地质力学学报  2019, Vol. 25 Issue (3): 440-452
引用本文
孔会磊, 栗亚芝, 李金超, 贾群子, 国显正, 张斌. 东昆仑希望沟橄榄辉石岩LA-ICP-MS锆石U-Pb定年及岩石地球化学特征[J]. 地质力学学报, 2019, 25(3): 440-452.
KONG Huilei, LI Yazhi, LI Jinchao, JIA Qunzi, GUO Xianzheng, ZHANG Bin. LA-ICP-MS ZIRCON U-PB DATING AND GEOCHEMICAL CHARACTERISTICS OF THE XIWANGGOU OLIVINE PYROXENOLITE IN EAST KUNLUN[J]. Journal of Geomechanics, 2019, 25(3): 440-452.
东昆仑希望沟橄榄辉石岩LA-ICP-MS锆石U-Pb定年及岩石地球化学特征
孔会磊1,2 , 栗亚芝1,2 , 李金超1,2 , 贾群子1,2 , 国显正3 , 张斌4     
1. 中国地质调查局西安地质调查中心, 陕西 西安 710054;
2. 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;
4. 宝鸡西北有色七一七总队有限公司, 陕西 宝鸡 721012
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,对青海东昆仑东段希望沟镁铁-超镁铁质杂岩体中橄榄辉石岩进行岩相学、地球化学及年代学分析,以期为东昆仑晚古生代-早中生代构造岩浆演化提供新的约束。利用LA-ICP-MS锆石U-Pb定年,获得希望沟橄榄辉石岩加权平均年龄为(262.4±1.6)Ma,表明岩体侵位于中二叠世。岩石地球化学研究显示,希望沟橄榄辉石岩具有较高的Mg#值(85.31~86.27),m/f比值介于5.50~6.18,属于铁质超基性岩类;具有低TiO2(0.41%~0.52%)和低Al2O3(3.04%~4.57%),属钙碱性系列。岩石富集大离子亲石元素(Rb、Th、U)和Pb,相对亏损高场强元素(Nb、Ta、P、Ti),ΣREE为29.79×10-6~38.96×10-6,(La/Yb)N为1.71~2.51,平均2.15,δEu=0.73~0.86,具有弱的Eu负异常。岩石成因分析表明,岩石源区为被俯冲改造的岩石圈地幔,岩浆在演化过程中遭受了上地壳物质的同化混染作用。结合东昆仑区域构造演化,认为希望沟橄榄辉石岩为阿尼玛卿古特提斯洋俯冲阶段的岩浆记录。
关键词地球化学    锆石U-Pb定年    希望沟橄榄辉石岩    东昆仑    
DOI10.12090/j.issn.1006-6616.2019.25.03.041     文章编号:1006-6616(2019)03-0440-13
LA-ICP-MS ZIRCON U-PB DATING AND GEOCHEMICAL CHARACTERISTICS OF THE XIWANGGOU OLIVINE PYROXENOLITE IN EAST KUNLUN
KONG Huilei1,2 , LI Yazhi1,2 , LI Jinchao1,2 , JIA Qunzi1,2 , GUO Xianzheng3 , ZHANG Bin4     
1. Xi'an Center of Geological Survey, CGS, Xi'an 710054, Shannxi, China;
2. Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, MNR, Xi'an 710054, Shannxi, China;
3. Institute of Geological Survey, China University of Geosciences, Wuhan 430074, Hubei, China;
4. Baoji 717 Corps Limited of the NWME, Baoji 721012, Shannxi, China
Abstract: The mafic-ultramafic rocks in Paleo-Tethys domain of East Kunlun are not well documented. Researches on petrography, geochronology and geochemistry of the olivine pyroxenolite in Xiwanggou mafic-ultramafic intrusion, located in the Eastern Section of East Kunlun in Qinghai Province, can provide new constraints for the Late Paleozoic-Early Mesozoic tectono-magmatic evolution in East Kunlun. LA-ICP-MS zircon U-Pb dating indicates that the weighted mean age of Xiwanggou olivine pyroxenolite is (262.4±1.6) Ma, in the Middle Permian. Lithogeochemical studies show that the olivine pyroxenolite, which is calc-alkaline series, is characterized by high content of Mg#(85.31~86.27), low content of TiO2(0.41%~0.52%) and low Al2O3(3.04%~4.57%). The m/f ratios range from 5.50 to 6.18, indicative of iron series of ultrabasic rocks. This suite of rocks are enriched in LILE (such as Rb, Th, U) and Pb, relatively depleting in HFSE (such as Nb, Ta, P, Ti). The rocks have low REE contents, with slightly negative Eu anomaly(∑REE=29.79×10-6~38.96×10-6, (La/Yb)N=1.71~2.51, δEu=0.73~0.86). The geochemical characteristics show that the parental magma of olivine pyroxenolite derived probably from the mantle influenced by the subducton. The parental magma is developed by the upper crustal assimilation contamination during the magma evolution. Combined with regional evolutionary characteristics in East Kunlun, the authors consider that the Xiwanggou olivine pyroxenolite ought to be the magmatism records in the subduction of Anyemaqen-Paleo-Tethys ocean.
Key words: geochemistry    zircon U-Pb dating    Xiwanggou olivine pyroxenolite    East Kunlun    
0 引言

