Monazite LA-ICP-MS U-Pb Dating to Constrain the Metamorphic Age of the Muzidian Gneiss Complex in the Northern Dabie Orogen
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摘要:
北大别构造带木子店片麻杂岩是目前华南陆块范围内出露的最古老岩石，但有关其变质时代和构造性质等问题仍缺乏研究。对这些岩石的变质时代开展研究，对于了解华南陆块早期地壳演化过程具有重要意义。本文利用LA-ICP-MS方法对木子店片麻杂岩中两件片麻岩样品进行了独居石的微区原位U-Pb定年和微量元素分析，采用独居石标准物质44069对年龄进行校正，独居石标准物质Trebilcock作为监控，测试结果与其推荐值在误差范围内相一致。结果显示，大多数独居石具有均一、无环带特点，且存在重稀土元素明显亏损以及负Eu异常，暗示其可能形成于变质成因。两件片麻岩样品的独居石U-Pb年龄分别为129±1Ma和130±1Ma，代表了木子店片麻杂岩发生变质作用的时间，这一年龄与木子店地区早白垩世花岗岩的侵位年龄基本一致，也与北大别造山带内大量出露的混合岩时代相吻合。研究认为大别造山带早白垩世加厚地壳的伸展垮塌和造山山根拆沉，引起了变质作用和同时代的岩浆活动，将北大别构造带从挤压向伸展的构造转换时间限定在～130Ma。
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关键词:
- 木子店片麻杂岩 /
- 变质作用 /
- 独居石 /
- 北大别构造带 /
- 英云闪长片麻岩；LA-ICP-MS
要点(1) 高空间分辨率LA-ICP-MS独居石U-Pb定年可对具有复杂演化历史的古老地壳岩石变质时代进行测定。
(2) 木子店片麻杂岩独居石LA-ICP-MS定年结果表明其经历了～130Ma的高压变质作用。
(3) 木子店片麻杂岩变质年龄与区域大规模岩浆作用时间一致，将北大别构造带从挤压向伸展的构造转换时间限定为～130Ma。
HIGHLIGHTS(1) High spatial resolution LA-ICP-MS monazite U-Pb dating can be used to determine the metamorphic age of ancient crustal rocks with complex evolutionary histories.
(2) The LA-ICP-MS dating results of monazite from Muzidian gneiss complex indicate that the complex experienced high-pressure metamorphism at ~130Ma.
(3) The metamorphic age of Muzidian gneiss complex is consistent with the time of regional magmatic activity, constraining the tectonic regime switch time from compression to extension in the Northern Dabie orogenic belt to be ~130Ma.
Abstract:The Muzidian gneiss complex (MGC) in the Northern Dabie Orogen is the oldest exposed rock formation in the South China block. However, there is still a lack of research on its metamorphic age and tectonic implications. LA-ICP-MS in situ U-Pb dating and trace element analysis of monazite were carried out on two gneiss samples from the MGC. The results show that most monazite grains exhibit homogeneous features with no zoning, evident depletion of HREEs and negative Eu anomalies, suggesting that they may be of metamorphic origin. The monazite U-Pb ages of the two gneiss samples are 129±1Ma and 130±1Ma, respectively, which represent the time of metamorphism of the MGC. These ages are broadly consistent with the emplacement age of the early Cretaceous granitoids in the Muzidian area, as well as the large amount of exposed migmatites in the Northern Dabie Orogen. Our study shows that the extensional collapse of the thickened continental crust and the delamination of the orogenic crustal root during the early Cretaceous of the Dabie Orogen caused contemporaneous metamorphism, migmatation, and magmatism, which constrained the tectonic regime switch time of the Northern Dabie Orogen from compression to extension to ~130Ma. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202403050032.
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Keywords:
- Muzidian gneiss complex /
- metamorphism /
- monazite /
- Northern Dabie Orogen /
- tonalitic gneiss; LA-ICP-MS
BRIEF REPORTSignificance: The South China Block (SCB) is one of the important continental blocks in eastern Asia, and thus understanding its formation and evolutionary history is of great significance for fully recognizing the generation, growth, and evolution of the Asian continental crust. The Muzidian gneiss complex (MGC) in the Northern Dabie Orogen is the oldest rock known so far in the SCB, but there remains a lack of research on its metamorphic age and tectonic affiliation. Therefore, conducting research on the metamorphic age of the MGC is of great significance for understanding the early crustal evolution processes of the SCB.
Zircon U-Pb dating is currently the most commonly used method for constraining the timing of high-grade metamorphism[5]. However, using this method to determine the metamorphic age of high-grade metamorphic rocks often fails to yield satisfactory results. Under certain specific metamorphic conditions, it is difficult to form metamorphic zircon. Moreover, some zircon grains have too small metamorphic overgrowth areas to be measured, which is particularly evident in zircons from ancient basement rocks[2,4,6], as these rocks typically undergo multiple episodes of metamorphism of varying degrees, leading to complexity in the genesis of zircon. When using the zircon U-Pb dating method for measurement, it may result in “mixing age” that does not have meaningful geological significance.
In this study, high-precision LA-ICP-MS in situ monazite U-Pb dating with a spot size of 13m on the Eoarchean MGC from the Northern Dabie Orogen was conducted, and the metamorphic age and tectonic significance of these rocks in combination with the trace element composition of monazite was discussed. Compared to zircon, monazite is more likely to form during amphibolite- to granulite-facies metamorphism. Besides, the extremely high Th and U content (usually exceeding 10000μg/g), relatively low common Pb content (generally below 100μg/g), and exceptionally slow Pb diffusion rate make it difficult to produce “Pb loss” during the metamorphism, thus making it an ideal method for constraining the metamorphic age of high-grade metamorphic rocks.
Methods: Two samples used for monazite dating, 22MW11 and 22MW01, were both collected from the felsic part of the MGC in the Muzidian area of Macheng City (Fig.2a,b). The samples were collected by a geological hammer, with each sample weighing ~5kg. After crushing the samples with a jaw crusher, the monazites were separated using traditional heavy-magnetic techniques. They were then mounted on an epoxy resin. After polishing, the monazite grains were evaluated by backscattered electron (BSE) imaging to detect their internal structures.
The U-Pb age and trace element measurements of monazite were carried out at the Isotopic Laboratory at the Wuhan Center, China Geological Survey. The RESOlution S155 laser ablation system equipped with a 193nm ArF-excimer laser was used coupled with an Icap-Q type ICP-MS. Helium was used as the carrier gas and mixed with the make-up gas argon in cell. The laser spot diameter was 13m, with a laser frequency of 3Hz and an energy density of 3.5J/cm. The analysis time for each point was 90s, including a background acquisition (gas blank) time of 15s, followed by 45s of data acquisition from the sample, and a wash time of 30s.
During the measurement, the standard monazite 44069 was used for age calibration. The error for each single data point is 1σ. Monazite 44069 was analyzed twice in every 10 analyses of the unknowns. Reference monazite Trebilcock was analyzed as “unknowns” to act as the independent controls for reproducibility and instrument stability.
Trace element content of monazite was calibrated against multiple USGS reference materials (BCR-2G, BIR-1G, and BHVO-2G) without applying an internal standard to eliminate the influence of matrix effects[11]. Offline processing of the analytical data (including the selection of sample and blank signals, correction of instrument sensitivity drift, and calculation of element content) was performed using the ICPMSDataCal software (ver. 10.9)[11]. Concordia diagrams and weighted mean calculations were made using the software ISOPLOT[12].
Data and Results: The LA-ICP-MS monazite U-Pb dating results of the Muzidian gneiss samples are shown in Table 1. For the sample 22MW11, 33 U-Pb analyses were performed on 33 monazite grains, and all analyses yielded concordant ages. Except for one analysis with a significantly older age (150±2Ma), the remaining 32 analyses exhibited relatively consistent age, with a weighted mean 206Pb/238U age of 129±1Ma (MSWD=1.8) (Fig.4a). For the sample 22MW01, 18 monazite grains were analyzed, all of which were concordant, with a weighted mean 206Pb/238U age of 130±1Ma (MSWD=2.1), with Th/U ratios ranging from 25.2 to 101. Two monazite grains had significantly lower Th/U ratios (8.2 and 19.2, respectively) and older 206Pb/238U ages (236±4Ma and 377±4Ma, respectively), which may represent inherited ages (Fig.4b).
