• Economic and Environmental Geology
• /
• v.15 no.4
• /
• pp.177-188
• /
• 1982

The Chungju and Seosan Groups have been known usually as Precambrian formations in Korea. But their relative and absolute ages have been controvericial problem in relation with other geologic system such as so-called Ogcheon and Yeoncheon Systems in Korea. This study has mainly focused on the corelation of the Chungju Group with the Seosan Group in their stratigraphy, structure, metamorphism, and iron ore deposits. In the process of study, the auther surveyed and reclassified the Chungju and Seosan Groups and corelated with Gyeonggi and Ogch cheon metamorphic belts and got some new data. The Chungju iron-bearing formations showing transtitional relation with the Gyeonggi Gneiss Complex and the Jangamri Formation consisting mainly of pebble bearing calcarious phyllite, should be seperated from the Gyemyeongsan formation which is mainly composed of metavolcanic rocks. The Jangamri Formation and the coaly phyllite, which can be corelated respectively with the Hwaggangri Formation and Changri Formation in Ogcheon Group, are repeated in the Gyemyeonsan and Munjuri Formations with the overturned anticlinal folding(F1). So the Chungju Group which was defined as an indipendant geologic unit from the Ogcheon Group should be limited only on the Chungju iron Formation. The Seosan Group can be classified stratigraphically such as Seosan Formation consisting of iron-bearing quartzite and mica schist, Daesan Formation overlying unconformably on the Seosan Formation and Gyeonggi Gneiss Complex. Taean Formation overlying unconformably on the Daesan Formation should be seperated from Seosan Group. There are many similarity in the stratigrphy, structure, and metamorphic facies between Chungju and Seosan Groups exept the metavolcanic rocks in the Gyemyeongsan and Munjuri Formations and the pebble bearing calcareous phyllite in the Jangamri Formation. The two Groups were deformed with two kinds of differant stages, the first shows $N30^{\circ}-40^{\circ}E$ trend of fold axis, the second $N70^{\circ}-80^{\circ}W$ respectively. The Seosan Formation, which is the lowest formation in Seosan Group and bearing the iron formation, was metamorphosed at 2500 m. y. before. These age is similar with the metamorphic age of Gyeonggi metamorphic belt and with the age of Algoman and Kenoran Orogenies which devide the Precambrian into Archean and Proterozoic Era. So the Seosan Formation, which is included in some migmatitic rocks of Gyeonggi Gneiss Complex, is the oldest formation in Korea and can be corelated with the Anshan Group which bears the oldest iron formation in China. The metamorphic facies of the Precambrian metamorphism in Seosan area is simillar with that of Chungju area, showing high temperature-low pressure amphibolite facies which is corelated with the Gyeonggi metamorphic belt, the oldest metamorphic belt in Korea ($650^{\circ}-680^{\circ}C$, 3.2-4.4 Kb). The high temperature intermediate pressure amphibolite facies in Seosan area with the low temperature-intermediate presure greenschist facies of Taean formation is corelated with that of Ogcheon Group ($590^{\circ}-640^{\circ}$ C, 5.2-6.3 Kb). The Chungju and Seosan iron formations were deposited in Archean, showing geochemical composition of Precambrian iron formations. The Chungju iron formation was mainly formed by the chemical precipitation, on the other hand, the Seosan iron formation was formed by alternated action of chemical and detrital depositions.

