MAGMA OF PEGMATITES FROM VOLHYNIA - COMPOSITION AND CRYSTALLIZATION PARAMETERS DETERMINED BY MAGMATIC INCLUSION STUDIES

Recrystallized melt inclusions (relies of the pegmatitic magma) were s tudied in topaz from the chamber pegmatites of Volhynia, Ukraine. The melt inclusions homogenize into silicate melts, which form glasses upo n quenching. The composition of pegmatitic magma was determined by mea ns of an electron and ion microprobe, along with the technique of loca l infrared spectroscopy. The major-element composition of this magma i s close to that of a strongly fractionated ongonitic melt poor in SiO2 and rich in Al2O3 and F. The first direct data on the trace element c ontents of the homogenized melt inclusions indicate that the pegmatiti c magma was rich in Li, Rb, Be, Sn, Nb, Cs, U, and Th, and poor in REE , Zr, Sr, Ba, and Ti. These geochemical features, typical of ongonites , are intrinsic to the pegmatitic magma rather than caused by overprin ted processes. The physicochemical parameters of the pegmatitic magma are as follows: liquidus temperature 650-700 degrees C, solidus temper ature 530 degrees C, pressure of the water-dominated fluid 2.6-3.0 kba r, magma was composed of melt, crystals, and fluid. The direct determi nation gave the following contents: 7 wt % H2O, 157 ppm CO2, 5.1 wt % F, 0.3 wt % Cl. Judging by the daughter minerals and inclusion texture s, the melt inclusions are an approximate model of the chamber pegmati tes. The assemblage of the daughter mineral association of the inclusi ons is essentially the same as the rock-forming and accessory minerals of the pegmatite, a fact indicative of their crystallization from the pegmatitic melt. Grains of the daughter minerals increase toward the contact with the fluid phase, and this provides evidence for the influ ence of fluid on the origin of the coarse-grained pegmatitic textures. The occurrence of syngenetic melt, crystal, and fluid inclusions sugg est the boiling of the pegmatitic magma, from which all main and acces sory pegmatite minerals crystallized. The magma was saturated in some minerals of Nb (columbite), Sn (cassiterite), W (wolframite), Be, and U. The partitioning coefficient of Cl between the fluid and melt was 3 3. The crystallization of pegmatitic magma can be approximated by the model for ongonitic magma, and the increase in Li, Rb, Cs, Sn, Nb, U, Th, H2O, F, and Cl, and decrease in Ti, Zr, REE, Sr, and Ba contents w ere caused by fractional differentiation.