The LA-ICP-MS trace element results of the monazite from the Muzidian gneisses are listed in Table 2. All monazite grains have similar trace element compositions, characterized by strong enrichment of light rare earth elements (LREEs) and depletion of heavy rare earth elements (HREEs) (La/Yb=1610−5390), as well as an evidently negative Eu anomaly (Eu/Eu*=0.14−0.32) (Fig.5).
Most monazites in this study are euhedral to subhedral, and core-rim structures cannot be observed, suggesting that they might be formed during metamorphic processes. The Th/Ce ratio of monazites in the Muzidian gneiss ranges from 0.59 to 1.61, which precludes the hydrothermal origin of monazites. Additionally, in the monazite classification diagrams of Si/Ce, Ca/Ce, and Y/LREEs[18], all monazites from the Muzidian gneiss fall into the metamorphic monazite field (Fig.6), also indicating that all monazites in this study are of metamorphic origin. The trace element features of monazites can further constrain the nature of the metamorphism. The monazites exhibit similar geochemical features, enrichment of LREEs, depletion of HREEs, and evidently negative Eu anomalies (Fig.5). The depletion of HREEs and the high fractionation between LREEs and HREEs are evidence of monazite formation under relatively high-pressure conditions. Calculations of the Y temperature for the monazites from the Muzidian gneiss indicate that these monazites might be formed at temperatures ranging from 583℃ to 962℃ (average 735℃) under a pressure of 0.5GPa. Therefore, it is suggested that the Mesozoic monazite from the Muzidian gneiss may have formed under granulite-facies metamorphism.
The U-Pb isotopic ages of monazites in this study are essentially consistent with the ages of zircons obtained from the migmatites and widely exposed granitoids in the Dabie Orogen, indicating that there were almost contemporaneous metamorphic, magmatic, and migmatization processes in the North Dabie Orogen at ca.130Ma. This suggests that the Mesozoic metamorphism of the Muzidian gneisses may be related to partial melting processes, thus constraining the time of tectonic switch from compression to extension in the North Dabie Orogen to ~130Ma. It should be noteworthy that this period of the tectono-thermal event may have a significant influence on the geochemical compositions, especially for the whole-rock isotopic compositions, of the Archean—Proterozoic ancient basement rocks represented by the MGC in the Dabie Orogen. Therefore, caution should be exercised in future research on ancient rocks in the North Dabie Orogen.
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华南陆块是亚洲东部重要的组成构造单元之一，有关其形成和演化历史对充分认识亚洲大陆地壳形成、生长和演化都具有重要意义，因而受到国内外研究者的广泛关注。然而，相比于毗邻的华北克拉通，由于显生宙巨厚沉积地层的覆盖所导致前寒武纪地质体出露极为有限，导致目前有关华南陆块早期基底岩石的研究程度仍显相对薄弱，有关其古老物质组成、早期地壳岩石的形成时代和演化过程等一些关键科学问题也尚未完全探索清楚。
大别造山带北大别构造带长期以来被认为是华南陆块北缘卷入秦岭—大别碰撞造山带的部分［1-4］。近年来越来越多的研究证据显示，在该区域内可能存在太古宙乃至冥古宙的古老地壳物质残余［1-4］，其中最为值得注意的是，本研究团队最近在北大别构造带木子店地区发现一套TTG片麻岩和斜长角闪岩的岩石组合(命名为“木子店片麻杂岩”)。已开展的锆石U-Pb年代学工作表明，这套岩石的原岩形成年龄为3.8～3.6Ga，代表了目前华南陆块范围内已知的最古老地壳岩石［4］。因此，对木子店片麻杂岩开展相应的研究工作，将对大别造山带乃至华南陆块古老地壳基底岩石的变质历史进行有效约束。另一方面，碰撞造山带内高级变质岩石往往保存了造山带构造-热事件的记录，木子店片麻杂岩位于秦岭—大别碰撞造山带，因而有关其变质时代和构造性质等同样也是深入了解造山带演化历史的关键问题。锆石和独居石U-Pb测年是目前最常用的对高级变质作用进行限定的测试方法［5］。其中，锆石可在高级变质作用的不同阶段通过变质重结晶或增生等方式形成，通过对这些变质锆石生长区域开展定年分析，理论上可获得相对应的岩石发生变质作用过程的年龄。然而，许多情况下利用锆石U-Pb定年方法对高级变质岩进行变质年龄测定往往无法获得理想的结果。例如，在某些特定变质作用条件下(如缺乏变质流体的封闭体系)较难形成变质锆石；此外还存在部分锆石的变质增生区域过小而无法对其开展测量，这在古老基底岩石的锆石中尤为明显［2，4，6］。同时，古老基底岩石通常经历了多期次不同程度的变质作用叠加，导致其锆石成因往往具有复杂性，在利用锆石U-Pb定年方法进行测量时可能会得到并无实际地质意义的“混合年龄”。另外，高级变质岩中锆石发生“Pb丢失”现象而导致的不谐和问题也值得注意。因此，高级变质岩中锆石U-Pb年龄多用来对这些岩石的原岩时代进行测定。但与锆石不同的是，独居石在角闪岩相-麻粒岩相变质作用过程中相对更容易形成，加之其具有极高的Th、U含量(通常超过104 μg/g)，以及相对低的普通Pb含量(一般低于100μg/g)和异常缓慢的Pb扩散速率，使得它在岩石变质作用过程中难以产生“Pb丢失”，因而成为对高级变质岩石变质时代进行限定的重要测试手段［7-8］。
本文对北大别始太古代木子店片麻杂岩开展了13μm束斑大小的高精度LA-ICP-MS微区原位独居石U-Pb同位素年龄测定，结合独居石的微量元素组成，对该套岩石的变质时代及其构造指示意义进行了探讨。
1. 区域地质概况
秦岭—桐柏—大别—苏鲁造山带是亚洲东部最为重要的碰撞造山带之一，是华北克拉通和华南陆块于中生代最终碰撞拼合的结果。大别造山带位于该造山带中部，并可依据变质程度进一步分为北淮阳、北大别、中大别、南大别以及宿松5个构造带。其中，北大别构造带主体为一套混合岩化长英质片麻岩，夹少量镁铁质-超镁铁质岩、榴辉岩、麻粒岩、磁铁石英岩以及大理岩透镜体。前人所开展的锆石年代学研究结果显示，北大别构造带内大部分混合岩的原岩可能形成于新元古代，并在白垩纪经历了混合岩化作用［9］。近年来，随着同位素年代学研究工作的逐步积累，在该构造带内亦陆续发现一批太古宙岩石，例如黄土岭麻粒岩、团风混合岩、贾庙花岗片麻岩以及木子店花岗片麻岩等［1-4］，锆石U-Pb定年结果显示，这些岩石的原岩形成时间主要集中在2.7～2.5Ga的新太古代，并经历了古元古代的构造-热事件［1-4］。
