岩石学报  2020, Vol. 36 Issue (11): 3414-3426, doi: 10.18654/1000-0569/2020.11.10   PDF    
引用本文
闫慧慧, 朱光有, 陈志勇, 张纯, 侯政, 孙琦森. 2020. 塔里木板块东北缘Ⅰ型花岗岩年代学与地球化学研究. 岩石学报, 36(11): 3414-3426, doi: 10.18654/1000-0569/2020.11.10  
Yan HH, Zhu GY, Chen ZY, Zhang C, Hou Z and Sun QS. 2020. Geochronology and geochemistry of Ⅰ-type granites in the northeastern margin of Tarim Plate. Acta Petrologica Sinica, 36(11): 3414-3426(in Chinese with English abstract)  
塔里木板块东北缘Ⅰ型花岗岩年代学与地球化学研究
闫慧慧1,2, 朱光有1, 陈志勇1, 张纯3, 侯政2, 孙琦森1     
1. 中国石油勘探开发研究院, 北京 100083;
2. 中国石油西南油气田公司川西北气矿, 江油 621741;
3. 中国石油西南油气田公司重庆气矿, 重庆 401147
2020-04-01 收稿; 2020-10-23 改回.
基金项目: 本文受中国石油天然气股份有限公司科学研究与技术开发项目(2019B-04、2018A-0102)资助
第一作者简介: 闫慧慧, 男, 1995年生, 硕士, 构造地质学方向, E-mail:yanhuihui2018@163.com
通讯作者: 朱光有, 男, 1973年生, 博士, 教授级高级工程师, 主要从事油气成藏与地球化学研究, E-mail:zhuguangyou@petrochina.com.cn.
摘要: 塔里木板块广泛发育新元古代岩浆岩，它们对研究塔里木新元古代构造演化及塔里木对Rodinia超大陆汇聚和裂解的响应具有重要意义。本文报道了塔里木板块东北缘Ⅰ型花岗岩的U-Pb年代学、原位Hf同位素以及全岩地球化学研究结果。该花岗岩具有较高的SiO2含量(平均值为71.61%)，低的镁铁质含量(FeOT和MgO平均值为1.52%和0.72%)；Na2O/K2O比值介于0.51~3.76，平均值为1.12；铝饱和度(A/CNK)介于0.69~1.16，属于碱性-钙碱性过渡型花岗岩。花岗岩表现为高场强元素(HFSE，如：Nb、Ta、Ti、Th等)的亏损以及大离子亲石元素(LILE，如：Rb、Sr、Ba、U等)的相对富集。样品Nb/Ta、Zr/Hf平均值分别为10.93、36.86，说明该花岗岩形成时有流体的参与。受斜长石的影响，花岗岩表现出明显的Eu正异常，Eu/Eu*平均值1.39。该花岗岩206Pb/238U加权平均年龄为748.8±6.1Ma，其原位Hf同位素εHf(t)值范围在-16.5~-9.7之间，对应两阶段模式年龄tDM2介于2.30~2.74Ga，指示该期花岗岩可能为古元古代地壳的部分熔融，可能形成于古洋壳向南俯冲塔里木板块产生的大陆弧后环境。结合之前在塔北缘的研究工作，提出了塔里木板块北缘新元古代弧后伸展体系，并且认为该体系主要受控于Rodinia超大陆的裂解。
关键词: Ⅰ型花岗岩    新元古代    构造背景    库鲁克塔格地区    
Geochronology and geochemistry of Ⅰ-type granites in the northeastern margin of Tarim Plate
YAN HuiHui1,2, ZHU GuangYou1, CHEN ZhiYong1, ZHANG Chun3, HOU Zheng2, SUN QiSen1     
1. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China;
2. Chuanxibei Division of PetroChina Southwest Oil and Gas Field Company, Jiangyou 621741, China;
3. Chongqin Division of PetroChina Southwest Oil and Gas Field Company, Chongqing 401147, China
Abstract: Neoproterozoic igneous rocks are widely exposed in Tarim Plate and they play a key role in understanding the Neoproterozoic tectonic evolution of the plate and its coupling with the assemblage and breakup of Rodina. In this paper,we reported zircon U-Pb ages and Lu-Hf isotope compositions as well as whole-rock geochemistry of the Neoproterozoic granitic plutons outcropped in the northeastern margin of the Tarim Plate. The granites have high contents of SiO2(averaged at 71.61%),low FeOT and MgO (1.52% and 0.72%) and variable Na2O/K2O ratios (0.51~3.76) and A/CNK values(0.69~1.16),which defining a calcium alkaline natures. As for trace elements,the granites are characterized by depletion of high field strength elements (HFSEs,such as Nb,Ta,Ti,Th,etc.) and relative enrichment of large ion lithophile elements (LILEs,e.g. Rb,Sr,Ba,U,etc.),with average Nb/Ta and Zr/Hf ratios of 10.93 and 36.86,respectively,indicating that they experienced fluid alteration. They have high Eu/Eu* values (averaged at 1.39),possibly due to accumulation of plagioclase. Zircon U-Pb dating revealed the granite crystallized at ca. 748.8Ma,and in situ Hf isotope analysis show their pronounced negative εHf(t) values ranging from -16.5 to -9.7,with corresponding two-stage model age tDM2 values ranging from 2.30Ga to 2.74Ga,demonstrating that they were derived from partial melting of a Paleoproterozoic crust. It may be formed in the back-arc environment of the old ocean crust that subducted under the Tarim Plate to the south. Combined with previous regional research works,we proposed that there is a post-stretching system,the Neoproterozoic arc,occurred in the northern margin of the Tarim Plate,and this system is mainly controlled by the break-up of the Rodinia supercontinent.
Key words: Ⅰ-type granite    Neoproterozoic    Tectonic setting    Kuruketage area    

