PALEOZOIC ARC AND DEFORMATION ZONE DISTRIBUTION AROUND EASTERN AVALONIA AND ITS TECTONIC SIGNIFICANCE

The eastern part of the Avalonian microcontinent (independent 462-443 Ma b.p.) is preserved in north-central Europe, comprising neoproterozoic basement underlying SE Ireland, England, Wales and Flanders. Its stable core is the Midlands Microcraton in Central England, to the northwest of which strongly deformed Lower Paleozoic rocks of the British and Irish Caledonides crop out. They include on the Avalonian side of the Iapetus Suture Tremadocian andesitic volcanics in South and Mid-Wales, and, further to the NW the Caradocian arc-backarc system of the English Lake District, North Wales and Leinster with abundant, well-exposed magmatism. Backarc extension was not oceanic: extension was apparently less than in the near-correlative Gander and Exploits arc-backarc systems of Newfoundland.

Concealed beneath younger rock in Eastern England, but recorded in several borehole sections, the continuation of the Lake District magmatic arc curves southeast and extends as the Anglo-Brabant Deformation Belt (ABDB) to the Brabant Massif in Belgium. East of the ABDB the southern North Sea - Luneberg Terrane (SNSLT) is also thought to be part of Avalonia because basement is exposed as end-Proterozoic ("Panafrican") granite gneisses in inliers from NW Germany, and borehole faunal and sediment provenance evidence from Rugen indicates the arrival of Panafrican age detritus, with Gondwana-related fossils in the Ashgill, coeval with convergence of established Avalonian crust with Laurentia and Baltica. The transpressional Heligoland-Pomerania Deformation Belt, marking the contact of the SNSLT with Baltica lacks significant magmatism.

Finally, Siluro-Devonian sporadic calc-alkaline magmatism along the "southern" Avalonian margin of Laurussia developed above a subducting Rheic Ocean plate.