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The wirter have prepared this preliminary report on the results of microscopic observation of the samples which he had collected during the 9 days of field survey in Ulnung Island, from Sept. 19, 1952 to Sept. 25, 1952. The writer intends to make a more detail survey on the istand on another occasion. The Ulnung Island is one of the isolated islands on the Japan Sea with two dependant islands, Chuk Do and Kwan Um Do, and, when Namyangdong the main Ⅵllege of this island is considered as a representatⅣe point, is located on 37°30'N. lat. and 130°50'E. long.. This island is consisted of alkali rocks, belonging to the Japan Sea Alkali-rock Petrographic ProⅥnce together with Paektoo Mountain, Kilchoo-Myonchon Area and Chejoo Island, Korea and Oki Island, Japan, where the calcalkali rock are distributed in general. The rocks which are found in this island are olⅣin basalt, aggromerate, hornblend trachyte, biotite trachyte aegirinaugite trachyte, aegirin augite phonolith, trachyandesitic trachyte, Yonchon trachyandesite, dykes and volcanic ejectas. The trachytes contain sanidines and the aegirin augite phonolith composed of phenocrysts of aegirin augite, aegirin, barkeⅥkite, common augite, magnetite, nepheline, sanidin, orthoclase and sanidine which covered albite, and groundmass. The volcanic rocks of this island have been extruded during the period, from early Oligocene to Pleistocene, in the order of basalt, trachytes, trachyandesite. The extrusion was accompanied by a huge explesion, and the depression of the apical part of the trachytic rock body has formed a depression caldera and also created the Naridong Basin. The cone shaped Albong which is located in the northwest of Naridong Basin, consists of trachyandesite and several kinds of volcanic ejectas. The ridge line connected in the order of peaks, 05m, 900m, 910m,983m, 967m, 813m and 482m which distributed around Naridong Basin falls under an somma and, it is appeared to form a double volcano relating with before-mentioned Allong cone.
The present study was proceeded to see a possibility of the age determination of Ogcheon group by means of 1) stratigraphic review of the relationship between Majeonri formation and Changni formation, the lower formation, 2) chronological dating of granite pebbles in pebble bearing formation, Hwanggangri formation, 3) review of structural features on particularly selected cross geologic profile lines in the Ogcheon geosynclinal zone, 4) finding of paleontological remains in the group, and 5) subdivision of Gyemyeongsan formation, by which the geological relation of the formation to the Hyangsanri dolomite can be found. The majeonri formation exposed along the southeastern flank of the Ogcheon zone contacts, through out its exposure, with the Changni formation which consisted mainly of black slate and accompanies with some calcareous beds. The field serveys in the areas, Jinsan-Jucheon, and Majeonri-Sinpyeong of S. Chungcheong Province and Annam-Imgogni and Yeonpoog-Songgyeri of N. Chungcheong Province, indicate that the Majeonri formation, an alternation of shale (or chect, or both) and calcareous beds overlies the on Changni formation in the relation of gradual lithologic change. An absolute dating on a muscovite granite pebble collected from the Hwanggangni formation revealed that its alee is 1,191m.y. (D.S. Lee, 1971) and the author have mentioned in the paper that the deposition age of Hwanggangni formation is older than 181m.y. at least, it means that the formation is older than Jurassic. For the structural review of Ogcheon zone, following four geologic profile lines were selected, loom northeast to southwest, (1) Sooanbo-Yeongpoong line, (2) Gwangi-Cheontaegsan line, (3) Doosanri-Doyulri line and (4) Yeonsan-Jucheon line. The results of (1) to (3) lines are shown in Fig. 2 to 4. A well preserved Archaeocyatha was found from the middle part of the Hyangsanri dolomite at Hyangsanri village located 5km south of Chungju city, N-Chungcheong Province. The occurrence confirms the Early (to Middle) Cambrian age of the country formation and in turn, indicates the phanerozoic age of a part, at least, of the Ogcheon Group (D.S- Lee et al. 1972). Lithologically the Gyemyeongsan formation was subdivided into four zones, (1) Amphibolite-dolomite, (2) migmatitic gneiss, (3) feldspar schist and (4) biotite schist, in ascending order. The subdivision suggested that the contact between the Gyemyeongsan formation and the Hyangsanri dolomite seems to be in low-angle clinounconformity (Fig 6). Summerizing the present and author's previous works on Ogcheon gene, the author proposes a geologic correlation of Ogcheon Group as shown in Table 14 and to be called "Eugeosynclinal deposition" to the southwestern part of the Ogcheon zone and "Miogeosynclinal deposition" to the northeastern part. They had developped in almost contemporally from Late Pre Cambrian to Early Jurassic before the invasion of Daebo Orogenic movement of Middle Jurassic.
