IMMISCIBILITY IN ACIDIC MAGMAS - EVIDENCE FROM MELT INCLUSIONS IN QUARTZ PHENOCRYSTS OF IGNIMBRITES

Unusual melt inclusions occur in quartz phenocrysts of the Lashkerek i gnimbrites in the Kurama Mountains, Central Asia; the inclusions provi de evidence of immiscibility in high-temperature acidic magmas, leadin g to the separation of iron-rich globules from the silicate melt. Incl usions contain gas (10-14 vol.%), translucent glass (85-90 vol.%) and one (sometimes several) spherical or semispherical ore globules (0.4 t o 1.0 vol.%). Average composition of the globules from microprobe anal yses is (in wt.%): 81.2 FeO, 9.9 SiO2, 2.2 Al2O3, 0.36 K2O, 0.34 Na2O, 0.27 CaO, 0.19 ZnO, 0.16 MnO, 0.01 TiO2, 0.01 Cl. E.M.Raster scan pro files of the characteristic radiations of Si, Fe and Al with a magnifi cation of 8000 x reveal a homogeneous distribution of the elements in the globules. The siliceous glass matrix to the iron-rich globules con tains (in wt.%): 70.2 SiO2, 17.7 Al2O3, 0.32 FeO, 0.66 CaO, 3.7 Na2O, 6.7 K2O, 0.29 Cl. The ore globules dissolve in the siliceous melt at 1 050-1100-degrees-C, and total homogenization occurs at 1140-1250-degre es-C.