LATE ARCHEAN INTRUSIVE COMPLEXES IN THE OLEKMA GRANITE GREENSTONE TERRAIN (EASTERN SIBERIA) - GEOCHEMICAL AND ISOTOPIC STUDY

Two granitoid intrusions, the Amnunnakta and Oldongso massifs in the O lekma 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 f ollowing ages were obtained for the Amnunnakta massif: U-Pb zircon 298 4 +/- 22 Ma (2sigma), Pb-Pb WR 2991 +/- 57 Ma, Sm-Nd WR 3094 +/- 430 M a and for the Oldongso massif: U-Pb zircon 2999 +/- 51 Ma, Pb-Pb WR 28 55 +/- 170 Ma, Sm-Nd WR 2923 +/- 144 Ma. The similar ages suggest pene contemporaneous formation of the two intrusive series. The observed re setting of the Rb-Sr WR systems (2767 +/- 116 Ma for Amnunnakta massif and 2597 +/- 82 Ma for the Oldongso massif ) and opening of the plagi oclase Pb isotopic systems are associated with superimposed metamorphi c processes of the greenschist and epidote-amphibolite facies. Source regions of the massifs are characterized by long-term depletion in inc ompatible elements: the initial epsilon(Nd)(T) value is +2.4 +/- 0.4 f or both massifs. The mu1 values are mantle-like at 7.68 for the Amnunn akta 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 deltaO-18 values (2.7-4 .7) and elevated Sr(i) = 0.7052 +/- 7 which may be caused by both a pr imary low deltaO-18, high Sr-87/Sr-86 magma or later interaction with a hydrothermal system. There is no evidence for recycled sedimentary m aterial in the granitoids of either massif. One of the rocks, a fine-g rained garnet-bearing trondhjemite, previously assigned to the Amnunna kta massif, yielded a T(Nd) Model age of 3700 Ma and reflects the pres ence of older sialic material. Moderately fractionated REE patterns wi th well pronounced negative Eu anomalies in low-alumina trondhjemites of the Amnunnakta massif suggest their formation by a process of diffe rentiation of high-alumina basaltic melts with fractionation of plagio clase. High-alumina tonalite-trondhjemites of the Oldongso massif are characterized by fractionated and depleted HREE patterns suggesting th e residual presence of garnet and hornblende during partial melting of a basic source.