东昆仑造山带是位于青藏高原东北缘的一条规模巨大的构造-岩浆岩带,是中国中央造山系的重要组成单元,历来为国内外地质学界所重视[1-2]。晚古生代—早中生代东昆仑处于古特提斯演化阶段[3],是形成东昆仑造山带最重要的一个阶段[4]。自东昆仑造山带发现夏日哈木超大型铜镍硫化物矿床以来,掀起了新一轮镁铁-超镁铁质岩研究的热潮。近年来,东昆仑二叠纪—三叠纪幔源岩浆事件开始受到关注,陆续发现了白日其利镁铁质岩墙群[5]、白日其利辉长岩[6]、中灶火镁铁质岩墙群[7]、小尖山辉长岩[8-9]、拉陵高里沟脑辉长岩[9]、加当辉长岩[10]、阿拉思木辉长岩[11]。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。

2014年以来,在矿产地质专项调查过程中,在青海省都兰县东南察汗乌苏河中游希望沟—哈陇休玛一带发现多处镁铁-超镁铁质岩的出露。对部分岩体进行研究,获得哈陇休玛辉石橄榄岩年龄为525.9±5.9 Ma,认为其属于蛇绿岩的一部分[12];同时获得加当一带辉长岩锆石U-Pb年龄为262.5±2.5 Ma,认为其形成于古特提斯洋俯冲阶段[10]。但是对规模相对较大的希望沟镁铁-超镁铁质杂岩体还缺乏研究,文章通过LA-ICP-MS锆石U-Pb定年确定其中橄榄辉石岩的形成时代,并结合岩石岩相学及岩石地球化学等方面的研究,对其岩石成因、形成时代进行探讨,以期对东昆仑造山带古特提斯构造演化提供新的制约。

1 地质背景

东昆仑造山带位于青藏高原的东北缘、柴达木盆地南缘[13],西以阿尔金走滑断裂为界与西昆仑相接,向东与秦岭—大别连接构成中国著名的中央造山带。东昆仑造山带由东向西延伸长达1500 km[1, 3],为古亚洲与特提斯构造域的交汇部位[14],构造-岩浆活动极其发育,主要经历了早古生代与晚古生代—早中生代两期洋陆构造演化阶段[15]。东昆仑造山带内由北往南依次发育昆北、昆中、昆南三条岩石圈深大断裂,以此为界可将东昆仑划分为东昆北、东昆中、东昆南三个构造单元[16-17],研究区位于东昆中构造带的东段(图 1a)。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白沙河组变质岩,岩性主要为片麻岩、片岩、斜长角闪岩、大理岩等,原岩为泥砂质碎屑岩—基性火山岩—碳酸盐岩建造,变质程度可达角闪岩相—麻粒岩相[16],形成时代可约束在(2468±46)~1846 Ma[3, 18]。随着古特提斯洋的关闭,从中二叠世到晚三叠世,东昆仑带产生了大量的与俯冲—碰撞相关的岩浆作用。中酸性侵入岩主要有花岗闪长岩、二长花岗岩、英云闪长岩、石英闪长岩和正长花岗岩等[10, 12],镁铁-超镁铁质岩体主要出露于加当、小尖山、拉陵高里沟脑、白日其利、中灶火、巴隆等地。

