Discovery and Significance of Early Silurian Andesites in Wuwamen Area, Southern Margin of Central Tianshan Block
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摘要: 安山岩是俯冲相关构造环境的特征岩石;对安山岩的研究,可以获得俯冲作用发生时代和俯冲过程的相关信息.报道了中天山地块南缘乌瓦门地区早志留世安山岩的年代学和地球化学特征,探讨了其岩石成因和构造属性,为南天山洋及中天山陆块南缘构造演化提供制约.研究表明,乌瓦门安山岩具有安山结构,组成矿物为普通辉石、钙质角闪石、斜长石和钾钠长石,为高钾钙碱性粗面安山岩;全岩SiO2=56.23%~59.28%,K2O=2.70%~3.37%,Na2O=3.32%~4.11%.其锆石U-Pb年龄为430 Ma,形成于早志留世晚期.微量元素组成上,富集轻稀土、亏损重稀土,(La/Yb)N=18.5~20.9;Eu负异常不明显,δEu=0.82~0.88;富集Ba、Th、U,亏损Nb、Ta、Ti;初始87Sr/86Sr比值为0.706 2~0.707 5,εNd(t)=+2.96~+3.01;这些数据揭示乌瓦门安山岩为安第斯型陆弧岩石,起源于被俯冲带相关流体交代的地幔楔,并在上升过程中受到了古老地壳的改造.早志留世晚期,南天山洋向中天山陆块下俯冲,构成成熟的洋-陆俯冲体系;中天山陆块南缘为活动大陆边缘,发育典型的安第斯型陆弧岩石组合.Abstract: Andesite is a typical rock type in subduction-related environment, and can reveal important information on the subduction processes. In this paper, it is presented of zircon U-Pb geochronology, whole-rock major and trace element and Sr-Nd isotope data for the Wuwamen andesites in the southern margin of Central Tianshan block, to provide constraints on the evolution processes of the South Tianshan ocean and Central Tianshan block. The andesites are high-K, calc-alkaline trachyandesite composed of augite, calcic amphibole, plagioclase and K-Na feldspar, with SiO2=56.23%-59.28%, K2O=2.70%-3.37%, and Na2O=3.32%-4.11%. They are characterized by fractionated rare earth element patterns with (La/Yb)N=18.5-20.9, and show negligible Eu anomalies with δEu=0.82-0.88; they are highly enriched in Rb, Ba and Sr, and depleted in Nb, Ta and Ti. Their initial 87Sr/86Sr ratios are in the range of 0.706 2-0.707 5 and εNd(t) in the range of 2.96-3.01. The data suggest that the Wuwamen andesites are typical Andean-type continental arcs, which originated from the partial melting of the mantle wedge that had been previously metasomatized by subduction-related fluids; the parental magma has been modified by the old crustal rocks during the magmas ascent. The origin and tectonic affinity of the Wuwamen andesites indicate that the South Tianshan ocean has subducted northward beneath the southern margin of the Central Tianshan block at the end of Early Silurian, forming a mature oceanic-continental subduction system; the southern margin of the Central Tianshan block has been a typical active continental margin characterized by development of Andean-type continental arc rocks.
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天山造山带在我国境内是一条东西长1 500 km的巨大山链,是由准噶尔地块、塔里木地块和其间的伊犁-中天山地块长期相互作用形成的复合型造山带(Gao et al., 1998; Shu et al., 2004; Kröner et al., 2007; Wang et al., 2007a, 2007b; Windley et al., 2007).天山造山带是中亚造山带的重要组成部分;中亚造山带是夹在北面的欧洲和西伯利亚克拉通与南面的塔里木和华北克拉通之间的一条巨大增生型造山带,记录了古亚洲洋复杂的演化历史(Sengör and Natal'in, 1996; Xiao et al., 2003; Li, 2006; Charvet et al., 2011; Han et al., 2011).
构造上,以中天山北缘边界断裂和中天山南缘边界断裂为界,天山造山带又分为北天山、中天山和南天山三部分(Shu et al., 1999, 2002; Xiao et al., 2009; Dong et al., 2011).在北缘主断裂带内巴音沟、干沟等处的蛇绿岩代表了古天山洋的闭合(Shu et al., 1999);沿中天山南缘断裂带在长阿吾子、古洛沟、乌瓦门及库米什一带出露的蛇绿岩,代表了南天山洋的闭合(郝杰和刘小汉,1993;汤耀庆等,1995;李茂松等,1996;董云鹏等,2005;徐向珍等,2011;杨经绥等,2011).
关于南天山洋的俯冲闭合,存在一些争议.首先,关于南天山洋的俯冲极性,一些学者根据构造变形和运动学研究,认为南天山洋向南俯冲到塔里木陆块之下导致了南天山洋的闭合(Shu et al., 2002, 2004; Wang et al., 2007a, 2007b; Lin et al., 2009; Charvet et al., 2011; Wang et al., 2011);另外一些学者根据沉积岩和岩浆岩记录,认为南天山洋向北俯冲到伊犁-中天山陆块之下导致了南天山洋的闭合(Gao et al., 1998; Xiao, 2004;杨天南等,2006; Dong et al., 2011;贺振宇等,2012; Xiao et al., 2013;郭春涛等,2017);还有一些学者认为存在双向俯冲(Gao et al., 2009; Ge et al., 2012).其次,关于南天山洋的俯冲时代,目前获得的南天山洋俯冲相关岩浆岩年代学数据有:西天山陆弧火山岩361~313 Ma(朱永峰等,2005;王博等,2006;龙灵利等,2008; Zhu et al., 2009);西南天山岛弧火山岩427~420 Ma(蒲晓菲等,2011);西南天山陆弧花岗岩类479~366 Ma(朱志新等,2006;龙灵利等,2007);中天山南缘巴仑台地区陆弧中酸性侵入岩430~340 Ma(杨天南等,2006;陈义兵等,2012; Ma et al., 2014;赵一珏等,2015);库米什地区陆弧花岗岩423~422 Ma(张成立等,2007;杨经绥等,2011).这些数据揭示南天山洋的俯冲发生在479~313 Ma较大的时间跨度内.
笔者在中天山南缘巴仑台以南乌瓦门地区识别出了一期早志留世安山岩岩浆作用.安山岩是俯冲相关构造环境的特征岩石,对安山岩的研究,可以获得俯冲作用发生时代和俯冲过程的相关信息.本文报道了这期安山岩的锆石U-Pb年龄、全岩主微量元素地球化学和Sr-Nd同位素数据;并将其与当今典型的大洋岛弧和陆弧安山岩进行比较研究,解析其岩石成因,判别其构造属性,将今论古,为南天山洋的俯冲时限和过程提供制约.
1. 地质背景及样品产出
研究区位于新疆和静县北部乌瓦门地区,构造位置属于中天山陆块南缘,紧邻中天山南缘断裂带(图 1).
图 1 中天山及周边地质简图CTB.中天山地块;STB.南天山;①中天山北缘边界断裂;②中天山南缘边界断裂.据Ma et al.(2014)修改Fig. 1. Sketch map of the Central Tianshan and its surroundings中天山地块可以识别出的地质单元,从老到新包括:元古代基底岩石、奥陶纪-早泥盆世弧相关岩浆岩和石炭纪及石炭纪以来的沉积岩(Shu et al., 2002; Wang et al., 2011).元古代基底岩石主要出露于巴仑台地区;其中,花岗质片麻岩的年龄为900~940 Ma(陈新跃等,2009; Gao et al., 2015),变质辉长岩和花岗质岩墙的年龄分别为733 Ma和730 Ma(Gao et al., 2015).
本文所研究的安山岩分布于乌瓦门河道沟口处,GPS位置为北纬42°39′3.38″,东经86°10′41.61″.野外产出如图 2所示,呈灰绿色,细粒-隐晶结构,厚约为2~3 m.其周围岩石为绿片岩,呈暗土红色,中粗粒结构.安山岩露头向西约10 m处出露新元古代花岗质片麻岩(时代为739 Ma;本文未发表数据).
