片俯冲有关，而中-低压TTG则可能与下地壳的部分熔融有关(Moyen, 2011)。根据地球化学特征和岩石类型，可将阿尔金北缘新太古代TTG片麻岩划分为中-低压TTG类型。
尽管TTG岩石是由含水的基性岩石部分熔融形成的这一观点已基本达成共识(Arth and Hanson, 1972; Martin, 1987; Drummond and Defant, 1990; Atherton and Petford, 1993; Rapp and Watson, 1995; Winther, 1996; Foley et al., 2002; Rapp et al., 2003; Nair and Chacko, 2008; Moyen and Martin, 2012; Martin and Arndt, 2015)，但是其形成的构造背景却一直存在争议(Moyen and Martin, 2012)。这主要是因为在高压条件下含水的基性岩部分熔融都有可能产生具有类似TTG特征的岩浆，这样的构造环境甚至包含洋底高原(Willbold et al., 2009)和洋中脊(Rollinson, 2009)。鉴于太古宙发育大量的TTG岩石，俯冲带和板内构造背景被认为是最合理的解释(Moyen and Martin, 2012)。俯冲带模式下，TTG岩石由俯冲洋壳部分熔融形成。
这些熔体与上覆地幔楔发生反应导致TTG岩石具有高MgO(Mg#)、Cr和Ni含量(Martin, 1999; Martin et al., 2005; Martin and Moyen, 2002; Smithies et al., 2009; Moyen, 2009)。而在板内模式下，由于TTG岩石是基性下地壳部分熔融形成，因此它们的MgO、Mg#、Cr和Ni含量均相对较低(Atherton and Petford, 1993; Rapp et al., 1999; Rapp and Watson, 1995)。结合前人的研究数据(Long et al., 2010; Zhang et al., 2014)，阿尔金北缘新太古代TTG片麻岩显示较高的Sr/Y和(La/Yb)N比值(图 8a, b)，MgO和Mg#变化较大，分别为0.65%~3.50%(平均值1.79%)和26~60(平均值44)，大部分样品点落在下地壳部分熔融区域(图 5c)。较低的Cr含量(1.19×10-6~59.4×10-6，平均值25.7×10-6)和Ni含量(1.14×10-6~37.0×10-6，平均值17.3×10-6)同样暗示了熔体没有与地幔楔参与反应。此外，阿尔金北缘新太古代TTG片麻岩的Nd同位素和锆石Hf同位素两阶段模式年龄均为3.6~3.1Ga，与岩石的结晶年龄相差0.9~0.4Ga。表明阿尔金北缘新太古代TTG片麻岩并非来自与俯冲熔体相关的新生地壳。所以，阿尔金北缘新太古代TTG片麻岩来源于古太古代基性下地壳部分熔融。阿尔金北缘新太古代TTG片麻岩属于中-低压TTG同样证实了这一观点。
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图 8 阿尔金北缘新太古代TTG片麻岩Sr/Y-Y图解(a)和(La/Yb)N-YbN图解(b)(据Moyen and Martin, 2012修改; 熔融曲线来自Drummond and Defant, 1990) Fig. 8 Y vs. Sr/Y diagram (a) and YbN vs. (La/Yb)N diagram (b) (modified after Moyen and Martin, 2012; melting curves from Drummond and Defant, 1990) for the Neoarchean TTG gneisses in the North Altyn Tagh area |
