Mafic inclusions from the Glenbog and Blue Gum granite suites, southeastern Australia

Abstract

Mafic inclusions from the I‐type granites of the Glenbog and Blue Gum suites of the Lachlan Fold Belt of southeastern Australia are less than 1 m and mostly less than 10 cm. in size and are generally in sharp contact with the surrounding host granites (unless otherwise stated the term “granite” is used in this paper in the general sense to include granite senso stricto, adamellite, granodiorite, tonalite, etc.). Although some inclusions are irregular in shape, the majority have an elliptical or circular outline when viewed on exposed surfaces. Rounded and subrounded individuals outnumber those with an angular appearance. Abundances of inclusions in seven plutons of the Glenbog and Blue Gum suites range from about 6% to less than 0.1%. Their abundance is negatively correlated with the SiO2 content of the host rock. Mafic inclusions contain the same mineral species as the host granite but plagioclase and hornblende are more abundant, and quartz and K‐feldspar are less abundant in inclusions. Mafic inclusions all contain more than 54% SiO2 and are mostly intermediate in composition. The abundances of many elements in the mafic inclusions follow those of that element in the host granites. Sr and Nd isotopic compositions in both the granites and the inclusions are similar and relatively evolved. Various models for the origin of the inclusions are considered. It is concluded that, of those considered, the only model capable of explaining all data is that the inclusions represent fragments of more refractory source material that were carried up with the magma. It is suggested that this more refractory character resulted from a lesser degree of hydration of the source rocks of the magmas, prior to partial melting.