Paleoproterozoic crustal evolution of the Oki belt, Japan: provenance and
tectonic significance for Northeast Asia

T Imayama, K Sakura, T Shibata, A Sato, K Aoki, S Aoki, R Kawabata

The Oki belt preserves a rare Paleoproterozoic continental basement in the Japanese Islands. The belt is
composed mainly of migmatitic paragneisses and granitic gneisses. This study constrains the provenance,
depositional age, and tectonothermal evolution of the Oki belt using zircon U–Pb geochronology, whole-rock
geochemistry, and Nd isotopic data. Detrital zircon age spectra from migmatitic paragneisses are dominated
by Paleoproterozoic populations between ca. 2.16 and 1.99 Ga, indicating source mainly from contemporaneous
Paleoproterozoic crust. The youngest concordant zircon cores constrain the maximum depositional ages to ca.
1.99–1.93 Ga, possibly as young as ca. 1.85 Ga locally. The high alumina saturation index values, variable
negative Nb anomalies, and εNd(t) values of − 5.1 to +0.1, taken together, suggest partial melting of arc-related,
old terrigenous sediments deposited along an active continental margin. Granitic gneiss and anatectic leucosomes
in the migmatite reflect ca. 1.87 Ga igneous activity and ca. 1.84 Ga anatexis related to the previously
recognized regional M1 granulite-facies metamorphism. The Oki belt was subsequently overprinted during the
Triassic at ca. 233–226 Ma, recording the renewed partial melting during the M2 high-grade metamorphism.
These results indicate that the Oki belt is tectonically correlated with the northern Gyeonggi Massif on the basis
of comparable Paleoproterozoic high-grade metamorphism (ca. 1.86–1.84 Ga), coeval ca. 1.87 Ga magmatism,
and Triassic anatexis. However, unlike the northern Gyeonggi Massif, its detrital zircon spectra, dominated by
2.16–1.99 Ga ages, indicate derivation from Paleoproterozoic continental crust, including arc-related magmatic
sources, in the Jiao–Liao–Ji Belt and the Nangrim Massif, implying a more northeasterly paleogeographic position
along the eastern margin of the North China Craton.

Precambrian Research