Are microstructures in plutonic rocks primary or secondary?: a re-examination of the metasomatism hypothesis for the roof-sourced autoliths in the Skaergaard intrusion

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Journal of Petrology Pub Date : 2024-01-17 DOI:10.1093/petrology/egae001
Marian B Holness, Jens C Ø Andersen, Olivier Namur, Troels F D Nielsen
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Abstract

The roof-derived autoliths in the floor cumulates of the Skaergaard Intrusion have been argued to have been extensively metasomatized and recrystallised, forming the foundation of the hypothesis that microstructures in plutonic rocks are essentially metamorphic. However, the augite-plagioclase-plagioclase dihedral angles and plagioclase core composition of the autoliths match with those of the roof rocks, demonstrating that they were generally solid on arrival at the floor, with no subsequent microstructural or compositional modification. Many autoliths have mafic rinds, which were used as evidence of metasomatism: these rinds fall into two groups. The rarely developed rind rock of Irvine et al. (1998) is most likely chilled magma infiltrating along fractures in the roof rocks, either associated directly with detachment of roof material, or occurring before final detachment. Thin mafic rims are widespread in LZc and MZ, present at the tops of the more elongate autoliths, with a corresponding felsic rim at the base of the most elongate. The close correspondence of thin rim development with autolith shape, rather than composition, is argued to be evidence that they formed as a result of differential migration of immiscible conjugate interstitial liquids: the dense Fe-rich liquid flowed downwards and ponded on the tops of impermeable autoliths, whereas its buoyant Si-rich conjugate flowed upwards and was trapped underneath. Any differences in microstructure and bulk composition of the autoliths compared to the remaining exposures of the roof sequence reflect the wider range of lithologies in the now-eroded regions of the roof.
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深成岩中的微结构是原生的还是次生的?
斯卡加尔德侵入体底板堆积物中的顶板自成岩被认为是经过了广泛的变质和重结晶过程,这构成了关于深成岩的微结构本质上是变质岩的假说的基础。然而,自成岩的辉石-斜长石-斜长石二面角和斜长石核心成分与顶板岩石相吻合,这表明自成岩在到达底板时一般是固体,其后没有发生微结构或成分的改变。许多自结石都有黑云母岩皮,这些岩皮被用作变质作用的证据:这些岩皮可分为两类。Irvine 等人(1998 年)发现的很少发育的边缘岩很可能是沿着顶板岩石裂缝渗入的冷岩浆,要么与顶板材料的剥离直接相关,要么发生在最终剥离之前。在 LZc 和 MZ 中,稀薄的黑云母边缘非常普遍,出现在较长的自岩石顶部,而在最长的自岩石底部则有相应的长英岩边缘。薄缘的形成与自岩石的形状(而不是成分)密切相关,这证明它们是由于不相溶的共轭间隙液体的不同迁移而形成的:致密的富Fe液体向下流动,积聚在不透水的自岩石顶部,而其具有浮力的富Si共轭液体向上流动,被困在自岩石底部。自结石的微观结构和块体成分与顶板序列其余出露部分相比有任何差异,都反映了顶板现已侵蚀区域的岩性范围更广。
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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