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首页> 外文期刊>Contributions to Mineralogy and Petrology >Geochemical and isotopic zoning patterns of plagioclase megacrysts in gabbroic dykes from the Gardar Province, South Greenland: implications for crystallisation processes in anorthositic magmas
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Geochemical and isotopic zoning patterns of plagioclase megacrysts in gabbroic dykes from the Gardar Province, South Greenland: implications for crystallisation processes in anorthositic magmas

机译：南格陵兰加尔达省辉长岩脉中斜长石巨晶的地球化学和同位素分区模式：对无硅浆岩结晶过程的影响

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摘要

Chemical and Sr isotopic zoning patterns in plagioclase megacrysts from gabbroic dykes in the Gardar Province can be used to elucidate magma-chamber and emplacement processes. The megacrysts occur either as single crystals or assembled as anortho-site xenoliths. The size of the megacrysts varies from <1 cm to 1 m. They consist of a large core with variable zonation (An_(58-39)) and a relatively small (<600 mu m), normally zoned rim (An_(62-27)). The contact between core and rim is sharp and marked by a sharp increase in anorthite content which can reach 11 mol percent An. This gap is interpreted as having formed during dyke emplacement due to a sudden pressure release. Some of the megacryst cores show a fairly constant composition whereas others exhibit an unusual wavy-oscillatory zoning which has not been reported elsewhere to our knowledge. The oscillatory zoning has wavelengths of up to 2,500 mu m and a maximum amplitude of 7 mol percent An. It is interpreted as reflecting movements of the crystals in the magma reservoir. The Sr isotopic composition of one crystal shows a radiogenic inner core ((~(87)Sr/~(86)Sr)_i=0.7044) and a less radiogenic outer core ((~(87)Sr/~(86)Sr)_i=0.7039-0.7036). The lack of a significant change between outer core and rim ((~(87)Sr/~(86)Sr)_i=0.7037) is consistent with formation of the more An-rich rim due to pressure release. Variations in the core may be related to movements of the crystal and/or magma mixing. A trace-element profile across a megacryst shows a small increase in Sr and small decreases in Ba and La contents of the recalculated melt composition across the core-rim boundary, whereas P, Ce, Nd and Eu remain constant. Melt compositional changes upon emplacement are therefore considered to be of minor importance. Constant ratios of incompatible trace elements in the megacryst cores indicate a dominant influence of a lower crustal source on trace-element budgets.
机译：Gardar省辉长岩脉斜长石巨晶的化学和Sr同位素分区模式可用于阐明岩浆腔和沉积过程。巨晶要么以单晶形式出现，要么以原位异位结石的形式存在。巨晶的大小从<1 cm到1 m不等。它们由一个带可变分区的大岩心（An_（58-39））和一个相对较小（<600μm）的通常划定的轮辋（An_（62-27））组成。芯和轮辋之间的接触非常尖锐，并且以钙长石含量的急剧增加为特征，其可以达到11摩尔％An。该间隙被解释为在堤坝安置期间由于突然的压力释放而形成。一些巨晶岩心显示出相当恒定的成分，而另一些则表现出不同寻常的波浪振荡带，据我们所知，这还没有其他报道。振荡分区的波长最大为2500微米，最大振幅为7摩尔百分比An。它被解释为反映了岩浆储层中晶体的运动。一个晶体的Sr同位素组成显示放射源的内核（（〜（87）Sr /〜（86）Sr）_i = 0.7044）和放射源较少的外核（（〜（87）Sr /〜（86）Sr） _i = 0.7039-0.7036）。外芯和轮辋之间没有明显变化（（〜（87）Sr /〜（86）Sr）_i = 0.7037）与由于压力释放而形成的更富An的轮辋相一致。岩心的变化可能与晶体和/或岩浆混合的运动有关。横跨大晶的痕量元素分布图表明，在芯缘边界处重新计算的熔体成分中Sr少量增加，Ba和La含量少量减少，而P，Ce，Nd和Eu保持恒定。因此，就位而言，熔体成分的变化并不重要。巨晶岩心中不相容的痕量元素的比率恒定，表明较低的地壳来源对痕量元素的预算具有显着影响。

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