This article describes the petrology on the six different granitic rock bodies which distributed in the middle zone and south eastern flank in the central part of the Okcheon System. The granitic rocks in the area were divided into three different groups from older to younger, as follows: older granitic rocks, younger granitic rocks I and younger granitic rock II. The granitic rocks of Chungsan, Iwon, Backnok bodies belong to older granitic rocks and they emplaced in metasediments of the south eastern Hank of the Okcheon System as well as in argilaceous sediments of Marori Formation (Permian?). However they seem to be overlain by the upper mesozoic formation; Woli-san Formation an and Hyoedong-ni Formation in unconformity. Granitic rocks of Okcheon and Boun bodies belong to Younger granitic rocks I and they intruded the middle zone of the Okcheon System and the Jangnae-ri Formation of the upper mesozoic era Lithological correlation indicates that the above formations may be contemporaneous. The Sokni-san granite belongs to younger granitic rock II and it also emplaced in the same zone and mesozoic formation. The geological ages of younger granitic racks I and younger granitic rocks II were inferred to middle Jurassic and upper Cretaceous respectively by absolute age dating. The lithogical properties of the granitic rocks are a3 follows:. The Chungsan rock is a porphyritic biotite granodiorite accompanied with migmatitic margnal zone, in the contact band with Maro-ri formation. Generally microcline crystals dominated as phenocryst and they align in a direction parallel to the contact zone. The marginal zone near the Marori formation is migmatitic and preserves structural relicts of sedimeetary origin. The geological feature of the zone suggests that the emplacement of the Chungsan granite body is syntectonic with crustal movement which took place in the area. Structurally the Iwon granitic body is located on the trend of south western extention of Chungsan granitic mass. The migmatitic structure and schistosity of Chungsan marginal zone contineous to the Iwon-body. However the structural changes of the body took place often thourghout the body, partly it show mylonitic, porphyroblastic, granular granitoidal structure or anatectic feature. Backnok granitic mass is distributed in the area of north eastern extention of Chungsan mass and it represents relatively massive structure. Backnok mass is a medium grained gray hornblende gronodiorite. Boun granitic mass is a stock of gray medium biotite granodiorite which has nomnal massive granitoid texture, and containes coarse grained microcline as characteristic mineral. its north west corner wedged into Okcheon System in the direction of perpendicular to general trend of the system. It is clear that the emplacement is discordant. Boun granite affected contact metasomatism to surunded rocks in the forms of epidotization, sericitization or spotted thermal alteration. Okcheon granitic mass is also a stuck which is quite similler characteristics to the Boun granitic mass. Under the microscope it shows some endometasomatic alterations of silicification albitizaton and myrmekitization in the contact of microcline (or orthoclase) and plagioclase. It graduates into porphyritic granodiorite along the southern margin. Sokni-san granitic mass is a batholith extended mort-easthward to Danyang area. In this study the southwestern part of the mass was surveyed. The mineral composition of the mass indicates that it is an alkali granite to adamellite. Texturally the mass classified into three rock types: miarolitic alkali granite, adamellitic porphyry, and porphy ritic adantellite. Commonly these rocks predominate in microperthite, micrographic symplectite and myrmekite, whereas, colored minerals such as biotite and muscovite are found scarecely. The data of modal analyses of the samples from six granitic bodies were plotted in Q-Pl-Kf and F-Q-M diagrams and the average Q-Pl-Kf compositions of each body and sub· divided rock types was plotted in the diagram of Figure 26. In terms of Q-Or-Pl index of F. Chafes, the figure indicates that most of the average compositions lie in (321) area and some others in (321) and (132). However exceptionally the average composition of the main body of Soknisan granite lies in (213) area. Conclusively, if the granitic rocks above mentioned and other igneous rocks such as hornblendite, quartz porphyrys and felsites which were not stated in this report are thought to be a co-magmatc origin existed for long geologie period, a inferred trend of magmatic differentiation was traced along the course on the arrow shown in figure 24.