Complex geochronological record of an emblematic Variscan eclogite (Haut-Allier, French Massif Central)

IF 3.5 2区 地球科学 Q1 GEOLOGY
Luc de Hoÿm de Marien, Pavel Pitra, Marc Poujol, Nathan Cogné, Florence Cagnard, Benjamin Le Bayon
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引用次数: 1

Abstract

Two eclogite samples from the Haut-Allier record a prograde evolution from ~20 kbar, 650°C to 750°C, 22–23 kbar followed by heating up to 850–875°C and partial melting. Incipient decompression in high-pressure granulite facies conditions (19.5 kbar, 875°C) was followed by exhumation to high-temperature amphibolite facies conditions (<9 kbar, 750–850°C). Following a detailed geochemical, petrological, and geochronological investigation using trace-element data and laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircon, apatite, and rutile, the eclogites reveal an Ordovician (c. 490 Ma) rifting event followed by Devonian (c. 370–360 Ma) subduction and Carboniferous (c. 350 Ma) exhumation in this part of the French Massif Central. The previously proposed Silurian age for the subduction, which strongly influenced many tectonic models, is definitively rejected. In the light of other geological data from the French Massif Central, including the lithological and geochemical zoning of calc-alkaline Devonian volcanism, we propose a southward polarity of the subduction and question the very existence of the so-called Massif Central Ocean. Furthermore, we infer that following subduction, the eclogites were relaminated to the upper plate and exhumed at the rear of the magmatic arc pointing to similarities with the geodynamics of the Bohemian Massif.

The petrochronological record of zircon is particularly complex. Metamorphic zircon with clear eclogitic rare-earth elements patterns (no Eu anomaly and flat heavy rare-earth elements) and inclusions (garnet, rutile, and omphacite) shows concordant apparent ages that spread from c. 380 down to c. 310 Ma. This apparent age pattern strongly contrasts with the well-defined age of apatite and rutile of c. 350 Ma. Apparent zircon ages younger than 350 Ma unequivocally testify that zircon can recrystallize outside the conditions of the eclogite facies, which resets the U–Pb while preserving an apparent eclogitic signature. Local fractures filled by analcite, thomsonite, plagioclase, and biotite testify to late interaction of the eclogites with alkaline fluids at relatively low temperatures. This interaction, possibly at c. 310 Ma or later, could lead to the recrystallization of zircon while leaving apatite unaffected.

Abstract Image

一个具有象征意义的瓦里西斯榴辉岩的复杂地质年代学记录(法国中部地块上阿列)
上阿列勒的两个榴辉岩样品记录了从~20 kbar, 650°C到750°C, 22-23 kbar的渐进演化过程,然后加热到850-875°C和部分熔化。高压麻粒岩相条件(19.5 kbar, 875°C)开始减压,随后挖掘到高温角闪岩相条件(<9 kbar, 750-850°C)。利用微量元素数据和激光烧蚀感应耦合等离子体质谱对锆石、磷灰石和金红石进行了详细的地球化学、岩石学和年代学调查,发现榴辉岩揭示了法国中部地块的奥陶纪(约490 Ma)裂谷事件,随后是泥盆纪(约370-360 Ma)俯冲和石炭纪(约350 Ma)发掘。先前提出的对许多构造模式产生强烈影响的志留纪俯冲时代被彻底否定。根据法国中央地块的其他地质资料,包括钙碱性泥盆纪火山活动的岩性和地球化学分带,我们提出了一个向南俯冲的极性,并质疑所谓的中央地块海洋的存在。此外,我们推断,在俯冲之后,榴辉岩被重新层析到上板块,并在岩浆弧的后方被发掘出来,这与波西米亚地块的地球动力学相似。
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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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