The origin of arfvedsonite in metabasites from contact aureole of the Kharlovo gabbro intrusion

Hornblende and arfvedsonite were formed at T = 525-550 degrees C and P = 1-1.5 kbar in the middle part of the contact aureole of the Kharlovo gabbro massif according to the following reactions: (1) 0.8 Chl + 0.27 Bt + 0.83 Cal + 1.38 Mag + 1.68 Spn + 1.4 Ab = 1.5 Hbl + 1.58 Ilm + 4.65 H2O + 0.83 CO2 (2) 0.02 Chl + 1.18 Ab + 0.21 Cal + 0.14 Mag = 0.1 Arf + 1.0 Olg + 0.02 H2O + 0.21 CO2 A rare combination of heterogeneous rock composition, PT-parameters and thermal metamorphism of short duration led to the formation of arfvedsonite in metabasites. Arfvedsonite formed under local saturation of pore solution by Si, Na and Fe in a very small volume as a result of a local metasomatic reaction. These reactions were controlled mainly by sodium and silicon leaching during incongruent dissolution of acid plagioclase. Mass balance analysis and the variation of composition and amount of coexisting mineral show that the transfer of the main petrogenetic components was limited to volumes as small as 0.06-0.1 mm(3).