A. Soesoo, Fractional crystallization of mantle-derived melts as a mechanism for someI-type granite petrogenesis: an example from Lachlan Fold Belt, Australia, J GEOL SOC, 157, 2000, pp. 135-149
The Mt Buller igneous suite in the southeastern Lachlan Fold Belt encompass
es 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, suggestin
g they are derived from the same parent as the gabbros. Most of the Mt Bull
er rocks do not show a positive correlation between silica and initial Sr-8
7/Sr-86, Or between silica and Nd-143/(1)44Nd values, indicating that neith
er crustal contamination nor mixing between mantle- and crustal derived mel
ts has been important and that these rock types are likely to have formed b
y fractional crystallization from a common mantle-derived parent. Some scat
ter in compatible trace elements, particularly in granites and diorites, ma
y be explained by mixing. Geochemical modelling confirms a comagmatic origi
n of the suite. Magma generation probably took place in the subduction-modi
fied upper mantle. This mechanism may play an important role in I-type gran
ite petrogenesis in particular tectonic settings such as during an efficien
t slab roll-back or sinking of oceanic crust in divergent subduction condit
ions.