| 超级喷发(超级侵入)后成矿作用 |
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| 引用本文： | 罗照华,周久龙,黑慧欣,刘翠,苏尚国.超级喷发(超级侵入)后成矿作用[J].岩石学报,2014,30(11):3131-3154. |
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| 作者姓名： | 罗照华 周久龙 黑慧欣 刘翠 苏尚国 |
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| 作者单位： | 中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083;中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083;中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083;中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083;中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083 |
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| 基金项目： | 本文受中国地质调查局地质调查项目(1212011220921、1212011121266、12120113094100、1212011121075、12120114020901)、973项目(2011CB808901)和中俄国际合作项目(RFBR14-05-91162-NSFC)联合资助. |
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| 摘 要： | 本文仿照超级喷发的概念定义了超级侵入,并将超级火山对应于大型岩基.文章聚焦于这样一个科学问题:为什么大规模成矿作用发生在紧接着超级喷发和超级侵入之后？为此,首先探讨了峨嵋山地幔柱系统的活动规律.尽管少数学者对玄武质岩浆大规模喷出之前的千米级地壳隆升提出了质疑,峨嵋山火山岩系第一旋回底部玄武岩直接覆盖在喀斯特之上的新观察支持千米级隆升的认识.这表明,峨嵋山地幔柱快速上涌之初期,岩石圈子系统在相当长一段时间没有作出伸展响应,尽管局部已经发生了地壳岩石的部分熔融.因此,岩浆通道形成之后,首先喷出了巨厚层玄武岩,并且后者裹挟了部分长英质岩浆.此后,岩浆喷发的规模振荡性减小,直至消失和地表沉降.斜长石巨斑玄武岩和苦橄岩中橄榄石斑晶与基质间的不平衡表明这些晶体属于循环晶,暗示岩浆曾经在深部岩浆房滞留了相当长的时间,这将导致岩石圈受热膨胀和再次隆升以及岩浆的冻结.因此,下一阶段岩浆活动的开始要求有一个冻结岩浆房的活化机制.依据野外地质学和岩相学观察,文章详细描述了流体活化机制,并强调了提出这种机制的必要性.虽然多数作者偏好升温活化机制,流体活化机制对长英质和镁铁质岩浆成矿系统都是必需的.进而,结合地幔名义无水矿物的H2O丰度及其对岩浆产生过程的贡献,提出岩浆产量与减压速率正相关而与流体产量反相关的观点.尽管水流体可以有效降低地幔橄榄岩的固相线温度从而有可能提高岩浆产量,新生代玄武岩中橄榄岩包体依然含有未分解的角闪石和云母且名义无水矿物依然含有较多的H2O,表明快速减压条件下含水暗色矿物的分解反应和名义无水矿物的脱水作用都是低效的.将这种认识与峨嵋山地幔柱系统的振荡性运动结合在一起,结合成矿作用的基本解是成矿金属从流体中析出的认识,可以得出超大型矿床必然形成于超级喷发和超级侵入之后.攀枝花式铁矿的观察表明,两类代表性矿床都具有铁矿浆侵位发生在成矿系统演化最后阶段的特点.因此得出结论:超大型矿床的形成取决于岩浆通道向流体通道的转换.如果岩浆通道在尚未完全封闭之前被含矿流体所利用,大规模流体快速上升将产生超大型矿床.含矿流体透过残留于通道中的熔体上升,不仅冲刷通道中的残留熔体并使其聚集在火山岩系之下或侵位于其下部形成含矿小岩体,而且持续注入于小岩浆体中的含矿流体可以导致岩浆强烈分异形成层状岩体.当通道中残留熔体被消耗殆尽,沿着通道上升的只有含矿流体.这些含矿流体充填在自生长裂隙中并强烈排气,最终可形成矿浆型富矿体.考虑到通道的规模与关闭速率的关系,推测超级喷发/侵入发生时的岩浆主通道更容易转换为含矿流体通道,因而是圈定找矿靶区的首选目标.该模型似乎与观察结果相吻合,并可与岩浆成矿系统的复杂性、小岩体成大矿理论、透岩浆流体成矿理论和通道成矿假说有机地结合在一起,较合理解释了超级喷发/侵入后成矿作用的地球动力学背景和成矿过程.由于长英质和镁铁质岩浆系统中均可见岩基,我们建议将这类成矿作用统称为岩基后成矿作用.
