CRYSTALLIZATION CONDITIONS AND HIGH-ALUMI NA TRACHYTE MAGMA COMPOSITION (PANTELLERIA ICELAND, ITALY) BASED ON INCLUSION STUDIES

Crystalline, melt and fluid primary inclusions in phenocrysts of pyrox ene and anorthoclase in high alumina trachytes were studied. Optical, thermometric and cryometric measurements as well as compositional dete rminations of glass and crystalline phases on the microprobe were used . Among the crystalline inclusions apatite, clinopyroxene, olivine (Fo 38-29), ilmenite, titanomagnetite and sulphide were identified. The m elt inclusions in anorthoclase are presented by crystalline phases inv olving ''daughter'' anorthoclase, pyroxene with 1,2 wt.% Of ZrO2, amph ibole, ilmenite and titanomagnetite. The homogenization temperature of such inclusions was measured as 985-1070-degrees-C for anorthoclase a nd 1045-1180-degrees-C for pyroxene. Only carbon dioxide was identifie d in fluid inclusions providing the pressure not above 250 bar. High a lumina trachyte melts are characterised by high chlorine content (0,44 wt.%), ZrO2 (0,22 wt.%) and low water content (<0,3 wt.%). The predom inant fraction of chlorine (about 80%) was presumably removed in the p rocess of volcanic extrusion of high alumina trachytic melts.