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首页> 外文期刊>Journal of Metamorphic Geology >Inverted metamorphic field gradient towards a Variscan suture zone (Champtoceaux Complex, Armorican Massif, France)
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Inverted metamorphic field gradient towards a Variscan suture zone (Champtoceaux Complex, Armorican Massif, France)

机译：倒向Variscan缝合带的变质场梯度（Champtoceaux Complex，Armorican Massif，法国）

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摘要

We describe, date and constrain the P–T conditions of a syntectonic inverted metamorphic sequence associated with continental collision and crustal-scale thrusting in one of the key regions of the late Palaeozoic Variscan belt of Western Europe – the Champtoceaux Complex (Armorican Massif, France), interpreted as a trace of the Variscan suture zone between Laurussia and Gondwana. The Complex consists of several stacked units, some of them eclogite-bearing, that are sandwiched between two main pieces of continental crust – the Parautochthon and the Upper Allochthon. Moderately to steeply dipping foliation parallels the main lithological boundaries. From the bottom to the top of the metamorphic rock pile, the following sequence testifies to the syntectonic temperature increase: chlorite– biotite-bearing metagreywackes (Parautochthon); orthogneisses with eclogite lenses; micaschists with chloritoid–chlorite–garnet; orthogneisses; micaschists with staurolite–biotite–garnet with chloritoid inclusions (Lower Allochthon); and migmatites with boudins of eclogite and kyanite–biotite–garnetbearing metapelitic lenses (Upper Allochthon). Mylonitic amphibolites with lenses of serpentinized peridotite mark the boundary between the Lower Allochthon and the overlying Upper Allochthon, suggesting the presence of a major thrust. It is inferred that the latter is responsible for the development of the inverted metamorphic zoning. Multiequilibrium thermobarometry and pseudosections calculated with THERMOCALC indicate that equilibration temperatures of the syntectonic peak metamorphic assemblages increase upwards in the rock pile from <500 C in the Parautochthon to >650 C in the Upper Allochthon. All units equilibrated at similar pressures between 7 and 10 kbar. In the Upper Allochthon, chronological results on muscovite suggest initial cooling from c. 343 Ma (muscovite Rb–Sr) to c. 337 Ma (muscovite 40Ar–39Ar). A subsequent very rapid temperature decrease is suggested by the synchronous closure of the muscovite and biotite K–Ar and biotite Rb–Sr isotopic systems (c. 337–335 Ma). This cooling is also recorded in the Upper Micaschists of the Lower Allochthon and in the Parautochthon with muscovite 40Ar–39Ar ages of c. 336–334 and 332 Ma, respectively. Ages of c. 343 Ma inferred from disturbed muscovite spectra from the Parautochthon are possibly linked to a previous higher pressure metamorphic event in this unit. It is suggested that the development of the inverted metamorphic zoning in the Champtoceaux Complex is due to the emplacement of a hot nappe over colder units and is contemporaneous with major crustal thrusting and associated pervasive ductile deformation. The preservation of this inverted field gradient was possible because of fast cooling, tentatively associated with the syn-compressional denudation of the tectonic pile, expressed by the detachment at the top of the nappe pile. The efficiency of cooling is best shown by the near-coincidence of Rb–Sr and 40Ar–39Ar ages, obtained on both sides of the major thrust. Finally, we highlight similarities with other regions of the West-European Variscan belt (Iberian massif, French Massif Central) and suggest that inverted metamorphic zoning is systematically associated with the contact between the Lower and Upper Allochthons.
机译：我们描述，标定和约束与西欧晚期古生代瓦里斯卡特带的主要区域之一——Champtoceaux Complex（法国阿莫里奇山地块）的大陆碰撞和地壳尺度逆冲有关的构造逆转变质序列的PT条件。 ），解释为月桂与冈瓦纳之间的瓦里斯坎缝合带的痕迹。该综合大楼由几个堆叠的单元组成，其中一些单元带有榴辉岩，它们被夹在两大主要的大陆壳之间，即Parautochthon和Upper Allochthon。中度至陡倾的岩层平行于主要岩性边界。从变质岩桩的底部到顶部，以下序列证明了构造温度的升高：绿泥石–含黑云母的变质巨石（Parautochthon）;带有榴辉石片的正片麻岩;含类胡萝卜素-亚氯酸盐-石榴石的云母;直片麻痹;云母，云母，辉石-黑云母-石榴石，内含类胡萝卜素（下异源石）;以及带有榴辉岩和蓝晶石-黑云母-石榴石的变质晶状体的迁徙岩（上Allochthon）。带有蛇纹石化橄榄岩晶状体的Mylonitic角闪石标志着下异形体和上覆上异形体之间的边界，表明存在主要的逆冲作用。可以推断，后者负责反向变质带的发展。多平衡热压法和用THERMOCALC计算的伪剖面表明，岩石堆中同构峰变质组合的平衡温度从Parautochthon中的<500 C上升到Allallthon中的> 650C。所有单位均在7至10 kbar的相似压力下达到平衡。在上Allochthon，白云母的年代学结果表明从c开始冷却。 343 Ma（白云母Rb–Sr）至c。 337马（白云母40Ar–39Ar）。随后白云母和黑云母K-Ar和黑云母Rb-Sr同位素系统的同步关闭表明温度随后迅速降低（约337-335 Ma）。这种冷却也记录在下Allochthon的上层云母岩中，以及年龄为c的白云母40Ar–39Ar的Parautochthon中。分别为336–334和332 Ma。 c。的年龄从Parautochthon的干扰白云母光谱推断出的343 Ma可能与该单元先前的高压变质事件有关。有人认为，在Champtoceaux Complex中，倒置变质带的形成是由于较热的单元位于较冷的单元上，这与主要的地壳逆冲作用和相关的普遍延性变形同时发生。倒转场梯度的保持是有可能的，因为快速冷却，暂时与构造桩的同压缩剥蚀有关，这由推覆桩顶部的剥离表示。在主推力的两边获得的Rb-Sr和40Ar-39Ar年龄的近似吻合可以最好地表明冷却的效率。最后，我们强调了与西欧Variscan带的其他地区（伊比利亚地块，法国地块中部）的相似性，并提出了倒置变质带与下和上异源地带的接触系统地联系在一起。

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