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

• Economic and Environmental Geology
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• v.32 no.4
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• pp.339-351
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• 1999

Petrographic and palaeomagnetic studies have been conducted to understand the origin of dark-colored enclaves occurring in the hornblende biotite granite within the Wondong caldera. The enclaves are commonly elongted and rounded in shape. They are finer-grained and darker-colored than enclosing granite, but they have simliar mineral assemblage to that of granite. Under the microscope the enclaves show microstructures reflecting an igneius origin. The microstructures of igneous origin include elongate grain shapes of plagioclase, hornblende, apatite, poikilitic anhedral quartz and zoning of plagiclase. All above features indicate that the enclaves belong to the 'microgranitoid enclaves' of Vernon (1984). a total of 91 orirnted samples were drilled from 4 sites, and their AMS (anisotropy of magnetic susceptibility) axis directions and remance components were analyzed. The result shows thar both enclave and host granite have same AMS priniciple axis direction and same remanence magnetic direction in each site. The result of this study suggests that the anclaves were derived from the globules of magma, which were formed by pillowing of synpltonic dike of magma that is more mafic than the host granite magma.

• Journal of the Korean earth science society
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• v.38 no.7
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• pp.546-551
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• 2017

We examine the morphology and texture of microlites included in the obsidians from the Baekdusan (Mt. Baekdu) and Kyushu (SW Japan) by using an electron microscope. According to the morphological types of microlites, the microlites in the Baekdusan obsidians are classified as Arculites, Asteroidal and Crenulite, whereas those in the Kyushu obsidians as various types of Arculites, Bacillite, Belonites, Crenulite, Furculite, Lath, Margarite, and Scopulites. Specifically, the Arculites in both obsidians show very distinctive crystal relationships, though they are mainly composed of magnetite and pyroxene. The Baekdusan Arculites exhibit the poikilitic texture of pyroxene crystals enclosing multiple magnetite crystals, whereas the Kyushu ones demonstrate the granular and/or intergranular texture of interlocking between magnetite and pyroxene crystals. This distinction can be used to discriminate the provenance of the obsidian artefacts from the prehistoric sites in the Korean Peninsula.

• The Journal of the Petrological Society of Korea
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• v.28 no.4
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• pp.299-306
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• 2019

We estimated the provenance of the obsidian artifacts from Samgeori site at Yeoncheon, one of the prehistoric sites in South Korea. Pyroxene microlites are of hedenbergite to augite compositions, and intergrown and/or overgrown with Fe-oxides showing poikilitic texture. Major oxides contents for the matrix of the obsidian artifacts exhibit a narrow compositional range, especially SiO₂ contents being 73.0~75.5 wt.% of acidic rhyolitic composition. Also, rare earth element (REE) contents are relatively constant in the obsidian artifacts, and the chondrite-normalized REE patterns show a strong Eu negative anomaly. These mineralogical and geochemical features of the Samgeori obsidian artifacts were compared with those from both the Baekdusan obsidians and Japanese Kyushu obsidians which have been thought to be two major obsidian provenances around South Korea. It is suggested that the Samgeori obsidian artifacts were possibly originated from the Baekdusan obsidians.

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

• Economic and Environmental Geology
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• v.55 no.1
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• pp.77-95
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• 2022

The Gonamsan gabbroic complex in the Pocheon area, northwestern region of South Korea consists of a variety types of gabbroic rocks and associated Fe-Ti oxide deposits caused by magmatic differentiation. Post-magmatic intrusions (i.e., gabbroic pegmatite and pyroxene-apatite-zircon rocks) partly intruded into the gabbroic rocks. The gabbroic pegmatite occurs in monzodiorite and oxide gabbro of the complex, intimately and spatially associated with high-grade lenticular Fe-Ti oxide mineralization. The pegmatite can be subdivided into plagioclase-amphibole and pyroxene-olivine pegmatite, in which the contact surface is sharp. The plagioclase-amphibole pegmatite comprises plagioclase and amphibole, with lesser amount of pyroxene, ilmenite, sphene, apatite, and biotite. The pegmatite shows plagioclase-amphibole intergranular texture, in which the open space formed by large plagioclase laths (An_2-26Ab_72-98Or_0-2) are infilled by amphibole. The pyroxene-olivine pegmatite is dark gray to black in color and also contains magnetite, ilmenite, spinel, apatite, and calcite as a minor component. The pyroxene (En_35-36Fs_8-9Wo₅₅) and olivine (Fo_84-85Fa_15-16) partly show a poikilitic texture defined by smaller euhedral olivine enclosed by coarser clinopyroxene. Fe-Ti oxide minerals consist mainly of magnetite and ilmenite that are found interstitially to earlier formed silicates. Subsequently, they are encompassed by reaction rim (almost of amphibole and biotite) along the boundary with surrounding silicate minerals. Under the microscope, magnetite contains a lot of oxyexsolved ilmenite (trellis type) and spinel, and thereby is weakly enriched in magnetite-compatible elements such as Ti, Al, Mg, and V. The structure and textures at the contact zone as well as mineralogical disequilibrium between gabbroic pegmatite and the host gabbroic rocks suggest that the pegmatite may form as a result of accumulation from Fe-rich melt (or liquid) that occurred somewhere rather than in situ form from the host gabbroic rock during the magmatic differentiation. Consequently, the preliminary study suggests that further study on the post-magmatic activities can not only help us improve our understanding on magmatic fractionation but also provide critical information on Fe-Ti oxide mineralization in gabbroic rocks resulting from the magmatic differentiation.