Late Archaean intrusive complexes in the Olekma granite-greenstone terrain (eastern Siberia): geochemical and isotopic study

Abstract

Two granitoid intrusions, the Amnunnakta and Oldongso massifs in the Olekma granite-greenstone terrain of the Aldan shield have been studied using Sm-Nd, Rb-Sr and Pb-Pb whole rock (WR) and U-Pb zircon methods. The isotopic composition of Pb in plagioclase, as well as whole-rock oxygen isotope compositions and REE patterns were also measured. The following ages were obtained for the Amnunnakta massif: U-Pb zircon 2984±22 Ma (2σ), Pb-Pb WR 2991±57 Ma, Sm-Nd WR 3094±430 Ma and for the Oldongso massif: U-Pb zircon 2999±51 Ma, Pb-Pb WR 2855±170 Ma, Sm-Nd WR 2923±144 Ma. The similar ages suggest penecontemporaneous formation of the two intrusive series. The observed resetting of the Rb-Sr WR systems (2767±116 Ma for Amnunnakta massif and 2597±82 Ma for the Oldongso massif) and opening of the plagioclase Pb isotopic systems are associated with superimposed metamorphic processes of the greenschist and epidote-amphibolite facies. Source regions of the massifs are characterized by long-term depletion in incompatible elements: the initial ϵ Nd(T) value is +2.4±0.4 for both massifs. The μ 1 values are mantle-like at 7.68 for the Amnunnakta and 7.71 for the Oldongso massifs and the initial Sr composition is low (Sr i=0.7015±5) for the Oldongso massif. Basic rocks of the Amnunnakta massif are characterized by low δ 18O values (2.7–4.7) and elevated Sr i=0.7052±7 which may be caused by both a primary low δ 18O, high rm 87Sr 86Sr magma or later interaction with a hydrothermal system. There is no evidence for recycled sedimentary material in the granitoids of either massif. One of the rocks, a fine-grained garnet-bearing trondhjemite, previously assigned to the Amnunnakta massif, yielded a T Nd model age of 3700 Ma and reflects the presence of older sialic material. Moderately fractionated REE patterns with well pronounced negative Eu anomalies in low-alumina trondhjemites of the Amnunnakta massif suggest their formation by a process of differentiation of high-alumina basaltic melts with fractionation of plagioclase. High-alumina tonalite-trondhjemites of the Oldongso massif are characterized by fractionated and depleted HREE patterns suggesting the residual presence of garnet and hornblende during partial melting of a basic source.