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  中国地质 2021, Vol. 48 Issue (1): 173-188  
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孔会磊, 栗亚芝, 李金超, 贾群子, 国显正, 王宇, 姚学钢. 2021. 东昆仑希望沟橄榄辉长岩的岩石成因:地球化学、锆石U-Pb年龄与Hf同位素制约[J]. 中国地质, 48(1): 173-188.  
Kong Huilei, Li Yazhi, Li Jinchao, Jia Qunzi, Guo Xianzheng, Wang Yu, Yao Xuegang. 2021. Petrogenesis of Xiwanggou olivine gabbro in East Kunlun Mountains: Constraints from geochemistry, zircon U-Pb dating and Hf isotopes[J]. Geology in China, 48(1): 173-188. (in Chinese with English abstract).  

东昆仑希望沟橄榄辉长岩的岩石成因:地球化学、锆石U-Pb年龄与Hf同位素制约
孔会磊1,2, 栗亚芝2, 李金超2, 贾群子2, 国显正2,3, 王宇2, 姚学钢2,3    
1. 中国地质科学院, 北京 100037;
2. 中国地质调查局西安地质调查中心自然资源部岩浆作用成矿与找矿重点实验室, 陕西 西安 710054;
3. 中国地质大学(武汉)地质调查研究院, 湖北 武汉 430074
摘要:东昆仑古特提斯域镁铁-超镁铁质岩石的研究极为薄弱,文章报道了青海东昆仑东段希望沟橄榄辉长岩的岩相学、锆石U-Pb年代学、岩石地球化学及锆石Hf同位素资料,以确定该岩体的形成时代、岩石成因及构造环境,为东昆仑晚古生代-早中生代构造岩浆演化提供新的约束。岩石地球化学研究表明,希望沟橄榄辉长岩具有低SiO2(40.91%~42.14%)、低TiO2(0.29%~0.39%)、高MgO(28.18%~30.66%)、贫碱(K2O+Na2O=1.09%~1.36%)的特征,属亚碱性系列岩石,m/f比值介于5.03~5.39,属于铁质超基性岩类。岩石微量元素总体上富集大离子亲石元素(Rb、Th、U、K)和Pb,相对亏损高场强元素(Nb、P、Ti),ΣREE为28.1710-6~30.9510-6,(La/Yb)N为3.77~4.98,显示轻稀土富集的特征,δEu=0.80~0.95,具有弱的Eu负异常。利用LA-ICP-MS锆石U-Pb定年技术,获得橄榄辉长岩加权平均年龄为(264.9±1.2)Ma(n=26,MSWD=0.71),属中二叠世。锆石176Hf/177Hf比值为0.282709~0.283152,对应的εHft)=3.7~19.3,锆石单阶段Hf模式年龄TDM为135~753 Ma,平均为414 Ma,大于锆石U-Pb年龄。研究认为,橄榄辉长岩的岩浆源区主要为亏损地幔,可能有早期流体交代的岩石圈地幔组分的加入,并经历了地壳物质的混染。结合东昆仑区域构造演化,认为希望沟橄榄辉长岩是阿尼玛卿古特提斯洋俯冲阶段的产物,说明古特提斯洋在中二叠世已北向俯冲。
关键词东昆仑    希望沟橄榄辉长岩    LA-ICP-MS锆石U-Pb测年    地球化学    Hf同位素    地质调查工程    
中图分类号:P597;P588.12+.4            文献标志码:A             文章编号:1000-3657(2021)01-0173-16
Petrogenesis of Xiwanggou olivine gabbro in East Kunlun Mountains: Constraints from geochemistry, zircon U-Pb dating and Hf isotopes
KONG Huilei1,2, LI Yazhi2, LI Jinchao2, JIA Qunzi2, GUO Xianzheng2,3, WANG Yu2, YAO Xuegang2,3    
1. Chinese Academy of Geological Sciences, Beijing 100037, China;
2. Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, MNR, Xi'an Center of Geological Survey, CGS, Xi'an 710054, Shaanxi, China;
3. Institute of Geological Survey, China University of Geosciences, Wuhan 430074, Hubei, China
Abstract: The mafic-ultramafic rocks in Paleo-Tethys domain of East Kunlun Mountains are not well documented. In this paper, the authors present petrographical, geochronological, lithogeochemical and Hf isotopic data for the Xiwanggou olivine gabbro located in the eastern section of East Kunlun Mountains, Qinghai Province, with the purpose of constraining its formation age, petrogenesis and tectonic setting and providing new constraints for the Late Paleozoic-Early Mesozoic tectono-magmatic evolution in East Kunlun Mountains. Lithogeochemical studies show that the olivine gabbro, which is of sub-alkaline series, is characterized by low SiO2 (40.91%-42.14%), low TiO2 (0.29%-0.39%) and alkali content (K2O+Na2O=1.09%-1.36%) but high MgO content (28.18%-30.66%). The m/f ratios range from 5.03 to 5.39, falling into the field of ferrous-ultrabasic rocks. This suite of rocks are enriched in LILE (such as Rb, Th, U and K) and Pb, and relatively depleted in HFSE (such as Nb, P and Ti). The rocks have low REE content, with LREE-rich pattern and slightly negative Eu anomaly (ΣREE=28.1710-6-30.9510-6, (La/Yb)N=3.77-4.98, δEu=0.80-0.95). LA-ICP-MS zircon U-Pb dating indicates that the weighted mean age of olivine gabbro is (264.9±1.2)Ma(n=26, MSWD=0.71), suggesting Middle Permian. Zircon 176Hf/177Hf values are in the range of 0.282709-0.283152 with corresponding εHf(t) values of 3.7-19.3. and Lu-Hf single-stage modal ages (TDM) vary from 135 to 753 Ma with mean age being 414Ma, older than U-Pb age. The authors hold that the parental magma of olivine gabbro was likely derived from the depleted mantle, with the probable addition of fluid-modified lithospheric mantle components, and was contaminated by crustal material. Combined with evolutionary characteristics of regional structures in East Kunlun Mountains, the authors consider that Xiwanggou olivine gabbro was formed during the subduction of Anyemaqen-Paleo-Tethys Ocean, and the northward subduction of Anyemaqen-Paleo-Tethys Ocean started at least in Middle Permian.
Key words: East Kunlun Mountains    Xiwanggou olivine gabbro    LA-ICP-MS zircon U-Pb dating    geochemistry    Hf isotopes    geological survey engineering    

