• Journal of the Mineralogical Society of Korea
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• v.22 no.1
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• pp.1-11
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• 2009

Fine-grained peridotite xenoliths are rarely trapped in the basaltic rocks from the southeastern part of Jeju Island. Based on textural characteristics of the constituent phases showing uniform-sized, fine-grained tabular to mosaic grains with rare porphyroclastic relics, the studied samples can be defined as fine-grained, foliated porphyroclastic peridotites (FPP). Almost no significant difference among the FPPs in textures and major element compositions implies that the FPPs were derived from a structural domain, experiencing similar deformation events and deformation patterns. Moreover, the bimodal distribution with kink-banded porphyroclasts ($2{\sim}3mm$) and stain-free neoblasts ($200{\sim}300{\mu}m$), straight to gently curved grain boundaries with triple junctions, interstitial melt pockets, and microstructures for migrating grain boundary suggest that the studied samples went through dynamic recrystallization (${\pm}$ static recrystallization) in the presence of melt/fluid movement along foliation planes. No notable difference between the FPP and common protogranular xenoliths in major element compositions and geochemical evolution also implies that the FPP and protogranular xenoliths were from a similar horizon. Thus, the textural and geochemical characteristics of the FPPs reflects deformation events occurred at a localized and narrow zone within the lithospheric mantle beneath the Jeju Island. Although further detailed studies are necessary to define deformation events, the most possible process which could trigger deformation in the FPP in the rigid upper mantle was the ascending basaltic magma forming high-stress deformation zones. The suggested high-stress deformation zones in the lithosphere beneath the Jeju Island may be produced by paleo-faulting events related to the ascent of basalt magma before Jeju Island was formed.

• The Journal of the Petrological Society of Korea
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• v.19 no.3
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• pp.227-244
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• 2010

Abundant spinel lherzolite xenoliths showing distinctively different textural types such as protogranular, porphyroclastic, and mylonitic texture are trapped in the basaltic rocks from southeastern part of Jeju Island. These xenoliths show the textural spectrum from coarse-grained protogranular through porphyroclastic with bimodal grain size to fine-grained and foliated mylonitic texture. They tend to decrease in grain sizes and show more linear grain boundaries and more frequent triple junctions from protogranular through porphyroclastic to mylonitic. Spinel has different occurrence mode according the textural type. Spinel is always associated with orthopyroxene in protogranular texture, whereas it is scattered and independent of orthopyroxene in mylonitic texture. Additionally, porphyroblast from porphyroclastic and mylonitic textures has internal deformation features such as kink band, undulatory extinction and curved lamella, whereas neoblast is strain-free. These textural features indicate increasing degree of static/dynamic recrystallization from protogranular through porphyroclastic to mylonitic texture. The mg#[$=100{\times}Mg/(Mg+Fe_t)$] of olivine, orthopyroxene and clinopyroxene is relatively constant (ol: 88-91; opx: 89-92; cpx: 89-92) regardless of textural differences. The mg# of constituent minerals, NiO content (0.3~0.4 wt%) and MnO content (0.1~0.2 wt%) of olivine are similar to those of mantle xenoliths worldwide, also indicating that studied spinel lherzolite xenoliths were mantle residues having experienced 20~25% partial melting. The geochemical and textural characteristics have close relations showing that LREE and incompatible trace elements content of orthopyroxene and clinopyroxene increases from protogranular through porphyroclastic to mylonitic. These observations suggest that the studied mantle xenoliths experienced metasomatism by LREE enriched melt or fluid after partial melting, indicating a close relation between deformation and metasomatism. The metasomatism was possibly confined to narrow shear zones from where porphyroclastic and mylonitic textured xenoliths originated. These shear zones might favorably drive the percolation of LREE-enriched melts/fluids responsible for the metasomatism in the lithospheric mantle below the Jeju Island.

• Korean Journal of Mineralogy and Petrology
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• v.35 no.3
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• pp.173-182
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• 2022

Dongsuak crater, located in the mid-mountainous region of Jeju Island, is located at an altitude of about 700 m, and the newly discovered Dongsuak spinel peridotites was enclosed in Dongsuak alkaline basalt. The Dongsuak spinel peridotites are composed of olivine, orthopyroxene, clinopyroxene, and spinel with porphyroclastic texture under the an equilibrium state. The variations of mineral major and trace compositions indicates that the Dongsuak spinel peridotites originate at depth from 66 to 88 km under an equilibrium temperature of about 960℃~1068℃. The Dongsuak spinel peridotites have been undergone about 1~3% fractional melting. The LREE-enriched characteristics indicate that the Dongsuak spinel peridotites have been undergone cryptic metasomatism by silicate melt without new minerals.

