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东昆仑五龙沟地区猴头沟二长花岗岩年龄、成因、源区及其构造意义
严威1, 邱殿明2, 丁清峰1, 刘飞1    
1. 吉林大学地球科学学院, 长春 130061;
2. 吉林大学学报编辑部, 长春 130026
摘要: 对东昆仑造山带五龙沟地区的猴头沟二长花岗岩开展了详细的岩相学、地球化学、锆石U-Pb年龄及Hf同位素的分析测试和研究工作。LA-ICP-MS锆石U-Pb测年表明,猴头沟二长花岗岩的206Pb/238U加权平均年龄值为(419.0±1.9)Ma,属于晚志留世。岩石地球化学数据表明:猴头沟二长花岗岩属于高钾钙碱性系列的A2型花岗岩,富SiO2、K2O、Y(>33×10-6)和Yb,贫Al2O3和Sr(<100×10-6),具有强烈的负铕异常;Rb、Th、U、La、Ce、Nd相对富集,Nb、Ta、Ba、Sr、P、Ti亏损。锆石的Hf同位素研究表明,其εHf(t)值为0.2~5.1,对应二阶段模式年龄(TDM2)为1066~1371 Ma,由此推测花岗岩源区来自中元古代镁铁质下地壳部分熔融。微量元素及其特征比值的构造判别图解表明,猴头沟二长花岗岩形成于早古生代晚志留世东昆仑造山旋回的造山后伸展阶段。据此认为,原特提斯洋在东昆仑地区的最晚闭合时限应该不晚于晚志留世末期(~419 Ma),而不是前人认为的早泥盆世。
关键词: 猴头沟二长花岗岩     岩石地球化学     锆石U-Pb定年     Hf同位素     东昆仑    
Geochronology, Petrogenesis, Source and Its Structural Significance of Houtougou Monzogranite of Wulonggou Area in Eastern Kunlun Orogen
Yan Wei1, Qiu Dianming2, Ding Qingfeng1, Liu fei1    
1. College of Earth Sciences, Jilin University, Changchun 130061, China;
2. Editorial Department of Journal of Jilin University, Changchun 130026, China
Abstract: This paper presents a detailed research work on LA-ICP-MS zircon U-Pb geochronology, geochemistry, and zircon Hf isotope of the Houtougou monzogranite in Wulonggou area of the eastern Kunlun orogen. The LA-ICP-MS zircon dating of the Houtougou monzogranite yields an average 206Pb/238U age of (419.0±1.9) Ma, belonging to Late Silurian. Its geochemical features show that the Houtougou monzogranite belongs to a high-K calc-alkaline series of A2-type granite, and is characterized by high SiO2, high K2O, high Y (>33×10-6) and Yb, low Al2O3 and Sr (<100×10-6), with a significant negative Eu anomaly, enriched in Rb, Th, U, La, Ce, and Nd, depleted of Nb, Ta, Ba, Sr, P, and Ti. The zircon Hf Isotopic data demonstrate that their εHf(t) values range from 0.2 to 5.1 with the model ages (TDM2) of 1066 to 1371 Ma. It implies that their primary magma was likely derived from partial melting of lower Mesoproterozoic mafic crust. The trace elements and their characteristic ratios show that the Houtougou monzogranite emplaced in the post-orogenic stage of the eastern Kunlun orogen in Late Silurian. In view of the above facts, the closure of Proto-Tethys occurred in Late Silurian (~419 Ma) rather than Early Devonian in the eastern Kunlun orogen.
Key words: Houtougou monzogranite     lithological geochemistry     zircon U-Pb geochronology     zircon Hf isotopic systematic     eastern Kunlun orogen    

0 引言

五龙沟地区位于青海省都兰县境内,大地构造位置处于东昆仑造山带中东段。东昆仑造山带内出露大量早古生代和晚古生代早中生代两个时期的侵入岩,分别是原特提斯和古特提斯两期构造演化的产物[1]。因此,对这两套不同时期侵入岩开展细致的研究,能够为反演东昆仑造山带原特提斯和古特提斯具体的构造演化过程提供重要的岩石学和年代学证据。相比晚古生代早中生代花岗岩而言,前人对东昆仑造山带内志留纪早泥盆世花岗岩的研究略显薄弱。据现有资料报道,东昆仑造山带至少从早泥盆世开始,已经进入了后碰撞的伸展阶段。如,前人在西段祁曼塔格地区和东昆中断裂带附近测出的花岗岩年龄为386~413 Ma[2,3,4,5,6,7],可见,前人限定东昆仑地区原特提斯洋闭合后进入造山后阶段时限为不早于413 Ma。当然,前人也曾报道过老于413 Ma花岗岩的存在,如西段祁曼塔格地区白干湖花岗岩年龄为430 Ma[8],并指出其可能与早古生代大洋的碰撞造山有关,但由于没有详细的岩石地球化学资料作依据,无法具体判断其构造意义,所以这些老于413 Ma的花岗质岩无法限定原特提斯闭合时限。不过,这也从一个侧面反映,东昆仑地区原特提斯洋的闭合可能要比413 Ma早,但到底能提早到何时值得进一步研究。

