Exploration of Issues Related to the LA–ICP–MS U–Pb Dating Technique
-
摘要:
LA–ICP–MS U–Pb定年技术是地质科学中被广泛应用的重要手段。发展至今,该技术已相对成熟,但在实际工作中仍需要注意一些关键问题。笔者就该技术的样品准备、定年结果的取舍、铅丢失问题、普通铅问题和定年结果投图与解释等5个方面进行简要探讨。研究认为,对于复杂矿物进行U–Pb定年研究建议不分选出单矿物,而是采用矿物识别定位手段和LA–ICP–MS仪器相结合的技术手段,直接在岩石光片或探针片上进行原地原位微区定年分析,但要注意样品准备过程中可能存在的铅污染问题。在碎屑矿物定年结果选择方面,对于大于1.5 Ga的定年测点,笔者建议采用207Pb/206Pb年龄代表该颗粒的结晶年龄,而对于小于1.5 Ga的定年测点则应采用206Pb/238U年龄。对沉积岩最大沉积年龄的判断和选择主要依靠统计学方法,必要时需要结合地球化学数据和地质背景信息作为辅助判断依据。对于连续分布在谐和线上的年轻样品要提高警惕,需要采用谐和图、加权平均图、CL图像和元素含量等多种手段识别是否存在铅丢失不一致线。针对普通铅校正问题,笔者重点介绍了一种专用于碎屑矿物U–Pb定年的普通铅校正方法,并给出了计算过程。关于对矿物U–Pb定年结果加权平均值数据质量的评价,笔者着重讨论MSWD越接近于1表示数据质量越高的理论基础。总之,应用LA–ICP–MS 技术对矿物进行U–Pb定年研究需要综合考虑多个因素,才能得出准确、可靠和地质意义明确的定年结果。
-
关键词:
- LA–ICP–MS U–Pb定年 /
- 最大沉积年龄 /
- 铅丢失 /
- 普通铅 /
- MSWD
Abstract:The LA–ICP–MS U–Pb dating technique is a crucial tool that has been widely employed in geological sciences. Despite its relative maturity, several critical issues still require attention in practical applications. This article provides a brief discussion of five critical aspects of this technique, including sample preparation, selection of dating results, lead loss, common lead, and presentation and interpretation of dating results. Firstly, for the U–Pb dating of complex minerals, it is recommended to employ mineral identification and location techniques in combination with LA–ICP–MS instruments for in–situ analysis directly on rock thin sections or polished sections, without mineral separation. However, attention must be paid to Pb contamination in sample preparation. Secondly, for the selection of dating results for detrital minerals, the 206Pb/207Pb age is utilized to represent the crystallization age of grains older than 1.5 Ga, while the 206Pb/238U age is employed for grains younger than 1.5 Ga. The determination and selection of the maximum depositional age of sedimentary rocks mainly rely on statistical methods and sometimes require the combination of geochemical data and geological background information as supplementary criteria. Thirdly, for young samples with continuous age distribution on the concordia line, various methods such as the concordia diagram, weighted mean diagram, CL images, and element contents should be used to identify whether there is an inconsistent lead loss line or not. Furthermore, this article focuses on a rarely used common lead correction method for detrital mineral U–Pb dating. Finally, this article emphasizes the theoretical basis that, when evaluating the quality of the weighted mean value, the closer the MSWD is to 1, the higher the data quality is. In summary, to conduct U–Pb dating studies on minerals using LA–ICP–MS technology, it is essential to consider multiple factors comprehensively to obtain accurate, reliable, and geologically meaningful dating results.
