• The Journal of the Petrological Society of Korea
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• v.2 no.2
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• pp.122-129
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• 1993

We report Rb-Sr whole rock, K-Ar and fission track mineral ages for the Chuncheon granite in the Precambrian Gyeonggi massif. The Rb-Sr whole rock define an age of $196{\pm}9$ Ma with an initial ratio of $0.7159{\pm}0.0006$, suggesting that the granitic magma might have been generated from crustal sources (S-type), or probably mixed mantle and crustal materials, and emplaced into the massif in the late Triassic or the early Jurassic. K-Ar mineral ages of hornblende, muscovite and biotite are ~210 Ma, ~180 Ma and 166-170 Ma respectively, and fission track zircon and apatite ages are 65-70 Ma, ~35 Ma respectively. These ages indicate that the granitic magma might have been emplaced at about 7 to 9 km from the paleosurface, and rapidly cooled down up to $300^{\circ}C$ until middle Jurassic (~170 Ma) with a rate of about $10^{\circ}C$/Ma, due to thermal difference between the magma and the wall rock. During middle Jurassic to late Cretaceous (about 170-70 Ma), the granite pluton is assumed to have uplifted to 4 to 6 km level under the paleosurface with a rate of 30 m/Ma and slowly cooled down with a rate of about $1^{\circ}C$/Ma owing to relatively slow denudation of the massif. In late Cretaceous to the present, the pluton might have more rapidly uplifted to the present level with a rate of 85 m/Ma and rapidly cooled down with a rate of about $3^{\circ}C$/Ma compared to those of middle Jurassic to late Cretaceous time because of extensive igneous activities accompanied by tectonism in the Gyeonggi massif.

• Journal of the Korean earth science society
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• v.30 no.1
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• pp.33-39
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• 2009

This paper presents the occurrence and characteristics of the basaltic andesite-siltstone peperite in the lower part of the Gyehwari Formation (Cretaceous), Buan-gun, Jeonbuk province, SW Korea. The peperite is associated with tabular basaltic andesite body, concordantly intercalated with red siltstone and silty sandstone interbeds of floodplain facies. Development of the peperite along the upper margin of the andesite and its textural transition from a dispersed blocky type inward into a closely packed type collectively indicate an intrusive origin (?sill) of the andesite. Magma intrusion and subsequent peperite formation suggest an active syndepositional volcanism since the early stage of evolution of the Gyehwa Basin. The andesite is dated at Late Cretaceous (Santonian) by K-Ar whole-rock radiometric method.

• Economic and Environmental Geology
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• v.26 no.4
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• pp.519-539
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• 1993

A total of 264 independently oriented core samples were collected from 26 sites in the southwestern part (the Naktong Trough) of the Cretaceous $Ky{\check{o}}ngsang$ Basin in south Korea. The sampled formations comprise the sedimentary Shindong and the Hayang Groups of the Lower Cretaceous age. Alternating field and thermal demagnetizations were conducted. Characteristic remanent magnetization (ChRM) was relatively easily isolated in each formation except in the Chinju formation, from which only remagnetization circles were observed. Even though an extensive use of the fold test was not possible due to the nearly homoclinal nature of the strata in the area, we believe that the ChRM of each formation is of primary origin based on the following grounds: The in-situ ChRM direction of each formation is different from the present geomagnetic field direction. Fisherian precision parameter becomes enhanced through the tilt correction in all formations, closely to the values required for a positive fold test. Three out of the five studied formations pass the reversal test. The mean palaeomagnetic pole position from the studied area is found to be statistically different from the contemporary pole from the Chinese block exclusive of the Shandong area. The difference in magnetic declination suggests a $14.5^{\circ}$ (${\pm}10.5^{\circ}$) clockwise rotation of the studied area relative to the Chinese block comprising the west of the Tan-Lu fault. On the other hand, any significant difference in magnetic inclination and concurrent palaeolatitude is not observed between the studied area and China as well as the other area (Taegu-Andong area) in the $Ky{\check{o}}ngsang$ Basin. The dual nature of the magnetic polarity confirmed in all formations suggests an older than 124 Ma (Neocomian or older) age of the studied sedimentary strata.

