Aluminum in hornblende: An empirical igneous geobarometer

Electron-microprobe analyses of hornblendes from five calc-alkalic plutonic complexes representing low- and high-pressure regimes define a tightly clustered linear trend in terms of total Al (A1^T) and tetrahedral Al (Al^iv) contents per 23 oxygens: Al^iv = 0.15 + 0.69A1^T, r² = 0.97. Hornblendes from shallow-level intrusions predominate for A1^T ≦ 2.0; hornblendes from epidote-bearing intrusions within the accreted terranes of the western Cordillera all have A1^T ≧ 1.8. Rocks from both pressure regimes span similar ranges in bulk-rock SiO₂ content, have a common magmatic mineral assemblage (plagioclase, hornblende, biotite, K-feldspar, quartz, sphene, magnetite or ilmenite, ± epidote), and probably crystallized at similar temperatures.

Data collated from the literature on calcic amphiboles from other plutonic complexes and from phase equilibrium experiments using natural rocks or synthetic analogue compositions show a similar A1^T-Al^iv trend and systematic pressure effects. High-pressure calcic amphiboles have high A1^T, independent of composition. Although Al^iv is temperature dependent, temperature alone cannot account for the observed differences in A1^T between low- and high-pressure compositions.

The relation between A1^T in hornblende and pressure (in kilobars), for data from calc-alkalic plutons with the common mineral assemblage, is P = –3.92 + 5.03A1^T, r² = 0.80. The A1^T content of hornblende is suggested as an indicator of pressure to within ±3 kbar for crystallization of plutonic rocks of appropriate bulk composition and mineral assemblage.