新元古代发生的全球超大陆裂解事件引发了大规模的构造岩浆活动(Zhu et al., 2008; Xu et al., 2009, 2013b; Zhao and Zhai, 2013; Xiao et al., 2014; Zhang et al., 2016; Chen et al., 2020)。而岛弧的时空结构对于理解和研究地球历史上造山带的形成以及超大陆聚合-裂解事件扮演了极其重要的角色，并且对黑色生油岩系的形成与油气勘探具有重要的启示意义(Liu et al., 2019; Zhu et al., 2020a)。塔里木板块作为中国最古老的克拉通之一，其东北缘库鲁克塔格地区广泛出露的新元古代岩浆岩，可以为研究塔里木新元古代构造和沉积演化提供重要的约束条件(Lu et al., 2008; Xiao et al., 2014; Zhu et al., 2018, 2020b;朱光有等，2020)。然而，由于该地区地质记录的不完整，塔里木盆地北部火山弧环境下的花岗岩形成的时间和方式仍然值得深入研究。

前人研究表明，塔里木板块东北缘发育了与新元古代Rodinia超大陆裂解相关的四期岩浆事件，分别为820~800Ma(Zhu et al., 2008; Shu et al., 2011)、780~760Ma(Zhang et al., 2009)、740~735Ma(Zhang et al., 2012b)和650~635Ma(Zhang et al., 2009)，其岩石类型主要包括花岗岩、双峰式火山岩、基性-超基性杂岩和基性岩墙群等(Zhu et al., 2008; Xu et al., 2009; Zhang et al., 2012b)。大部分地质学家认为这一裂解活动主要与地幔柱或局部热点活动有关(Lu et al., 2008; Zhu et al., 2008; Xu et al., 2013a; Zhang et al., 2013; He et al., 2014)，也有少部分学者认为塔里木盆地的长期俯冲和增生导致这一结果(Ge et al. 2014)。由此引发对该区火成岩形成的地球动力学背景认识方面的争议，特别是多种类型花岗岩的成因问题。

为了更好地厘清新元古代塔里木板块的构造演化，本文通过LA-ICP-MS锆石U-Pb定年、全岩主微量元素测定以及锆石Hf同位素分析等测试手段，针对塔里木盆地北缘库鲁克塔格地区雅尔当山剖面花岗岩的年代学、岩石学和地球化学特征进行了系统研究。不仅为新元古代塔里木板块构造演化以及塔里木板块拼合事件提供依据，同时为理解造山带的形成以及新元古代Rodinia超大陆的裂解事件提供参考。

