Polymetamorphism during the Grenvillian Orogeny in SE Ontario: Results from trace element mapping, in situ geochronology, and diffusion geospeedometry

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2023-09-16 DOI:10.1111/jmg.12745
F. Gaidies, T. Mccarron, A. D. Simpson, R. M. Easton, S. Glorie, B. Putlitz, K. Trebus
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Abstract

The Flinton Group is a metasedimentary succession of the Grenville Province in SE Ontario, potentially allowing insight into the tectono-thermal evolution of continental crust during the Mesoproterozoic. At its Green Bay locality, Flinton Group metapelites of the staurolite zone contain abundant, post-kinematic garnet porphyroblasts. Whereas the larger garnet crystals are typically impinged, smaller crystals are isolated from each other, occasionally exhibiting elongated shapes with apparently trigonal morphology. Central sections of the garnet population of a representative sample reveal that garnet is composed of different compositional and microstructural domains. In the largest crystals of the population, garnet contains rectangular to rhombic domains, marked by sharp increases in the concentrations of Nb, V, Ti, and Cr. These domains are associated with irregularly shaped patches, characterized by spatially heterogenous enrichments of Ca and LREE, and depletions in the contents of P, Y, MREE, and HREE, accompanied by increased densities of comparatively coarse-grained quartz inclusions hosting apatite. Microstructural relationships indicate that these domains correspond to portions of garnet that pseudomorphed biotite, with the enrichments of Nb, V, Ti, and Cr outlining the original biotite shapes. The compositional patterns formed by Ca, P, Y, and REE indicate that apatite participated in the grain-fluid interactions that operated during the metasomatic replacement of biotite by garnet. The statistical analyses of the garnet number and size distributions confirm that garnet initially nucleated on biotite, controlled by the kinetics of attachment and detachment processes at the garnet/biotite interface, resulting in the typical impingement habit. In situ Lu–Hf garnet geochronology applied to garnet that did not pseudomorph biotite, and hence is enriched in HREE, points to a first metamorphic event at c. 1080 ± 31 Ma. Subsequent pseudomorphism of staurolite by white mica in a Al2O3- and FeO-mobile system resulted in the concomitant crystallization of a new garnet generation, forming overgrowths on the first garnet generation and nuclei in the fine-grained matrix. Garnet that nucleated during this event grew to isolated and elongated crystals with apparently trigonal morphology, aligned in a direction c. perpendicular to the rock matrix foliation. The open-system behaviour during this event limits the use of whole-rock-based geochronological and thermobarometrical applications. However, previously published in situ U–Pb ages of monazite included in the rims of the garnet crystals and in the rock matrix indicate that this event took place at c. 976 ± 4 Ma, likely associated with a period of increased hydrothermal activity late in the metamorphic history of the Grenvillian Orogeny. Diffusion geospeedometry calculations indicate that garnet growth during this hydrothermal event lasted for less than 6 Myr.

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安大略东南部格伦维利造山运动期间的多变质作用:痕量元素绘图、原位地质年代学和扩散地质速度测量的结果
弗林顿组是安大略省东南部格勒维尔省的一个元古代演替,有可能让人们了解中新生代大陆地壳的构造-热演化过程。在格林湾矿区,闪长岩带的弗林顿组玄武岩含有丰富的后激变石榴石斑块。较大的石榴石晶体通常呈撞击状,而较小的晶体则彼此孤立,偶尔会呈现出明显呈三棱状的拉长形状。一个代表性样本中石榴石群的中心切面显示,石榴石由不同的成分和微结构域组成。在最大的晶体群中,石榴石包含矩形至菱形结构域,其特征是 Nb、V、Ti 和 Cr 的浓度急剧增加。这些域与形状不规则的斑块有关,其特征是钙和 LREE 在空间上异质富集,而 P、Y、MREE 和 HREE 的含量则减少,同时还伴随着寄生磷灰石的相对粗粒石英包裹体密度的增加。显微结构关系表明,这些区域对应于石榴石假象生物玢岩的部分,Nb、V、Ti 和 Cr 的富集勾勒出原始生物玢岩的形状。由 Ca、P、Y 和 REE 形成的成分模式表明,磷灰石参与了石榴石对生物橄榄石的元气置换过程中的晶粒-流体相互作用。对石榴石数量和尺寸分布的统计分析证实,石榴石最初成核于黑云母,受石榴石/黑云母界面附着和脱离过程动力学的控制,形成典型的撞击习性。原位 Lu-Hf 石榴石地质年代学应用于没有假象生物橄榄岩的石榴石,因此富含 HREE,这表明第一次变质事件发生在约 1080 ± 31 Ma。随后,白云母在 Al2O3- 和 FeO- 移动体系中对白云母进行假变质,导致新一代石榴石随之结晶,在第一代石榴石上形成过生层,并在细粒基质中形成晶核。在这一过程中成核的石榴石长成了孤立的细长晶体,具有明显的三棱柱形态,其排列方向与岩石基质折线垂直。这一事件中的开放系统行为限制了基于整块岩石的地质年代和温压计应用。不过,之前公布的石榴石晶体边缘和岩石基质中的独居石的原位 U-Pb 年龄表明,这一事件发生在约 976 ± 4 Ma,很可能与格勒维利造山带变质历史晚期热液活动增加的时期有关。扩散测地计算表明,在这一热液活动期间石榴石的生长持续了不到 6 Myr。
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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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