• Economic and Environmental Geology
• /
• v.43 no.1
• /
• pp.53-66
• /
• 2010

We report petrologic characteristics including $^{40}Ar-^{39}Ar$ absolute ages of the subsurface lavas recovered from borehole cores in two islets, Marado and Gapado, off the southwestern coast of Jeju in southernmost Korea and discuss on the volcanism in the region. The lavas in Gapado are apparently divided into one unit with bright colored, aphanitic texture and sheet jointed, and another unit with dark colored and massive. The outcrops often show differentially weathered pattern due to textural difference. While, the lavas in Marado have vesicular and glomerporphyric texture, even though each lava flow unit in Marado has slight unique texture with variation of vesicularity and phenocrysts. The chemical composition of rock core samples from Gapa borehole and Mara borehole shows that the lavas from Gapado and Marado are classified into basaltic trachyandesite($SiO_2$ 52.6-53.6 wt%, $Na_2O+K_2O$ 7.3-7.5 wt%) and tholeiitic andesite($SiO_2$ 51.7-52.8 wt%, $Na_2O+K_2O$ 3.6-4.1 wt%), respectively. The measured $^{40}Ar-^{39}Ar$ plateau ages range from $824{\pm}32\;Ka$(MSL -69 m) to $758{\pm}\;Ka$(MSL 19 m) for core samples of Gapa borehole and $259{\pm}168\;Ka$(MSL -26 m) for a core sample of Mara borehole, respectively. The absolute age of Gapado basaltic trachyandesite is well correlated with that of Sanbangsan trachyte(Won et al., 1986). Meanwhile, the age of a sample in Marado has $259{\pm}168\;Ka$(MSL -26 m) with poor plateau age formation and high error range. We report the data in caution but the rock composition and absolute age of Marado tholeiitic andesite are relatively correlated with those of lava units from Duksu and Sangmo-2 boreholes, indicating the volcanism during 260-150 Ka. On the basis of interpretation of occurrences of exposed and subsurface volcanic rocks of the study area, stratigraphic relationship with adjacent borehole cores and the bathymetry chart of surrounding area, it indicates that the lavas in Gapado were formed around 800 Ka during relatively early stage of volcanic activity in Jeju Island. Meanwhile, Marado may have originated around 260-150 Ka during relatively young stage of volcanism in Jeju Island. It is inferred that the volcanisms have originated in land and these islets were individual ancient volcanoes. The apparent topography has been re-shaped by tidal erosion due to transgression.

• Economic and Environmental Geology
• /
• v.16 no.1
• /
• pp.19-36
• /
• 1983

The study revealed that the sequence of volcanism in Ulrung island can be classified into 5 stages, and the volcanic history is summerized as follow: 1st stage: Eruption of basaltic agglomerates, tuffs and lavas, 2nd stage: Eruption of trachytic and trachyandesitic agglomerates and tuffs, 3rd stage: Eruption of trachyte lavas and their lapilli tuffs, 4th stage: Eruption of trachyte lavas and nepheline phonolites, 5th stage: Eruption of pumice, trachytic ash and lapilli, and plutonic ejecta (fragments of alkali gabbro, monzonite and alkali feldspar syenite) and a subsequent caldera formation. Finally, a small scale eruption of leucite bearing trachyandesite lava in the caldera. Several evidences show that there have been long erosional intervals between the 1st and 2nd stages and between the 4th and 5th stages. A K-Ar age for trachybasalt lava of the 1st stage was determined to be 1.8 Ma, and a $C^{14}$ age, 9300Y. (Machida, 1981) is available for these volcanic events. Therefore, it is considered that volcanic activity of the island above sea level began at least in early Pleistocene, and continued to until 9300 years ago exploding large amount of pumice, prior to pouring out of leucite bearing trachyandesite from the inner caldera. Using solidification index (SI) of Kuno, microscopic texture and mineral composition as criteria of the classification, the volcanic rocks are classified into alkali basalt, trachybasalt, trachyandesite, trachyte and phonolite. These are mostly prophyritic in texture. Main constituent minerals of alkali basalt and trachybasalt are plagioclase, olivine, Ti-augite and magnetite. Principal minerals of trachyandesite are plagioclase, anorthoclase, clinopyroxenes, kaersutite, biotite and magnetite. Trachyte and phonolite consist mainly of anorthoclase, clinopyroxene and magnetite, showing typical trachytic texture in groundmass. In solidification index, alkali basalt ranges from 39 to 27, trachybasalt 17 to 14, trachyandesite 12 to 9 and trachyte 8.15 to 0.72. A trend of compositional variation showing a typical alkali volcanic rock series is revealed on $SiO_2$-oxides and SI-oxides diagrams. In $SiO_2$-total alkali diagram, alkali lime index and An-Ab'-Or diagram, the samples fall into the fields of potassic series of the alkali volcanic rock series, whereas in A-F-M diagram show a trend toward the alkali enrichment with a curve approaching toward the iron apex. In particular, trachybasalt lavas in this island have higher total iron contents which is comparable to alkali rocks in other areas, e. g. as Gough and Tristan volcanic islands located near the Mid-Oceanic ridge in South Atlantic Ocean.