前人在该区域的研究普遍通过开展锆石U-Pb年代学、岩石地球化学、同位素研究等地球化学方法来探讨该区域岩石的岩浆来源及构造背景等[9,10,11]。故本文借鉴前人的经验方法,通过对五龙沟地区猴头沟二长花岗岩开展详尽的锆石U-Pb年代学、岩石地球化学、Hf同位素组成及其构造意义研究,确定其属于典型的造山后伸展阶段A型花岗岩,年龄为419 Ma,从而将原特提斯洋闭合时限提早到晚志留世,这对于反演东昆仑原特提斯洋的构造演化过程有重要意义。

1 地质背景及岩体地质特征

东昆仑造山带位于青藏高原北部,南邻巴颜喀拉松潘甘孜造山带,北邻柴达木陆块,东西延伸约1 500 km,记录青藏高原拼合的早期历史[12,13,14](图 1)。区内发育3条近东西向的区域性断裂,即东昆北、东昆中和东昆南断裂;姜春发等[16]据此将其划分为东昆北带、东昆中带和东昆南带3个次级构造单元。五龙沟地区就位于东昆中带中东段(图 1b)。沿着东昆中断裂和东昆南断裂发育两条蛇绿岩带,一般认为其分别代表了原特提斯洋和古特提斯洋的存在[16,17],其存在时间分别大致为新元古代早泥盆世和石炭纪晚三叠世[16,17,18,19,20,21]

a 据文献[15]修编;b据文献[13]修编。 图 1 中国区域构造略图(a)和东昆仑地质简图(b) Fig.1 Tectonic sketch map of China(a); schematic geological map of the eastern Kunlun orogen(b)

五龙沟地区以发育大面积花岗岩、多个造山型金矿床和矽卡岩型铜多金属矿点为特征[22]。研究区内地层自北东向南西,依次为古元古代金水口岩群斜长片岩、石英片岩夹大理岩,中元古代长城纪小庙组大理岩、角闪片麻岩、石英片岩,中、新元古代青白口纪丘吉东沟组变砾岩、千枚岩夹大理岩和早古生代奥陶纪祁曼塔格群变凝灰岩、火山角砾岩。矿区断裂构造最发育的是3条规模较大的NWWNW向剪切带(图 2),它们是矿区内最重要的金矿导矿、容矿构造[23,24,25]。研究区以发育大量花岗质侵入岩为主要特征,由老到新依次为:在研究区北东部新元古代花岗质岩体侵位于古元古代金水口群中;在研究区南东部,大面积寒武纪花岗质岩体侵位于中元古代小庙组中;中部则是大面积志留纪花岗质岩体侵位;另外还有少量二叠纪和泥盆纪花岗质岩体侵位[22]

据文献[22]修编。 图 2 东昆仑五龙沟地区地质矿产简图 Fig.2 Geological map of the Wulonggou area in the eastern Kunlun orogen

本文研究的猴头沟二长花岗岩就是五龙沟地区志留纪花岗质岩体的一部分,岩性较为单一,主要为肉红色二长花岗岩,不具分带现象,出露面积为30 km2,呈北西向侵位于五龙沟地区中部,并受北西向断裂构造所夹持。该花岗岩体与周围岩体及地层主要为侵入接触关系或断层接触关系。其中,南东侧侵入到中元古界长城系小庙组;南侧侵入到古元古界金水口群;北东侧与奥陶系祁曼塔格群主要为断层接触;南西侧与中元古界小庙组也为断层接触;该岩体后期被早泥盆世斜长花岗岩及早二叠世石英闪长岩、二长花岗岩等花岗质岩体及后期基性岩脉侵入(图 23)。陆露等[26]曾在黄龙沟和深水潭地区分别对该岩体开展锆石U-Pb定年工作,获得年龄为418~420 Ma。

