河北武安坦岭多斑斜长斑岩的成因:冻结岩浆房活化机制

发布时间：2018-11-07 14:11

【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。
[Abstract]:Rheological experiments show that when the volume fraction of crystals in the magma reaches about 50 vols, the magmatic body is in a frozen state and no longer has the ability to migrate as a whole. But in nature, there are still a lot of porphyry-bearing epigenetic igneous rocks and volcanic rocks. Therefore, the ascending process and emplacement mechanism of rich-crystal magma are one of the hot topics in the field of earth science in recent years. At present, there are two main activation mechanisms of frozen magma chamber: heating activation mechanism and fluid activation mechanism. The newly discovered multi-plagiocline porphyry provides an opportunity to reveal the activation of frozen magma chamber. Field observation and (CSD) analysis of crystal grain size distribution show that the plagioclase porphyry in the Tan-ling plagioclase is as high as 70 vols and the matrix is micrystalline structure. The particle size distribution of plagioclase porphyry is uniform and the size is about 3.1 脳 1.7 mm. Microscopic observation and backscatter images show that plagioclase porphyry has a ring structure, which is composed of striped feldspar edges with variable width of the broad plagioclase core, and has no melting phenomenon. The electron microprobe (EPMA) profile analysis shows that the composition of the macular nucleus is more feldspar (An_ (27) Ab_ (71) Or_2), the mantle is An_ (13) Ab_ (83) Or_4, and the edge is striped feldspar. The composition of edge stripe feldspar changed to a certain extent, from inside to outside, the composition of main crystal albite changed from Ab_ (53) Or_ (47) to Ab_ (99) Or_1, the composition of potassium feldspar changed from Ab_ (48) Or_ (51) to Ab3Or97.. In the core of porphyry plagioclase, there are fine stripe or spotted potash feldspar, and the more close to the center, the less the number of potash feldspar spots. These characteristics indicate that the marginal stripe feldspar is the origin of metasomatism. The analysis of rare earth elements and trace elements showed that the fringed feldspar had weak positive Eu anomaly, relatively enriched in LREE and K\ + Rb\ +\ {} b\} ~ (2 +)\%, etc., and depleted the characteristics of Th,Zr,Nb. The correlation diagram of CSD and the above characteristics showed that, Plagioclase porphyry was formed in a stable and closed crystalline environment and was modified by late alkali metasomatism. The matrix is mainly composed of microcalcareous amphibole, striped feldspar, quartz, potassium feldspar and albite, and contains a small amount of magnetite and ilmenite, apatite, sphene. 11 mineral compositions, such as rutile and zircon, indicate that the matrix was formed in an extremely unstable crystalline environment, in contrast to the porphyry formation conditions of plagioclase. Based on the mineral composition of the matrix, it is assumed that the magma forming the matrix is rich in K ~ (2 +) Na ~ (2 +) Fe ~ (2 +) Si and volatile compounds. This characteristic is consistent with the above judgment on the formation condition of the fringed feldspar ring. Based on this, it is concluded that the magma that produced plagioclase porphyry has been stuck in the deep crust for too long, which leads to the stable crystallization of plagioclase, increases the viscosity and density of magma, and makes the magma freeze; The injection of alkali-rich high iron melt-fluid flow has greatly reduced the total viscosity of magma and increased the buoyancy of magma, thus promoting the rapid activation and emplacement of frozen magma chamber. At the same time, the alkali-rich high iron melt-fluid flow strongly replaced the preexisting plagioclase porphyry, resulting in the formation of striped feldspar at the edge of the plagioclase. The melt-fluid flow is rapidly exhaled and crystallized rapidly during cooling, resulting in a micrystalline matrix with unbalanced mineral assemblage. In the deeper part of the magmatic intrusion, the alkali-rich high iron melt-fluid experienced a very slow consolidation process, while the fluid produced by phase separation may extract the iron from the carrying magma to form the Fe rich fluid. The latter may play an important role in the formation of "iron ore" type iron ore in the region.
【作者单位】：中国地质大学地球科学与资源学院;中国黄金集团资源有限公司;
【基金】：中国地质调查局项目(12120115069701) 国家自然科学基金项目(41272105) 教育部博士学科点基金联合资助
【分类号】：P588.13

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