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GRANITOIDS
OF THE OLENEK UPLIFT (NORTH OF SIBERIAN CRATON):
AGE, PETROGENESIS AND
TECTONIC SETTING
Gladkochub D.P., Donskaya T.V., Mazukabzov A.M.
Institute of Earth’s
Crust SB RAS, Irkutsk, Russia, dima@crust.irk.ru
The Olenek Uplift is part of the
Paleoproterozoic Olenek superterrane – one of the building
blocks of the Siberian craton, which was assembled between 2.1 and
1.8 Ga. The Paleoproterozoic rocks consist of folded,
greenschist-grade metasediments of the Aekit Formation, intruded by
post-tectonic granitoids that yielded K-Ar ages of c. 2080-2050 Ma.
We have
studied granitoids of the Olenek complexes. ASI index of the
granitoids fits in the range 1.06-1.35, a normative corundum
concentrations in the studied granitoids vary from 1.02 up to 3.75.
The granitoids demonstrate Rb (134 - 250 ppm), Sr (55 - 150 ppm), Zr
(42 - 225 ppm), Y (14 - 27 ppm). Such indexes values and trace
element concentrations are typical for S-type granites according to
(Chappell, White, 1992). The granitoids have fractionated spectra of
REE (La/Yb)n = 7-21 and negative Eu anomaly (Eu/Eu* = 0,48-0,73).
According to Pearce's (1996) criteria, the studied rocks could be
considered as post-collisional granitoids. Nd(T)
in the granitoids is +0,5, their Nd model age T(DM) = 2524 Ma.
For the age
determination, we analyzed the sample 03138. Sample 03138 is
coarse-grained biotite granite, and it yielded abundant subhedral to
euhedral zircons up to 400 microns long. Most of them are equant in
shape, although a few elongate crystals have aspect ratios up to 8:1.
They range from clear and colourless to brown and turbid. Concentric
growth zoning is common. Thirty-three analyses of 22 zircons were
conducted. Uranium concentrations vary widely, from 66 to 4046 ppm,
and Th varies from 36 to 581 ppm, with Th/U ratios between 0.04 and
1.01. The proportion of common 206Pb
in total measured 206Pb
averages 0.5%.
The analyzed zircons are divided
into two main groups: a concordant group at 2036 ± 11 Ma (MSWD
= 0.35), and a second at 2111 ± 20 Ma (MSWD = 0.58). We
interpret the older concordant result of 2111 ± 20 Ma to
represent the age of granite crystallization, and consider the older
crystals to be inherited.
Based on the
obtained results and previously reported data (Krilov et al., 1963;
Rosen et al., 2000; Gladkochub et al., 2006), we propose the
following principal stages of the area evolution: 1) between 2560 –
2200 Ma – accumulation of the sedimentary sequence (Eekit
suite); 2) ~ 2150 Ma – the collision along the eastern margin
of Birekte terrane; 3) 2111 Ma – intrusion of the
post-collisional granitoids; 4) 1970 Ma – amalgamation of
Birekte terrane and Anabar shield.
The combination of new data: Nd
model age (2524 Ma) of the analyzed granites and U/Pb age (2564 Ma)
of one grain of inherited zircon from Mesoproterozoic mafic intrusion
of the Olenek uplift could be used as significant evidence of the
Archean protholith presence in the basement of the Olenek uplift
(never reported before).
References
Chappell B. W., White A. J. R. Two contrasting granite
types // Pacif. Geol. 1974. V. 8. P. 173-174.
Gladkochub D.P., Pisarevsky S.A.,
Donskaya T.V. et al. Siberian Craton and its evolution in terms of
Rodinia hypothesis, Episodes 29 (2006) P.169–174.
Krylov A.J., Vishnevsky A.N., Silin
Yu.I. Absolute age of the rocks of the Anabar Shield, Geochimia 12
(1963) P.1140–1144
(in Russian.)
Rozen, O. M., Zhuravlev, D. Z.,
Sukharov, M. K., Bibikova, E. V. and Zlobin, V. L., 2000, Early
Proterozoic terranes,
collision zones and associated anorthosites in the north-east of
Siberian craton: isotope geochemistry and age characteristics.
Russian Geology
and
Geophysics.
vol.41,
no
2, P.163 –180.
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