Pressure–Temperature–Time Evolution of a Polymetamorphic Paragneiss With Pseudomorphs After Jadeite From the HP–UHP Gneiss‐Eclogite Unit of the Variscan Erzgebirge Crystalline Complex, Germany

IF 3.5 2区 地球科学 Q1 GEOLOGY
Hans‐Joachim Massonne
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引用次数: 0

Abstract

A quartz‐rich paragneiss from the Variscan Erzgebirge Crystalline Complex (ECC) was studied in detail because of abundant millimetre‐sized and clearly oriented pseudomorphs after a sodic mineral interpreted to have been jadeite. This mineral, or pseudomorphs after it, is rarely found in extensive high‐pressure (HP)–ultrahigh‐pressure (UHP) terranes worldwide despite reported pressure–temperature (P–T) conditions suitable for the formation of jadeite in common paragneisses and orthogneisses. In the studied rock, which contains abundant large and oriented potassic white mica flakes and minor millimetre‐sized garnet grains, the pseudomorphs consist of clusters of small albite grains with thin phengitic muscovite flakes in between. X‐ray maps for Ca and Mg in garnet demonstrate that an early generation of this mineral (Gt1) was corroded and subsequently overgrown by a Ca‐richer generation (Gt2). White mica is phengite with maximum Si contents of 3.42 atoms per formula unit. P–T conditions of 0.85 GPa and 650°C and 1.7 GPa and 660°C were derived for the formation of Gt1 and Gt2 rim + Si‐rich phengite, respectively, using pseudosection modelling. The latter conditions representing the pressure peak experienced by the paragneiss are compatible with the original presence of jadeite and possibly paragonite as well. This metamorphic peak occurred at 338.4 ± 2.3 (2σ) Ma based on in situ dating of monazite grains with the electron microprobe. A single monazite age of 386.4 ± 10.5 (2σ) Ma is related to the formation of Gt1. Thus, a Late Devonian metamorphism is suggested here for the first time to have occurred in ECC gneisses before the major HP event in the Early Carboniferous. Furthermore, the study demonstrates that the eclogite‐facies gneisses of the Gneiss‐Eclogite Unit of the ECC experienced peak pressures of not more than 2 GPa in contrast to recent proposals of an extensive UHP area in this unit. In addition, it is suggested that the localized occurrence of UHP rocks surrounded by other lithologies otherwise lacking evidence for UHP conditions should be interpreted with caution with respect to their regional extent and significance.
德国瓦里斯坎-埃尔茨盖比尔格结晶复合体 HP-UHP Gneiss-Eclogite 单元中具有翡翠后假形态的多变质副辉石的压力-温度-时间演化
我们对瓦里斯坦时代埃尔茨盖比尔格结晶群(ECC)中的一种富含石英的片麻岩进行了详细研究,因为其中有大量毫米大小、取向清晰的假象,而这种假象被解释为翡翠的钠长石矿物。尽管有报道称在普通片麻岩和正长片麻岩中存在适合翡翠形成的压力-温度(P-T)条件,但在全球广泛的高压(HP)-超高压(UHP)地层中很少发现这种矿物或其假象。在所研究的岩石中,含有丰富的大型定向钾长石白云母片和少量毫米大小的石榴石颗粒,假象由小白云母颗粒群组成,中间夹着薄薄的黝帘石片。石榴石中钙和镁的 X 射线图显示,这种矿物的早期一代(Gt1)被腐蚀,随后被钙富集的一代(Gt2)所覆盖。白云母是辉绿岩,每式单位的最大硅含量为 3.42 个原子。利用假吸力建模法推导出 Gt1 和 Gt2 边缘+富硅辉绿岩形成的 P-T 条件分别为 0.85 GPa 和 650°C 以及 1.7 GPa 和 660°C。后一种条件代表了辉绿岩所经历的压力峰值,与原来存在的翡翠以及可能存在的副皂石相吻合。根据电子显微镜对独居石晶粒的现场测年,变质峰值出现在 338.4 ± 2.3 (2σ) Ma。Gt1 的形成与 386.4 ± 10.5 (2σ) Ma 的单一独居石年龄有关。因此,这里首次提出了晚泥盆世变质作用发生在早石炭世主要HP事件之前的ECC片麻岩中。此外,该研究还表明,ECC片麻岩-蚀变片麻岩单元的蚀变片麻岩经历的峰值压力不超过2 GPa,这与最近关于该单元存在大面积超高压区的建议形成了鲜明对比。此外,研究人员还建议,对于局部出现的超高压岩石,以及周围缺乏超高压条件证据的其他岩性,在解释其区域范围和意义时应谨慎从事。
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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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