Sorger Dominik, Hauzenberger Christoph A, Finger Fritz, Linner Manfred, Skrzypek Etienne, Schorn Simon
Institute of Earth Sciences, NAWI Graz Geocenter University of Graz Graz Austria.
Geoscience Center University of Goettingen Goettingen Germany.
J Metamorph Geol. 2024 Jan;42(1):3-34. doi: 10.1111/jmg.12744. Epub 2023 Sep 14.
Two types of aluminous paragneiss from the Loosdorf complex (Bohemian Massif, NE Austria) contain coarse-grained granulite assemblages and retrograde reaction textures that are investigated to constrain the post-peak history of the Gföhl unit in the southern Bohemian Massif. Both types have a peak assemblage garnet-biotite-sillimanite-plagioclase-K-feldspar-quartz-granitic melt ± kyanite ± ilmenite ± rutile, recording peak metamorphic conditions of 0.9-1.1 GPa and 780-820°C estimated by isochemical phase equilibrium modelling. The first sample type (Ysper paragneiss) developed (i) cordierite coronae around garnet and (ii) cordierite-spinel and cordierite-quartz reaction textures at former garnet-sillimanite interfaces. Calculated chemical potential relationships indicate that the textures formed in the course of a post-peak near-isothermal decompression path reaching 0.4 GPa. Texture formation follows a two-step process. Initially, cordierite coronae grow between garnet and sillimanite. As these coronae thicken, they facilitate the development of local compositional domains, leading to the formation of cordierite-spinel and cordierite-quartz symplectites. The second sample type (Pielach paragneiss) exhibits only discontinuous cordierite coronae around garnet porphyroblasts but lacks symplectites. The formation of cordierite there also indicates near-isothermal decompression to 0.4-0.5 GPa and 750-800°C. This relatively hot decompression path is explained by the contemporaneous exhumation of a large HP-UHT granulite body now underlying the Loosdorf complex. The timing of regional metamorphism in the granulites and the southern Bohemian Massif in general is well constrained and has its peak at 340 Ma. Monazite from Loosdorf paragneiss samples yield a slightly younger age of 335 Ma. Although the ages overlap within error, they are interpreted to reflect near-isothermal decompression and exhumation resulting in the formation of the observed reaction textures.
来自洛斯多夫杂岩体（奥地利东北部波希米亚地块）的两种含铝副片麻岩含有粗粒麻粒岩组合和退变质反应结构，对其进行研究以限制波希米亚地块南部格福尔单元峰期后的演化历史。这两种类型都具有峰期组合石榴石-黑云母-矽线石-斜长石-钾长石-石英-花岗质熔体±蓝晶石±钛铁矿±金红石，通过等化学相平衡模拟估计其峰期变质条件为0.9 - 1.1吉帕和780 - 820℃。第一种样品类型（伊斯珀副片麻岩）发育出（i）石榴石周围的堇青石晕圈，以及（ii）在原石榴石-矽线石界面处的堇青石-尖晶石和堇青石-石英反应结构。计算得出的化学势关系表明，这些结构是在峰期后近等温降压至0.4吉帕的过程中形成的。结构形成遵循两步过程。最初，堇青石晕圈在石榴石和矽线石之间生长。随着这些晕圈变厚，它们促进了局部成分域的发展，导致堇青石-尖晶石和堇青石-石英交生体的形成。第二种样品类型（皮拉赫副片麻岩）仅在石榴石斑晶周围显示出不连续的堇青石晕圈，但没有交生体。那里堇青石的形成也表明近等温降压至0.4 - 0.5吉帕和750 - 800℃。这种相对高温的降压路径是由现在位于洛斯多夫杂岩体之下的一个大型高压-超高温麻粒岩体的同期隆升所解释的。一般来说，麻粒岩和波希米亚地块南部区域变质作用的时间受到很好的限制，其峰期为340百万年。来自洛斯多夫副片麻岩样品的独居石给出了稍年轻的335百万年的年龄。尽管这些年龄在误差范围内重叠，但它们被解释为反映了导致观察到的反应结构形成的近等温降压和隆升过程。
