The Sindong Group forms the lowermost basin-fill of the Gyeongsang Basin, the largest Cretaceous nonmarine basin located in southeastern Korea, and comprises the Nakdong, Hasandong, and Jinju Formations with decreasing age. The depositional age of the Sindong Group has not yet been determined well and the reported age ranges from the Valanginian to Albian. Detrital zircons from the Sindong Group have been subjected to U-Pb dating using laser ablation inductively coupled plasma mass spectrometry. The Sindong Group contains noticeable amounts of detrital magmatic zircons of Cretaceous age (138-106 Ma), indicative of continuous magmatic activity prior to and during deposition of the Sindong Group. The youngest detrital zircon age of three formations becomes progressively younger stratigraphically: 118 Ma for the Nakdong Formation, 109 Ma for the Hasandong Formation, and 106 Ma for the Jinju Formation. Accordingly, the depositional age of the Sindong Group ranges from the late Aptian to late Albian, which is much younger than previously thought. Lower Cretaceous magmatic activity, which supplied detrital zircons to the Sindong Group, changed its location spatially through time; it occurred in the middle and northern source areas during the early stage, and then switched to the middle to southern source areas during the middle to late stages. This study reports first the Lower Cretaceous magmatic activity from the East Asian continental margin, which results in a narrower magmatic gap (ca 20 m.y.) than previously known.

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The Sindong Group (Aptian-Albian) in southeastern Korea is a 2-3-km-thick fluvio-lacustrine sedimentary package deposited in an elongated basin (the Nakdong Trough), which was formed by extension in an active-continental-margin setting. The provenance of the Sindong Group was studied to understand spatial and temporal variation in composition in three different parts of the basin by using integrated data on petrography, quartz SEM-cathodoluminescence (CL) analysis, and zircon Zr/Hf analysis. Although Sindong Group sandstones display wide variation in the composition of framework grains, they generally have increasing amounts of feldspar and decreasing amounts of quartz up sequence. Significant amounts of volcanic rock fragments and volcanic quartz are observed in the late-stage sediments. Metamorphic quartz is predominant in all Sindong Group sandstones, indicative of the exposure of metamorphic rocks in the source terrane, mostly from Precambrian basement and Triassic granites. The occurrence of significant amounts of plutonic quartz from early-stage sediments suggests that Jurassic granites were widely exposed in the source terranes. In addition, the episodic increase in volume of plutonic quartz content in the sequence, especially in the northern part of the basin, suggests that episodic tectonic activity occurred in the catchment. The increasing feldspar content up sequence could support the occurrence of tectonic activity in the catchments. The Zr/Hf analysis of detrital zircons revealed that the majority of zircons are of continental-crust origin formed in orogenic settings, but zircons in the lowest strata in the northern part of the basin were derived largely from anorogenic magmatic rocks of mantle origin. Our results demonstrate that source terranes for the Sindong Group have heterogeneous spatial and temporal distribution and were composed mainly of Precambrian basement and Triassic to Jurassic granitic rocks, with minor (meta)sedimentary rocks and syndepositional volcanic rocks. The detritus derived from syndepositional volcanic rocks became significant in the late stage of basin filling, representing the transition from extensional tectonism to continental-arc magmatism. The differences in characteristics of quartz SEM-CL and zircon Zr/Hf ratios in different parts of the basin are best explained by deposition on different alluvial fans and river systems.

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U-Pb ages of detrital zircons collected from the sediments of seven major South Korean rivers were analysed to infer the temporal and spatial distribution of Mesozoic arc magmatism related to the subduction of the palaeo-Pacific plates. Mesozoic detrital zircon U-Pb ages indicate continuous arc magmatic activity in the Korean Peninsula throughout the Mesozoic era (249-72 Ma), except for a magmatic gap during 120-162 Ma, with NW-SE spatial migration of the arc magmatic centres: trenchward migration in both the Triassic and the Cretaceous periods, and inland-directed migration in the Jurassic period.A comparison of these results with the magmatic ages in the adjacent regions such as northeastern China and southwestern Japan suggests that subduction modes of the palaeo-Pacific plates were changed in the Mesozoic East Asian continental margin due to buoyancy differences caused by the subduction of the Farallon-Izanagi plate ridge or oceanic plateaus and their subducting location shifting with time.