The Saxothuringian-Rhenohercynian boundary underneath the Vogelsberg volcanic field: evidence from basement xenoliths and U-Pb zircon data of trachyte

Abstract

The present study is focusing on Variscan basement xenoliths, which are present inside pyroclastic rocks of the Miocene Vogelsberg volcanic field (VVF). The investigated samples have been collected from quarries situated at the SW margin of the Vogelsberg (Nickel quarry, Nidda Ober-Widdersheim) and at the NE margin of the Vogelsberg (Rauher Berg quarry, Alsfeld-Brauerschwend). Moreover, we investigated different types of trachyte, which are present (1) as xenoliths inside an alkali basaltic tuff breccia of the Nickel quarry, (2) as trachytic lava from an outcrop at the Hauserhof (Nidda OberWiddersheim), and (3) as part of a trachytic/phonolitic lava dome drilled in the central VVF near Schotten-Sichenhausen. Apart from one garnet-bearing sample, the investigated basement xenoliths consist of phyllite and basic to felsic metavolcanics. Both the metamorphic index minerals and the quartz deformation microfabrics suggest that these rocks underwent greenschist facies metamorphism, and thus can be attributed to the Northern Phyllite Zone of the Variscan chain. These findings allow tracing the Northern Phyllite Zone from the Rhenish Massif via the VVF to the Harz Mountains. Variscan basement underneath the VVF is also documented by inherited U-Pb zircon ages obtained from the Hauserhof trachyte (366 ± 8.5 Ma and 294 ± 3 Ma) and from the trachyte of the Sichenhausen-Eschwald drilling (323 ± 12 Ma to 344 ± 6 Ma and ages ranging from the Middle Proterozoic to Cambrian/Ordovician boundary). The Permian and Carboniferous ages correlate with widespread late-Variscan volcanic activity in Central Europe at ~290 Ma, and with granitoid intrusions along the Rheic suture during the main phase of the collision of the Rhenohercynian and Saxothuringian domains at ~340 Ma, respectively. For the Latest Devonian ages (366 ± 8.5 Ma) several scenarios are possible. Middle Proterozoic to Cambrian/Ordovician ages are regarded as reworking of pre-Variscan sedimentary and magmatic material. The new ages and the garnetbearing xenolith, the metamorphic grade of which does not fit with the greenschist facies rocks of the Northern Phyllite Zone, suggest that rocks of both the Northern Phyllite Zone and the Mid-German Crystalline Zone are present underneath the volcanic rocks of the VVF. The U-Pb zircon ages of trachytic xenoliths of the Nickel quarry suggest the emplacement of a trachytic lava dome at 66.7 ± 0.4 Ma. Similar ages have been obtained from camptonitic dykes and trachyte intrusions within Permian and Triassic sediments exposed at the southwestern margin of the VVF. The large amount of trachyte together with a significant positive magnetic anomaly in the southwestern VVF might indicate a magma chamber that developed at shallow crustal levels below the southwestern part of the present Vogelsberg during the Upper Cretaceous. The new results have been used to develop a model for the early evolution of the southwestern Vogelsberg. Kurzfassung: Die vorliegende Arbeit befasst sich mit variszischen Grundgebirgsxenolithen, die sich in vulkanischen Gesteinen des tertiaren Vogelsberg-Vulkangebiets (VVG) befinden. Die untersuchten Proben stammen aus Steinbruchen am SW-Rand des Vogelsberges (Steinbruch Nickel, Nidda Ober-Widdersheim) sowie am NE-Rand des Vogelsberges (Steinbruch Rauher Berg, Alsfeld-Brauerschwend). Auserdem untersuchten wir verschiedene Arten von Trachyt, der (1) als xenolithische Einschlusse in einer alkalibasaltischen Tuffbrekzie des Steinbruchs Nickel, (2) als trachytische Lava an einem Aufschluss nahe des Hauserhofs (Nidda Ober-Widdersheim) und (3) als Teil eines trachytisch-phonolitischen Lavadoms in einer Bohrung nahe Schotten-Sichenhausen vorkommt. © 2014 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany DOI: 10.1127/1860-1804/2014/0079 www.schweizerbart.de 1860-1804/0079 $ 9.90 Z. Dt. Ges. Geowiss. (German J. Geosci.), 165 (3), p. 373–394, 15 figs., 5 tables Article Stuttgart, September 2014