Isotope Lu-Hf composition of detrital zircon from paragneisses of the Sharyzhalgai uplift: evidence for the Paleoproterozoic crustal growth

We present results of study of the trace-element and Lu-Hf isotope compositions of zircons from Paleoproterozoic high-grade metasedimentary rocks (paragneisses) of the southwestern margin of the Siberian craton (Irkut terrane of the Sharyzhalgai uplift). Metamorphic zircons are represented by rims and multifaceted crystals dated at similar to 1.85 Ga. They are depleted in either LREE or HREE as a result of subsolidus recrystallization and/or synchronous formation with REE-concentrating garnet or monazite. In contrast to the metamorphic zircons, the detrital cores are enriched in HREE and have high (Lu/Gd)(n) ratios, which is typical of igneous zircon. The weak positive correlation between Lu-176/Hf-177 and Hf-176/Hf-177 in the zircon cores evidences that their Hf isotope composition evolved through radioactive decay in the closed system. Therefore, the isotope parameters of these zircons can give an insight into the provenance of metasedimentary rocks. The Paleoproterozoic detrital zircon cores from paragneisses, dated at similar to 2.3-2.4 and 2.0-1.95 Ga, are characterized by a wide range of epsilon(Hf) values ( from +9.8 to -3.3) and model age T-Hf(C) = 2.8-2.0 Ga. The provenance of these detrital zircons included both rocks with juvenile isotope parameters and rocks resulted from the recycling of the Archean crust with a varying contribution of juvenile material. Zircons with high positive epsilon(Hf) values were derived from the juvenile Paleoproterozoic crustal sources, whereas the lower epsilon(Hf) and higher T-Hf(C) values for zircons suggest the contribution of the Archean crustal source to the formation of their magmatic precursors. Thus, at the Paleoproterozoic stage of evolution of the southwestern margin of the Siberian craton, both crustal recycling and crustal growth through the contribution of juvenile material took place. On the southwestern margin of the Siberian craton, detrital zircons with ages of similar to 2.3-2.4 and 1.95-2.0 Ga are widespread in Paleoproterozoic paragneisses of the Irkut and Angara-Kan terranes and in terrigenous rocks of the Urik-Iya graben, which argues for their common and, most likely, proximal provenances. In the time of metamorphism (1.88-1.85 Ga), the age of Paleoproterozoic detrital zircons (2.4-2.0 Ga), and their Lu-Hf isotope composition (epsilon(Hf) values ranging from positive to negative values) the paragneisses of the southwestern margin of the Siberian craton are similar to the metasedimentary rocks of the Paleoproterozoic orogenic belts of the North China Craton. In the above two regions, the sources of detrital zircons formed by both the reworking of the Archean crust and the contribution of juvenile material, which is evidence for the crustal growth in the period 2.4-2.0 Ga. (C) 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.