• The Journal of the Petrological Society of Korea
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• v.8 no.3
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• pp.131-148
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• 1999

Characteristics and complex structures in the northwest Nevada, U.S.A. are de-veloped due to relative tectonic movement of major tectonostratigraphic terranes. Theresearch area is composed of autochthonous rocks of both Early Triassic Koipato Group and Middle Triassic Star Peak Group, which is located in the Humboldt Range, northwest Nevada, U.S.A. The present research is focused on deformation history, related fabric development, and state of regional paleostress during the Jurassic to Late Cretaceous. The Triassic autochthonous rocks in the Humboldt Range, Nevada, U.S.A. display polyphase deformation due to E- to ESE-directed tectonic transport of the Fencemaker allochthon over autochthonous rocks of the Humboldt Range. Structures involving the Mesozoic foreland deformation are development of intense foliation, different styles of folds, minor thrusts, transposed layering, and strong mylonitization. These tectonic structures are mostly developed along the western flank of the Humboldt Range, and are reported as the first deformation of the Mesozoic foreland in the Humboldt Range, Nevada, U.S.A. Regional principal stress(${\sigma}_1$) is interpreted to be E to ESE between the Jurassic and Early Cretaceous on the basis of orientations of strongly developed $D_1$ structures. The deformation during the Middle to Late Cretaceous, is characterized by development of consistent N- to NNE-trending metamorphic quartz veins, and shear zones parallel to pre-existing $D_1$ foliation. Orientations of metamorphic quartz veins as well as other kinematic indicators are N to NNE and are interpreted as those of regional principal stress(${\sigma}_1$) during the Late Cretaceous. The sense of shear applied in the Humbololt Range is dextral and is caused by reactivation of early-formed $D_1$ structures. These results reflect counterclockwise rotation of regional principal paleostress in the Humboldt Range from the Jurassic to Late cretaceous. Finally, development of both shear band cleavage and S/C mylonitic fabrics indicates that the shear zones in the Humboldt Range reflect involvement of enhanced non-coaxial flow during bulk shortening in mylonitic formation.

• Economic and Environmental Geology
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• v.19 no.spc
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• pp.175-191
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• 1986

The Geodo granodiorite intruded into the Joseon Supergroup is fine-grained at the marginal part, and medium-grained and more leucocratic at the central part. The Quartz monzonite porphyry intruded inte Precambrian granite and Geodo granodiorite has abundant plagioclase phenocryst. The Imog granite intruded into the Yulri Group and the Joseon Supergroup is mediumgrained biotite granite with partly pinkish feldspar phenocryst. The K/Ar ages obtained from the biotite of the Geodo granodiorite and Imog granite are Early ($111{\pm}1{\sim}107{\pm}1$ Ma) and Late ($93{\pm}1{\sim}92{\pm}1$ Ma) Cretaceous, respectively. The K/Ar sericite age of the quartz-sericite zone of the lower Jangsan quartzite occuring in the western area gave much younger age (about 170 Ma) than that of the Jangsan quartzite, that might be reset due to the regional metamorphism of the Daebo orogeny. The granitic rocks of the area are felsic to mafic, metaluminous to peraluminous, calc-alkalic (alkali-lime index${\fallingdotseq}$ 57) and I-type (magnetite-series) based on the chemical data_ And they appear to have been fractionated at the order of Geodo granodiorite, Quartz monzonite porphyry and Imog granite. In terms of mineralogy, geochemistry and K/Ar biotite age, a rock suite of monzodiorite, quartz monzodiorite and quartz monzonite-granodiorite in the Geodo stock was fractionally differentiated from a magmatic body from its margin to inward.

• Economic and Environmental Geology
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• v.21 no.4
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• pp.335-348
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• 1988

The author aimed to describe the volcano-stratigraphy and petrology of the volcanic mass in the Koheung peninsula, South Cheolla province. The volcanic mass is composed of the volcanics and intrusives of late Cretaceous which extruded the Pre-cambrian metamorphic(Jirisan gneiss complex) and the early Cretaceous sedimentary(Duwon Formation) basement. The volcanic pile consists of, in ascending order, Bibongsan andesite, Koheung tuff and breccia, and Palyeongsan welded tuff, and are intruded by ring intrusives( intrusive breccia, andesite porphyry, intrusive rhyolite and fine-grained quartz-diorite) and central pluton(diorite, quartz monzodiorite, biotite granite and micrographic granite). Bibongsan andesite mainly consists of andesite tuff and lava. Koheung tuff consists of alternation of fine tuff, coarse tuff and lapilli tuff, and Palyeongsan welded tuff which overlies Koheung tuff, comprises K-feldspar and quartz phenocrysts, elongated brown fiamme, lithic fragments in matrix of devitrified brown glass shards, and mainly consists of rhyodacite to rhyolite vitric ash-flow tuff. The results of petrochemical studies of the igneous rocks suggest that the rocks were a serial differentiational products of fractional crystallization of calc-alkaline magma series. This study reveals that the volcanic mass in this area is inferred to the remnant of the resurgent cauldron, measuring 30 by 25 km in diameter. The cauldron block was lowered at least 1,000 m by ring fault displacement.

