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Journal of the Geological Society

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Very low-grade metamorphic Sr isotopic resettings of magmatic rocks and minerals: evidence for a late Givetian strike-slip division of the Brabant Massif, Belgium

Laboratoires associés de géologie-pétrologie-géochronologie, Université Libre de Bruxelles, avenue F. D. Roosevelt, 50, B1050 Brussels, Belgium

Rb-Sr isochron ages recorded by the Ashgillian ignimbrites from Deerlijk and Harelbeke and the Caradocian quartz diorites from Quenast and Bierghes significantly post-date their emplacement. At Quenast, two mineral isochrons (biotite-WR-plagioclase: 372 ± 13 Ma; K-feldspar-WR-epidote: 375 ± 14 Ma) reveal that the Brabantian branch of the Caledonian belts was affected by a post-Siegenian, pre-Tournaisian event. This overprint turns out to be contemporaneous with the development of the Oudenaarde–Bierghes fault zone which is dated to 373 ± 11 Ma and 379 ± 13 Ma on foliated rocks from Bierghes, Deerlijk and Harelbeke. This c. 375 Ma cluster of events is interpreted as representing late Givetian stresses developed along the Oudenaarde–Bierghes fault zone for which the balance of geological data favours strike-slip movements. Since this compressive phase of deformation is posterior to the Givetian opening of the Borain trough, the Brabant Massif appears to have been affected by successive crustal extension and shortening events in the Middle Devonian.

Petrological evidence demonstrates the very low-grade character of the metamorphism in the metasediments on both sides of the Oudenaarde–Bierghes fault zone. From this, we deduce that Sr isotopic homogenization of whole-rock ignimbrite and the loss of radiogenic Sr from biotite and K-feldspar could be operative even at temperatures below 250°C.

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