大部分TTG岩石都具有高Sr、低Y和Yb的特点，在稀土元素配分图上无明显Eu异常或Eu正异常。这些特点都指示了TTG岩浆的熔融深度位于斜长石的稳定区或斜长石的堆晶。阿尔金北缘新太古代TTG片麻岩中部分样品显示微弱的负异常，显示有少量斜长石的分离结晶。而高(La/Yb)N比值和低YbN比值都暗示源区有石榴石或(和)角闪石的残留(Martin et al., 2005)。由于不同元素在同一矿物中的分配系数不同，因此TTG岩石中一些特定的元素比值(如Nb/Ta、Zr/Hf、Zr/Sm等)可以示踪岩浆源区的成分(Foley et al., 2002; Klemme et al., 2002; Xiong et al., 2005;熊小林等, 2007)。比如稀土元素在角闪石中的分配系数大小关系为MREE>HREE>LREE(Bottazzi et al., 1999)，因此，源区角闪石的残留会导致熔体中La/Yb的比值升高，而Gd/Dy和Dy/Yb比值降低。石榴石的残留不仅会导致Sr/Y、La/Yb比值的升高，还会造成Gd/Yb和Dy/Yb比值的增加(Davidson et al., 2007)。阿尔金北缘新太古代TTG片麻岩的Sr/Y比值和La/Yb比值显示明显的正相关关系(图 9a)，指示了源区石榴石的存在；而在Dy/Yb和La/Yb图中，二者却没有明显的线性关系，并且大多数岩石的Dy/Yb比值都介于2~3之间(图 9b)，暗示了熔体除受到石榴石控制外，还受到角闪石的作用。所以，阿尔金北缘新太古代TTG片麻岩的岩浆源区有石榴石和角闪石的残留。此外，样品还显示强烈的Nb-Ta负异常，而Zr-Hf却显示明显的正异常(图 6b)，指示了源区可能还存在金红石。
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图 9 阿尔金北缘新太古代TTG片麻岩Sr/Y-La/Yb图解(a)和Dy/Yb-La/Yb图解(b)阿尔金北缘文献数据来源于Long et al. (2014)和Zhang et al. (2014) Fig. 9 Plots of La/Yb vs. Sr/Y diagram (a) and La/Yb vs. Dy/Yb diagram for the Neoarchean TTG gneisses in the North Altyn Tagh area Data for North Altyn Tagh TTG from Long et al. (2014) and Zhang et al. (2014) |
综上所述，阿尔金北缘新太古代TTG片麻岩岩浆来源于古太古代基性下地壳，并且在源区有石榴石、角闪石和金红石的残留。估计其熔融压力超过1.5GPa，深度超过45km(Rapp et al., 1991;熊小林等, 2007)。
4.2 TTG岩浆活动与变质事件通过对锆石核部具有振荡环带的锆石进行精确的SHRIMP U-Pb定年获得其207Pb/206Pb年龄的加权平均值为2740±19Ma(图 4b)，表明该TTG片麻岩的形成年龄为2.74Ga，与前人在阿尔金北缘东段阿克塔什塔格地区获得的TTG片麻岩的年龄(2.60~2.74Ga，平均值~2.70Ga)一致(陆松年和袁桂邦, 2003; Long et al., 2014; Zhang et al., 2014)。
新太古代TTG片麻岩在塔里木克拉通广泛分布，库鲁克塔格辛格尔地区和兴地地区的TTG片麻岩年代学显示其形成时代为2.46~2.64Ga(平均值~2.57Ga, 胡蔼琴和韦刚健, 2006; Long et al., 2010; Zhang et al., 2012b)。敦煌地区的TTG片麻岩主要分布在石包城、红柳峡、水峡口和东巴兔一带，年龄为2.50~2.71Ga(平均值~2.59Ga, 赵燕等, 2013;梅华林等, 1998; Zhang et al., 2013b; Zhao et al., 2015; Zong et al., 2013)。因此，敦煌地区的TTG片麻岩的形成时代与库鲁克塔格地区的基本一致，而比阿尔金北缘地区的晚100~300Myr。
