• The Journal of the Petrological Society of Korea
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• v.1 no.2
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• pp.104-123
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• 1992

We present petrography, mineral chemistry of amphibole and plagioclase, and major and trace element chemistry for the Ogcheon metabasites occurring in the Poun and Mungyong areas to understand metamorphism, and to define chemical characteristics of parental rocks and their implication for tectonic environment. The Ogcheon metabasites often preserve relict igneous textures, although no primary phases are observed. They are mainly composed of amphibole (actinolite+hornblende)+plagioclase+epidote+chlorite+sphene+opaque oxides, indicating epidote amphibolite facies metamorphism. Coarse-grained amphiboles frequently have actinolitic composition in the core, and hornblende along the margin and cleavage, which can be interpreted either as miscibility gap or as result of polymetamorphism. Although presumed polymetamorphic events in the Ogcheon supergroup favor the latter possibility, further metamorphic studies are necessary to solve the problem. Amphibole and plagioclase chemistries suggest greenschist (epidote-amphibolite, if miscibility gap is present) to amphibolite facies metamorphism of possibly medium pressure. The major and trace element data of whole rocks indicate that the Ogcheon metabasites are transitional to tholeiitic basalts belonging to within-plate environment. Absence of evidences indicating deep sea environment suggests that the Ogcheon metabasites emplaced in an intra-cratonic, possibly rift environment which failed to proceed to an oceanic rift. Chemical variation of the metabasites toward a granitic pluton indicates K loss closer to the pluton, suggesting that caution should be taken when K is involved in a discussion.

• Proceedings of the Mineralogical Society of Korea Conference
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• 2001.06a
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• pp.143-144
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• 2001

경기육괴의 남서부에 해당하며 옥천구조대의 북쪽에 위치한 백동-홍성 일대의 충청남도 일원에는 선캠브리아기의 편마암복합체내에 초염기성암체들이 소규모 렌즈상의 암체로 이 지역에 우세한 구조선 방향인 북북동 방향으로 배열되어 산출된다. 이들 초염기성암채들이 조산대에 형성되는 알파인형이며 전형적인 상부 맨틀기원의 암석이 부분용융된 맨틀 잔여물로서 지표로의 이동과정에서 변성작용을 받아 대부분은 사문석화 혹은 활석화된 것으로 보고되어 있다. 본 연구의 주대상인 변성염기성암을 포함하는 백동지역의 초염기성암은 홍성도폭내 덕정리 편암암체 내에 주변 편마암체와 단층으로 접하면서 나타난다. 덕정리 편마암체는 주로 각섬석 화강편마암으로 구성되어있다. 백동지역의 초염기성암내에는 변성염기성암이 층상으로 협재되어 나타난다. 이들 변성염기성암은 석류석, 단사휘석, 각섬석, 사장석으로 주로 이루어진 석류석 각섬암이며 금홍석도 포함하고 있다. 많은 석류석 주변에는 사장석과 각섬석으로 이루어진 심플렉타이트가 발달되어있다. 석류석 각섬암내 심플렉타이트가 형성되지 않은 석류석 가장자리로부터 계산된 변성 온도-압력 조건은 590-72$0^{\circ}C$, 9-l0kb이며 심플렉타이트가 잘 형성된 석류석 주변부로부터는 490-$600^{\circ}C$, 4-6.3kb의 변성 온도-압력 조건이 계산되었다. 이에 반해 변성염기성암을 포함한 초염기성암체 주변에 나타나는 석류석-각섬석 화강편마암으로 부터는 570-78$0^{\circ}C$, 11.6-l6kb의 변성 온도-압력 조건을 얻었다. 석류석 각섬암에서 석류석내에 포획물로 나타나는 휘석(Xjd, 4-11)이 기질에 나타나는 휘석(Xid, 1-6)보다 높은 제이다이트 성분을 갖는다. 이는 석류석 중심부가 석류석 주변부보다 변성압력 조건이 높았을 가능성을 지시한다. 즉 연구지역의 석류석 각섬암은 주변의 각섬석 화강편마암과 같이 에콜로자이트상-상부각섬암상-고압백립암상의 점이대에 해당하는 변성작용을 받은후 초염기성암의 사문암화 작용시기에 1차 후퇴변성작용을 그리고 그후의 활석화 시기에 심플렉타이트를 형성하는 2차 후퇴변성작용을 받았다. 이러한 후퇴변성작용시기에 많은 유체가 침입하였고 그 결과 초염기성암내의 변성염기성암이 주변 화강편마암보다 후퇴변성작용을 강하게 받았을 것으로 예상된다. 변성염기성암의 변성연대를 구하기 위해 두 석류석 각석암 시료에 대해 석류석-전암 Sm-Nd 등시선 연령을 구하였다. 두 개의 석류석 각섬암으로부터 계산된 전암-석류석 연령은 297.9$\pm$5.7(2$\sigma$)Ma 그리고 268.7$\pm$3.3(2$\sigma$)Ma를 나타낸다. 석류석의 Sm-Nd 폐쇄온도가 700-75$0^{\circ}C$임을 고려할 때 두 석류석 각섬암으로부터 계산된 연대는 최고 변성작용 혹은 초기 냉각 시기를 지시할 가능성이 높다. 한편 전암의 두 동위원소 자료로부터 계산된 전암의 연령은 408Ma이다. 이러한 연구 결과는 연구지역의 석류석 각섬암이 시루리아기 후기 혹은 데본기 초기에 관입 후 석탄기 후기에서 페름기 중기에 주변 화강편마암과 함께 섭입작용을 받았을 가능성을 지시한다.

