Application of the Al-in-hornblende geobarometer and the hornblende-plagioclase geothermometer to the St. George Batholith, in SW New Brunswick

Abstract

The Al-in-hornblende geobarometer and amphibole-plagioclase geothermometer have been applied to selected samples from plutons in the Saint George Batholith, located in the southwestern corner of New Brunswick. These plutons are the Magaguadavic Pluton, the Evansdale Pluton and the Bocabec Bay Pluton, all of which met the proposed criteria for application of both the geobarometer and the geothermometer. The geothermometer and geobarometer are based upon the following Al dependent exchanges in hornblende: ANa IV Al A-1 IVSi-1 (temperature dependent edenite exchange vector) VI Al IV Al VlMg-1 IVSi-1 (pressure dependent tschermaks exchange vector). The following results were obtained for average pressures (±0.6 kbar) and temperatures (±75 °C): Magaguadavic Pluton, 1.0-2.1 kbar and 645 - 676°C; Evansdale Pluton, 0.9 kbar and 648°C; Bocabec Bay Pluton, 2.3 - 2.9 kbar and 700 - 746°C. Since both temperature and pressure are calculated based on the Al content of hornblende, it is necessary to come to grips with the factor that control the composition of hornblende. A determination of the exchange vectors controlling the Al content in hornblende should be done. Doing so on the samples from this study reveals that temperatures from 5 samples in this study may be acceptable, to the best of the geothermometer's accuracy. The stoichiometries of these samples suggest that the temperature dependent edenite exchange was dominant in their crystallization history, and therefore, the amphibole-plagioclase geothermometer is applicable. The other results are not very different from previous estimates (not using the methods presented in this study), but the lack of cation exchange correlation and evidence which suggests subsolidus alteration of the Al content of hornblende are factors that raise uncertainty in the results of this study. In order to successfully apply the Al-in-hornblende geobarometer to the plutons in this study, subsolidus changes in the Al-content of hornblende need to be investigated and the calibration of the geobarometer should be extended to low pressures (0 - 2.5 kbar) This should also include improvement in the precision of the barometer, something which may not be possible with the present thermodynamic data base on hornblende.