Empirical phengitic geobarometer: substantiation, calibration and practical application

Kamzolkin V. A., Ivanov S. D., Konilov A. N.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2015. V. 144. N 5. P. 1-14

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Language: Russian


Phengite is a wide spread metamorphic mineral, which is stable in wide pressure range. Dependence of pressure on silicon content that was established in the middle of 20th century enabled us to consider the phengite based geobarometer. Recently phengite geobarometer was calibrated by Caddik and Thompson but for a limited range of pressure, however there are some attempts to extend its range. We have analyzed the large number of datasets on phengite composition published by different authors. Data under consideration included both natural and experimental specimens of well defined PT-condition. For moderate temperature (T < 750 °C) two groups of phengite can be marked. Groups are separated by silicon content value 3.25 p.f.u. Different geobarometer equations were suggested for both groups. Precisions of these geobarometers are ±0.34 GPa and ±0.56 GPa, respectively. There is no evidence of using phengite as geobarometer at high temperature (T > 750 °C). Derived dependences were applied to study conditions of the gneiss and schist metamorphism of Blyb metamorphic complex at the North Caucasus. This study shows that pressure peak of gneiss and schist metamorphism is 2.0—2.2±0.56 GPa. The latter agrees with previous works on the Blyb metamorphic complex eclogite data.

Key words: phengite geobarometry, the Caucasus, Blyb metamorphic complex.