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

• Proceedings of the Mineralogical Society of Korea Conference
• /
• 2001.06a
• /
• pp.75-75
• /
• 2001

We report Sr, Nd and Pb isotope data of clinopyroxene separates from ultramafic xenoliths and their host basaltic rocks in Jeju Island, Baekryeong Island, Boeun and Ganseong, Korea. The isotopic data of the xenoliths and host basalts are distinctly different from those of Korean basement rocks. Except for two xenoliths from Ganseong, all samples in this study have isotopic ratios within the combined range of MORB-OIB data. All basaltic rocks have Nd-Sr-Pb isotope compositions different from those of xenoliths, indicating that the host basaltic magma did not derive from the lithospheric mantle where the xenoliths originated. The range of isotopic composition of xenoliths is much greater than that observed in host basalts, which reflects small-scale heterogeneity of the lithospheric mantle. The greater isotopic heterogeneity of the lithospheric mantle probably reflects its long-term stability. The spinel peridotite xenolith data of Jeju Island, Baekryeong Island and Boeun display mixing hyperbolas between DMM and EM II end members. Since Jeju basalts have EM II-like isotopic signature, the mixing relationship shown by the isotopic data of the Jeju xenoliths can be interpreted as the result of infiltration of metasomatic fluid or melt derived from basaltic magma into DMM-like lithospheric mantle. In contrast to other xenolith sites, the Ganseong xenoliths are dominantly clinopyroxene megacryst and pyroxenite. Clinopyroxene megacrysts have different isotopic ratios from their host basalt, reflecting its exotic origin. Two Ganseong xenoliths (wherlite and clinopyroxenite) have much enriched Sr and Nd isotopic ratios and Nd model ages of 2.5-2.9 Ga, and plot in an array away from the MORB-OIB field. The mantle xenoliths from Korean Peninsula have similar $\^$87/Sr/$\^$86/Sr,$\^$143/Nd/$\^$144/Nd and $\^$207/Pb/$\^$204/Pb ratios to, but higher $\^$208/Pb/$\^$204/Pb ratios than, those from eastern China, indicating that Korean xenoliths are derived from the lithospheric mantle with higher Th/U ratio compared with Chinese ones. The isotopic data of xenolith-bearing basalts of Baekryeong Island and Ganseong, along with Ulreung and Dok Islands, show a mixing trend betlveen DMM and EM I in Sr-Nd-Pb isotopic correlation diagrams, which is also observed in tile northeastern Chinese basalts. However, the Jeju volcanic rocks show an EM II signature that is observed in southeastern Chinese basalts. The isotopic variations in volcanic rocks from the northern and southern portions of the East Asia reflect a large-scale isotopic heterogeneity in their source mantle.

• The Journal of the Petrological Society of Korea
• /
• v.26 no.4
• /
• pp.361-371
• /
• 2017

We investigated geochemical characteristics of the Dodong Basaltic Rocks in the lower part of the Ulleung Island. They have lithological range of alkali basalt to trachybasalt, belonging to Na and K subseries of alkaline series. They mostly fall within the field of alkalline within-plate basalts on tectonic discrimination diagrams, and then plot in the field of oceanic island basalt (OIB). Geochemically, extension of lithospheric mantle and asthenospheric upwelling after East Sea under an Cenozoic extensional tectonic setting might be a heat source for partial melting of the enriched lithospheric mantle, which might generate the basaltic magma. But we cannot exclude that mantle plume might also be a heat source for melting of the lithospheric mantle.

• The Journal of the Petrological Society of Korea
• /
• v.15 no.2 s.44
• /
• pp.106-117
• /
• 2006

Re-Os isotopic system, based on the long-lived $\beta^-$ transition of $^{187}Os(\lambda=1.67\times10^{-11}year^{-1})$ is being widely used in cosmochemistry and geochemistry. Along with the development of elemental separation and mass-spectrometric technique, the Re-Os isotopic system, like Sm-Nd, Rb-Sr, U-Th-Pb isotopic system, is now conventionally applied as a useful tool for absolute dating and isotoptc tracers. This paper introduces brief principles of Re-Os isotopic system and presents the general methodology fur dating the formation age of the subcontinental lithospheric mantle, based on the Re-Os isotopic data of the mantle xeonliths from South Korea.

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

• The Journal of the Petrological Society of Korea
• /
• v.25 no.3
• /
• pp.261-281
• /
• 2016

Our knowledge of the lithosphere beneath the Korean Peninsula has been improved through petrologic and geochemical studies of upper mantle xenoliths hosted by Quaternary intraplate alkali basalts from Jeju Island. The xenoliths are mostly spinel lherzolites, accompanied by subordinate harzburgite and pyroxenites. The mantle xenoliths represent residual mantle material showing textural and geochemical evidence for at least a three-stage evolution, fractional partial melting, recrystallization, and metasomatism. Their composition primarily controlled by early fractional melt extraction and porphyroclastic and mylonitic fabrics formed in a shear-dominated environment, which was subsequently modified by residual slab-derived fluids (or melts). Modal metasomatic products occur as both anhydrous phase(orthopyroxene) and hydrous phase (phlogopite). Late-stage orthopyroxene is more common than phlogopite. However, chemical equilibrium is evident between the primary and secondary orthopyroxene, implying that the duration of post-metasomatic high temperatures enabled complete resetting/reequilibration of the mineral compositions. The metasomatic enrichment pre-dates the host Jeju Quaternary magmatism, and a genetic relationship with the host magmas is considered unlikely. Following enrichment in the peridotite protolith in the mantle wedge, the upper mantle beneath proto-Jeju Island was transformed from a subarc environment to an intraplate environment. The Jeju peridotites, representing old subarc fragments, were subsequently transported to the surface, incorporated into ascending Quaternary intraplate alkali basalt. The result of this study implies that long term material transfer in the transformation of geotectonic setting from a subarc to intraplate may have played a significant role in the evolution of lithospheric mantle, resulting in the enriched mantle domains, such as EM I or EM II in the lithospheric mantle beneath East Asia.