• Journal of the Mineralogical Society of Korea
• /
• v.31 no.1
• /
• pp.57-65
• /
• 2018

The geology of Hidalgo state in Mexico is formed by a Precambrian metamorphic rock base which discordantly supports a strong Paleozoic sedimentary rock sequence. Triassic-Jurassic and Cretaceous sedimentary rocks overlie the above-mentioned rocks at a discordant angle. These Mesozoic rocks are in turn covered by a Cenozoic structure which is marine at its base. At the top, the structure changes to andesitic and basaltic composition volcanic rocks. And, a great variety of mineral deposits, both metallic and nonmetallic, is present in Hidalgo state. The host rocks of these deposits are also very varied in age from Mesozoic to Tertiary. Mineralization age corresponds to Tertiary in 90% of the area. Hidalgo state occupies as an important place in national silver and manganese production. Main mineral deposits correspond to argentiferous veins hosted in sedimentary rocks. Following in order of importance are lead and zinc, as well as some small iron deposits. There is evidence of tin and molybdenum mineralization, but these deposits have not been exploited because of their low grade and volume. And, Hidalgo state has different types of nonmetallic mineral deposits such as sedimentary, hydrothermal, metamorphic and volcanic origins.

• Geophysics and Geophysical Exploration
• /
• v.18 no.4
• /
• pp.171-180
• /
• 2015

Electrical resistivity of 23 core samples from Ulleung Island at dry or saturated condition has been measured along with dry density and effective porosity, and the relations between the properties has also been discussed. Upper and lower bounds of electrical resistivity at room temperature can be provided by the dry- and saturated-resistivity, respectively. Injecting nitrogen gas to the pore space at the very end of drying process can prevent humid air from getting into the pore space, so that measurement of dry-resistivity can be less affected by humidity in the air. Dry density and porosity have very close correlation; the ratio between increase of porosity and the decrease of density showed distinct relation to the rock types, such that basaltic rocks showed higher ratio while trachytic rocks showed lower. Saturated resistivity showed close correlation to density and effective porosity of the rock sample, while dry resistivity didn't.

• The Journal of the Petrological Society of Korea
• /
• v.22 no.2
• /
• pp.83-115
• /
• 2013

Erroneous fission-track (FT) ages caused by an inappropriate calibration in the initial stage of FT dating were redefined by re-experiments and zeta calibration using duplicate samples. Revised FT zircon ages newly define the formation ages of Yucheon Group rhyolitic-dacitic tuffs as Late Cretaceous to Early Paleocene ($78{\pm}4$ Ma to $65{\pm}2$ Ma) and Gokgangdong rhyolitic tuff as Early Eocene ($52.1{\pm}2.3$ Ma). In case of the Early Miocene volcanics, FT zircon ages from a dacitic tuff of the upper Hyodongri Volcanics ($21.6{\pm}1.4$ Ma) and a dacitic lava of the uppermost Beomgokri Volcanics ($21.3{\pm}2.0$ Ma) define chronostratigraphies of the upper Beomgokri Group, respectively in the southern Eoil Basin and in the Waeup Basin. A FT zircon age ($19.8{\pm}1.6$ Ma) from the Geumori dacitic tuff defines the time of later dacitic eruption in the Janggi Basin. Based on FT zircon ages for dacitic rocks and previous age data (mostly K-Ar whole-rock, partly Ar-Ar) for basaltic-andesitic rocks, reference ages are recommended as guides for stratigraphic correlations of the Miocene volcanics and basements in SE Korea. The times of accumulation of basin-fill sediments are also deduced from ages of related volcanics. Recommended reference ages are well matched to the whole stratigraphic sequences despite complicated basin structures and a relative short time-span. The Beomgokri Group evidently predates the Janggi Group in the Eoil-Waeup basins, while it is placed at an overlapped time-level along with the earlier Janggi Group in the Janggi Basin. Therefore, the two groups cannot be uniformly defined in a sequential order. The Janggi Group of the Janggi Basin can be evidently subdivided by ca. 20 Ma-basis into two parts, i.e., the earlier (23-20 Ma) andesitic-dacitic and later (20-18 Ma) basaltic strata.

