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首页> 外文期刊>Journal of Volcanology and Geothermal Research >A Late Paleozoic Snake River-type ignimbrite (Planitz vitrophyre) in the Chemnitz Basin, Germany: Textural and compositional evidence for complex magma evolution in an intraplate setting
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A Late Paleozoic Snake River-type ignimbrite (Planitz vitrophyre) in the Chemnitz Basin, Germany: Textural and compositional evidence for complex magma evolution in an intraplate setting

机译：德国开姆尼茨盆地晚古生代蛇河型火成岩（Planitz vitrophyre）：板内环境复杂岩浆演化的构造和成分证据

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An Early Permian VEI 6 eruption (ca. 42 km(3)) deposited an ash flow tuff into the Chemnitz Basin (southeastern Germany), which exhibits typical characteristics of a Snake River-type ignimbrite. The peculiarity of the Planitz ignimbrite are well-preserved vitroclastic domains with fresh minerals, which provide a unique opportunity for investigation of a relatively crystal-poor (similar to 20 vol% crystals) magmatic system of Late Paleozoic Central Europe. Combined textural and chemical analyses on whole rock, glass shards and mineral phases were acquired to unravel the physical characteristics of the vitrophyre and the pre-eruptive evolution of its multistage magma system.The greyish-black vitrophyre is classified as a well-sorted coarse ash flow tuff (95 vol% of crystals 2 mm) with predominant cuspate and microvesicular fabrics in the groundmass. The latter comprise colorless glass shards; subordinately, brown glass shards (= 1 vol%). The felsic modal mineralogy comprises Ab-rich plagioclase (similar to 7 vol%) followed in abundance by sanidine (similar to 5 to 6 vol%) and quartz (similar to 2 vol%), whereas the ferromagnesian cumulate fragments are dominated by pyroxene (3 to 4 vol%). Bulk ignimbrite composition corresponds to a peraluminous rhyolite that shows characteristics of crustal assimilation by pronounced negative Nb and Ta anomalies.The ferromagnesian silicates have compositions indicating crystal growth in alkaline to calc-alkaline magma (diopside, augite, annite) as well as peraluminous one (siderophyllite). Some glomerocrysts contain alkaline diopside with high Mg-numbers from 80 to 84 and plagioclase with An-content from 44 to 56 mol%, which are evidence for input of mafic melts in the Planitz magma reservoir. Oscillatory zonation in plagioclase crystals points to a repetition of this process. Melt inclusions in pyroxene (57 to 67 wt% SiO2) indicate the presence of intermediate to acidic magmas in mid crustal reservoirs. Thermo- and barometry calculations based on pyroxene suggest crystallization depths around 25 km for lower crustal dry magma, whereas the occurrence of biotite, sanidine and quartz has been interpreted as entrained material from a rhyolitic cap of an upper crustal batholithic reservoir at depths around 6 km.A genetic connection between the Planitz vitrophyre and the intracaldera Rochlitz ignimbrite of the North Saxon Volcanic Complex, as it was suggested in previous studies, is unlikely because (1) biotite in the Planitz vitrophyre is Fe-rich annite showing Al-IV -Mn exchange trends, whereas the Rochlitz biotite is siderophyllite with Fe-Mg substitution and (2) the Planitz vitrophyre lacks textures like granophyric intergrowth, which are abundant in the Rochlitz ignimbrites.During and after deposition of the ash flow tuff (= 30 m thick), high-temperature processes like moderate welding compaction and formation of lithophysae were followed by high-temperature perlite formation and glass hydration. The latter was associated with depletion of Na and K in colorless glass shards when compared to the whole rock composition. Moreover, the hydration reduced Mg, Fe, and Ca in evolved brown glass shards. (C) 2018 Elsevier B.V. All rights reserved.
机译：早二叠世> VEI 6喷发（约42 km（3））将灰烬凝灰沉积到开姆尼茨盆地（德国东南部）中，具有典型的蛇河型火成岩特征。普朗兹火成岩的特殊性是保存有新鲜矿物的保存完好的碎屑岩域，为研究中古生代中相对晶体贫乏（类似于20％体积百分比的晶体）岩浆系统提供了独特的机会。对整个岩石，玻璃碎片和矿物相进行了结构和化学分析相结合，以揭示玻璃体的物理特征及其多阶段岩浆系统的喷发前演化。灰黑色玻璃体被分类为粗粒灰流动凝灰岩（95％（体积）的晶体<2 mm），基体中有主要的尖状和微囊状织物。后者包括无色玻璃碎片。次要是棕色玻璃碎片（<= 1 vol％）。长英质模态矿物学包括富含Ab的斜长石（约7％（体积）），其后是Sanidine（约5-6％（体积））和石英（约2％（体积）），而铁锰矿的堆积碎片主要由辉石（ 3至4％（体积）。大量的火成岩成分对应于一种高铝质流纹岩，其表现出明显的负Nb和Ta异常地壳同化的特征。铁镁硅酸盐的成分表明，碱性至钙碱性岩浆（透辉石，镁铝矿，闪铁矿）以及高铝质流辉岩（铁锈石）。一些珠光岩含有镁离子数高的镁，含量从80到84，斜长石的An含量从44到56 mol％，这是在Planitz岩浆储层中输入铁镁铁质熔体的证据。斜长石晶体的振荡带化表明该过程的重复。辉石中的熔体夹杂物（SiO2含量为57至67 wt％）表明在中地壳储层中存在着中等至酸性岩浆。基于辉石的热力和气压计计算表明，下地壳干岩浆的结晶深度约为25 km，而黑云母，山梨和石英的出现已被解释为上地壳岩性储层的流纹岩帽夹带了约6 km深度的物质。如先前研究中所暗示的，普朗兹玻璃体与北部撒克逊火山群的火山口内Rochlitz着火点之间的遗传联系不大可能，因为（1）普朗兹玻璃体中的黑云母是富含铁的黑闪石，显示出Al-IV-Mn交换趋势，而Rochlitz黑云母是铁铁镁铁取代的铁镁橄榄石，（2）Planitz玻璃体缺乏像粒状共生的质地，在Rochlitz着火岩中含量丰富。在烟灰凝灰沉积期间和之后（<= 30 m厚），高温过程（如中等程度的焊接压实和碎石的形成）之后是高温珍珠岩的形成和玻璃化水合。与整个岩石成分相比，后者与无色玻璃碎片中Na和K的消耗有关。此外，水合作用减少了生成的棕色玻璃碎片中的Mg，Fe和Ca。 （C）2018 Elsevier B.V.保留所有权利。

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