An Oxygen and Hydrogen Isotope Study of the Skaergaard Intrusion and its Country Rocks: a Description of a 55 M.Y. Old Fossil Hydrothermal System
An Oxygen and Hydrogen Isotope Study of the Skaergaard Intrusion and its Country Rocks: a Description of a 55 M.Y. Old Fossil Hydrothermal System
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DOI：
10.1093/petrology/20.3.355
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发表时间：
1979-08
影响因子：
3.9
通讯作者：
H. Taylor;R. W. Forester
中科院分区：
文献类型：
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作者：
H. Taylor;R. W. Forester
从Skaergaard层状辉长岩侵入体及其围岩的约400个样品中获得了岩石和共存矿物（主要是斜长石和单斜辉石）的氧同位素分析。斜长石的δ^（18）O值在侵入体中向上降低，从下部带和中部带部分地区的约+6.0到+6.4的“正常”值，降低到上部边界群的-2.4。斜长石的^（18）O亏损均发生在亚固相条件下，是由该岩体建立的始新世大气-热液体系产生的。单斜辉石比斜长石更耐^（18）O交换，也经历了^（18）O的亏损，但程度较轻（δ^（18）O = +5.2至+3.5）。贫^（18）O的岩石通常表现出反向的Δ^（18）Oplag−px分馏，但在上部带的顶部除外，那里的辉石是在铁硅灰石之后假像的非常细的颗粒聚集体;这些反向的辉石更容易进行亚固相^（18）O交换（δ^（18）O = +3-9至+0.7）。对绿泥石化玄武岩围岩和岩体中少量蚀变矿物（δD = −116至−149）的D/H分析证实了这些解释，表明岩石与原始δD为<$−100的大气降水相互作用。和δ^（18）O <$−14。在前寒武纪基底片麻岩的黑云母中也发现了低δD值（δ-125），这意味着少量的水向下渗透到至少6至10公里的深度。这些数值，加上片麻岩中没有^（18）O亏损，意味着该单元（400-500 °C）的总体水/岩比非常小。在流入辉长岩之前，这些流体与玄武岩围岩中的类似矿物组合交换，解释了辉长岩缺乏化学蚀变。顶板玄武岩捕虏体和上界群淡色辉长岩捕虏体在落入岩浆房底部并被纳入层状系列之前，热液系统强烈地耗尽了^（18）O。这证明热液系统在Skaergaard侵入体侵位时就已形成。然而，在辉长岩岩浆中没有检测到可测量的^（18）O亏损，这表明，尽管在整个结晶过程中，热液循环系统完全包裹了岩浆房至少10万年，但只有很少的H_2O直接渗入液态岩浆。只有在结晶过程中，热液系统才能向内塌陷，并与辉长岩的固化和断裂部分相互作用。尽管如此，一些H_2O显然是通过采出的蚀变顶板岩块脱水而直接加入到岩浆中的。少量的大气降水直接扩散到岩浆中也是合理的，最合乎逻辑的是在主要断裂带附近，这些断裂带渗透到靠近或位于后期分异的铁辉石岩浆片层之下。这些结果表明，这种大气沃茨的流入是边缘群中常见的辉长岩伟晶岩体的形成原因;此外，这种H_2O可能导致岩浆中Fe^（+3）/Fe^（+2）的局部增加，这反过来又可以解释岩浆房中的一些不对称结晶效应。液相线温度的局部降低也会发生，也许会导致岩浆房底部的地形不规则（例如槽带？）。
Oxygen isotope analyses have been obtained on rocks and coexisting minerals, principally plagioclase and clinopyroxene, from about 400 samples of the Skaergaard layered gabbro intrusion and its country rocks. The δ^(18)O values of plagioclase decrease upward in the intrusion, from ‘normal’ values of about +6.0 to +6.4 in the Lower Zone and parts of the Middle Zone, to values as low as −2.4 in the Upper Border Group. The ^(18)O depletions of the plagioclase all took place under subsolidus conditions, and were produced by the Eocene meteoric-hydrothermal system established by this pluton. Clinopyroxene, which is more resistant to ^(18)O exchange than is plagioclase, also underwent depletion in ^(18)O, but to a lesser degree (δ^(18)O = +5.2 to +3.5). The ^(18)O-depleted rocks typically show reversed Δ^(18)Oplag−px fractionations, except at the top of the Upper Zone, where the pyroxenes are very fine-grained aggregates pseudomorphous after ferrowollastonite; these inverted pyroxenes were much more susceptible to subsolidus ^(18)O exchange (δ^(18)O = +3–9 to +0.7). D/H analyses of the chloritized basalt country rocks and of the minor quantities of alteration minerals in the pluton (δD = −116 to −149) confirm these interpretations, indicating that the rocks interacted with meteoric groundwaters having an original δD ≈ −100. and δ^(18)O ≈ −14. Low δD values (≈ −125) were also found throughout the biotites of the Precambrian basement gneiss, requiring that small amounts of water penetrated downward to depths of at least 6 to 10 km. These values, together with the lack of ^(18)O depletion of the gneiss, imply that the overall water/rock ratios were very small in that unit (400–500 °C). Prior to flowing into the gabbro, these fluids had exchanged with similar mineral assemblages in the basaltic country rocks, explaining the lack of chemical alteration of the gabbro. Xenoliths of roof rock basalt and of Upper Border Group leucogabbro were strongly depleted in ^(18)O by the hydrothermal system prior to their falling to the bottom of the magma chamber and being incorporated in the layered series. This proves that the hydrothermal system was established very early, at the time of emplacement of the Skaergaard intrusion. However, no measurable ^(18)O depletion of the gabbro magma could be detected, indicating that very little H_2O penetrated directly into the liquid magma, in spite of the fact that a hydrothermal circulation system totally enveloped the magma chamber for at least 100,000 years during its entire period of crystallization. Only as crystallization proceeded was the hydrothermal system able to collapse inward and interact with the solidified and fractured portions of the gabbro. Nevertheless some H_2O was clearly added directly to the magma by dehydration of the stoped blocks of altered roof rock. It is also plausible that small amounts of meteoric water diffused directly into the magma, most logically in the vicinity of major fracture zones that penetrated close to, or were underneath, the late-stage sheet of differentiated ferrodiorite magma. It is suggested that such influx of meteoric waters was responsible for many of the gabbro pegmatite bodies that are common in the Marginal Border Group; also, such H_2O might have produced local increases in Fe^(+3)/Fe^(+2) in the magma that in turn could explain some of the asymmetric crystallization effects in the magma chamber. Local lowering of the liquidus temperature would also occur, perhaps leading to topographic irregularities on the floor of the magma chamber (e.g. the trough bands?).