The metamorphic evolution from ultrahigh-temperature to amphibolite facies metamorphism in the Odaesan area after the collision between the North and South China Cratons in the Korean Peninsula

Lee, B.C.,Oh, C.W.,Kim, T.S.,Yi, K. Universitetsforlaget ; Elsevier Science Ltd 2016 Lithos Vol.256 No.-

<P>The Odaesan Gneiss Complex (Odesan Gneiss Complex) is the eastern end of the Hongseong-Odaesan collision belt in the Korean Peninsula, which is an extension of the Dabie-Sulu collision belt between the North and South China cratons. The Odaesan Gneiss Complex mainly consists of banded and migmatitic gneisses with porphyritic granitoids and amphibolites. The garnet-bearing banded gneisses can be subdivided into garnet-biotite and garnet-orthopyroxene banded gneisses. At the beginning of the post-collision stage, the banded gneisses underwent regional ultrahigh-temperature metamorphism (902-950 degrees C/8.8-9.4 kbar) at ca. 247-245 Ma due to the heat supplied from underplated basic magma, which was generated by the partial melting of the litho spheric mantle caused by the heat supplied from the asthenospheric mantle. As a result of the continuous extensional force, the study area (lower crust) uplifted onto the middle crust depths, and then the study area underwent prograde granulite facies metamorphism from 660 degrees C and 8.7 kbar to 750-760 degrees C and 6.3-6.5 kbar at ca. 227 Ma, causing migmatization, which erased the ultrahigh-temperature metamorphism in most of the study area. The ultrahigh-temperature metamorphism was preserved only in the garnet-orthopyroxene banded gneisses due to their very low water contents. During migmatization, the garnet-biotite banded gneisses were retrograded into upper granulite facies due to the relatively abundant water compared with the garnetorthopyroxene gneisses. Finally, the study area was uplifted to a shallow depth and locally underwent amphibolite fades retrograde metamorphism (575-680 degrees C and 3.1-4.5 kbar). In addition, Paleoproterozoic metamorphic (ca. 1930-1886 Ma) and post-collisional magmatic events (ca. 1847 Ma) are identified based on SHRIMP age dating. These ages agree well with the regional Paleoproterozoic metamorphic and post-collisional magmatic activities reported from other areas of the Gyeonggi Massif. (C) 2016 Elsevier B.V. All rights reserved.</P>

Fluid-present disequilibrium melting in Neoarchean arc-related migmatites of Daeijak Island, western Gyeonggi Massif, Korea

Lee, Y.,Cho, M. Universitetsforlaget ; Elsevier Science Ltd 2013 Lithos Vol.179 No.-

The melting process of meta-igneous rocks was investigated via field, petrographic and geochemical analyses of the Neoarchean (~2.51Ga) migmatite complex in Daeijak Island, western Gyeonggi Massif. This complex consists primarily of garnet-free amphibolites and tonalitic migmatites, both of which contain hornblende, plagioclase and quartz as major constituents. Neosomes and leucosomes in the migmatite have dioritic-tonalitic and tonalitic-trondhjemitic compositions, respectively. Compositions of hornblende (X<SUB>Fe</SUB>=0.39-0.42) and plagioclase (An<SUB>24-27</SUB>) vary little between the neosomes and leucosomes. The amphibolites show distinct depletions in Nb, Ta, Zr, and Ti relative to large ion lithophile elements, suggesting an arc-related origin for their basaltic protolith. Leucosomes have lower contents of K<SUB>2</SUB>O, MgO, FeO<SUP>@?</SUP>, TiO<SUB>2</SUB>, Zr, Rb, and rare earth elements (REE) than amphibolites and neosomes, but are higher in SiO<SUB>2</SUB>, Na<SUB>2</SUB>O, and Sr contents. Leucosomes and neosomes have positive [(Eu/Eu<SUP>@?</SUP>)<SUB>N</SUB>=1.32-7.26] and negative (0.71-0.97) Eu anomalies, respectively, which are attributed to the variable degree of plagioclase fractionation during the partial melting. The P-T condition for the migmatite formation was estimated to be ~700-730<SUP>o</SUP>C and 4.7-5.5kbar, primarily based on the hornblende-plagioclase thermobarometry and phase equilibria. Various lines of textural evidence, such as the channel flow of melt along migmatitic layers and the segregation of melt into shear bands or boudin necks suggest a syn-deformation crystallization of melt. Chemical disequilibrium in migmatites is documented not only by petrographic and geochemical data but also by the REE modeling between melt product and source rock. Disequilibrium process is most likely attributed to the rapidity of melt extraction or migration, compared to chemical diffusion rate. In summary, the fluid-present disequilibrium melting of dioritic-tonalitic protoliths has produced tonalitic-trondhjemitic leucosomes in a dynamic arc environment prevalent in the Neoarchean crust of the Korean Peninsula as well as the Shandong Peninsula, North China Craton.