• The Journal of Engineering Geology
• /
• v.13 no.4
• /
• pp.429-444
• /
• 2003

Domestic marbles are mostly distributed in Gyeonggi and Yeongnam Massifs, southwest and northeast Ogcheon Belts, which belong to Precambrian, age-unknown, Cambrian-Ordovician ages, respectively. The former marbles occur as interbedded rocks in metasediments and xenoliths in granitic gneisses. Age-unknown ones occur as interbedded in the formations of Hyangsanri, Gyeomyeongsan, Hwajeonri and Munjuri, and some in metasedimentary rocks. The latter ones occur as interbedded in Pungchon Limestone, and in Jeongseon Limestone, Hwacheonri Formation and Great Limestone Group, respectively. Among physical properties, porosity shows irregular patterns to density and compressive strength, respectively. Absorption ratio has a linear pattern of positive trend to porosity, and compressive strength mostly shows a positive trend to tensile strength. Compressive strengths of the marbles are as follows : Precambrian $1,106{\;}kg/\textrm{cm}^2$, age-unknown $935{\;}kg/\textrm{cm}^2$. Cambrian $1,162{\;}kg/\textrm{cm}^2$ and Ordovician $1,560{\;}kg/\textrm{cm}^2$, respectively. Tensile strengths have decreasing trends as the above order of geologic age. In diagrams of major elements, $Al_2O_3,{\;}Fe_2O_{3(t)}{\;}and{\;}Na_2O+K_2O$ generally show positive trends with increasing $v_2$. MgO/CaO of Precambrian and age-unknown marbles have much higher values than Cambrian and Ordovician marbles as follows, Precambrian 0.31, age-unknown 0.30, Cambrian 0.03 and Ordovician 0.08. And MgO shows a negative trend with increasing CaO, which nay be caused by dolomitization. By MgO contents they can be classified into calcitic dolomite, dolomitic limestone, limestone and dolomitic limestone, respectively.

• Proceedings of the KSEEG Conference
• /
• 2005.04a
• /
• pp.314-316
• /
• 2005

• Economic and Environmental Geology
• /
• v.3 no.1
• /
• pp.9-15
• /
• 1970

This paper describes the definition of the Ogcheon Group and its characters, and indicates that it is neither the Precambrian nor metamorphosed Cambro-Ordovician sediments, and its geological age might fall in Post-Ordovician on the bases of lithostratigraphy, correlation and geological structures.

• Economic and Environmental Geology
• /
• v.3 no.3
• /
• pp.187-191
• /
• 1970

There is a discrepancy in opinion regarding geological age of Okchon system among professor C.M. Son and the writer who represent the two school of thought in precambrian stratigraphy in Korea as a whole. This brief article is a reply to the recent paper by C.M. Son titled "On the geological age of the Ogcheon Group" The discrepancy in opinion on the age Okchon system is based mainly on the difference in opinion about the age of Majeonri, Hwachonri and Kounri formations, the age of which professor Son believes as post-ordovician and regards them as a part of the Great Limestone series and the base of the Ogchon Group. The writer is in a opinion that Okchon system belong to precambrian in age and Majeonri-, Hwachonri-, and Kounri formations are the same formation composing an upper member of Okchon system. The writer's opinion is based on the facts that i) stratigraphical sequence of Okchon system established by the writer was accepted by C.M. Son who used believed the reverse order in sequence and confessed his mistake in his article; and ii) regional stratigraphy and structure strongly support's to writer's opinion. The writer pointed out and discussed in this paper various facts which do not support Son's idea by any means.

• Economic and Environmental Geology
• /
• v.26 no.4
• /
• pp.499-505
• /
• 1993

The Imgye hornblendites occur as intrusive sills or dykes within the mylonite zone developed along the contact boundary between Precambrian Jungbongsan granite and Cambrian Jangsan quartzite or Myobong slate formations. The hornlendites belong to the subalkaline and tholeiitic series. In tectonomagmatic discrimination diagrams such as $TiO_2-K_2O-P_2O_5$, 2Nb-Zr/4-Y and $TiO_2-10MnO-10P_2O_5$, the hornblendites are classified into continental- and island-arc tholeiites. The hornblendites show fractionated REE patterns with $(La/Yb)_{CN}$ ranging from 3.73-4.56. In incompatible element abundance variations, the hornblendites show distinctive positive and negative anomalies for Rb and Nb, respectively, and unfractionated patterns of immobile incompatible elements such as Y and Yb. The REE patterns of the hornblendites are also similar to those of typical continental back-arc tholeiites and those of the Precambrian Okbang amphibolites in the Socheon-meon, Bonghwa-gun. Accoiding to geochemical characteristics above-mentioned, the hornblendites seem to have been formed from tholeiitic magmas of depleted upper mantle source, contaminated by crustal material en route to continental back-arc basin.