乌瓦门安山岩的显微结构如图 3所示,以发育斑状结构为特征,斑晶已经绿泥石化和绿帘石化.根据矿物晶型及蚀变矿物种类,推测原生斑晶矿物可能为辉石.基质呈现交织结构或安山结构,斜长石微晶呈不定向排列,微晶之间充填辉石、角闪石、磁铁矿和隐晶质.显微镜下较难识别镁铁矿物的种类及含量,但据电子探针分析结果(详见3.2),基质中的镁铁矿物主要为角闪石,其次为辉石.
2. 分析方法
锆石的分选是在河北省地矿局廊坊区调队实验室完成.利用重液和磁选相结合的方法从粉碎的岩石样品中把锆石分选出来,再在双目镜下提纯,将锆石嵌于树脂样靶中,并打磨、抛光.锆石的阴极发光图像在北京锆年领航科技有限公司电子探针实验室采用扫描电镜完成.锆石测年采用LA-ICPMS分析方法,在中国地质调查局天津地质调查中心完成.分析仪器为Finnigan Neptune型MC-ICP-MS及与之配套的Newwave UP 193激光剥蚀系统,激光剥蚀斑束直径为35 μm,剥蚀深度为20~40 μm.锆石年龄计算采用国际标准锆石91500作为外标,元素含量采用人工合成硅酸盐玻璃NIST SRM610作为外标,29Si作为标准元素进行校正,数据处理采用ICPMSDataCal 4.3程序.年龄计算及谐和图绘制采用Isoplot 3.0完成(Ludwig, 2003).普通Pb校正方法同Andersen(2002).
矿物电子探针元素分析在中国地质科学院地质研究所电子探针实验室完成.仪器型号为JXA-8100,加速电压15 kV,束流1×10-8 A,束斑1 μm.采用PRZ方法校正,分析标样为美国SPI公司的53种矿物,测试精度优于1%.
全岩主微量元素分析在国家地质实验测试中心完成.样品粉末熔成玻璃饼后,应用X射线荧光光谱仪(PW4400)测定主量元素组成.采用两酸(HNO3+HF)高压反应釜溶样方法对样品粉末进行溶解,采用等离子质谱仪(PE300D)测定微量元素含量.
全岩Sr-Nd同位素化学分析及测试在中国科学院南京土壤研究所技术服务中心完成.样品粉末用混合酸溶解,利用传统的阳离子交换柱法实现元素的分离和纯化.采用热电离同位素质谱方法测定元素比值,仪器为英国制造的VG354多接收质谱计.详细的化学分析过程和质谱测定方法可参考王银喜等(2007).
3. 分析结果
3.1 锆石U-Pb定年
用于U-Pb定年的锆石选自样品12YX11-30.锆石均无继承核,发育明显的岩浆振荡环带(图 4),指示它们为岩浆成因.锆石U-Pb定年分析结果见表 1,图 5a和图 5b分别为206Pb/238U-207Pb/235U年龄谐和图及206Pb/238U年龄加权平均统计图.这些锆石的Th含量为28×10-6~739×10-6,U含量为297×10-6~1 167×10-6,Th/U比值为0.1~0.8. 32颗锆石给出的206Pb/238U年龄分布在(427±3)~(433±3)Ma之间,加权平均年龄为430.0±0.9 Ma(MSWD=0.16).
表 1 乌瓦门安山岩锆石LA-ICP-MS U-Pb定年分析结果Table Supplementary Table Zircon LA-ICP-MS U-Pb isotopic data for the Wuwamen andesites分析点号 含量(10-6) Th/U 同位素比值 年龄(Ma) U Th 206Pb/238U 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/235U 1σ 1 583 273 0.5 0.069 0 0.000 5 0.528 6 0.005 5 430 3 431 5 2 1 167 541 0.5 0.069 0 0.000 4 0.529 5 0.004 0 430 3 432 3 3 726 332 0.5 0.068 9 0.000 4 0.524 4 0.004 5 430 3 428 4 4 567 257 0.5 0.069 0 0.000 4 0.528 7 0.004 8 430 3 431 4 5 706 311 0.4 0.069 0 0.000 5 0.530 0 0.004 9 430 3 432 4 6 685 458 0.7 0.069 0 0.000 5 0.529 4 0.004 7 430 3 431 4 7 663 351 0.5 0.069 0 0.000 5 0.530 3 0.004 5 430 3 432 4 8 637 363 0.6 0.069 0 0.000 5 0.526 2 0.004 5 430 3 429 4 9 709 472 0.7 0.069 0 0.000 5 0.523 8 0.004 6 430 3 428 4 10 492 259 0.5 0.069 0 0.000 4 0.529 0 0.005 3 430 3 431 4 11 680 400 0.6 0.069 1 0.000 4 0.528 0 0.004 8 431 3 430 4 12 827 542 0.7 0.069 1 0.000 4 0.529 8 0.004 6 430 3 432 4 13 767 475 0.6 0.069 0 0.000 5 0.527 6 0.004 6 430 3 430 4 14 951 705 0.7 0.069 0 0.000 4 0.526 5 0.004 2 430 3 430 3 15 1 072 739 0.7 0.069 0 0.000 4 0.527 4 0.004 4 430 3 430 4 16 569 315 0.6 0.069 0 0.000 4 0.528 8 0.005 8 430 3 431 5 17 385 253 0.7 0.069 0 0.000 4 0.521 7 0.007 5 430 3 426 6 18 626 368 0.6 0.068 9 0.000 4 0.531 2 0.007 8 430 3 433 6 19 453 28 0.1 0.068 6 0.000 4 0.529 0 0.008 8 427 3 431 7 20 727 411 0.6 0.068 7 0.000 4 0.528 8 0.004 4 429 3 431 4 21 695 378 0.5 0.068 4 0.000 4 0.532 5 0.004 5 427 3 433 4 22 352 175 0.5 0.069 1 0.000 4 0.525 6 0.006 9 430 3 429 6 23 943 712 0.8 0.068 8 0.000 4 0.528 6 0.004 0 429 3 431 3 24 509 285 0.6 0.069 2 0.000 5 0.531 9 0.005 2 431 3 433 4 25 655 337 0.5 0.069 2 0.000 4 0.529 9 0.004 8 431 3 432 4 26 563 304 0.5 0.069 2 0.000 4 0.527 0 0.005 4 432 3 430 4 27 691 328 0.5 0.069 1 0.000 4 0.530 2 0.004 9 431 3 432 4 28 297 111 0.4 0.069 4 0.000 4 0.522 0 0.009 7 433 3 426 8 29 583 249 0.4 0.068 8 0.000 4 0.529 3 0.004 8 429 3 431 4 30 790 365 0.5 0.069 0 0.000 5 0.527 4 0.004 5 430 3 430 4 31 876 381 0.4 0.069 0 0.000 5 0.528 6 0.004 2 430 3 431 3 32 668 309 0.5 0.068 8 0.000 4 0.529 5 0.004 7 429 3 432 4 3.2 矿物化学成分
对乌瓦门安山岩主要组成矿物(辉石、角闪石和长石)进行了电子探针主量元素分析,分析结果见表 2~3.
