• Journal of the Korean earth science society
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• v.33 no.5
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• pp.434-449
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• 2012

The Mesozoic leucocratic granite in the northeastern margin of the Taebaeksan Basin was transformed to protomylonite and mylonite. Mylonitic foliations generally strike to NWWNW and dip to NE with the development of a sinistral strike-slip (top-to-the-northwest) shear sense. Grain-size reduction of feldspar in the mylonitized leucocratic granite occurred due to fracturing, myrmekite formation and neocrystallization of albitic plagioclase along the shear fractures of K-feldspar porphyroclasts. As the deformation proceeded, compositional layering consisting of feldspar-, quartz- and/or muscovite-rich layers developed in the mylonite. In the feldspar-rich layer, fine-grained albitic plagioclase and interstitial K-feldspar were deformed dominantly by granular flow. On the other hand, quartz-rich layers containing core-mantle and quartz ribbons structures were deformed by dislocation creep. Based on calculations from conventional two-feldspar and ternary feldspar geothermometers, mylonitization temperatures of the leucocratic granite range from 360 to $450^{\circ}C$. It thus indicates that the mylonitization has occurred under greenschist-facies conditions. Based on the geochemical features and previous chronological data, the leucocratic granite was emplaced during the Middle Jurassic at volcanic arc setting associated with crustal thickening. And then the mylonitization of the granite occurred during the late Middle to Late Jurassic (150-165 Ma). Therefore, the mylonitization of the Jurassic granitoids in the Taebaeksan Basin was closely related to the development of the Honam shear zone.

• The Journal of the Petrological Society of Korea
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• v.27 no.4
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• pp.223-233
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• 2018

In this study, SHRIMP zircon U-Pb dating was performed on deformed granitic rocks in the Sancheong area in the northeastern part of the Jirisan area, southwest of Yeongnam Massif. Until now, these have been known as Precambrian or age-unknown old igneous rocks, but the U-Pb concordant ages obtained from two samples are $237.8{\pm}4.0Ma$ and $230.2{\pm}3.4Ma$, respectively, showing their emplacements in Early to Middle Triassic. These results indicate that the deformed granite was emplaced at about 238~230 Ma. The study area shows the characteristics of ductile deformation with prominent development of foliation, augen structure, and lineation. It is observed that the deformed granites occur as xenoliths within the syenite, indicating that the time of deformation is earlier than the intrusion of the syenite of about 220 Ma. The emplacement and deformation periods of the deformed granite is similar to that of Permo-Triassic granite gneisses distributed in the Gimcheon and Andong areas of the Yeongnam Massif. Taken together, the eastern part of the Yeongnam Massif, extending from the central part to the southwestern part, granite intrusions occurred at about 260-230 Ma, followed by metamorphism-deformation of about 230-220 Ma.

• Economic and Environmental Geology
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• v.49 no.2
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• pp.147-153
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• 2016

SHRIMP U-Pb age determination was carried out for deformed granites in the Aneui quadrangle, SW Yeongnam Massif. Dating of zircons from a highly deformed mylonitic granite with banded structure and a relatively less deformed porphyritic to augenic granites, that were known as Precambrian gneisses, yielded the same age of ca. 195 Ma. On the basis of this result and previous age data, Early to Middle Mesozoic igneous activity around the Aneui area was interpreted as follows; Subduction-related granitic magmatism started with the intrusion of the Hamyang Granite in the middle Triassic (ca. 225-219 Ma) mainly in the west of the area and ended with syenitic intrusion at the end of Triassic period (ca, 220-210 Ma). After a relatively short period of quiescency, granitic magmatism restarted with the intrusion of magma forming deformed granites dated in this study at the Early Jurassic of ca. 195 Ma and continued to ca. 189 Ma and dioritic intrusion was associated around the late stage of granitic magmatism.