1 区域背景

库鲁克塔格地区位于塔里木盆地东北缘，大地构造位置上属于塔里木板块与中亚造山带系统的结合部位(Xiao et al., 2014)。区内获得的最古老岩石年龄来自新太古代的托格杂岩(2.56Ga; 胡霭琴和韦刚健，2006)，主要由表壳岩系和变质深层岩组成；古元古代兴地塔格群不整合覆盖于托格杂岩之上，以中高级变质岩为主，包括石英岩、黑云斜长片麻岩、角闪斜长片麻岩、石榴云母片麻岩、变粒岩等(高振家等，1993；魏震等，2017)，这一时期岩浆岩主要包括片麻状花岗岩、闪长岩、蓝石英花岗岩、花岗闪长岩、二长花岗岩等(曹晓峰等，2012)。中元古代地层不整合覆盖于兴地塔格群之上，这一时期主要出露长城系波瓦姆群和杨吉布拉克群、蓟县系爱尔基干群，岩性包括大理岩、片岩等中-低级副变质岩和变质砾岩、砂岩、泥岩等沉积岩。

新元古代早期地层为青白口系帕尔岗塔格群，主要由塞纳尔塔格组细碎屑岩和北塞纳尔塔格组灰岩、白云岩、少量钙质板岩等组成。不整合在帕尔岗塔格群之上的南华-震旦系，从东向西主要出露于赛马山、辛格尔、兴地、西山口地区(徐备等，2008)(图 1)。以兴地断裂为界，库鲁克塔格分为南区和北区，南区为火山岩喷发中心，主要为巨厚的火成岩高地；北区以裂谷盆地沉积为主，发育多套滨海相沉积旋回，南北地区地层差异较大。本次花岗岩样品来自于库鲁克塔格南区雅尔当山剖面(40°44′12″N、88°55′41″E)。

图 1 塔东北库鲁克塔格地区大地构造位置图(a, 据Xiao et al., 2013)和区域地质图(b, 据徐备等，2008) TC-塔里木克拉通；NCC-华北克拉通；SCC-华南克拉通 Fig. 1 Sketch map showing tectonic location(a, after Xiao et al., 2013) and regional geology(b, after Xu et al., 2008)of the Kuruktag area in the northeastern margin of Tarim TC-Tarim Craton; NCC-North China Craton; SCC-South China Craton
2 样品与实验方法

雅而当山剖面花岗岩呈现肉红色，岩石为半自形粒状结构，块状构造(图 2a, b)，主要矿物为钾长石、斜长石、石英及少量的黑云母(图 2c)。其中钾长石含量约35%，呈板柱状；斜长石含量约为25%~30%，可见有聚片双晶；石英含量约为25%，呈他形充填于较为自形的长石及暗色矿物空隙之间；暗色矿物主要为黑云母及少量的角闪石，其中黑云母的含量约为6%~8%，自形程度较好，可见呈蓝绿-橙红干涉色；其他副矿物分别有锆石、磷灰石和榍石等，在岩体边缘可见到石榴子石，晶形较好，大小为0.5~1cm。

图 2 库鲁克塔格地区雅尔当山剖面花岗岩野外以及显微照片 Pl-斜长石；Kf-钾长石；Bt-黑云母；Hb-角闪石；Qz-石英 Fig. 2 Petrographic characteristics of granites in Kuruketage area Bt-biotite; Hb-hornblende; Kf-K-feldsper; Pl-plagioclase; Qz-quartz
2.1 全岩主微量元素

样品在南京宏创地质勘查技术服务有限公司进行全岩主微量元素测试。将机械粉碎至200目的花岗岩粉末在烘箱中烘干2小时，设置温度为110℃；在马弗炉中对于0.5~1.0g烘干后样品1000℃灼烧90min，自然晾干至25℃左右，称重后计算烧失量；之后将6.0000g助熔剂(49.75% Li2B4O7:49.75% LiBO2:0.5% LiBr)与0.6000g烘干后样品均匀混合，称取助熔剂和样品时误差控制在0.3mg之内。在熔样炉中1100℃下熔融，程序运行结束，在气泡赶出之后，将熔体冷却，取出玻璃片，贴标签，利用帕纳科AxiosMAX XRF分析完成。

全岩微量样品处理时，首先将40mg干燥后的粉末置于聚四氟乙烯溶样罐，之后加入0.5mL浓硝酸与1.0mL氢氟酸，在钢套的保护下加热72小时。待样品被彻底消解后，稀释2000倍，在Agilent 7700x ICP-MS测定(雾化形式)。安山岩AGV-2、花岗闪长岩GSP-2被当做质控盲样。