表 2 乌瓦门安山岩镁铁矿物(角闪石和辉石)的电子探针分析结果(%)Table Supplementary Table Microprobe analyses of amphibole and pyroxene from the Wuwamen andesites点号 A1 A2 A3 A4 A5 A6 A7 A8 A9 P1 P2 P3 矿物种类 Parg Eden Eden Eden Eden Parg Parg Eden Eden Augi Augi Augi SiO2 43.20 45.58 44.33 44.27 45.65 43.00 42.48 44.48 47.09 54.25 50.52 53.64 TiO2 2.94 2.69 2.81 2.59 2.12 2.98 2.78 1.29 0.92 0.18 1.46 0.11 Al2O3 9.43 9.15 8.92 9.06 7.93 9.79 10.46 9.39 7.68 1.92 3.58 1.68 Cr2O3 0.05 0.06 0.03 0 0 0.02 0.00 0.06 0 0.03 0.07 0.05 FeOT 11.78 13.05 11.90 12.31 11.87 12.17 12.54 12.45 11.41 12.21 11.59 11.60 MnO 0.27 0.28 0.17 0.25 0.21 0.20 0.16 0.20 0.18 0.30 0.23 0.22 MgO 14.60 13.44 14.89 14.58 14.83 14.15 14.07 15.66 16.70 15.79 15.27 16.13 CaO 10.91 10.76 11.07 10.92 11.28 11.07 10.96 10.26 10.53 12.60 12.41 12.70 Na2O 2.16 2.08 2.10 2.16 1.82 2.28 2.28 1.83 1.88 0.26 0.62 0.21 K2O 1.08 0.84 0.93 1.01 0.88 1.11 1.07 0.75 0.66 0.13 0.31 0.47 NiO 0 0 0 0 0.01 0.07 0.01 0.10 0.01 0 0 0.01 总计 96.44 97.93 97.15 97.15 96.60 96.82 96.81 96.47 97.05 97.66 96.07 96.81 基于23个氧原子计算的离子个数 基于6个氧原子计算的离子个数 Si 6.41 6.65 6.51 6.51 6.73 6.38 6.30 6.61 6.79 2.03 1.94 2.03 Ti 0.33 0.30 0.31 0.29 0.24 0.33 0.31 0.15 0.10 0.01 0.04 0 Al 1.65 1.57 1.54 1.57 1.38 1.71 1.83 1.64 1.31 0.09 0.16 0.08 Cr 0.01 0.01 0 0 0 0 0 0.01 0 0 0 0 Fe3+ 0.34 0.27 0.32 0.37 0.29 0.31 0.40 0 0.63 0 0 0 Fe2+ 1.13 1.33 1.14 1.14 1.17 1.20 1.16 1.55 0.75 0.38 0.37 0.37 Mn 0.03 0.04 0.02 0.03 0.03 0.03 0.02 0.03 0.02 0.01 0.01 0.01 Mg 3.23 2.92 3.26 3.20 3.26 3.13 3.11 3.47 3.59 0.88 0.87 0.91 Ca 1.74 1.68 1.74 1.72 1.78 1.76 1.74 1.64 1.63 0.51 0.51 0.52 Na 0.62 0.59 0.60 0.62 0.52 0.66 0.65 0.53 0.53 0.02 0.05 0.02 K 0.20 0.16 0.17 0.19 0.17 0.21 0.20 0.14 0.12 0.01 0.02 0.02 ∑ 15.79 15.59 15.74 15.76 15.65 15.82 15.85 15.74 15.68 3.93 3.97 3.95 注:Parg.韭闪石;Eden.浅闪石;Augi.普通辉石. 表 3 乌瓦门安山岩长石的电子探针分析结果(%)Table Supplementary Table Microprobe analyses of feldspar from the Wuwamen andesites点号 F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 F13 F14 F15 矿物种类 Ab Ab Ab Ab Ab Ab Ab Or Or Or Or Or Or Or Or SiO2 67.81 68.24 68.30 69.31 67.56 68.32 67.96 66.23 69.76 65.98 65.97 64.43 70.01 65.93 66.47 TiO2 0.01 0.07 0.02 0.03 0.03 0.01 0 0 0.08 0 0 0.03 0 0.04 0.06 Al2O3 17.64 19.65 19.43 19.08 20.16 19.68 19.41 17.36 15.38 17.44 17.56 18.01 15.53 17.57 17.01 Cr2O3 0.0 0 0.02 0.05 0.01 0 0 0 0.01 0.01 0 0 0.01 0 0.01 FeOT 1.23 0.18 0.18 0.14 0.27 0.18 0.14 0.16 0.21 0.10 0.16 0.11 0.23 0.14 0.09 MnO 0.053 0 0 0.007 0 0 0 0.007 0.021 0.012 0 0.028 0.006 0.004 0 MgO 1.29 0.01 0.00 0.01 0.06 0.03 0.01 0 0 0 0 0 0 0 0 CaO 1.61 1.41 1.08 0.61 1.15 1.23 1.03 0.05 0.03 0 0.07 0.01 0.07 0.05 0.06 Na2O 10.06 10.21 10.50 10.86 9.84 10.37 10.38 0.36 0.19 0.16 0.16 0.14 0.30 1.54 0.57 K2O 0.09 0.10 0.21 0.11 0.74 0.16 0.34 15.93 14.04 16.39 16.36 16.61 13.68 14.05 15.61 NiO 0 0 0.01 0 0.01 0 0 0.03 0.02 0 0 0 0 0 0 总计 99.80 99.86 99.75 100.20 99.82 99.98 99.29 100.11 99.73 100.09 100.27 99.38 99.82 99.32 99.88 基于8个氧原子计算的离子个数 Si 2.98 2.98 2.99 3.02 2.96 2.98 2.99 3.05 3.17 3.04 3.04 3.00 3.17 3.04 3.06 Al 0.91 1.01 1.00 0.98 1.04 1.01 1.01 0.94 0.82 0.95 0.95 0.99 0.83 0.95 0.92 Ca 0.08 0.07 0.05 0.03 0.05 0.06 0.05 0 0 0 0 0 0 0 0 Na 0.86 0.87 0.89 0.92 0.84 0.88 0.89 0.03 0.02 0.01 0.01 0.01 0.03 0.14 0.05 K 0.01 0.01 0.01 0.01 0.04 0.01 0.02 0.94 0.81 0.96 0.96 0.99 0.79 0.83 0.92 ∑ 4.97 4.94 4.96 4.95 4.95 4.95 4.96 4.96 4.83 4.97 4.97 5.00 4.82 4.96 4.96 An 8 7 5 3 6 6 5 0 0 0 0 0 0 0 0 Ab 91 92 93 96 90 93 93 3 2 1 1 1 3 14 5 Or 1 1 1 1 4 1 2 96 98 99 98 99 96 85 94 乌瓦门安山岩中的辉石为单斜辉石,属Ca-Mg-Fe系列中的普通辉石(Wo28-29En50-51Fs20-22;图 6a),具体组成为:SiO2=50.52%~54.25%,Al2O3=1.68%~3.58%,FeOT=11.21%~ 11.60%,MgO=15.27%~16.13%,CaO=12.41%~12.70%,Na2O=0.21%~0.62%,K2O=0.13%~0.47%.
图 6 乌瓦门安山岩的矿物分类图解a.Ca-Mg-Fe单斜辉石系列的硅灰石(Wo)-顽火辉石(En)-斜铁辉石(Fs)分类图解,据Morimoto(1988);b.钙质角闪石分类图解,据Leake(1997);c.长石分类图解Fig. 6. Classification diagrams of the minerals from the Wuwamen andesites角闪石为钙质角闪石亚组中的浅闪石和韭闪石(图 6b),具体组成为:SiO2=42.48%~47.09%,Al2O3=7.68%~10.46%,FeOT=11.41%~13.05%,MgO=13.44%~16.70%,CaO=10.26%~11.28%,Na2O=1.82%~2.28%,K2O=0.66%~1.11%.
长石包括斜长石和钾钠长石两类(图 6c).斜长石在化学成分上集中在钠长石端元(An3-8Ab90-96Or1-4),具体组成为:SiO2=67.56%~69.31%,Al2O3=17.64%~20.16%,CaO=0.61%~1.61%,Na2O=9.84%~10.86%,K2O=0.09%~0.74%.钾钠长石则主要集中在钾长石端元(An0Ab1-5Or94-99),具体组成为:SiO2=64.43%~70.01%,Al2O3=15.38%~18.01%,CaO=0~0.70%,Na2O=0.14%~1.52%,K2O=13.68%~16.61%.