1 引言

东昆仑造山带位于青藏高原东北缘,是中国中央造山带的重要组成部分,晚古生代—早中生代东昆仑处于古特提斯演化阶段(莫宣学等,2007),是形成东昆仑造山带最重要的一个阶段(罗照华等,1999),东昆仑的构造格架基本形成,是贵金属、铁、有色金属等矿产形成的重要时期,历来为研究者所重视。东昆仑古特提斯域广泛发育与俯冲-碰撞相关的岩浆作用,前人进行了大量研究,并取得了丰硕成果。然而,对于阿尼玛卿古特提斯洋俯冲作用的时限还存在争议,主要有以下几种观点:(1)古特提斯洋于晚二叠世开始俯冲(郭正府等,1998杨经绥等,2005莫宣学等,2007李瑞保等,2012李瑞保,2012Zhang et al., 2012Xiong et al., 2012, 2013陈国超,2014罗明非等,2015);(2)东昆仑古特提斯洋在晚二叠世就已经关闭,三叠纪花岗岩为碰撞-碰撞后环境产物(李荣社等,2008陈守建等,2010Huang et al., 2014);(3)古特提斯洋俯冲起始的时间为早—中二叠世(杨延乾等,2013Liu et al., 2014甘彩虹,2014熊富浩,2014Xiong et al., 2014Li et al., 2015Peng et al., 2016孔会磊等,2017aChen et al., 2017Dong et al., 2018);(4)古特提斯洋俯冲一直持续到晚三叠世,三叠纪花岗岩类均为弧岩浆作用的产物(Yin and Harrison, 2000Liu,2005Yuan et al., 2009Wang et al., 2011Ding et al., 2014)。近年来,东昆仑二叠纪—三叠纪幔源岩浆事件开始受到关注,并取得了一系列成果(熊富浩等, 2011a, 2011b菅坤坤等,2015奥琮等,2015王亚磊等,2017孔会磊等,2017a张玉等,2017赵旭等,2018)。然而,相对于中酸性岩,对区内镁铁-超镁铁质岩石的研究还较为薄弱,这极大制约了对东昆仑晚古生代—早中生代构造岩浆演化的研究。

因此,本文选取青海东昆仑东段察汗乌苏河地区希望沟橄榄辉长岩为研究对象,进行了详细的岩石学、地球化学、LA-ICP-MS锆石U-Pb年代学及Hf同位素研究,对其岩石成因、构造环境进行探讨,以期对东昆仑造山带古特提斯构造演化提供新的制约。