-
Keywords:
- LA–ICP–MS U–Pb dating /
- maximum depositional age /
- lead loss /
- common lead /
- MSWD
-
南天山造山带是天山造山带以及中亚造山带的重要组成部分,蕴含着天山洋盆演化的重要信息,一直是区域研究的热点(Huang et al., 2000;肖文交等,2006;龙灵利等,2006;Xiao et al., 2008;左国朝等,2011;郭瑞清等2013;张斌等,2014;魏强等,2017;郭春涛等,2018)。众多学者和地质工作者在南天山北缘和中天山北缘相继识别出了一条西起查干萨依东至库米什孔雀沟长达数百千米的早古生代辉长岩-闪长岩-花岗岩为岩性组合的钙碱性花岗岩带(Zhang et al., 2007; Yang et al., 2009; Yang et al., 2011; Chen et al., 2015),为研究早古生代南天山洋的构造演化提供了便利。但关于南天山洋盆的闭合时限一直存在较大争议,其中引用较多的几种认识有:Han等(2004)基于中天山南缘发现的铝质A型花岗岩认为南天山洋在490~380 Ma已经进入了造山晚期;Zhong等(2015)基于中天山南缘发现的钾长花岗岩认为417 Ma时南天山洋已经闭合;田亚洲等(2014)基于中天山南缘的辉长岩认为南天山洋在316 Ma时仍在俯冲;郭瑞清(2018)等基于对塔里木北缘库鲁克塔格辉长岩脉认为南天山洋在419 Ma时仍处于向南的俯冲–消减作用。这些及其他基于不同类型的花岗岩(杨莉等,2016;于新慧等,2020;刘桂萍等,2021;刘传朋等,2022)以及相邻构造带中辉长岩得出来南天山洋盆不同闭合时限观点显然具有较大的分歧,既矛盾也缺乏全南天山造山带岩浆活动的支持,因此,加强南天山早古生代辉长岩等基性岩成因、构造环境的研究将为解决这些问题提供重要的地球化学证据和约束条件。
1. 区域地质背景
红柳沟辉长岩体位于新疆和静县正西方向约65 km处,在区域上位于中天山南缘-南天山北缘早古生代钙碱性花岗岩带的东部,处于南天山造山带哈尔克-萨阿尔明晚古生代沟弧带之中,北侧为中天山南缘断裂带,南侧为南天山前断裂带(图1b)。
研究区内通过1∶2 000地质测量发现3条断层(图1c),主要为区域大断裂(大山口断裂F1)与NNE向断裂(F2、F3)。区内地层主要出露在中部,呈EWW向带状分布。主要为上志留统—下泥盆统大山口组(S3-D1d),主要为一套浅变质碎屑岩,岩性为灰-浅灰黑色砂质板岩。区内出露的岩浆岩主要有辉长岩、闪长岩、花岗闪长岩以及少量的晚志留世—早泥盆世凝灰岩,并且受后期构造的变形影响,多呈条带状平行分布,整体与区域大断裂F1方向一致呈NWW向。前人对红柳沟地区的岩浆岩研究较少,集中在大山口金矿的成矿流体及矿床成因方面(鲍庆中等,2006;董丰新等,2011),基础地质研究极为薄弱。
2. 岩体地质和岩相学特征
辉长岩主要在研究区北部和东部大面积出露(图1c),受后期构造变形影响,多呈条带状平行分布,出露长度为100~1 000 m、宽度为50~200 m,整体与韧性剪切带方向一致呈NWW向,与大山口组砂质板岩接触界线明显,呈侵入接触关系,接触界限呈锯齿状(图2a)。
红柳沟辉长岩新鲜面呈灰绿-灰黑色,半自形细粒结构,块状构造(图2b、图2c)。岩石主要由斜长石(±48%)、普通辉石(±42%)、黑云母(±5%)、角闪石(±5%)及副矿物组成。显微镜下观察,斜长石呈半自形板状,聚片双晶发育,粒径一般为0.8~1.2 mm,少数粒径为2.0 mm,整体呈杂乱分布,部分斜长石发生绢云母化(图2d);普通辉石呈自形-半自形短柱状,粒径一般为0.5~1.2 mm。黑云母呈片状,粒径一般为1.0 mm,杂乱分布,局部发生绿泥石化蚀变;角闪石呈半自形短柱状,粒度多在0.3~2.5 mm不等。副矿物(±1%)为锆石、磁铁矿、榍石等,矿物粒径大部分为0.1~1.2 mm,少量矿物粒径可达2 mm(图2d)。
3. 分析方法
笔者共选取9件辉长岩样品进行测试研究,其中8件样品进行主量、微量以及稀土元素分析,1件样品(H62-RZ1)进行锆石U-Pb测年。
主量、微量、稀土元素分析测试交由湖北省地质实验测试中心完成。测试样品在经历去污、碎样、去离子水洗涤和2~3次烘干等步骤后进行研磨(≤ 200目)。然后在恒定温度(21 ℃)与湿度(58%)的环境下进行测试分析。主量元素采用型号RXF-1800,编号为27-HY-2009-001 X荧光光谱仪进行分析测试,分析精度优于2%。微量与稀土元素采用型号为电感耦合等离子体质谱仪-X2进行分析,分析精度优于2%。
本次锆石U-Pb定年在武汉上谱分析科技有限责任公司进行测定,工作分为两部分,前期完成锆石单矿物的挑选与制靶、LA-ICP-MS锆石U-Pb同位素年龄测定,后期在室内进行数据处理及相关图件的绘制。前期具体操作为:将样品中挑选出的锆石粘于环氧树脂内并抛光,对抛光之后的靶样采用透射光与反射光进行拍照并收集阴极发光(CL)图像,开展综合判定,选择无裂隙、无包裹体、环带清晰、大小合适的锆石圈定激光剥蚀区域。测年仪器采用安捷伦电感耦合等离子体质谱仪(Agilent 7900)以及相干193 nm准分子激光剥蚀系统(GeoLas HD),剥蚀采用直径为32 um的束斑,频率选用5 Hz,激光能量为80 mJ,后期的数据处理工作采用ICPMS Data Cal10.8软件程序进行,相关图件(谐和图与加权年龄图)的绘制和谐和年龄与加权年龄的计算采用Isoplot3.0程序。
4. 分析结果
4.1 主量元素特征