3.3 全岩主量和微量元素组成
乌瓦门安山岩的主微量元素组成见表 4.其SiO2含量为56.23%~59.28%,TiO2含量为0.81%~0.88%,Al2O3含量为14.7%~16.19%,K2O含量为2.70%~3.37%,Na2O含量为3.32%~4.11%;Na2O含量普遍高于K2O含量,Na2O/K2O=0.98~1.49(只有一个样品的Na2O含量低于K2O).Fe2O3T含量为5.44%~7.65%,MgO的含量为3.52%~6.36%,Mg#=55~63.样品经历了后期蚀变作用,烧失量(LOI)介于2.04%~3.13%.扣除烧失量,对主量元素组成进行归一化计算后再投图.在火山岩TAS分类图解中,样品落在亚碱性系列的粗面安山岩范围内(图 7a);在区分拉斑质和钙碱性岩石的AFM图解中,样品落在钙碱性系列范围内(图 7b);在进一步划分钙碱性系列的K2O-SiO2图解中,样品落在高钾钙碱性系列区域内(图 7c).为比较研究,图 7还给出了南美安第斯陆弧中带火山岩(Winter, 2001及其中文献)、巴布亚新几内亚的Tabar-Lihir-Tanga-Feni(TLTF)高钾洋内岛弧岩石(Stracke and Hegner, 1998)的成分分布.乌瓦门安山岩与安第斯陆弧中带火山岩具有一致的主量元素组成.
表 4 乌瓦门安山岩的主量(%)和微量元素(10-6)组成Table Supplementary Table Major (%) and trace element (10-6) compositions of the Wuwamen andesites样品号 12YX11-23 12YX11-24 12YX11-25 12YX11-26 12YX11-27 12YX11-28 12YX11-29 12YX11-30 SiO2 56.4 58.5 58.6 56.8 56.2 57.4 58.9 59.3 TiO2 0.82 0.83 0.85 0.88 0.81 0.83 0.84 0.83 Al2O3 14.9 15.9 15.9 15.5 14.7 15.1 16.2 16.2 Fe2O3T 7.53 5.44 6.27 7.21 7.65 6.82 5.99 5.84 MnO 0.12 0.08 0.10 0.11 0.12 0.11 0.09 0.09 MgO 6.36 3.68 4.50 5.72 6.27 5.20 3.75 3.52 CaO 4.05 5.38 4.11 4.15 4.62 4.96 4.88 5.03 Na2O 3.22 3.99 3.96 3.77 3.24 3.73 3.94 3.94 K2O 3.27 2.99 2.95 2.76 3.00 2.68 2.74 2.64 P2O5 0.28 0.29 0.30 0.29 0.28 0.28 0.30 0.29 LOI 2.51 2.78 2.31 2.87 3.13 2.81 2.28 2.04 Total 99.42 99.9 99.82 100.11 100.05 99.91 99.87 99.68 Mg# 63 58 59 61 62 60 56 55 K2O+Na2O 6.49 6.98 6.91 6.53 6.24 6.41 6.68 6.58 Na2O/K2O 0.98 1.33 1.34 1.37 1.08 1.39 1.44 1.49 Cr 260 102 143 221 257 194 110 112 Mn 921 642 742 877 944 851 690 670 Co 25.0 15.8 21.3 24.6 20.7 23.6 16.5 15.4 Ni 159 64.6 83.8 127.0 153.0 114.0 65.4 67.0 Cu 8.75 7.83 9.18 19.60 9.87 39.00 11.30 7.68 Zn 66.9 44.3 53.0 62.4 66.6 58.9 44.3 42.4 Ga 18.0 18.5 19.1 18.3 17.9 18.2 19.4 19.4 Rb 157 58.4 61.1 63.0 123 57.1 56.3 52.9 Sr 405 619 476 421 393 435 652 706 Ba 1 286 1 074 1 176 1 132 1 142 995 968 919 Pb 4.55 10.1 8.43 5.49 4.03 7.64 9.68 9.30 Th 17.8 19.3 19.2 17.5 16.7 17.7 19.5 19.5 U 2.97 4.30 3.58 3.16 2.99 3.29 4.13 4.09 Nb 7.52 8.41 8.37 7.54 7.52 7.97 8.60 8.60 Ta 0.62 0.76 0.73 0.62 0.59 0.64 0.75 0.75 Zr 179 185 189 189 180 185 186 189 Hf 4.82 4.95 5.00 4.89 4.53 4.88 5.00 5.05 Ti 4 952 4 873 5 172 5 318 4 967 5 115 4 970 5 106 V 109 106 109 112 108 107 108 105 Y 17.3 18.3 18.0 18.1 17.6 17.7 17.8 18.1 La 46.0 48.3 48.0 45.9 43.9 45.9 49.3 48.8 Ce 81.0 85.2 84.4 81.1 78.3 80.4 86.9 85.1 Pr 9.64 10.1 10.0 9.73 9.43 9.55 10.2 9.88 Nd 37.3 40.2 40.0 37.8 37.0 37.9 38.7 38.2 Sm 6.01 6.47 6.41 6.19 5.88 5.94 6.46 6.26 Eu 1.52 1.74 1.70 1.53 1.54 1.53 1.70 1.65 Gd 5.10 5.40 5.42 4.95 5.02 4.89 5.64 5.20 Tb 0.68 0.72 0.70 0.69 0.64 0.67 0.70 0.70 Dy 3.52 3.79 3.68 3.59 3.48 3.46 3.62 3.71 Ho 0.65 0.67 0.66 0.67 0.64 0.63 0.66 0.67 Er 1.84 1.81 1.91 1.89 1.76 1.81 1.83 1.86 Tm 0.24 0.23 0.25 0.25 0.24 0.24 0.24 0.24 Yb 1.61 1.56 1.72 1.67 1.55 1.58 1.66 1.64 Lu 0.23 0.25 0.26 0.25 0.23 0.24 0.25 0.25 ∑REE 195.34 206.44 205.11 196.21 189.61 194.74 207.86 204.16 δEu 0.82 0.88 0.86 0.82 0.85 0.84 0.84 0.86 (La/Yb)N 19.3 20.9 18.8 18.5 19.1 19.6 20.0 20.1 图 7 乌瓦门安山岩的岩石分类图解a.火山岩TAS分类图解,据Le Maitre(2002);b.AFM图解,据Irvine and Baragar(1971);c.K2O-SiO2图解,据Le Maitre(2002);南美安第斯陆弧中带火山岩数据引自Winter(2001)及其中的文献;巴布亚新几内亚TLTF(Tabar-Lihir-Tanga-Feni)高钾洋内岛弧岩石引自Stracke and Hegner(1998)Fig. 7. Classification diagrams for the Wuwamen andesites乌瓦门安山岩球粒陨石标准化稀土元素配分模式和原始地幔标准化微量元素蛛网图见图 8.乌瓦门安山岩的稀土元素(REE)总量相对较高,为189×10-6~206×10-6;呈富集轻稀土(LREE)、亏损重稀土(HREE)的右倾型配分模式,(La/Yb)N=18.5~20.9;Eu负异常不明显,δEu=0.82~0.88.在原始地幔标准化微量元素蜘蛛网图中,样品以明显亏损Nb、Ta、Ti,而富集Rb、Ba、Th、U、K和Pb元素为特征.
图 8 乌瓦门安山岩的球粒陨石标准化稀土元素配分模式和原始地幔标准化微量元素蛛网图安第斯陆弧南带火山岩数据引自Winter(2001)及其中的文献;平均俯冲沉积物数据引自Plank and Langmuir(1998);平均陆壳数据引自Rudnick and Gao(2003);球粒陨石稀土元素数据采用Boynton(1984);原始地幔微量元素数据采用Sun and McDonough(1989);其他数据来源同图 7Fig. 8. Chondrite-normalized REE patterns and primitive mantle-normalized spider diagrams for the Wuwamen andesites为对比研究,图中还给出了安第斯陆弧中带、南带火山岩(Winter, 2001及其中文献)、巴布亚新几内亚的TLTF高钾洋内岛弧岩石(Stracke and Hegner, 1998)以及全球平均俯冲沉积物(Plank and Langmuir, 1998)和平均陆壳(Rudnick and Gao, 2003)的REE配分模式和微量元素蛛网图分布.乌瓦门安山岩的微量元素组成与安第斯陆弧中带火山岩比较一致,而与岛弧火山岩和安第斯陆弧南带岩石有明显差异.