在北大别构造带木子店地区，一系列花岗片麻岩、磁铁石英岩以及变超基性-基性岩零星出露，且被大规模早白垩世黑云二长花岗岩所侵入(图1)。木子店片麻杂岩零星出露于木子店地区，该套岩石原属“大别群”(部分文献亦称“大别山杂岩”)组成部分，以英云闪长片麻岩为主，伴少量花岗闪长片麻岩、斜长角闪岩以及磁铁石英岩和变基性-超基性岩岩块。这些岩石总体变质程度达麻粒岩相，略高于区域上新太古代—古元古代花岗片麻岩类。由于强烈构造变形及后期高级变质作用影响，除可见白垩纪花岗岩类明显侵入木子店片麻杂岩外，其余各岩性之间接触关系不明。Wang等(2023)［4］对木子店片麻杂岩开展的锆石U-Pb定年结果则表明，该套岩石形成于3.8～3.6Ga，代表了目前华南陆块范围内已知最古老岩石。
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2. 实验部分
2.1 样品采集与制备
两件用于独居石定年的样品均采自木子店片麻杂岩中的长英质部分(图2中a，b)，样品编号为22MW11(角闪斜长片麻岩)和22MW01(长英质片麻岩)，采用地质锤进行采集，每件样品质量约5kg。其中样品22MW11为角闪斜长片麻岩，呈灰白色，具有片状粒状变晶结构，片麻状构造。岩石主要由石英(含量50%～55%)、斜长石(25%～30%)、角闪石(～15%)以及少量单斜辉石(～3%)构成(图2c)，副矿物包括磁铁矿、锆石及少量独居石和磷灰石等。样品22MW01为长英质片麻岩，野外呈浅灰白色，野外可见变基性岩捕掳体，岩石具有片状粒状变晶结构，片麻状构造。主要由石英(55%～60%)、斜长石(25%～30%)、角闪石(13%～15%)以及少量黑云母(～2%)构成(图2d)，副矿物包括锆石、磁铁矿、独居石、磷灰石等。石英和斜长石颗粒以它形为主，均具长轴定向特点，斜长石粒径0.2～2mm，石英粒径变化范围较大，从0.2mm到1cm不等，与黑云母、角闪石等暗色矿物共同构成岩石片麻理。
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图 2 木子店片麻杂岩野外和镜下照片Pl—斜长石；Hb—角闪石；Bi—黑云母；Q—石英。Figure 2. Field outcrop and photomicrographs for Muzidian gneiss complex: Pl—Plagioclase; Hb—Hornblende; Bi—Biotite; Q—Quartz野外样品采集后，送至河北省廊坊市宇能岩石矿物分选技术服务有限公司进行独居石分选。样品采用颚式破碎机破碎后，通过传统重磁技术方法分离出独居石，随后将其粘在环氧树脂上进行制靶，经抛光等步骤处理后用于开展背散射(BSE)照相，并以此为基础选择合适区域进行独居石U-Pb定年分析。独居石制靶、透射光和反射光照相均在南京宏创科技有限公司完成。独居石BSE照相在中国地质调查局武汉地质调查中心实验测试室采用日本岛津公司EPMA-1600电子探针完成，拍照工作条件为：加速电压15kV，电流10nA。
2.2 独居石LA-ICP-MS定年和微量元素分析
2.2.1 标准物质、测试仪器及关键参数
独居石的U-Pb年龄和微量元素的同时测定，在中国地质调查局武汉地质调查中心实验测试室完成。测试仪器为RESOlution S155 193nm氟化氩准分子激光剥蚀系统与Icap-Q型ICP-MS联机使用。测试激光束斑直径为13μm，激光频率3Hz，能量密度3.5J/cm2。每个样品点的分析时间为90s，其中背景信号时间15s，激光剥蚀时间45s，尾吹时间30s。
2.2.2 分析方法特点
测定时采用独居石国际标准物质44069对年龄进行校正。单个数据点误差为1σ。每分析10个样品点分析两次标准物质44069，同时采用独居石标准物质Trebilcock作为未知样品对数据结果进行监控，测试结果与其推荐值(272±2Ma［10］)在误差范围内相一致。独居石中的微量元素含量以USGS参考玻璃(BCR-2G、BIR-1G和BHVO-2G)为校正标准，采用多外标-无内标法［11］对微量元素含量进行定量计算以扣除基体效应影响。USGS玻璃中元素含量的推荐值参考GeoReM数据库(http://georem.mpch-mainz.gwdg.de/)。对分析数据的离线处理(包括对样品和空白信号的选择、仪器灵敏度漂移校正、元素含量计算)采用ICPMSDataCal软件(Ver.10.9)进行处理［11］。年龄计算以及谐和图的绘制均采用ISOPLOT软件完成［12］。
3. 独居石形貌特征与分析结果
木子店片麻杂岩独居石样品的LA-ICP-MS U-Pb定年结果见表1。
表 1 北大别木子店片麻杂岩独居石U-Pb同位素组成和年龄值Table 1. U-Pb isotopic ratios and apparent ages of monazite from Muzidian gneiss complex in the Northern Dabie Orogen样品22MW11
测点Th含量
(μg/g)U含量
(μg/g)207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/206Pb 207Pb/235U 206Pb/238U 谐和度
(%)比值 1σ 比值 1σ 比值 1σ 年龄(Ma) 1σ 年龄(Ma) 1σ 年龄(Ma) 1σ MW11-1 186410 7332 0.0491 0.0012 0.1344 0.0037 0.0198 0.0002 154 56 128 3 126 2 98 MW11-2 197307 3778 0.0470 0.0013 0.1296 0.0037 0.0200 0.0002 50.1 72 124 3 128 1 96 MW11-3 172276 3779 0.0501 0.0017 0.1404 0.0047 0.0204 0.0002 211 76 133 4 130 2 97 MW11-6 227338 6906 0.0484 0.0011 0.1323 0.0030 0.0199 0.0002 120 54 126 3 127 1 99 MW11-7 192879 4041 0.0483 0.0017 0.1353 0.0047 0.0204 0.0002 117 82 129 4 130 1 98 MW11-9 180820 4645 0.0500 0.0016 0.1365 0.0044 0.0199 0.0003 198 105 130 4 127 2 97 MW11-10 213252 10626 0.0483 0.0012 0.1363 0.0035 0.0205 0.0002 122 57 130 3 131 2 99 MW11-11 174629 4962 0.0507 0.0023 0.1447 0.0067 0.0208 0.0003 228 106 137 6 133 2 96 MW11-12 198507 3882 0.0486 0.0014 0.1374 0.0040 0.0206 0.0002 128 69 131 4 131 2 99 MW11-14 179763 3510 0.0471 0.0015 0.1312 0.0045 0.0201 0.0002 53.8 74 125 4 128 2 97 MW11-15 207922 2505 0.0490 0.0018 0.1356 0.0050 0.0203 0.0003 146 89 129 5 129 2 99 MW11-16 177388 6678 0.0475 0.0012 0.1314 0.0035 0.0200 0.0002 76.0 94 125 3 128 1 98 MW11-17 211743 2839 0.0499 0.0017 0.1404 0.0046 0.0205 0.0002 191 112 133 4 131 2 98 MW11-18 184015 2839 0.0474 0.0016 0.1328 0.0046 0.0204 0.0002 77.9 69 127 4 130 2 97 MW11-19 213105 3124 0.0473 0.0017 0.1320 0.0049 0.0203 0.0002 61.2 85 126 4 129 2 97 MW11-20 167763 4389 0.0502 0.0017 0.1399 0.0046 0.0203 0.0002 206 80 133 4 130 2 97 MW11-22 192664 8380 0.0497 0.0017 0.1406 0.0050 0.0204 0.0002 189 78 134 4 130 1 97 MW11-23 243780 4535 0.0481 0.0018 0.1334 0.0049 0.0202 0.0002 102 91 127 4 129 2 98 MW11-24 184068 3900 0.0492 0.0017 0.1364 0.0048 0.0201 0.0003 167 80 130 4 128 2 98 MW11-25 188850 8382 0.0484 0.0012 0.1335 0.0033 0.0199 0.0002 120 56 127 3 127 1 99 MW11-26 180843 6387 0.0483 0.0015 0.1359 0.0043 0.0204 0.0002 122 74 129 4 130 1 99 MW11-27 207930 5238 0.0476 0.0010 0.1301 0.0028 0.0198 0.0002 79.7 50 124 3 126 1 98 MW11-28 197678 2753 0.0471 0.0016 0.1340 0.0046 0.0206 0.0002 53.8 78 128 4 131 2 97 MW11-30 195694 3999 0.0484 0.0020 0.1332 0.0053 0.0201 0.0003 117 96 127 5 128 2 98 MW11-32 