2.2 U-Pb测年和Hf同位素

锆石的挑选、阴极发光图拍照、U-Pb测年以及原位Hf同位素测试均在南京宏创地质勘查技术服务有限公司完成。193nm ArF准分子激光剥蚀系统由Australian Scientific Instruments制造，型号为RESOlution LR。四极杆型电感耦合等离子体质谱仪(ICP-MS)型号为Agilent 7700x，由安捷伦科技公司制造。样品测试时，首先准分子激光发生器产生的深紫外光束在锆石表面聚焦，能量密度为3.5J/cm2，束斑直径为33μm，频率为5Hz，时间为40秒，剥蚀气溶胶和氦气一起送入ICP-MS。测试过程中的外标为91500，用来对质量歧视与元素分馏进行校正；盲样为GJ-1，对U-Pb定年数据质量进行检验；在确定锆石中的Pb元素含量时采用NIST SRM 610(外表)、Si(内标)，确定锆石中其余微量元素含量时以Zr作为内标(Hu et al., 2011; Liu et al., 2010)。原始的测试数据经过ICPMSDataCal软件离线处理完成(Liu et al., 2010)，计算误差标准为1σ。

在处理原位Hf同位素时，通过179Hf/177Hf=0.7325获得质量歧视因子βHf；通过实测172Yb/173Yb获得Yb同位素质量歧视因子βYb，之后利用176Yb/172Yb=0.5887(Vervoort et al., 2004)扣除176Yb对176Hf的同量异位干扰；由于Lu只有175Lu与176Lu两个同位素，因此假定βLu=βHf，再采用176Lu/175Lu=0.02655 (Vervoort et al., 2004)扣除176Lu对176Hf的同质异位干扰。样品测试在多接收器型号电感耦合等离子体质谱仪(MC-ICP-MS)完成。测试过程中采用标准锆石(包括GJ-1、91500、Plešovice、Mud Tank、Penglai)来检验锆石Hf同位素数据质量。

3 分析结果 3.1 地球化学特征 3.1.1 主量元素

花岗岩主量元素分析结果和有关岩石化学分析参数见表 1。样品主量元素含量变化范围较大，SiO2含量65.72%~75.57%，平均值为71.61%；Al2O3含量较高，为12.15%~15.86%，平均含量为13.75%；FeOT在0.37%~3.04%之间，平均值为1.52%；MgO的含量0.12%~1.64%；全碱含量(K2O+Na2O)较高(图 3a)，为7.86%~10.77%，平均值为9.05%；K2O含量较高，为1.84%~6.08%，平均值为4.72%；Na2O/K2O比值介于0.51~3.76，平均值为1.12；指示出Ⅰ型花岗岩的特征。K2O/SiO2比值介于0.02~0.08之间，平均值为0.07，在K2O-SiO2图中，属于高钾系列(图 3c)。TiO2、MnO、P2O5含量较低，均小于1%。烧失量变化范围较小，为0.82%~3.71%，平均值为1.58%。铝饱和度(A/CNK)范围在0.69~1.16之间，平均值为0.96，在ANK/ACNK图中，所有样品均属于钙碱性花岗岩(图 3b)。在用里特曼指数(σ=(Na2O+K2O)2/(SiO2-43))判断岩性时，样品σ介于2~5之间，有15个样品σ小于3.3，为钙碱性花岗岩，而3个样品σ大于3.3，属于碱性花岗岩。

表 1 塔东北库鲁克塔格地区Ⅰ型花岗岩主量(wt%)和微量(×10-6)元素分析结果 Table 1 Major oxide compositions(wt%) and trace element(×10-6)concentrations of the Ⅰ-type granites in Kuruketage area in the northeastern Tarim

图 3 Ⅰ型花岗岩主量元素判别图 (a)Na2O+K2O与SiO2图解(据Middlemost, 1994)；(b)A/NK-A/CNK图解(据Maniar and Piccoli, 1989)；(c)K2O-SiO2图解(据Rollison, 1993)；(d)Na2O+K2O-CaO与SiO2图解(据Frost et al., 2001) Fig. 3 Discrimination diagrams of major elements in Ⅰ-type granites (a)Na2O+K2O vs. SiO2(after Middlemost, 1994);(b)A/NK vs. A/CNK(after Maniar and Piccoli, 1989);(c)K2O vs. SiO2(after Rollison, 1993);(d)(Na2O+K2O-CaO) vs. SiO2(after Frost et al., 2001)
3.1.2 微量元素和稀土元素