• The Journal of the Petrological Society of Korea
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• v.6 no.2
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• pp.133-149
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• 1997

In the metapelites around the Ogcheon granite, the metamorphic grade increases from the biotite zone through the andalusite zone to the sillimanite zone towards the intrusion contact. In the metabasites around the Boeun granite, the metamorphic grade increases from transitional zone between the greenchist and amphibolite facies through the amphibolite facies to the upper amphibolite facies towards the intrusion contact. In the Doiri area locating near the intrusion contact of the Boeun granite, sillimanite- and andalusite-bearing metapelites are found with in 500 m away from the contact. The evidence described above indicates that the Ogcheon and Boeun granites caused low-P/T type contact metamorphism to the country rocks. The P-T condition of contact metamorphism due to the intrusion of the Ogcheon granite is $540{\pm}40^{circ}C, 2.8{\pm}0.9$ kb. The temperature condition of contact metamorphism due to the intrusion of the Boeun granite is $698{\pm}28^{\circ}C$. The wide compositional range of amphibole and plagioclase in the metabasites around the Boeun granite is due to the immisibility gab of amphibole and plagioclase and unstable relict composition resulted from an incomplete metamorphic reaction. The compositional range of stable amphibole and plagioclase decreases as a metamorphic grade increases due to a close of immiscibility gab. The thermal effect of contact metamorphism due to the intrusion of the Ogcheon and Boeun granites, are calculated using the CONTACT2 program based on a two dimensional finite difference method. In order to estimate the thermal effect of an introduced pluton, a circle with 10 km diameter and a triangle with 20 km side are used for the intrusion geometries of the Ogcheon granite and the Boeun granite, respectively. The results from the field and modeling studies suggest that the intrusion temperatures of the Ogcheon granite close to $800^{\circ}C$ and the intrusion temperature of the Boeun granite is higher than $1000^{\circ}C$. However, the intrusion temperatures can be lower than the suggested temperature, if the geothermal gradient prior to the intrusion of the Ogcheon and Boeun granites was higher than the normal continental grothermal gradient.

• The Journal of the Petrological Society of Korea
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• v.11 no.3_4
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• pp.234-249
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• 2002

The Miwon-Boeun area in the central and northern part of Okcheon metamorphic zone, Korea, is composed of Okcheon Supergroup and Mesozoic Cheongju and Boeun granitoids which intruded it. The Okcheon Supergroup consists mainly of quartzite (Midongsan Formation), meta-calcareous rocks (Daehyangsan Formation, Hwajeonri Formation), meta-psammitic rocks (Unkyori Formation), meta-politic rocks (Munjuri Formation), meta-conglomeratic rocks (Hwanggangni Formation) in the study area, showing a zonal distribution of NE trend. Its' general trend is locally changed into NS to EW trend in and around high-angle fault of NS or NW trend. This study focused on deformation history of the Okcheon Supergroup, suggesting that the geological structure was formed at least by four phases of deformation. (1) The first phase of deformation occurred under ductile shear deformation of top-to-the southeast movement, forming sheath fold or A-type fold, asymmetric isoclinal fold, NW-SE trending stretching lineation. (2) The second phase of deformation took place under compression of NW-SE direction, forming subhorizontal, tight upright fold of M trend in the earlier phase, and formed semi-brittle thrust fault (Guryongsan Thrust Fault) of top-to-the southeast movement and associated snake-head fold in the later phase. (3) The third phase of deformation formed subhorizontal, open recumbent fold through gravitational or extensional collapses which might be generated from crustal thickening and gravitational instability. (4) The fourth phase of deformation formed moderately plunging, steeply inclined kink fold related to high-angle faulting, being closely connected with the local change of NE-trending regional foliation into NS to EW direction of strike in the vicinity of the high-angle fault.

• The Journal of the Petrological Society of Korea
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• v.2 no.2
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• pp.95-103
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• 1993

The geology of the talc ore deposits in the Chungju area consists of the Kyemyeongsan Formation, the Munjuri Formation, the Daehyangsan Quartzite, the Hyangsanni Dolomite, and the basic rocks of the Ogcheon belt. The talc ore occurs in the Hyangsanni Dolomite near the Daehyangsan Quartzite The mineral assemblages in the Hyangsanni Dolomite are \circled1calcite-tremolite-talc-quartz, \circled2calcite-talc-quartz, \circled3tremolite-calcite-dolomite, and \circled4calcite-dolomite-phlogopite-chlorite. Talc has almost the ideal composition($X_{Mg}$=Mg/(Fe+Mg)=0.98). Talc was formed in siliceous dolomite by the medium-pressure type regional metamorphism. The evidences for contact metamorphism and/or hydrothermal reaction are not clear. The metamorphic grade of the Hyangsanni Dolomite and its adjacent pelitic or basic rocks near the deposits corresponds to epidote-amphibolite facies or greenschist facies based on the, mineral assemblages of \circled1hornblendebiotite-muscovite-epidote-quartz \circled2biotite-chlorite-quartz, and \circled3hornblende-actinolite-plagioclasequartz. The formation of the talc deposits were caused by the following reactions due to greenschist facies metamorphism of siliceous-dolomitic rocks in the Hyansanni Dolomite. (I) 3 dolomite+4 quartz+$H_2O$= talc+ 3 calcite +3 $CO_2$; (11) 3 tremolite+ 2 $H_2O$+ 6 $CO_2$= 5 talc+ 6 calcite + 4 quartz. The minimum temperature of the talc-tremolite-quartz assemblage is about $434^{\circ}C$ from calcite thermometry and the carbon dioxide mole fraction in metamorphic fulid($X_{$CO_2$}$) is about 0.1 at assumed pressure, 3 kbar.