• Korean Journal of Mineralogy and Petrology
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• v.36 no.4
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• pp.365-376
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• 2023

Songaksan, formed about 3800 year ago, is one of the tuff rings in the Jeju Island. Mantle xenoliths, spinel peridotites, are enclosed in the Songaksan Trachybasalt. The spinel peridotites are less than 2 cm in size and are composed of olivine, orthopyroxene, clinopyroxene, and spinel. The uniform compositions of the minerals from core to rim indicate that equilibrium was reached in the spinel peridotites before these were enclosed in the host magma. The spinel peridotites originated at depths between 55 and 60 km with equilibrium temperatures ranging from 915 to 968℃. The spinel peridotites from Songaksan reveal porphyroclastic texture with a lot of neoblast minerals. Olivines display strong kink banding, indicating that the upper mantle of Songaksan has been deformed. The spinel peridotites from Songaksan have undergone about 5~7% fractional melting, and cryptic metasomatism by an silicate melt. The period of entrainment and transport of the spinel peridotites in the host magma is about 15 days.

• Economic and Environmental Geology
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• v.35 no.6
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• pp.523-532
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• 2002

The rare earth elements (REE) and Nd, Sr and noble gas isotopic compositions eHer'He, 4$^{\circ}$Arp6Ar) for the Quaternary alkali basaltic rocks and mantle xenoliths in the basaltic rocks from the Baegryongdo were investigated to decipher the origin of alkali basaltic magma and xenolith beneath the Sino-Korean craton. Analytical results are summarized as follows; (1) The alkali volcanic rocks with voluminous xenoliths which are represented by the Mg-olivine and clinopyroxene dominant spinel-lherzolite in the Baegryongdo consist mainly of the basalt-mugearite and basaltic andesite. (2) The REE pattern of alkali basaltic rocks characterized by high HREE is similar to that of oceanic island basalt (OlB). Relatively concordant REE patterns of the basaltic rocks suggest that the alkali basaltic magma be formed by the identical source materials. (3) The Nd-Sr isotopic data of the alkali basaltic rocks suggest that the alkali basaltic magma be originated from the depleted mantle source with a little contamination of the continental crustal materials. (4) The $^3$He/ $^4$He ratios in olivines of xenoliths ranging from 5.0${\pm}$1.lRa to 6.7${\pm}$1.3Ra are lower than that of MORB (ca. 8.0Ra). It suggest that the xenolith be derived from the subcontinental lithospheric mantle. However, the high $^3$Her'He value of 16.8${\pm}$3.IRa at 1800$^{\circ}$C fraction (sample no OL-7) might be resulted from the post-eruptive cosmogenic $^3$He. The 4OAr/ 36 Ar ratios in olivines of mantle xenoliths are comparable to that of atmospheric argon, and are much lower than that of the MORB type mantle. These facts can lead to conclusion that the olivine of the xenolith in the Baegryongdo is affected by the post-eruptive atmospheric contamination during the slow degassing process.

• The Journal of the Petrological Society of Korea
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• v.4 no.2
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• pp.104-123
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• 1995

Mantle xenoliths in alkali basalt from Boun, Gansung area, and Baegryung island in S. Korea are spinel lherzolites composed of olivine, orthopyroxene, clinopyroxene, and spinel. The xenoliths generally show triple junctions among grams, kink-banding in olivine and pyroxenes, and protogranular and eqigranular textures having m orlentatron of specific direction. Anhedral brown spinels are disseminated in the intergranular spaces of minerals. Mineral compositions are very homogeneous without compositional zonation from rim to core in grains regardless different locahties. Olivine shows Fo. component of 89.0-90.2 and low CaO of 0.03-0.12wt%, orthopyroxene is enstatite with En component of 89.0 - 90.0 and $Al_2O_3$ of 4-5wt%, and clinopyroxene is diopside having En. component of 47.2-49.1 and $Al_2O_3$ of 7.42-7.64wt% from Boun and 4.70-4.91wt% from Baegryung showing local variation. Spinel shows the distinctive negative trend with increasing of A1 and decreasing of Cr, and Mg value and Cr number are 75.1-81.9 and 8.5-12.6, respectively. To estlmate T and P for these mantle xenoliths pyroxene-geothermometers (Wood and Banno, 1973; Wells, 1977; Mercier, 1980; Sachtleben and Seck, 1981; Bertrand and Mercier, 1985; Brey and Kohler, 1990) and Al-solubility geobarometer (Mercier, 1980; Lane and Ganguly, 1980) are used. Temperatures of Mercier (1980) and Sachtleben and Seck (1981) are compatible and equilibrium temperatures of xenoliths, average value of these two, aiie from $970^{\circ}C$ to $1020^{\circ}C$, and equihbrium pressures derived from Mercier (1980) are in the range of 12-19 Kb (42-63 Km). These temperatures and pressures seem to be reasonble wlth the consideration of Al-isopleths in MAS system (Lane and Ganguly, 1980) and Fe effect on Al-solubility in orthopyroxene (Lee and Ganguly, 1988). Equllibrium of temperatures and pressures of xenoliths in P-T space belong to ocenanic geothem among the Mercier's mantle geotherms (1980) and are completely different from continental geotherms of S. Africa (Lesotho) and S. India having different geologcal ages. anera1 compositions of spmel-lherzohtes in S. Korea and eastern China are primitwe and paleogeothems of both are very s~mllar, but degrees of depletion of upper mantle could be locally different from each other since eastern China has various depleted xenoliths due to different degrees of partial melting.