3.4 Sr-Nd同位素组成
乌瓦门安山岩的Sr-Nd同位素组成见表 5.据锆石U-Pb年龄,采用430 Ma对初始同位素组成进行计算,获得其初始87Sr/86Sr比值为0.706 2~0.707 5,初始143Nd/144Nd=0.512 235~0.512 238,εNd(t)=+2.96~+3.01.在(143Nd/144Nd)i-(87Sr/86Sr)i相关图解中(图 9),乌瓦门安山岩落在安第斯陆弧中带火山岩Sr-Nd同位素组成范围内.
表 5 乌瓦门安山岩的Sr-Nd同位素组成Table Supplementary Table Sr-Nd isotopic data of the Wuwamen andesites样号 Rb Sr 87Rb/
86Sr87Sr/
86Sr2σ (87Sr/
86Sr)iSm Nd 147Sm/
144Nd143Nd/
144Nd2σ (143Nd/
144Nd)iεNd(t) TDM
(Ma)12YX11-23 155 399 1.149 0 0.713 209 8 0.706 2 5.98 36.8 0.098 2 0.512 512 9 0.512 235 2.96 842 12YX11-25 59.8 470 0.375 5 0.709 770 7 0.707 5 6.51 41.3 0.095 8 0.512 508 7 0.512 238 3.01 830 12YX11-27 125 403 0.916 7 0.712 304 9 0.706 7 6.00 38.4 0.094 6 0.512 503 9 0.512 237 2.98 828 12YX11-29 57.9 659 0.258 4 0.708 685 8 0.707 1 6.50 39.7 0.099 2 0.512 516 8 0.512 237 2.98 844 注:根据锆石U-Pb年龄,采用430 Ma计算初始同位素组成. 图 9 乌瓦门安山岩的(143Nd/144Nd)i-(87Sr/86Sr)i相关图解MORB范围采用Gale et al.(2013);安第斯陆弧中带弧下地幔范围引自Lucassen et al.(2006);安第斯陆弧中带古生代地壳范围引自Lucassen et al.(1999);安第斯陆弧中带火山岩范围引自Winter(2001)Fig. 9. (143Nd/144Nd)i vs. (87Sr/86Sr)i diagram for the Wuwamen andesites4. 讨论
4.1 岩石成因
乌瓦门安山岩属于高钾钙碱性岩石系列,体系富水(角闪石是主要镁铁矿物);富集元素Rb、Ba、Th、U、K和Sr,亏损元素Nb、Ta和Ti;这是俯冲带环境弧岩浆的典型特征,岩浆源区为被俯冲板片蚀变洋壳(及上覆沉积物)相关流体交代过的地幔楔.
但与TLTF岛弧和安第斯陆弧南带火山岩相比,乌瓦门安山岩具有相对较高的LREE、Nb-Ta和Zr-Hf含量(图 8),暗示乌瓦门安山岩在形成过程中有陆壳物质参与.该认识得到了元素协变图解和其Sr-Nd同位素组成的支持:(1)在元素协变图解上(图 10),尽管乌瓦门安山岩的元素组成比较集中,但在样品内部,元素间的线性关系仍较明显;不相容元素Sr、Th、U、Nb、La、Ce与SiO2呈较好的线性正相关关系,即随着SiO2含量的增加,这些元素含量也明显增加,指示陆壳物质的参与;(2)在Sr-Nd同位素组成上,乌瓦门安山岩具有较低的初始143Nd/144Nd比值(0.512 235~0.512 238,εNd(t)=+2.96~+3.01)和较高的86Sr/87Sr比值(0.706 2~0.707 5);此外,Nd同位素组成较均一,而Sr同位素组成呈一定的范围(图 9);这些特征均是岩浆形成过程中有古老地壳物质参与的表现.乌瓦门地区确实存在古老地壳(Gao et al., 2015),安山岩向西10 m左右,即出露新元古代花岗岩(图 2).
图 10中,元素K2O和Ba与SiO2呈负相关关系,说明乌瓦门安山岩的初始岩浆即具有较高的K和Ba含量;代表HREE的元素Yb与SiO2关系不明显,说明样品的HREE组成主要反映了源区特征.乌瓦门安山岩的HREE含量低,且发生分异(即不平坦),说明岩浆源区很有可能残留矿物石榴石;即岩浆可能来源于石榴石稳定区(>100 km).
乌瓦门安山岩为高钾钙碱性岩石(图 7c),K2O含量相对较高(2.70%~3.37%).蚀变的洋壳在一定的P-T条件下,会产生含碳酸盐、具有霞石标准矿物的熔体.这种熔体与地幔矿物发生反应,大于2 GPa时(约70~80 km),生成金云母-单斜辉石;低于2 GPa时,会生成角闪石-单斜辉石(McInnes and Cameron, 1994).含金云母的源区部分熔融形成的熔体会富钾,含角闪石源区部分熔融形成的熔体钾含量会低些.乌瓦门安山岩的钾含量相对较高,可能反映了其源区含有金云母,深度较大.但是,最新研究发现,来自大陆的沉积物熔体与地幔橄榄岩反应后形成的岩浆也可以含有高达5%的K2O(Wang et al., 2017);因此,乌瓦门安山岩相对较高的K2O含量也可能是俯冲沉积物与地幔橄榄岩反应的结果.
综上所述,乌瓦门安山岩形成于俯冲带环境,起源于被俯冲带相关流体交代的、富集大离子亲石元素的地幔楔;重稀土元素分布特征支持其源区可能位于石榴石稳定区(>100 km).岩浆形成后在经过陆壳到达地表过程中,与陆壳物质相互作用,古老地壳岩石对岩浆的改造非常明显.
4.2 安第斯型活动陆缘弧岩浆作用
为确定乌瓦门安山岩的构造属性,本文将其与南美安第斯陆弧火山岩、巴布亚新几内亚的Tabar-Lihir-Tanga-Feni(TLTF)岛弧岩石进行了比较研究.
岛弧岩石成分范围较广,拉斑系列和钙碱性系列都会出现在岛弧岩石中;而且钙碱性系列中,低钾、中钾和高钾系列都普遍分布.本文选择巴布亚新几内亚的TLTF岛弧岩石进行对比,是因为它们代表了岛弧“家族”中的高钾系列,相较其他类型的岛弧岩石,它们与乌瓦门安山岩成分更为接近.但是,乌瓦门安山岩与TLTF岛弧岩石仍有明显不同:(1)主量元素组成上,乌瓦门安山岩更富SiO2;TLTF岛弧主要为粗面玄武岩-玄武粗安岩,而乌瓦门安山岩则为粗面安山岩(图 7);(2)REE组成上,乌瓦门安山岩更富LREE(图 8e);(3)微量元素蛛网图模式明显不同,乌瓦门安山岩明显更加富集Ba、Th和U,而Nb-Ta的亏损程度相对岛弧岩石则弱很多;另外,乌瓦门安山岩Zr-Hf含量明显高于TLTF岛弧,而Sr含量低于后者.
南美洲大陆西缘自500 Ma至今连续发育由俯冲作用导致的岩浆作用,而且未遭受明显碰撞构造事件的改造,是研究活动大陆边缘弧岩浆作用的天然实验室.需要说明的是,南美安第斯陆弧岩浆作用活跃区自北向南分为北带(分布在哥伦比亚和厄瓜多尔,介于5°N和2°S)、中带(主要分布在秘鲁南部和智利北部,介于16°S和27°S)和南带(主要分布在智利南部,介于33°S和55°S);3个带之间为岩浆作用稳定区.岩浆作用活跃区的板片俯冲角度相对较大(25°~30°),而岩浆作用稳定区下的板片俯冲角度相对较小(10°~15°).这3个带的区别在于它们的地壳厚度和地壳类型:北带和南带的地壳以中生代和新生代地壳为主,而且可能主要为增生洋壳和岛弧地体,地壳厚度约为30~40 km;而中带出露前寒武变质岩基底,地壳厚度达75 km.地壳岩石组成和地壳厚度的不同,导致这3个带岩浆岩具有不同的地球化学特征(Winter, 2001).
