Amfibolok petrogenetikai jelentősége a Ditrói alkáli masszívum ultramafikus kumulátumkőzeteiben

Amphiboles are the main rock-forming minerals in the ultramafic cumulates of the Ditrău Alkaline Massif, Eastern Carpathians, where they appear both as cumulus and intercumulus phases. In olivine-pyroxene hornblendite amphibole represents an intercumulus phase, in plagioclase-bearing pyroxene hornblendite it is both intercumulus and cumulus phases, while in plagioclase-bearing hornblendite it becomes a cumulus phase. We used the composition of amphiboles to estimate the pressure and temperature conditions of their crystallization. The best results were given by the thermobarometer calibrated also for alkaline systems. Cumulus and intercumulus amphiboles of the Ditrău ultramafic cumulates are supposed to have crystallized at mid to lower crust levels in ~25 km depth indicated by the calculated crystallization temperature and pressure of 900–1050 °C and 6±1 kbar, and 950–1050 °C and 7±1 kbar, respectively, from a magma containing 6–7% H2O. Based on their composition, the amphiboles are inferred to have crystallized from a Si-undersaturated, differentiated basaltic-basanitic melt. Intercumulus amphiboles and the enclosed olivine and clinopyroxene cumulus crystals of the Ditrău ultramafic cumulates crystallized froma more primitive melt (mg#amp=0.46–0.48; mg#ol=0.43–0.46; mg#cpx=0.42–0.58), while cumulus amphibole phases were formed from a more fractionated melt (mg#=0.24–0.33).