本文报道的猴头沟二长花岗岩位于五龙沟地区猴头沟南的五龙沟西侧,由北向南,依次采集了6件样品(HTG-01-B1、HTG-02-B1、HTG-03-B1、HTG-04-B1、HTG-05-B1、HTG-06-B1,图 3)。所有样品均进行了岩相学研究与主量元素、微量元素和稀土元素分析研究,其中HTG-03-B1进行了LA-ICP-MS锆石U-Pb定年和Hf同位素研究工作。

据文献[22]简化修编。 图 3 青海东昆仑五龙沟地区猴头沟地质简图 Fig.3 Geological map of Houtougou area in the Wulonggou area in the eastern Kunlun orogen

样品野外呈肉红色,均发生弱风化,粗粒自形结构,块状构造。主要矿物组成为条纹长石(36%~40%)、斜长石(20%~24%)、石英(30%左右)和黑云母(8%~12%),副矿物为锆石、磷灰石等(图 4)。结合岩石地球化学分析结果,确定其类型为二长花岗岩。其中条纹长石自形程度较好,主要是自形半自形,可见长柱状,卡氏双晶,条纹明显;斜长石镜下聚片双晶明显,部分发生黏土化,单偏光下明显模糊不干净;石英以他形为主;黑云母自形程度较差,主要为他形、片状,单偏光下显淡绿色和黄绿色,多色性较明显,镜下观察发现部分黑云母存在绢云母化现象。

a.块状构造(野外照片);b.块状构造(手标本照片);cf.自形半自形晶长石和石英等组成花岗结构、条纹长石卡氏双晶及条纹结构(正交偏光下显微照片)。Pth.条纹长石;Pl.斜长石;Bt.黑云母;Qtz.石英;Afs.碱性长石。 图 4 猴头沟二长花岗岩野外、手标本和镜下照片 Fig.4 Photographs of field,specimen,and microscope of Houtougou monzogranite
2 测试方法 2.1 锆石制靶和CL照相

首先将样品粉碎,通过常规的磁选和重力分选方法挑选出锆石颗粒,接着在双目镜下挑选;然后,将分选出的锆石用双面胶粘在载玻片上,罩上PVC环;将环氧树脂和固化剂进行充分混合后注入PVC环中,待树脂充分固化后将样品从载玻片上剥离,并对其进行打磨和刨光;最后对靶上样品进行显微镜下的反射光和透射光照相以及阴极发光(CL)照相。本次锆石阴极发光(CL)照相工作在北京锆石领航科技有限公司进行。

2.2 LA-ICP-MS锆石U-Pb测年

LA-ICP-MS锆石U-Pb测年在中国冶金地质总局山东局测试中心完成。实验仪器为装配193 nm激光剥蚀系统的美国热电公司生产的Xseries2型等离子体质谱仪(LA-ICP-MS)。激光剥蚀过程中采用氦气作载气、氩气为补偿气以调节灵敏度,二者在进入ICP之前通过一个T型接头混合。在等离子体中心气流(Ar+He)中加入了少量氮气,以提高仪器灵敏度、降低检出限和改善分析精密度[27]。另外,激光剥蚀系统配置了一个信号平滑装置,即使激光脉冲频率低达1 Hz,采用该装置后也能获得光滑的分析信号[28]。激光束斑直径选用40 μm,每个时间分辨分析数据包括20~30 s的空白信号和50 s的样品信号。U-Pb同位素定年中采用锆石标准91500作外标进行同位素分馏校正,每分析5个样品点,分析2次91500。对分析数据的离线处理(包括对样品和空白信号的选择、仪器灵敏度漂移校正、元素含量及U-Th-Pb同位素比值和年龄计算)采用软件ICPMSDataCal[29,30]完成。详细的仪器操作条件和数据处理方法同Liu等[29,30,31]。年龄计算及谐和图的绘制采用Ludwig编写的Isoplot程序[32]

2.3 岩石地球化学测试

岩石地球化学测量在澳实分析检测(广州)有限公司完成。主量元素由荷兰PANalytical生产的Axios仪器利用熔片X-射线荧光光谱法(XRF)测定,将准备好的样品与包含助溶剂(硝酸锂)的四硼酸锂-偏硼酸锂混合后倒入铂金模具中,利用XRF进行分析。除稀土元素(REE)之外的微量元素测试用美国生产的电感耦合等离子体-原子发射光谱(Varian VISTA ICP-AES)和电感耦合等离子体-质谱分析方法(Perkin Elmer Elan 6000 ICP-MS)完成,制备好的样品用含氮、高氯的氢氟酸进行溶解,残渣利用稀盐酸进行过滤并稀释;然后利用ICP-AES和ICP-MS进行分析,结果利用光谱元件间的相互干扰进行校正。稀土元素(REE)利用美国生产的Agilent 7700x电感耦合等离子体-质谱仪(ICP-MS)进行分析测试,将样品加入到硼酸锂中进行熔融,并混合均匀,然后放到1 000 ℃的熔炉中进行熔融,再将熔融物冷却,并用100 mL4%的硝酸进行溶解,最后用ICP-MS进行分析。主量元素分析精度和准确度优于5%,微量和稀土元素分析精度和准确度为5%~10%,具体流程可参见文献[33]。

