Palaeoproterozoic Kimban mobile belt, Eyre Peninsula: Timing and significance of felsic and mafic magmatism and deformation

Abstract

Within the Kimban mobile belt on the eastern margin of the Gawler Craton, two separate tectono‐thermal and up to four separate magmatic events can be recognised. The first (inferred) magmatic event involved underplating of the mafic, lower crustal protolith of the Donington Granitoid Suite at 2100–2000 Ma. Emplacement, around 1850 Ma, of the Donington and Colbert Granitoid Suites and the mafic, syn‐plutonic Jussieu dykes, together forming the Lincoln Batholith, comprise the second magmatic event. Donington Suite emplacement was accompanied by high‐temperature meta‐morphic conditions, indicated by syn‐intrusive migmatitic shearing in host rock of the Lincoln Batholith and by the generation of voluminous felsic selvages around the Jussieu dykes. This first tectonother‐mal event in the Lincoln Batholith is associated with D₁ in Hutchison Group metasediments. The mafic Tournefort dykes, emplaced during the third magmatic event, are structurally and compositionally distinct from the Jussieu dykes. Their time of emplacement is constrained by the 1850 Ma Donington Suite rocks which they post‐date, and by ca 1720 Ma peak metamorphic assemblages contained within them. The main deformation within the Lincoln Batholith occurred during a second tectono‐thermal event, broadly associated with D₂ and D₃ in the Hutchison Group. This event occurred after Tournefort dyke emplacement, perhaps in the order of 100 million years after the first tectonothermal event. The Kalinjala Shear Zone was active during this second tectonothermal event, juxtaposing the Lincoln Batholith to the east with the Hutchison Group to the west. Since there is no evidence to date that indicates that the first tectonothermal event was associated with orogenic activity or that it was an integral part of the orogeny associated with the second tectonothermal event, the term ‘Kimban Orogeny’ should be restricted to the latter. The fourth magmatic event is represented by emplacement of the Moody Granitoid Suite into the Hutchison Group, pre‐ to syn‐D₃. Within the Kimban mobile belt on southern Eyre Peninsula, only the Moody Suite represents true syn‐orogenic magmatism. The term ‘Lincoln Complex’, which is synonymous with a common genesis for intrusives of the second, third, and fourth magmatic events, is inappropriate and should be abandoned. Whereas the Donington Suite is chemically distinct from plate‐margin type calc‐alkaline magmatism, the Lincoln Batholith has some physical aspects in common with plate‐margin batholiths, notably the occurrence of mafic enclaves and the emplacement of mafic syn‐plutonic dykes, indicating the coexistence of mafic and felsic magma. Until the underlying mechanisms that control the chemical evolution and physical architecture of magmatic systems are understood, it is dangerous to use any of these characteristics to infer a tectonic setting by comparison with modern examples.

Key words:

• deformation
• magmatism
• mobile belt
• Palaeoproterozoic