Late Paleozoic to Early Mesozoic arc-related magmatism in southeastern Korea: SHRIMP zircon geochronology and geochemistry

Yi, K.,Cheong, C.s.,Kim, J.,Kim, N.,Jeong, Y.J.,Cho, M. Universitetsforlaget ; Elsevier Science Ltd 2012 Lithos Vol.153 No.-

Phanerozoic granitoids are widespread in the Korean Peninsula and form a part of the East Asian Cordilleran-type granitoid belt extending from southeastern China to Far East Russia. Here we present SHRIMP zircon U-Pb ages and geochemical and Nd isotopic compositions of Late Paleozoic to Early Jurassic granitoid plutons in the northern Gyeongsang basin, southeastern Korea; namely the Jangsari, Yeongdeok, Yeonghae, and Satkatbong plutons. The granite and associated gabbroic rocks from the Jangsari pluton were coeval and respectively dated at 257.3+/-2.0Ma and 255.7+/-1.4Ma. This result represents the first finding of a Late Paleozoic pluton in South Korea. Three granite samples from the Yeongdeok pluton yielded a slightly younger age span ranging from 252.9+/-2.5Ma to 246.7+/-2.1Ma. Two diorite samples from the Yeonghae pluton gave much younger ages of 195.1+/-1.9Ma and 196.3+/-1.6Ma. An Early Jurassic age of 192.4+/-1.6Ma was also obtained from a diorite sample from the Satkatbong pluton. The mineral assemblage and Al<SUB>2</SUB>O<SUB>3</SUB>/(Na<SUB>2</SUB>O+K<SUB>2</SUB>O) versus Al<SUB>2</SUB>O<SUB>3</SUB>/(CaO+Na<SUB>2</SUB>O+K<SUB>2</SUB>O) relationship indicate that all the analyzed plutons are subduction zone granitoids. Enrichments in large-ion-lithophile-elements and depletions in high-field-strength-elements of these plutons are also concordant with geochemical characteristics typical for the subduction zone magma. The presence of Late Permian to Early Triassic arc system is in contrast with the conventional idea that the arc magmatism along the continental margin of the Korean Peninsula has commenced from Early Jurassic after the termination of Triassic collisional orogenesis. The ε<SUB>Nd</SUB>(t) values of the granitoid plutons are consistently positive (2.4-4.6), suggesting that crustal residence time of the basement beneath the Gyeongsang basin is relatively short. Moreover, the reevaluation of previously-published data reveals that geochemical compositions of the Yeongdeok pluton are compatible with those of high-silica adakites; La/Yb=37.5-114.6, Sr/Y=138.2-214.0, SiO<SUB>2</SUB>=62.9-72.0wt.%, Al<SUB>2</SUB>O<SUB>3</SUB>=15.5-17.0wt.%, Sr=562-1173ppm, MgO=0.4-1.6wt.%, Y=3-6ppm, Yb=0.18-0.45ppm, and Eu/Eu*=0.92-1.31. The occurrence of adakites in southeastern Korea, and presumably in the Hida belt of central-western Japan, is indicative of a hot subduction regime developing at least partly along the East Asian continental margin during the Permian-Triassic transition period.

