• Economic and Environmental Geology
• /
• v.26 no.4
• /
• pp.445-454
• /
• 1993

The Keumseongsan caldera is composed of the Cretaceous sedimentary rocks of the Gyeonesang Supergroup, volcanic rocks of the Yucheon Group and basic dykes. The Keumseongsan caldera is formed by subsidence of volcanic rocks, and arc fault developed late. Also, synistral strike-slip fault ($N60^{\circ}W$) developed. Volcanic rocks belong to subalkaline rocks and calcalkaline magma series. First tuffaceous breccia erupted before 71.4 Ma and cavity of magma chamber caused subsidence, which formed arc fault. Basaltic lava erupted at 71.4 Ma and residual fluids containing Fe, As, Pb, Zn and Cu metal elements built the Ohto deposits, which are dated to be 70.5 Ma based on K-Ar age for sericite. Tuffaceous breccia and tuff erupted between 70.5 and 67 Ma. When volcanic eruption became weakened, cavity in site of magma chamber brought subsidence. Rhyolite intruded and erupted at 67 Ma, and intrusive rhyolite intruded according to arc faults, also. Hydrothermal fluids containing Fe, As, Pb, Zn, Cu, Sb, Bi, Au and Ag formed the Tohyeon deposits. K-Ar age for sericite from the Tohyeon mine gives 66.0 Ma. Results of field exploration, geochemical analyses of volcanic rocks support mineralization possibility by volcanism. Especially, age of volcanism and mineralization are well in coincidence with results of K-Ar age dating. By these results, Ohto Cu mineralization is regarded to be associcated with basaltic rocks, while Tohyeon Cu mineralization with rhyolitic rocks.

• Economic and Environmental Geology
• /
• v.34 no.6
• /
• pp.617-625
• /
• 2001

The Skaergaard intrusion is widely considered a type example of a strongly fractionated, layered intrusion that has undergone extensive in situ igneous differentiation. The Intrusion, therefore, should be a good locality for modeling trace element vriation in a closed system. Previous studios (Haskin and Haskin, 1968; Faster et al., 1974), however, have suggested thats the rare earth elements in whole rocks and mineeral separates from the Intrusion did not fellow the expected trend for closed system crystatllization. Trace element modeling using published distribution coefficients, modal abundances of the coexisting minerals, and the concentration of trace elements In whole rocks and mineral separates from the Skaergaard Intrusion, reveals that the rare earth elements were significantly Influenced by the crystallization of abundant apatite in the Layered Series suring the final stages of crystallization. The results of trace element modeling also suggcsts that apatite, which appears sporadically in the UBS, is not a primary liquidus phase in these samples as previously suggested (Naslund, 1984) but an interstitial phase that (lid not directly effect trace element abundances In the evolving magma As the Skaergaard magma coaled convection, or convected as small Isolated cells during the final stages of differentiation, an elebated $P_{H2O}$ Induced by accumulation of volatile elements near the roof of the magma chamber ingibited or delayed the precipitation of primary apatite in the UBS If the Skaergaard differentiation Is modeler assuming primary apatite crystallization In the upper par of the LS where abundant modal apatite is present, and only late stage crystallization of apatite In the UBS where apatite Is less abundant, rare earth elements abundances follow a closed system variation trend. These results rule but any differentiation model for the Skaergaard Intrusion that Includesvolumetrically significant injections or discharges of magma Into or out of the chamber during the final 20% of the crystallization history.

• Journal of the Mineralogical Society of Korea
• /
• v.20 no.2 s.52
• /
• pp.103-114
• /
• 2007

Gabbroic xenoliths in poikilitic and equigrnular textures and plagioclase megacrysts, up to 15 cm in size, are trapped in alkaline basalts from Sinsanri and Sangeumburi, Jeju island. Gabbroic xenoliths are gabbro norite in composition. Plagioclase is the most dominantly present $(42{\sim}94vol%)$, while olivine crystals are absent. Pokilitic xenoliths, interpreted as cumulates, include euhedral opx+cpx in oikocryst plagioclase. Equigranular xenoliths include subhedral to anhedral opx+cpx+pl. Based on the textural and geochemical natures, occurrence mode at the outcrops, poikiltic and equigranular xenoliths and plagioclase megacrysts were from rigid zone, mushy zone and crystal-suspended zone of the magma chamber, respectively. The gabbroic xenoliths in alkaline basalts of the Juju island represent crystallization products of relatively evolved basaltic magma at the Jeju magma reservoir beneath the Jeju Island. They are gabbro fragments that represent crystallization-isolation-capture processes associated with magma batches temporarily occupying reservoirs.

