Magmatic-Hydrothermal History of Bayan Obo REE-Rich Carbonatites Revealed by Monazite Th-Pb Geochronology of the Wu Dyke
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摘要: 越来越多的证据表明，白云鄂博超大型稀土矿床与区内分布的火成碳酸岩有密切的成因联系，因此精确约束碳酸岩的侵位时代对研究成矿历史和机制非常关键. 然而，由于后期复杂的热液叠加影响，以往的年代学研究结果大多得到后期扰动事件的年龄，而对碳酸岩岩浆活动的启动时间的制约则比较缺乏. 为重建白云鄂博碳酸岩型稀土矿的岩浆-热液成矿历史，我们利用SHRIMP离子探针对一号碳酸岩墙（吴氏岩墙）及其蚀变围岩样品开展了薄片原位独居石Th-Pb年代学研究. 碳酸岩中最老的独居石记录了其侵入时代1 401±39 Ma（MSWD=0.57，n=7），这是首次从白云鄂博的独居石得到碳酸岩初始岩浆年龄，证实白云鄂博碳酸岩可能代表了华北克拉通从Nuna超大陆裂解过程中早期岩浆作用产物. 同时，碳酸岩和霓长岩化围岩中的独居石还获得了格林威尔期到加里东期的一系列年龄数据，反映了后期构造热事件的叠加-扰动记录. 尤其是，本研究识别出泛非期构造事件的独居石年代学记录（529±17 Ma，MSWD=1.01，n=6）并对其地质意义进行了初步探讨.本文的研究结果和认识为理解白云鄂博碳酸岩型稀土矿的岩浆-热液历史及成矿构造动力学背景提供了新的关键证据，所采用的研究方法对解决类似碳酸岩和稀土矿的年代学问题具有启示意义.Abstract: Growing evidence suggests that mineralisation of the world-renowned Bayan Obo REE deposit is genetically linked to the intrusion of carbonatite dykes in Archean-Paleoproterozoic sedimentary rocks. Hence, accurate determination of the emplacement age of the carbonatite dykes is critically important for understanding the genesis and geodynamic drivers of the giant REE deposits. Dating carbonatites has proven challenging because they are commonly overprinted by later tectono-thermal events. Previous attempts to date the Bayan Obo carbonatites have yielded younger ages reflecting post-magmatic overprinting (typically 1 300 – 420 Ma). In this study, we collected samples from the No. 1 carbonatite dyke (Wu dyke) and its fenitised wall-rocks and carried out insitu (in polished thin section) U-Th-Pb dating of monazite by Sensitive High-Resolution Ion Microprobe (SHRIMP), which enables us to precisely reconstruct the magmatic and hydrothermal history of the dyke. Monazites in the calcite carbonatite, fenite and metasomatised quartz conglomerate are characterised by extreme depletion of U and high Th/U ratios. The oldest cluster of monazite analyses yielded a weighted mean 208Pb/232Th age of 1 401±39 Ma (MSWD=0.57, n=7), which is the oldest monazite age hitherto obtained from Bayan Obo, and is taken to approximate the onset of intrusion of the carbonatite dykes. This age confirms that the carbonatite magmatism at Bayan Obo predates the 1.33-1.30 Ga Yanliao Large Igneous Province, but coincides with the separation of North China Craton from the Western Australian Craton as part of the greater breakup of the Nuna/Columbia supercontinent (1 450-1 380 Ma). Our results also reveal repeated monazite growth during the late Mesoproterozoic through to the early Paleozoic (1 300-420 Ma) as widely documented in previous studies. Notably, our data define a prominent age peak at 529±17 Ma (MSWD=1.01, n=6) indicating Pan-African overprinting, supportive of recent reconstruction placing North China at the northern margin of Gondwana during the earliest Paleozoic. In addition, zircon crystals in the quartz conglomerate were also analysed in this study and yielded concordant 207Pb/206Pb dates between 2.1 Ga and 1.9 Ga, which are interpreted as detrital ages on the basis of crystal morphology and Th-U chemistry of the zircon. In summary, the new monazite geochronology data from the Wu dyke significantly refine the magmatic-hydrothermal timeframe of the Bayan Obo carbonatites, affording new insights into the geodynamic drivers and genesis of the giant REE deposits.
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Key words:
- Bayan Obo /
- REE deposit /
- Wu dyke /
- carbonatite /
- monazite /
- Th-Pb geochronology /
- geochemistry
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图 2 （a, b）白云鄂博矿区一号岩墙野外露头照片; （c~e）白云鄂博一号岩墙样品岩相学显微照片
c.碳酸岩，正交偏光；d. 霓长岩，正交偏光；e. 石英砾岩，砾间填隙矿物霓长岩化，单偏光; 矿物缩写：Aeg. 霓石，Afs. 碱性长石，Agt. 霓辉石，Arf. 钠铁闪石，Cal. 方解石，Qz. 石英
Fig. 2. Outcrop photos (a, b) and representative photomicrographs (c-e) of the Bayan Obo No.1 carbonatite dyke (Wu dyke)
图 3 薄片中的独居石背散射（BSE）图像，来自白云鄂博一号岩墙（Wu dyke）方解石碳酸岩（a）及其交代围岩霓长岩（b）和石英砾岩（c）
图中亮白色为独居石（Mnz），红色椭形圈为SHRIMP分析点位，旁边标注相应的Th-Pb年龄；（b）中可见独居石与重晶石（BSE亮度比独居石略低）共生；（c）中显示锆石（Zrn）与独居石共同出现在碎屑石英粒间蚀变区域；矿物缩写同图 2
Fig. 3. Backscattered electron (BSE) images of monazite (Mnz) in carbonatite (a), fenite (b), and quartz conglomerate (c) from the Bayan Obo No.1 carbonatite dyke
图 4 白云鄂博一号岩墙独居石Th-U含量及比值特征（a）及独居石Th/U与年龄和寄主岩性相关图（b）
Fig. 4. (a) Th-U contents and ratios of monazite from the Bayan Obo No. 1 carbonatite dyke (Wu dyke); (b) Correlation of monazite Th/U with age and host-rock lithology
图 5 白云鄂博一号岩墙独居石Th-Pb年代学结果分布图（a）和碎屑锆石年代学协和图（b）.
（b）中的BSE图像显示代表性锆石颗粒在薄片中的产出特征及SHRIMP分析点位和207Pb/206Pb年龄（1σ），协和图中空心圆代表的不协和数据点不参与年龄解释.
Fig. 5. (a) Results of monazite Th-Pb geochronology from the Bayan Obo No. 1 carbonatite dyke (Wu dyke). (b) Tera-Wasserburg U-Pb concordia diagram for detrital zircon in the quartz conglomerate
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