• The Journal of the Petrological Society of Korea
• /
• v.19 no.2
• /
• pp.123-139
• /
• 2010

The properties of fracture system in Precambrian Jangbong schist and Mesozoic granites from Seokmo-do, Ganghwa-gun were investigated and analyzed. Most of the fractures measured at outcrops are nearly vertical or steeply dipping. Orientations of fracture sets in terms of frequency order are as follows: Set $1:N2^{\circ}E/77^{\circ}SE$, Set $2:N17^{\circ}E/84^{\circ}NW$, Set $3:N26^{\circ}E/64^{\circ}SE$, Set $4:N86^{\circ}W/82^{\circ}SW$, Set $5:N80^{\circ}W/77^{\circ}NE$, Set $6:N60^{\circ}W/85^{\circ}SW$, Set $7:N73^{\circ}E/87^{\circ}NW$, Set $8:N82^{\circ}W/53^{\circ}NE$, Set $9:N23^{\circ}W/86^{\circ}SW$, Set 10: $N39^{\circ}W/61^{\circ}NE$. Especially, the rose diagram of fracture strikes(N:240) indicates that there are two dorminant directions of N-S~NNE and WNW. These distribution pattern of fractures from Seokmo-do correponds with those of major lineaments from South Korea suggested in previous study. Meanwhile, the scaling properties on the length distribution of fracture populations have been investigated. First, fracture sets from Precambrian Jangbong schist and Mesozoic granites(north and south rock body) has been classified into five groups(group I~V) based on strike and frequency. Then, the distribution chart generalized the individual length-cumulative frequency diagram for above five groups were made. From the related chart, five subpopulations(group I~V) that closely follow a power-law length distribution show a wide range in exponents(-0.79~-1.53). These relative differences in exponent among five groups emphasizes the importance of orientation effect. From the related chart, the diagram of group III occupies an upper region among five groups. Finally, the distribution chart showing the chracteristics of the length frequency distribution for each rock body were made. From the related chart, the diagram of each rock body shows an order of porphyritic biotite granite < hornblende granodiorite < medium-grained biotite granite(south rock body) < medium-grained biotite granite(north rock body) < Precambrian Jangbong schist. From the related chart, the diagram of more older rock body in the formation age tends to occupy an upper region. Especially, the diagram of Precambrian Jangbong schist occupies an upper region compared with the diagrams of Mesozoic granites. These distributional chracteristics suggests that coexistence of new fracture initiation and growing of existing fractures corresponding with stress field acted since the formation of rock body.

• Economic and Environmental Geology
• /
• v.13 no.2
• /
• pp.91-103
• /
• 1980

In spite of the fact that the Okcheon system has been rather intensively studied by many geologists since 1945, it still remains as a controversal problem as to its stratigraphy and geologic age. Present study has mainly focused on the upper members of the Okcheon system, namely the Hwanggangri and the Kunjasan formations so as to clarify the lithology, the depositional environment and the tectonic evolution of the formations. The Kunjasan formation lying unconformably over the Hwanggangri formation which is supposed to be a meta-tillite is interpreted as a metamorphosed calcareous argillaceous and/or arenaceous sediments on contract to the idea postulated by some geologists that it was a derivative of silicified Hwanggangri formation. Lithology of the Kunjasan and the Hwanggangri formation is quite different in that the former is white in color, contains few pebbles, and mostly composed of diopside and detrital quartz, whereas the latter is black to dark in color, contains abundant and variable kinds of pebbles, and composed of more argillacous matrix that has been metamorphosed to hornfels. The Hwanggangri and the Kunjasan formations were deposited in the rather deep sea which has transgressed toward northeast from southwest in the late Precambrian time, and the writer (1970) had formerly designated it as Okcheon Paleogeosyncline. With the beginning of Paleozoic era, Okcheon neogeogyncline was formed to the northeast of the old paleogeogyncline area. The transgression of the sea had proceeded toward southwest in which Cambrian strata were accumulated. During this period the area occupied formerly by the paleogeosyncline was uplifted, so that most of the Hwanggangri and the Kunjasan formations were eroded away except in the area close to the neogeosyncline sea coast. This is the reason why the Hwanggangri and the Kunjasan formations are cropped out presently in the area of the vicinity of contact zone of the paleo- and neogeosyncline zones. The age of the Okcheon system has been reconfirmed to be Precambrian from the view of the facts that 1) the Hanggangri formation, the upper member of the Okcheon system is meta-tillite and correlated to the Precambrian tillite in the Yantze basin in China, 2) the Okcheon system has been moderately metamorphosed while other formations of the same age, if it is Paleozoic or later, have not been metamorphosed, and 3) tectonic history and limited areal distribution of the Hwanggangri and the Kunjasan formations is suggestive of Precambian age.

