Fractional crystallization of mantle-derived melts as a mechanism for some I-type granite petrogenesis: an example from Lachlan Fold Belt, Australia

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 Sr-87/Sr-86 values, 0.7037-0.7045, and epsilon(Nd) 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 Sr-87/Sr-86, Or between silica and Nd-143/(1)44Nd 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.