在原始地幔标准化微量元素蛛网图中可以观察到(图 4a)，花岗岩样品的元素分布特征总体一致。与相邻元素相比，Nb、Pr、Nd、Ti等元素显著亏损，Ti含量为223.7×10-6~2989×10-6，Pb含量为2.32×10-6~15.04×10-6，Cs含量为0.25×10-6~0.74×10-6，Ta含量为0.12×10-6~2.40×10-6，Sr含量为205.6×10-6~354.9×10-6，Sr/Y比值为18.01~102.6，Nb/Ta比值为5.03~17.53，平均值为10.93，低于地壳平均值12.22(Taylor and McClennan, 1985)和原始地幔平均值17.4(Sun and McDonough, 1989)，说明其具有大陆地壳物质的参与；Zr/Hf比值32.73~39.00，平均值为36.86，高于原始地幔平均值(36.25，Taylor and McClennan, 1985)而小于地壳平均值(Taylor and McClennan, 1985)，反映出岩浆演化过程中Zr/Hf分馏明显；Rb/Sr比值0.20~0.45，平均值为0.36，接近于全球上地壳平均值(0.32，Taylor and McClennan, 1985)。Rb/Nb比值介于5.67~101.3之间，平均值为27.75。球粒陨石标准化稀土元素配分模式图表现出轻稀土轻微富集，重稀土平坦的右倾模式(图 4b)。稀土总量较底，含量在7.52×10-6~51.16×10-6之间，平均值为26.45×10-6，LREE/HREE=2.73~7.17；(La/Yb)N=2.13~8.10，(La/Sm)N=1.92~3.88，HREE分异较弱，(Gd/Yb)N=10.73~1.51，Eu表现出显著的正异常(Eu/Eu*平均值=1.39)，Ce无明显正负异常(Ce/Ce*=0.83~1.19)。

图 4 库鲁克塔格地区Ⅰ型花岗岩的原始地幔标准化微量元素蛛网图(a)和球粒陨石标准化稀土元素配分图(b)(标准化值据Sun and McDonough, 1989) Fig. 4 The primitive mantle-normalized multi-element spidergrams (a) and chondrite-normalized REE patterns (b) of the Ⅰ-type granites in Kuruketage area (normalization values after Sun and McDonough, 1989)
3.2 U-Pb年代学研究

样品中锆石颗粒较大，粒径介于45~200μm之间，且长宽比为1~2；自形程度较差，颜色以褐色、浅褐色为主。CL图中均显示清晰的震荡环带(图 5)，磨圆分选较差。除12号锆石Th/U为0.21之外，其余Th/U比值为均大于0.4，平均值为0.71 (表 2)，为典型的岩浆锆石(Hoskin and Schaltegger, 2003)。花岗岩样品中挑选18颗锆石进行年代学分析，其结果见表 2，这些锆石存在2个协和年龄，分别为826Ma和748Ma(图 6)。较老的协和年龄826Ma为继承锆石的年龄，其年龄要大于岩浆的结晶年龄；年轻的协和年龄748.6±3.1Ma (MSWD=0.025)，误差小，精度高，可靠性大，代表了该岩体的形成年龄。

图 5 塔东北库鲁克塔格地区花岗岩部分锆石阴极发光图 Fig. 5 Cathodoluminescence(CL)images of selected zircons of the granites in Kuruketage area in the northeastern Tarim

表 2 塔东北库鲁克塔格地区Ⅰ型花岗岩U-Pb年龄分析结果 Table 2 Zircon U-Pb isotopic data for the type-Ⅰ granites in Kuruketage area in the northeastern Tarim

图 6 塔东北库鲁克塔格地区花岗岩U-Pb年龄协和图 Fig. 6 U-Pb concordia plots for the granites in Kuruketage area in the northeastern Tarim
3.3 Hf同位素

