Petrology and P-T Evolution of the Granulite-facies Rocks of the Angara-Kanskiy Uplift Block (Eastern Siberia)

Olga Kalichka (kalichka@ggd.nsu.ru) & Nikolai Popov (popov@uiggm.nsc.ru)

Koptyug Pr. 3, Novosibirsk 630090, Institute of Geology, lab 922, Russia

The Kanskaya formation in the Angara-Kanskiy uplift block is an excellent example of high-grade metamorphic terranes. Coronites derived from olivine-Fe-Ti -oxide-bearing gabbro and metapelitic rocks are common in such area. Primary igneous and secondary metamorphic minerals record textural and chemical changes, if rocks bulk chemistry has not change during metamorphic process. Ilminite inclusions and lamellar structure in low-calcium and aluminium (up to 4 wt.%) orthopyroxenes indicate relatively higher Ti-concentration and the extremely high crystallization temperature (>1300°C) of primary igneous pyroxenes of gabbro. There are two types of coronitic textures between plagioclase and olivine and plagioclase and clinopyroxenes. The former consist of clinopyroxene and spinel- orthopyroxene symplectites, the later consist of mainly garnet. The two groups of PT-parameter values were performed using the program THERMOCALC, which computes all possible end-member equilibria for a given assemblage using internally consistent thermodynamic database for minerals and Ol-Cpx, Cpx-Opx, Grt-Bt-Opx-Qtz, Grt-Cpx, Grt-Bt, Grt-Sil-Bt-Qtz, Grt-Ilm, Grt-Spl-Sil-Qtz, Grt-Cam-Pl-Qtz geothermometers and geobarometers. Temperatures of metapelitic rocks formation are approximately 100-200°C lower and are characterized by a wide variation of values (T = 580+200°C, P = 6+2.5 kbar), than corresponding parameters for coronites (T = 780+60°C, P = 7.7+1 kbar). Inasmuch as the coronites and metapelitic rocks form a banded complex, it is impossible to state that such P-T condition difference took place during their formation. Such variation of P-T condition values is evidently stipulated by initial rock composition and by the nature of rock protoliths. Parental sedimentary rocks are supposed to contain more water in fluid than initial magmatic rocks, which crystallized under granulite facies conditions. Undoubtedly, rates of component diffusion increase at the presence of water fluid. Thus, distribution of components in gneiss minerals corresponds to temperatures, reflecting the regressive stages of metamorphism.

Liquid Immiscibility on the Phase Diagram for a Multicomponent Aluminosilicate System

Valery M. Kalugin (kalugin@uiggm.nsc.ru)

Institute of Geology, Koptyuga pr. 3, Novosibirsk, 630090, Russia

Liquid immiscibility was detected in the diversified magmatic rocks of the Earth and Moon, however our knowledge about this process remains too primitive. There is only one Lc-Fa-SiO₂ phase diagram (Roedder, 1951), which can be used as pattern for tracing of the melt division in a multicomponent aluminosilicate system. Naslund (1983) has found out experimentally that addition of CaO, TiO₂ and P₂O₅ to this system result in, probably, to broadening of liquid immiscibility field, however till now there is no acceptable phase diagram for explanation of this phenomenon.

Experimental investigations of a liquid immiscibility in ternary systems such as MeO-SiO₂-Al₂O₃ have shown, that on the diagrams with CaO, MgO and FeO components metastable liquid immiscibility fields are very similar (Galakhov, 1985). The minor differences are in direct relation to the radius and charge of a network modifier cation (Galakhov, 1969). Thus, in a multicomponent system the size and critical temperature of liquid immiscibility field change unsignificantly and depending on a magnitude relation of these cations.

A field of stable low-temperature liquid immiscibility appears as a result in intersection of liquidus surface and a volume of metastable liquid immiscibility. It can take place only above the SiO₂-(Me₂SiO₄-Me₂Si₂O₆) cotectic curve and only in those systems, where an eutectic and accordingly cotectic temperatures are low enough. Therefore it was detected only in Lc-Fa-SiO₂ system and misses in similar Lc-Fo-SiO₂ and Lc-Di-SiO₂ systems. The attachment of CaO and MgO components to the Lc-Fa-SiO₂ system will reduce its eutectic temperature, and area of stable low-temperature liquid immiscibility can be found there too.

It's the most convenient to use a pseudoternary diagram with following oxide components: SiO₂, Me²⁺O, and (Me¹⁺, Me²⁺_0.5)Me³⁺O₂, which are structural units of a silicate melt.

Roedder E, Am. Miner, 36, 282-286, (1951).

Naslund HR, Am. Journ. Sci, 283, 1034-1059, (1983).

Galakhov FYa & Vavilonova VT, Phys. Chem. Glass., 11, 276-281, (1985).

Galakhov FYa & Varshal BG, Liquid Immiscibility in Glasses, Nauka, Leningrad, 6-11, (1969).