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首页> 外文期刊>Contributions to Mineralogy and Petrology >Formation of corundum megacrysts during H_2O-saturated incongruent melting of feldspar: P-T pseudosection-based modelling from the Skattora migmatite complex, North Norwegian Caledonides
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Formation of corundum megacrysts during H_2O-saturated incongruent melting of feldspar: P-T pseudosection-based modelling from the Skattora migmatite complex, North Norwegian Caledonides

机译：H_2O饱和长石不均匀融化过程中刚玉大晶的形成：基于P-T假剖面的北挪威喀里多内斯Skattora migmatite复杂构造模型

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摘要

Corundum megacryst-bearing rocks associated with the high-pressure migmatites of the Skatt0ra migmatite complex (SMC) belonging to the Nakkedal Nappe Complex, North Norwegian Caledonides, display a classical example of incongruent melting of plagioclase under water-saturated conditions. Petrography and micro-textures suggest that several centimetre long corundum megacrysts formed from the silicate melt along with amphibole (pargasite) and plagioclase (X_(An) ～ 0.47). The corundum-bearing leucosomes are rich in biotite compared to the other mafic units of SMC. Locally, margarite occurs in coronas around corundum megacrysts. Geochemically, the corundum-bearing rocks are enriched in Al, K, Rb and Ba and depleted in Fe, Mg and Ca compared to the leuco-gabbroic host rock. A P-T pseudosection of the leuco-gabbro indicates that feldspar breakdown and corundum formation occurred at temperatures >850 °C and pressure >1.2 GPa. The calculated equilibrium P-T of the corundum-bearing rock corresponds to 750-825 °C and 0.9-1.1 GPa. The P-T pseudosection of margarite indicates that margarite formed after cooling and decompression to P-T conditions corresponding to 600 °C at 0.5 GPa. Based on geochemical and mineral chemical analysis coupled with thermodynamic modelling, we suggest that formation of corundum occurred as a result of high-pressure incongruent melting of plagioclase in the presence of a K-, Rb- and Ba-rich external fluid. It is also suggested that the external fluid transported out portions of Ca, Fe and Mg, resulting in an increase of the peraluminousity of the melt and promoting further growth of corundum.
机译：属于北挪威喀里多内德Nakkedal Nappe复杂构造的Skatt0ra migmatite复杂构造（SMC）的高压蒙脱石伴生的刚玉超大晶岩，显示了在水饱和条件下斜长石不相容融化的经典例子。岩石学和微观纹理表明，硅酸盐与闪石（辉石）和斜长石（X_（An）〜0.47）一起融化形成了几厘米长的刚玉大晶。与SMC的其他黑手党单位相比，含刚玉的白质体富含黑云母。玛格石在刚玉大晶周围的日冕中局部存在。从地球化学角度看，与无色长石宿主岩相比，含刚玉的岩石富含Al，K，Rb和Ba，而贫化了Fe，Mg和Ca。隐色球的P-T假剖面表明，长石分解和刚玉的形成发生在温度> 850°C和压力> 1.2 GPa。计算得出的刚玉岩石的平衡P-T对应于750-825°C和0.9-1.1 GPa。玛格石的P-T假剖面表明，在0.5 GPa冷却和减压至对应于600°C的P-T条件后形成的玛格石。基于地球化学和矿物化学分析以及热力学建模，我们认为刚玉的形成是由于在富含K，Rb和Ba的外部流体存在下高压斜长石不相容融化的结果。还建议外部流体运出部分Ca，Fe和Mg，导致熔体的铝含量增加，并促进刚玉的进一步生长。

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