177606 5946 0.0506 0.0017 0.1398 0.0044 0.0201 0.0002 233 76 133 4 128 2 96 MW11-34 200061 3514 0.0493 0.0021 0.1388 0.0058 0.0206 0.0002 161 98 132 5 131 2 99 MW11-35 194798 5650 0.0476 0.0014 0.1329 0.0039 0.0203 0.0002 79.7 70 127 3 130 2 97 MW11-36 174154 2157 0.0498 0.0014 0.1358 0.0038 0.0199 0.0002 183 69 129 3 127 1 98 MW11-37 178861 2757 0.0493 0.0016 0.1365 0.0044 0.0202 0.0002 161 74 130 4 129 2 99 MW11-38 180591 6144 0.0474 0.0013 0.1363 0.0037 0.0210 0.0002 77.9 136 130 3 134 1 96 MW11-39 175177 6068 0.0469 0.0010 0.1313 0.0031 0.0203 0.0002 55.7 39 125 3 129 1 96 MW11-40 187657 4194 0.0473 0.0017 0.1348 0.0050 0.0208 0.0003 64.9 85 128 5 133 2 96 样品22MW01
测点Th含量
(μg/g)U含量
(μg/g)207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/206Pb 207Pb/235U 206Pb/238U 谐和度
(%)比值 1σ 比值 1σ 比值 1σ 年龄(Ma) 1σ 年龄(Ma) 1σ 年龄(Ma) 1σ MW1-1 166645 3043 0.0490 0.0018 0.1400 0.0050 0.0207 0.0003 150 85 133 5 132 2 99 MW1-5 262630 7005 0.0489 0.0010 0.1346 0.0029 0.0199 0.0002 143 45 128 3 127 1 99 MW1-7 191746 5815 0.0507 0.0018 0.1448 0.0051 0.0208 0.0003 233 82 137 5 133 2 96 MW1-8 133410 2873 0.0474 0.0019 0.1310 0.0050 0.0202 0.0002 77.9 83 125 5 129 2 96 MW1-9 181982 3623 0.0499 0.0014 0.1382 0.0039 0.0201 0.0003 191 67 131 4 128 2 97 MW1-10 242641 3159 0.0489 0.0019 0.1346 0.0053 0.0201 0.0002 143 91 128 5 128 2 99 MW1-12 252773 9255 0.0505 0.0011 0.1374 0.0031 0.0198 0.0002 217 84 131 3 126 2 96 MW1-14 240269 4057 0.0496 0.0029 0.1370 0.0087 0.0201 0.0003 189 135 130 8 128 2 98 MW1-18 139956 3108 0.0495 0.0015 0.1397 0.0045 0.0204 0.0003 169 70 133 4 130 2 98 MW1-21 263686 10469 0.0471 0.0010 0.1310 0.0031 0.0202 0.0003 53.8 149 125 3 129 2 97 MW1-22 226722 3006 0.0507 0.0018 0.1433 0.0050 0.0206 0.0003 228 82 136 5 131 2 96 MW1-23 224741 3177 0.0492 0.0017 0.1415 0.0053 0.0208 0.0002 167 82 134 5 133 2 98 MW1-25 230108 2332 0.0482 0.0021 0.1381 0.0060 0.0210 0.0003 109 95 131 5 134 2 98 MW1-26 144067 1911 0.0488 0.0022 0.1376 0.0062 0.0207 0.0003 200 103 131 6 132 2 99 MW1-29 268982 2672 0.0512 0.0020 0.1420 0.0052 0.0204 0.0003 250 86 135 5 130 2 96 MW1-30 156794 5440 0.0466 0.0017 0.1322 0.0047 0.0207 0.0003 31.6 91 126 4 132 2 95 MW1-6 69266 3613 0.0521 0.0025 0.2683 0.0128 0.0373 0.0006 300 105 241 10 236 4 97 MW1-27 34270 4206 0.0551 0.0013 0.4559 0.0107 0.0602 0.0006 417 58 381 8 377 4 98 注：235U衰变常数为9.8485×10−10a，238U衰变常数为1.55125×10−10a；238U/235U=137.88。 样品22MW11中，独居石多为半自形到它形，呈淡黄色、透明，长度范围为30～60μm。独居石BSE照片显示(图3)大多数独居石颗粒没有明显的核边结构，仅少数颗粒具有狭窄的边缘。对该样品中33个独居石颗粒33个点进行了U-Pb年龄测定，所有分析均获得谐和年龄。独居石具有相对高的Th和U含量，对应Th/U比值为20.1～83.0，所获得分析点的年龄数据相对其原岩形成时代具有明显偏年轻的特点，除一个分析点具有明显偏老的年龄(150±2Ma)以外，剩余32个分析点的年龄数据集中，具有相对一致的年龄，数据点的206Pb/238U加权平均年龄为129±1Ma (MSWD=1.8)(图4a)。
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图 3 北大别木子店片麻杂岩代表性独居石背散射图像Figure 3. BSE images of representative monazite from Muzidian gneiss complex in the Northern Dabie Orogen: (a) Amphibole-plagioclase gneiss (sample 22MW11); (b) Felsic gneiss (sample 22MW01).![]()
图 4 北大别木子店片麻杂岩独居石U-Pb年龄谐和图Figure 4. U-Pb concordia of monazite from Muzidian gneiss complex in the Northern Dabie Orogen: (a) Amphibole-plagioclase gneiss (sample 22MW11); (b) Felsic gneiss (sample 22MW01).样品22MW01中，独居石为半自形到自形，颜色为淡黄色，BSE图像显示多数独居石为均质的，仅少部分颗粒显示具有斑点(图3b)。对该样品中18颗独居石18个数据点进行了分析，所有分析点均为谐和独居石。结果显示，所获得的年龄数据均远远小于岩石原岩的形成年龄，其中16个独居石颗粒年龄组成接近，其206Pb/238U加权平均年龄为130±1Ma(MSWD=2.1)，Th/U比值介于25.2～101之间。剩余2个独居石颗粒具有明显偏低的Th/U比值(分别为8.2和19.2)，以及偏老的206Pb/238U年龄(分别为236±4Ma和377±4Ma)，可能为继承独居石年龄(图4b)。
木子店片麻杂岩独居石样品的LA-ICP-MS微量元素测试结果见表2。数据显示，所有独居石颗粒具有相似的微量元素组成，稀土元素配分模式图中表现为强烈的轻稀土元素富集，重稀土元素亏损(La/Yb=1610～5390)以及明显的Eu负异常(Eu/Eu*=0.14～0.32)。与样品22MW11相比，样品22MW01中独居石显示出更低的重稀土元素含量、更高的轻重稀土元素分异，以及更为明显的负Eu异常的特点(图5)。