• Korean Journal of Mineralogy and Petrology
• /
• v.35 no.3
• /
• pp.313-331
• /
• 2022

Natural or native abiotic molecular hydrogen (H₂) is a major component in natural gas, however yet its importance in the global energy sector's usage as clean and renewable energy is underestimated. Here we review the occurrence and geological settings of native hydrogen to demonstrate the much widesprease H₂ occurrence in nature by comparison with previous estimations. Three main types of source rocks have been identified: (1) ultramafic rocks; (2) cratons comprising iron (Fe²⁺)-rich rocks; and (3) uranium-rich rocks. The rocks are closely associated with Precambrian crystalline basement and serpentinized ultramafic rocks from ophiolite and peridotite either at mid-ocean ridges or within continental margin(Zgonnik, 2020). Inorganic geological processes producing H₂ in the source rocks include (a) the reduction of water during the oxidation of Fe²⁺ in minerals (e.g., olivine), (b) water splitting due to radioactive decay, (c) degassing of magma at low pressure, and (d) the reaction of water with surface radicals during mechanical breaking (e.g., fault) of silicate rocks. Native hydrogen are found as a free gas (51%), fluid inclusions in various rock types (29%), and dissolved gas in underground water (20%) (Zgonnik, 2020). Although research on H₂ has not yet been carried out in Korea, the potential H₂ reservoirs in the Gyeongsang Basin are highly probable based on geological and geochemical characteristics including occurrence of ultramafic rocks, inter-bedded basaltic layers and iron-copper deposits within thick sedimentary basin and igneous activities at an active continental margin during the Permian-Paleogene. The native hydrogen is expected to be clean and renewable energy source in the near future. Therefore it is clear that the origin and exploration of the native hydrogen, not yet been revealed by an integrated studies of rock-fluid interaction studies, are a field of special interest, regardless of the presence of economic native hydrogen reservoirs in Korea.

• The Journal of the Petrological Society of Korea
• /
• v.27 no.3
• /
• pp.121-138
• /
• 2018

We have examined the petrotectonic setting and magmatic evolution from petrochemical characteristics of major and trace elements for the Cretaceous volcanic rocks in and around the Mireukdo Island. The volcanic rocks, can be devided into Jusasan, Unmunsa, Yokji and Saryang subgroups on the ascending order, are classified as basalt, basaltic andesite, andesite, dacite and rhyolite on TAS diagram. Petrochemical data show that the rocks are calc-alkaline series, and suggest that erupted earlier medium-K series and later high-K series. The volcanic rocks provide a case in which the calc-alkaline magma are formed, not only from separate protoliths, but following separate paths from source to surface. Earlier and later subgroups take different paths to the surface respectively, and are emplaced in the shallow crust as a series of discrete magma chambers through the volcanic processes. After emplacement, each chamber evolves indepently through fractional crystallization with a little assimilation of wall rock. The volcanic rocks have close petrotectonic affinities with orogenic suite and subduction-related volcanic arc. The rhyolitic magma can be derived from calc-alkaline andesitic magma by fractional crystallization with crustal assimilation, which may be derived from a partial melt of peridotite in the upper mantle.