阿尔金北缘TTG片麻岩除了其结晶年龄，还记录了两期变质事件的形成时代，分别为~2.5Ga和~1.96Ga(图 4c, d)。目前，对阿尔金北缘~2.5Ga变质事件的认识还非常有限，前人在对TTG片麻岩进行定年时，也发现了少量~2.5Ga的锆石(辛后田等, 2013; Long et al., 2014; Zhang et al., 2014)，这些锆石在形态上和CL图像上与~2.7Ga的锆石并没有太大差别，以至于有学者将其解释为TTG片麻岩的形成年龄。除此之外，前人还对侵入TTG片麻岩中的片麻状花岗岩进行了详细的锆石U-Pb定年工作，发现它的形成时代也是~2.5Ga，并将其解释为~2.7Ga TTG片麻岩部分熔融形成的浅色体(新成体)(Zhang et al., 2014)。同时，~2.5Ga锆石的εHf(~2.7Ga)与TTG片麻岩中~2.7Ga的εHf(t)相近也证实了混合岩化作用的存在。因此，阿尔金北缘~2.5Ga的变质事件可能代表了~2.7Ga TTG片麻岩的混合岩化作用(深熔作用)。阿尔金北缘太古代岩中(包括TTG和花岗片麻岩)广泛发育~1.96Ga的锆石变质增生边(辛后田等, 2013; Long et al., 2014; Zhang et al., 2014)，且这一事件被解释为与Columbia超大陆的聚合有关(Zhang et al., 2014)。虽然前人对片麻岩中的麻粒岩进行了报道，但并没有年代学数据(辛后田等, 2013)。近年来，Wu et al. (2019)在阿克塔什塔格地区首次报道了二辉石麻粒岩的形成时代为~1.97Ga，建立了顺时针P-T轨迹，证实了阿克塔什塔格地区存在古元古代碰撞造山带，并认为该造山事件可能与Columbia超大陆聚合有关。而且对石榴角闪岩和泥质片麻岩的研究也得出了相同的结果(Zhang et al., 2019)。因此，很显然该地区的TTG片麻岩也同样记录了这一期变质事件。
前人的研究表明敦煌地区和库鲁克塔格地区的TTG片麻岩不仅具有相似的形成时代，而且它们记录的变质事件也相近。敦煌地区TTG片麻岩锆石变质增生边的U-Pb年龄为变化较大(1856~2003Ma)，主要集中在1.85Ga左右(Zhang et al., 2013b; Zong et al., 2013)。这一年龄与区域内高压麻粒岩的锆石年龄一致，顺时针P-T轨迹揭示了敦煌地区~1.85Ga的碰撞造山事件(Zhang et al., 2012a)。前人推测这一事件可能与Columbia超大陆聚合有关，且认为敦煌地块是华北克拉通的一部分(Zhang et al., 2012a; Zhao et al., 2015)。库鲁克塔格地区的TTG片麻岩同样记录了~1.85Ga变质事件(Zhang et al., 2012a)，但目前还没有同时代的高压变质岩报道。
总之，阿尔金北缘TTG片麻岩与敦煌-库鲁克塔格TTG片麻岩的形成时代略有不同，并且它们记录的变质事件也有所差异。
4.3 塔里木克拉通前寒武纪基底组成塔里木一直被认为是具有统一前寒武纪基底的克拉通(辛后田等, 2013)。然而，越来越多的年代学及岩石学证据都显示塔里木克拉通早前寒武纪基底可能是由两个或多个独立的块体组成(Xu et al., 2013, Yang et al., 2018; Ye et al., 2016; Zhang et al., 2014)。本文通过对阿尔金北缘新太古代TTG片麻岩详细的年代学和系统的岩石学研究，结合前人的研究成果显示阿尔金北缘地区的太古代基底与敦煌-库鲁克塔格地区的基底并不相同。首先，大量的年代学资料表明阿尔金北缘地区的大量基底岩石的形成时间比敦煌-库鲁克塔格地区的早100~300Myr；其次阿北地块的TTG片麻岩经历了两期变质事件(~2.5Ga混合岩化作用和~1.96Ga麻粒岩相变质作用)，而敦煌-库鲁克塔格地区只识别出了一期早前寒武纪变质事件(~1.85Ga)，并且它们的时间相差~100Myr；最后，阿尔金北缘地区TTG片麻岩的Hf两阶段模式年龄显示其地壳生长发生峰期发生在~3.3Ga，而敦煌为~2.8Ga、~2.9Ga和~3.4Ga(Zhang et al., 2013b; Zong et al., 2013)，库鲁克塔格为~2.6Ga和3.2Ga(Long et al., 2010)。