乌瓦门安山岩具有与安第斯陆弧中带火山岩一致的地球化学特征:均为高钾钙碱性粗安岩(图 7);具有相似的Rb、Ba、Th、U、K和Pb富集程度及Nb、Ta和Ti亏损程度(图 8a, 8b);具有一致的Sr-Nd同位素组成(图 9).乌瓦门安山岩(和安第斯陆弧中带火山岩)与安第斯陆弧南带火山岩在REE和微量元素组成上不同,后者表现为LREE含量低,HREE相对平坦,Rb、Ba、Th富集程度低,Nb-Ta亏损程度强;这些表现与其地壳类型和地壳厚度是一致的:乌瓦门地区和安第斯中带发育前寒武古老基底,地区厚度较大;而安第斯南带地壳主要为增生洋壳和岛弧地体,厚度较小.
因此,乌瓦门安山岩可能代表了活动大陆边缘弧岩浆作用(即安第斯型陆弧),与当今安第斯陆弧中带岩浆作用情形比较一致.
微量元素判别图解常用来判别岩石的构造属性.在花岗岩类的Rb-(Y+Nb)和Nb-Y判别图解上(图 11a, 11b),乌瓦门安山岩与安第斯陆弧中带、南带岩石和TLTF岛弧岩石一同落在火山弧花岗岩(VAG)范围内;在Rb-(Y+Nb)图解中,部分安第斯陆弧南带岩石落在了板内花岗岩(WPG)内.这两个图解一定程度上可以有效地识别弧岩浆,但不能够区分是陆弧还是岛弧.
图 11 乌瓦门安山岩的微量元素判别图解a, b.花岗岩类的Rb-(Y+Nb)和Nb-Y判别图解,据Pearce et al.(1984):VAG.火山弧花岗岩;ORG.洋脊花岗岩;WPG.板内花岗岩;Syn-COLG.同碰撞花岗岩;c, d.中酸性岩石的Th/Yb-Ta/Yb和Th/Ta-Yb判别图解,据Gorton and Schandl(2000):Oceanic arcs.岛弧;ACM.活动大陆边缘;WPVZ.板内火山带;WPB.板内玄武岩;MORB.大洋中脊玄武岩Fig. 11. Discrimination diagrams for the Wuwamen andesitesGorton and Schandl(2000)利用元素Ta、Th和Yb建立了两个判别图解(图 11c, 11d),来识别岛弧(oceanic arcs)、活动陆缘弧(active continental margins)和板内火山(within-plate volcanic zones).在这两个图解中,乌瓦门安山岩和个别安第斯陆弧中带和南带样品落在了岛弧范围内;大部分安第斯陆弧和TLTF岛弧样品一同落在了活动陆缘弧范围内;投图结果与样品的实际产出环境不一致.说明这两幅图存在局限性,其划分的构造环境可能不准确,应用时须谨慎.
4.3 构造意义
如上所述,乌瓦门安山岩具有与南美安第斯陆弧中带火山岩一致的成因过程,即大洋岩石圈俯冲到具有古老陆壳的大陆岩石圈之下,俯冲蚀变板片(及上覆沉积物)发生脱水(或部分熔融作用),形成的流体(或熔体)交代地幔楔使之发生部分熔融,形成的熔体上涌,经过地壳时,受到古老陆壳物质的混染.乌瓦门安山岩的成因过程,可以为南天山洋及中天山地块的演化提供下列限制:(1)乌瓦门安山岩产出在中天山地块南缘,与大量的中天山陆弧花岗岩和火山岩一起,证实了南天山洋向中天山地块下的俯冲;(2)乌瓦门安山岩的形成时代是430 Ma,与区域上同时代的陆弧花岗岩一起(赵一珏等,2015),标志着在早志留世末期,南天山洋向伊犁-中天山陆块下的俯冲已经构成成熟的洋-陆俯冲体系;中天山陆块南缘为活动大陆边缘,发育典型的安第斯陆弧岩石组合;(3)乌瓦门安山岩为高钾钙碱性岩石,K2O含量高,HREE含量低且发生分异,岩浆可能来源于石榴石稳定区;这些特征暗示南天山洋板片向中天山陆块下俯冲时的角度不会太小,俯冲深度应该较大(大于2 GPa).
5. 结论
(1) 中天山南缘巴仑台以南乌瓦门安山岩为典型的高钾钙碱性安第斯型陆弧岩石,以高钾、高Th-U含量和高LREE含量为特征.
(2) 乌瓦门安山岩起源于被俯冲带相关流体交代的地幔楔;上升过程中,古老地壳物质对其进行了成分改造.
(3) 早志留世末期(430 Ma左右),南天山洋向中天山陆块下俯冲,构成成熟的洋-陆俯冲体系,与当今南美安第斯造山带中段(秘鲁南部和智利北部,16°S和27°S之间)情形一致;此时中天山陆块南缘为活动大陆边缘,发育典型的安第斯陆弧岩石组合.
致谢: 本文野外工作得到了田亚洲、赵一珏、张岚和高健的帮助;锆石测年及数据处理得到了天津地质调查中心耿建珍的帮助;矿物电子探针分析得到了戎合和毛小红的帮助;两位审稿人对论文进行了认真的审阅,提出了建设性的修改意见;在此一并表示感谢! -
图 1 中天山及周边地质简图
CTB.中天山地块;STB.南天山;①中天山北缘边界断裂;②中天山南缘边界断裂.据Ma et al.(2014)修改
Fig. 1. Sketch map of the Central Tianshan and its surroundings
图 6 乌瓦门安山岩的矿物分类图解
a.Ca-Mg-Fe单斜辉石系列的硅灰石(Wo)-顽火辉石(En)-斜铁辉石(Fs)分类图解,据Morimoto(1988);b.钙质角闪石分类图解,据Leake(1997);c.长石分类图解
Fig. 6. Classification diagrams of the minerals from the Wuwamen andesites
图 7 乌瓦门安山岩的岩石分类图解
a.火山岩TAS分类图解,据Le Maitre(2002);b.AFM图解,据Irvine and Baragar(1971);c.K2O-SiO2图解,据Le Maitre(2002);南美安第斯陆弧中带火山岩数据引自Winter(2001)及其中的文献;巴布亚新几内亚TLTF(Tabar-Lihir-Tanga-Feni)高钾洋内岛弧岩石引自Stracke and Hegner(1998)
Fig. 7. Classification diagrams for the Wuwamen andesites
图 8 乌瓦门安山岩的球粒陨石标准化稀土元素配分模式和原始地幔标准化微量元素蛛网图
安第斯陆弧南带火山岩数据引自Winter(2001)及其中的文献;平均俯冲沉积物数据引自Plank and Langmuir(1998);平均陆壳数据引自Rudnick and Gao(2003);球粒陨石稀土元素数据采用Boynton(1984);原始地幔微量元素数据采用Sun and McDonough(1989);其他数据来源同图 7
Fig. 8. Chondrite-normalized REE patterns and primitive mantle-normalized spider diagrams for the Wuwamen andesites
图 9 乌瓦门安山岩的(143Nd/144Nd)i-(87Sr/86Sr)i相关图解
MORB范围采用Gale et al.(2013);安第斯陆弧中带弧下地幔范围引自Lucassen et al.(2006);安第斯陆弧中带古生代地壳范围引自Lucassen et al.(1999);安第斯陆弧中带火山岩范围引自Winter(2001)
Fig. 9. (143Nd/144Nd)i vs. (87Sr/86Sr)i diagram for the Wuwamen andesites
图 11 乌瓦门安山岩的微量元素判别图解
a, b.花岗岩类的Rb-(Y+Nb)和Nb-Y判别图解,据Pearce et al.(1984):VAG.火山弧花岗岩;ORG.洋脊花岗岩;WPG.板内花岗岩;Syn-COLG.同碰撞花岗岩;c, d.中酸性岩石的Th/Yb-Ta/Yb和Th/Ta-Yb判别图解,据Gorton and Schandl(2000):Oceanic arcs.岛弧;ACM.活动大陆边缘;WPVZ.板内火山带;WPB.板内玄武岩;MORB.大洋中脊玄武岩
Fig. 11. Discrimination diagrams for the Wuwamen andesites
表 1 乌瓦门安山岩锆石LA-ICP-MS U-Pb定年分析结果
Table 1. Zircon LA-ICP-MS U-Pb isotopic data for the Wuwamen andesites