1—第四系;2—上三叠统鄂拉山组火山岩;3—古元古界白沙河组变质岩系;4—早侏罗世正长花岗岩;5—晚三叠世二长花岗岩;6—二叠纪石英闪长岩;7—二叠纪花岗闪长岩;8—二叠纪二长花岗岩;9—晚奥陶世石英闪长岩;10—辉长岩;11—橄榄岩;12—橄榄辉长岩;13—橄榄辉石岩;14—地质界线;15—不整合界线;16—性质不明断层;17—逆断层;18—采样位置;①—柴达木北缘断裂;②—哇洪山-温泉断裂;③—东昆北断裂;④—东昆中断裂;⑤—东昆南断裂 图 1 希望沟—哈陇休玛一带镁铁-超镁铁岩地质略图 Fig. 1 The sketch geological map of the mafic-ultramafic intrusions in Xiwanggou-Halongxiuma area
2 样品和测试方法

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、橄榄辉石岩、辉石橄榄岩等,岩浆分异较好(图 1b),岩石后期发生了强烈的蛇纹石化与磁铁矿化。地表出露面积约2.1 km2,杂岩体露头不好,多覆盖。杂岩体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,东部被早侏罗世正长花岗岩侵入,南部与上三叠统鄂拉山组呈不整合接触。

文中样品均采自镁铁-超镁铁质杂岩体中的橄榄辉石岩,尽量采集蚀变较弱的块状样品,GPS坐标位置为35°52′57″N,98°40′51″E。橄榄辉石岩体在平面上呈长条状,近东西向展布(图 1b)。岩石露头较差,多被第四系黄土所覆盖(图 2a)。岩石露头风化面呈黑色,新鲜面为灰黑色,块状构造,中细粒粒状结构,变余网环状结构。岩石的矿物成分主要由橄榄石、辉石、斜长石及少量褐色角闪石组成(图 2b),副矿物有少量金属矿物。橄榄石含量约30%,矿物晶体为粒状,粒径大小为1~3 mm,常沿晶体裂隙被纤维蛇纹石交代,呈网环状结构(图 2c)。辉石含量约60%,种属为斜方晶系紫苏辉石与单斜晶系透辉石两种类型。紫苏辉石镜下特征为高突起,微具粉色色调,平行消光,一级灰干涉色,二轴晶负光性。辉石晶体呈粒状,粒径大小在1~2.5 mm之间,少部分晶体被褐色角闪石交代(图 2b)。斜长石含量小于10%,矿物晶体粒径<1.2 mm,多呈填隙状产出(图 2d)。

Pl—斜长石;Opx—斜方辉石;Cpx—单斜辉石;Ol—橄榄石;Hbl—角闪石
a—含长橄榄辉石岩野外露头;b—辉石被褐色角闪石交代(正交偏光);c—橄榄石常呈网环状结构(正交偏光);d—斜长石多呈填隙状产出(正交偏光)
图 2 希望沟橄榄辉石岩的野外露头及显微照片 Fig. 2 Outcrop photos and microphotograghs of the Xiwanggou olivine pyroxenolite

采集1件岩石大样(15XWGUPb03)(>30 kg),用于锆石U-Pb年代学研究。锆石的挑选在河北省区域地质矿产调查研究所实验室完成。锆石制靶、透反射光、阴极发光照相在西北大学大陆动力学国家重点实验室完成。在双目镜下仔细挑选表面平整光洁且颗粒大、透明的锆石颗粒进行制靶。通过透反射光和阴极发光(CL)图像详细观察锆石的晶体形态和内部结构特征,选择无明显裂隙及包体的锆石进行定年工作。LA-ICP-MS锆石U-Pb测年在中国地质调查局西安地质调查中心岩浆作用成矿与找矿重点实验室完成,采用193 nm ArF准分子(excimer)激光器的Geo Las200M剥蚀系统,ICP-MS为Agilent7700x,激光束斑直径24 μm,所得锆石同位素比值和年龄数据应用Glitter(ver4·0,Mac QuarieUniversity)程序进行计算和处理,采用Ludwig编写的Isoplot程序[19]进行年龄计算及谐和图的绘制。

