阿巴拉契亚造山带北部片麻岩丘带逆变质作用、地体增生、逆冲堆积和韧性流动的岩石年代学约束

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2023-08-22 DOI:10.1111/jmg.12741
Ian W. Hillenbrand, Michael L. Williams, Emily M. Peterman, Michael J. Jercinovic, Craig W. Dietsch
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引用次数: 1

摘要

片麻岩圆顶是许多造山带的组成部分,通常为深入地壳提供构造窗口。阿巴拉契亚造山带新英格兰段的片麻岩圆顶在经典上与底辟作用和褶皱干涉有关,但也提出了涉及韧性流的替代模型。我们用U‐Th‐Pb独居石、异长岩、锆石和钛矿岩石年表以及主元素和微量元素热气压测量法对新英格兰西部片麻岩穹隆带的这些模型进行了评估。这些数据限制了片麻岩穹隆带构造地层学中每个单元的不同压力-温度-时间(P‐T‐T)路径。结构最低的单元,Waterbury穹隆的Laurentia衍生的混合岩片麻岩,记录了两个变质阶段(455-435和400-370 Ma),峰值阿卡迪亚变质条件为约1.0–1.2 GPa,温度为750–780°C,温度为391 ±7至386 ±4 Ma。下一个结构更高的单元,冈瓦纳大陆衍生的泰恩山组,记录了Taconic(峰值条件:0.6 GPa,600°C,441 ±4 Ma)和阿卡迪亚(峰值:0.8–1.0 GPa,在377时为650°C ±4 Ma)变质作用。上覆的Collinsville地层产生473 ±5Ma结晶年龄和436时650°C变质条件的证据 ±4 Ma和1.2–1.0 GPa,温度为750–775°C,温度为397 ±4至385 ±6 Ma。Collinsville组结构较高的Sweetheart Mountain段仅产出Acadian锆石、独居石和异长岩时代,以及380–375年左右高压麻粒岩相变质作用(1.8 GPa,815°C)的证据 Ma。圆顶覆盖层的盖层岩石海峡片岩在386年记录了约1 GPa、700°C的峰值条件 ±6至380 ±4 Ma。石榴石分解为独居石和/或异长岩发生在375–360和345–330左右的所有单元中 马。阿卡迪亚峰变质岩压力从结构最低单元到最高单元(从1.0到1.8GPa)有系统地增加。这种反向变质岩序列与底辟和褶皱干涉模型不兼容,后者预测结构最低水平的最高压力。基于P‐T‐T和结构数据,我们更喜欢一个首先涉及大约380 Ma逆冲叠加,随后是同碰撞造山带的平行伸展、韧性流动和380至365年之间的穹隆上升 马,大约345–330年石榴石击穿 马被解释为反映了阿卡迪亚造山高原崩塌期间的进一步剥露。这些结果突出了将岩石学约束与来自多个计时器的成对地球化学和地质年代数据相结合的力量,以测试结构和构造模型,并表明同收敛造山带平行韧性流极大地改变了新英格兰早期的吸积相关结构。此外,片麻岩穹隆带记录了同碰撞厚地壳环境中的片麻岩穹隆发育,提供了现代喜马拉雅山脉和帕米尔山脉可能发生的中下部地壳过程的一个古老例子。
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Petrochronologic constraints on inverted metamorphism, terrane accretion, thrust stacking, and ductile flow in the Gneiss Dome belt, northern Appalachian orogen

Gneiss domes are an integral element of many orogenic belts and commonly provide tectonic windows into deep crustal levels. Gneiss domes in the New England segment of the Appalachian orogen have been classically associated with diapirism and fold interference, but alternative models involving ductile flow have been proposed. We evaluate these models in the Gneiss Dome belt of western New England with U-Th-Pb monazite, xenotime, zircon, and titanite petrochronology and major and trace element thermobarometry. These data constrain distinct pressure–temperature–time (P-T-t) paths for each unit in the gneiss dome belt tectono-stratigraphy. The structurally lowest units, Laurentia-derived migmatitic gneisses of the Waterbury dome, document two stages of metamorphism (455–435 and 400–370 Ma) with peak Acadian metamorphic conditions of ~1.0–1.2 GPa at 750–780°C at 391 ± 7 to 386 ± 4 Ma. The next structurally higher unit, the Gondwana-derived Taine Mountain Formation, records Taconic (peak conditions: 0.6 GPa, 600°C at 441 ± 4 Ma) and Acadian (peak: 0.8–1.0 GPa, 650°C at 377 ± 4 Ma) metamorphism. The overlying Collinsville Formation yielded a 473 ± 5 Ma crystallization age and evidence for metamorphic conditions of 650°C at 436 ± 4 Ma and 1.2–1.0 GPa, 750–775°C at 397 ± 4 to 385 ± 6 Ma. The structurally higher Sweetheart Mountain Member of the Collinsville Formation yielded only Acadian zircon, monazite, and xenotime dates and evidence for high-pressure granulite facies metamorphism (1.8 GPa, 815°C) at circa 380–375 Ma. Cover rocks of the dome-mantling The Straits Schist records peak conditions of ~1 GPa, 700°C at 386 ± 6 to 380 ± 4 Ma. Garnet breakdown to monazite and/or xenotime occurred in all units at circa 375–360 and 345–330 Ma. Peak Acadian metamorphic pressures increase systematically from the structurally lowest to highest units (from 1.0 to 1.8 GPa). This inverted metamorphic sequence is incompatible with the diapiric and fold interference models, which predict the highest pressures at the structurally lowest levels. Based upon P-T-t and structural data, we prefer a model involving, first, circa 380 Ma thrust stacking followed by syn-collisional orogen parallel extension, ductile flow, and rise of the domes between 380 and 365 Ma. Garnet breakdown at circa 345–330 Ma is interpreted to reflect further exhumation during collapse of the Acadian orogenic plateau. These results highlight the power of integrating petrologic constraints with paired geochemical and geochronologic data from multiple chronometers to test structural and tectonic models and show that syn-convergent orogen parallel ductile flow dramatically modified earlier accretion-related structures in New England. Further, the Gneiss Dome belt documents gneiss dome development in a syn-collisional, thick crust setting, providing an ancient example of middle to lower crustal processes that may be occurring today in the modern Himalaya and Pamir Range.

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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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