与阿尔金北缘地区和敦煌-库鲁克塔格地区不同，迄今为止塔里木西南地区未发现可靠的太古宙TTG基底。该区最古老的岩系为赫罗斯坦杂岩，其形成时代为2.41~2.34Ga，并清楚记录了~1.90Ga变质事件(Zhang et al., 2014; Ye et al., 2016)。这些证据都表明阿尔金北缘地区前寒武纪基底与敦煌-库鲁克塔格地区以及塔里木西南地区具有显著的区别，它们可能来源于不同的大陆块体。
基于全球广泛分布的2.1~1.8Ga的碰撞造山带，前人对Columbia超大陆进行了复原和重建(如: Zhao et al., 2002; Rogers and Santosh, 2002)。这些造山带主要包括南美和西非陆块之间的Transamazonian和Eburnean造山带(2.1~1.8Ga, Alkmim and Marshak, 1998)；北美的Trans-Hudson造山带(1.95~1.85Ga, Hoffman, 1989)；南非的Limpopo碰撞带(2.0~1.9Ga, Kröner et al., 1999)；澳大利亚西部的Capricorn碰撞带(2.0~1.9Ga, Myers, 1990)；格陵兰的Nagssugtoqidian造山带(1.9~1.8Ga, Kalsbeek, 2001)；西伯利亚的Akitkan造山带(1.9~1.8Ga, Rosen et al., 2005)和中国的华北中部造山带(1.85Ga, Zhao et al., 2001)等。根据塔里木不同地区变质事件的时代可以推测阿尔金北缘地体、库鲁克塔格-敦煌地体和塔里木西南地体分别与扬子和澳大利亚克拉通、华北克拉通以及西伯利亚和劳伦克拉通具有亲缘性(图 10, Zhang et al., 2014; Ye et al., 2016)。然而，它们在Columbia超大陆的具体位置还需要更多地质学和地球物理学证据。这些块体随后经历了复杂的构造事件，最终于新元古代拼贴在一起，形成统一的塔里木克拉通(叶现韬和张传林, 2020)。
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图 10 阿尔金北缘地体、库鲁克塔格-敦煌地体以及塔里木西南地体在Columbia超大陆中的位置(据Zhao et al., 2002; Ye et al., 2016修改) Fig. 10 Possible positions of the North Altyn terrane, Kuluketage-Dunhuang terrane and Southwest Tarim terrane in the Columbia supercontinent (modified after Zhao et al., 2002; Ye et al., 2016) |
(1) 阿尔金北缘TTG片麻岩的形成时代为~2.74Ga，并遭受了两期不同程度的变质事件：~2.5Ga混合岩化作用和~1.96Ga麻粒岩相变质作用。
(2) 阿尔金北缘TTG片麻岩来源于古太古代基性下地壳，并且在源区残留有石榴石、角闪石和金红石，估计其熔融深度超过45km。
(3) 阿尔金北缘地区前寒武纪基底与敦煌-塔里木北缘地区具有显著的区别，它们可能来源于不同的大陆块体。
致谢 野外工作得到福建闽西地质大队倪康高级工程师的大力帮助；中国科学院地球化学研究所漆亮研究员和胡静实验师在主微量元素分析上提供了指导和帮助；SHRIMP U-Pb年代学测试得到北京离子探针中心颉颃强副研究员的大力帮助；同时主编、两位审稿人和俞良军编辑对本文提出了很多有益的意见，对本文质量的提升有非常重要的作用；我们在此一并感谢。
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