Abstract
We have tested the proposed hornblende geobarometer of Hammarstrom and Zen (1985) by analyzing and averaging only rim compositions of hornblende from plutons for which we have independently determined pressures of intrusion from the metamorphic country rocks. Over a range in pressure of 2-8 kbar, the average hornblende rim content of A1 at nine localities correlates linearly with pressure. The data points fall within error on the linear relation proposed by Hammarstrom and Zen (1986), which was based primarily on data near 2 and 8 kbar. Our results confirm that the relation is valid for intermediate pressures (4-6 kbar), and the addition of our intermediate-pressure data reduces the error in pressure determined from the A1 content of hornblende from ±3 kbar, proposed by Hammarstrom and Zen (1986), to ±1 kbar, provided due caution is exercised in sample selection. The geobarometer works because calc-alkaline plutons seem to be bivariant at the end of crystallization and because the temperature of solidification is fairly pressure insensitive above 2 kbar.