2 地质背景

东昆仑造山带位于柴达木盆地与巴颜喀拉—松潘—甘孜地体之间(图 1a),是青藏高原内可与冈底斯带相媲美的另一条巨型岩浆岩带(莫宣学等,2007)。西以阿尔金走滑断裂为界与西昆仑相接,向东与秦岭—大别连接构成了中国著名的中央造山带。东昆仑造山带由东向西延伸可达1500 km(莫宣学等,2007Dai et al., 2013),为古亚洲与特提斯构造域的交汇部位(殷鸿福等,1997),构造-岩浆活动极其发育,主要经历了早古生代原特提斯与晚古生代—早中生代古特提斯两期洋陆构造演化阶段(许志琴等,2007)。东昆仑造山带内由北往南依次发育昆北、昆中、昆南3条岩石圈大断裂,以此为界可将东昆仑划分为东昆北、东昆中、东昆南3个构造单元(姜春发等, 1992, 2000)(图 1b)。造山带内广泛发育加里东—印支期中酸性岩类及前寒武纪变质基底。基底主要为古元古代金水口群(形成时代可约束在(2468±46)~1846 Ma,陆松年,2002莫宣学等,2007)和小庙岩群(锆石U-Pb年龄为1683~ 1554Ma,陈有炘等,2011)。研究区位于东昆仑造山带东段,处于昆北与昆中断裂之间,属东昆中构造带(图 1b)。从二叠纪到晚三叠世,东昆仑带产生了巨量的与俯冲-碰撞相关的岩浆作用。中酸性侵入岩主要有花岗闪长岩、石英闪长岩、二长花岗岩、英云闪长岩和正长花岗岩等(孔会磊等, 2017a, 2017b),基性-超基性岩体主要出露于希望沟、小尖山、拉陵高里沟脑、白日其利、中灶火、巴隆等地。

图 1 东昆仑造山带构造位置(a据Xia et al., 2015b)与岩浆岩分布图(b据Xia et al., 2015a Fig. 1 Geotectonic framework (a, after Xia et al., 2015b) and magmatite distribution (b, after Xia et al., 2015a) of East Kunlun orogenic belt
3 岩体地质及岩石学特征

希望沟镁铁质-超镁铁质杂岩体岩性主要为辉长岩、橄榄辉长岩、辉石橄榄岩、橄榄二辉岩、辉石岩等,岩浆分异较好(图 2),岩石后期发生了强烈的蛇纹石化与磁铁矿化。地表露头规模较小,出露面积约2.1 km2,形态多为不规则长条状、不规则三角形状及不规则椭圆状。杂岩体露头不好,多覆盖,北部与古元古代白沙河岩组变质岩呈断层接触,西部与晚奥陶世石英闪长岩侵入接触,东部被侏罗纪正长花岗岩侵入,南部与晚三叠世鄂拉山组呈不整合接触。杂岩体中橄榄辉石岩和橄榄辉长岩锆石U-Pb年龄分别为(262.4±1.6)Ma(孔会磊,2019)和(249.7±3.0)Ma(孔会磊等,2018),形成时代为中二叠世和早三叠世,为东昆仑晚古生代—早中生代构造旋回俯冲阶段多期次岩浆活动的产物。杂岩体m/f比值为3.38~6.18,为铁质系列基性-超基性岩。

图 2 希望沟一带镁铁-超镁铁质岩地质略图 1—第四系;2—上三叠统鄂拉山组英安岩段;3—古元古界白沙河组片麻岩段;4—早侏罗世正长花岗岩;5—中三叠世二长花岗岩;6—晚奥陶世石英闪长岩;7—顶志留世英云闪长岩;8—晚志留世花岗闪长岩;9—超镁铁质岩;10—榴闪岩;11—辉长岩;12—钾长花岗岩脉;13—橄榄辉长岩;14—断层;15—地质界线;16—不整合界线;17—采样位置 Fig. 2 The sketch geological map of the mafic-ultramafic intrusions in Xiwanggou area 1-Quaternary; 2-Dacite member of Upper Triassic Elashan Formation; 3-Gneiss member of Paleoproterozoic Baishahe Formation; 4-Early Jurassic syenogranite; 5-Middle Triassic monzonitic granite; 6-Late Ordovician quartz diorite; 7-Top Silurian tonalite; 8-Late Silurian granodiorite; 9-Undivided ultramafic rocks; 10-Amphibole eclogite; 11-Gabbro; 12-Syenogranite vein; 13-Olivine gabbro; 14-Fault; 15-Geological boundary; 16-Unconformable boundary; 17-Sampling position