本次实验所采集的8件辉长岩样品主量元素数据见表1。红柳沟辉长岩SiO2含量为44.24%~46.29%,平均值为45.39%,Al2O3含量为12.26%~13.03%,平均值为12.68%。CaO含量为5.21%~7.56%,平均值为6.87%;Na2O含量为2.49%~3.08%,平均值为2.83%;K2O含量为1.6%~2.4%,平均值为2.02%;里特曼指数σ=1.78~2.09,均小于3.3;分异指数DI介于50.3~57.06,平均值为53.071。在TAS图解中(图3a),8件辉长岩样品都落入辉长岩区域和碱性范围内;根据Zr-Y图解(图3b),红柳沟8件样品均落入拉斑-钙碱性系列岩石范围内。
表 1 红柳沟辉长岩主量元素(%)分析结果表Table 1. Major elements (%) analytic data table of the Hongliugou gabbro样品号 H62-YQ1 H62-YQ2 H62-YQ3 H62-YQ4 H62-YQ5 H62-YQ6 H62-YQ7 H62-YQ8 SiO2 45.76 46.29 44.24 45.31 45.96 45.86 44.52 45.24 TiO2 0.833 0.940 0.848 0.859 0.860 0.964 0.772 0.813 Al2O3 12.26 12.67 12.78 12.93 12.90 13.03 12.26 12.58 Fe2O3 8.24 10.11 9.64 8.21 8.62 8.66 7.72 8.38 FeO 5.27 6.78 5.88 6.09 6.21 6.00 5.73 6.23 MnO 0.151 0.122 0.144 0.143 0.132 0.134 0.158 0.142 MgO 10.71 11.62 11.39 11.10 11.06 11.33 10.35 11.33 CaO 7.56 5.21 6.64 6.94 6.44 6.18 8.84 7.17 Na2O 3.08 2.61 2.95 3.03 2.85 2.89 2.75 2.49 K2O 1.60 2.14 2.09 2.10 2.40 1.79 1.93 2.13 P2O5 0.346 0.362 0.333 0.350 0.352 0.359 0.319 0.339 LOI 3.98 0.99 2.90 2.79 2.06 2.62 4.46 2.97 总量 99.79 99.85 99.84 99.86 99.83 99.81 99.82 99.81 Mg# 45.77 42.27 43.91 45.17 44.19 45.11 44.95 45.14 4.2 微量、稀土元素特征
红柳沟辉长岩的8件样品微量、稀土元素分析结果见表2。在稀土元素球粒陨石标准化模式图上(图4a),曲线整体呈平行不对称的右倾型。根据表2,辉长岩的稀土元素总量∑REE=213.80×10−6~237.27×10−6,平均值为224.93×10−6;LREE=195.28×10−6~217.51×10−6,HREE=18.52×10−6~20.82×10−6;LREE/HREE=10.2~11.0,平均值为10.64。(La/Yb)N为12.45~13.86,平均值为13.11,(Gd/Yb)N为2.1~2.3,平均值为2.21,说明轻稀土富集且分馏程度高;δEu=0.8~1.09,平均为0.92,无明显的Eu异常,说明辉长岩轻稀土元素呈富集型。
表 2 红柳沟辉长岩微量和稀土元素(10−6)分析结果表Table 2. Trace and REE element (10−6) analytic data table of the Hongliugou gabbro样品号 H62-YQ1 H62-YQ2 H62-YQ3 H62-YQ4 H62-YQ5 H62-YQ6 H62-YQ7 H62-YQ8 Rb 108 144 134 132 160 109 120 130 Ba 1670 649 746 548 1279 1114 1103 1121 Th 7.76 8.43 8.30 7.56 8.56 8.38 8.00 7.54 U 2.49 2.35 2.11 2.27 2.22 2.09 2.18 2.00 Ta 0.54 0.76 0.64 0.87 0.49 0.78 0.97 0.46 Nb 9.63 10.0 9.29 9.57 10.1 10.3 9.12 9.17 Sr 303 195 239 230 239 236 305 257 Zr 145 178 179 176 152 180 170 156 Hf 4.24 4.72 4.94 5.11 4.14 4.86 4.89 4.41 Y 22.3 24.9 22.8 23.1 22.7 26.5 22.0 21.3 La 46.4 49.0 45.9 46.1 48.2 48.4 45.3 45.1 Ce 89.4 94.7 86.9 87.9 94.6 91.7 84.9 85.2 Nd 44.9 49.6 46.2 46.1 47.8 47.9 43.2 44.6 Sm 8.43 9.27 8.76 8.95 9.01 9.16 8.06 8.31 Pr 11.6 12.7 11.9 12.0 12.5 12.5 11.5 11.5 Eu 2.74 2.14 2.12 2.12 2.48 2.72 2.40 2.38 Gd 6.44 6.69 6.56 6.66 6.87 6.91 6.12 6.48 Tb 1.02 1.11 1.02 1.06 1.06 1.15 0.99 1.00 Dy 4.62 5.28 4.93 5.10 4.93 5.57 4.92 4.72 Ho 0.85 0.94 0.90 0.90 0.95 1.06 0.85 0.88 Er 2.74 2.53 2.49 2.63 2.84 2.81 2.53 2.46 Tm 0.34 0.36 0.37 0.35 0.35 0.39 0.35 0.34 Yb 2.26 2.49 2.37 2.50 2.46 2.54 2.35 2.30 