Kyounghee Yang

Magmatic-Hydrothermal Evolution in the “Bottoms” of Porphyry Copper Systems: Evidence from Silicate Melt and Aqueous Fluid Inclusions in Granitoid Intrusions in the Gyeongsang Basin, South Korea

The Gyeongsang Basin of southeastern Korea contains numerous Cretaceous-early Tertiary (120–40 Ma) granitoid intrusions formed at a convergent plate boundary. The geotectonic setting is similar to that associated with porphyry-type mineralization elsewhere in the Circumpacific region. However, erosion has removed higher-level economic mineralization and exposed deeper levels of the granitoids, representing the poorly mineralized “bottoms” of porphyry copper systems. The intrusions of the Gyeongsang Basin thus provide a unique opportunity to advance our understanding of magmatic-hydrothermal evolution in the roots of porphyry-type systems, below the level of economic mineralization. The physical and chemical environment during crystallization of the magmas has been characterized through studies of silicate melt and aqueous fluid inclusions in the granitoids. Two different types of silicate melt inclusions were recognized based on occurrence and room-temperature appearance. Type-I inclusions contain one or ...

Tectonic Evolution of the Gyeongsang Basin, Southeastern Korea from 140 Ma to the Present, Based on a Strike-Slip and Block Rotation Tectonic Model

The geometric model involving two conjugate strike-slip fault sets with opposite-sense block rotations synthesized by structural, petrological, geochronologic, and paleomagnetic data from the Gyeongsang Basin from Cretaceous to Tertiary time is placed in the tectonic framework of East Asia. As a result, the birth and evolution of the Gyeongsang Basin might reflect a regional continental strike-slip zone in a convergent plate boundary such as the Tan-Lu wrench tectonic system. According to this model, we suggest six major geotectonic stages in the Gyeongsang Basin since the Cretaceous—e.g., 140-120, 120-110, 110-99, 99-80, 80-50, and after 50 Ma—which include the collision of an accretionary plateau with proto-Japan, subduction of the Izanagi-Pacific ridge, collision of India-Eurasia, northward approach of the Philippine plate, and the East Sea opening. Over more than 90 m.y., the Gyeongsang Basin apparently underwent three events of block rotations in opposite directions, and two events of clockwise rotation of the whole basin or the Korean Peninsula. The first series of events rotated with respect to Eurasia and the second rotated together with Eurasia. We present the tectonic evolution of the Gyeongsang Basin as a model for the tectonic development of East Asia from the Cretaceous to the present.

Cenozoic Strike-Slip Displacement along the Yangsan Fault, Southeast Korean Peninsula

Granitic rocks astride the Yangsan fault on the southeast Korean Peninsula can be classified into five rock types in two groups, based on field relationships and petrographic features. Group I rocks are granodiorite, enclave-rich porphyritic granite, and enclave-poor porphyritic granite. Mafic microgranular enclaves (MME) and/or mafic clots in rocks of this group imply magma mixing. Group II rocks are equigranular and micrographic granite indicative of shallow emplacement. Group II rocks intrude Group I rocks without magma mingling at several outcrops. The granitic rock facies on both sides of the Yangsan fault are similar, suggesting post-50 Ma dextral motion of approximately 21 km in a N20°E direction.

Tectonic Implication of A-type Granites across the Yangsan Fault, Gigye and Gyeongju Areas, Southeast Korean Peninsula

Based on field relations and petrographic characteristics, alkali-feldspar granite in the Gigye area on the western side of the Yangsan fault is classified as an A-type granite. It is mainly composed of quartz, perthitic K-feldspar, and interstitial ferri-annite plus sodic amphiboles such as riebeckite and arfvedsonite, indicating crystallization under hypersolvus, anhydrous conditions. These petrographic features are the same as those of the A-type granite in the Gyeongju area on the east side of the Yangsan fault. These distinctive, pre-Eocene granites are separated by about 21.3 km, indicating Cenozoic dextral strike-slip of this amount along the Yangsan fault.

Hydrous Minerals (Phlogopite and Amphibole) from Basaltic Rocks, Jeju Island: Evidences for Modal Metasomatism

Phlogopite and kaersutite, showing distinctively different textural characteristics compared to the common phenocrysts, are observed in alkali basalt from Jeju Island. They occur as large crystals (2-10 mm) in host basalts, whereas fine-grained phlogopite and kaersutite occur in ultramafic mantle xenoliths and mafic gabbroic xenoliths, respectively, as an interstitial and microvein phases, or in corona textures ((4.1-6.9 wt%) and F(2.8-4.6 wt%) and relatively high mg#[

Two different magma series imply a Palaeogene tectonic transition from contraction to extension in the SE Korean Peninsula

Late Cretaceous–early Tertiary granites in the Gyeongsang Basin have distinctly different bulk-rock compositions. Calc-alkaline I-type metaluminous granites display petrographic features implying magma mixing, whereas A-type granites are hypersolvus and peralkaline. I-type plutons mainly consist of enclave-rich granodiorites and enclave-poor porphyritic granites typified by abundant plagioclase phenocrysts; these granitoids contain various mafic clots and magmatic/microgranular enclaves (MMEs). A-type bodies are perthitic alkali-feldspar granites characterized by interstitial annite + riebeckite-arfvedsonite. New SHRIMP-RG zircon U–Pb age dating of an I-type enclave-poor porphyritic granite and an A-type alkali-feldspar granite yielded ages of 65.7 ± 0.7 and 53.9 ± 0.3 million years, respectively. Based on prior geochronologic data and these contrasting ages of granitic magma genesis, SE Korea may have evolved tectonically from latest Cretaceous compression to late Palaeocene extension (i.e. orogenic collapse). The later part of the 66–54 Ma magmatic gap apparently includes the time of tectonic inversion in the SE Korean Peninsula, a far-field effect of the collision of the Indian subcontinent with Eurasia. This process is also reflected in the 69–52 Ma NNE-trending Eurasian apparent polar wandering path.