表 2 北大别木子店片麻杂岩独居石微量元素组成Table 2. Trace element contents of monazite from Muzidian gneiss complex in the Northern Dabie Orogen样品编号 元素含量(μg/g) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Th U Si MW11-1 138521 199176 15840 46140 4077 272 3066 173 573 80.0 160 12.7 56.9 6.34 2124 186410 7332 41727 MW11-2 143070 196675 14714 42334 3158 259 2589 121 364 49.5 100 8.18 36.5 4.26 1333 197307 3778 42534 MW11-3 138555 202502 16616 49693 4628 272 3447 207 703 95.7 185 14.5 68.5 6.74 2550 172276 3779 40078 MW11-6 137308 179216 12685 33671 2345 241 2247 91.9 288 40.0 85.3 6.54 34.8 4.10 1080 227338 6906 57053 MW11-7 140932 193833 15030 43731 3458 250 2770 134 418 56.2 112 8.65 37.6 4.11 1485 192879 4041 45845 MW11-9 140270 197158 15210 43927 3466 261 2779 137 450 63.4 127 10.0 43.4 4.35 1655 180820 4645 44196 MW11-10 138311 180924 12586 34588 2619 255 2446 112 373 50.4 103 8.28 35.3 3.94 1346 213252 10626 54893 MW11-11 133330 195509 15880 49487 4911 275 3691 238 862 117 223 17.7 76.2 8.04 3001 174629 4962 40191 MW11-12 128565 188117 15211 47303 4100 266 2922 159 506 68.9 135 10.4 47.7 4.97 1783 198507 3882 46016 MW11-14 138239 193244 15258 45785 3612 259 2779 133 413 54.4 110 7.74 35.9 3.82 1427 179763 3510 42572 MW11-15 125427 182828 14895 46925 3931 258 2809 140 434 55.1 112 8.01 34.5 3.83 1463 207922 2505 52004 MW11-16 135816 191316 14721 45225 3953 265 3100 168 581 78.5 153 12.0 53.3 5.81 2054 177388 6678 42943 MW11-17 129835 182962 14042 42579 3309 250 2523 115 348 46.4 95.9 6.78 33.8 3.39 1239 211743 2839 55636 MW11-18 136910 189546 14882 44646 3560 258 2766 133 411 54.7 109 7.87 36.7 3.94 1436 184015 2839 43140 MW11-19 128190 181294 14211 42072 3233 252 2499 116 347 45.9 94.0 7.21 35.1 3.29 1228 213105 3124 53381 MW11-20 135145 195703 15825 46799 4180 252 3259 184 625 88.3 170 14.0 61.8 6.45 2270 167763 4389 34881 MW11-22 132014 182367 13817 40792 3535 253 2938 156 525 72.4 140 11.3 49.1 5.29 1877 192664 8380 50744 MW11-23 114156 166619 13063 39294 2906 255 2167 98.8 282 36.8 78.1 5.72 25.5 2.92 982 243780 4535 65177 MW11-24 136312 189089 14052 40820 3094 248 2565 121 397 53.9 111 8.17 42.4 4.22 1439 184068 3900 43658 MW11-25 128308 183106 14339 43132 3672 256 2857 157 526 72.1 148 11.5 51.3 5.60 1942 188850 8382 46737 MW11-26 126226 185572 14703 44785 4464 271 3364 219 812 112 220 17.7 78.4 8.31 2975 180843 6387 40228 MW11-27 128463 176009 13386 38839 2958 244 2407 110 340 45.9 95.0 7.10 34.0 3.81 1213 207930 5238 50059 MW11-28 129218 181035 14100 41784 3095 255 2396 108 322 42.5 87.8 6.66 29.6 3.34 1142 197678 2753 47381 MW11-30 130004 180582 13751 41552 3166 246 2497 114 352 46.0 94.4 7.12 34.7 3.33 1237 195694 3999 45829 MW11-32 126189 182886 14535 44930 3789 252 2955 160 555 77.4 154 12.3 56.2 5.95 2012 177606 5946 42733 MW11-34 118846 170988 13533 41324 3394 245 2608 136 435 58.5 119 9.45 37.5 4.34 1566 200061 3514 60236 MW11-35 126117 178981 13751 41292 3120 255 2497 115 359 48.9 97.9 7.20 35.6 3.63 1295 194798 5650 47697 MW11-36 126109 183416 14810 48039 4040 255 2959 152 472 61.3 120 9.08 38.9 4.05 1556 174154 2157 42681 MW11-37 126057 180966 14600 45086 3688 249 2822 139 434 58.1 113 8.52 36.3 3.55 1490 178861 2757 45309 MW11-38 128255 179239 13777 40359 3255 252 2675 135 480 68.4 137 11.2 47.9 5.27 1786 180591 6144 43495 MW11-39 126166 183025 14320 43528 3707 257 2909 160 548 76.9 149 11.8 50.1 5.73 2044 175177 6068 41464 MW11-40 131049 177956 13209 38550 2909 232 2433 108 343 46.9 97.2 7.59 36.5 3.94 1258 187657 4194 45326 MW11-29 128500 180131 14261 43697 3424 241 2591 123 380 51.0 104 7.40 34.5 3.65 1348 191806 3201 48000 样品编号 元素含量(μg/g) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Th U Si MW1-1 158157 212653 15817 44459 3476 159 3116 146 445 59.4 115 8.86 36.1 4.02 1633 166645 3043 36883 MW1-5 123671 173235 13143 39498 2992 207 2474 115 350 47.4 96.8 7.53 32.1 3.50 1194 262630 7005 66654 MW1-7 156116 202450 14113 38157 2734 153 2721 113 358 47.1 99.2 7.67 33.4 3.31 1345 191746 5815 46068 MW1-8 170321 226809 16461 47368 3828 170 3456 173 539 72.1 137 10.1 42.8 4.50 1855 133410 2873 28844 MW1-9 149310 205327 15638 46605 3903 180 3248 168 543 69.5 136 10.4 44.0 4.18 1817 181982 3623 42624 MW1-10 129912 183044 14344 42810 3291 218 2623 125 382 51.6 106 8.17 34.5 3.79 1334 242641 3159 55240 MW1-12 127926 174960 13094 37233 2711 215 2326 99.3 302 40.7 83.1 6.35 28.2 3.19 1066 252773 9255 66267 MW1-14 129500 180618 13711 40271 2985 209 2443 113 344 47.5 94.4 7.94 36.9 4.69 1211 240269 4057 57984 MW1-18 165035 222645 15924 44681 3452 158 3143 145 454 60.8 119 8.75 39.0 3.99 1623 139956 3108 29700 MW1-21 121663 170826 12812 36653 2650 214 2237 100 311 42.8 86.2 7.09 30.2 3.12 1111 263686 10469 66797 MW1-22 135213 186713 14368 42543 3259 223 2590 117 353 45.7 94.3 6.77 30.5 2.86 1184 226722 3006 54088 MW1-23 128995 185314 14897 45653 3949 188 3060 171 561 76.1 145 10.9 46.3 4.84 1969 224741 3177 58256 MW1-25 133317 184376 14544 43366 3269 220 2615 123 379 52.1 103 7.82 34.9 3.41 1311 230108 2332 54878 MW1-26 168153 221184 15725 44612 3569 153 3163 149 439 56.8 107 7.57 31.2 3.38 1569 144067 1911 35698 MW1-29 120654 166938 13169 39012 3006 215 2364 111 348 46.8 94.5 6.89 31.3 3.49 1178 268982 2672 68041 MW1-30 164943 215486 15035 40775 2993 145 2873 128 389 51.0 101 7.42 31.5 3.02 1423 156794 5440 36654 MW1-6 169915 257423 20702 63531 5832 146 4160 211 605 73.9 137 9.41 39.2 3.80 1978 69266 3613 14800 MW1-27 189722 268973 20096 57313 4524 262 3735 189 590 76.3 145 9.28 39.7 3.84 1938 34270 4206 8600 ![]()