2.4 锆石Hf同位素测试方法

Hf同位素研究选取对象为LA-ICP-MS锆石U-Pb测年所用锆石。测试是在南京大学内生金属矿床成矿机制国家重点实验进行,所用仪器为Neptune II MC-ICP-MS,该仪器配有NewWave UP213激光剥蚀探针。仪器条件设置和数据采集详见文献[34,35]。对锆石中测试点采用35 μm直径的激光进行原位分析,以氦气当做载体气体,同时向气相载体中加入少许氮气以获得更高的灵敏度。

3 分析结果 3.1 锆石U-Pb定年结果

猴头沟二长花岗岩中分离出的锆石多为短柱状,晶形完好,颗粒大小不均,多数为 100~200 μm,阴极发光均具有清晰的生长环带和韵律结构(图 5),具有典型岩浆锆石的特点。锆石Th和U的质量分数变化范围较大,分别为(70.9~502 .0)×10-6和(130~922)×10-6,样品Th/U值较高,为0.40~0.93,且锆石群形态单一,为岩浆活动一次结晶而形成的,能代表花岗岩的形成年龄(表 1)。选择环带结构清晰的 25个代表性颗粒进行锆石U-Pb年龄测定,其中9号测试点数据不谐和,其余24个测试点206Pb/238U表面年龄为415~427 Ma(表 1),锆石U-Pb谐和图显示24个测试点在谐和图上分布比较集中,其加权平均年龄值为(419.0±1.9)Ma(MSWD=0.59) (图 6),代表了猴头沟地区二长花岗岩的结晶年龄,属于晚志留世末期。

实线圆圈为LA-ICP-MS U-Pb定年位置,虚线圆圈为Hf同位素测试位置。 图 5 猴头沟二长花岗岩锆石阴极发光(CL)典型图像和U-Pb年龄/εHf(t)值 Fig.5 CL images and U-Pb ages/εHf(t) values of zircons in Houtougou monzogranite
表 1 猴头沟二长花岗岩HTG-03-B1样品LA-ICP-MS锆石U-Pb测年结果 Table 1 LA-ICP-MS zircon U-Pb data for the sample HTG-03-B1 from Houtougou monzogranite
测点wB/10-6Th/U同位素比值年龄/Ma
ThU207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ
1247.04440.560.051 00.001 40.474 20.012 40.067 10.000 824356.53948.54194.6
2264.04720.560.053 10.001 60.493 81.012 40.067 30.000 8345102.84079.64204.9
3502.09220.540.056 30.001 10.520 42.012 40.066 50.000 646544.44257.04153.8
4199.02990.660.052 30.001 30.494 93.012 40.068 30.000 730255.64087.94264.5
5139.02570.540.055 50.001 60.511 64.012 40.066 50.000 943264.841910.34155.2
6220.02850.770.052 30.001 40.489 65.012 40.067 60.000 829867.64059.14224.9
7268.03630.740.055 40.001 40.523 36.012 40.068 30.000 942859.34278.94265.2
8234.04670.500.055 80.001 80.519 87.012 40.067 00.000 945674.142511.64185.6
10353.05850.600.053 60.001 30.500 88.012 40.067 40.000 736753.74127.84204.3
11236.05280.450.057 90.001 30.548 39.012 40.068 30.000 852850.04448.04264.6
12285.04590.620.056 80.001 20.527 010.012 40.067 10.000 748348.14307.44194.3
13232.04330.540.055 30.001 30.517 311.012 40.067 50.000 743351.84237.94214.0
14241.04950.490.054 90.001 40.511 512.012 40.067 20.000 740955.64198.84194.4
15212.05320.400.055 30.001 20.513 313.012 40.067 10.000 843348.14217.84194.8
16176.03030.580.063 20.001 80.586 814.012 40.067 50.000 972260.046910.34215.3
1781.21300.620.054 20.001 90.503 115.012 40.068 00.001 438979.641411.74248.4
18187.02260.830.055 30.001 70.507 616.012 40.066 90.000 843370.441710.24175.1
19234.04530.520.056 20.001 40.518 117.012 40.066 50.000 646158.34248.74153.9
20125.02100.590.062 70.001 80.580 018.012 40.067 20.000 869865.04649.84194.7
21439.07980.550.054 70.001 20.510 219.012 40.067 20.000 639850.04197.64193.8
22345.03710.930.053 60.001 60.498 520.012 40.067 00.000 935468.541110.64185.5
23329.06040.550.053 40.001 20.494 821.012 40.066 90.000 634649.14087.24183.8
24264.04490.590.058 00.001 30.552 522.012 40.068 50.000 753254.64478.64274.5
2570.91600.440.053 20.002 10.493 723.012 40.067 10.001 133986.140713.44186.9
图 6 猴头沟二长花岗岩锆石U-Pb年龄加权图和谐和图< Fig.6 Zircon U-Pb Concordia and weighted mean diagrams of Houtougou monzogranite
3.2 主量、微量和稀土元素