The Indosinian collision-extension event between the South China Block and the Palaeo-Pacific plate: Evidence from Indosinian alkaline granitic rocks in Dashuang, eastern Zhejiang, South China

Mao, J.,Ye, H.,Liu, K.,Li, Z.,Takahashi, Y.,Zhao, X.,Kee, W.S. Universitetsforlaget ; Elsevier Science Ltd 2013 Lithos Vol.172 No.-

This study reports on the first comprehensive analysis of the geology, petrology and origin of the Dashuang pluton in Jinhua, eastern Zhejiang, South China, which is predominantly composed of quartz monzonite and subordinate quartz syenite that includes variable amounts of aegirine-augite. The quartz monzonite has a porphyritic texture defined by K-feldspar phenocrysts, whereas the quartz syenite shows considerable variation in grain size and is categorised into fine- and coarse-grained types. Zircons from the quartz monzonite and fine-grained quartz syenite yield LA-MC-ICP-MS (laser ablation-multi-collector-inductively coupled plasma-mass spectrometry) U-Pb ages of 231.60+/-0.86Ma and 231.7+/-1.1Ma, respectively, indicating crystallisation in the Middle Triassic. The chemistry of the quartz monzonite indicates a calc-alkaline to alkaline evolutionary trend, which may reflect partial melting of upper mantle contaminated by Proterozoic basement rocks, subjected to fractional crystallisation during ascent. Both the fine- and coarse-grained quartz syenites are alkaline and have high rare earth element (REE) concentrations, especially light rare earth elements (LREE), and are relatively enriched in large ion lithosphere elements (LILE). The Al<SUB>2</SUB>O<SUB>3</SUB> and NaO<SUB>2</SUB> contents of the quartz syenite increase proportionally with SiO<SUB>2</SUB>, owing to greater amounts of aegirine-augite and feldspar. The fine-grained quartz syenite has the lowest initial <SUP>87</SUP>Sr/<SUP>86</SUP>Sr ratio and lowest Nd model age, and the highest ε<SUB>Nd</SUB> value compared with the quartz monzonite. The quartz syenite compositions are best explained by fractional crystallisation of an enriched mantle-derived alkaline magma. Slight chemical variations result from source heterogeneities, as well as the spatially variable degrees of melting, assimilation, and other factors. Our new age and geochemical data for the alkaline rocks in eastern Zhejiang, considered together with collisional granites from South Korea, support a history of collision and extension between the Palaeo-Pacific plate and the South China Block during the Indosinian. We use these data to refine the geodynamic model for Indosinian multi-plate convergence in South China.

Deformation fabrics of olivine in Val Malenco peridotite found in Italy and implications for the seismic anisotropy in the upper mantle

Universitetsforlaget ; Elsevier Science Ltd 2009 Lithos Vol.109 No.3

An exhumed huge body of ultramafic rocks is preserved in Val Malenco, Italy. In this study, petrofabrics and deformation microstructures of olivine in the Val Malenco peridotite were examined, and new observations of lattice preferred orientation (LPO) of olivine such as type-B and type-E LPO in the peridotite are reported. Three types, depending on the grain size, of olivine fabrics were observed: type-A fabric exhibiting the (010)[100] slip system in the large relic grain area, and type-B and type-E fabrics exhibiting the (010)[001] and (001)[100] slip systems, respectively, in the widely distributed recrystallized-grain area. The slips occurred mainly on (010)[001], and the (001)[100] slip system has only rarely been reported to be the dominant slip system of olivine found in natural rocks. Further, seismic velocities and seismic anisotropies were calculated for the three types of fabrics found. The seismic anisotropies in the upper mantle can be attributed to the type-B, type-E, or type-A LPOs of olivine. Moreover, the anomaly in seismic low velocity in the mantle wedge can be attributed to the serpentinization as well as the mixed fabrics of the type-A, type-B, and type-E.

Spinel compositions and tectonic relevance of the Bibong ultramafic bodies in the Hongseong collision belt, South Korea

Oh, C.W.,Rajesh, V.J.,Seo, J.,Choi, S.G.,Lee, J.H. Universitetsforlaget ; Elsevier Science Ltd 2010 Lithos Vol.117 No.1