Lu 0.37 0.35 0.38 0.36 0.39 0.39 0.40 0.35 LREE 203.38 217.51 201.74 203.10 214.48 212.30 195.28 196.99 HREE 18.64 19.77 19.02 19.55 19.85 20.82 18.52 18.54 ∑REE 222.02 237.27 220.76 222.65 234.33 233.11 213.80 215.53 δEu 1.09 0.79 0.82 0.8 0.93 1.0 1.0 0.96 LREE/HREE 10.91 11.00 10.61 10.39 10.81 10.20 10.55 10.63 (La/Yb)N 13.86 13.25 13.07 12.45 13.19 12.87 12.97 13.18 (Gd/Yb)N 2.30 2.17 2.24 2.15 2.25 2.20 2.10 2.27 在微量元素原始地幔标准化蛛网图上(图4b),Rb、Ba、Th、U等大离子亲石元素相对富集,而Ta、Nb、Ti等高场强元素相对亏损,表明该辉长岩可能受到地壳混染或发生变质作用。
4.3 锆石U-Pb定年
笔者对新疆红柳沟辉长岩样品H62-RZ1进行LA-ICP-MS锆石U-Pb测年,分析结果见表3,Th、U含量值分别为3×10−6~653×10−6、94×10−6~613×10−6,Th/U值范围为0.03~1.06。在阴极发光(CL)图上,锆石颗粒粒径较小,一般为80~120 μm,长宽比为1.2∶1~1.8∶1,晶形主要呈自形-半自形短柱状,具有较清晰的地震荡环带,为岩浆锆石(图5)。本次选取25颗清晰的锆石进行测年分析,结果表明,锆石206Pb/238U谐和年龄为401~419 Ma,加权平均年龄为(410.4±2.3) Ma(MSWD=0.51),代表该辉长岩的结晶年龄(图6),形成于早泥盆世。
表 3 红柳沟辉长岩LA-MC-ICP-MS锆石U-Pb定年结果Table 3. Ziron U-Pb dating data for the Hongliugou gabbro测点号 Th(10−6) U(10−6) Th/U 同位素比值 年龄(Ma) 207Pb/206Pb 1δ 207Pb/235U 1δ 206Pb/238U 1δ 207Pb/206Pb 1δ 207Pb/235U 1δ 206Pb/238U 1δ H62-RZ1-03 120 204 0.59 0.0880 0.0047 0.7657 0.0340 0.0641 0.0014 1383 103.7 577 19.5 401 8.8 H62-RZ1-04 9 94 0.10 0.0774 0.0048 0.7048 0.0477 0.0660 0.0017 1132 122.7 542 28.4 412 10.2 H62-RZ1-05 9 251 0.04 0.0735 0.0024 0.6662 0.0295 0.0655 0.0013 1028 62.8 518 18.0 409 7.9 H62-RZ1-06 234 294 0.79 0.0727 0.0029 0.6518 0.0264 0.0651 0.0006 1006 81.5 510 16.2 406 3.9 H62-RZ1-07 297 364 0.82 0.0718 0.0041 0.6459 0.0384 0.0652 0.0007 989 115.3 506 23.7 407 4.5 H62-RZ1-09 37 146 0.26 0.0697 0.0031 0.6280 0.0302 0.0653 0.0008 920 90.7 495 18.8 408 4.6 H62-RZ1-10 652 613 1.06 0.0613 0.0020 0.5492 0.0295 0.0652 0.0033 650 72.2 444 19.3 407 19.9 H62-RZ1-11 282 348 0.81 0.0607 0.0019 0.5462 0.0172 0.0653 0.0006 628 66.7 442 11.3 408 3.9 H62-RZ1-12 197 242 0.81 0.0607 0.0020 0.5563 0.0214 0.0664 0.0008 628 72.2 449 14.0 414 5.1 H62-RZ1-13 382 519 0.74 0.0612 0.0023 0.5587 0.0169 0.0666 0.0015 656 81.5 451 11.0 416 9.2 H62-RZ1-14 188 291 0.65 0.0596 0.0022 0.5369 0.0211 0.0652 0.0007 587 113.9 436 13.9 407 4.2 H62-RZ1-15 434 418 1.04 0.0593 0.0022 0.5327 0.0211 0.0652 0.0007 576 78.7 434 14.0 407 4.3 H62-RZ1-16 446 500 0.89 0.0596 0.0023 0.5504 0.0200 0.0672 0.0011 587 115.7 445 13.1 419 6.7 H62-RZ1-17 191 252 0.76 0.0594 0.0015 0.5464 0.0127 0.0669 0.0010 583 55.5 443 8.3 417 5.8 H62-RZ1-18 10 95 0.10 0.0529 0.0040 0.4717 0.0341 0.0656 0.0014 324 170.3 392 23.6 409 8.2 H62-RZ1-19 653 764 0.86 0.0593 0.0044 0.5333 0.0284 0.0662 0.0011 576 162.9 434 18.8 