| 河北武安坦岭多斑斜长斑岩的成因:冻结岩浆房活化机制 |
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| 引用本文： | 刘璐璐,苏尚国,侯建光,谢玉淳.河北武安坦岭多斑斜长斑岩的成因:冻结岩浆房活化机制[J].岩石学报,2017,33(1):204-220. |
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| 作者姓名： | 刘璐璐 苏尚国 侯建光 谢玉淳 |
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| 作者单位： | 中国地质大学地球科学与资源学院, 北京 100083,中国地质大学地球科学与资源学院, 北京 100083,中国黄金集团资源有限公司, 北京 100011,中国地质大学地球科学与资源学院, 北京 100083 |
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| 基金项目： | 本文受中国地质调查局项目（12120115069701）、国家自然科学基金项目（41272105）和教育部博士学科点基金联合资助. |
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| 摘 要： | 流变学实验表明,当岩浆中晶体体积分数达到约50vol%时,岩浆体实际上处于冻结状态,不再具有整体迁移的能力。但在自然界中仍存在含大量斑晶的浅成火成岩和火山岩。因此,富晶体岩浆的上升过程和侵位机制是近年来地球科学领域关注的热点之一。目前,冻结岩浆房的活化机制主要有二种:升温活化机制和流体活化机制。河北武安坦岭地区新发现的多斑斜长斑岩为揭示冻结岩浆房的活化提供了契机。野外观察和晶体粒度分布(CSD)分析表明,坦岭斜长斑岩中斜长石斑晶高达70vol%,基质为显微晶质结构。斜长石斑晶粒径分布均一,大小约为3.1×1.7mm;显微镜观察和背散射图像揭示,斜长石斑晶具环带结构,由宽广的斜长石核部+宽度可变的条纹长石边部组成,且无熔蚀现象;电子探针成分剖面分析表明,斑晶核部成分为更长石(An_(27)Ab_(71)Or_2),幔部为更长石(An_(13)Ab_(83)Or_4),边部为条纹长石。边部条纹长石的成分有一定变化,从内侧到外侧,主晶钠长石成分由Ab_(53)Or_(47)变为Ab_(99)Or_1,客晶钾长石成分由Ab_(48)Or_(51)变为Ab3Or97。斑晶斜长石核部存在细长条状或斑点状钾长石,且越靠近中心,钾长石斑点的数量越少。这些特点表明,边部条纹长石为交代成因。稀土和微量元素分析则显示,边部条纹长石具弱正Eu异常,相对富集LREE和K、Rb、Ba、Sr等大离子亲石元素,亏损Th、Zr、Nb的特点。CSD相关图解及以上特征表明,斜长石斑晶形成于稳定,封闭的结晶环境,并受到晚期碱交代作用的改造。基质主要由微粒钙质角闪石,条纹长石,石英,钾长石和钠长石组成,含少量自形-半自形磁铁矿和钛铁矿、磷灰石、榍石、金红石和锆石等11种矿物组成。11种矿物相和结构特征暗示基质形成于极端不稳定的结晶环境,与斜长石斑晶形成条件鲜明对照。根据基质的矿物组成,推测形成基质的岩浆具有富含K、Na、Fe、Si和挥发分的特征。这种特征与上述关于条纹长石环边形成条件的判断一致。据此,本文认为:产生斜长石斑晶的岩浆曾经在地壳深部作过长时间滞留,导致了斜长石的稳定结晶,增加了岩浆的粘度和密度,使岩浆处于冻结状态;富碱高铁熔体-流体流的注入大幅降低了岩浆的总粘度,并提高了岩浆的浮力,从而促使冻结岩浆房迅速活化和上升侵位;同时,富碱高铁熔体-流体流强烈交代了先存的斜长石斑晶,使其边部形成条纹长石;这种熔体-流体流则在快速排气,冷却过程中迅速结晶,形成了具有不平衡矿物组合的显微晶质基质。在岩浆侵入体较深部位,富碱高铁熔体-流体经历了很缓慢的固结过程,而相分离产生的流体有可能萃取携带岩浆中的铁质,形成富Fe流体流,后者可能对区内"铁矿浆"型铁矿的形成具有重要的贡献。
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| 关 键 词： | 河北武安 多斑斜长斑岩 晶体粒度分析 碱交代 冻结岩浆房活化 |
| 收稿时间： | 2016/5/31 0:00:00 |
| 修稿时间： | 2016/9/10 0:00:00 |
Genesis of Tanling plagioporphyry in Wuan, Hebei: The remobilizing mechanism of frozen magma chambers |
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| LIU LuLu,SU ShangGuo,HOU JianGuang and XIE YuChun.Genesis of Tanling plagioporphyry in Wuan, Hebei: The remobilizing mechanism of frozen magma chambers[J].Acta Petrologica Sinica,2017,33(1):204-220. |
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| Authors: | LIU LuLu SU ShangGuo HOU JianGuang and XIE YuChun |
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| Institution: | School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China,School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China,China Gold Group Resources Co., Ltd, Beijing 100011, China and School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China |
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| Abstract: | Rheological experiments show that crystal-rich magma ("crystal mushes") will remain stable when the content of crystal is up to 50vol%. However, crystal-rich hypabyssal rocks and volcanic rocks are founded in the world. So the emplacement mechanism of crystal mush''s ascending is one of the hot spot in earth sciences. There are two models about the remobilizing mechanism of frozen chambers:(1) thermal remobilizing mechanism; and (2) fluid-induced remobilizing mechanism. Plagioporphyry, which was founded recently in Tanling Wu''an, presents a new example for the study on Remobilizing Mechanism of Frozen Chambers (RMFC). Field observations and the analysis of crystal size distribution (CSD) show that Tanling plagioporphyry contains up to 70vol% plagioclase phenocrysts, and the size of phenocryst is~3.1×1.7mm. The size of groundmass in plagioporphyry is very small (0.05~0.30mm), and it can be seen under microscope. Core content of plagioclase phenocrysts:the central part content of plagioclase phenocryst is An27Ab71Or2; the middle part content of plagioclase phenocryst is An13Ab83Or4, the rim of plagioclase phenocrysts consists of perthite, the composition of host crystal is Ab53Or47-Ab99Or1, and the content of guest crystal is Ab48Or51-Ab3Or97. The spots amount of potassium feldspar decreases from the mantle to center of plagioclase phenocryst. The LA-ICP-MS trace element analysis reveals that the perthite rim of plagioclase phenocryst has weak positive Eu anomaly, much richer in K, Rb, Ba, Sr and poorer in Th, Nb and Zr. The relevant diagrams of CSD and all characteristics suggest that plagioclase phenocrysts have crystallized in a stable and closed environment. The minerals in groundmass consist of calcic amphibole, perthite, quartz, potassium feldspar, magnetite, ilmenite, apatite, sphene, rutile and zircon. Therefore, it is easy to judge that magma of groundmass was rich in K, Na, Fe, Si and volatile, which was identical with the forming condition of reaction rim. These features show that the perthite maybe formed by metasomation. We think that a fluid rich in alkali and iron injected into crystal-rich chamber, decreased the density of magma (crystal mushes) and accelerated the rapid reactivation of frozen magma. |
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| Keywords: | Wuan Hebei Poly-phenocryst plagioporphyry Crystal size analysis Alkali-metasomatism Remobilizing mechanism of frozen magma chambers |
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