• Economic and Environmental Geology
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• v.26 no.3
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• pp.383-393
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• 1993

Paleomagnetic data have been obtained from the Lower Triassic Tonggo formation which is exposed in the E-W trending Baekunsan syncline comprising the Pyongan Supergroup in eastern Korea. Two ancient components of magnetization are recovered in this formation by detailed thermal demagnetization: a post-folding component and a pre-folding component The post-folding component ($D/I=58.8/55.5^{\circ}$) is normally magnetized and appears to acquire in the Cretaceous Normal Superchron. It is a magnetic signature of the Daebo Orogeny and has been rotated clockwise since this magnetization has been acquired, in common with the main synclinal axis. The pre-folding component ($D/I=1.1/19.4^{\circ}$, Paleopole at $306.1^{\circ}E$, $63.2^{\circ}N$) passes fold and reversal tests and is inferred to be a post-depositional or early chemical diagenetic remanence of Lower-Middle Triassic age. This paleopole corresponds only with the Lower Triassic poles from the North China Block: it is removed from the contemporary poles from the South China Block. If the result of this study is corrected for the clockwise rotation deduced from the Cretaceous overprint, the enhanced agreement with the Lower Triassic poles from the North China block can be achieved. Therefore, a first order correlation between the Korean Peninsula and North China at least since Lower Triassic times is identified in this study.

• Economic and Environmental Geology
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• v.21 no.1
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• pp.17-27
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• 1988

The Cretaceous Jinsan gold-bearing hydrothermal veins occur within the Late Proterozoic to Mid Ordovician metasedimentary rocks, intruded by Early Cretaceous pink-feldspar granite($142{\pm}2.0m.y.$). Electrum-galena-sphalerite mineralization was deposited in three stages of quartz and calcite veins. Quartz sulfide-bearing stage I and II evolved from initial high temperatures (near $360^{\circ}C$)to later lower temperatures(near $220^{\circ}C$)from lower salinity fluids(1.0 to 3.2 wt.% NaCl eq.). Fluid inclusion data from the post ore carbonate stage reflects much cooler($110^{\circ}$ to $180^{\circ}C$). Evidence of boiling indicates pressure of<85 bars, corresponding to depths of 400m to 1050m assuming lithostatic and hydrostatic loads. Au-deposition was likely a result of boiling, coupled with declining temperatures. The ${\delta}^{34}S$ $H_2S$ values calculated for sulfides are consistent with an igneous source of sulfur with a ${\delta}^{34}S_{{\Sigma}S}$ value near 4.0‰.

• The Journal of the Petrological Society of Korea
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• v.26 no.1
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• pp.63-72
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• 2017

We report an Early Cretaceous zircon U-Pb age ($113.9{\pm}1.2Ma$) for the Heuksan-do granite located about 90km from Mokpo offcoast of the southwestern Korean peninsula. At this Aptian/Albian boundary, widespread igneous activities occurred not only in the Korean peninsula but also in the eastern China and Japan. We raise the possibility that the flat-slab subduction and delamination triggered such an episodic igneous activity over the large areas of East Asia.

• Journal of the Korean Geophysical Society
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• v.9 no.3
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• pp.273-286
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• 2006

The Kyongsang Basin is the most representative Cretaceous basin in the Korean Peninsula where extensive crustal deformation and non-marine sedimentation took place in the early Cretaceous period. The lithology of the basement of the basin and adjacent areas is comprised of mainly Precambrian gneiss complex and Mesozoic granite intrusions. We have carried out magnetotelluric (MT) surveys to investigate the deep geoelectric structure around the Kyongsang Basin. The MT data were collected in the frequency range from 0.00042 to 320 Hz at 24 sites along a profile across the northern part of Kyongsang Basin. The results of MT inversion show that the thickness of sediments is estimated about 3 km to 9 km and the depth to base of granite intrusion is about 20 km. A remarkable discovery in this study is the highly conductive layer beneath the basin, having the resistivity of 1 ohm-m to 30 ohm-m and the thickness of about 3 km to 4 km or more. Although we are not able to reveal the nature of this layer, the result of this study could provide some basic information with respect to the formation process and deposit environment of the proto-Kyongsang Basin.