6件样品(15XWGH1—15XWGH6)用于岩石主量元素、稀土元素、微量元素分析,在中国地质调查局西安地质调查中心实验测试中心完成,其中主元素采用荷兰帕纳科公司Axios 4.0 kW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)测定,分析精度优于5%~10%。

3 测试结果 3.1 锆石LA-ICP-MS U-Pb年代学

橄榄辉石岩样品中(15XWGUPb03)锆石多呈短柱状或浑圆状(长70~150 μm),长宽比为1:1~2:1。多数锆石内部可见清晰的震荡环带(图 3),37个有效分析点测试结果见表 1。锆石的U、Th含量较高,分别为128.1×10-6~11366.4×10-6和209.8×10-6~4057.6×10-6,Th/U比值为0.36~1.74,大部分均大于0.4,应属于典型的岩浆成因锆石[20]。37个有效测点206Pb/238U表面年龄集中在271.0~253.1 Ma,其加权平均年龄为(262.4±1.6) Ma,MSWD=1.02(图 4)。37个锆石点数据较集中,均落在谐和线上及其附近,表明希望沟橄榄辉石岩结晶年龄为(262.4±1.6) Ma,属于中二叠世岩浆活动的产物。

图 3 希望沟橄榄辉石岩锆石阴极发光照片 Fig. 3 CL images of zircons from Xiwanggou olivine pyroxenolite

表 1 希望沟橄榄辉石岩锆石LA-ICP-MS测年结果 Table 1 LA-ICP-MS isotopic data of zircon from Xiwanggou olivine pyroxenolite

图 4 希望沟橄榄辉石岩锆石U-Pb年龄谐和图和加权平均年龄图 Fig. 4 Zircon U-Pb concordia diagram and weighted mean ages diagram from Xiwanggou olivine pyroxenolite
3.2 地球化学特征 3.2.1 主量元素含量

希望沟6件橄榄辉石岩样品全岩地球化学数据见表 2,样品的SiO2含量为46.84%~47.44%,平均47.14%,Al2O3含量为3.04%~4.57%,平均值为3.90%。CaO含量为10.78%~11.89%,TFeO含量为7.06%~7.36%,MgO含量为23.16%~25.29%,平均为24.02%,对应的Mg#值较高,为85.31~86.27,整体大于与橄榄岩平衡的原生岩浆Mg#值范围(68~75,据文献[21])。样品的m/f比值介于5.50~6.18,属于铁质超基性岩类。样品的TiO2含量均较低,介于0.41%~0.52%,平均值为0.48%,相对接近岛弧型玄武岩TiO2含量(0.98)[22];P2O5含量较低,为0.035%~0.051%,平均值为0.04%。岩石的全碱(K2O+Na2O)含量为0.69%~0.83%,且Na2O含量均大于K2O含量,所有岩石样品在SiO2-(Na2O+K2O)图解中全部落在亚碱性的范围(图 5a),在SiO2-TFeO/MgO图解中岩石落在钙碱性系列范围内(图 5b)。

表 2 希望沟橄榄辉石岩主量元素(%)和微量元素含量(×10-6) Table 2 Contents of major elements(%)and trace elements(×10-6)of Xiwanggou olivine pyroxenolite

图 5 希望沟橄榄辉石岩的SiO2-(Na2O+K2O)和SiO2-TFeO/MgO图解(a底图据文献[23];b底图据文献[24]) Fig. 5 SiO2-(Na2O+K2O) and SiO2-TFeO/MgO plots for Xiwanggou olivine pyroxenolite (a is after reference[23]; b is after reference [24])
3.2.2 微量元素和稀土元素含量

原始地幔标准化微量元素蛛网图(图 6a)显示,希望沟橄榄辉石岩微量元素配分模式近一致,显示了同源岩浆演化分异特征。相对于原始地幔,岩体明显富集大离子亲石元素(Rb、Th、U)和Pb,相对亏损高场强元素(如Nb、Ta、P、Ti)。样品的ΣREE含量较低(表 2),为29.79×10-6~38.96×10-6,平均为34.72×10-6。轻重稀土比值(LREE/HREE)为2.68~3.17,平均为2.96,(La/Yb)N为1.71~2.51,平均2.15,(La/Sm)N为1.03~1.44,平均1.22,(Gd/Yb)N为1.44~1.71,平均1.55,表明轻重稀土之间和轻重稀土元素内部分馏较弱(图 6b)。δEu为0.73~0.86,平均为0.81,显示弱的Eu负异常,说明斜长石发生了一定程度的分离结晶。

