• Journal of the Mineralogical Society of Korea
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• v.16 no.1
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• pp.19-31
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• 2003

Chemical composition, crystal structure, refractive index, specific gravity, color, and luster were studied fur pyrope-almandine series garnets. The main coloring agents determining the reddish or brownish garnets were also investigated. It was also examined if there is any relationship between mineralogical properties with respect to the various chemical compositions in the solid solution, in the hope to figure out the existing classification values of R.I. and S.G. using gem- testing facilities to distinguish pyrope from almadine. It was found that 17 out of the 24 specimens belong to pyrope and the rest almandine. R.I. of pyrope goes up to 1.77 and that of almandine is higher than the value.5.5. of pyrope reaches to 3.88 and that of almandine is greater than the value of pyrope. X-ray diffraction data revealed that pyrope-almandine garnets are isometric with space group Ia3d, and also show that the variation of cell parameters are not significant enough to parallel with the chemical compositions of the series. R.I. and S.G. increase with FeO content. Fe and Mn are most responsible to the red-purple and orange coloration of the specimens, respectively. Both zircon and rutile crystals are most common inclusions in the reddish stones.

• The Journal of the Petrological Society of Korea
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• v.8 no.1
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• pp.34-45
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• 1999

Mafic granulite xenoliths are found in precambrian porphyroblastic gneiss of the Mt. Jiri area, SW Sobaegsan massif, Korea. The xenoliths are rounded to ellipsoidal in shape, 50-100 cm in length and coarse-grained with granoblastic and foliated texture. The xenoliths consist of orthopyroxene, garnet, biotite, plagioclase, quartz, ilmenite and secondary orthoamphibole. Orthopyroxene is mostly resorbed and rimmed by coronitic orthoamphiboles. Garnets occur as porphyblasts and are zoned with higher pyrope content in cores than in rims. Geothermo-barometry results yield conditions of about $800-850^{\circ}C$, 6 kb and $500^{\circ}C$, 4 kb for early and retrograde stages of equilibration, respectively. According to available geochronological data, it is suggested that the granulite facies metamorphism occurred prior to 2.1-1.9Ga and that the area was superimposed by the high-grade (over $600-700^{\circ}C$) metamorphism between 1.9-1.7Ga, followed by cooling during uplift.

• The Journal of the Petrological Society of Korea
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• v.1 no.1
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• pp.34-41
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• 1992

The Precambrian granitic gneisses are widely distributed in the Danyang-Yecheon area, eastern part of Korea, where the Ryeongnam massif borders the Ogcheon fold belt. They are composed of migmatitic, biotite granitic, garnet-bearing and granoblastic granitic gneisses. The common joint sets of the granitic gneiss are NE and NS directions, which are probably related to the effects of Daebo orogeny and Bulgugsa disturbance, respectively. Mineral assemblages of the banded gneiss xenolith in the garnet-bearing granitic gneiss are quartz-plagioc1ase-biotite-mus-covite-orthoclase and quartz-plagioc1ase-biotite-garnet, belonging to the amphibolite facies. The granoblastic granitic gneiss is felsic, metaluminous, and granitic, and shows subalkaline trend. The garnet-biotite geothermometry of garnet-bearing granitic gneiss yields 640$^{\circ}$-708$^{\circ}C$ at pressure of 4 kb.

• Journal of the Korean earth science society
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• v.22 no.6
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• pp.528-547
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• 2001

The blastoporphyritic granite gneiss (BPGN) including much alkali-feldspar megacrysts occurs in Jiri mountains area, southwestern part of Sobaegsan massif, Korea. The BPGN is formed gneiss complexes with other gneisses in Precambrian. The BPGN was named as porphyroblastic gneiss with porphyroblasts of alkali-feldspar megacrysts by other researchers, but the BPGN includes of euhedral alkali-feldspars (microcline), and the boundary with the granitic gneiss represents sharp contact as intrusive relationship. The BPGN mainly composes of alkali-feldspar megacrysts, quartz, plagioclase, K-feldspar and biotite some almandine and accessary minerals are muscovite, chlorite, apatite, zircon and opaques. The alkali-feldspar is microcline with perthitic texture. An content of plagioclases show 30 to 40. Biotites occur two type, one is Brown biotite which shows compositional ranges of Mg/Fe+Mg ratios from 0.38 to 0.52, the other is Green Bt. which is retrograde product. Camels to be various sizes and shapes have composition of almandine with 73 to 80 mole percent, but represent retrogressive zoning from core (X$_{pyr}$: 15.9${\sim}$20.8) to rim (X$_{pyr}$:13.7${\sim}$15.9) to be evidence of retrograde metamorphism. Megacrysts of alkali-feldspar in the BPGN show rectangular shape of euhedral and some become ellipsoidal or spheroidal in shape and the average size up to 20 cm long. The megacryst includes of biotite, plagioclase and quartz, and rarely euhedral apatite as inclusions. In petrochemistry the BPGN represents granodiorite composition, characteristics of peraluminous S-type granitoid and calc-alkaline features.

