Mt Stuart amphibole: intra-sample variation and Al-in-hornblende barometry reassessed.

Abstract

The Mt Stuart batholith has been key in demonstrating the use of the Al-in-hornblende (AH) barometer for estimating the crystallization depth for plutonic rocks. Previous studies have used the AH barometer to infer regional tilting, magma loading, and to restore paleomagnetic data. Previous studies have, however, failed to fully consider the range of amphibole textures and compositions within the batholith and the significance of intra-sample variation on AH barometry. We have sampled across the different age and compositional zones of the batholith and have collected texturally-controlled major and trace element data for pyroxene, plagioclase and amphibole. Amphibole textures indicate direct magmatic crystallization, growth from pyroxene + melt reaction, reequilibration with a fluid phase, and the presence of xenocrystic amphibole. Of particular note is the common occurrence of re-equilibrated amphibole which occurs not just along cracks or near chloritized biotite as previously reported [1], but is common along the rims of most amphibole grains. Amphibole rims were analysed exclusively by [1] and it is likely that the pressure determinations are affected by the analysis of compositions that do not represent equilibrium magmatic growth. Magmatic amphibole composition is distinct from fluidreequilibrated rims. The latter, relative to unaltered magmatic compositions, are Al and REE depleted. On a A Na+ A K+ A Ca vs T Al– A Na– A K–2 A Ca plot, amphibole compositions lie along trend-lines connecting magmatic hornblende and fullyreequilibrated actinolite, indicating varying degrees of fluidreequlibration. In terms of AH barometry, a range of calculated pressures are attained, even for a single thin section, ranging from magmatic crystallization pressures to lower pressures estimated from fluid-altered rims. Data from the analyzed samples indicate that the Mt Stuart magma crystallized at pressures of 3–4 kbar at 730–810 °C and that fluid-reequilibration occurred at high temperatures not far below the wet-granite solidus. This result is consistent with a recent oxygen isotope study [2]. We conclude that AH barometry alone does not provide compelling evidence for batholith tilting.