413 6.7 H62-RZ1-20 195 312 0.63 0.0529 0.0018 0.4840 0.0195 0.0664 0.0017 324 71.3 401 13.4 414 10.0 H62-RZ1-24 91 184 0.50 0.0570 0.0024 0.5131 0.0234 0.0657 0.0019 500 99.1 421 15.7 410 11.5 H62-RZ1-25 208 334 0.62 0.0569 0.0014 0.5206 0.0145 0.0665 0.0012 487 55.6 426 9.7 415 7.3 H62-RZ1-26 29 393 0.07 0.0565 0.0014 0.5217 0.0149 0.0669 0.0009 472 53.7 426 9.9 417 5.4 H62-RZ1-27 3 105 0.03 0.0536 0.0044 0.4858 0.0332 0.0655 0.0022 367 185.2 402 22.7 409 13.2 H62-RZ1-28 254 339 0.75 0.0547 0.0017 0.4978 0.0190 0.0658 0.0009 398 66.7 410 12.9 411 5.4 H62-RZ1-29 71 139 0.51 0.0555 0.0027 0.5008 0.0225 0.0659 0.0012 432 109.2 412 15.2 411 7.0 H62-RZ1-30 241 321 0.75 0.0558 0.0017 0.5071 0.0149 0.0661 0.0009 443 66.7 416 10.0 413 5.3 H62-RZ1-31 318 438 0.72 0.0548 0.0016 0.5008 0.0142 0.0665 0.0010 467 69.4 412 9.6 415 5.9 4.4 Lu-Hf同位素
笔者在LA-ICP-MS锆石U-Pb测年基础上,选取25颗锆石在测年点近似相同的位置上进行Hf同位素分析(图5),数据结果显示(表4):红柳沟辉长岩锆石176Hf/177Hf为
0.282491 ~0.282788 ,平均值为0.282627 ,可以代表锆石颗粒原始176Hf/177Hf值。εHf(t)变化范围为−1.27~8.75,平均值为3.30,对应的二阶段模式年龄(TDM2),分布范围介于0.84~1.50 Ga。表 4 红柳沟辉长岩锆石Lu-Hf同位素分析结果Table 4. Zircon Hf isotopic data for the Hongliugou gabbro测点号 176Yb/177Hf 2σ 176Lu/177Hf 2σ 176Hf/177Hf 2σ 年龄
(Ma)εHf (0) εHf (t) TDM1
(Ma)TDM2
(Ma)fLu/Hf H62-RZ1-03 0.116058 0.001210 0.003363 0.000048 0.282657 0.000024 401 −4.07 3.86 898.6 1146.0 −0.899 H62-RZ1-05 0.046197 0.000359 0.001391 0.000012 0.282542 0.000033 409 −8.15 0.48 1015.0 1367.1 −0.958 H62-RZ1-10 0.040480 0.000183 0.001150 0.000006 0.282705 0.000046 407 −2.38 6.27 778.1 997.6 −0.965 H62-RZ1-11 0.033934 0.000073 0.001120 0.000002 0.282491 0.000024 408 −9.94 −1.27 1079.3 1477.1 −0.966 H62-RZ1-12 0.110041 0.000667 0.003024 0.000017 0.282578 0.000069 414 −6.88 1.42 1008.1 1311.7 −0.909 H62-RZ1-13 0.114382 0.001270 0.003538 0.000037 0.282745 0.000039 416 −0.94 7.24 769.4 942.5 −0.893 H62-RZ1-14 0.053794 0.000894 0.001635 0.000028 0.282534 0.000105 407 −8.43 0.08 1033.2 1390.6 −0.951 H62-RZ1-15 0.097250 0.001810 0.002924 0.000046 0.282529 0.000074 407 −8.61 −0.44 1077.7 1423.4 −0.912 H62-RZ1-16 0.060179 0.000714 0.001907 0.000021 0.282689 0.000028 419 −2.94 5.76 816.7 1039.0 −0.943 H62-RZ1-17 0.106043 0.001360 0.003139 0.000037 0.282690 0.000055 417 −2.91 5.42 843.8 1059.7 −0.905 H62-RZ1-18 0.154616 0.002070 0.004256 0.000057 0.282649 0.000049 409 −4.36 3.50 934.5 1175.4 −0.872 H62-RZ1-19 0.050009 0.000531 0.001423 0.000020 0.282631 0.000024 413 −4.97 3.73 888.2 1163.7 −0.957 H62-RZ1-20 0.049060 0.000338 0.001494 0.000006 0.282728 0.000044 414 −1.55 