猴头沟二长花岗岩岩石地球化学分析结果见表 2。主量元素组成: w(SiO2)为73.08%~75.03%,w(TiO2)为0.17%~0.21%,w(Al2O3)为12.38%~12.82%,w(TFeO)为1.43%~1.82%,w(MnO)为0.01%~0.02%,w(MgO)为0.08%~0.21%,w(CaO)为0.33%~0.95%,w(Na2O)为3.29%~3.44%,w(K2O)为5. 09%~5.39%,w(P2O5)为0.031%~0.034%,w(Na2O+K2O)为8.38%~8.82%,K2O/Na2O为1.53~1.59。据w(SiO2)-w(K2O)岩石系列图解(图 7a)判断,属高钾钙碱性系列向钾玄岩性系列的过渡。A/CNK值为0.970~1.066,A/NK>1,A/NK-A/CNK判别图解(图 7b)显示样点落入准铝质过铝质花岗岩区。分异指数(ID)为93.05~95.33,说明岩浆分离结晶作用强烈,分异程度很高。在各种SiO2和氧化物哈克图解中,无明显变化趋势(图未附)。

表 2 猴头沟二长花岗岩HTG-03-B1样品主梁元素、微量元素和稀土元素分析结果表 Table 2 Major,trace elements and REE(ppm) for the sample HTG-03-B1 from Houtougou monzogranite
a.底图据文献[36]; b.底图据文献[37]。 图 7 猴头沟二长花岗硅钾图解(a)和A/NK-A/CNK图解(b) Fig.7 Chemical variation diagrams of w(K2O) vs. w(SiO2)(a),and A/NK-A/CNK (b) of Houtougou monzogrante

猴头沟二长花岗岩稀土总量质量分数为339.5×10-6~378.0×10-6,轻稀土质量分数为 313.5×10-6~346.4×10-6,重稀土质量分数为26.0×10-6~32.7×10-6,轻稀土元素较重稀土元素富集明显,轻重稀土比值为9.67~12.12,LaN/YbN值为11.80~15.83(表 2),配分曲线为右倾型分布模式(图 8)。δEu为0.08~0.12,Eu强烈亏损。猴头沟样品花岗岩微量元素原始地幔标准化蜘蛛图(图 9)显示,Rb、Th、U、La、Ce、Nd等元素相对富集,Nb、Ta、Ba、Sr、P、Ti、Eu等亏损而呈现"V"型谷,其中Eu、P、Ba、Sr显示强烈的亏损异常,表明花岗质岩浆形成过程中长石为残留相,Nb、Ta、Ti亏损通常表明与俯冲作用有成因联系[41]

微量元素球粒陨石标准化值据文献[39]。 图 8 猴头沟二长花岗岩稀土元素配分模式图 Fig.8 Chondrite-normalized REE patterns of Houtougou monzogranite
微量元素原始地幔标准化值据文献[40]。 图 9 猴头沟二长花岗岩微量元素蛛网图 Fig.9 Primitive-mantle-normalized variation spidergrams of Houtougou monzogranite
3.3 锆石Hf同位素

本次测试标样为锆石MT和锆石Plai,其176Hf/177Hf值分析结果分别为0.282 502±0.000 005 (2σ,n=26)和 0.282 914±0.000 012 (2σ,n=24)。对于εHf(t)的计算,采用常数如下:176Lu为1.867×10-11[42],(176Lu/177Hf)CHUR=0.033 6,(176Hf/177Hf)CHUR=0.282 785[43],亏损地幔Hf模式年龄(TDM1)的计算应用的是测量的176Lu/177Hf锆石比值,并且假设现在球粒陨石地幔176Hf/177Hf值为0.283 250,亏损地幔的176Lu/177Hf比值为0.038 4[44]。锆石两阶段模式年龄计算是将锆石初始176Hf/177Hf值投影到地幔增长曲线上,其中所用平均大陆地壳176Lu/177Hf平均值为0.015。