• The Journal of the Petrological Society of Korea
• /
• v.8 no.2
• /
• pp.57-70
• /
• 1999

The volcanic rocks of Ulleungdo reveal very high alkali element abundances and most of them have high K20/Na20 ratios and belong to potassium-series. Ulleungdo volcanics show very wide range of variation in their composition from basalts to trachytic basalt, basaltic trachyandesite, trachyandesite, and finally to trachyte on total alkali-silica diagram. Such a general trend of compositional variation can be explained well by differentiation due to fractional crystallization of various minerals. Olivine, clinopyroxene, plagioclase, ilmenite, and apatite are suggested as the major fractionated minerals. Ulleungdo volcanics show Nb/U and PbICe ratios similar to oceanic volcanics such as MORB and OIB, but significantly different .from volcanic rocks of island arc environments, which suggest that they are not directly related with subduction along the Japanese arc. LREE abundances of Ulleungdo volcanics are highly enriched compared with HREE abundances ((La)N=193-420, (L~)~=7.5-19.5).O nly trachyte-1 show appreciable negative Eu anomalies among various rock types, which suggests significant amount of plagioclase were fractionated. However, trachyte-2, trachyte-3, phonolite, and pumice reveal quite different variation trend of trace and rare earth element abundances from trachyte-1, which suggest that they have originated from different magma batches and have experienced different differentiation processes. A prominent bimodal distribution, thus lacking of intermediate composition, is observed from the Ulleundo volcanics.

• The Journal of the Petrological Society of Korea
• /
• v.9 no.3
• /
• pp.142-168
• /
• 2000

The granites distributed in the Kyongsang basin contain the rocks which are different from the host rocks, and they are known as magic microgranular enclaves. The genesis of the magic micro-granular enclaves can be divided into four types: (1) rock fragments from country rocks; (2) cumulation of the early crystals in host magma or disruption of early chilled borders; (3) magma mingling; and (4) restite. These enclaves can be easily found in the granites around Mt. Wonhyo, Yangsan city. They are ellipsoidal in shape, and have phenocrysts might be originated from the host rocks and sharp contacts with the granites. Under the microscope, textures such as oscillation zoning, horn-blende-mantled quartz, rapakivi texture, and acicular apatite are observed, and these indicate that the enclaves were originated from magma and then produced by chilling. The evidences showing that the enclaves were formed by magma mingling are: (1) petrographical characteristics; (2) similarity of the compositions between the rim of plagioclase in the enclave and plagioclase in the granite; (3) linear trends of the major elements; (4) total REE content of the enclaves; and (5) Textural and compositional variations from rim to core in zoned enclaves. The magic end member of the enclave is regarded as the aphyric basaltic andesite in Mt. Sinbul-Youngchui area. The granites around Mt. Wonhyo experienced the magma mingling process which was produced by the injection of mafic magma at about 70 Ma, during the crystal differentiation, and then continued the crystallization. The equigranular granites and the micrographic granites in the study area are considered as the results after the magma mingling process.

• Journal of the Korean Society of Groundwater Environment
• /
• v.1 no.1
• /
• pp.33-50
• /
• 1994

The Hydrogeologic data of 455 water wells comprising geologic log and aquifer test were analyzed to determine hydrogeologic characteristics of the Cheju island. The groundwater. of the Cheju island is occurred in unconsolidated pyroclastic deposits and crinker interbedded in highly jointed basaltic and andesic rocks as high level, basal and parabasal types under unconfined condition. The average transmissivity and specific yield of the aquifer are at about 29,300㎡/day and 0.12 respectively, The total storage of groundwater is estimated about 44 billion cubic meters. Average annual precipitation is about 3,390 million ㎥ among which average recharge is estimated for 1,494 million ㎥ being equivalent 44.1% of total annual precipitation with 638 million ㎥ of runoff and 1,256 million ㎥ of evapotranspiration. Based on groundwater budget analysis, the sustainable yield is about 620 million ㎥(41% of annual recharge)and rest is discharging into the sea. The geologic logs of recently drilled thermal water wells indicate that very low-permeable marine sediments(Sehwa-ri formation) composed of loosely cemented sandy silt derived from mainly volcanic ashes at the 1st stage volcanic activity of the area is situated at the 120${\pm}$68m below sea level. Another low-permeable sedimentary rock called Seogipo-formation which is deemed younger than the former marine sediment is occured at the area covering north-west and western part of the Cheju island at the ${\pm}$70m below sea level. If these impermeable beds art distributed as a basal formation of fresh water zone of the Cheju island, the most of groundwater in the Cheju island will be para-basal type. These formations will be one of the most important hydrogeologic boundary and groundwater occurences in the area.