7.17 751.7 946.2 −0.955 H62-RZ1-25 0.088510 0.000480 0.002601 0.000024 0.282631 0.000018 415 −5.00 3.42 918.3 1184.8 −0.922 H62-RZ1-26 0.056476 0.000804 0.001639 0.000024 0.282557 0.000022 417 −7.59 1.14 999.2 1331.2 −0.951 H62-RZ1-27 0.092919 0.000309 0.002561 0.000009 0.282575 0.000056 409 −6.97 1.34 999.1 1312.4 −0.923 H62-RZ1-28 0.017302 0.000215 0.000572 0.000007 0.282492 0.000027 411 −9.89 −1.00 1061.7 1462.5 −0.983 H62-RZ1-29 0.114128 0.000995 0.003262 0.000024 0.282788 0.000066 411 0.57 8.75 699.0 843.1 −0.902 H62-RZ1-30 0.076111 0.000460 0.002223 0.000012 0.282695 0.000088 413 −2.73 5.76 815.2 1034.7 −0.933 5. 讨论
5.1 岩石成因与源区性质
研究显示,可以利用基性岩的(Na2O+K2O)/TiO2值来判断岩浆源区,并对地幔源区、角闪石岩和碳酸盐化榴辉岩地幔源区的界线进行清楚的划分。图7a显示,在(Na2O+K2O)/TiO2>1的范围内,随着TiO2含量的增高,碳酸盐化橄榄岩地幔源区全碱含量表现出大幅度上升趋势;而在(Na2O+K2O)/TiO2<1的范围内,随着TiO2含量的增高,角闪石岩和碳酸盐化榴辉岩地幔源区全碱含量表现出缓慢上升。红柳沟辉长岩在图中投点落在(Na2O+K2O)/TiO2>1的范围内,显示有碳酸盐化橄榄岩部分熔融趋势的特征。
通常来说,基性岩浆通常来自岩石圈地幔或者软流圈地幔岩石的部分熔融,利用岩石中不易受后期蚀变和低于角闪石相变质作用的高场强元素和重稀土元素可以判断岩浆源区的性质,源于软流圈地幔的玄武岩La/Nb <1.5,La/Ta <22,而源于岩石圈地幔的玄武岩与之相反(Huang et al., 2000)。文中辉长岩La/Nb=4.7~4.97,平均为4.86,均大于1.5,La/Ta=46.78~99.12,平均为72.58,均大于22,指示红柳沟辉长岩岩浆源区为岩石圈地幔。笔者通过对红柳沟辉长岩19颗锆石Lu-Hf同位素分析,176Hf/177Hf 初始值介于
0.282491 ~0.282788 ,εHf(t) 的变化范围为−.27~8.75,平均值为3.30(>0),εHf(t)⁃年龄图解(图7b)中红柳沟辉长岩全部落入地壳和亏损地幔演化线之间范围,因此表明红柳沟辉长岩岩浆源区可能来自受到地壳混染的亏损地幔。前人研究提出(Th/Nb)PM值和Nb/La值是判断岩石形成过程中是否受到地壳物质混染的两个重要指标,若(Th/Nb)PM远大于1,Nb/La <1,则说明基性岩形成过程中有地壳物质加入。红柳沟辉长岩(Th/Nb)PM的范围为6.62~7.49,Nb/La的范围为0.20~0.21;在(Th/Nb)PM-Nb/La的图解中(图7c),红柳沟辉长岩落在遭受地壳混染的区域内,显示岩浆在上升的过程中受到地壳的污染;在(Th/Nb)PM-(Th/Ta)PM图解(图7d)中显示红柳沟辉长岩均落入上地壳区域内,表明红柳沟辉长岩在形成过程中原始岩浆受到上地壳的污染。
综上所述,红柳沟辉长岩岩浆可能来自岩石圈地幔的部分熔融,源区为碳酸盐化地幔橄榄岩混合物,并在上升过程中与地壳物质发生同化混染。
5.2 构造地质背景
一般来说,玄武质岩浆用来探讨构造环境是相对可靠的,因为玄武质岩浆具有独特的地球化学特征和特定的构造环境,文中红柳沟辉长岩总体显示轻稀土元素较重稀土元素相对富集以及Ta、Nb、Ti等元素明显的负异常,说明红柳沟辉长岩可能与弧环境相关的岩浆作用的产物密切相关。通常利用Zr与Y元素可以有效判别大陆弧与洋内弧玄武质岩浆,红柳沟辉长岩的Zr/Y值介于6.5~7.86,为典型的大陆弧(Zr/Y>3);Nb/Th- Nb图解(图8a)与Hf/3-Th-Ta图解(图8c)均显示红柳沟辉长岩样品均为岛弧质玄武岩;TiO2-Zr图解(图8b)显示红柳沟辉长岩为火山弧玄武岩。综上所述,红柳沟辉长岩形成于与大陆岛弧相关的俯冲背景。
6. 结论
(1)红柳沟辉长岩体为钙碱性辉长岩,锆石U-Pb获得的年龄为(410.4±2.3)Ma,补充了南天山构造带早古生代基性岩浆活动的地球化学和年代学证据。
(2)红柳沟辉长岩为富集Rb、Ba、Th、U等大离子亲石元素,亏损Ta、Nb、Ti等高场强元素的活动陆缘弧基性侵入岩,具负的和正的εHf (t),为亏损地幔的部分熔融在上升过程中受到地壳物质的混染。
(3)研究结果表明,早泥盆世南天山洋仍处于洋陆俯冲作用之下,南天山洋的闭合时间在早泥盆世之后。
-
图 1 锆石U–Pb定年测试数据统计图(37358个)(Voice et al.,2011;Spencer et al., 2016)
A. 207Pb/206Pb表面年龄与不确定度相关性投图;B. 206Pb/238U表面年龄与不确定度相关性投图;C. 207Pb/206Pb表面年龄–不确定度相关曲线与206Pb/238U表面年龄–不确定度相关曲线相交于1.5 Ga
Figure 1. Statistical results of 37358 zircon U–Pb dating data
-
