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The Al-in-homblende barometer potentially offers a basis for estimating crystallization pressure for granitic batholiths. However, owing to the simplicity of its formulation, misuse of the barometer can occur. Many granitic intrusions are emplaced at conditions inconsistent with those of the existing experimental calibrations, including fO2 < NNO and/or variable to high temperature. The barometer is sensitive to variations in both fO2 and temperature: low fO2 can cause calculated pressures to be high by a factor of two or more, and the effect of temperature is up to 2 kbar per 100 °C, depending on total Al abundance.

Batholiths emplaced at elevated temperature and portions of plutons that crystallized below H2O saturation yield artificially high pressures relative to near-solidus experiments conducted at H2O saturation. Al-in-homblende pressures within the tonalitic Mount Stuart batholith of Washington, for example, define a 1–4 kbar domal structure that is exaggerated by emplacement crystallization temperatures that range from 620 to 760 °C. Pressure correlates with temperature (r2 = 0.86), indicating that temperature-sensitive edenite exchange has greatly influenced observed pressure variations. Data for other plutons also exhibit a marked positive correlation between temperature and hornblende pressure. If corrections are not made for temperature, such large apparent pressure variations can lead to overly high estimates of pluton thickness and tilt in addition to invalid tectonic interpretations.

Using experimental data at ~675 °C (Schmidt, 1992) and at ~760 °C (Johnson and Rutherford, 1989), a revised expression for the barometer incorporating the effect of temperature is P (±0.6 kbar) = 4.76Al − 3.01 − {[T(°C) − 675]/85} × {0.530Al + 0.005294[T (°C) − 675]}. For a pluton emplaced at 100 °C above wet-solidus temperatures, this reformulation of the barometer lowers derived pressures by 1.3 to >2 kbar at typical crustal pressures. For the Mount Stuart batholith, consideration of temperature yields revised pressures that are in agreement with pressures obtained from wall rocks and eliminates much of the apparent domal structure.

Low-/fO2 granites have amphibole Fe/(Fe + Mg) ratios that exceed the typical 0.40−0.65 range used in most experimental and empirical calibrations. Examples from anorogenic granitic batholiths of mid-Proterozoic age yield pressures that are too high by a factor of two to three in comparison with pressures obtained from adjacent metamorphic assemblages. Hornblende in these granites not only has high Fe/(Fe + Mg) but also low ratios of Fe3+ to Fe2+. The anomalously high Al in Fe-rich, Fe3+-poor hornblende is inferred to be the result of increased [6]Al occupancy of the M2 site not buffered by the Mg and Fe3+ abundances typical of amphiboles in calc-alkaline and other high-fO2 plutonic rocks.

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GeoScience World


McLean, VA

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American Mineralogist