• The Journal of the Petrological Society of Korea
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• v.25 no.4
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• pp.349-360
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• 2016

The Bunam Stock ($29.5km^2$ area) is an outcrop of plutonic complex classified four facies: coarse-grained granite, quartz monzodiorite, granodiorite and fine-grained granite. Three facies except the last one exhibit very irregular boundaries with gradational compositional variations between both facies and show concentric zoning from the central quartz monzodiorite through granodiorite to outer coarse-grained granite. Mafic microgranular enclaves (MME) commonly occur in granodiorite. Some MMEs, have very fine-grained chilled margins and indentedly crenulate contacts, and display horizontally circular and vertically elongate shapes. Their shape and granularity indicate coeval flow and mingling of partly crystalline felsic and mafic magmas. MMEs exhibit dark fine-grained margins giving them a ellipsoidal form that has been attributed to undercooling of a mafic magma as blobs intruded into a felsic magma. The observed relations in the Bunam Stock identify that two endmembers are coarse-grained granite from a felsic magma and quartz monzodiorite from a mafic magma, and hybrid is granodiorite including MMEs. So they exhibit concentric zoning that lays the center on the mafic endmember due to magma mixing at the contacts of two magmas, when mafic magma injected into felsic magma. Thus the quartz monzodiorite may probably represent an ancient conduit of mafic magma transport through a granitic magma chamber. Mafic magma would rise through the conduit in which favorable conditions for magma mixing occurred. All these features suggest that they formed from mixing processes of calc-alkaline magma in the Bunam Stock.

• The Journal of the Petrological Society of Korea
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• v.26 no.3
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• pp.235-254
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• 2017

We investigate the geological history that formed geology and landscapes of the Juwangsan National Park and its surrounding areas. The Juwangsan area is composed of Precambrian gneisses, Paleozoic metasedimentary rocks, Permian to Triassic plutonic rocks, Early Mesozoic sedimentary rocks, Late Mesozoic plutonic and volcanic rocks, Cenozoic Tertiary rhyolites and Quaternary taluses. The Precambrian gneisses and Paleozoic metasedimentary rocks of the Ryeongnam massif occurs as xenolithes and roof-pendents in the Permian to Triassic Yeongdeok and Cheongsong plutonic rocks, which were formed as the Songrim orogeny by magmatic intrusions occurring in a subduction environment under the northeastern and western parts of the area before a continental collision between Sino-Korean and South China lands. The Cheongsong plutonic rocks were intruded by the Late Triassic granodiorite, which include to be metamorphosed as an orthogneiss. The granodiorite includes geosites of orbicular structure and mineral spring. During the Cretaceous, the Gyeongsang Basin and Gyeongsang arc were formed by a subduction of the Izanagi plate below East Asia continent in the southeastern Korean Peninsula. The Gyeongsang Basin was developed to separate into Yeongyang and Cheongsong subbasins, in which deposited Dongwach/Hupyeongdong Formation, Gasongdong/Jeomgok Formation, and Dogyedong/Sagok Formation in turn. There was intercalated by the Daejeonsa Basalt in the upper part of Dogyedong Formation in Juwangsan entrance. During the Late Cretaceous 75~77 Ma, the Bunam granitoid stock, which consists of various lithofacies in southwestern part, was made by a plutonism that was mixing to have an injection of mafic magma into felsic magma. During the latest Cretaceous, the volcanic rocks were made by several volcanisms from ubiquitous andesitic and rhyolitic magmas, and stratigraphically consist of Ipbong Andesite derived from Dalsan, Jipum Volcanics from Jipum, Naeyeonsan Tuff from Cheongha, Juwangsan Tuff from Dalsan, Neogudong Formation and Muposan Tuff. Especially the Juwangsan Tuff includes many beautiful cliffs, cayon, caves and falls because of vertical columnar joints by cooling in the dense welding zone. During the Cenozoic Tertiary, rhyolite intrusions formed lacolith, stocks and dykes in many sites. Especially many rhyolite dykes make a radial Cheongsong dyke swarm, of which spherulitic rhyolite dykes have various floral patterns. During the Quaternary, some taluses have been developed down the cliffs of Jungtaesan lacolith and Muposan Tuff.