在本次研究中，对于18个点做锆石原位Hf同位素分析(表 3)，获得Hf模式年龄(tDM2)分布范围2.3~2.74Ga，其对应的εHf(t)分布范围为-16.5~-9.7。所有的锆石都有相似的176Lu/177Hf和176Hf/177Hf值，并且几乎所有锆石的176Lu/177Hf比值均小于0.002，表明锆石在形成之后放射性成因Hf的积累较低，所以现今锆石176Hf/177Hf比值可以代表锆石形成时的176Hf/177Hf比值(Amelin et al., 1999)。花岗岩样品中176Hf/177Hf同位素主要集中在0.2818~0.2820，两阶段Hf模式年龄(tDM2)为2.30~2.74Ga，该花岗岩投点全部落在了早古元古代至新太古代未成熟地壳的位置(图 7)。

表 3 塔东北库鲁克塔格地区Ⅰ型花岗岩Hf同位素分析结果 Table 3 In situ Hf isotopic compositions of zircons from the granites in Kuruketage area in the northeastern Tarim

图 7 雅尔当山剖面748.8Ma花岗岩的Hf同位素模式图(底图据Ge et al., 2014) (a)锆石176Hf/177Hf-年龄图图解；(b)锆石εHf(t)-年龄图解.塔东北地区新太古代和早元古代地壳演化线引自Long et al.(2010)和龙小平等(2011) Fig. 7 Hf isotope values of zircons from 748.8Ma granites from the Yardang mountains (base map after Ge et al., 2014) (a)176Hf/177Hf ratio vs. age; (b)εHf(t)vs. ages. The line of Neoarchean basement of northeastern Tarim from Long et al.(2010, 2011)
4 讨论 4.1 源区特征

库鲁克塔格地区748Ma花岗岩表现为Nb、Ta、Zr、Ti等高场强元素(HFSE)的亏损以及Rb、Th、U等大离子亲石元素(LILE)的相对富集，表明其源区以陆壳成分为主，而P的亏损及K含量较高也反映出该花岗岩的壳源性。花岗岩的εHf(t)值范围介于-16.5~-9.7，Hf模式两阶段年龄(tDM2)为2.30~2.74Ga，表明该花岗岩为新太古代至古元古代壳缘岩石重熔作用的产物。花岗岩中亏损重稀土元素并且存在低的Sr(明显小于354.9×10-6)高的Yb含量(大于0.33×10-6)，表明该花岗岩的源区压力较浅(图 8a)。酸性岩浆中锆石结晶较早，其锆石的饱和温度近似等于岩浆的液相线温度。根据Miller饱和温度计算公式TZr=12900/(2.95+0.85M+lnDZr锆石/溶体)，获得该花岗岩的熔融温度为657.5~786.9℃(表 1)，平均值为731.2℃，说明其形成温度相对较低，属于冷花岗岩质岩浆(图 8b)。其中TZr为绝对温度，M=[(Na+K+2Ca)/(Al×Si)]，D为锆在锆石中的浓度与熔体中的浓度的比值，在未进行全岩锆石矿物的Zr、Hf校正时，用纯锆石中的Zr含量(496000×10-6)，及全岩的Zr含量分别代表锆石中的含量和溶体中的含量。

图 8 库鲁克塔格地区花岗岩形成时间与压力、锆石饱和温度关系图(a据Ge et al., 2014修改；b据Miller et al., 2003) Fig. 8 (Ho/Yb)N ratio vs. age (a, modified after Ge et al., 2014) and zircon saturation temperature (TZr) vs. age (b, after Miller et al., 2003) diagrams for the granitoids from the Kuruktag area
4.2 构造背景分析

本次研究的Ⅰ型花岗岩在微量元素Rb-(Y+Nb)和Rb-(Yb+Ta)图解中全部落在了火山弧花岗岩区(图 9a)，与利用微量元素组合的Rb/10-Hf-Ta×3图解中(Harris et al., 1986)的结果基本一致(图 9b)。其明显的Nb-Ta亏损和大离子亲石元素(如Rb、Cs、Sr、Ba)的富集(图 4a)，也支持其形成于俯冲环境。该花岗岩不具有同碰撞环境花岗岩的强过铝质特征(岩体的A/CNK=1.01~1.13)，不支持同碰撞环境成因。其较低的Ga/Al值和FeO/MgO比值(表 1)，明显不同于板内伸展环境下的A型花岗岩的地球化学特征。

