西秦岭天水关子镇变玄武岩的地球化学特征及大地构造环境

操雨森1),黄始琪2),周炜鉴1),卢洋3),刘志强2),黄德志1)

1)中南大学地球科学与信息物理学院,长沙,410083;2)中国地质科学院,北京,100037; 3)湖南常德市地震局,湖南常德,415000

内容提要: 天水关子镇蛇绿岩分布于西秦岭造山带北缘,一般认为是商南—丹凤缝合带西延部分,代表了华北板块与扬子板块的结合部位。本文通过关子镇蛇绿岩中变玄武岩的主量、微量和稀土元素及Sr、Nd同位素综合分析,对关子镇蛇绿岩的源区特征及构造背景进行分析和探讨。研究表明,蛇绿岩中变玄武岩隶属于低碱、富钠低钾的拉斑玄武岩系列;稀土元素分异不明显,微量元素亏损Ba、Th、P,高场强元素无亏损现象。εNd(t)值均为正值,与MORB相近。

关键词:蛇绿岩;Sr—Nd同位素;微量元素;关子镇

秦岭造山带是分隔中国南北大陆的复合型大陆造山带,其形成演化记录了我国华北克拉通与华南陆块多阶段拼合过程, 具有复杂的演化历史和多阶段多构造体制的特征。其中西秦岭是秦岭造山带的西延,向北衔接祁连造山带,具有继承性演化历史和复杂的构造体系,为研究中国大陆不同时期构造格局、构造演化及华北板块与扬子板块拼贴、碰撞关系的关键地段,受到国内外地质学家们的广泛关注。西秦岭天水地区分布较多不同的火成岩,裴先治等(2004)已经识别出早古生代关子镇蛇绿岩;而张国伟等(1995)认为关子镇蛇绿岩是东秦岭商丹(商南—丹凤)早古生代古缝合带的向西延伸部分,并且张国伟等(1995)对西秦岭及其邻区的中元古代浅变质火山岩系进行了综合分析,发现具明显裂谷火山岩特征,认为区内基底在中元古代存在多个裂谷构造;王宗起等(2009)对西秦岭地区的地层和火成岩进行了系统分析,划分出13个主要构造单元;李永军等(2008)对天水地区震旦纪以来的花岗岩进行了总结分析,认为天水地区多处花岗岩具有不同的大地构造背景,存在多种构造单元类型,其源区多数为下地壳,少数与壳幔混溶作用有关;裴先治等(2004)首先在天水关子镇地区拟定了一套蛇绿岩,并进行了年代学和地球化学分析(裴先治等,2007),认为天水地区关子镇蛇绿岩和洋脊型拉斑玄武岩的厘定对重建秦岭造山带与祁连造山带之间的构造关系和华北陆块与扬子板块之间相互构造作用过程等均具有重要的大地构造意义。蛇绿岩是鉴别板块边界的最重要标志之一,可形成于板块边界的多种构造环境,关子镇蛇绿岩地球化学特征及反映的大地构造背景的确定,对重塑华北与扬子间古构造格局及恢复古构造演化具有十分重要的意义。

自关子镇蛇绿岩拟定以来,前人通过锆石SHRIMP/LA- ICP- MS定年给出U- Pb年龄,限定关子镇蛇绿岩的形成时代,对岩石地球化学进行了初步研究,并据此对源区特征及构造环境进行了探讨。裴先治等(2007)测得关子镇蛇绿岩中辉长岩的锆石U-Pb年龄为499.7±1.8 Ma,认为代表洋盆扩张时期,形成于岛弧环境;李王晔等(2007)获得蛇绿岩中辉长岩和闪长岩的锆石SHRIMP U-Pb年龄分别为534±9 Ma和517±8 Ma,认为关子镇蛇绿岩形成于洋中脊环境;董云鹏等(2008)认为其为秦岭古洋盆洋壳残片,是商丹缝合带的西延。随着全球岩石地球化学数据库的应用开发,发现很多传统的利用主量及微量元素进行的构造环境判别图解不能给出准确的结论(王金荣等,2016, 2017;第鹏飞等, 2017),因此,利用主量、微量及同位素地球化学综合分析,联合示踪岩石源区特征及构造背景,显得十分重要。

