EVOLUTION OF THE ARCHEAN AULIAN-GNEISS COMPLEX, MIDDLE DNIEPER GNEISS-GREENSTONE TERRAIN, UKRAINIAN SHIELD - SHRIMP U-PB ZIRCON EVIDENCE

The Aulian Gneiss Complex (AGC) forms the main part of the Middle Dnie per gneiss-greenstone terrain (Ukrainian Shield) as domal blocks betwe en greenstone belts. The Complex consists of three main units. The old est consists of interlayered tonalitic gneisses, amphibolites and ultr amafic schists and are grouped into the supracrustal Aulian Series. Th e second unit comprises coarse-grained tonalitic and trondhjemitic gne isses which formed as small intrusive bodies within Aulian Series rock s. The third unit contains diverse migmatites and gneissic granitoids collectively named the Dnieprope trovsk granitoids and forming the mai n component of the AGC. SHRIMP U-Pb geochronology has been undertaken on these units of the AGC. Weighted mean (207)pb/Pb-206 (2 sigma) ages for different zircon types in each sample are as follows. (1) Aulian Series: tonalitic gneisses 3196 +/- 13 and 3079 +/- 9 Ma, coarse-grain ed amphibolite (metagabbro) of tholeiitic composition 3181 +/- 5 Ma, f ine-grained melanoamphibolite (metavolcanic) of basalt komatiitic comp osition 3078 +/- 17 Ma. (2) Small intrusions into the Aulian Series: t rondhjemitic gneiss 3122 +/- 2 and 3081 +/- 5 Ma. (3) Dniepropetrovsk granitoids: tonalite 3074 +/- 7 Ma, trondhjemite similar to 3070 Ma. T hese results allow the following synthesis for the AGC. 3200-3180 Ma a go, a supracrustal dacite-basalt (komatiite) sequence was formed and t hen 3120 Ma ago it was intruded by trondhjemites and tonalites. At sim ilar to 3080 Ma metamorphism of amphibolite facies and perhaps local i n situ partial melting caused growth of new zircon in the older lithol ogies and was accompanied by intrusion of younger of tonalites and tro ndhjemites. These U-Pb zircon results have not substantiated a previou s interpretation of Sm-Nd whole-rock isotopic data that very early Arc haean (similar to 3900 Ma) mafic components are present in the Aulian Series of AGC.