图 9 库鲁克塔格地区花岗岩构造背景判别图(a、b, 底图据Pearce et al., 1984；c, 底图据Harris et al., 1986) Fig. 9 Discrimination diagrams of tectonic setting for the granites in Kuruketage area (a, b, base map after Pearce et al., 1984; c, base map after Harris et al., 1986)

前人研究表明，在塔里木北缘与中天山板块发育有新元古代蛇绿混杂岩、高压-超高压变质带和岩浆弧，因而认为塔里木北缘在新元古代存在古大洋(现今坐标)的俯冲活动(Xiao et al., 2013; Ge et al., 2014)。本研究发现的新元古代花岗岩的形成可能与该俯冲作用有关，俯冲带中沉积物的脱水反应产生的流体参与了下地壳的部分熔融，导致低温花岗岩的形成。

4.3 构造演化

本次研究获得的Ⅰ型花岗岩两阶段Hf模式年龄分布在2.30~2.74Ga，这与塔里木板块最古老的陆核形成事件以及初期大陆地壳生长事件(Zhang et al., 2012a: 2.5~2.8Ga；高振家等, 1993：2.4~2.3Ga)一致。古老陆核主要为辉长岩-英云闪长岩-奥长花岗岩及稍晚期的钾质花岗岩组成，可能代表了聚合、碰撞、造山后拉伸复合构造作用的产物(Zhang et al., 2012a)；2.4~2.3Ga左右广泛发育的岩浆事件，标志着塔里木板块地壳的进一步增生和生长(高振家等, 1993)；大量发育的2.0~1.8Ga角闪岩-麻粒岩代表了塔里木板块的结晶基底的变质事件，普遍认为该期次变质事件与Columbia超大陆的聚合相关(Long et al., 2012; Ge et al., 2013)。之后在中元古时期1.4Ga左右广泛发育的岩浆事件代表了塔里木板块受到了Columbia超大陆裂解的影响(Xu et al., 2013b)。新元古代早期(1.0~0.9Ga)，伴随着Rodinia超大陆的聚合，南塔里木地块和北塔里木地块逐渐靠拢，最终汇聚时间可能一直持续到780Ma(He et al., 2013; Yong et al., 2013; Zhang et al., 2014)。

本文研究所获得的748~726Ma花岗岩年龄，代表了塔里木板块北部新元古代期间大洋板块的持续俯冲。该Ⅰ型花岗岩岩浆熔融深度相对较浅，显然与北塔里木板块北部的洋壳俯冲的活动构造环境相关。邓兴梁等(2008)在库鲁克塔格地区发现的新元古代韧性逆冲推覆和右旋韧性变形(820~800Ma，兴地断裂带)进一步证实俯冲背景下强烈的构造活动。伴随着Rodinia超大陆的解体，塔里木板块北部的古洋壳向南发生俯冲，780Ma之后，构造环境逐渐由挤压向伸展转换(Ge et al., 2014)。748.8Ma时，在库鲁克塔格地区形成火山弧环境，在740Ma时，大陆弧向弧后裂谷盆地发展，广泛发育双峰式火山岩(Xu et al., 2009)，为现今库鲁克塔格地区基本构造格局奠定了基础(图 10)。

图 10 塔里木板块北缘新元古代构造模式图(据Li et al., 2018修改) Fig. 10 Neoproterozoic tectonic evolution of the northern margin of the Tarim Plate (modified after Li et al., 2018)
5 结论

本文测得库鲁克塔格地区雅尔当山剖面中的花岗岩锆石U-Pb同位素年龄为748.8Ma，确定了塔里木盆地北缘存在与青白口系帕尔冈塔格岩群同期花岗岩的存在；该Ⅰ型花岗岩属于下地壳的部分熔融，反映当时塔里木板块北部洋壳的一次俯冲活动，其动力学背景可能受控于Rodinia超大陆裂解；该花岗岩发育于火山弧环境。

致谢      中国地质大学杜柏松、张士全在样品处理分析时提供了热心帮助；河海大学张传林教授、中国石油大学(北京)刘汇川教授等审稿人对本文提出了宝贵的指导性意见；本刊编辑认真审阅全文并提出了细致的修改意见；在此一并表示感谢！

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