| 大兴安岭南段北大山含锡石与不含锡石花岗岩特征对比 |
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| 引用本文： | 丁磊磊,毛启贵,王玉往,管育春,李婷婷.大兴安岭南段北大山含锡石与不含锡石花岗岩特征对比[J].地球科学,2022,47(9):3371-3388. |
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| 作者姓名： | 丁磊磊 毛启贵 王玉往 管育春 李婷婷 |
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| 作者单位： | 1.北京矿产地质研究院有限责任公司, 北京 100012 |
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| 基金项目： | 中国地质调查局国家二级项目DD20190815 |
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| 摘 要： | 近年来，人们在大兴安岭南部发现了多处与白垩纪花岗岩相关的锡多金属矿床，但并非所有该时期的花岗岩都与锡矿伴生.为了解花岗岩伴生锡矿的形成条件，本文对内蒙古北大山岩体中的含锡石（磨盘山）与不含锡石（窟窿山）花岗岩开展了锡石U-Pb年龄、锆石U-Pb年龄、全岩地球化学及矿物地球化学分析测试，并进行岩石学、年代学和岩浆演化物理化学条件的对比.结果表明，窟窿山石英正长斑岩和磨盘山黑云母花岗岩的锆石U-Pb加权平均年龄分别为140.2±0.7 Ma和139.9±0.7 Ma，磨盘山黑云母花岗岩锡石U-Pb加权平均年龄为134.9±1.4 Ma，时代均属于早白垩世.全岩地球化学分析结果表明，这些岩石具有高硅（SiO2=64.96%~76.71%），富碱（Na2O+K2O=8.28%~9.03%），过铝质（Al2O3=12.42%~15.88%）的特点.相对富集Th、Pb、Hf等元素，亏损Nb、Ta、Ti、Sr、P等元素.轻稀土相对重稀土明显富集，Eu异常明显.窟窿山石英正长斑岩与磨盘山黑云母花岗岩在哈克图解上，随着SiO2含量增加，TiO2、FeOT、Al2O3、CaO、Na2O、P2O5含量逐渐降低，呈现较好的负相关关系，暗示二者具有同源性.但窟窿山样品的DI值为89，Sn含量为2×10?6；磨盘山样品具有较高的DI值（96~98），Sn含量高（15×10?6~36×10?6）.从矿物学特征反演的物理化学条件推测，窟窿山岩体经历了温度降低、氧逸度升高的过程，而磨盘山岩体在降温过程中氧逸度进一步降低.另外，在流体卤素含量上，黑云母的Ⅳ（F）、Ⅳ（Cl）指示磨盘山花岗岩（Ⅳ（F）=0.95~1.15，Ⅳ（Cl）=-3.66~-3.54）相较于窟窿山石英正长斑岩（Ⅳ（F）=1.24~1.28，Ⅳ（Cl）=-2.96~-2.52）具有更高富集程度的Cl和F含量.综上，低氧逸度、高演化程度和高的F、Cl丰度是影响北大山地区花岗岩是否含锡的重要原因.
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| 关 键 词： | 大兴安岭 白垩纪 花岗岩 锡 锆石 矿物地球化学 物理化学条件 岩石学 |
| 收稿时间： | 2021-11-29 |
Comparison on the Characteristics of Cassiterite-Bearing and Barren Granites in the Beidashan Region,Southern Great Xing'an Range |
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| Abstract: | Recently, many tin polymetallic deposits related to Cretaceous granite have been discovered in the southern part of the Great Xing'an Range. However, not all granites of this period are associated with tin deposits. To obtain a better understanding of the formation conditions of granite associated tin deposits, (Method) in this paper, the cassiterite U-Pb age, zircon U-Pb age, whole rock geochemistry and mineral chemistry analysis of the cassiterite-bearing granites (the Mopanshan area) and barren granites (the Kulongshan area) in the Beidashan pluton were carried out. The geochronology, geochemistry and physical-chemical conditions of magma evolution were compared. The U-Pb weighted average zircon ages of the Kulongshan porphyritic quartz syenite and the Mopanshan biotite granite are 140.2±0.7 Ma and 139.9±0.7 Ma, respectively, and the cassiterite U-Pb weighted average age of the Mopanshan area is 134.9±1.4 Ma. All of them are Early Cretaceous in age. The whole rock geochemical analysis results show that these rocks are rich in silicon (SiO2=64.96%-76.71%), alkali (Na2O+ K2O=8.28%-9.03%), and aluminum (Al2O3=12.42%-15.88%). For trace elements, they are relatively enriched in elements such as Th, Pb, and Hf, and deficient in elements such as Nb, Ta, Ti, Sr, and P. For rare earth elements, they show significant enriched HREEs than LREEs, and Eu negative anomaly is unusually obvious. As the content of SiO2 increases, the content of TiO2, FeOT, Al2O3, CaO, Na2O, and P2O5 gradually decreases, showing a good negative correlation, suggesting a homogeneous magmatic source. However, the DI value of the Kulongshan sample is 89 and the Sn content is 2×10-6; the Mopanshan samples have a higher DI value (96-98) and higher Sn content (15×10-6~36×10-6). Based on the physical- chemical conditions inverted from mineralogy, the Kulongshan granite has experienced a process of oxygen fugacity increase during cooling, while the oxygen fugacity of the Mopanshan granite has further decreased during the cooling process. In addition, in terms of fluid halogen content, the Ⅳ (F) and Ⅳ (Cl) of biotite indicate the Mopanshan granite (Ⅳ(F)=0.95-1.15; Ⅳ(Cl)=-3.66--3.54) compared to the Kulongshan porphyritic quartz syenite (Ⅳ(F)=1.24-1.28; Ⅳ(Cl)=-2.96--2.52) has a higher concentration of Cl and F. In summary, low oxygen fugacity, high degree of evolution, and high F and Cl abundance, are the main factors affecting enrichment of cassiterite in granite in the Beidashan area. |
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