• Journal of the Korean earth science society
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• v.29 no.3
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• pp.221-245
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• 2008

The precambrian granitic gneiss and porphyroblastic gneiss are widely distributed in the Gwangyang-Hadong area of Korea. This study focuses on the geochemical properties and metamorphic P-T conditions of these gneisses. These gneisses are plotted according to granodiorite domain on an IUGS silica-alkali diagram. Geochemical properties of major elements suggest that these rocks are of the sub-alkalic rock series, and were farmed from S-type magmas which were generated in a syn-collision tectonic environment. The amounts of trace elements (Zn, Sc, Sr, V, etc.) decreased as $SiO_2$ concentrations increased. Almandine and spessartine mol%'s and XFe are higher in garnet rims, while pyrope mol%'s are higher in the garnet cores. This seems to be the result of garnet growth and retrogressive metamorphism. Metamorphic zones are divided into sillimanite-cordierite, sillimanite, garnet, and biotite zones. Metamorphic P-T conditions estimated from the gneisses indicate high temperature and low to medium pressure metamorphism (689-757$^{\circ}C$, 5.0-5.6 kbar), followed by medium temperature, low pressure retrorade metamorphism (579-628$^{\circ}C$, 3.1-4.5 kbar), and overprinted retrogade metamorphism (502-558$^{\circ}C$, 1.6-2.3 kbar).

• The Journal of the Petrological Society of Korea
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• v.8 no.2
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• pp.106-124
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• 1999

In the Cheongju-Minwon area which occupies the middle part of the Ogcheon Metamorphic Belt, three metamorphic events(M1, M2, M3) had occurred. Intermediate P/T type M2 regional metamorphism formed prevailing mineral assemblages in the study area. Low PIT type M3 contact metamorphism occurred due to the intrusion of granites after M2 metamorphism. M1 metamorphism is recognized by inclusions within garnet. During M2 metamorphism, the metamorphic grade increased from the biotite zone in the southeastern part to the garnet zone in the northwestern part of the study area. This result is similar to the metamorphic evolution of the southwestern part of the Ogcheon Metamorphic Belt. Garnets in the garnet zone are classified into two types; Type A garnet has inclusions whose trail is connected to the foliation in the matrix and Type B garnet has inclusion rich core and inclusion poor rim. Type A garnet formed in the mica rich part with crenulation cleavage whereas Type B garnet formed in the quartz rich part with weak crenulation cleavage. In some outcrops, two types garnets are found together. Compared to the rim of Type A garnet, the rim of Type B garnet is lower in grossular and spessartine contents but higher in almandine and pyrope contents. In some Type B garnets, the inclusion poor part is rimmed by muddy colored or protuberant new overgrowth. In the inclusion poor part and new overgrowth, a rapid increase in grossular and decrease in spessartine is observed. However, the compositional patterns of Type A and B are similar; Ca increases and Mn decreases from core to rim. Two types garnets formed mainly due to the difference of bulk chemistry instead of metamorphic and deformational differences. The metamorphic P-T conditions estimated from Type A garnets are 595-690 OC15.7-8.8 kb, which indicates M2 metamorphism is intermediate P/T type metamorphism. On the other hand, a wide range of P-T conditions is calculated from Type B garnets. The P-T conditions from most Type B garnet rims are 617-690 OC16.2-8.9 kb which also indicates an intermediate P/T type metamorphism. However, at the rim part with flat end or weak overgrowth, grossular content is low and 573-624OC14.7-5.8 kb are estimated. The P-T conditions calculated from plagioclase and biotite inclusions in garnet are 460-500 0C/1.9-3.0 kb. The P-T conditions from rim part with weak overgrowth and inclusions within garnet, indicate that low P/T type M1 regional metamorphism might have occurred before intermediate P/T type M2 regional metamorphism. The P-T conditions estimated from samples which had undergone low PIT type M3 metamorphism strongly, are 547-610 0C/2.1-5.0 kb.