Seung Ryeol Lee

Age, geochemistry, and tectonic significance of Neoproterozoic alkaline granitoids in the northwestern margin of the Gyeonggi massif, South Korea

Alkaline meta-granitoids, ranging in composition from syenite to alkali granite, occur in the northwestern Gyeonggi massif. Ion microprobe U–Pb zircon analyses indicate that the granitoids were emplaced at 742±13 Ma, and are corroborated by a Rb–Sr whole rock age of 770 ± 40 Ma. Major and trace element characteristics, together with Sr and Nd isotopic data, suggest that the granitoid magma was derived from ancient (TDM = 2.6–2.2 Ga) continental crust with addition of juvenile mantle-derived basaltic magma. The generation of the alkaline granitoid is attributed to crustal thinning induced by deep-seated thermal activity such as mantle upwelling or mafic magma influx. Alkaline igneous activity at 742 Ma is coeval with Neoproterozoic rift-related magmatism prevalent in South Korea and the South China Block but lacking in the North China Block. Thus, we suggest that the Gyeonggi massif is correlative with the South China Block and has experienced a rifting event during Rodinia breakup.

Early Proterozoic Granulites in Central Korea: Tectonic Correlation with Chinese Cratons

A coherent granulite complex has been discovered in the central part of the Korean Peninsula, formerly regarded as the eastern Imjingang belt. This granulite complex (Hwacheon granulite complex [HGC]) experienced two cycles of tectonometamorphic events. The first event corresponded to crustal thickening, followed by peak granulite‐facies metamorphism and associated partial melting. The quasi‐isobaric cooling path, suggested by the occurrences of secondary garnet mantling orthopyroxene and secondary kyanite in mafic and pelitic granulites, respectively, indicates substantial residence time at midcrustal levels prior to exhumation of the HGC. The timing of peak metamorphism was dated from the unzoned overgrowth rims on zircons in a migmatitic granulite at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Paleostress analysis of the Pohang-Ulsan area, Southeast Korea: tectonic sequence and timing of block rotation

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An early Proterozoic Sm-Nd age of mafic granulite from the Hwacheon area, South Korea

\end{document} (2&sgr;) Ma, using an ion microprobe. Detrital cores of zircon, however, give diachronous U‐Pb ages of ca. 2.30, 2.45, 2.65, and 2.90 Ga, attesting to the presence of Archean protoliths in South Korea. Sm‐Nd model ages of migmatitic granulites (ca. 2.8–2.6 Ga) further suggest that a significant addition of juvenile materials from the mantle took place prior to granulite‐facies metamorphism. The second tectonometamorphic event may be responsible for the final exhumation of the HGC toward the surface. The age of exhumation is interpreted to be Permo‐Triassic on the basis of available isotopic data. These results suggest correlation of the Gyeonggi massif with the Yangtze craton, and as a corollary, that both this massif and the Imjingang belt correspond to the eastward continuation of the Chinese collisional belt.

Hf isotopic evidence for Paleoarchean (> 3.5 Ga) crustal components in the Korean Peninsula

From the 3000 km2 Eoarchean Itsaq Gneiss Complex (IGC) of Greenland, zircon U-Pb dating of numerous meta-granitoid and orthogneiss samples is integrated with geologic observations, whole rock geochemistry and a strategic subset of zircon Hf and whole rock Nd isotopic measurements. This shows that there are multiple episodes of TTG suite formation from ∼3890 to 3660 Ma, characterized by zircon initial εHf≈0 and whole rock initial εNd of > +2. These rocks mostly have geochemical signatures of partial melting of eclogitized mafic sources, with a subset of high magnesian, low silica rocks indicating fusion by fluid fluxing of upper mantle sources. The TTG suites are accompanied by slightly older gabbros, basalts and andesites, which have geochemical signatures pointing to magmas originating from fluid fluxing of upper mantle sources. The data show the formation of juvenile crust domains in several discrete events from ∼3900 to 3660 Ma, probably at convergent plate boundaries in an environment analogous, but not identical to, modern island arcs. In the Isua area, a northern ∼3700 Ma terrane formed distal from a predominantly ∼3800 Ma terrane. These terranes were juxtaposed between 3680 and 3660 Ma—respectively the age of the youngest rocks unique to the northern terrane and the lithologically distinctive ultramafic-granitic Inaluk dykes common to both terranes. This shows the assembly of different domains of juvenile rocks to form a more expansive domain of “continental” crust. A rare occurrence of high-pressure granulite is dated at ∼3660 Ma, demonstrating that assembly involved tectonic crustal thickening. This continental crust was then reworked in the 3660 to 3600 Ma Isukasian orogeny. In the northern part of the Isua area, 3660 to 3600 Ma granites were emplaced into ∼3700 Ma tonalites. The earliest granites are nebulous, and sigmoidal schlieric inclusions within them demonstrate ductile extension. Younger granite sheets were emplaced into extensional ductile-brittle fractures. These granite-tonalite relationships are overprinted by widespread development of late Eoarchean (pre-3500 Ma Ameralik dyke) brittle-ductile extensional cataclastic textures, together demonstrating that extension was polybaric. The southern part of the Isua area largely escaped 3660 to 3600 Ma high temperature processes and has sparse granite sheets commonly focused into coeval shear zones. In the rest of the complex, deeper crustal levels during the Isukasian orogeny are widely preserved. These experienced upper amphibolite to granulite facies moderate- to low-pressure syn-kinematic metamorphism, forming complex migmatites rich in granitic-trondhjemitic neosome. The migmatites were intruded by composite ferrogabbro and granite bodies, in which syn-magmatic extensional features are locally preserved. Thus 3660 to 3600 Ma crustal recycling involved elevated crustal thermal gradients in an extensional regime. Crustal melts formed in the Isukasian orogeny have zircon initial εHf<0 and whole rock initial εNd of ≤0, showing incorporation of slightly older Eoarchean juvenile crust. A Phanerozoic example of collisional orogeny followed by crustal thinning is explored as an analog for the Isukasian orogeny.

