Reversal of Ages of the Kalkadoon/Leichhardt Complex and the Magna Lynn Metabasalt, NW Queensland

One of the central tenets of interpretation of the Mount Isa Inlier, North West Queensland, is that there is a median underlying belt of Paleoproterozoic acid volcanics (Leichhardt Volcanics) and granites (Kalkadoon Supersuite), 1850–1860 Ma, commonly referred to as the ‘Kalkadoon-Leichhardt Basement’. A primary requirement of this interpretation is that one of the main boundaries, that between the felsic Leichhardt complex and the Magna Lynn Metabasalt, is an unconformity. This boundary is everywhere serrated and complex, and the unconformity interpretation would require it to have been deformed by a system of variably plunging, refolded folds. Mapping of this boundary in the field, and using previous mapping and remotely sensed images, shows it to be better interpreted as intrusive, with isolated bodies of Magna Lynn Metabasalt within the Leichhardt complex interpreted as relict mega xenoliths, rather than fault blocks or refolded synforms. An intrusive relationship of the Kalkadoon/Leichhardt complex calls into question the relationships of the other mafic volcanic sequences across the Inlier.