U-Pb ages from SW Poland: evidence for a Caledonian suture zone between Baltica and Gondwana

Faunal and palaeomagnetic evidence suggests the existence of a c. 3000 km-wide Tornquist Sea between Gondwana and Baltica in Early Ordovician times, which narrowed to <1000 km by the Late Ordovician. The inferred suture zone between sequences with ‘Baltic’ faunas in Poland and the ‘Gondwana’ faunas in Czechoslovakia is characterized by a collage of six terranes which have distinct Cambrian to Carboniferous histories. They have geochemical characteristics and sedimentary associations of volcanic rocks in marginal basins, island arcs, ophiolites and volcanic arc granites, give Tremadoc and Arenig protolith U–Pb zircon ages, and have been regionally metamorphosed. A syn-metamorphic granite is dated as pre-Lower Ashgill (461 −50 –2 Ma). An unmetamorphosed ophiolite gives a 420 −20 –2 Ma age, whilst in two other terranes amphibolite-facies island-arc lavas have fossiliferous Lower Ludlow (424–415 Ma) sequences: all are unconformably overlain by unmetamorphosed Upper Devonian conglomerates. A sinistral transpressive regime is observed in regionally extensive mylonite zones. Titanite and zircon ages (338 −2 –3 and 339 ± 4 Ma) record Variscan magmatism. The data suggest considerable narrowing of the Tornquist Sea during the Ordovician, continuing ocean floor and island arc activity in the Silurian, and final sinistral transpressive closure by the Mid-Devonian.