• The Journal of the Petrological Society of Korea
• /
• v.7 no.3
• /
• pp.161-176
• /
• 1998

There are exposed Samrangjin Tuff and intracaldera intrusions, of which rhyolitic rocks emplaced as postcollapsed central and ring intrusions within the Samrangjin caldera, and fine-grained granodiorite and biotite granite as regional tectonic intrusions nearby. The Samrangjin Tuff and the rhyolitic rocks are of a single Samrangjin magmatic system. Flow-banded rhyolite among rhyolitic rocks was emplaced in the outer part of the ring intrusions, rhyodacite in the inner part of the eastern ring, and porphyritic dacite and dacite porphyry in the inner part of the northwestern ring. Totally the Samrangjin Tuff and the rhyolitic rocks range from rhyolite to dacite in chemical composition. The Rb-Sr isotopic data of the Samrangjin Tuff and the rhyolitic rocks yield an age of $80.8{\pm}1.5(2{\sigma})$ Ma with the initial $^{87}Sr/^{86}Sr$ ratio of $0.70521{\pm}0.00010(2{\sigma})$. The continuous compositional zonations generally define a large stratified magma system in the postcollapse magma chamber. The Sr isotopic data suggest that the compositional zonations might have resulted from the fractional crystallization of a parental dacitic magma.

• The Journal of the Petrological Society of Korea
• /
• v.6 no.2
• /
• pp.96-110
• /
• 1997

The Wondong Caldera, formed by the voluminous eruption of the rhyolitic ashflows of the Wondong Tuff which is about 1,550 m thick at the intracaldera and 550 m at the outflow, is a resurgent caldera which shows a dome structure on the central exposure of the caldera. The Wondong caldera volcanism eviscerated the magma chamber by a series of explosive eruptions during which rhyolitic magma was ejected, as small fallouts and voluminous ash-flows, to form the Wondong Tuff. The explosive eruptions began with ash-falls, progressed through pumice-falls and transmitted ash-flows. During the ash-flow phase the initial central vent eruption transmitted into late ring-fissure eruption which accompanied with caldera collapse. Contemporaneous collapse of the roop of the chamber resulted in the formation of the Wondong Caldera, a subcircular depression subsiding about 1,930 deep. Following the collapse, quartz porphyry was intruded as ring dykes along the ring fracture near the southwestern caldera rim. Subsequently the central part of the caldera floor began to be uplifted into a circular resurgent dome by the rising of residual magma. Concurrent with the resurgent doming, the volcaniclastic sediments of Hwajeri Formation were accumulated in the caldera moat and then rhyodacite lava erupted from the initial central resurgent dome and another ash-flow tuff from the northern ring fracture. After the sedimentation, the find-grained granodiorite was intruded as an arc along the eastern ring fracture of the caldera. Finally in the central part, the resurgent magma was emplaced as a hornblende biotite granite stock that formed the central dome.

• Economic and Environmental Geology
• /
• v.54 no.1
• /
• pp.35-48
• /
• 2021

The Gumi basin, situated in the mid-southeastern Yeongnam Massif, has the Cretaceous stratigraphy that is divided into Gumi Formation, andesitic rocks (Yeongamsan Tuff, Busangni Andesite), rhyolitic rocks (Obongni Tuff, Doseongul Rhyolite, Geumosan Tuff) and Intrusives (ring dikes, other dikes) in ascending order. The Geumosan Tuff is composed mostly of many ash-flow tuffs which are associated with Geumosan caldera along with the ring dikes. The caldera is outlined by ring faults and dikes and has about 3.5 × 5.6 km in diameters. The intracaldera volcanics show a downsag structure that is dipped inward in their flow and welding foliations. The caldera block represent an asymmetric subsidence, which drops 350 m in the northern margin and 600 m in the southern one. Based on these data, the Geumosan caldera is geometrically classified as an asymmetric piston subsidence caldera that suggests a single caldera cycle. The caldera reflects the piston subsidence of the caldera block bounded by the outward-dipping ring faults following a voluminous eruption of magma from the chamber. The downsag in the caldera block refers to the downsagging during the initial subsidence at the same time as the full development of the bound fault. In the ring fissures following the sagging, magma was injected due to the overpressure of magma chamber caused by subsidence.