本次研究成功获得猴头沟二长花岗岩样品(HTG-03-B1)中共23个锆石的Hf同位素组成(表 3),其176Yb/177Hf和176Lu/177Hf范围分别为0.036 845~0.135 098和0.000 743~0.002 618。176Lu/177Hf非常接近或小于0.002,表明源区具有极低的放射性成因176Hf的积累,分析获得的176Hf/177Hf值能够代表源区的Hf同位素组成[46]。23个锆石分析点的(176Hf/177Hf)i值为0.282 525~0.282 660,εHf(t)值为0.2~5.1,相对变化较小,显示了Hf同位素组成的均一性。与此相对应,样品也显示出变化较小的二阶段模式年龄(TDM2)为1 066~1 371 Ma。

表 3 猴头沟二长花岗岩HTG-03-B1样品LA-ICP-MS锆石U-Hf同位素数据 Table 3 LA-MC-ICP-MS of zircon Lu-Hf isotopic composition in situ analysis of the sample HTG-03-B1 from Houtougou monzogranite
测点年龄/Ma176Yb/177Hf176Lu/177Hf176Hf/177Hf2σ(176Hf/177Hf)iεHf(0)(a)εHf(t)2σTDM1/MaTDM2/MafLu/Hf
14190.060 4620.001 1860.282 5900.000 0210.282 580-6.92.10.79421 249-0.96
24200.036 8450.000 7430.282 6330.000 0190.282 627-5.43.80.78701 143-0.98
34150.094 4030.001 7920.282 6310.000 0230.282 617-5.53.30.88981 169-0.95
44260.086 0390.001 6710.282 6730.000 0270.282 660-4.05.11.08351 066-0.95
54150.075 2560.001 4950.282 5940.000 0270.282 582-6.82.11.09441 247-0.96
64220.053 5880.001 0740.282 6380.000 0220.282 630-5.23.90.88711 136-0.97
74260.078 6030.001 5760.282 6310.000 0250.282 619-5.43.60.98931 159-0.95
84180.060 2320.001 2050.282 6470.000 0230.282 638-4.94.10.88611 121-0.96
104200.084 3030.001 6390.282 6720.000 0260.282 659-4.04.90.98361 072-0.95
114260.078 3190.001 4950.282 6170.000 0190.282 605-5.93.10.79111 189-0.96
124190.049 5600.000 9850.282 6220.000 0210.282 614-5.83.30.78911 172-0.97
134210.056 5170.001 1320.282 6640.000 0210.282 655-4.34.80.78361 081-0.97
144190.052 5390.001 0370.282 6130.000 0190.282 605-6.13.00.79051 193-0.97
154190.037 4490.000 7550.282 5620.000 0190.282 556-7.91.20.79701 303-0.98
164210.076 6870.001 5300.282 6400.000 0270.282 628-5.13.80.98791 140-0.95
184170.075 8760.001 4670.282 6230.000 0270.282 612-5.73.20.99011 179-0.96
194150.068 4170.001 3380.282 6160.000 0220.282 605-6.02.90.89091 195-0.96
204190.045 5980.000 9200.282 5330.000 0250.282 525-8.90.20.91 0151 371-0.97
214190.085 4150.001 6340.282 5600.000 0210.282 547-8.00.90.89961 323-0.95
224180.135 0980.002 6180.282 6070.000 0240.282 587-6.32.30.99541 235-0.92
234180.069 6390.001 3430.282 5900.000 0230.282 580-6.92.00.89451 251-0.96
244270.053 9190.001 0610.282 6370.000 0240.282 628-5.24.00.98731 137-0.97
254180.093 2080.001 8220.282 6460.000 0260.282 631-4.93.90.98781 135-0.95
注:εHf(0)和εHf(t) 值利用现在的(176Hf/177Hf)CHUR=0.282 785和(176Lu/177Hf)CHUR=0.033 6[43]计算获得。TDM1值利用现在的(176Hf/177Hf)DM=0.283 25和(176Lu/177Hf)DM=0.038 4[44]计算获得。TDM2值利用现在的(176Hf/177Hf)DM=0.283 250,(176Lu/177Hf)DM=0.038 4[44]和(176Lu/177Hf)cc=0.015[45]计算获得。
4 讨论 4.1 岩石成因类型