图1 西秦岭天水关子镇变玄武岩地质简图(据裴先治等, 2004修改)
Fig. 1 Geological Map of metabalts in Guanzizhen, Tianshui (modified after Pei Xianzhi et al., 2004)

1 区域地质背景及样品描述

图2 天水关子镇变玄武岩镜下照片(样品坐标:34°36′147″N,105°57′26.9″E)
Fig. 2 Microscope photographs of themetabalts from Guanzizhen, Tianshui (sampling location: 34°36′147″N, 105°57′26.9″E)
(a1)、(a2)、(a3) 单偏光镜下显微照片(揭示矿物组成及结构构造);(b1)、(b2)、(b3) 正交镜下显微照片(揭示矿物组成及结构构造); Pl—斜长石;Hbl—角闪石;Cal—方解石(碳酸盐化);Px—辉石;Qtz—石英;Mag—磁铁矿
(a1),(a2),(a3) Microscope photographs (single-polarized,to show mineral composition and structure); (b1)、(b2)、(b3) microscope photographs (cross-polarized to show mineral composition and structure); Pl—plagioclase;Hbl—hornblende;Cal—calcite (carbonatization);Px—pyroxen;Qtz—quartz;Mag—magnetite

关子镇变玄武岩为关子镇蛇绿岩的重要组成部分,关子镇蛇绿岩分布于甘肃省天水市NWW约35 km处的关子镇南侧,呈NWW—SEE方向带状展布,南侧以大型韧性剪切带与早古生代“李子园群”中解体出来的太阳寺组相邻,北侧以片麻状变质辉长—闪长岩体与古元古代秦岭岩群相隔(图1)。关子镇混杂岩带内出露岩性主要为变玄武岩、变辉长岩、闪长岩等,被认为是秦岭造山带商丹缝合带的西延部分(杨钊等,2006),为一个成因复杂的构造混杂岩带,向西被温泉花岗岩体截断,至武山一带又出现并与武山蛇绿岩带相连(Li Yuan et al., 2015)。关子镇蛇绿岩带中的变基性火山岩后期经历了低角闪相变质,在变质岩的形成过程中,如无交代作用,除H2O和CO2外,变质岩的化学成分基本取决于原岩的化学成分,同时,氧化物和硅酸盐的生成热较高,因此,在区域变质作用过程中,当温度升高时,亲石元素(包括主要造岩元素K、Na、Fe、Mg、Al、Si)保持其稳定,这也部分地解释了在区域变质作用过程中,岩石的主要造岩元素可以保持不变或稍有变化的原因。从主、微量及同位素测试数据看,不同样品含量及比值稳定,表明变质作用对主、微量及Sr同位素影响不大。辉长岩结晶年龄显示关子镇蛇绿岩可能形成于470~530 Ma(杨钊等, 2006;裴先治等,2004,2007)。

本文研究的样品产于关子镇蛇绿岩带中的变玄武岩(图1)。岩石呈黑灰色,主要以角闪石、斜长石为主,含少量磁铁矿及石英,块状构造。样品薄片单偏光和正交偏光镜下显微照片揭示了其主要矿物组成和结构特征(图2),斜长石:含量约45%~55%,粒径50×200~100×500 μm2,自形程度较高,可见简单双晶、聚片双晶。角闪石:含量约25%~35%,粒径50×200~200×500 μm2,由辉石变质形成,自形程度低,充填于斜长石骨架中,呈变余辉长结构。石英:含量约5%,粒径20×200~50×500 μm2,呈细脉状分布,脉宽约50×500 μm2,镜下可见部分石英细脉被斜长石所截切。辉石:含量5%左右,粒径50×200~500×500 μm2,自形程度高,为变质后的少量残留。方解石:含量约5%,粒径100×200~500×500 μm2,他形,呈团块状分布,可见明显三组解理发育,为后期蚀变产物。

表1 天水关子镇变玄武岩主量元素(%)及微量、稀土元素(×10-6)分析结果
Table1 Analytic results of major elements (%), trace and rare earth elements (×10-6) of metabasalts in Guanzizhen, Tianshui