图 5 北大别木子店片麻杂岩独居石稀土元素配分模式图Figure 5. REE patterns of monazite from Muzidian gneiss complex in the Northern Dabie Orogen4. 木子店片麻杂岩的变质时代及其意义
4.1 木子店片麻杂岩的变质时代
秦岭—大别造山带作为世界上规模最大的高压-超高压变质带，且经历了多阶段构造演化，成为研究高压-超高压变质作用和造山带演化的热点地区。北大别构造带大规模出露早白垩世混合岩和后碰撞花岗岩，表明该时期存在显著的构造-岩浆事件，该过程对构造带的物质组成、地壳再造等均具有重要影响［9］。然而，作为华南陆块已知最古老岩石，木子店片麻杂岩是否亦受该时期变质作用影响，目前仍缺乏了解。已有研究显示，木子店片麻杂岩中的锆石U-Pb同位素年龄记录了这些岩石的原岩主要形成于3.8～3.6Ga的始太古代，但仅少数锆石颗粒记录了零星的可能变质时代信息［4］。与之相比，在变质流体存在的情况下，独居石相比锆石更容易发生再沉淀过程，加之其具有极低的铅扩散速率，因此其相对能够更准确地记录高级变质岩所经历的变质年龄。
本研究中，木子店片麻杂岩中独居石颗粒以半自形到自形为主，且未见显示明显的核-边结构，表明它们可能主要为变质作用过程中形成的。最近研究显示，变质和岩浆独居石相比热液独居石而言，具有明显更低的Th以及更低的Ce含量，通常Th/Ce比值不超过0.1［13］。木子店片麻杂岩独居石的Th/Ce比值为0.59～1.61，排除了独居石的热液成因。同时，独居石Si/Ce、Ca/Ce和Y/LREEs成因分类图中［14］，岩浆成因的独居石分布范围很广，集中在该分类图的中间位置，而变质成因独居石往往具有相对低的Y/LREEs比值。木子店片麻杂岩独居石全部位于变质独居石区域内(图6)，证明了本研究所有独居石均为变质成因独居石。两件片麻岩样品的独居石U-Pb年龄分别为129±1Ma和130±1Ma，具有高度一致性。独居石的微量元素特点可进一步对其变质作用性质进行约束。木子店片麻杂岩样品的独居石均显示了相似的地球化学特点，即轻稀土元素富集、重稀土元素亏损及显著的负Eu异常(图5)，重稀土元素亏损以及轻、重稀土元素的高度分异是独居石形成在相对高的压力条件下的证据，而明显的Eu负异常则表示在独居石形成过程中可能存在斜长石。样品23MW11中独居石具有相对更明显的负Eu异常，表明其源区斜长石含量可能更高。变质独居石中Y含量被认为可能与其结晶温度相关［16-17］。
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本文对木子店片麻杂岩独居石开展了Y元素温度的计算，结果显示这些独居石在0.5GPa压力条件下的形成温度变化范围在583～962℃之间(平均735℃)。此外，前人对北大别构造带同时代混合岩中淡色体开展的变质温压条件研究也表明，其形成的P-T条件在520～550MPa、760～820℃［18］。因而，本文认为来自木子店片麻杂岩的中生代独居石可能是在麻粒岩相变质作用下形成。
4.2 北大别构造带早白垩世构造-热事件
碰撞造山带地壳演化通常包含三个阶段，即最初的地壳增厚、随后的地壳根部变质，以及最后的加厚地壳伸展和垮塌。已有研究显示，大别造山带在三叠纪华南陆块和华北克拉通的陆陆碰撞过程中大陆地壳曾增厚至50km以上，但现今该地区地壳厚度仅35～40km，且缺失了造山带地壳根部，暗示部分岩石圈已拆沉到地幔之中［18］，该观点也得到大别地区地震层析图像研究结果的支持［19-20］。随后，因受侏罗纪古太平洋板块的西北向俯冲，以及早白垩世(135～130Ma)古太平洋板块后撤过程所导致相应区域发生挤压及之后的伸展过程［21］，在大别造山带发育大规模区域性地壳深熔作用，导致大量晚中生代构造-岩浆作用形成。
Wu等(2007)［9］对北大别构造带漫水河、凤凰关、燕子河等多处混合岩开展了详细的锆石U-Pb年代学和Hf同位素研究，结果显示大别造山带中生代可能存在两期混合岩化作用，时间分别在139±1Ma和123±1Ma，其认为前者可能形成于区域伸展拉张的构造背景，后者则与大别造山带岩石圈垮塌和拆沉过程相关。胡俊良等(2018)［22］对木子店地区出露最广泛的二长花岗岩进行了系统研究，结果显示这些花岗岩主要形成于131±1Ma，并具有明显的埃达克质岩石的地球化学特点，认为它们可能是后碰撞伸展过程中加厚地壳部分熔融形成的。本文通过独居石U-Pb同位素定年方法，在北大别构造带木子店片麻杂岩中获得～130Ma的变质作用，进一步支持了在北大别构造带早白垩世存在强烈的构造-热事件。
大别造山带中生代花岗岩类大多数具有早白垩世年龄［22-23］。前人研究结果表明，这些后碰撞成因的花岗岩大多属于Ⅰ型花岗岩，并可依据时代、结构和地球化学组成等进一步分为两类，其中早期(144～130Ma)花岗岩多变形，且具有典型埃达克质岩石的地球化学组成，而晚期(130～116Ma)花岗岩则未变形且没有埃达克质岩石的地球化学特点［23］。早期花岗岩被认为来自三叠纪陆陆碰撞所导致的加厚地壳部分熔融，晚期花岗岩则多认为来自岩石圈拆沉所诱发的地壳部分熔融过程［24］。在这些过程中，地壳部分熔融或软流圈地幔上涌过程中释放出来的流体在控制地壳变形、变质以及岩浆活动中都起到了关键作用。木子店片麻杂岩独居石U-Pb同位素年龄和大别造山带混合岩以及木子店地区广泛出露的黑云母二长花岗岩中获得的锆石U-Pb年龄结果基本相一致，显示北大别构造带在～130Ma存在几乎同时的变质、岩浆和混合岩化作用，表明木子店片麻杂岩中生代变质作用可能与部分熔融过程相关。结合大别造山带两类中生代花岗岩类在变形特点和地球化学组成上的差异，本文将大别造山带北大别构造带由挤压向伸展的构造转换时间大致限定为～130Ma。值得注意的是，该期构造-热事件可能对以木子店片麻杂岩为代表的大别造山带太古宙—元古宙古老基底岩石的地球化学组成，尤其是全岩同位素组成产生显著影响，因而在后续对北大别构造带古老岩石开展进一步研究工作时需予以谨慎。
5. 结论
对木子店片麻杂岩中两件片麻岩样品开展了LA-ICP-MS独居石U-Pb定年和微量元素测定，结果显示大多数独居石可能形成于变质作用条件下，两件片麻岩样品的独居石U-Pb年龄分别为129±1Ma和130±1Ma，代表了木子店片麻杂岩发生变质作用的时间。
大别造山带早白垩世加厚地壳的伸展垮塌和造山山根拆沉，引起变质作用和同时代的岩浆活动，将北大别构造带从挤压向伸展的构造转换时间限定在～130Ma。大别造山带太古宙—元古宙古老基底岩石的地球化学组成，尤其是全岩同位素组成产生了显著影响，建议后续对北大别构造带古老岩石开展进一步研究工作时需谨慎予以考虑。
致谢：在野外工作和论文写作期间，与中国地质调查局武汉地质调查中心彭练红正高级工程师、田洋正高级工程师、徐大良正高级工程师和金巍高级工程师进行了有益讨论；论文修改过程中，审稿专家给予了宝贵的修改意见。在此一并表示感谢！
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图 2 木子店片麻杂岩野外和镜下照片
Pl—斜长石；Hb—角闪石；Bi—黑云母；Q—石英。
Figure 2. Field outcrop and photomicrographs for Muzidian gneiss complex: Pl—Plagioclase; Hb—Hornblende; Bi—Biotite; Q—Quartz
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图 3 北大别木子店片麻杂岩代表性独居石背散射图像
Figure 3. BSE images of representative monazite from Muzidian gneiss complex in the Northern Dabie Orogen: (a) Amphibole-plagioclase gneiss (sample 22MW11); (b) Felsic gneiss (sample 22MW01).
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图 4 北大别木子店片麻杂岩独居石U-Pb年龄谐和图
Figure 4. U-Pb concordia of monazite from Muzidian gneiss complex in the Northern Dabie Orogen: (a) Amphibole-plagioclase gneiss (sample 22MW11); (b) Felsic gneiss (sample 22MW01).