本文年龄及地球化学样品均采自镁铁-超镁铁质杂岩体东约1 km的橄榄辉长岩(图 1b),地理坐标为35°52′51.87″N,98°42′27.43″E。岩体在平面上呈长条状南北向展布,南北长约400 m,东西宽20~30 m,侵入于古元古界金水口群白沙河组片麻岩段(图 2图 3a)。岩石为灰黑色,块状构造(图 3b),变余粒状结构。矿物成分主要由斜长石(>50%)、辉石(25%±)、橄榄石(20%±)及少量金属矿物(<5%)组成(图 3c)。斜长石晶体呈板状,大部分晶体已次生蚀变,主要被绿泥石交代,少部分被细小闪石或绢云母交代,常呈交代残留结构。橄榄石晶体呈粒状,常沿边缘被角闪石、蛇纹石、少量滑石交代,少数呈交代残留结构(图 3d),经电子探针测定为贵橄榄石。辉石晶体多被褐色角闪石、纤维闪石或细小滑石交代,仅见少量单斜辉石残留体(图 3e),电子探针测定为透辉石。金属矿物主要为磁铁矿,另可常见有磁黄铁矿、黄铜矿、镍黄铁矿在一起连生(图 3f)。

图 3 希望沟橄榄辉长岩的野外露头、手标本及显微照片 a—橄榄辉长岩野外露头;b—橄榄辉长岩手标本;c—橄榄辉长岩中的橄榄石、单斜辉石、斜长石(正交偏光);d—橄榄石沿边缘被闪石、蛇纹石等交代呈残留结构(正交偏光);e—单斜辉石被角闪石交代(正交偏光);f—矿化橄榄辉长岩中的镍黄铁矿、磁黄铁矿、黄铜矿(反射光);Pl—斜长石;Ol—橄榄石;Cpx—单斜辉石;Hbl—角闪石;Pn—镍黄铁矿;Po—磁黄铁矿;Ccp—黄铜矿 Fig. 3 Outcrop and hand specimen photos and microphotographs of the Xiwanggou olivine gabbro a- Outcrop photo of olivine gabbro; b- Hand specimen photo; c- The olivine, clinopyroxene, plagioclase in olivine gabbro (crossed nicols); d- The edge of olivine altered to amphibole and serpentine, showing relict texture (crossed nicols); e- The clinopyroxene metasomatized by amphibole (crossed nicols); f- The pentlandite, pyrrhotite and chalcopyrite in the mineralized olivine gabbro (reflected light); Pl- Plagioclase; Ol-Olivine; Cpx-Clinopyroxene; Hbl-Hornblende; Pn-Pentlandite; Po-Pyrrhotite; Ccp-Chalcopyrite
4 测试分析方法

锆石的挑选、制靶、透反射光、阴极发光照相均在北京前寒武纪科技有限公司实验室进行。经过双目镜下仔细挑选表面平整光洁且具不同长宽比例、不同柱锥面特征、不同颜色的锆石颗粒,再将这些锆石黏在双面胶上,用无色透明环氧树脂固定,待环氧树脂固化之后对其表面抛光至锆石中心。在原位分析之前,通过透反射光和CL图像详细研究锆石的晶体形貌和内部结构特征,选择无明显裂痕及包裹体的锆石进行测年。锆石微量元素含量、U-Pb同位素年龄和Hf同位素测试是在中国地质调查局西安地质调查中心自然资源部岩浆作用成矿与找矿重点实验室利用7700x型四级杆等离子体质谱仪、Neptune Plus型多接收等离子体质谱仪两台仪器同时和Geolas Pro型激光剥蚀系统联机来完成的。测试束斑直径为32 μm,激光剥蚀的样品气溶胶由氦气作为载气输送到质谱仪中进行测试,为了调节和提高仪器灵敏度,气路中间引入了氩气和少量氮气。样品气溶胶经过匀化器匀化之后被分成两路,一路被输送到四极杆型等离子体质谱仪中进行锆石微量元素含量和U-Pb同位素年龄测试,另一路被输送到多接收等离子体质谱仪中进行Hf同位素测试。每分析6个点样品分析一次标准样品NIST610、91500和GJ-1,GJ-1同时作为U-Pb年龄和Hf同位素测试监控样品,本次实验GJ-1标准样品176Hf/177Hf同位素的测试精准度为0.282030±20(2SD)。锆石微量元素含量和U-Pb年龄校正计算采用Glitter4.4.4来完成,Hf同位素校正计算采用实验室自己开发的计算机程序Hllow1.0来完成。详细测试流程可参照Meng et al. (2017)。年龄计算及谐和图的绘制采用Ludwig(2003)编写的Isoplot程序。