样品号H045-1H045-2H045-3H045-4H045-5SiO249.47 49.65 49.84 49.75 50.08 TiO21.33 1.35 1.32 1.35 1.61 Al2O314.17 14.17 13.94 14.22 14.76 Fe2O32.19 2.37 2.34 2.06 2.92 FeO9.18 9.15 9.30 9.55 8.19 MnO0.20 0.20 0.20 0.20 0.21 MgO7.91 7.84 8.34 7.72 8.50 CaO10.00 10.17 9.94 10.46 8.44 K2O0.39 0.39 0.29 0.30 0.58 Na2O2.55 2.18 2.34 1.90 1.26 P2O50.11 0.10 0.11 0.11 0.13 烧失1.32 1.26 0.87 1.15 2.28 总量98.81 98.83 98.82 98.77 98.96 σ1.33 0.99 1.01 0.72 0.48 DI24.49 22.43 22.32 20.62 22.00 Mg#58.4 57.9 59.2 57.3 60.9 Rb16.30 16.40 9.70 11.20 26.40 Sr194.00 188.00 192.00 230.00 142.00 Ba57.30 30.20 31.90 30.60 49.80 Nb6.01 6.00 6.25 6.50 7.18 Ta0.44 0.44 0.44 0.46 0.52 Zr79.00 79.80 81.50 84.20 99.40 Hf2.48 2.50 2.58 2.63 3.09 样品号H045-1H045-2H045-3H045-4H045-5U0.36 0.40 0.24 0.25 0.43 Th0.63 0.58 0.58 0.60 1.28 La6.24 6.21 6.28 6.08 6.90 Ce13.40 13.50 13.70 13.40 16.60 Pr2.32 2.36 2.37 2.37 2.98 Nd11.70 11.80 12.00 12.00 14.70 Sm3.37 3.51 3.49 3.56 4.09 Eu1.20 1.24 1.25 1.26 1.40 Gd3.31 3.44 3.48 3.50 3.80 Tb0.68 0.70 0.71 0.71 0.74 Dy4.48 4.66 4.69 4.72 4.76 Ho0.92 0.95 0.97 0.98 0.96 Er2.43 2.48 2.55 2.55 2.47 Tm0.38 0.39 0.39 0.40 0.38 Yb2.40 2.52 2.52 2.53 2.40 Lu0.33 0.34 0.34 0.35 0.33 Y24.40 25.20 25.50 25.30 25.00 δEu1.10 1.09 1.10 1.09 1.09 ΣREE77.56 79.30 80.24 79.71 87.51 ∑LREE∑HREE0.97 0.95 0.95 0.94 1.14 LaN/YbN1.86 1.77 1.79 1.72 2.06 LaN/SmN1.20 1.14 1.16 1.10 1.09

注:取样位置(Sampling location):X=34°36′147″N;Y=105°57′26.9″E;岩性(lithology):变玄武岩(metabasalts) 。

2 测试方法及结果

由于样品有少量后期的石英脉及方解石脉,分析前样品处理时,将其剔除。样品测试由国土资源部中南矿产资源监督测试中心(武汉)完成。主量元素分析使用AXIOS型X-荧光光谱仪(荷兰帕拉科生产,端窗铑靶X光管,4W),采用熔片—X荧光光谱法,将样品烘干后置于Pt—Au坩埚中,在1100~1200℃条件下用2∶1的Li2B4O7与Li3BO2混合溶液溶融制得玻璃片,通过测定特征谱线强度,经由计算机SuperQ分析软件自动分析得出样品中元素含量的定量。微量和稀土元素分析使用由美国热电公司XⅡSeries型等离子体质谱仪(功率为1350 W),将样品加酸溶液制成溶液采用ICP-MS法测定,以跳峰的测试方式脉冲计数,共计扫描15次,停留10 ms,采样深度为135 mm,选择外标法和Rh元素内标法校正。用标准样 BHVO-1 、AGV-1 和 G-2 做分析样品元素浓度的校正标准。所有元素分析相对误差均小于 5%。主量和微量及稀土元素测试结果见表1。

Sm-Nd同位素分析使用热电离质谱仪(Triton)在超净化实验室完成,通过同位素稀释法计算来得到Sm、Nd的含量,校正使用n(146Nd)/ n(144Nd)=0.7219,以GBW04419和JNdi-1标准物质对测试分析过程进行监控;Rb-Sr同位素分析使用热电离质谱仪(MAT261)在超净化实验室完成,通过同位素稀释法计算来得到Rb、Sr同位素比值,以NBS987、NBS607和GBW04411标准物质对测试分析过程进行监控。测试结果见表2。