花岗岩类岩石成因类型与其生成的构造背景密切相关,不同构造演化阶段形成的岩石成因类型有所差异。其具有高硅(w(SiO2)为73.08%~75.03%)和高碱(w(K2O+Na2O)为8.38%~8.82%)、相对富铝(A/CNK为0.970~1.066)、高TFeO/MgO(8.65~20.58)和104Ga/Al值(3.039~3.304)、富集轻稀土、明显的负Eu异常和相对原始地幔明显富集Zr、Nb、Y、Hf等高场强元素,并且强烈亏损 Ba、Sr、P和 Ti等低场强元素的特征,这些特征与A型花岗岩类的地球化学特征一致[47],在Whalen等[47]提出的判别图解(图 10)中,所有样品点均落入A型花岗岩区域。其高的Rb/Nb值和Y/Nb值等进一步表明,猴头沟二长花岗岩属于A2型花岗岩[48](图 11)。

底图据文献[47]。 图 10 A型花岗岩判别图解 Fig.10 Discrimination diagrams of A-type granite
底图据文献[48]。 图 11 Rb/Nb-Y/Nb图解(a);Nb-Y-3Ga图解(b) Fig.11 Rb/Nb-Y/Nb diagram(a); Nb-Y-3Ga diagram(b)
4.2 岩石岩浆源区讨论

A型花岗岩的成因前人总结大致有以下4种观点:地幔碱性岩浆的分离结晶作用[49,50];熔出含水长英质岩浆之后的富 F、Cl麻粒岩相下地壳的低程度部分熔融[51,52];幔源岩浆与深熔形成的壳源岩浆的混合与交代作用[53,54];低压下钙碱性岩石的部分熔融[55]。猴头沟二长花岗岩可以排除是幔源岩浆来源的可能,因为花岗质岩浆不能直接由幔源橄榄岩直接形成,幔源橄榄岩形成的岩浆酸碱度不会高于闪长质(w(SiO2)为55%)[56,57]。猴头沟二长花岗岩并不发育镁铁质包体,没有明显幔源岩浆与地壳熔体混合成因的证据。所以可以判定,该区域二长花岗岩不是地幔成因,故可以推测是壳源成因。

原始地幔标准化蛛网图上,强烈的Nb、Ta、Ti异常表明猴头沟二长花岗岩与板块俯冲有成因联系。在花岗质岩浆形成过程中,幔源镁铁质熔体或者是初生幔源镁铁质下地壳的部分熔融形成高176Hf/177Hf值(即εHf(t)>0)[58]。约为419 Ma的猴头沟二长花岗岩的锆石εHf(t)值为0.2~5.1,可以推断猴头沟二长花岗岩是镁铁质下地壳部分熔融的产物(图 12)。强烈亏损的P、Sr和明显的负Eu异常表明源区可能斜长石富集。样品也显示二阶段模式年龄(TDM2)为1 066~1 371 Ma,表明为花岗岩源区应为中元古代镁铁质下地壳。锆石的饱和温度提供了一个简单有效的长英质花岗岩从原岩分离结晶时温度的估算方法[43]。猴头沟二长花岗岩的锆石饱和温度(TZr)为715~727 ℃,其平均温度为720 ℃,相对较高,这与A型花岗岩石大都属于高温成因的认识一致[52,59,60,61]

图 12 猴头沟二长花岗岩εHf(t)-206Pb/238U年龄图解 Fig.12 εHf(t)-206Pb/238U diagram of zircons from Houtougou monzogranite
4.3 构造环境和动力学意义

由于五龙沟地区研究程度很低,关于猴头沟二长花岗岩的侵位机制等前人没有任何报道;本文也未能对此开展过系统调查,所以只能从其地球化学特征判断其侵位时的构造背景。通常而言,A1型花岗岩主要形成于造山后或非造山伸展的构造环境[48,62,63,64,65],而A2型花岗岩通常代表花岗质岩浆起源于陆陆碰撞或岛弧环境下的大陆壳部分熔融[48]w(Rb)-w(Yb+Ta)图解(图 13a)、w(Rb)-w(Y+Nb)图解(图 13b)及w(Y)-w(Nb)图解(图 13c)等均无法有效判断其构造环境,暗示为造山后花岗岩,且从R1-R2图解(图 13d)可知,猴头沟二长花岗岩大致落入造山后环境的A型花岗岩区域内,可以推测猴头沟二长花岗岩应为造山后A2型花岗岩。该二长花岗岩锆石U-Pb年龄((419.0±1.9) Ma)属于早古生代晚志留世,可见其为早古生代晚志留世造山旋回末后造山的产物,说明东昆仑造山带在早古生代晚志留世造山旋回末后造山阶段,在五猴头地区形成了A2型花岗岩。

a-c底图据文献[66] ;d底图据文献[67]。 图 13 w(Rb)-w(Yb+Ta)图解(a); w(Rb)-w(Y+Nb)图解(b); w(Y)-w(Nb)图解(c); R1-R2构造判别图解(d) Fig.13 w(Rb)-w(Yb+Ta) diagram(a); w(Rb)-w(Y+Nb) diagram(b); w(Y)-w(Nb) diagram(c); R1-R2 diagram(d)