岩石主元素、稀土元素、微量元素分析在西安地质矿产研究所实验测试中心完成,其中主元素采用荷兰帕纳科公司Axios 4.0kW顺序式X射线荧光光谱仪(XRF)进行分析,分析精度优于5%;稀土和微量元素利用美国热电公司Series Ⅱ型SX50型电感耦合等离子质谱仪(ICP-MS)进行测定,分析精度优于5%~10%。

5 测试结果 5.1 LA-ICP-MS锆石U-Pb年龄

橄榄辉长岩(17XWGUPb01)中锆石均为无色透明或浅黄色,多为短柱状及浑圆状,长轴直径为70~130 μm,长宽比为1:1~2:1。锆石的阴极发光图像显示部分锆石颜色偏暗,内部结构不明显,但多数锆石内部可见清晰的岩浆韵律环带(图 4)。所测锆石的Th、U含量较高,分别为124.2×10-6~8502.3× 10-6、267×10-6~3878×10-6,Th/U比值为0.37~4.27,基本都大于0.4(表 1),指示其为岩浆结晶的产物(Hoskin and Schaltegger, 2003)。一些锆石的阴极发光照片具有白色的亮边,可能为变质重结晶的结果。26个有效锆石分析点206Pb/238U表面年龄集中在270.6~256.5 Ma,落在谐和线上及其附近,其加权平均年龄为(264.9±1.2)Ma(MSWD=0.71),谐和年龄值(266.0±1.5)Ma(MSWD=0.18),二者在误差范围内基本一致(图 5)。表明希望沟橄榄辉长岩结晶年龄为(264.9±1.2)Ma,属于中二叠世岩浆活动的产物。

图 4 希望沟橄榄辉长岩锆石阴极发光照片(圈内数字代表U-Pb分析点,其他数字代表206Pb/238U表面年龄) Fig. 4 CL images of zircons from Xiwanggou olivine gabbro (numbers in the circles represent analytical spots of U-Pb, and the values represent ages of 206Pb/238U)
表 1 希望沟橄榄辉长岩锆石LA−ICP−MS测年结果 Table 1 LA−ICP−MS isotopic data of zircon from Xiwanggou olivine gabbro
图 5 希望沟橄榄辉长岩锆石U-Pb谐和图(a)和加权平均年龄图(b) Fig. 5 Zircon U-Pb concordia diagram (a) and weighted mean ages diagram (b) of Xiwanggou olivine gabbro
5.2 元素地球化学 5.2.1 主量元素

5件橄榄辉长岩样品全岩地化分析数据见表 2。样品的SiO2含量变化范围较小,含量较低,为40.91%~42.14%,平均41.62%;Al2O3含量为6.34%~ 8.08%,平均值为7.09%。CaO含量为4.14% ~ 5.05%,MgO含量较高,为28.18%~30.66%,平均为29.37%,对应的Mg#值为83.64~84.53,整体大于原生岩浆的Mg#值范围(68~75;Wilson,1989)。样品的m/f比值介于5.03~5.39,属于铁质超基性岩类。样品的TiO2含量均较低,介于0.29%~0.39%,平均值为0.32%,明显低于大洋拉斑玄武岩(2.63%)和板内玄武岩(2.23%),相对接近岛弧型玄武岩(0.98%)(Pearce,1982);P2O5含量较低,为0.03%~0.06%,平均值为0.05%。Na2O含量为0.46%~0.74%,K2O含量为0.58%~0.71%,岩石的全碱(K2O+Na2O)含量为1.09%~1.36%,所有岩石样品在(Na2O+K2O) - SiO2图解中全部落在亚碱性橄榄辉长岩的范围(图 6a),在TFeO/MgO-SiO2图解中岩石落在拉斑系列范围内(图 6b)。总体上看,希望沟橄榄辉长岩具有低硅、低钛、高镁、贫碱的特征。