• Journal of the Korean Geographical Society
• /
• v.47 no.3
• /
• pp.328-346
• /
• 2012

This study provides quantitative constraints on Neogene uplift in the Korean peninsula using onshore paleo-shoreline records and seismic data. The eastern margin of Northeast Asia including Korea sits in the back-arc system behind the Western Pacific Subduction Zone, a complex trench triple junction of the Philippine Sea, Pacific, and Eurasian (Amurian) plates. An analysis of seismic data in the subduction zone shows that the pattern of uplift in the peninsula mirrors the extent of deep seismicity in subducting Pacific plate beneath. Combined with previous tomographic studies it is proposed that uplift is partly driven by asthenospheric upwelling caused by a sinking slab during the Neogene. In addition, the SHmax orientations of E-W and N-S trends in the peninsula are consistent with the prevailing in-situ stress fields in the eastern Eurasian continent generated by various plate boundary forces. The uplift in Korea during the Late Neogene is attributed, in part, to lithospheric failure relating to faulting movements, thus providing a link between dynamic effects of mantle upwelling at sinking slab edge and lithospheric responses driven by plate boundary forces.

• The Journal of the Petrological Society of Korea
• /
• v.20 no.1
• /
• pp.61-75
• /
• 2011

Mantle-derived xenoliths, corresponding to spinel harzburgite and lherzolite in alkali basalts from Jeju Island, are metasomatized to various extents. They contain distinctive secondary orthopyroxene, forming corona or poikilitic textures. It clearly indicate that this secondary orthopyroxene has been produced at the expense of olivine along the grain boundaries and margins, suggesting silica-enriched metasomatic melt infiltrated through grain boundaries. Based on the geotectonic characteristics of Jeju Island and textural characteristics and major elements composition of mantle xenoliths, it is suggested that the silica-enriched melt/fluid could have derived from the ancient subducted slab, possibly in the mantle wedge, implying that the high $SiO_2$ activity in the lithospheric upper mantle beneath Jeju Island at that time.

• Proceedings of the Mineralogical Society of Korea Conference
• /
• 2000.05a
• /
• pp.19-19
• /
• 2000

;Ultramafic xenoliths found in alkali basalts from Jeju-do, Korea are mostly spinel Iherzolites composed of olivine, orthopyroxene, clinopyroxene and spinel. A subordinate amount of spinel harzburgites and pyroxenites are also found. Temperatures for these xenoliths were estimated from the compositions of coexisting pyroxenes (Wood '||'&'||' Banno 1973; Wells 1977; Bertrand '||'&'||' Mercier 1985; Brey '||'&'||' Kohler 1990), the AI-solubility in orthopyroxene coexisting with olivine and spinel (Sachtleben '||'&'||' Seck 1981; Webb '||'&'||' Wood 1986), and from Fe/Mg partitioning between olivine and spinel (Ballhaus et al. 1991). Temperature estimates from the thermometers by Wells (1977) and Brey and Kohler (1990) are compatible. Average equilibrium temperatures by these two methods for spinel peridotites range from 890 to 1030$^{\circ}$C. Pressures for spinel peridotites were estimated from the geobarometer by Kohler and Brey (1991) derived from the equilibrium Ca content of olivine coexisting with clinopyroxene, and fall within the range of 12.9 to 26.3 kbar. The combination of the thermometer by Brey and Kohler (1990) and the geobarometer by Kohler and Brey (1991) yields P- T estimates for Jeju-do spinel peridotites that fall in experimentally determined spinel lherzolite field in CFMASCr system (O'Neill 1981). These P-T data sets have been used to construct the Quaternary Jeju-do geotherm, which is significantly different from the conventional conductive geotherm. The xenolith-derived geotherm has a higher T gradient at low P (13 kbar) than at high P, which may be due to perturbation of the conductive heat flow by magma underplating or overplating at the crust-mantle boundary. Temperature estimates and statistics on the xenoliths indicate that the crust/mantle boundary in Jeju-do lies at about 11 kbar (~39 km). Spinellherzolite is inferred as a main constituent rock of the uppermost lithospheric mantle beneath Jeju-do. Pyroxenites were intercalated in peridotites in similar depth and temperature as re-equilibrated veins or lens.

• Journal of The Geomorphological Association of Korea
• /
• v.19 no.3
• /
• pp.153-163
• /
• 2012

The Earth's surface deforms vertically in response to a variety of sources relating to lithospheric and sub-lithospheric processes, and distinguishing the continental mechanisms for vertical motions of the lithosphere remains a fundamental challenge in geosciences. A key prerequisite to the challenge is documentation of the temporal and spatial pattern of vertical motions in different tectonic settings. This study is aimed at elucidating the geodynamic factors that can contribute to vertical motions of the Earth's surface in intraplate continental settings including the Neogene uplift in the Korean peninsula based on numerous recent achievements in relevant fields. Ultimately, deciphering the interplay between the Earth's surface and the Earth's interior processes leads us to the notion of "the importance of geomorphic landscape" as a prism to view the dynamics of the Earth's inside.