表2 天水关子镇变玄武岩Sr、Nd同位素分析结果
Table 2 Analytic results ofSr and Nd isotopes of metabasalts in Guanzizhen, Tianshui

样品号H045-1H045-2H045-3H045-4H045-5Sm(×10-6)3.163.253.213.273.81Nd(×10-6)1111.2511.111.2613.82Rb(×10-6)14.8714.738.059.2323.35Sr(×10-6)189.7185.9178.8213.3137.4n(87Rb)/ n(86Sr)0.226130.228440.129830.124710.48988n(87Sr) n(86Sr)测值0.707620.707250.706280.706510.70923±1σ0.000060.000070.000030.000030.00009n(147Sm)/ n(144Nd)0.1737340.1747350.1747620.1757350.166714n(143Nd) n(144Nd)测值0.5128130.5128410.5128770.5128570.512635±1σ0.0000030.0000030.0000050.0000050.000005[n(87Sr)/ n(86Sr)]i0.70600.70560.70530.70560.7057εNd(t)4.935.185.295.150.88

注:依裴先治等(2007),计算时t

3 岩石地球化学特征

3.1主量元素

关子镇变玄武岩SiO2含量为49.47%~50.08%,变化范围很小,均落于基性岩浆岩区;全碱—SiO2图解显示本区变玄武岩样品均为亚碱系列玄武质岩石。Na2O含量为1.26%~2.55%,K2O含量为0.29%~0.58%,全碱(Na2O+K2O)含量较低,介于1.84%~2.94%,表现出低碱的特点;K2O/Na2O值介于0.12~0.46,均为Na2O>K2O,岩石具明显富钠贫钾特征。TFeO值变化小(11.11%~11.64%);MgO为7.72%~8.50%,TiO2为1.33%~1.61%,Al2O3介于13.94~14.76%,总体显示低Ti、Al的特征。

变玄武岩样品里特曼指数(σ)为0.48~1.33,明显小于3.3,表明为钙碱性岩浆岩。TAS分类采用国际地科联建议的方案(Le Maitre et al., 1989; Le Maitre, 2002;邓晋福等,2015),TAS图解上,样品一致落入玄武岩区域(图3),SiO2—K2O图解表明岩石隶属于拉斑玄武岩系列(图4)。Mg#值介于57.3~60.9之间,低于由地幔熔融形成的原始玄武岩浆(Mg#=68~72)。分离结晶作用作为岩浆分异演化的重要机制之一,当岩浆来源于深部地幔或在岩浆作用的早期阶段,往往发挥着至关重要的作用。在主量元素的哈克图解中(图5),MgO与SiO2、Al2O3、、Na2O及TiO2无明显的线性关系,表明其未经历强烈的分离结晶作用。MgO与FeO、CaO、TiO2呈微弱的负相关关系,指示了岩浆演化早期经历了少量结晶分异作用。

3.2 微量及稀土元素

变玄武岩稀土总量∑REE较高(77.56×10-6~87.51×10-6),样品稀土配分特征一致,均表现出略微右倾(图6),反映样品具有相同源区。轻重稀土间分异不明显,LREE/HREE值接近于1(0.95~1.14),Eu异常不明显(δEu=1.086~1.098),具有与MORB相似的特征。微量元素蛛网图中(图7),变玄武岩显示Ba、Th和P亏损外,整体表现分异不明显,可能同时指示了源区的特征及地壳混染作用微弱。

在岩浆演化过程中,地壳混染往往能在一定程度上改变岩石的元素和同位素组成。研究表明,地壳物质一般亏损Nb、Ta而富集Zr、Hf、Th、Pb(Sun and McDonough, 1989;Rudnick and Gao, 2003),因此受地壳混染的岩浆不仅会显示负的Nb—Ta异常,还会显示Zr—Hf的正异常。关子镇变玄武岩无Nb—Ta负异常,也没有Zr—Hf的正异常,而显示有微弱的Zr富异常,因此,地壳混染微弱。

图3 天水关子镇变玄武岩TAS图解(底图据Le Maitre et al.,1989; Le Maitre, 2002)
Fig. 3 TAS diagram of metabasalts in Guanzizhen, Tianshui (after Le Maitre et al.,1989; Le Maitre, 2002)

图4 天水关子镇变玄武岩SiO2—K2O(据Miyashiro,1975)
Fig. 4 Diagram of SiO2 vs K2O of metabasalts in Guanzizhen, Tianshui(after Miyashiro,1975)