• The Journal of the Petrological Society of Korea
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• v.24 no.2
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• pp.125-139
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• 2015

Rock structural and microstructural analyses on the deformed Cheongsan granite, which is characterized by abundant feldspar megacrystals, have been carried out to understand the microstructural change during the mylonitization by ductile shear deformation. In K-feldspars, the characteristic microstructures are recognized as microkinks, microfractures, myrmekites, flame perthites, and core-and-mantle structures without the development of subgrains in outer core-zone. Microkinks are observed in both the microfractured and unmicrofractured K-feldspars and the directions of their axes are generally extended across the adjacent K-feldspar fragments bounded by microfractures. Myrmekites and flame perthites are found on the strain-localized boundaries of the microfractured K-feldspars. In plagiclases, microfractures, deformation twins and kink bands are predominant. Grain size reduction of plagioclase megacrysts also occurs by microfracturing but the core-and-mantle structures like the case of K-feldspars are uncommon in the microfractured plagioclases. The deformation twins, which overlap the igneous zoning structures, are often found in less deformed rocks. The twin lamellae in more deformed rocks generally bisect the obtuse angles of conjugate kink-band boundaries, and are microfractured or microfaulted and randomly oriented. From such characteristic microstructures, thus, it can be suggested that the micostructures during the mylonitzation of Cheongsan granite was developed as follows: production of microkinks in the K-feldspar megacrysts and of deformation twins and kink bands in the plagioclase megacrysts, and then grain-size reduction of the feldspar megacrysts through microfracturing, and then production of core-and-mantle structures (grain-size reduction of the microfractured K-feldspars through grain boundary migration), myrmekites and flame perthites in the microfractured K-feldspars.

• The Journal of the Petrological Society of Korea
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• v.7 no.1
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• pp.37-52
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• 1998

The granites in the Jeomchon area can be divided into hornblende biotite granite (Hbgr), deformed biotite granite (Dbgr), deformed pinkish biotite granite(Dpbgr), biotite granite (Btgr), and granite porphyry(Gp). These granites show metaluminous, 1-type and calc-alkaine characteristics from their whole-rock chemistry. Hbgr and Dbgr belong to ilmenite-series granitoids, but Gp to magnetite-series. Dpbgr and Btgr show the intermediate nature between ilmenite- and magnetite-series. Tectonic discriminations indicate that Hbgr and Dbgr were formed in active continental margin environment, whereas Dpbgr, Btgr, and Gp in post-orogenic and/or anorogenic rift-related environment. From the Harker diagrams major oxide contents of Hbgr and Dbgr show a continuous variation with $SiO_2$, indicating that they are genetically correlated with each other. On the other hand, any correlation of major oxides variation cannot be recognized among Dpbgr, Btgr and Gp. It seems like that Hbgr and Dbgr were derived from a same parent granitic magma, judging from their occurrence of outcrop, mineral composition as well as whole-rock chemistry. Variation trends of major oxide contents between Hbgr and Baegnok granodiorite are very similar and continuous. If the two granites were derived from a cogenetic magma, there exists a possibility that the granitic bodies had been separated by Btgr and Gp of Cretaceous age. Three stages of the granitic intrusions are understood in the Jeomchon area. After the intrusion of Hbgr and Dbgr during middle to late Paleozoic time, Dpbgr emplaced into the area next, and finally Btgr and Gp intruded during Cretaceous time. Tectonic movement accompanying shear and/or thrust deformation seems likely to have occurred bewteen the intrusions of Dpbgr and Btgr.

