Abstract

The aluminum-in-hornblende (AH) barometer has been widely used to estimate the emplacement pressure (P) of granitic rocks. Application of the barometer has remained controversial because the controls on the aluminum content of hornblende imposed by temperature (T), magma composition, and fluid composition remain incompletely understood. In this paper, the P-T conditions of equilibration for 19 rocks from the Mesozoic batholiths of California are estimated by simultaneous evaluation of (1) the P-sensitive reaction tremolite + phlogopite + 2 anorthite + 2 albite = 2 pargasite + 6 quartz + K-feldspar and (2) previously published amphibole-plagioclase thermometer equilibria. P estimates for the batholiths range from 1.2 to 7.4 kbar and are consistent with independent geologic evidence. The highest P samples (5.1–7.4 kbar) are located near the San Andreas fault. Furthermore, it appears that regional P gradients determined by previous AH barometry studies are valid, but that the absolute values of the P estimates will need to be revised somewhat (±1.5 kbar). I suggest that the method of this study provides a viable, thermodynamics-based alternative to conventional AH barometry that requires no a priori assumptions about T or bulk composition.

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