Cenozoic wrench tectonics and oroclinal bending in SE Korea

Based on fault geometry, petrography, and geochronology of granitic rocks as well as palaeomagnetic data from the Gyeongsang Basin, two conjugate fault sets are explained as a reflection of NNE-trending right-lateral wrench tectonics. According to this interpretation, the Gaum and Yangsan fault sets correspond to antithetic faulting by R′-shear and synthetic faulting by R-shear, respectively; they have rotated clockwise and counterclockwise, respectively, due to NE–SW compression (shortening), as a result of a NNE-trending wrenching force (simple shear). During progressive deformation, NS- or NNW-trending strike–slip faulting by P-shear occurred in the Yeongyang sub-basin, and finally the Yangsan fault formed as a wrench fault bisecting the P-shear and R-shear directions. Extension of the faults (R-shear, striking ∼N22°E) generated by block rotation on the east side of the Yangsan fault (wrench fault, striking ∼N13°E) resulted in convex eastward deflections. We suggest that this was caused by oroclinal bending of the existing faults generated by block rotations in opposite directions and is inferred to have been closely related to the East Sea (i.e. Sea of Japan) opening.

The Relationship between the Mineral Characteristics and Spectral Induced Polarization for the Core Rock Samples from the Gagok Skarn Deposit

In order to develop the evaluation techniques for the potential sulfide ore reserves, the relationships between the modal vol.%, grain sizes and textural characteristics of the constituent minerals (e.g., sulfides, oxides and skarn minerals) and the Spectral Induced Polarization (SIP) phase differences are examined for the nine rock cores collected from the Gagok Pb-Zn skarn deposit. The Gagok Pb-Zn skarn deposit occurs mainly along the intrusive contact between the Cretaceous granitic rocks and Cambrian Myobong slate and Pungchon limestone. The nine rock cores have been grouped into three showing distinctive SIP phase differences: the highest (Group I), intermediate (Group II) and lowest (Group III). In relation with the modal vol.% of minerals, Group I is characterized by higher pyrrhotite (25-38 vol.%) and amphibole (40-55 vol.%); Group II by intermediate pyrrhotite (7-13 vol.%) and higher garnet (44-68 vol.%); and lower pyrrhotite (1-7 vol.%) and higher pyroxene (24-66 vol.%) stand for Group III. Furthermore, the grains of all the major constituent minerals become smaller from Group I (

A fluid inclusion study of an amethyst deposit in the Cretaceous Kyongsang Basin, South Korea

Abstract The Eonyang amethyst deposit is thought to be spatially and temporally associated with the biotite granite of the Cretaceous Kyongsang Basin, South Korea. The euhedral quartz crystals in cavities in the aplite which intrudes biotite granite are colour-zoned from white at the base to amethystine at the top. Fluid inclusions from rock-forming quartz in granitic rocks and euhedral quartz crystals in cavities were examined. Three types of primary inclusions were observed and three isochores for inclusions representing each type are constructed to constrain the trapping conditions and fluid evolution involved during the formation of the amethyst. The intersection of the isochore representing the early fluid inclusions with solidus temperature of the host granite indicates initial quartz formation at ~600°C and 1.0−1.5 kbar. Intermediate quartz formation, associated with the high-salinity inclusions, occurred at somewhat lower temperatures (400°C) and pressures of ~1 kbar. The amethystine quartz formed from H2O-CO2-NaCl fluids at temperatures between 280 and 400°C, and pressures of ~1 kbar. Based on the texture and mineralogy of host minerals and on the fluid inclusion characteristics, the euhedral quartz began growing at near solidus conditions of the granite and the pressure did not vary significantly until the end of crystallization of amethystine quartz crystals in cavities. Early quartz in cavities formed from moderately saline fluids that either exsolved from or were in equilibrium with the granite, whereas the amethystine quartz apparently grew from fluids of at least partial sedimentary origin. The granite crystallized at considerable depth under relatively low water pressures probably in the root zones of porphyry-type systems. Hydrothermal activities, fluid compositions and erosion factors combined to provide favourable conditions for the formation of the Eonyang amethyst deposit and its presence near the Earth’s surface today.

A Technical Note on Monitoring Methods for Volcanic Gases

The monitoring methods for volcanic gases are divided into remote sensing and direct gas sampling approaches. In the remote sensing approach, COSPEC and Li-COR are used to measure and , respectively, with FT-IR for detection of a range of volcanic gases. However, the remote sensing approach is not applicable to Mt. Baegdu, where the atmospheric contents of volcanic gases are very low as a result of the strong interaction of volcanic gases with the nearby surface water and groundwater. On the other hand, the direct gas sampling approach involves the collection of volcanic gases from volcanic vents or fumaroles and the subsequent laboratory analysis, thus making it possible to measure even very low levels of volcanic gases. The direct sampling approach can be subdivided into the evacuated bottle method and the flow-through bottle method. In applying both methods, sampling bottles typically contain reaction media to trap specific volcanic gases. For example, NaOH solution(Giggenbach bottle), solution, and acid condensates have been experimented for volcanic gas sampling. Once taken from vents and fumaroles, the samples of volcanic gases are pretreated and subsequently analyzed for volcanic gases using GC, IC, HPLC, titrimetry, TOC-IC, or ICP-MS. Recently, there has been the increasing number of evidences on the potential volcanic activity of Mt. Baegdu. However, little technical development has been made for the sampling and analysis of volcanic gases in Korea. In the present work, we reviewed various volcanic gas monitoring methods, and provided the detailed information on the monitoring methods applied to Mt. Baegdu.