Paper Number: 1950
In situ U-Pb zircon dating on sedimentary and granitoid rocks in Imjingang Belt, North Korea
Zhang, Y. 1, Zhai, M.1, Wu, F.1, Hou, Q.2, Peng, P.1, Zhang, X.1 and Li, L.1
1 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P. O. Box 9825, Beijing 100029, China
2 University of Chinese Academy of Sciences, No.19A Yuquan Road, Beijing 100049, China
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The Imjingang belt occurring in the middle part of the Korean Peninsula consists of Archaean–Proterozoic basement rocks and Devonian-Carboniferous sedimentary series “Imjin Group” [1-2]. It lies between the Pyongnam basin and Gyeonggi massifs, and is separated on all sides by faults or shear zones from surrounding Precambrian basement rocks. Until now, the Imjingang belt is still enigmatic, and whether it is correlated with the Qinling-Dabie-Sulu belt in China is also debated [3-9]. In order to provide geochronological constraints on tectonic affinity of the Imjingang belt, we carry out zircon U-Pb dating and Hf isotopic composition analyses on strata and basement rocks.
The basement rocks in Imjngang belt have been divided into the Archean Nangrim group, early Paleoproterozoic Jungsan group and a large volume of granitoid plutons. SHRIMP U-Pb zircon analyses on three porphyritic monzogranite samples reveal that they were emplaced at 1884 ± 10 Ma, 1875 ± 10 Ma and 1885 ± 10 Ma, respectively. Synchronous granitic magmatism has also been recorded in the Eastern Block of the North China Craton, and in Nangrim, Gyeonggi and Yeongnam massifs of Korean Peninsula [10-13], which suggests that 1.8 Ga granitic magmatic event occurred in both the Korean Peninsula and the Eastern Block of the North China Craton.
The Imjin group is a thick volcano-clastic sedimentary strata over 2000 m in thickness, and is divided into three formations with conformable contact relationships. The three formations are Anhyop, Puap and Sannyong formation from bottom up. Two arenitic phyllite and one sandstone samples were collected from the Puap formation. Detrital ages of the three samples can be divided into five groups: 2.5-2.3 Ga (~20%), 2.1-1.5 Ga (~17%), 1424-1176 Ma (~8%), 1108-676 Ma (~26%) and 591-388 Ma (~29%). The youngest group gives a weighted mean 206Pb/238U age of 401 ± 3 Ma (n=15), confirming maximum age of sedimentation as Early Devonian of the Puap Formation. Together with the fossils of charophyte and crinoid stem in Anhyop Formation, the sedimentary age of the Imjin group is Lower-Middle Devonian. The overall distribution pattern of detrital zircon ages of the Puap formation is consistent with that of the Silurian Taean Formation in western Gyeonggi massif, implying that they have same sedimentary sources. Namely, the Nangrim and Gyeonggi massif were connected at least before Early Devonian. So, the Imjingang belt may neither be a suture zone, nor it is the eastward extension of the Qinling-Dabie-Sulu collisional belt of China.
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