Blundy and Holland (1990)
recently proposed a pressuredependent geothermometer based on the tetrahedral aluminum content of calcic amphibole and the albite content of coexisting plagioclase. Their formulation of these equilibria as barometers, when set at a fixed temperature of 700~ gave an equation quite similar to the equation that we proposed as an empirical geobarometer (Hammarstrom and Zen 1986). Blundy and Holland suggest that the previously proposed hornblende geobarometers, including ours as well as the empirical version of Hollister et al. (1987) and the experimentally calibrated version of Johnson and Rutherford (1988, 1989), are unreliable unless one can assume a magmatic temperature and rule out subsolidus reequilibration. These assumptions are important because: (I) tetrahedral aluminum in calcic amphibole is strongly temperature dependent; (2) tetrahedral aluminum accounts for most of the aluminum present in calcic amphibole encountered in quartz-saturated rocks. Contrary to Blundy and Holland's inference, we did not ignore the temperature dependence of tetrahedral aluminum in calcic amphibole, and, in fact, our Fig. 7b is remarkably similar to Blundy and Holland's Fig 6a . We showed that changes in both tetrahedral and total aluminum followed a positively correlated linear trend for the data set we considered and that for a restricted temperature range of about 100~ which we consider appropriate for the final stages of crystallization, the total aluminum content of calcic amphiboles encountered in experimental studies exhibits a pressure dependence. Johnson and Rutherford (1989) recently confirmed this pressure dependence for three bulk compositions appropriate for application of the hornblende geobarometer at 740 to 800~
We tested the Blundy and Holland geothermometer on representative samples of tonalites and granodiorites from our original data set for which we had coexisting plagioclase analyses. The pressures of emplacement of the three "8-kbar" plutons that we used to calibrate our equation since have been verified by independent pressure