| 云南卓潘碱性杂岩体岩浆演化与稀土富集机制——来自磷灰石地球化学的证据 |
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| 引用本文：马飞,王蝶,丁志丹,刘俊鹏,田昊丰.2024.云南卓潘碱性杂岩体岩浆演化与稀土富集机制——来自磷灰石地球化学的证据[J].地球学报,45(4):547-560. |
| DOI：10.3975/cagsb.2024.052301 |
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| 基金项目:国家自然科学基金项目(编号: 41963003);云南省科技计划项目(编号: 202202AG050006) |
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| 中文摘要:云南卓潘碱性杂岩体具有较高的稀土元素含量(约1 319×10–6), 具有较高程度的稀土元素找矿前景, 然而, 对于卓潘碱性岩的岩浆演化过程和稀土元素富集机制有待深入研究。本研究选取卓潘岩体中的出露岩性(正长辉石岩和辉石正长岩), 对其中主要的富稀土矿物磷灰石进行主、微量测试, 深入剖析磷灰石的地球化学特征, 旨在探讨岩浆演化过程和稀土元素富集机制。电子探针(EPMA)和激光剥蚀等离子质谱(LA-ICP-MS)分析结果表明, 正长辉石岩和辉石正长岩中磷灰石均为氟磷灰石, F含量分别为2.92%和3.31%; 磷灰石稀土元素含量存在差异, 平均值分别为12 288×10–6和16 503×10–6; 稀土模式配分图均表现出富集轻稀土元素, 强烈亏损重稀土元素特征。据磷灰石Mn温度计估算结果结合SO3含量的相对高低, 正长辉石岩氧逸度相对低于辉石正长岩; 磷灰石中高的Ce/Pb和低的Th/U特征, 表明岩浆演化过程中流体活动性较强, 并且前者流体活动性高于后者; 磷灰石中的挥发分(F、Cl)含量及其比值特征指示岩浆源区受到大洋俯冲相关的交代作用。基于上述研究结果, 卓潘碱性杂岩体稀土元素的富集过程: (1)18~10 km岩浆房阶段, 由于硅酸盐矿物的大量结晶, 岩浆房中磷酸盐含量逐渐饱和并结晶生成大量磷灰石, 此时岩浆体系具有较高流体活动性和较低的挥发分F, 导致正长辉石岩中结晶的磷灰石稀土元素富集程度较低; (2)~10 km岩浆房阶段, 由于早期磷灰石的大量结晶, 岩浆房中磷酸盐含量降低并生成少量磷灰石, 此时岩浆体系具有较低流体活动性和较高的挥发分F, 导致辉石正长岩中结晶的磷灰石稀土元素富集程度较高。因此, 卓潘碱性杂岩体中正长辉石岩和辉石正长岩均具有一定程度的稀土找矿前景。 |
| 中文关键词:磷灰石 主微量元素成分 稀土富集 卓潘碱性杂岩体 |
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| Rare-earth Enrichment Mechanism of Magmatic Evolution in the Zhuopan Alkaline Complex, Yunnan Province:Evidence from Apatite Geochemistry |
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| Abstract:The Zhuopan alkaline complex in Yunnan is characterized by a relatively high content of rare earth elements (REE) (approximately 1 319×10–6), showing high potential for REE exploration. However, the magmatic evolution process and the enrichment mechanism of REEs within the Zhuopan alkaline rocks warrant further investigation. This study selected the exposed lithology of the Zhuopan intrusion (syenite-pyroxenolite and gabbro-syenite), and conducted major and trace element tests on the main rare earth mineral apatite, delving into the geochemical characteristics of apatite, aiming to explore the magmatic evolution process and the mechanism of REE enrichment. The results from Electron Probe Microanalysis (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analyses indicated that the apatite found in both syenite-pyroxenolite and gabbro-syenite was fluorapatite, with fluorine contents of 2.92% and 3.31%, respectively. There were differences in the REE contents of apatite between the two rock types, with average values of 12 288×10–6 and 16 503×10–6, respectively. The REE distribution patterns for both types of apatite show an enrichment in light rare earth elements (LREEs) and a strong depletion in heavy rare earth elements (HREEs). The estimation results of the apatite Mn thermometer, combined with the relative levels of SO3 content, suggest that the oxygen fugacity in the syenite-pyroxenolite is relatively lower than that in the gabbro-syenite. The high Ce/Pb and low Th/U characteristics in apatite indicate a strong fluid activity during the magmatic evolution process, with the former exhibiting higher fluid activity than the latter. The content and ratio characteristics of volatiles (F, Cl) in apatite indicate that the magma source region was influenced by metasomatism associated with oceanic subduction. Based on the aforementioned research results, the enrichment process of rare earth elements in the Zhuopan alkaline complex can be delineated as follows: (1) During the magma chamber stage at depths of 18 to 10 kilometers, due to extensive crystallization of silicate minerals, the phosphate content in the magma chamber gradually saturates, leading to the crystallization of abundant apatite. At this point, the magma system exhibits higher fluid activity and lower volatile content, resulting in relatively lower enrichment of rare earth elements in the apatite crystallized within the syenite-pyroxenolite. (2) During the magma chamber stage at around 10 kilometers, due to the extensive crystallization of early apatite, the phosphate content decreases in the magma chamber, leading to the formation of a small amount of apatite. At this stage, the magma system exhibits lower fluid activity and higher volatile content, resulting in a higher enrichment of rare earth elements in the apatite crystallized within the gabbro-syenite. Therefore, both the syenite-pyroxenolite and gabbro-syenite in the Zhuopan alkaline complex show promising prospects for rare earth exploration to a certain extent. |
| keywords:apatite main and trace element composition rare earth enrichment Zhuopan alkaline complex |
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