Chemistry of the minerals of the granitic Murrumbidgee Batholith, Australian Capital Territory

Abstract

Sixty-six mineral concentrates from the granitic Murrumbidgee Batholith, A.C.T., were analysed for major and trace elements to explore their chemical variation. Analyses of eighteen microcline samples show that Ba, Sr, Rb and Ga vary in a regular manner which can be correlated with the SiO 2 content of their host rock, but Pb and Y vary irregularly; all six trace elements vary independently of the major element composition of the microcline. Twenty-two analyses show a high correlation between the anorthite content of plagioclase and the CaO and SiO 2 contents of the host rocks. Ba and Sr are significantly correlated with the Ca content of the plagioclase, and therefore with the SiO 2 content of the host rocks, but Pb, Rb and Y behave irregularly. Ga content of the plagioclase decreases with increasing SiO 2 content of the host rock and shows a high correlation with Al content of the plagioclase. All of twenty-two biotites analysed are rich in Al and the Al content of both tetrahedral and octahedral sites appears to be related to host rock paragenesis and, therefore, to bulk rock composition. Fe 2+, Mn, Li, Nb, Zn, Ga, Rb and Pb contents of the biotites increase with increasing SiO 2 content of their host rocks, whilst Mg, V, Cr, Co and Na decrease; Cu, K and Ba show no correlation with host rock SiO 2 content. Analyses of two muscovites indicate preferential enrichment in Nb and Ga. Chemical similarities between hornblendes occurring in xenoliths and in enveloping tonalite at an isolated locality in the batholith suggest a xenocrystal origin for the hornblende in the tonalite. Almandine-spessartine garnet occurs in two leucogranite intrusions and is probably a primary mineral phase precipitated from Mn-rich magma. Regular patterns discerned in the partition of trace elements between coexisting mineral pairs suggest that a close approach to equilibrium has been attained in the intrusions, but that three different types of intrusions recognised in the batholith have equilibrated at different temperatures.