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| 关 键 词： | 岩基后成矿作用 熔体-流体强相互作用 透岩浆流体 岩浆型铁矿 峨嵋山地幔柱 四川攀枝花 |
| 收稿时间： | 2014/2/28 0:00:00 |
| 修稿时间： | 2014/6/15 0:00:00 |
Post-supereruption (-superintrusion) metallogenesis |
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| LUO ZhaoHu,ZHOU JiuLong,HEI HuiXin,LIU Cui and SU ShangGuo.Post-supereruption (-superintrusion) metallogenesis[J].Acta Petrologica Sinica,2014,30(11):3131-3154. |
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| Authors: | LUO ZhaoHu ZHOU JiuLong HEI HuiXin LIU Cui and SU ShangGuo |
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| Institution: | State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China |
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| Abstract: | Following the idea for proposing supereruption, a term to describe processes building up supervolcanos, we provide, in this article, a new term called as superintrusion to describe processes assembling batholiths. The article has a principal focus of why the giant ore deposits have formed immediately after supereruption or superintrusion. Accordingly, the behaviors of the Emeishan mantle plume system are discussed, firstly. Though a few authors query that a kilometre-scale domal uplift took place before onset of volcanism in the large igneous province, the new observation of that base basalt of the first cycle in the Emeishan volcanic series fills immediately in the ancient karst supports the view point about kilometre-scale domal uplift. This suggests that the lithosphere subsystem bas not an extending response for significantly long time when the Emeishan plume is rapidly ascending, despite of that the crust may be partially melted in some depth levels. Therefore, after opening the magma conduits, the plume system firstly blew out a significantly thick stratum of basaltic lava, and the later wraps some felsic lavas. Then, the volcanic eruption is characterized by oscillatory decrease of its lava volume, even till to disappear, accompanied by surface subsidence. The giant plagioclase phenocrysts in basalts and the un-equilibrium relationships between the olivine phenocrysts and the host matrix in the so-called picrite show that these crystals are of antecrysts instead of that crystallized from the host lava. This suggests that the primary magma in which such crystals have grown was detained in the deep chamber for a long period. Such retention should induce the lithosphere to dilate due to conductive heating and the surface to swell up again and hence the retained magma should be frozen. Subsequently, onset of the next stage of magmatism requires a reasonable mechanism for rejuvenation and reactivation of the frozen chamber. According to the field and petrographical observations, the fluid reactivation mechanism is described in details, and the necessity of proposing such mechanism is emphasized. Although most authors incline to the heating reactivation mechanism, the fluid reactivation mechanism is necessary for both felsic and mafic magmatic mineral system. Furthermore, combining the estimated H2O concentrations of the mantle nominal anhydrate minerals and their contributions to magma production, it is discovered that the volume of magma production is positively relative to the depression rate of mantle sources, but negatively relative to the volume of fluid production. As well-known, the aqueous fluid may efficiently decrease the solidus temperature of mantle peridotite, and hence could increase the volume of magma production. However, in a condition of rapid upwelling of mantle source, for instance, in the mantle plume condition, the aqueous fluid could not be added in large amount to the magma system. The facts that the un-decomposed amphibole and mica remain in the peridotite xenoliths in the Cenozoic basalts and the nominal anhydrate minerals in such xenoliths contain more H2O suggest that the decomposition reaction of the water-bearing minerals and the dehydration of the nominal minerals are of lower effective. Accordingly, combining the oscillatory movement of the Emeishan plume system and that the essential process of metallogenesis is the separation of the ore-forming metals from the fluids, it can be concluded that the giant ore deposits must be formed immediately behind the supereruption or superintrusion. The observations in the Panzhihua-style iron deposits display that the two representative types of iron deposit show an identical feature: the iron magma emplaces in the last stage of the history of mineral systems. Subsequently, it is concluded that formation of a giant ore deposit is decided by changing of the magma conduits to the fluid conduits. If the conduit is used by the ore-bearing fluid before entirely closing, the ore-bearing fluid would rapidly upwell in large-scale, and produces the giant ore deposits. Percolating through the magmas remained in the conduits and ascending, the ore-bearing fluid not only washes out the magmas in conduits and induces them to form the ore-bearing minor intrusions under and in the lower part of the volcanic strata, but its continuous injection causes violent differentiation to generate layered intrusions. If the remained magmas in the conduits are completely consumed and the conduits are not closed, the upwelling materials along the conduits are only the ore-bearing fluid. Such fluid fills into the self-propagation fractures and experiences phase separation to produce iron-rich melt, solute-rich solution and vapour, the violet degassing ultimately induces to form enriched ore bodies of the so-called ore-magma type of deposit. Considering the relation between the conduit scale and its closing rate, it can be inferred that the major conduits used by supereruption/superintrusion are easy to be transformed to the fluid conduits, and hence they can be surveyed as the first destination in choice of the exploration targets. The model seems to be consistent with the observations, complexity of the magmatic mineral system, the theory of metallogenesis by small intrusion, the theory of metallogenesis by transmagmatic fluid, and the hypothesis of conduit, and gives a reasonable interpretation for the geodynamic background and ore-forming process of the supereruption/superintrusion metallogenesis. Due to the batholiths can be produced in both the mafic and felsic magmatic mineral systems, we recommend to use an identical term "post-batholith" metallogenesis. |
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| Keywords: | Post-batholith metallogenesis Strong melt-fluid interaction Transmagmatic fluid Magmatic iron deposit Emeishan mantle plume Panzhihua in Sichuan Province |
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