表 2 希望沟橄榄辉长岩主量元素(%)和微量元素(10-6)含量 Table 2 Content of major elements(%) and trace elements (10-6)of Xiwanggou olivine gabbro
图 6 希望沟橄榄辉长岩的(K2O+Na2O)−SiO2(a据Middlemost,1994)和TFeO/MgO−SiO2图解(b据Miyashiro,1974 Fig. 6 (K2O+Na2O)–SiO2(a, after Middlemost, 1994) and TFeO/MgO−SiO2 (b, after Miyashiro, 1974) plots for the Xiwanggou olivine gabbro
5.2.2 微量和稀土元素

原始地幔标准化微量元素蛛网图(图 7a)显示,橄榄辉长岩微量元素配分曲线近一致,显示了同源岩浆演化分异特征。岩石样品明显富集大离子亲石元素(LILE: Rb、Th、U、K)和Pb,相对亏损高场强元素(HFSE: Nb、P、Ti),显示具有弧岩浆的特征。

图 7 希望沟橄榄辉长岩的微量元素原始地幔标准化蛛网图(a)及稀土元素球粒陨石标准化配分曲线图(b)(标准化数值据Sun and McDonough, 1989 Fig. 7 Primitive mantle-normalized trace element patterns (a) and chondrite-normalized REE patterns (b) for the olivine gabbro (normalizing values after Sun and McDonough, 1989)

橄榄辉长岩的稀土总含量ΣREE为28.17×10-6~ 30.95 × 10-6,平均为29.75 × 10-6。轻重稀土比值LREE/HREE=4.32~5.06,平均为4.64,(La/Yb)N为3.77~4.98,平均4.64,属轻稀土富集型。(La/Sm)N为2.01~2.37,(Gd/Yb)N为1.41~1.62,稀土元素配分曲线总体显示出轻稀土元素富集重稀土元素平坦的配分形式(图 7b)。δEu为0.80~0.95,平均为0.89,样品显示弱的Eu负异常,表明深部岩浆房中发生了一定程度斜长石的分离结晶作用。

5.3 锆石原位Lu-Hf同位素

锆石Hf同位素分析与U-Pb测试在锆石颗粒同一位置联机同时进行。26个分析点的176Yb/177Hf和176Lu/177Hf比值范围分别为0.000981~0.090236和0.000036~0.003528(表 3),除测点24、35、39外,176Lu/177Hf比值均小于0.002,表明这些锆石在形成以后,仅具有较少的放射性成因176Hf的积累,因而分析获得的176Hf/177Hf比值能够代表源区的Hf同位素组成(吴福元等,2007)。26个分析点的(176Hf/177Hf)i的变化范围在0.282709~0.283152(表 3),对应的εHf(t)=3.7~19.3,变化范围较大(图 8a),加权平均值为12.3,数据点均落在球粒陨石Hf同位素演化线之上,更靠近亏损地幔(图 8b);计算得到锆石单阶段Hf模式年龄TDM为135~753 Ma,平均值为414 Ma。

表 3 希望沟橄榄辉长岩锆石Lu−Hf同位素组成 Table 3 Zircon Lu−Hf isotopic compositions of Xiwanggou olivine gabbro
图 8 希望沟橄榄辉长岩的锆石Hf同位素组成图解 Fig. 8 Hf isotopic compositions of zircons from Xiwanggou olivine gabbro
6 讨论 6.1 岩浆源区和岩石成因

一般情况下,如果锆石εHf(t)为正值,说明岩浆源区为亏损地幔或从亏损地幔中新增生的年轻地壳;而εHf(t)值为负值,说明源区为富集地幔或以古老地壳物质占主导(Vervoort et al., 2000; Kinny and Maas, 2003Griffin et al., 2004)。橄榄辉长岩中锆石176Hf/177Hf比值为0.282709~0.283152,对应的εHf(t)=3.7~19.3,表明岩体形成于Hf同位素亏损的地幔源区。对于幔源岩浆而言,如果锆石母岩浆直接来源于未受任何影响的亏损地幔,那么锆石结晶年龄应近似等于锆石Hf模式年龄。而橄榄辉长岩锆石单阶段Hf模式年龄TDM为135~753 Ma,平均值为414 Ma,大于锆石结晶年龄265 Ma,可能是因为源区有富集岩石圈地幔的加入或遭受了地壳物质的混染(吴福元等,2007)。