图 6 希望沟橄榄辉石岩的微量元素原始地幔标准化蛛网图及稀土元素球粒陨石标准化配分曲线图(标准化数值据文献[25]) Fig. 6 Primitive mantle-normalized trace element patterns and chondrite-normalized REE patterns for Xiwanggou olivine pyroxenolite (Normalized data are from reference [25])
4 讨论 4.1 岩浆源区和岩石成因

希望沟橄榄辉石岩样品均具有相对含量低的TiO2(0.41%~0.52%),相对于原始地幔富集LILE(Rb、Th、U),亏损HFSE(如Nb、Ta、P和Ti),表明橄榄辉石岩应起源于岩石圈地幔。同时样品具有较低的SiO2含量(46.84%~47.44%)、较高的Mg#值(85.31~86.27)和Cr、Ni含量(Cr=1850×10-6~2210×10-6、Ni=249×10-6~378×10-6),也显示了幔源岩浆的成分特征。由样品的Mg#值整体大于与橄榄岩平衡的原生岩浆Mg#值范围(68~75)可知,橄榄辉石岩应主要由岩浆早期结晶的矿物相聚集而成,而非演化的岩浆。样品的Sr含量(56.3×10-6~116×10-6,平均值为79×10-6)显著高于地幔值(17.8×10-6)[26],指示其岩浆源区并非单一来自地幔,可能受到围岩混染或俯冲板片流体交代作用的影响[27-28]

希望沟橄榄辉石岩样品的Nb/Ta比值为12.67~13.69,平均为13.19,Zr/Hf比值变化于30.69~37.84之间,平均值为33.29,分别与大陆地壳值相近(Nb/Ta=11、Zr/Hf=33)[26],而低于洋中脊玄武岩值(Nb/Ta=17.7、Zr/Hf=36.1)[25],反映橄榄辉石岩明显受到地壳同化混染的影响。Neal等[29]提出可以用(La)PM/(Nb)PM与(Th)PM/(Ta)PM图解来区分上地壳和下地壳物质对原始岩浆的混染作用。从图 7a可以看出,岩石遭受了上地壳物质的混染。

N-MORB—正常的大洋中脊玄武岩;E-MORB—富集的大洋中脊玄武岩;OIB—洋岛玄武岩 图 7 希望沟橄榄辉石岩源区性质与地壳混染判别图解(a底图据文献[29];b底图据文献[31]) Fig. 7 Discrimination diagram of the source characteristics and crustal contamination for Xiwanggou olivine pyroxenolite

希望沟橄榄辉石岩样品Th/Yb比值为0.57~1.27,在Th/Yb-Nb/Yb图解中投点均明显偏离MORB-OIB演化线(图 7b),暗示其形成明显受到俯冲组分的影响[30]。综上所述,希望沟橄榄辉石岩原生岩浆为被俯冲改造的岩石圈幔源岩浆,同时,岩浆在演化过程中遭受了上地壳物质的同化混染。

4.2 构造背景

希望沟橄榄辉石岩明显富集大离子亲石元素(Rb、Th、U)和Pb,相对亏损高场强元素(Nb、Ta、P、Ti),显示了岛弧岩浆岩的特征。样品的TiO2含量均较低,平均值为0.48%,相对接近岛弧型岩浆岩(0.98%),明显低于洋中脊玄武岩(1.5%)和洋岛玄武岩(大于2.0%)[22];希望沟橄榄辉石岩Th/Ta比值为8.41~16.16,平均值为11.66,远高于3,指示应形成于岛弧环境[22]。样品的微量元素含量中Nb/La=0.30~0.38(< 1),Hf/Ta=10.55~13.26(>5),La/Ta=34.36~44.55(>15),Th/Yb=0.57~1.27(>0.1),Th/Nb=0.62~1.21(>0.07),进一步显示了岛弧的性质[32]

