Эмпирический фенгитовый геобарометр: обоснование, калибровка и применение

Протестированы фенгитовые геобарометры Масона—Шрайера и Каддика—Томпсона с использованием большого массива данных по составам природных и синтезированных фенгитов с известными условиями формирования. Выделены три группы фенгитов: 1 — Si 3.0—3.25 к.ф., 2 — Si > 3.25, T<750 °C, 3 — Si> 3.25, T>750 °C. Для двух первых групп выведены новые уравнения геобарометра. Использование этих уравнений для изучения гнейсов и сланцев блыбского метаморфического комплекса Северного Кавказа показало, что пиковое давление при метаморфизме достигало 2—2.2 ГПа.

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. Accuracies 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 gneisse and schist metamorphism of Blyb metamorphic complex at the North Caucasus. This study shows that pressure peak of gneisse and schist metamorphism is 2.0–2.2 ±0.4 GPa. The latter agrees with previous works on the Blyb metamorphic complex eclogite data.