Magmatic flare-up at the Carboniferous/Permian boundary in the NE German Basin revealed by SHRIMP zircon ages

Abstract

SHRIMP ages of 12 volcanic samples indicate intense magmatic activity to have occurred at the Carboniferous/Permian boundary throughout much of the NE German Basin. Rhyolitic crystal-rich samples have been taken from quarries in the Halle Volcanic Complex (HVC) and in the Flechtingen Ignimbrite, and from drill cores of the Kotzen, Mirow, Friedland and Penkun areas. Ten samples yielded 206Pb/ 238U ages between 302 and 297 Ma (±3 Ma, respectively) which indicate that the magmatic activity took place concentrated in a relatively short time span throughout much of the NE German Basin. Two HVC samples have ages of 307 and 294 Ma. This remarkably synchronous magmatic activity occurred during the initial phase of the basin development. Considering published volume estimates (48,000 km 3), extrusion rates during this time period probably were in the order of 0.01 km 3 a −1, implying magma production rates of 0.1 km 3 a −1. The calc-alkaline SiO 2-rich volcanic rocks originated from magmas that presumably formed during anatexis and subordinate magma mixing in an intra-continental transtensional setting. The basaltic magma which must have provided the thermal input into the lower crust probably formed during decompressional melting of lithospheric mantle fertilised by previous magmatic processes. The dated volcanic rocks occur in three geotectonic provinces, namely the (a) Mid German Crystalline Rise, which forms the northern part of the Internal Variscides, the (b) External Variscides and the Variscan foreland which is considered as (c) Eastern Avalonia. Many of the old zircons found in the HVC samples reflect the magmatic activity of the Mid German Crystalline Rise (325–400 Ma). However, Cadomian (500–650 Ma) and older Gondwanian elements (1674–2373 Ma) are also present. Although the Flechtingen and Kotzen areas are located within the External Variscides, it seems more likely that the middle to lower crust, that experienced anatexis, formed part of the Eastern Avalonia Plate overridden by the Variscan Front. Thus the old zircons found in Flechtingen (350 and 538 Ma) and Kotzen (345 Ma), but certainly those present in the Penkun drill cores (1483 Ma) represent Eastern Avalonia tectonic history. The anatectic magmas in the Friedland area probably formed partly from Baltic crust thrusted below Eastern Avalonia during the Caledonian Orogeny. Among the old zircons from the Friedland samples, in addition to Proterozoic ages (1456 Ma), we found testimony of Caledonian (443 Ma) and of clearly post-Caledonian (387 Ma) magmatism.