Antonio S, Larry M H, Thomas C, et al. In situ petrographic thin section U–Pb dating of zircon, monazite, and titanite using laser ablation–MC–ICP-MS[J]. International Journal of Mass Spectrometry, 2006, 253, 87–97. doi: 10.1016/j.ijms.2006.03.003
Becquerel H. Sur les radiations émises par phosphorescence[J]. Comptes Rendus, 1896, 122, 420-421.
Bowes D R. The geology and geochemistry of lead ore deposits[J]. Earth-Science Reviews, 1977, 13(4), 315-384.
Cawood P A, Nemchin A A, Strachan R A. Provenance record of Laurentian passive-margin strata in the northern Caledonides: Implications for paleodrainage and paleogeography[J]. Geological Society of America Bulletin, 2007, 119, 993-1003. doi: 10.1130/B26152.1
Cawood P A, Hawkesworth C J, Dhuime B. Detrital zircon record and tectonic setting [J]. Geology, 2012, 40(10), 875−878.
Condon D J, Bowring S A. A user’s guide to Neoproterozoic geochronology[A]. In Arnaud E, Halverson G P, Shields-Zhou G (Eds.). The Geological Record of Neoproterozoic Glaciations [R]. Geological Society of London, 2011: 135-146.
Chiarenzelli J, Kratzmann D, Selleck B, et al. Age and provenance of Grenville supergroup rocks, Trans-Adirondack Basin, constrained by detrital zircons[J]. Geology, 2014, 43, 183-186.
David J, Chew D, Petrus J. Apatite U-Pb dating with LA-ICP MS[J]. Chemical Geology, 2011, 280(1-2), 1-20. doi: 10.1016/j.chemgeo.2010.07.008
Dickinson W R, Gehrels G E. Use of U–Pb ages of detrital zircons to infer maximum depositional ages of strata: A test against a Colorado Plateau Mesozoic database[J]. Earth and Planetary Science Letters, 2009, 288(1-2), 115–125. doi: 10.1016/j.jpgl.2009.09.013
Fleet M E. The geochemistry of lead[A]. In Holland H D, Turekian K K (Eds. ). Treatise on geochemistry[M]. Oxford: Elsevier, 2003, 9: 1–51.
Fripiat J J. Lead isotopes in minerals[A]. In Henderson P (Ed. ). Rare earth element geochemistry[M]. Amsterdam: Elsevier, 1984: 571–584.
Hiess J, Condon D J, McLean N, et al. 238U/235U systematics in terrestrial uranium-bearing minerals[J]. Science, 2012, 335, 1610–1614. doi: 10.1126/science.1215507
Herriott T M, Crowley J L, Schmitz M D, et al. Exploring the law of detrital zircon: LA-ICP-MS and CA-TIMS geochronology of Jurassic forearc strata, Cook Inlet, Alaska, USA[J]. Geology, 2019, 47(11), 1044-1048. doi: 10.1130/G46312.1
Horstwood M S A, Košler J, Gehrels G, et al. Community-Derived Standards for LA-ICP-MS U-(Th-)Pb Geochronology – Uncertainty Propagation, Age Interpretation and Data Reporting[J]. Geostandards and Geoanalytical Research, 2016, 40(3), 311−332.
