The Proterozoic I-type Kalkadoon and Ewen Batholiths and their comagmatic extrusive equivalents, the Leichhardt suite, form an association covering at least 5000 km2 in the central part of the Mount Isa Inlier. U-Pb zircon data and some Rb-Sr total rock data show that these rocks crystallised from melts emplaced between 1840 and 1870 m.y. ago and are the oldest dated igneous rocks in the Inlier. Chemically and isotopically, these granites are relatively uniform and, compared with most other Mount Isa granites , they have higher Sr and Al2O3 contents, and lower TiO2 , Zr, Nb, and Th contents . These chemical characteristics appear to be restricted to felsic igneous rocks known to be older than 1800 m .y. and may be useful in identifying the older felsic melts of the Mount Isa Inlier. The source for the rocks of the Kalkadoon- Ewen- Leichhardt association is estimated to have had an SiO2 content of 55-60 per cent. Relative to other large Palaeozoic and Mesozoic I-type batholiths elsewhere, this Mount Isa association is enriched in K20, Rb, Th, U, La, Ce, Zr, and Nb, and depleted in CaO, MgO, Ni, and Cr. The least isotopically disturbed granites of the association have relatively low initial 87Sr/86Sr ratios (about 0.704), which implies that the age of the source for these melts was not much older than the age of their emplacement. As chemically and isotopically similar granites occur in most Proterozoic areas of Northern Australia, it is inferred that during the period 1900-2100 m.y. a significant mantle differentiation event took place, during which large volumes of material were accreted to the base of the crust in these areas. Post-emplacement metamorphism and deformation , which have a maximum age of 1640 m.y. , caused significant textural and mineralogical changes in the Kalkadoon Batholith, but had a lesser effect on the Ewen Batholith . Igneous textures are commonly preserved in the Ewen Batholith, but the Kalkadoon Batholith, which has been metamorphosed from lower greenschist to upper amphibolite grade, shows significant isotopic disturbances.