Neoproterozoic Bimodal Volcanism in the Okcheon Belt,South Korea,and Its Comparison with the Nanhua Rift,South China:Implications for Rifting in Rodinia

摘要

A systematic geochemical examination and zircon U-Pb dating of Neoproterozoic bimodal volcanic rocks from the Chungju area in the northeast segment of the Okcheon Metamorphic Belt (OMB),South Korea,allow a comparison with similar rocks from the Nanhua Rift,South China.The bimodal metavolcanics of the Chungju area comprise the subalkaline to alkaline basalts (hereafter "mafic metavolcanic rocks") and trachytes (hereafter "felsic metavolcanic rocks").The mafic metavolcanic rocks are characterized by light rare earth element (LREE)-enriched and "humped" trace-element patterns with moderate depletions in Sr and Ti and variable but reasonably low eNd(T) values between +0.83 and +2.99.These geochemical features are consistent with the area's origin in an ocean island basalt (OIB) mantle source with minor crustal contamination.The felsic metavolcanic rocks are characterized by LREE-enriched patterns with a remarkable negative Eu anomaly.They display an overall enriched trace-element pattern with significant depletions of Sr,P,Eu,and Ti.They have the geochemical characteristics typical of A_1-type granites and epsilon Nd(T) values between +1.30 and +2.54.The incompatible element versus incompatible element diagrams for both rocks exhibit nearly smooth positive trends.In the Y/Nb versus Yb/Ta diagram,all the felsic rocks plot within the OIB field.Hence,our data imply a genetic linkage between the mafic and the felsic rocks.The felsic rocks were most likely generated from basaltic protoliths through extensive fractional crystallization plus minor crustal contamination/ assimilation.Igneous zircon from a felsic metavolcanic rock from the Munjuri Formation in the OMB gives a SHRIMP U-Pb age of 762 +- 7 Ma,providing a temporal link to the Neoproterozoic rift-related volcanism in South China.Rare detrital zircons extracted from a felsic sample from the Gyemyeongsan Formation yield a mixture of ages: approx 1.9 Ga,approx 870 Ma,and approx 250 Ma.The presence of some Triassic metamorphic overgrowths reflects a high-grade metamorphic event coeval with that recently identified in the Gyeonggi and Yeongnam massifs and linked to an Early Triassic collision event in South Korea.The lithological and geochemical data from the Neoproterozoic metavolcanic rocks in the OMB of South Korea show similarities with the Nanhua Rift in South China,pointing to a North Asian connection associated with the disruption of Rodinia.