3.3 Sr、Nd同位素

关子镇变玄武岩Sr的初始比值[n(87Sr)/ n(86Sr)]i为0.7053~0.7060,稍高于来自亏损地幔的的N-MORB (0.70229~0.70316,Saunders et al., 1988),与一类富集地幔(EMI, 约0.705, Zindler et al., 1986)相当。εNd(t)值均为正值,除一个样品稍低(0.88)以外,其余样品在4.93~5.29之间,与MORB相近。此外,[n(87Sr)/ n(86Sr)]iεNd(t)值变化很小,也佐证了地壳混染微弱。

4 大地构造环境

对关子镇蛇绿岩的形成大地构造环境,目前研究不多,且主要基于岩石的主、微量及稀土元素的研究。裴先治等(2007)研究得出蛇绿岩中的变质基性火山岩具有N-MORB的地球化学特征,认为关子镇蛇绿岩是洋脊型蛇绿岩;董云鹏等(2003)通过对变玄武岩主、微量及稀土元素研究,认为其源于亏损地幔源区,形成于古洋中脊构造环境。丁仨平(2008)对李子园群中的变基性—中基性火山岩组合研究表明,岩石具有岛弧拉斑玄武岩和钙碱性玄武岩特征。

图5 天水关子镇变玄武岩主量元素哈克图解(据Winter, 2001)
Fig. 5 Harker diagram formetabasalts in Guanzizhen, Tianshui (after Winter, 2001)

图6 天水关子镇变玄武岩稀土元素配分图(球粒陨石标准值据Sun and McDonough, 1989)
Fig. 6 REE diagram of metabasalts in Guanzizhen, Tianshui (values of chondrites for normalization from Sun and McDonough, 1989)

本次研究显示,关子镇变玄武岩富钠低钾、高镁低钛,微量不相容元素富集不明显和无高场强元素亏损,稀土元素呈平坦型,这些地球化学特征大多数显示关子镇变玄武岩原岩具有MORB的特征,同位素εNd(t)平均值为4.14,总体与MORB接近,然而,另一些地球化学参数显示与正常MORB相比,具有明显富集特征。[n(87Sr)/ n(86Sr)]i明显高于N-MORB,不活动元素的比值Ce/Zr、Th/Yb、Th/La、La/Yb、La/Sm、Zr/Nb、Hf/Nb及Y/Nb均一致地显示,关子镇变玄武岩与N-MORB差异明显,与E-MORB十分接近,仅一个元素比值(La/Nb)与N-MORB相似(表3)。在[n(87Sr)/ n(86Sr)]iεNd(t)协变图解(图8)中,样品落在洋岛玄武岩边界,具有来自富集地幔的特征。

表3 天水关子镇变玄武岩及典型背景基性岩的元素比值
Table 3 Some element ratios of metabasalts in Guanzizhen, Tianshui,and typical basic rocks

样品号Ce/ZrTh/YbTh/LaLa/YbLa/SmLa/NbZr/NbHf/NbY/NbH045-10.17 0.26 0.10 2.60 1.85 1.04 13.14 0.41 4.06 H045-20.17 0.23 0.09 2.46 1.77 1.04 13.30 0.42 4.20 H045-30.17 0.23 0.09 2.49 1.80 1.00 13.04 0.41 4.08 H045-40.16 0.24 0.10 2.40 1.71 0.94 12.95 0.40 3.89 H045-50.17 0.53 0.19 2.88 1.69 0.96 13.84 0.43 3.48 IAT0.130.190.171.081.243.8477.12.2533.6N-MORB0.10.040.050.820.951.0731.80.8812E-MORB0.20.250.12.662.420.768.90.242.7

注: IAT数据引自Pearce 等(1995); N-MORB和E-MORB数据引自Sun and McDonough(1989)。

图7 天水关子镇变玄武岩微量元素蛛网图(原始地幔标准值据Sun and McDonough, 1989)
Fig. 7 Spider diagram of trace elements of metabasalts in Guanzizhen, Tianshui(primitive mantle values for normalization from Sun and McDonough, 1989)