Hisatoshi I, Shimpei U, Futoshi N, et al. Zircon U–Pb dating using LA-ICP-MS: Quaternary tephras in Yakushima Island, Japan[J]. Journal of Volcanology and Geothermal Research, 2017, 338, 92-100. doi: 10.1016/j.jvolgeores.2017.02.003
Jaffey A H, Flynn K F, Glendenin L E, et al. Precision measurement of half-lives and specific activities of 235U and 238U[J]. Physical Review C, 1971, 4(5), 1889-1906. doi: 10.1103/PhysRevC.4.1889
Li Y G, Song S G, Yang X Y, et al. Age and composition of Neoproterozoic diabase dykes in North Altyn Tagh, northwest China: implications for Rodinia break-up[J]. International Geology Review, 2023a, 65(7): 1000-1016. doi: 10.1080/00206814.2020.1857851
Li Y, Yuan F, Simon M J, Li X, et al. (2023). Combined garnet, scheelite and apatite U–Pb dating of mineralizing events in the Qiaomaishan Cu–W skarn deposit, eastern China[J]. Geoscience Frontiers, 2023b, 14(1): 17-32.
Lin M, Zhang G, Li N, et al. (2021). An Improved In Situ Zircon U‐Pb Dating Method at High Spatial Resolution (≤ 10 μm spot) by LA‐MC‐ICP‐MS and its Application[J]. Geostandards and Geoanalytical Research, 2021, 45(2): 265-285. doi: 10.1111/ggr.12374
Lin J, Liu Y, Yang Y, et al. Calibration and correction of LA-ICP-MS and LA-MC-ICP-MS analyses for element contents and isotopic ratios[J]. Solid Earth Sciences, 2016. 1(1), 5-27. doi: 10.1016/j.sesci.2016.04.002
Liu E, Zhao J X, Wang H, et al. LA-ICPMS in-situ U-Pb Geochronology of Low-Uranium Carbonate Minerals and Its Application to Reservoir Diagenetic Evolution Studies[J]. Journal of Earth Science, 2021, 32, 872–879. doi: 10.1007/s12583-020-1084-5
Liu Y S, Hu Z C, Li M, et al. Applications of LA-ICP-MS in the elemental analyses of geological samples. Chinese Science Bulletin, 2013, 58, 3863-3878.
Merriman R J. Lead[A]. In Linnen R L, Samson I M (Eds. ). Rare element geochemistry and mineral deposits[R]. Geological Association of Canada Short Course Notes, 2007, 17: 201-230.
Patterson C. Age of meteorites and the Earth[J]. Geochimica et Cosmochimica Acta, 1956a, 10, 230-237. doi: 10.1016/0016-7037(56)90036-9
Patterson C. Isotopic ages of the Earth and Moon[J]. Proceedings of the National Academy of Sciences, 1956b, 42(4), 194-199. doi: 10.1073/pnas.42.4.194
Rutherford E. A Radioactive Substance emitted from Thorium Compounds[J]. Philosophical Magazine, 1900, 49(293), 1-14.
Richard A C, Derek H C W. U–Pb dating of perovskite by LA-ICP-MS: An example from the Oka carbonatite, Quebec, Canada[J]. Chemical Geology, 2006, 235, 21–3. doi: 10.1016/j.chemgeo.2006.06.002
Schaltegger U, Schmitt A K, Horstwood M S A. U-Th-Pb zircon geochronology by ID-TIMS, SIMS and laser ablation ICP-MS: Recipes, interpretations and opportunities[J]. Chemical Geology, 2015, 402, 89-110. doi: 10.1016/j.chemgeo.2015.02.028
Spencer C J, Kirkland C L, Taylor R J M. Strategies towards statistically robust interpretations of in situ U–Pb zircon geochronology[J]. Geoscience Frontiers, 2016, 7(4): 581-589. doi: 10.1016/j.gsf.2015.11.006
Spencer C J, Prave A R, Cawood P A, et al. Detrital zircon geochronology of the Grenville/Llano foreland and basal Sauk Sequence in west Texas, USA[J]. Geological Society of America Bulletin, 2014, 126(7-8), 1117-1128. doi: 10.1130/B30884.1
Stacey J S, Kramers J D. Approximation of terrestrial lead isotope evolution by a two-stage model[J]. Earth and Planetary Science Letters, 1975, 26(2), 207-221. doi: 10.1016/0012-821X(75)90088-6
Tang Y, Cui K, Zheng Z, et al. LA-ICP-MS UPb geochronology of wolframite by combining NIST series and common lead-bearing MTM as the primary reference material: Implications for metallogenesis of South China[J]. Gondwana Research, 2020, 83, 217-231. doi: 10.1016/j.gr.2020.02.006
Vermeesch P. Maximum depositional age estimation revisited[J]. Geoscience Frontiers, 2021, 12(2), 843-850. doi: 10.1016/j.gsf.2020.08.008
Vermeesch P. Multi-sample comparison of detrital age distributions[J]. Chemical Geology, 2013, 341, 140-146. doi: 10.1016/j.chemgeo.2013.01.010
Voice P J, Kowalewski M, Eriksson K A. Quantifying the timing and rate of crustal evolution: global compilation of radiometrically dated detrital zircon grains[J]. The Journal of Geology, 2011, 119: 109-126. doi: 10.1086/658295
Wendt I, Carl C. The statistical distribution of the mean squared weighted deviation. Chemical Geology: Isotope Geoscience section, 1991, 86(4), 275-285.