分析点号 含量(10-6) Th/U 同位素比值 年龄(Ma) U Th 206Pb/238U 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/235U 1σ 1 583 273 0.5 0.069 0 0.000 5 0.528 6 0.005 5 430 3 431 5 2 1 167 541 0.5 0.069 0 0.000 4 0.529 5 0.004 0 430 3 432 3 3 726 332 0.5 0.068 9 0.000 4 0.524 4 0.004 5 430 3 428 4 4 567 257 0.5 0.069 0 0.000 4 0.528 7 0.004 8 430 3 431 4 5 706 311 0.4 0.069 0 0.000 5 0.530 0 0.004 9 430 3 432 4 6 685 458 0.7 0.069 0 0.000 5 0.529 4 0.004 7 430 3 431 4 7 663 351 0.5 0.069 0 0.000 5 0.530 3 0.004 5 430 3 432 4 8 637 363 0.6 0.069 0 0.000 5 0.526 2 0.004 5 430 3 429 4 9 709 472 0.7 0.069 0 0.000 5 0.523 8 0.004 6 430 3 428 4 10 492 259 0.5 0.069 0 0.000 4 0.529 0 0.005 3 430 3 431 4 11 680 400 0.6 0.069 1 0.000 4 0.528 0 0.004 8 431 3 430 4 12 827 542 0.7 0.069 1 0.000 4 0.529 8 0.004 6 430 3 432 4 13 767 475 0.6 0.069 0 0.000 5 0.527 6 0.004 6 430 3 430 4 14 951 705 0.7 0.069 0 0.000 4 0.526 5 0.004 2 430 3 430 3 15 1 072 739 0.7 0.069 0 0.000 4 0.527 4 0.004 4 430 3 430 4 16 569 315 0.6 0.069 0 0.000 4 0.528 8 0.005 8 430 3 431 5 17 385 253 0.7 0.069 0 0.000 4 0.521 7 0.007 5 430 3 426 6 18 626 368 0.6 0.068 9 0.000 4 0.531 2 0.007 8 430 3 433 6 19 453 28 0.1 0.068 6 0.000 4 0.529 0 0.008 8 427 3 431 7 20 727 411 0.6 0.068 7 0.000 4 0.528 8 0.004 4 429 3 431 4 21 695 378 0.5 0.068 4 0.000 4 0.532 5 0.004 5 427 3 433 4 22 352 175 0.5 0.069 1 0.000 4 0.525 6 0.006 9 430 3 429 6 23 943 712 0.8 0.068 8 0.000 4 0.528 6 0.004 0 429 3 431 3 24 509 285 0.6 0.069 2 0.000 5 0.531 9 0.005 2 431 3 433 4 25 655 337 0.5 0.069 2 0.000 4 0.529 9 0.004 8 431 3 432 4 26 563 304 0.5 0.069 2 0.000 4 0.527 0 0.005 4 432 3 430 4 27 691 328 0.5 0.069 1 0.000 4 0.530 2 0.004 9 431 3 432 4 28 297 111 0.4 0.069 4 0.000 4 0.522 0 0.009 7 433 3 426 8 29 583 249 0.4 0.068 8 0.000 4 0.529 3 0.004 8 429 3 431 4 30 790 365 0.5 0.069 0 0.000 5 0.527 4 0.004 5 430 3 430 4 31 876 381 0.4 0.069 0 0.000 5 0.528 6 0.004 2 430 3 431 3 32 668 309 0.5 0.068 8 0.000 4 0.529 5 0.004 7 429 3 432 4 表 2 乌瓦门安山岩镁铁矿物(角闪石和辉石)的电子探针分析结果(%)
Table 2. Microprobe analyses of amphibole and pyroxene from the Wuwamen andesites
点号 A1 A2 A3 A4 A5 A6 A7 A8 A9 P1 P2 P3 矿物种类 Parg Eden Eden Eden Eden Parg Parg Eden Eden Augi Augi Augi SiO2 43.20 45.58 44.33 44.27 45.65 43.00 42.48 44.48 47.09 54.25 50.52 53.64 TiO2 2.94 2.69 2.81 2.59 2.12 2.98 2.78 1.29 0.92 0.18 1.46 0.11 Al2O3 9.43 9.15 8.92 9.06 7.93 9.79 10.46 9.39 7.68 1.92 3.58 1.68 Cr2O3 0.05 0.06 0.03 0 0 0.02 0.00 0.06 0 0.03 0.07 0.05 FeOT 11.78 13.05 11.90 12.31 11.87 12.17 12.54 12.45 11.41 12.21 11.59 11.60 MnO 0.27 0.28 0.17 0.25 0.21 0.20 0.16 0.20 0.18 0.30 0.23 0.22 MgO 14.60 13.44 14.89 14.58 14.83 14.15 14.07 15.66 16.70 15.79 15.27 16.13 CaO 10.91 10.76 11.07 10.92 11.28 11.07 10.96 10.26 10.53 12.60 12.41 12.70 Na2O 2.16 2.08 2.10 2.16 1.82 2.28 2.28 1.83 1.88 0.26 0.62 0.21 K2O 1.08 0.84 0.93 1.01 0.88 1.11 1.07 0.75 0.66 0.13 0.31 0.47 NiO 0 0 0 0 0.01 0.07 0.01 0.10 0.01 0 0 0.01 总计 96.44 97.93 97.15 97.15 96.60 96.82 96.81 96.47 97.05 97.66 96.07 96.81 基于23个氧原子计算的离子个数 基于6个氧原子计算的离子个数 Si 6.41 6.65 6.51 6.51 6.73 6.38 6.30 6.61 6.79 2.03 1.94 2.03 Ti 0.33 0.30 0.31 0.29 0.24 0.33 0.31 0.15 0.10 0.01 0.04 0 Al 1.65 1.57 1.54 1.57 1.38 1.71 1.83 1.64 1.31 0.09 0.16 0.08 Cr 0.01 0.01 0 0 0 0 0 0.01 0 0 0 0 Fe3+ 0.34 0.27 0.32 0.37 0.29 0.31 0.40 0 0.63 0 0 0 Fe2+ 1.13 1.33 1.14 1.14 1.17 1.20 1.16 1.55 0.75 0.38 0.37 0.37 Mn 0.03 0.04 0.02 0.03 0.03 0.03 0.02 0.03 0.02 0.01 0.01 0.01 Mg 3.23 2.92 3.26 3.20 3.26 3.13 3.11 3.47 3.59 0.88 0.87 0.91 Ca 1.74 1.68 1.74 1.72 1.78 1.76 1.74 1.64 1.63 0.51 0.51 0.52 Na 0.62 0.59 0.60 0.62 0.52 0.66 0.65 0.53 0.53 0.02 0.05 0.02 K 0.20 0.16 0.17 0.19 0.17 0.21 0.20 0.14 0.12 0.01 0.02 0.02 ∑ 15.79 15.59 15.74 15.76 15.65 15.82 15.85 15.74 15.68 3.93 3.97 3.95 注:Parg.韭闪石;Eden.浅闪石;Augi.普通辉石. 表 3 乌瓦门安山岩长石的电子探针分析结果(%)
Table 3. Microprobe analyses of feldspar from the Wuwamen andesites
点号 F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 F13 F14 F15 矿物种类 Ab Ab Ab Ab Ab Ab Ab Or Or Or Or Or Or Or Or SiO2 67.81 68.24 68.30 69.31 67.56 68.32 67.96 66.23 69.76 65.98 65.97 64.43 70.01 65.93 66.47 TiO2 0.01 0.07 0.02 0.03 0.03 0.01 0 0 0.08 0 0 0.03 0 0.04 0.06 Al2O3 17.64 19.65 19.43 19.08 20.16 19.68 19.41 17.36 15.38 17.44 17.56 18.01 15.53 17.57 17.01 Cr2O3 0.0 0 0.02 0.05 0.01 0 0 0 0.01 0.01 0 0 0.01 0 0.01 FeOT 1.23 0.18 0.18 0.14 0.27 0.18 0.14 0.16 0.21 0.10 0.16 0.11 0.23 0.14 0.09 MnO 0.053 0 0 0.007 0 0 0 0.007 0.021 0.012 0 0.028 0.006 0.004 0 MgO 1.29 0.01 0.00 0.01 0.06 0.03 0.01 0 0 0 0 0 0 0 0 CaO 1.61 1.41 1.08 0.61 1.15 1.23 1.03 