ULTRAMETAMORPHISM WITHIN THE AMPHIBOLITE GRANULITE TRANSITION ZONE, UPPER ALDAN RIVER, SIBERIA

The sequence of metamorphic events and granite formation in the Nimnyr block of the Aldan shield is described. Metamorphic assemblages, poly migmatites and granite varieties developed in the upper Aldan River ar e the results of two tectonothermal events corresponding to the second , so called ''Aldan'', cycle (approximately 3.3-3.1 Ga) and the fourth , or ''Ungra'', cycle (2 Ga) occurring during the five-cycle history o f the Aldan shield. For the second cycle a prograde sequence from the amphibolite facies through a transition zone to the biotite-garnet-cor dierite-orthoclase subfacies and then the garnet-cordierite-hypersthen e-orthoclase subfacies of the granulite facies is established. Tempera tures and pressures are in the range of 680-850-degrees-C and 5-7 kbar (biotite-garnet-cordierite geothermobarometer data). Metamorphism of the Ungra cycle is also inhomogeneous, and similar to that of the seco nd cycle. Enderbite- and tonalite-gneisses, charnockite- and granite-g neisses (gamma1), stromatic and stockwork migmatites with leucosomes g amma2 and gamma3, respectively, anatectic and removed charnockites and granites were formed during the Aldan cycle. Migmatites with leucosom es gamma4 and removed charnockites (gamma5) are related to the Ungra c ycle. The mineral assemblages of granitoids and migmatite leucosomes v ary systematically with the metamorphic assemblages. Comparison of the simultaneous leucosomes of the different metamorphic zones suggests a decrease in Si and increase in Ti, Ca, Mg, Fe from amphibolite to gra nulite facies. The biotite compositions and lattice ordering of K-feld spar also change regularly. Three stages are recognized in the sequenc e of granite formation related to the second cycle: (1) replacement of initial rocks by granitoids, (2) leucosome formation in stromatic and stockwork migmatites, and (3) mobilization of granite materials. Thes e stages are also recognized in the fourth cycle. We consider the pred ominant processes of granite formation: metasomatism, anatexis togethe r with replacement, and rheomorphism. Densities of CO2-inclusions sugg est that a decompressional process forms all the more or less displace d granites, whereas conditions generating granite- and charnockite-gne isses were closer to those of metamorphism. During the formation of th e metamorphic zone, pressure decreased by at least 3 kbar., correspond ing to an isothermal uplift up to 10-11 km.