ABSTRACT

The Precambrian basement of the Korean Peninsula, located at the east margin of Asian continent, consists of the Nangrim Massif, the Gyeonggi Massif and the Sobaegsan (Yeongnam) Massif. Two kinds of orthogneisses (biotite gneiss and granitic gneiss) from the central part of the Sobaegsan Massif show the particular difference in geochemical features suggesting the discrimination in formation history. The biotite gneisses give an Sm-Nd whole rock isochron age of 819 ± 114 Ma (2σ) with ε_Nd (0.82 Ga) = −15, and the granitic gneisses give an age of 1484 ± 810 Ma (2σ) with ε_Nd (1.48 Ga) = +7. The ε_Nd and ε_Ce, values for the granitic gneisses show characteristics of highly depleted mantle material and those of the biotite gneisses show their sources having light-REE enriched continental-like feature. Chondrite-normalized REE patterns are also different. Biotite gneisses have less fractionated patterns with (La/Yb)_N ratio of 10∼16 and negative Eu anomalies, while granitic gneisses have fractionated patterns with (La/Yb)_N ratio of 30∼180 with no/or positive Eu anomalies. The Ce-Nd isotopic results obtained here indicate that the Sobaegsan biotite gneisses were derived from continental-like sources having light-REE enriched feature, while the Sobaegsan granitic gneisses were derived from MORB-like sources having light-REE-depleted feature. These isotopic and REE data are interpreted as recording LREE fractionation events associated with major episodes of crust formation in East Asia. And the comparison of the initial ε_Nd value of the Sobaegsan biotite gneiss at 0.82 Ga with those of the Chinese and Japanese Precambrian rocks implies that the sources for the Sobaegsan biotite gneisses and these Chinese and Japanese rocks had common light-REE enriched pattern, e.g., ¹⁴⁷Sm/¹⁴⁴Nd ratio ranging from 0.09–0.13. Furthermore, it suggests that these sources having light-REE enriched patterns were derived from depleted mantle at the late Archean.