• The Journal of the Petrological Society of Korea
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• v.19 no.3
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• pp.199-208
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• 2010

The Cenozoic alkali basalts are distributed over Korea, both on central part as Bangnyeongdo, Ganseong, Pyeongtaek-Asan and Jogongni and also on southernmost part Jejudo. The ultramafic mantle xenoliths carried by Korean alkali basalts are spinel lherzolites. Garnet lherzolite that is more stable at the deeper level has not been reported so far, indicating that the lithospheric thickness under Korea does not reach deep enough to the stable zone of garnet lherzolite. The crustal evolution history of the Korean peninsula, at least some part of it, seemingly started since the Archean, it normally should have lithospheric thickness greater than 150 km. However, the mantle xenoliths carried by the Cenozoic alkali basalts indicate the maximum depth of origination in the much shallower range of 60-90 km. Such significantly thinner lithospheric thickness of the Korean peninsula than expected is quite similar to the case of North China Craton having lithospheric thickness of ca. 80 km in average, suggesting thinning of the lithospheric mantle in a depth scale of a few tens of kilometers during the past geologic time. The main causal events for such significant thinning of the lithospheric mantle can be continental collisional events of Paleoproterozoic and early Mesozoic similar to the case of North China Craton, which are also supported by Paleoproterozoic igneous and metamorphic events during the 1.9-2.0 Ga occurring all over the Korean peninsula and also early Mesozoic continental collisional event which has been discussed on lively arguments.

• The Journal of the Petrological Society of Korea
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• v.13 no.1
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• pp.33-45
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• 2004

• The Journal of the Petrological Society of Korea
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• v.21 no.1
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• pp.13-30
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• 2012

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 (<1 mm). This textural characteristics of fine-grained grains clearly indicates secondary in origin. Phlogopite contains high $TiO_2$(4.1-6.9 wt%) and F(2.8-4.6 wt%) and relatively high mg#[=100Mg/(Mg+$Fe^t$) in mols, where $Fe^t$ is total iron](88-80), whereas kaersutite has high $TiO_2$(5.6-6.11 wt%) and much lower mg#s(68-64). Our textural observations and the geochemical character of these hydrous minerals suggest that they were unrelated to each other and mica formation happened early in the upper mantle before the mantle xenoliths had been trapped. In contrast, kaersutite formation has happened later, probably during the late stage of crystallization as intracrustal processes. The presence of phlogopite and kaersutitic amphibole is a direct evidence for K-, Ti-, F- and $H_2O$-bearing fluid/melt percolation in the lithosphere beneath Jeju Island, indicating that they are product of interaction between host rock/peridotite/fluid-melt. Thus, the upper mantle/lower crust beneath Jeju Island are metasomatized to various extents, characterized by a change in major metasomatic hydrous minerals from phlogopite to amphibole with decreasing depth.

• The Journal of the Petrological Society of Korea
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• v.17 no.3
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• pp.144-153
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• 2008

Sr, Nd, Pb isotopic compositions of the Cenozoic basaltic rocks distributed in Pyeongtaek-Asan area display significantly enriched values compared with mid-ocean ridge basalts just like other Cenozoic basalts of Korea. The isotopic compositions of most of the Cenozoic basaltic rocks of Korea including those from Pyeongtaek-Asan area can be explained as mixing between enriched mantle component with relatively low $^{206}Pb/^{204}Pb$ ratios and depleted mantle component. In contrast, Jejudo basalts can be explained as mixing between enriched mantle component with realtively higher $^{206}Pb/^{204}Pb$ ratios and depleted mantle componsnt. Combined with that very similar division of enriched mantle components is applied to the Cenozoic basalts of northeast China and southeast China, it is suggested that subcontinental lithospheric mantle of central and southern parts of Korea represents eastern extension of North China Block and South China Block respectively. The indentation model for the late Paleozoic to early Mesozoic continental collision of China contradicts to such an interpretation, because it cannot explain occurrence of subcontinental lithospheric mantle component of South China Block-affinity under the Jejudo area. Instead, it is more probable that suture zone of the two continental blocks crosses between central and southern Korea and its location is further south from the Pyeongtaek-Asan area. Such distinct location compared with Imjingal belt, supposedly collisional boundary suggested before, suggests that mantle boundary may not be coincide with crustal boundary for the continental collision.