Garnet crystallization mechanisms and localized polymetamorphism in the southwestern Meguma Terrane, Nova Scotia, Canada

IF 3.5 2区 地球科学 Q1 GEOLOGY
Alexandra B. Nagurney, Mark J. Caddick, Chris E. White
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引用次数: 0

Abstract

We present data on the pressure and temperature (P–T) conditions experienced by metamorphic rocks of the Meguma Terrane, Nova Scotia, Canada, also utilizing three-dimensional microstructural data on one sample to better constrain the mechanisms that controlled garnet crystallization. Inverse and forward thermodynamic modelling place peak P–T conditions in the southwestern Meguma Terrane at ~650°C and 4.5 kbar. Interpretation of these results with petrographic observations and previous P–T constraints across the terrane suggests that amphibolite facies metamorphism occurred during the Devonian Neoacadian orogeny (406–388 Ma). Integration of quantitative 3D textural data with an estimated metamorphic heating rate of <5°C/Myr is consistent with amphibolite facies metamorphism resulting from tectonic loading during the Neoacadian orogeny, though the exact nature of the orogeny is still not well understood. Further, the intrusion of granitic plutons into the Meguma metasediments at 373 Ma likely locally drove metamorphic recrystallization (polymetamorphism). The 3D size, shape, and location of garnet crystals in one sample reveal that the rate-limiting step for garnet crystallization was likely the diffusion of aluminium through the intergranular matrix at length scales less than the mean nearest neighbour distance between garnet crystals. Nucleation was aided by epitaxial overgrowth onto a muscovite substrate, though it appears there may have been a decoupling between minerals providing a substrate and those providing nutrients during garnet growth.

加拿大新斯科舍省Meguma地块西南部石榴石结晶机制及局部多变质作用
我们提供了加拿大新斯科舍省Meguma Terrane变质岩所经历的压力和温度(P-T)条件的数据,并利用一个样品的三维微观结构数据来更好地约束控制石榴石结晶的机制。反演和正演热力学模拟表明,在~650°C和4.5 kbar时,Meguma地块西南部的P-T条件达到峰值。这些结果与岩石学观测和以往跨地体的P-T约束解释表明,泥盆纪-新阿卡德造山运动(406-388 Ma)期间发生了角闪岩相变质作用。定量三维纹理数据与估计的5°C/Myr的变质升温速率相结合,与新阿卡德造山运动期间构造负荷导致的角闪岩相变质相一致,尽管造山运动的确切性质仍未得到很好的了解。此外,373 Ma花岗岩岩体侵入Meguma变质沉积层可能局部驱动了变质再结晶(多变质作用)。一个样品中石榴石晶体的三维尺寸、形状和位置表明,石榴石结晶的限速步骤可能是铝在小于石榴石晶体之间平均最近邻距离的长度尺度上通过晶间基体的扩散。在白云母基底上的外延过度生长有助于成核,尽管在石榴石生长过程中,提供基底的矿物和提供营养物质的矿物之间似乎存在解耦。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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