• The Journal of the Petrological Society of Korea
• /
• v.23 no.2
• /
• pp.61-73
• /
• 2014

The dextral strike-slip Yecheon Shear Zone of (E)NE-(W)SW trend is developed in the Pukhumyeon-Pyeongeunmyeon area, Gyeongsangbukdo, in the central part of Sobaeksan Massif, Korea which mainly consists of Precambrian metasedimentary rocks and Mesozoic granite. This paper researched the relative formation time, the deformation temperature, and the distribution of ultramylonite zone of Yecheon Shear Zone from the analysis of their deformed rock structures and microstructures. There are the mylonite foliation and stretching lineation by shear deformation, and the (E)SE-vergence gentle plunging, moderately inclined tight fold defined by the gneissosity and mylonite foliation as ductilely deformed main rock structures. Flame perthites, myrmekites, microfractures, microkinks, kinkband, and undulatory extinction are recognized as characteristic microstructures in feldspars during ductile shear deformation. Patterns of c-axis fabrics of recrystallized quartz aggregate display single girdle and type I crossed girdle where the rhomb and basal slip systems are predominant. The shearing senses on sense-of-shear plane show top-to-the-NE and top-to-the-SW on the predominant NW-dipping and subordinate SE-dipping mylonite foliations, respectively. These characteristic rock structures and microstructures suggest that the Yecheon ductile shearing occurred under $350{\sim}450^{\circ}C$ deformation temperature after the tight folding. The (E)NE trending ultramylonite zone of Yecheon Shear Zone, which passes through the Pulnobong and the north part of Pakdalsan, is established from the zonal distribution of Precambrian metasedimentary rocks and the grain-size distribution of feldspars in the Precambrian metasedimentary rocks and quartzs in the Mesozoic granite.

• The Journal of the Petrological Society of Korea
• /
• v.14 no.3 s.41
• /
• pp.141-148
• /
• 2005

Lead isotopic compositions are analyzed from the sulfide minerals of the Yeonhwa, Janggun and Uljin deposits and from host limestone, intrusives, and basement rocks to reveal the source of Pb in these deposits. In the $^{206}Pb/^{204}Pb$ vs $^{207}Pb/^{204}Pb$ plot, Galenas from the Yeonhwa mine display relatively well defined positive linear array, similar to the Precambrian basement rocks of the Korean peninsula. A galena sample from the Uljin mine, Janggun limestone and the basement rocks also follow the variation of Yeonhwa mine. However, ore minerals from the Janggun mine, having relatively low $^{206}Pb/^{204}Pb$ values, reveal offset from such trend toward lower $^{207}Pb/^{204}Pb$ values. Considering the fact that Mesozoic igneous rocks and ores within the Gyeongsang basin display considerably lower $^{207}Pb/^{204}Pb$ values than basement rocks of the Korean peninsula, the deviation of Janggun ore minerals can be interpreted as to reflect mixing between leads from old continental crustal materials and from Mesozoic igneous rocks with more mantle signature. The lead of the Yeonhwa and Uljin mine, following trend of Precambrian basement rather well, seems to have been originated mostly from such basement. However, regarding that they occupy low $^{207}Pb/^{204}Pb$ side of the variation trend of the basement, the possibility of having some leads derived from the Mesozoic igneous rocks cannot be excluded.