| 东南极拉斯曼丘陵硼硅酸盐矿物组合硅硼镁铝矿-硼柱晶石-电气石的形成过程及其岩石学意义 |
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| 引用本文： | 任留东,宗师,王彦斌,刘平.东南极拉斯曼丘陵硼硅酸盐矿物组合硅硼镁铝矿-硼柱晶石-电气石的形成过程及其岩石学意义[J].岩石学报,2021,37(2):575-588. |
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| 作者姓名： | 任留东 宗师 王彦斌 刘平 |
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| 作者单位： | 中国地质科学院地质研究所, 北京 100037;江苏地质矿产设计研究院(中国煤炭地质总局检测中心), 中国煤炭地质总局煤系矿产资源重点实验室, 徐州 221006 |
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| 基金项目： | 本文受国家自然科学基金项目（41472172、41530209、41941004）和中国地质调查项目（DD20190358）联合资助. |
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| 摘 要： | 东南极拉斯曼丘陵长英质片麻岩中产出大量的电气石-硼柱晶石-硅硼镁铝矿之硼硅酸盐矿物组合,这些矿物(电气石除外)的形成晚于变质峰期一般的硅酸盐矿物。电气石可多次出现,硅硼镁铝矿之后形成硼柱晶石,很少见两种以上的硼硅酸盐矿物能够同时结晶,各种硼硅酸盐矿物在同一期、甚至同一阶段内呈递进关系。在硼硅酸盐矿物的结晶过程中,B2O3和Al2O3较为活动从而SiO2的活度相对受到抑制,即存在组分的分异和活性波动,表明络阴离子SiO44-、PO43-、BO33-活动高峰并非同步因而,挥发分组分对深熔作用的影响可能是有限的;同时,结晶的金属阳离子组分不断发生分异。不同的硼硅酸盐矿物形成的介质条件有所差异:电气石形成于富钙的弱酸性溶液,硅硼镁铝矿应为近中-碱性溶液介质,而硼柱晶石形成于含少量氟的偏碱性介质环境。除温压因素外,流体挥发分种类和介质条件也影响到硼硅酸矿物的多期次、多阶段的变化,从而造成矿物组合的复杂性。本区长英质岩石中硼(B)及其他挥发分组分的较高含量可由原岩成分决定,亦可经由变质-深熔作用引起,即深熔作用时熔体的整体优先吸收硼(硼的第一次富集)及随后熔体结晶阶段的局部残留和富集硼(硼的第二次富集)。Gdd-Prs-Trn硼硅酸盐矿物组合的存在,表明发生了以脱水为主的高级变质作用,同时伴随强烈的深熔作用。挥发分组分在露头尺度体系中可能属于开放性质,而在更大尺度和范围内则基本封闭;在深熔作用中,硼等挥发分的存在影响了熔体的组成使得岩浆熔点降低、熔融成分调整粘度有所降低,更容易运移;而且,熔体可以携带这些挥发分,当熔体结晶时挥发分析出、结晶局部富集而形成硼硅酸盐矿物,尤其是沿着一些构造有利部位发生显著的聚集。
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| 关 键 词： | 硼硅酸盐矿物 挥发分 介质组成 变质演化 东南极拉斯曼丘陵 |
| 收稿时间： | 2020/8/20 0:00:00 |
| 修稿时间： | 2020/11/17 0:00:00 |
Formation process and petrological implication of the borosilicate assemblage grandidierite, prismatine and tourmaline in the high-grade quartzofeldspathic gneisses in the Larsemann Hills, East Antarctica |
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| REN LiuDong,ZONG Shi,WANG YanBin,LIU Ping.Formation process and petrological implication of the borosilicate assemblage grandidierite, prismatine and tourmaline in the high-grade quartzofeldspathic gneisses in the Larsemann Hills, East Antarctica[J].Acta Petrologica Sinica,2021,37(2):575-588. |
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| Authors: | REN LiuDong ZONG Shi WANG YanBin LIU Ping |
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| Institution: | Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;Jiangsu Design Institute of Geology for Mineral Resources(the Testing Center of China National Administration of Coal Geology(CNACG)), Key Laboratory of Mineral Resources in Coal Measures, CNACG, Xuzhou 221006, China |
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| Abstract: | Borosilicate minerals, tourmaline, grandidierite and prismatine are present in the high-grade quartzofeldspathic gneisses in the Larsemann Hills, East Antarctica. The borosilicates (except tourmaline) generally don''t crystallize contemporaneously with the common rock-forming silicate minerals. Several generations of tourmaline have been discerned. As to the specific grandidierite and prismatine, rarely can be observed that two or more borosilicate minerals formed at the same time, and the common pattern is the progressive evolution between the borosilicates. In the crystallization of the borosilicates, the activity of SiO2 was constrained while that of the B2O3 and Al2O3 were enhanced, suggesting the component differentiation and fluctuation and the dissynchronization of the complex anions SiO44-, BO33-, and PO43- activity. Thus the effect of volatile components to anatexis may be limited. The media or solution condition of different borosilicate minerals may vary, i.e., tourmaline crystallized in the Ca-rich and slight acid solution, grandidierite in the near neutral to alkalescent, and prismatine in the F-bearing alkaline media. Meanwhile, the crystallizing metal cations may differentiate successively. Besides the pressure and temperature factors, the multi-stages or generations may also be controlled by the volatile components and the media compositions. This was finally responsible for the complexity of the mineral associations in metamorphism. The contents of boron and other volatile components in the high-grade quartzofeldspathic gneisses of the area can be inherited from the protoliths or derived from metamorphism-anatexis process, in which the components could be preferentially absorbed in the melt (the first enrichment of boron), and again enriched in the residues during cooling and crystallization of the melt (the second enrichment of boron). The occurrence of the borosilicates demonstrates the existence of the high-grade metamorphism with dehydration partial melting. The volatile components were in open system on the outcrops scale, but nearly closed on the wider system. In anatexis, the existence of the B, F can affect the composition of the melt, decreasing the melting point and viscosity. On the other hand, the volatiles carried in the melt could be exsolved in the cooling and concentrated along the structurally favorable positions. |
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| Keywords: | Borosilicate Volatile components Media composition Metamorphism evolution Larsemann Hills East Antarctica |
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