• 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.

• Journal of the Korean earth science society
• /
• v.34 no.6
• /
• pp.550-560
• /
• 2013

In this study, the variations of geoid measured by GRACE satellite are investigated in the 20 volcanic areas erupted since 2005, and it is recognized that a detailed geological study is necessary in using geoid data for a research of the magmatic activities under the volcano. Therefore, the relationship between the regional geoid variation obtained by GRACE satellite and the change of magma activity, is studied in Japan's Shinmoedake volcano in the Kirishima volcanic complex whose eruption in 2011 was studied in detail geologically. Throughout this study the increase of geoid from 2002 in the Shinmoedake volcanic area is confirmed to be caused by the increase of gravity under the volcano, which is well matched with geological interpretation of the continuous intrusion of basaltic magma into magma chamber during several years before the 2011 eruption. The result indicates that information of the geoid variation measured by GRACE satellite is useful for monitoring the possibility of volcanic eruption although there is a need to more study to be able to confirm the possibility.

• Journal of the Korean earth science society
• /
• v.43 no.1
• /
• pp.140-150
• /
• 2022

This paper focuses on introducing the concept of the multi-scale study on the Ulleungdo-Dokdo Volcanic Group in the East Sea and recent new findings from it. Multi-channel seismic reflection data reveals that the major volcanic activities of the Ulleungdo-Dokdo Volcanic Group took place between 5 and 2.5 Ma, which were propagated from Isabu Tablemount on the eastern end to the Ulleung Island on the western end. The terrestrial Ulleung Island was built via 5 stages, which eventually formed a 3 km-wide caldera, named Nari Caldera, and a volcanic dome, named Albong, within the caldera. The Albong and the unit N-1, the earliest phreatomagmatic explosive phase of the Albong volcano, were generated from a new magma injected into the existing phonolitic body. The generally trachyandesitic bulk rock composition of the pumice in unit N-1 and Albong is attributed to the contamination of the new magma by mafic cumulates at the base of the existing phonolitic chamber. The lines of evidence of a new magma injection point toward that Ulleung Island is an active volcano with a live subvolcanic magma plumbing system.

• The Journal of the Petrological Society of Korea
• /
• v.17 no.2
• /
• pp.57-82
• /
• 2008

The volcanic sequence of the late Cretaceous Moonyu volcanic mass which distributed in the southwestern part of Ryeongnam massif, can be divided into felsic pyroclastic rocks, andesite and andesitic pyroclastic rocks, rhyolite in ascending order. The earliest volcanic activity might commence with intermittent eruptions of felsic magma during deposition of volcaniclastic sediments. Explosive eruptions of felsic pyroclastic rocks began with ash-falls, to progressed through pumice-falls and transmitted with dacitic to rhyolitic ash-flows. Subsequent andesite and andesitic pyroclastic rocks were erupted and finally rhyolite was intruded as lava domes along the fractures near the center of volcanic mass. Petrochemical data show that these rocks are calc-alkaline series and have close petrotectonic affinities with subduction-related continental margin arc volcanic province. Major element compositions range from medium-K to high-K. Petrochemical variation within the volcanic sequence can be largely accounted for tractional crystallization processes with subordinate mixing. The most mafic rocks are basaltic andesite, but low MgO and Ni contents indicate they are fractionated by fractional crystallization from earlier primary mafic magma, which derived from less than 20% partial melting of ultramafic rocks in upper mantle wedge. Based on the stratigraphy, the early volcanic rocks are zoned from lower felsic to upper andesitic in composition. The compositional zonation of magma chamber from upper felsic to lower andesitic, is interpreted to have resulted from fractionation within the chamber and replenishment by an influx of new mafic magma from depth. Replenishment and mixing is based on observations of disequilibrium phenocrysts in volcanic rocks. REE patterns show slight enrichment of LREE with differentiation from andesite to rhyolite. Rhyolite in the final stage can be derived from calc-alkaline andesite magma by fractional crystallization, but it might have underwent crustal contamination during the fractional crystallization.