Sources and evolution of initial melts of Archean "gray gneisses" : trace-element composition and Lu-Hf isotope systematics of zircon from plagiogneisses of the Kola superdeep well and the surrounding area

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Trace element composition and Lu-Hf isotope systematics of zircon from plagiogneisses of the Kola superdeep well : contribution of a Paleoarchean crust in mesoarchean metavolcanic rocks

Zircons have been studied in three samples of Archean plagiogneisses from the Kola superdeep well (SG-3). The crystals consist of cores, magmatic shells, and metamorphic rims. The cores and shells are characterized by similar lowered concentrations of most trace elements, which is typical of zircons from plagiogranitoids, rocks of elevated basicity, and basites. At a wide range of Hf isotope characteristics, the cores and shells have similar average ¹⁷⁶Hf/¹⁷⁷Hfi, which determines the close composition of their sources. The metamorphic rims have close ¹⁷⁶Hf/¹⁷⁷Hfi ratio. The minimum age of the crustal contaminant of parental melts is estimated at 3.4 and 3.3 Ga for cores and 3.3–3.2 Ga for shells at almost equal proportions of mantle and crustal components in them. The contribution of Paleoarchean crust established in zircons from plagiogneisses of SG-3 using Lu-Hf isotope systematics is confirmed by the presence of 3.3and 3.4-Ga old zircons in surrounding TTG.20 page(s