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图 5 北大别木子店片麻杂岩独居石稀土元素配分模式图
Figure 5. REE patterns of monazite from Muzidian gneiss complex in the Northern Dabie Orogen
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表 1 北大别木子店片麻杂岩独居石U-Pb同位素组成和年龄值
Table 1 U-Pb isotopic ratios and apparent ages of monazite from Muzidian gneiss complex in the Northern Dabie Orogen
样品22MW11
测点Th含量
(μg/g)U含量
(μg/g)207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/206Pb 207Pb/235U 206Pb/238U 谐和度
(%)比值 1σ 比值 1σ 比值 1σ 年龄(Ma) 1σ 年龄(Ma) 1σ 年龄(Ma) 1σ MW11-1 186410 7332 0.0491 0.0012 0.1344 0.0037 0.0198 0.0002 154 56 128 3 126 2 98 MW11-2 197307 3778 0.0470 0.0013 0.1296 0.0037 0.0200 0.0002 50.1 72 124 3 128 1 96 MW11-3 172276 3779 0.0501 0.0017 0.1404 0.0047 0.0204 0.0002 211 76 133 4 130 2 97 MW11-6 227338 6906 0.0484 0.0011 0.1323 0.0030 0.0199 0.0002 120 54 126 3 127 1 99 MW11-7 192879 4041 0.0483 0.0017 0.1353 0.0047 0.0204 0.0002 117 82 129 4 130 1 98 MW11-9 180820 4645 0.0500 0.0016 0.1365 0.0044 0.0199 0.0003 198 105 130 4 127 2 97 MW11-10 213252 10626 0.0483 0.0012 0.1363 0.0035 0.0205 0.0002 122 57 130 3 131 2 99 MW11-11 174629 4962 0.0507 0.0023 0.1447 0.0067 0.0208 0.0003 228 106 137 6 133 2 96 MW11-12 198507 3882 0.0486 0.0014 0.1374 0.0040 0.0206 0.0002 128 69 131 4 131 2 99 MW11-14 179763 3510 0.0471 0.0015 0.1312 0.0045 0.0201 0.0002 53.8 74 125 4 128 2 97 MW11-15 207922 2505 0.0490 0.0018 0.1356 0.0050 0.0203 0.0003 146 89 129 5 129 2 99 MW11-16 177388 6678 0.0475 0.0012 0.1314 0.0035 0.0200 0.0002 76.0 94 125 3 128 1 98 MW11-17 211743 2839 0.0499 0.0017 0.1404 0.0046 0.0205 0.0002 191 112 133 4 131 2 98 MW11-18 184015 2839 0.0474 0.0016 0.1328 0.0046 0.0204 0.0002 77.9 69 127 4 130 2 97 MW11-19 213105 3124 0.0473 0.0017 0.1320 0.0049 0.0203 0.0002 61.2 85 126 4 129 2 97 MW11-20 167763 4389 0.0502 0.0017 0.1399 0.0046 0.0203 0.0002 206 80 133 4 130 2 97 MW11-22 192664 8380 0.0497 0.0017 0.1406 0.0050 0.0204 0.0002 189 78 134 4 130 1 97 MW11-23 243780 4535 0.0481 0.0018 0.1334 0.0049 0.0202 0.0002 102 91 127 4 129 2 98 MW11-24 184068 3900 0.0492 0.0017 0.1364 0.0048 0.0201 0.0003 167 80 130 4 128 2 98 MW11-25 188850 8382 0.0484 0.0012 0.1335 0.0033 0.0199 0.0002 120 56 127 3 127 1 99 MW11-26 180843 6387 0.0483 0.0015 0.1359 0.0043 0.0204 0.0002 122 74 129 4 130 1 99 MW11-27 207930 5238 0.0476 0.0010 0.1301 0.0028 0.0198 0.0002 79.7 50 124 3 126 1 98 MW11-28 197678 2753 0.0471 0.0016 0.1340 0.0046 0.0206 0.0002 53.8 78 128 4 131 2 97 MW11-30 195694 3999 0.0484 0.0020 0.1332 0.0053 0.0201 0.0003 117 96 127 5 128 2 98 MW11-32 177606 5946 0.0506 0.0017 0.1398 0.0044 0.0201 0.0002 233 76 133 4 128 2 96 MW11-34 200061 3514 0.0493 0.0021 0.1388 0.0058 0.0206 0.0002 161 98 132 5 131 2 99 MW11-35 194798 5650 0.0476 0.0014 0.1329 0.0039 0.0203 0.0002 79.7 70 127 3 130 2 97 MW11-36 174154 2157 0.0498 0.0014 0.1358 0.0038 0.0199 0.0002 183 69 129 3 127 1 98 MW11-37 178861 2757 0.0493 0.0016 0.1365 0.0044 0.0202 0.0002 161 74 130 4 129 2 99 MW11-38 180591 6144 0.0474 0.0013 0.1363 0.0037 0.0210 0.0002 77.9 136 130 3 134 1 96 MW11-39 175177 6068 0.0469 0.0010 0.1313 0.0031 0.0203 0.0002 55.7 39 125 3 129 1 96 MW11-40 187657 4194 0.0473 0.0017 0.1348 0.0050 0.0208 0.0003 64.9 85 128 5 133 2 96 样品22MW01
测点Th含量
(μg/g)U含量
(μg/g)207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/206Pb 207Pb/235U 206Pb/238U 谐和度
(%)比值 1σ 比值 1σ 比值 1σ 年龄(Ma) 1σ 年龄(Ma) 1σ 年龄(Ma) 1σ MW1-1 166645 3043 0.0490 0.0018 0.1400 0.0050 0.0207 0.0003 150 85 133 5 132 2 99 MW1-5 262630 7005 0.0489 0.0010 0.1346 0.0029 0.0199 0.0002 143 45 128 3 127 1 99 MW1-7 191746 5815 0.0507 0.0018 0.1448 0.0051 0.0208 0.0003 233 82 137 5 133 2 96 MW1-8 133410 2873 0.0474 0.0019 0.1310 0.0050 0.0202 0.0002 77.9 83 125 5 129 2 96 MW1-9 181982 3623 0.0499 0.0014 0.1382 0.0039 0.0201 0.0003 191 67 131 4 128 2 97 MW1-10 242641 3159 0.0489 0.0019 0.1346 0.0053 0.0201 0.0002 143 91 128 5 128 2 99 MW1-12 252773 9255 0.0505 0.0011 0.1374 0.0031 0.0198 0.0002 217 84 131 3 126 2 96 MW1-14 240269 4057 0.0496 0.0029 0.1370 0.0087 0.0201 0.0003 189 135 130 8 128 2 98 MW1-18 139956 3108 0.0495 0.0015 0.1397 0.0045 0.0204 0.0003 169 70 133 4 130 2 98 MW1-21 263686 10469 0.0471 0.0010 0.1310 0.0031 0.0202 0.0003 53.8 149 125 3 129 2 97 MW1-22 226722 3006 0.0507 0.0018 0.1433 0.0050 0.0206 0.0003 228 82 136 5 131 2 96 MW1-23 224741 3177 0.0492 0.0017 0.1415 0.0053 0.0208 0.0002 167 82 134 5 133 2 98 MW1-25 230108 2332 0.0482 0.0021 0.1381 0.0060 0.0210 0.0003 109 95 131 5 134 2 98 MW1-26 144067 1911 0.0488 0.0022 0.1376 0.0062 0.0207 0.0003 200 103 131 6 132 2 99 MW1-29 268982 2672 0.0512 0.0020 0.1420 0.0052 0.0204 0.0003 250 86 135 5 130 2 96 MW1-30 156794 5440 0.0466 0.0017 0.1322 0.0047 0.0207 0.0003 31.6 91 126 4 132 2 95 MW1-6 69266 3613 0.0521 0.0025 0.2683 0.0128 0.0373 0.0006 300 105 241 10 236 4 97 MW1-27 34270 4206 0.0551 0.0013 0.4559 0.0107 0.0602 0.0006 417 58 381 8 377 4 98 注：235U衰变常数为9.8485×10−10a，238U衰变常数为1.55125×10−10a；238U/235U=137.88。 
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表 2 北大别木子店片麻杂岩独居石微量元素组成
Table 2 Trace element contents of monazite from Muzidian gneiss complex in the Northern Dabie Orogen
