The Mt Buller igneous suite in the southeastern Lachlan Fold Belt encompasses a large variety of rock types, from gabbros through diorites to granites. Mafic rocks and enclaves have primitive, mantle‐like initial 87Sr/86Sr values, 0.7037–0.7045, and ϵNd values +5.6 to +4.1. The granites do not show distinctively more radiogenic initial 87Sr/86Sr values, suggesting they are derived from the same parent as the gabbros. Most of the Mt Buller rocks do not show a positive correlation between silica and initial 87Sr/86Sr, or between silica and 143Nd/144Nd values, indicating that neither crustal contamination nor mixing between mantle‐ and crustal derived melts has been important and that these rock types are likely to have formed by fractional crystallization from a common mantle‐derived parent. Some scatter in compatible trace elements, particularly in granites and diorites, may be explained by mixing. Geochemical modelling confirms a comagmatic origin of the suite. Magma generation probably took place in the subduction‐modified upper mantle. This mechanism may play an important role in I‐type granite petrogenesis in particular tectonic settings such as during an efficient slab roll‐back or sinking of oceanic crust in divergent subduction conditions.