为进一步探讨橄榄辉石岩的构造背景,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 8a)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 8b)中,样品均落在破坏板块边缘玄武岩区域;在Hf/3-Th-Ta图解(图 8c)中,岩石表现出岛弧拉斑玄武岩的特征;在Th/Yb-Ta/Yb图解(图 8d)中,样品点均落入大洋岛弧范围,暗示希望沟橄榄辉石岩为俯冲作用形成的岛弧岩浆岩。

图 8 希望沟橄榄辉石岩构造环境判别图解(a底图据文献[33];b、c底图据文献[34];d底图据文献[22]) Fig. 8 Tectonic discrimination diagrams for Xiwanggou olivine pyroxenolite

晚古生代—早中生代是东昆仑构造演化的一个重要阶段,相关学者进行了大量的研究。最新资料研究认为,东昆仑古特提斯洋从晚石炭世(309 Ma)开始打开[3, 35],至少自早二叠世278 Ma开始,阿尼玛卿古特提斯洋开始向北俯冲[36-38],并持续至中三叠世晚期(237 Ma)[39-41];晚三叠世之后东昆仑进入碰撞后伸展阶段(230~185 Ma)[37, 42-43]

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[5]获得白日其利角闪辉长岩的锆石U-Pb年龄为(248.9±4.2) Ma,认为其为早三叠世阿尼玛卿古特提斯洋俯冲阶段的产物。熊富浩等[6]与菅坤坤等[7]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,认为岩墙群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[8]获得小尖山中细粒辉长岩的锆石U-Pb年龄为(227.8±0.9) Ma,认为其形成于造山后伸展的构造环境。王亚磊等[9]获得小尖山中粗粒磁黄铁矿化辉长岩的锆石U-Pb年龄为(247±2.1) Ma,拉陵高里沟脑2号和3号岩体辉长岩年龄分别为(244.9±1.6) Ma与(241.6±1.8) Ma,认为东昆仑早中生代幔源岩浆为后碰撞伸展环境的产物。孔会磊等[10]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,认为其是古特提斯洋向北俯冲的产物。张玉等[11]获得巴隆地区阿拉思木辉长岩锆石U-Pb年龄为(241.1±1.2) Ma,认为其代表了东昆仑洋壳俯冲的最晚期记录。Xiong等[44]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。

文中获得希望沟橄榄辉石岩的加权平均年龄为(262.4±1.6) Ma,表明岩体形成时代为中二叠世晚期。区域上五龙沟花岗闪长岩的锆石U-Pb年龄为(260.1±1.8) Ma,与希望沟橄榄辉石岩年代十分接近,形成于古特提斯洋俯冲阶段活动大陆边缘的局部伸展环境[45]。另外,区域上东昆仑南缘阿尼玛卿发育一条显著的蛇绿岩带,带内共存在两期蛇绿岩,早期蛇绿岩形成时代为早寒武世[46],晚期形成时代为早石炭世至早中三叠世[47]。对于晚期蛇绿岩,相关学者研究获得玛积雪山岛弧火山岩的同位素年龄为260 Ma[16],并测得德尔尼超镁铁质岩北侧的德-恰花岗杂岩体的单颗粒锆石U-Pb年龄为(250±20) Ma,认为其形成于大陆边缘的岛弧环境[47]。阿尼玛卿晚期蛇绿岩与该研究的希望沟橄榄辉石岩在时空分布、构造背景方面均具有一致性。同时结合上文岩石成因及构造环境分析认为,希望沟橄榄辉石岩为阿尼玛卿古特提斯洋俯冲阶段的岩浆记录。

5 结论

(1) 希望沟橄榄辉石岩属于钙碱性铁质超基性岩,锆石U-Pb年龄为(262.4±1.6) Ma,形成于中二叠世。

(2) 岩石成因分析表明,岩石源区为被俯冲改造的岩石圈地幔,岩浆在演化过程中遭受了上地壳物质的同化混染。

(3) 结合东昆仑区域年代学资料及构造演化,认为希望沟橄榄辉石岩为阿尼玛卿古特提斯洋俯冲阶段的岩浆记录。

致谢: 样品测试和数据处理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、赵慧博工程师、李艳广工程师、汪双双博士的帮助,文章修改过程中审稿专家及编辑部老师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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