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首页> 外文期刊>Contributions to Mineralogy and Petrology >Petrogenesis of low-delta O-18 quartz porphyry dykes, Koegel Fontein complex, South Africa
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Petrogenesis of low-delta O-18 quartz porphyry dykes, Koegel Fontein complex, South Africa

机译：Low-Delta O-18石英斑岩堤，Koegel Fontin Complex，南非

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摘要

This paper investigates the origin of low-delta O-18 quartz porphyry dykes associated with the 144-133 Ma Koegel Fontein Igneous Complex, which was intruded during the initial phase of breakup of Africa and South America. The 25-km diameter Rietpoort Granite is the largest and youngest phase of activity, and is roofed by a 10-km diameter pendant of gneiss. Quartz porphyry (QP) dykes, up to 15 m in width, strike NW-SE across the complex. The QP dykes that intruded outside the granite have similar quartz phenocryst delta O-18 values (average 8.0%, +/- 0.7, n = 33) to the granite (average 8.3 +/- 1.0, n = 7). The QP dykes that intruded the roof pendant have quartz phenocrysts with more variable delta O-18 values (average 1.6%, +/- 2.1, n = 55). In some cases quartz phenocrysts have delta O-18 values as low as - 2.5%. The variation in delta O-18 value within the quartz crystal population of individual dykes is small relative to the overall range, and core and rim material from individual quartz phenocrysts in three samples are identical within error. There is no evidence that quartz phenocryst delta O-18 values have been affected by fluid-rock interaction. Based on a Delta(quartz-magma) value of 0.6%, magma delta O-18 values must have been as low as 3.1%. Samples collected along the length of the two main QP dykes that traverse the roof pendant have quartz phenocryst delta O-18 values that range from + 1.1 to + 4.6%, and - 2.3 to + 5.6 parts per thousand, respectively. These delta O-18 values correlate negatively (r = - 0.96) with initial Sr-87/Sr-86, which can be explained by the event that lowered delta O-18 values of the source being older than the dykes. We suggest that the QP dykes were fed by magma produced by partial melting of gneiss, which had been variably altered at high temperature by O-18-depleted meteoric water during global glaciation at similar to 550 Ma. The early melts had variable delta O-18 value but as melt pockets interconnected during melting, the delta O-18 values approached that of average gneiss. Variable quartz phenocryst delta O-18 values in the same dyke can be explained by vertical emplacement, at variable rates of ascent along the dyke. The lateral variation in quartz, and hence magma delta O-18 value at a particular point along a single dyke would depend on the rate of ascent of magma at that point along the dyke, and the 'age' of the particular magma batch.
机译：本文调查了与144-133 mA Koegel Fontein的低ΔO-18石英斑岩堤的起源，这在非洲和南美洲分析的初始阶段侵入。直径25公里的Rietpoort Granite是最大和最小的活性，并且由10公里的扁平吊坠屋顶。石英斑岩（QP）堤坝宽度，宽度高达15米，击打整个复合物的NW-SE。在花岗岩外侵入的QP染料具有类似的石英苯基晶醇ΔO-18值（平均8.0％，+/- 0.7，n = 33），适用于花岗岩（平均8.3 +/- 1.0，n = 7）。侵入屋顶侧膜的QP染料具有石英异晶，具有更多可变的δO-18值（平均1.6％，+/- 2.1，n = 55）。在某些情况下，石英苯基酯具有低于-2.5％的Delta O-18值。在单个堤坝的石英晶体群内的ΔO-18值的变化相对于整体范围小，并且在三个样品中的各个石英异晶中的核心和轮辋材料在误差内相同。没有证据表明石英苯杂交ΔO-18值受到流体岩石相互作用的影响。基于三角洲（石英 - 岩浆）值0.6％，Magma Delta O-18值必须低至3.1％。沿着横穿屋顶垂直的两个主要QP染料的长度收集的样品具有石英苯基杂交δO-18值，其范围为+ 1.1至+ 4.6％，分别为-2.3至+ 5.6份每千份。这些ΔO-18值与初始SR-87 / SR-86负相关（r = - 0.96），这可以通过降低源的Delta O-18值比Dykes较大的事件来解释。我们建议QP堤坝被岩浆喂养，岩浆生产的岩浆生产，在全球冰川期间在类似于550 mA的全球冰川期间通过O-18耗尽的陨石水分改变。早期的熔体具有可变的δO-18值，但随着在熔化过程中互连的熔体袋，Delta O-18值接近平均球茎。可以通过垂直施加，以沿着堤坝的可变速度来解释相同堤坝中的可变石英苯基杂交ΔO-18值。横梁的横向变化，以及沿着单个堤坝的特定点处的岩浆ΔO-18值将取决于沿着堤坝的该点处的岩浆的速度，以及特定岩浆批次的“年龄”。

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