样品编号 元素含量(μg/g) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Th U Si MW11-1 138521 199176 15840 46140 4077 272 3066 173 573 80.0 160 12.7 56.9 6.34 2124 186410 7332 41727 MW11-2 143070 196675 14714 42334 3158 259 2589 121 364 49.5 100 8.18 36.5 4.26 1333 197307 3778 42534 MW11-3 138555 202502 16616 49693 4628 272 3447 207 703 95.7 185 14.5 68.5 6.74 2550 172276 3779 40078 MW11-6 137308 179216 12685 33671 2345 241 2247 91.9 288 40.0 85.3 6.54 34.8 4.10 1080 227338 6906 57053 MW11-7 140932 193833 15030 43731 3458 250 2770 134 418 56.2 112 8.65 37.6 4.11 1485 192879 4041 45845 MW11-9 140270 197158 15210 43927 3466 261 2779 137 450 63.4 127 10.0 43.4 4.35 1655 180820 4645 44196 MW11-10 138311 180924 12586 34588 2619 255 2446 112 373 50.4 103 8.28 35.3 3.94 1346 213252 10626 54893 MW11-11 133330 195509 15880 49487 4911 275 3691 238 862 117 223 17.7 76.2 8.04 3001 174629 4962 40191 MW11-12 128565 188117 15211 47303 4100 266 2922 159 506 68.9 135 10.4 47.7 4.97 1783 198507 3882 46016 MW11-14 138239 193244 15258 45785 3612 259 2779 133 413 54.4 110 7.74 35.9 3.82 1427 179763 3510 42572 MW11-15 125427 182828 14895 46925 3931 258 2809 140 434 55.1 112 8.01 34.5 3.83 1463 207922 2505 52004 MW11-16 135816 191316 14721 45225 3953 265 3100 168 581 78.5 153 12.0 53.3 5.81 2054 177388 6678 42943 MW11-17 129835 182962 14042 42579 3309 250 2523 115 348 46.4 95.9 6.78 33.8 3.39 1239 211743 2839 55636 MW11-18 136910 189546 14882 44646 3560 258 2766 133 411 54.7 109 7.87 36.7 3.94 1436 184015 2839 43140 MW11-19 128190 181294 14211 42072 3233 252 2499 116 347 45.9 94.0 7.21 35.1 3.29 1228 213105 3124 53381 MW11-20 135145 195703 15825 46799 4180 252 3259 184 625 88.3 170 14.0 61.8 6.45 2270 167763 4389 34881 MW11-22 132014 182367 13817 40792 3535 253 2938 156 525 72.4 140 11.3 49.1 5.29 1877 192664 8380 50744 MW11-23 114156 166619 13063 39294 2906 255 2167 98.8 282 36.8 78.1 5.72 25.5 2.92 982 243780 4535 65177 MW11-24 136312 189089 14052 40820 3094 248 2565 121 397 53.9 111 8.17 42.4 4.22 1439 184068 3900 43658 MW11-25 128308 183106 14339 43132 3672 256 2857 157 526 72.1 148 11.5 51.3 5.60 1942 188850 8382 46737 MW11-26 126226 185572 14703 44785 4464 271 3364 219 812 112 220 17.7 78.4 8.31 2975 180843 6387 40228 MW11-27 128463 176009 13386 38839 2958 244 2407 110 340 45.9 95.0 7.10 34.0 3.81 1213 207930 5238 50059 MW11-28 129218 181035 14100 41784 3095 255 2396 108 322 42.5 87.8 6.66 29.6 3.34 1142 197678 2753 47381 MW11-30 130004 180582 13751 41552 3166 246 2497 114 352 46.0 94.4 7.12 34.7 3.33 1237 195694 3999 45829 MW11-32 126189 182886 14535 44930 3789 252 2955 160 555 77.4 154 12.3 56.2 5.95 2012 177606 5946 42733 MW11-34 118846 170988 13533 41324 3394 245 2608 136 435 58.5 119 9.45 37.5 4.34 1566 200061 3514 60236 MW11-35 126117 178981 13751 41292 3120 255 2497 115 359 48.9 97.9 7.20 35.6 3.63 1295 194798 5650 47697 MW11-36 126109 183416 14810 48039 4040 255 2959 152 472 61.3 120 9.08 38.9 4.05 1556 174154 2157 42681 MW11-37 126057 180966 14600 45086 3688 249 2822 139 434 58.1 113 8.52 36.3 3.55 1490 178861 2757 45309 MW11-38 128255 179239 13777 40359 3255 252 2675 135 480 68.4 137 11.2 47.9 5.27 1786 180591 6144 43495 MW11-39 126166 183025 14320 43528 3707 257 2909 160 548 76.9 149 11.8 50.1 5.73 2044 175177 6068 41464 MW11-40 131049 177956 13209 38550 2909 232 2433 108 343 46.9 97.2 7.59 36.5 3.94 1258 187657 4194 45326 MW11-29 128500 180131 14261 43697 3424 241 2591 123 380 51.0 104 7.40 34.5 3.65 1348 191806 3201 48000 样品编号 元素含量(μg/g) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Th U Si MW1-1 158157 212653 15817 44459 3476 159 3116 146 445 59.4 115 8.86 36.1 4.02 1633 166645 3043 36883 MW1-5 123671 173235 13143 39498 2992 207 2474 115 350 47.4 96.8 7.53 32.1 3.50 1194 262630 7005 66654 MW1-7 156116 202450 14113 38157 2734 153 2721 113 358 47.1 99.2 7.67 33.4 3.31 1345 191746 5815 46068 MW1-8 170321 226809 16461 47368 3828 170 3456 173 539 72.1 137 10.1 42.8 4.50 1855 133410 2873 28844 MW1-9 149310 205327 15638 46605 3903 180 3248 168 543 69.5 136 10.4 44.0 4.18 1817 181982 3623 42624 MW1-10 129912 183044 14344 42810 3291 218 2623 125 382 51.6 106 8.17 34.5 3.79 1334 242641 3159 55240 MW1-12 127926 174960 13094 37233 2711 215 2326 99.3 302 40.7 83.1 6.35 28.2 3.19 1066 252773 9255 66267 MW1-14 129500 180618 13711 40271 2985 209 2443 113 344 47.5 94.4 7.94 36.9 4.69 1211 240269 4057 57984 MW1-18 165035 222645 15924 44681 3452 158 3143 145 454 60.8 119 8.75 39.0 3.99 1623 139956 3108 29700 MW1-21 121663 170826 12812 36653 2650 214 2237 100 311 42.8 86.2 7.09 30.2 3.12 1111 263686 10469 66797 MW1-22 135213 186713 14368 42543 3259 223 2590 117 353 45.7 94.3 6.77 30.5 2.86 1184 226722 3006 54088 MW1-23 128995 185314 14897 45653 3949 188 3060 171 561 76.1 145 10.9 46.3 4.84 1969 224741 3177 58256 MW1-25 133317 184376 14544 43366 3269 220 2615 123 379 52.1 103 7.82 34.9 3.41 1311 230108 2332 54878 MW1-26 168153 221184 15725 44612 3569 153 3163 149 439 56.8 107 7.57 31.2 3.38 1569 144067 1911 35698 MW1-29 120654 166938 13169 39012 3006 215 2364 111 348 46.8 94.5 6.89 31.3 3.49 1178 268982 2672 68041 MW1-30 164943 215486 15035 40775 2993 145 2873 128 389 51.0 101 7.42 31.5 3.02 1423 156794 5440 36654 MW1-6 169915 257423 20702 63531 5832 146 4160 211 605 73.9 137 9.41 39.2 3.80 1978 69266 3613 14800 MW1-27 189722 268973 20096 57313 4524 262 3735 189 590 76.3 145 9.28 39.7 3.84 1938 34270 4206 8600
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