如前所述,东昆仑造山带记录青藏高原拼合的早期历史(图 1)[12,13,14]。沿着东昆中断裂和东昆南断裂发育两条蛇绿岩带,一般认为其分别代表了原特提斯洋和古特提斯洋的存在[16,17],其存在大致时间分别为新元古代早泥盆世和石炭纪晚三叠世[12,16,17,18,19,20,21]。大量的早泥盆世造山后花岗岩的报道也进一步限定原特提斯洋的闭合时限最晚应该是早泥盆世,如西段祁曼塔格地区喀雅克登塔格造山后环境形成的二长花岗岩为394 Ma[2];中段鲸鱼湖北侧东昆中断裂带附近造山后环境形成花岗岩年龄为413~403 Ma[3];西段祁曼塔格地区造山后环境形成的乌兰乌珠尔正长花岗岩为389 Ma[4];东段具有典型后碰撞特征的跃进山花岗岩体为407 Ma,并显示很强的幔源岩浆作用印记[1];西段祁曼塔格地区代表后碰撞阶段壳幔岩浆混合产物的野马泉二长花岗岩和花岗闪长岩年龄为393~386 Ma[5];东段具有典型的造山后伸展阶段A型花岗岩特征的冰沟正长花岗岩年龄为391 Ma[6];中段夏日哈木矿区构造后伸展体制下的花岗岩年龄为391 Ma[7]。可见,前人限定东昆仑地区原特提斯洋进入造山后阶段时限为不早于413 Ma。而根据本文的进一步研究表明,原特提斯洋沿着东昆中断裂带的最早闭合时限,可能要提早到晚志留世之前,因为形成于419 Ma的猴头沟二长花岗岩为具有典型造山后伸展环境形成的A2型花岗岩特征。

总之,最晚在晚志留世,东昆仑地区原特提斯洋盆已经闭合,东昆中和东昆南带拼合在一起,整个东昆仑造山带进入造山后伸展阶段,拼贴后的伸展作用阻碍了浅部洋壳的进一步俯冲,但深部的俯冲板片仍在继续向下俯冲,导致深部继续俯冲的板片发生断离,形成板片窗,引起深部软流圈的上涌。随后,上涌的软流圈物质对上覆地壳的直接加热导致低压下的中元古代镁铁质下地壳部分熔融最终形成本区高钾钙碱性系列的A2型花岗岩。

5 结论

1)五龙沟地区猴头沟二长花岗岩成因类型为造山后伸展阶段A2型花岗岩,富SiO2、K2O、贫Al2O3、贫Sr、富Y和Yb,具有强烈的负铕异常,Rb、Th、U、La、Ce、Nd相对富集,Nb、Ta高场强不相容元素亏损,Ba、Sr、P、Ti 强烈亏损。

2)猴头沟二长花岗岩锆石的Hf同位素研究表明,锆石εHf(t)值为0.2~5.1,对应二阶段模式年龄(TDM2)为1 066~1 371 Ma,推测花岗岩源区来自中元古代镁铁质下地壳部分熔融。

3)LA-ICP-MS锆石U-Pb测年表明,猴头沟二长花岗岩的206Pb/238U加权平均年龄值为(419.0±1.9)Ma,由此推测原特提斯洋在东昆仑地区的最晚闭合时限应该不晚于晚志留世末期(~419 Ma),而不是前人认为的早泥盆世。

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http://dx.doi.org/10.13278/j.cnki.jjuese.201602112
吉林大学主办、教育部主管的以地学为特色的综合性学术期刊
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文章信息

严威, 邱殿明, 丁清峰, 刘飞
Yan Wei, Qiu Dianming, Ding Qingfeng, Liu fei
东昆仑五龙沟地区猴头沟二长花岗岩年龄、成因、源区及其构造意义
Geochronology, Petrogenesis, Source and Its Structural Significance of Houtougou Monzogranite of Wulonggou Area in Eastern Kunlun Orogen
吉林大学学报(地球科学版), 2016, 46(2): 443-460
Journal of Jilin University(Earth Science Edition), 2016, 46(2): 443-460.
http://dx.doi.org/10.13278/j.cnki.jjuese.201602112

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收稿: 2015-08-14

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