Empirical phengite geobarometer: Background, calibration, and application

Abstract

Phengite is a common metamorphic mineral stable in a wide pressure range. The dependence of pressure on silicon content established in the mid-20th century allowed us to propose a phengite-based geobarometer. Recently, the phengite geobarometer was calibrated by Caddik and Thompson (2008) but in the narrow pressure range. However, there attempts have been made to extend this range. We have analyzed the large number of published datasets on phengite composition. These data included both natural and experimental specimens of well defined P–T-conditions. For moderate temperatures (T < 750°C), two groups of phengite are identified. These groups are divided by silicon content value of 3.25 apfu. Different geobarometer equations were suggested for both groups. The precision of these geobarometers is ±0.34 GPa and ±0.56 GPa, respectively. There is no evidence of phengite used as a geobarometer at high temperatures (T > 750°C). The derived dependences were applied to study the conditions of gneiss and schist metamorphism of the Blyb metamorphic complex in the Northern Caucasus. This study shows that the peak pressure of gneiss and schist metamorphism is 2.0–2.2 ± 0.56 GPa. The latter agrees with previous data on the Blyb metamorphic complex.