关于E-MORB的成因,目前尚存争议,普遍的观点认为E-MORB是有由OIB与N-MORB的混合而成。然而,大量E-MORB产于远离地幔柱的洋中脊或弧后盆地,也就意味着不仅E-MORB的成因复杂,其产出构造背景可以是大洋环境,也可以是弧后背景。利用主量及微量元素相关图解进行构造环境判别是一种传统的普遍方法,随着全球岩石地球化学数据库的应用开发,人们对这种方法的可靠程度提出了怀疑(王金荣等,2016,2017)。但通过利用全球地球化学数据库对相关构造环境判别图解的验证表明,有些图解是有效的。第鹏飞等(2017)研究表明,包括Th、Ta(Nb)元素的判别图可以把IAB有效地与MORB、OIB区分开。在Hf/3—Th—Nb/16判别图解中落入N-MORB与E-MORB之间,更偏于E-MORB,而与岛弧背景下岛弧拉斑玄武岩(IAT)及钙碱性玄武岩(CAB)具有明显差别(图9)。Ta/Hf及Th/Hf值,能较好地反映其源区的Th、Ta、Hf之间的分异特征,Th/Hf—Ta/Hf判别图能较好区分玄武岩形成的大地构造环境(汪云亮等,2001),在Th/Hf—Ta/Hf判别图中(图10),样品主要落入大洋板内洋岛和海山玄武岩区(Ⅲ),与岛弧背景相差较远。

图8 天水关子镇变玄武岩[n(87Sr)/ n(86Sr)]iεNd(t)图解(底图据Condie,1989; t=500 Ma,据裴先治等,2007)
Fig. 8 Diagram of[n(87Sr)/ n(86Sr)]iεNd(t) of metabasalts in Guanzizhen, Tianshui (after Condie,1989;t=500 Ma, after Pei Xianzhi et al., 2007&)

图9 天水关子镇变玄武岩Hf/3—Th—Nb/16构造判别图解(底图据Meschede,1986)
Fig. 9 Hf/3—Th—Nb/16 discriminant diagram of metabasalts in Guanzizhen, Tianshui(after Meschede,1986)

图10 天水关子镇变玄武岩Th/Hf—Ta/Hf构造判别图解(底图据汪云亮等, 2001)
Fig. 10 Th/Hf—Ta/Hf discriminant diagram of metabasalts in Guanzizhen, Tianshui (after Wang Yunliang et al., 2001&)

综上所述,关子镇变玄武岩具低碱、富钠贫钾的拉斑玄武岩特征,微量元素亏损Ba、Th、P,无高场强元素亏损现象,轻重稀土间分异不明显,[n(87Sr)/ n(86Sr)]iεNd(t)值相对N-MORB微富集,原始岩浆来源于富集地幔。此外,地壳混染微弱,与岛弧背景明显不同,据此认为关子镇变玄武岩形成于洋盆的初始发育阶段,产于小型洋盆的构造环境。结合前人在关子镇蛇绿岩中获得辉长岩锆石年龄(499.7±1.8) Ma(裴先治,2007b)及(471.0±1.4) Ma(杨钊等,2006),变闪长岩锆石年龄517±8 Ma(李王晔,2007),其代表的古洋盆的扩张时代为晚寒武世到早奥陶世,变玄武岩的成岩年龄限定在早古生代初,是西秦岭地区早古生代早期洋盆扩张的直接产物。

5 结论

根据西秦岭天水关子镇变玄武岩主、微量和稀土元素及Sr、Nd同位素研究,初步获得下列结论:

(1)关子镇变玄武岩具有低SiO2、富钠、低钾,和高镁低钛的特征,稀土分配模式总体为平坦型,高场强元素分异不明显,具亚碱性拉斑玄武岩的特征,[n(87Sr)/ n(86Sr)]iεNd(t)值变化范围小,显示了原岩形成过程经历的结晶分异作用及地壳混染微弱,代表原始岩浆成分。

(2) εNd(t)与MORB接近,[n(87Sr)/ n(86Sr)]i 值稍高,总体上Sr、Nd同位素与E-MORB接近,指示岩石的源区具富集地幔的特征。

(3)主微量和稀土元素及Sr、Nd同位素联合示踪表明,关子镇变玄武岩形成于洋盆的初始发育阶段,产于小型洋盆的构造环境。

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Geochemica.characteristic.an.tectoni.environmen.o.metabasalt.i.Guanzizhen, Tianshui, Wester.Qinling

