Polymetamorphism and metastability in Paleozoic schists of the central Appalachian Baltimore Terrane, USA

IF 2.9 2区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Freya R. George , Daniel R. Viete , George L. Guice , Kayleigh M. Harvey , Stephanie Walker , Ethan F. Baxter
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引用次数: 0

Abstract

Metapelitic assemblages are often considered highly reactive during prograde metamorphism, such that they effectively record a history of tectono-metamorphic processes. Across eastern North America, metamorphosed rift-to-drift stratigraphy has been central to disentangling the terminal history of Appalachian convergence. We report results from a single metapelitic outcrop in the Loch Raven Schist of Maryland's Baltimore Terrane, where regional metamorphism is historically interpreted to have derived only from the Ordovician Taconic orogeny. An aluminum-rich litho-horizon alludes to two phases of metamorphism: an early, low/medium-pressure (P) sillimanite-stabilizing phase largely overprinted by a subsequent medium/high-P kyanite-stabilizing event, terminating at c. 391–383 Ma. This provides evidence of substantial crustal disturbances in the central Appalachians during Avalonian collision of comparable timing to equivalent events in the north. In a subordinate and aluminum-poor litho-horizon, large (2–3.5 cm) garnet provides geochronological evidence only for a protracted phase of metamorphism at 440–424 Ma. We interpret this age as recording the early, low/medium-P phase of metamorphism that either: (a) extends the duration of the Taconic event; or (b) provides new evidence for metamorphism associated with Silurian tectonism and/or extension in the Central Appalachians. However, this assemblage appears to have been metamorphically unresponsive during Devonian overprinting, despite peak metamorphic conditions of ∼670 °C and ∼8 kbar. We suggest that a potential combination of the paucity of fluid, limited strain accumulation, and coarse refractive assemblages stabilized a kinetically sluggish and metastable composition.
美国阿巴拉契亚巴尔的摩地层中部古生代片岩中的多变质作用和易变性
变质岩集合体通常被认为在顺层变质过程中具有高度反应性,因此它们有效地记录了构造变质过程的历史。在整个北美东部,变质的裂谷-漂移地层对于厘清阿巴拉契亚汇聚的末期历史至关重要。我们报告了马里兰州巴尔的摩地层中 Loch Raven 片岩的一个变质露头的研究结果,根据历史解释,该地区的变质作用仅来自奥陶纪的塔科尼克造山运动。富铝岩性地层暗示了变质作用的两个阶段:早期的低/中压(P)矽线石稳定阶段在很大程度上被随后的中/高-P奇安岩稳定事件所覆盖,大约在 391-383 Ma 时结束。这提供了阿瓦隆碰撞期间阿巴拉契亚山脉中部地壳剧烈扰动的证据,其时间与北部的同等事件相当。在一个从属的贫铝岩层中,大块(2-3.5 厘米)石榴石仅为 440-424 Ma 的漫长变质阶段提供了地质年代证据。我们将这一年龄解释为记录了变质作用的早期、低/中P阶段,该阶段要么:(a)延长了塔科尼克事件的持续时间;要么(b)为与志留纪构造作用和/或阿巴拉契亚山脉中部延伸有关的变质作用提供了新的证据。然而,尽管峰值变质条件为 ∼670 °C和 ∼8 kbar,但在泥盆纪叠印期间,该组合似乎对变质没有反应。我们认为,流体稀少、有限的应变积累以及粗折射组合的潜在组合稳定了动力学上缓慢和易变的成分。
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来源期刊
Lithos
Lithos 地学-地球化学与地球物理
CiteScore
6.80
自引率
11.40%
发文量
286
审稿时长
3.5 months
期刊介绍: Lithos publishes original research papers on the petrology, geochemistry and petrogenesis of igneous and metamorphic rocks. Papers on mineralogy/mineral physics related to petrology and petrogenetic problems are also welcomed.
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