Tectonic evolution of Precambrian basement massifs and an adjoining fold-and-thrust belt (Gyeonggi Marginal Belt), Korea: An overview

In order to reconstruct the tectonic sequence in the Pohang-Ulsan area, eastern block of the Yangsan Fault, Southeast Korea, we analyzed the paleostresses for the Tertiary formations using fault-slip and tension gash data from 29 sites. Approximately ten episodic faulting events were recognized from fault populations classified from this study. Principal stress axes of the relatively younger faulting events are well correlated with those of the western block of the Yangsan Fault. However, this is not the case for the older events. A horizontal rotation for the principal stress axes of these older events at each site of the study area has produced a good correlation of the older principal stress axes between the two blocks. Consequently, this probably makes it possible to establish a regional tectonic sequence in the Pohang-Ulsan area. Because of the partial coexistence of neighboring coaxial events, the ten faulting events can be regrouped into five major tectonic events in a chronological order: (1) NW-SE compression and NE-SW extension; (2) NE-SW compression and NW-SE extension; (3) NNE-SSW extension and WNW-ESE compression; (4) E-W compression and N-S extension; and (5) NNW-SSE extension and ENE-WSW compression. Timing of the rotation of the principal stress axes at the sites is mainly related to the third tectonic event, NNE-SSW extension or WNW-ESE compression. The rotation found in the study area appears to have occurred in relation to a regional-scale block rotation. The timing falls on the moment when the SW Japanese Block converged into SE Korea (ca. 12 Ma). Orientations of syndepositional faults and sandstone dikes in the Chunbuk strata, and the dated ages of NE-SW trending dikes around the Guryongpo area support that the tectonic event with a NE-SW compression was active at least during the early-middle Miocene.

Contrasting source domains for the Phanerozoic granitoids in South Korea revealed by zircon Hf isotopic signatures

The Andong Fault is one of the E-W trending boundary faults that separate the Cretaceous Gyeongsan Basin from the surrounding basement. Several reverse faults are developed parallel to the Andong Fault, and these faults together with the Andong Fault are named collectively the Andong Fault System. The fault system is characterized by significant fault drags. Structural features enable us to analyze the geometry and slip vectors of faults, on the basis of which we can develop an understanding of the related tectonic setting. The results of the analysis of the fault drags indicate that the Andong Fault System had a largely reverse motion. The major movement of the hanging wall was mainly toward and southeast and partly toward the south. The ages of pre- and post-reverse faulting plutons indicate that the reverse movement of Andong Fault System occurred between the Cretaceous and the Early Tertiary times. In addition, considering the distribution of strata, the Andong Fault might have played the role of a transfer fault during the E-W extension of the Gyeongsang Basin, from the early stage of the opening and throughout the basin development.

Effect of Zircon on Rare-Earth Element Determination of Granitoids by ICP-MS

The Sm−Nd mineral isochron ages of 1897±120 and 1742±140 Ma were obtained from two mafic granulites of the Hwacheon granulite complex, east-central Korean Peninsula. These ages are interpreted to date the granulite-facies metamorphism and/or subsequent cooling to ca. 700°C. Our result apparently contradicts the Permo-Triassic age for the regional metamorphism that was reported in the Imjingang belt, and instead suggests an early Proterozoic tectonometamorphic event that may be related to the crustal formation of the Gyeonggi massif. Thus, the Hwacheon granulite complex is likely to represent Proterozoic crystalline basement that was reactivated during the Permo-Triassic orogeny, corresponding to the collision between the Sino-Korean and Yangtze cratons.