0.05 0.03 0 0.07 0.01 0.07 0.05 0.06 Na2O 10.06 10.21 10.50 10.86 9.84 10.37 10.38 0.36 0.19 0.16 0.16 0.14 0.30 1.54 0.57 K2O 0.09 0.10 0.21 0.11 0.74 0.16 0.34 15.93 14.04 16.39 16.36 16.61 13.68 14.05 15.61 NiO 0 0 0.01 0 0.01 0 0 0.03 0.02 0 0 0 0 0 0 总计 99.80 99.86 99.75 100.20 99.82 99.98 99.29 100.11 99.73 100.09 100.27 99.38 99.82 99.32 99.88 基于8个氧原子计算的离子个数 Si 2.98 2.98 2.99 3.02 2.96 2.98 2.99 3.05 3.17 3.04 3.04 3.00 3.17 3.04 3.06 Al 0.91 1.01 1.00 0.98 1.04 1.01 1.01 0.94 0.82 0.95 0.95 0.99 0.83 0.95 0.92 Ca 0.08 0.07 0.05 0.03 0.05 0.06 0.05 0 0 0 0 0 0 0 0 Na 0.86 0.87 0.89 0.92 0.84 0.88 0.89 0.03 0.02 0.01 0.01 0.01 0.03 0.14 0.05 K 0.01 0.01 0.01 0.01 0.04 0.01 0.02 0.94 0.81 0.96 0.96 0.99 0.79 0.83 0.92 ∑ 4.97 4.94 4.96 4.95 4.95 4.95 4.96 4.96 4.83 4.97 4.97 5.00 4.82 4.96 4.96 An 8 7 5 3 6 6 5 0 0 0 0 0 0 0 0 Ab 91 92 93 96 90 93 93 3 2 1 1 1 3 14 5 Or 1 1 1 1 4 1 2 96 98 99 98 99 96 85 94 表 4 乌瓦门安山岩的主量(%)和微量元素(10-6)组成
Table 4. Major (%) and trace element (10-6) compositions of the Wuwamen andesites
样品号 12YX11-23 12YX11-24 12YX11-25 12YX11-26 12YX11-27 12YX11-28 12YX11-29 12YX11-30 SiO2 56.4 58.5 58.6 56.8 56.2 57.4 58.9 59.3 TiO2 0.82 0.83 0.85 0.88 0.81 0.83 0.84 0.83 Al2O3 14.9 15.9 15.9 15.5 14.7 15.1 16.2 16.2 Fe2O3T 7.53 5.44 6.27 7.21 7.65 6.82 5.99 5.84 MnO 0.12 0.08 0.10 0.11 0.12 0.11 0.09 0.09 MgO 6.36 3.68 4.50 5.72 6.27 5.20 3.75 3.52 CaO 4.05 5.38 4.11 4.15 4.62 4.96 4.88 5.03 Na2O 3.22 3.99 3.96 3.77 3.24 3.73 3.94 3.94 K2O 3.27 2.99 2.95 2.76 3.00 2.68 2.74 2.64 P2O5 0.28 0.29 0.30 0.29 0.28 0.28 0.30 0.29 LOI 2.51 2.78 2.31 2.87 3.13 2.81 2.28 2.04 Total 99.42 99.9 99.82 100.11 100.05 99.91 99.87 99.68 Mg# 63 58 59 61 62 60 56 55 K2O+Na2O 6.49 6.98 6.91 6.53 6.24 6.41 6.68 6.58 Na2O/K2O 0.98 1.33 1.34 1.37 1.08 1.39 1.44 1.49 Cr 260 102 143 221 257 194 110 112 Mn 921 642 742 877 944 851 690 670 Co 25.0 15.8 21.3 24.6 20.7 23.6 16.5 15.4 Ni 159 64.6 83.8 127.0 153.0 114.0 65.4 67.0 Cu 8.75 7.83 9.18 19.60 9.87 39.00 11.30 7.68 Zn 66.9 44.3 53.0 62.4 66.6 58.9 44.3 42.4 Ga 18.0 18.5 19.1 18.3 17.9 18.2 19.4 19.4 Rb 157 58.4 61.1 63.0 123 57.1 56.3 52.9 Sr 405 619 476 421 393 435 652 706 Ba 1 286 1 074 1 176 1 132 1 142 995 968 919 Pb 4.55 10.1 8.43 5.49 4.03 7.64 9.68 9.30 Th 17.8 19.3 19.2 17.5 16.7 17.7 19.5 19.5 U 2.97 4.30 3.58 3.16 2.99 3.29 4.13 4.09 Nb 7.52 8.41 8.37 7.54 7.52 7.97 8.60 8.60 Ta 0.62 0.76 0.73 0.62 0.59 0.64 0.75 0.75 Zr 179 185 189 189 180 185 186 189 Hf 4.82 4.95 5.00 4.89 4.53 4.88 5.00 5.05 Ti 4 952 4 873 5 172 5 318 4 967 5 115 4 970 5 106 V 109 106 109 112 108 107 108 105 Y 17.3 18.3 18.0 18.1 17.6 17.7 17.8 18.1 La 46.0 48.3 48.0 45.9 43.9 45.9 49.3 48.8 Ce 81.0 85.2 84.4 81.1 78.3 80.4 86.9 85.1 Pr 9.64 10.1 10.0 9.73 9.43 9.55 10.2 9.88 Nd 37.3 40.2 40.0 37.8 37.0 37.9 38.7 38.2 Sm 6.01 6.47 6.41 6.19 5.88 5.94 6.46 6.26 Eu 1.52 1.74 1.70 1.53 1.54 1.53 1.70 1.65 Gd 5.10 5.40 5.42 4.95 5.02 4.89 5.64 5.20 Tb 0.68 0.72 0.70 0.69 0.64 0.67 0.70 0.70 Dy 3.52 3.79 3.68 3.59 3.48 3.46 3.62 3.71 Ho 0.65 0.67 0.66 0.67 0.64 0.63 0.66 0.67 Er 1.84 1.81 1.91 1.89 1.76 1.81 1.83 1.86 Tm 0.24 0.23 0.25 0.25 0.24 0.24 0.24 0.24 Yb 1.61 1.56 1.72 1.67 1.55 1.58 1.66 1.64 Lu 0.23 0.25 0.26 0.25 0.23 0.24 0.25 0.25 ∑REE 195.34 206.44 205.11 196.21 189.61 194.74 207.86 204.16 δEu 0.82 0.88 0.86 0.82 0.85 0.84 0.84 0.86 (La/Yb)N 19.3 20.9 18.8 18.5 19.1 19.6 20.0 20.1 表 5 乌瓦门安山岩的Sr-Nd同位素组成
Table 5. Sr-Nd isotopic data of the Wuwamen andesites
样号 Rb Sr 87Rb/
86Sr87Sr/
86Sr2σ (87Sr/
86Sr)iSm Nd 147Sm/
144Nd143Nd/
144Nd2σ (143Nd/
144Nd)iεNd(t) TDM
(Ma)12YX11-23 155 399 1.149 0 0.713 209 8 0.706 2 5.98 36.8 0.098 2 0.512 512 9 0.512 235 2.96 842 12YX11-25 59.8 470 0.375 5 0.709 770 7 0.707 5 6.51 41.3 0.095 8 0.512 508 7 0.512 238 3.01 830 12YX11-27 125 403 0.916 7 0.712 304 9 0.706 7 6.00 38.4 0.094 6 0.512 503 9 0.512 237 2.98 828 12YX11-29 57.9 659 0.258 4 0.708 685 8 0.707 1 6.50 39.7 0.099 2 0.512 516 8 0.512 237 2.98 844 注:根据锆石U-Pb年龄,采用430 Ma计算初始同位素组成. -
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