• Economic and Environmental Geology
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• v.8 no.4
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• pp.211-221
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• 1975

Weathering of granites has a geochemical role of great significance, because of their abundance and because of chemical instability near the surface of the earth, which is more pronounced than in most other rocks. On the other hand the granites are tectonically fragile and "react" to deformation with marked facility, giving rise to a whole gamut of deformed rocks. Therefore, the writer has studied on the weathering of granitic rocks at of Iri city and Jeonju city Jeollabukdo, Korea, The fresh and weathered rocks were used as material for the investigation. The results obtained by chemical analysis and observation are as follows. 1) The order of mobility in major elements was Ca, Na and K$H_2O$ was observed clearly and late stages of weathering processes. 3) The early stage of weathering is commenced by physical weathering and followed by chemical weathering. 4) The ratio of FeO/. $Fe_2O_3$, FeO/MgO, and $SiO_2/Al_2O_3 $ decreased uniformly from early to late stage of weathering processes. 5) It was proved that weathering potential of granite was larger than that of basaltic rocks.

• The Journal of the Petrological Society of Korea
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• v.4 no.2
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• pp.144-152
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• 1995

The Jangsan Quartzite of the Joseon Supergroup and the foliated granite (so-called granitlc gneiss of presumed Precambrian age) of the Yeongnam massif are in direct contact at Cheondong-ri area, 6 km @SE of Danyang. sllthough it has been thought traditionally that the Jangsan Quartzite overlies unconformably the f&ted granite, it is difficult to interpret the contact as an unconformity smce the basal conglomerate in- the lower part of the Jangsan Quartzite does not have any clast of the foliated granite, Rather, recent structural studies of this area indlcate that the contact is a ductile shear zone. However, the sense and age of the shear movement are still problematic. Our mesoscopic and microscopic studies of &tre Cheondong-11 semi-brittle shear zone involving foliated cataclasite and phyllonite, which is a pa& of the Ogdong fault, indlcate a top-to-the northeast shearing, i.e., dextral strike slip. We also performed Pb-Pb dating for the age-unknown foliated granite, since the age of deformed granite ccarr emtrain the maximum age of deformation. The whole rock and feldspar Pb isotape data for the foliated granite and a micaceous xenolith define an isoc chron age of $2.16{\pm}0.15$ Ga ($2{\sigma}$;MSWD=4.4) which is interpreted as the emplacement age of the granite. This early Proterozoic age agrees with those of Precambrian igneous activity In the Yeongnam massif reported previously. The obtaiPrfid gge confirms the traditional idea about the age of the foliated granite and indicates that other methd(s) should be employed to constrain the age of the shear movement.

• Economic and Environmental Geology
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• v.27 no.3
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• pp.247-261
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• 1994

Honam Shear Zone is a mylonite zone approximately parallel to the NE-SW trend located southern part of Korea peninsula. Geologic ages and petrogenesis of foliated granites in this zone are as follows: Igneous rocks of this zone are composed of granite gneiss, Paleozoic granites, Songrim granites, Jurassic granites and Cretaceous granites. Foliated granites show deformed phase of Paleozoic and Songrim granites during Daebo Orogeny. And isotopic ages obtained from foliated granites are early Permian to late Triassic period (276~200 Ma). Most of foliated granite masses are igneous complex consisting of a series of differential product of cogenetic magma. The individual rock mass of foliated granites plotted on Harker diagram shows mostly similar trend of calc-alkali series. REE diagram indicates that LREE amount of foliated granites are more enriched than HREE and negative Eu anomalies of them are weaker than those of the other granites. From these data, we suggest the rocks are generated from continental margin under syntectonic environment. Original magma type of foliated granites correspond to I-type, syn-collision type and Hercyano type. In compressive stress field between Ogcheon folded belt and Youngnam massif, foliated granites had formed due to mylonitic deformation. Those facts indicate that magma of foliated granites would had been generated by melting in lower crust or contamination in upper mantle.

• The Journal of the Petrological Society of Korea
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• v.24 no.2
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• pp.65-75
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• 2015

Around Harar in the northeastern part of the Ethiopia, the Precambrian granitic gneiss and gabbro bodies are developed with several pegmatites. The rock bodies in this area have been deformed by ductile and brittle deformations developing fold and ductile shear structure, and thrust and fault.

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