Metasomatism of a depleted granulite facies terrain in the Arunta Block, central Australia

Abstract

Geochemical investigations in the Utralanama Block, an intermediate pressure granulite facies terrain in the Arunta Block, central Australia, has revealed several anomalous features, not consistent with the depletion of granitophile components generally considered to accompany granulite facies metamorphism. However, other geochemical features are indicative of depletion. The mean K2O for the Utralanama Block is exceptionally low relative to most other granulite facies terrains, but Rb contents are comparatively high. Consequently, the mean K/Rb ratio is relatively low for granulite facies terrains as is the mean Ba/Rb ratio, whilst mean K/Sr and Rb/Sr ratios are much higher than usual for such terrains. Only the K/Ba ratio shows equivalent values to depleted terrains elsewhere. Comparison of these ratios for the three main compositional groups of rocks in the Utralanama Block reveals that for mafic rocks all the above ratios are characteristic of extreme depletion, whereas, for all but the K/Ba ratio, mean ratios for the pelitic rocks, and to a lesser extent for the quartzofeldspathic rocks approach normal crustal values or values for metasomatic rocks. The abnormally high Rb/Sr ratios of these rocks compared to average crustal rocks suggest, however, that metasomatism is the cause of the anomalous geochemical features of the Utralanama Block, and this is supported by field and microstructural evidence. Thus, Rb/Sr ratios appear to be useful indicators of metasomatism where no gross mineralogical or microstructural evidence for metasomatism is obvious, and under such conditions the K/Ba ratio may be more reliable than the K/Rb ratio for indicating prior depletion of the terrain.