希望沟橄榄辉长岩样品的Nb/Ta比值为6.00~ 6.96,平均为6.40,Zr/Hf比值变化于30.49~34.00,平均值为31.43,分别与大陆地壳值相近(Nb/Ta=11,Zr/Hf=33,据Taylor and McLennan, 1985),而低于洋中脊玄武岩值(Nb/Ta=17.7,Zr/Hf=36.1,据Sun and McDonough, 1989),反映橄榄辉长岩受到地壳同化混染的影响。地壳中Ce/Pb比值<15,而典型地幔Ce/Pb=25±5(Hofmann et al., 1986)。希望沟橄榄辉长岩中Ce/Pb比值为0.73~1.15,明确显示岩体受到同化混染作用;Nb/U比值可作为判别地壳混染的标志,洋中脊玄武岩和洋岛玄武岩Nb/U比值为47± 10,而大陆地壳和原始地幔Nb/U平均值分别为8.93和33.59(Taylor and McLennan, 1985),希望沟橄榄辉长岩Nb/U比值在4.46~5.80,远离原始地幔平均值,接近大陆地壳,反映岩浆在上升过程中有陆壳物质的加入。

希望沟橄榄辉长岩样品Th/Yb比值为1.25~ 1.80,在Th/Yb- Nb/Yb图解中投点均明显偏离MORB-OIB演化线(图 9a),暗示其形成明显受到俯冲组分的影响(Pearce and Peate, 1995)。在Nb/ Zr-Th/Zr图解中,样品显示了流体俯冲交代的趋势(图 9b)。以上特征可看出有含水流体进入地幔源区(Woodhead et al., 2001),这种岩石圈地幔主要是由早期大洋俯冲阶段流体交代形成。综上所述,希望沟橄榄辉长岩源区主要为亏损地幔,可能有早期流体交代的岩石圈地幔组分的加入,并经历了地壳物质的混染。

图 9 希望沟橄榄辉长岩源区性质判别图解(a据Pearce, 2008;b据Woodhead et al., 2001 Fig. 9 Discrimination diagram of the source characteristics for Xiwanggou olivine gabbro (a, after Pearce, 2008; b, after Woodhead et al., 2001)
6.2 希望沟橄榄辉长岩形成时代及其构造意义

希望沟橄榄辉长岩明显富集大离子亲石元素(Rb、Th、U、K)和Pb,相对亏损高场强元素(Nb、P、Ti),显示了岛弧岩浆岩的特征。样品的TiO2含量均较低,介于0.29%~0.39%,相对接近岛弧型玄武岩(0.98%),明显低于洋中脊玄武岩(1.5%)和洋岛玄武岩(大于2.0%)(Pearce,1982);样品的微量元素Nb/La=0.31~0.39(< 1),La/Ta=17.00~19.92(>15),Th/Yb=1.25~1.80(>0.1),Th/Nb=0.66~0.77(>0.07),进一步显示了岛弧的性质(Condie,1989)。

为进一步探讨橄榄辉长岩的构造环境,笔者利用不活动元素协变关系进行构造环境判别。在Nb× 2-Zr/4-Y图解(图 10a)中,岩石表现出火山弧玄武岩的特征;在Zr/117-Th-Nb/16判别图(图 10b)中,样品均落在破坏板块边缘玄武岩区域,暗示橄榄辉长岩为俯冲作用形成的岛弧岩浆岩。

图 10 希望沟橄榄辉长岩构造环境判别图解(a据Meschede, 1986;b据Wood et al., 1979 Fig. 10 Tectonic discrimination diagrams for Xiwanggou olivine gabbro (a, after Meschede, 1986; b, after Wood et al., 1979)