CAO Yusen1),HUANG Shiqi2), ZHOU Weijian1), LU Yang3), LIU Zhiqiang2), HUANG Dezhi1)

1)School of Geosciences and Info-Physics, Central South University, Changsha, Hunan, 410083;2)Sino-Probe Center, Chinese Academy of Geological Sciences, Beijing, 100037;3)Seismological Bureau of Changde City, Hunan Province, Changde, Hunan, 415000

Objectives:The ophiolite in Guanzizhen is located in the northern margin of the western Qinling Orogen, which is taken as the western extension part of the Shang-Dan (Shangnan—Danfeng) suture zone representing the joint position between North China and Yangtze Blocks. In this paper, we focus on the geochemical features of the ophiolite in Guanzizhen to analysis its source area characteristics and tectonic background.

Methods:Samples for major and trace element analysis were first crushed and ground to a particle size of less than 200 mesh. The oxides of major elements were measured by X-ray fluorescence spectrometry. The trace and rare earth elements were determined with an XⅡ Series ICP-MS. For rare earth elements content analyses, 50 mg samples were precisely weighed and reacted with 1 mL HF and 0.5 mL HNO3 in screw top PTFE-lined stainless bombs. The analytical precision was better than 5%.

Prior to n(87Sr)/n(86Sr) and n(144Nd)/n(143Nd) analysis, 50 mg of sample powder was weighed. The powder was then leached using 1.0 mol/L acetic acid to dissolve the carbonate fraction. After this step, Sr and Nd were separated and purified by conventional cation-exchange resin method respectively. Isotopic ratios of obtained Sr and Nd solutions were measured on a thermal ionization mass spectrometer (Triton, MAT261). The measured isotopic ratios were corrected for mass fractionation using n(86Sr)/n(88Sr) = 0.1194 for Sr and n(146Nd/n(144Nd) = 0.7219 for Nd. The Sr standard NBS SRM 987 yielded n(87Sr)/n(86Sr) = 0.710244 ± 4 (n = 10, 2σ), while the Nd standard JNDi-1 gave n(143Nd)/n(144Nd) = 0.512124 ± 2 (n = 20, 2σ). All analyses were performed at the Mineral Resources Supervision Testing Wuhan Centre of China Land & Resources Ministry.

Result.an.conclutions:Major elements of the rocks shows that the meta-basalts belong to tholeiitic basalt characterized by rich in sodium and poor in potassium; rare-earth element differentiation is not obvious, trace elements are deficient in Ba, Th, and P, and high field strength elements are not deficient. the values ofεNd (t) are all positive, similar to the mid-oceanic ridge basalt (MORB).

Keywords:ophiolite;Sr—Nd isotopes;trace elements;Guanzizhen

注: 本文为国家自然科学基金资助项目(编号:41702218),中国地质科学院基本科研业务费项目(编号:YWF20190302),中国地质调查局项目(编号:1212011120157,DD20190010), 国家重点研发计划项目(编号:2017YFC0601402)的成果。

收稿日期:2019-09-05;改回日期:2020-04-20;责任编辑:章雨旭。Doi: 10.16509/j.georeview.2020.03.007

作者简介:操雨森,男,1995年生, 硕士研究生,构造地质与地球化学专业; Email: yusim68@163.com。

通讯作者:黄始琪,男,1984年生,博士,助理研究员, 构造地质专业; Email: shi7huang@163.com。

Acknowledgements:This work is financially supported by the National Natural Science Foundation of China (No. 41702218), Chinese Academy of Geological Sciences ( No. YWF20190302), Chinese Geological Survey (Nos. 1212011120157,DD20190010), and Ministry of Science and Technology of the People's Republic of China (No. 2017YFC0601402). The experimental test and analysis of this study has been assisted by the Mineral Resources Supervision Testing Wuhan Centre of China Land & Resources Ministry; the 2 experts reviewed the papers and put forward valuable suggestions for revision. Thank them sincerely here.

Firs.author:CAO Yuseng, male, born in 1995, master degree student, mainly engaged in structure geology and geochemistry; Email: yusim68@163.com

Correspondin.author:HUANG Shiqi, male, born in 1984, Ph. D., assistant professor, mainly engaged in structure geology and geochemistry; Email: shi7huang@163.com

Manuscript received on: 2019-09-05;Accepted on: 2020-04-20; Edited by: ZHANG Yuxu