Lu-Hf isotope composition of zircon and geochemistry of Paleoproterozoic granites of Birjusa Terrane (South-Western Siberian Craton)

This paper presents whole-rock geochemical, Nd isotopic and zircon U-Pb and Hf isotopic data for Paleoproterozoic granites of the Birjusa terrane (SW Siberian craton). The leucogranites of the Toporok pluton are intruded the Paleoproterozoic metasedimentary-volcanic units (the Elash trough) whereas the granodiorite-granites of the Podporog massif cut the Archaean metamorphic complex of the Birjusa terrane. LA-ICP-MS U-Pb dating of zircon grains yield ages of 1884 and 1747 Ma for granites of the Toporok and Podporog massifs respectively. The Toporok granitic rocks are weakly peraluminous, high ferroan (FeO*/FeO* + MgO = 0,94-0,96) and calc-alkaline leucogranites. These rocks have moderate concentrations of Rb, Th, LREE and HFSE and correspond to evolved potassium I-granite. The granodiorite-granites of the Podporog massif are weakly peraluminous, ferroan (FeO*/FeO* + MgP = 0,85-0,89) rocks showing calc-alkalic to alkali-calcic composition. By contrast, the Podporog granitoids are more enriched in incompatible elements such as Th, LREE and HFSE that is typified them as A-granite. On chondrite-normalized REE patterns, both type granites show moderately fractionated patterns (La/Yb_n = 8-15) that are characterised by moderate to strongly negative Eu anomalies of Eu/Eu* = 0,44-0,48 and 0,18-0,29 for the Toporok and Podporog massifs, respectively. The zircon saturation temperatures of the granites of the Toporok and Podporog massifs of 800-820°С and 830-870°С, respectively suggest a high minimum initial magma temperatures. The overall geochemical features of these granitoids together with evidence from experimental results are consistent with their generation by partial melting of quartzofeldspathic sources at pressures of ~4 kbar. The plutons show significant variation in initial Hf isotope composition: the 1,88 Ga Toporok granites have wider range of initial e_Hf values between +3.0 to +0.8 and T^c Hf(DM) of 2,3-2,5 Ga, the 1.75 Ga Podporog granite have a relatively narrow range e_Hf values from -3,9 to -3,0 and T^c Hf(DM) of 2,6-2,7 Ga. The differences in the Hf isotope composition suggest that significantly different source rock contributed to the melts that produced two types of granites. Zircons of the Toporok leucogranite with positive g_Hf values suggest the melting of the Paleoproterozoic juvenile source and indicate minor contribution from an older continental crust. In contrast, the isotope characteristics of zircons and granite (g_Nd -4,8 and -5,3) of the Podporog massif indicate predominately the Archaean crustal source which is similar in isotope characteristics to gneisses of the Birjusa terrane. Thus, isotope characteristics of granitoids and their zircons indicate partial melting diverse an old Archaean and juvenile Paleoprotero-zoic crustal sources during formation of granites of the Birjusa terrane.

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