东昆仑造山带发育巨量与俯冲-碰撞相关的岩浆活动,前人进行了大量研究工作,然而,对于阿尼玛卿古特提斯洋俯冲作用起始的时限还存在争议。有学者认为古特提斯洋于晚二叠世开始俯冲,闭合于中三叠世,晚二叠世—中三叠世早期为俯冲造山阶段(260~238 Ma),中三叠世晚期—晚三叠世早期为同碰撞阶段(237~230 Ma)(郭正府等,1998杨经绥等,2005莫宣学等,2007李瑞保等,2012李瑞保,2012Zhang et al., 2012Xiong et al., 2012, 2013陈国超,2014罗明非等,2015)。也有学者认为古特提斯洋俯冲起始的时间可能早于260 Ma,杨延乾等(2013)认为古特提斯洋268 Ma左右就已经进入俯冲阶段;Liu et al.(2014)获得小庙镁铁质岩墙群角闪石Ar-Ar年龄为(277.76±2.72)Ma,认为阿尼玛卿古特提斯洋北向俯冲的起始时间不晚于278 Ma;随后其他一些学者也相继提出了古特提斯洋俯冲开始于早二叠世的观点(熊富浩,2014Xiong et al., 2014Li et al., 2015Peng et al., 2016Chen et al., 2017陈邦学等,2019)。一般认为,中晚三叠世具有典型磨拉石堆积特征的不整合面是阿尼玛卿大洋闭合的标志,东昆仑阿尼玛卿洋闭合时限接近晚三叠世,晚三叠世之后东昆仑进入碰撞后伸展阶段(230~185 Ma)(熊富浩,2014)。

晚古生代—早中生代是东昆仑构造演化的一个重要阶段,近年来,东昆仑二叠纪—三叠纪幔源岩浆事件逐渐受到研究者重视,为古特提斯构造演化研究提供了新的方向。熊富浩等(2011a)获得白日其利角闪辉长岩的锆石U-Pb年龄为(248.9±4.2) Ma,认为是早三叠世阿尼玛卿古特提斯洋俯冲阶段岩浆活动的产物。熊富浩等(2011b)菅坤坤等(2015)分别对白日其利及中灶火镁铁质岩墙群进行了研究,获得年龄分别为(251±2)Ma与(249±1) Ma,认为是俯冲环境中弧后伸展作用影响下板片流体交代富集地幔的产物。奥琮等(2015)获得小尖山中细粒辉长岩的锆石U-Pb年龄为(227.8±0.9) Ma,认为形成于造山后伸展环境。孔会磊等(2017a)对位于希望沟西北的加当辉长岩进行了研究,获得其结晶年龄为(262.5±2.5)Ma,认为是古特提斯洋北向俯冲的产物。张玉等(2017)赵旭等(2018)分别获得阿拉思木辉长岩及按纳格角闪辉长岩锆石U-Pb年龄为(241.1±1.2)Ma与(242±2)Ma,均代表了东昆仑洋壳俯冲的晚期记录。Xiong et al.(2012)获得巴隆地区瑙木浑花岗闪长岩体中MME结晶年龄为(263±2)Ma,认为其形成于俯冲环境。从上可以看出,东昆仑二叠纪—三叠纪幔源岩浆活动主体形成于古特提斯洋俯冲阶段,少部分形成于后碰撞伸展阶段。

本文获得希望沟橄榄辉长岩的加权平均年龄为(264.9±1.2)Ma,表明岩体形成时代为中二叠世,同时结合前文岩石成因及构造环境分析,笔者认为希望沟橄榄辉长岩为阿尼玛卿古特提斯洋俯冲阶段的产物,说明至少在中二叠世265 Ma古特提斯洋就开始北向俯冲,这为东昆仑古特提斯构造演化提供了新的年代学证据,并进一步印证了东昆仑造山带广泛发育的二叠纪—三叠纪幔源岩浆事件。

7 结论

(1)岩石地球化学特征表明,希望沟橄榄辉长岩具有低硅、低钛、高镁、贫碱的特征,属亚碱性系列岩石。岩石富集LILE(Rb、Th、U、K)和Pb,相对亏损HFSE(Nb、P、Ti),显示轻稀土富集的特征,具有弱的Eu负异常。

(2)橄榄辉长岩锆石176Hf/177Hf比值为0.282709~0.283152,对应的εHf(t)=3.7~19.3。研究认为其源区主要为亏损地幔,可能有早期流体交代的岩石圈地幔组分的加入,并经历了地壳物质的混染。

(3)本文获得希望沟橄榄辉长岩LA-ICP-MS锆石U-Pb年龄为(264.9±1.2)Ma,形成于中二叠世,结合东昆仑区域构造演化,认为是阿尼玛卿古特提斯洋俯冲阶段的产物,说明古特提斯洋在中二叠世已北向俯冲。

致谢: 薄片鉴定过程中得到了中国地质调查局西安地质调查中心叶芳研究员的指导,锆石U-Pb年龄和Hf同位素测试及数据处理得到自然资源部岩浆作用成矿与找矿重点实验室李艳广工程师、靳梦琪工程师的热心帮助,文稿修改过程中审稿专家提出了宝贵的修改意见,在此一并表示衷心的感谢。

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