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

Autor: Bernhard Schulz, Frank Fischer, Christoph Breitkreuz, Alexander Repstock, Franziska Heuer, Jihyeon Im, Sabine Gilbricht, Manuel Lapp, Marcel Hübner
Rok vydání: 2019
Předmět:
Zdroj: Journal of Volcanology and Geothermal Research. 369:35-49
ISSN: 0377-0273
Popis: An Early Permian >VEI 6 eruption (ca. 42 km3) 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 (~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 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 AlIV-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.
Databáze: OpenAIRE
Externí odkaz: