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Russian Geology and Geophysics

2013 year, number 3

THE ISOTOPE COMPOSITION OF Hf IN ZIRCON FROM PALEOARCHEAN PLAGIOGNEISSES AND PLAGIOGRANITOIDS OF THE SHARYZHALGAI UPLIFT ( southern Siberian craton): IMPLICATIONS FOR THE CONTINENTAL-CRUST GROWTH

O.M. Turkina, I.N. Kapitonov, S.A. Sergeev
Keywords: Paleoarchean, tonalitetrondhjemitegranodiorite complex, zircon, UPb dating, LuHf isotope composition, crustal growth, Sharyzhalgai uplift
Pages: 272-282

Abstract

This paper presents results of UPb dating (SHRIMP-II) and LuHf (LAICP MS) isotope study of zircon from Paleoarchean plagiogneisses and plagiogranitoids of the Onot and Bulun blocks of the Sharyzhalgai uplift. Magmatic zircons from the Onot plagiogneiss and Bulun gneissic trondhjemite are dated at 3388 11 and 3311 16 Ma, respectively. Magmatic zircons from plagiogneisses and plagiogranitoids of the studied tonalitetrondhjemitegranodiorite (TTG) complexes are characterized mainly by positive values of ε Hf indicating that felsic melts were generated mainly from juvenile (mafic) sources, which are derived from a depleted mantle reservoir. The variable Hf isotope composition in magmatic zircons and the lower average ε Hf values in comparison with the depleted mantle values suggest the contributions of both mafic and more ancient crustal sources to magma formation. Metamorphic zircons from the gneissic plagiogranite and migmatized plagiogneiss either inherited the Hf isotope composition from magmatic zircon or are enriched in radiogenic Hf. The more radiogenic Hf isotope composition of metamorphic zircons from the migmatized plagiogneisses is due to their interaction with melt during partial melting. Variations in the LuHf isotope composition of zircon from the Bulun rocks in the period 3.333.20 Ga are due to the successive melting of mafic crust or the growing contribution of crustal material to their genesis. Correlation between the LuHf isotope characteristics of zircon and the SmNd parameters of the Onot plagiogneisses points to the contribution of ancient crustal material to their formation. The bimodal distribution of the model Hf ages of zircons reflects two stages of crustal growth in the Paleoarchean: 3.453.60 and ~3.35 Ga. The isotope characteristics of zircon and rocks of the TTG complexes, pointing to recycling of crustal material, argue for the formation of plagiogneisses and plagiogranitoids as a result of melting of heterogeneous (mafic and more ancient crustal) sources in the thickened crust.