The Hongseong area in the western Gyeonggi Massif, South Korea, is characterized by the occurrence of several isolated ultramafic lenses in close association with metabasites within the granitic gneiss. Most of these bodies suffered various degrees of serpentinization and also have been highly deformed and metamorphosed. The Bibong ultramafic rock is assumed to be a mantle section of a probable Neoproterozoic supra-subduction zone (SSZ) and its evolution involved two igneous stages overprinted by later stage metamorphism. The compositions of the chromian spinel cores and olivines representative of the igneous stages were used to deduce the petrogenesis and tectonic environments for the formation of the Bibong ultramafic rocks. Spinel Cr# (100@?Cr/(Cr+Al)) for the igneous stages in lherzolite, harzburgite and chromitite are 24.5-33.3, 37.4-61.2 and 37.3-50.9, respectively. During the first stage, magma formed in the nascent back-arc basin spreading center by a small degree of partial melting of wedge mantle intruded the preexisting lithospheric mantle causing the melt/rock interaction that resulted in lherzolite with Al-rich chromian spinel. During the second stage more evolved magma formed in the mature back-arc basin by high degrees of partial melting due to the increased supply of fluid or melt from the subducting oceanic crust and intruded the lherzolitic mantle forming harzburgite with chromian spinel (with high Cr#) through the coupled process of olivine precipitation and dissolution of pyroxene. The Bibong ultramafic rocks have later undergone three metamorphic episodes. The first stage is evidenced by development of medium to fine grained olivine, orthopyroxene, clinopyroxene and amphibole around orthopyroxene porphyroblasts. The second stage is serpentinization which is followed by the third stage amphibolite facies metamorphism as evidenced by Fe-rich rims around Al-rich chromian spinel due to the diffusional exchange of Al, Mg, Cr, and Fe between chromian spinel and adjacent silicate minerals. The spinel composition shows that the transition from the nascent back-arc basin to mature back-arc basin tectonic setting may have been caused by trench roll back during the Neoproterozoic in the Hongseong area, and the serpentinization and amphibolite facies metamorphism could have occurred during uplift after Triassic collision between the North and South China blocks.

Geochemical and mineralogical characteristics of the Yonghwa phoscorite-carbonatite complex, South Korea, and genetic implications

Seo, J.,Choi, S.,Park, J.,Whattam, S.,Kim, D.W.,Ryu, I.,Oh, C.W. Universitetsforlaget ; Elsevier Science Ltd 2016 Lithos Vol.262 No.-

<P>The Yonghwa phoscorite-carbonatite complex occurs as an isolated individual body with an inclined pipe shape within the Precambrian Gyeonggi Massif, South Korea. The phoscorite consists mainly of olivine, apatite, magnetite, carbonates, amphibole, and phlogopite, and can be subdivided into two types, olivine-rich and apatite-rich. The carbonatite is composed of calcite, Mg-rich dolomite, Fe-rich dolomite, magnetite, apatite, and/or siderite. Intensive fenitization occurred along the boundary between the complex and the wall rocks of leucocratic banded gneiss and garnet-bearing metabasite. The paragenetic sequences of the phoscorite-carbonatite complex demonstrate that the early crystallization of silicate minerals was followed by the crystallization of carbonates as the carbonatitic melt cooled. Magnetite occurs within the complex, and the carbonatites have Fe contents that are higher than typical ferrocarbonatites, due to the high magnetite contents. The rare earth elements (REEs) in the phoscorites and carbonatites are weakly fractionated and show enrichments of LREEs and Nb relative to HREEs. Furthermore, the apatites reflect the fractionated trends of LREEs relative to HREEs. Phoscorite apatites are enriched in Sr and show substitutions between Ca and Sr. Mica chemistry reflects the evolutionary trend of Fe2+ and Mg2+ in the phoscorite-carbonatite melt without Al substitution. Micas exhibit high values of Mg# in the phoscorite-carbonatite complex, but lower values in fenites. Via thermodynamic analysis, the early stability fields of magnetite-pyrrhotite-graphite-carbonate assemblages indicate that the Yonghwa phoscorite and carbonatite crystallized under conditions of 600 degrees C, 2 kbar, and X-CO2 = 0.2. Afterward, melts underwent an evolution to the late stability fields of magnetite-pyrite-pyrrhotite-ilmenite assemblages. The delta C-13 and 8180 isotopic compositions of carbonates in the Yonghwa phoscorite-carbonatite complex are -8.2 parts per thousand to -3.4 parts per thousand and 6.6 to 11.0 parts per thousand., respectively, and together with the sulfur isotope compositions of the sulfides (delta S-34 values of about 0.2 parts per thousand to 2.2 parts per thousand) indicate a primary mantle source of the magmas. Phlogopites from the fenites yielded K-Ar ages of 193.4 +/- 4.9 and 195.0 +/- 5.1 Ma, which demarcate the timing of the cooling of the phoscorite-carbonatite intrusion, and indicate that the phoscorite-carbonatite may be related to a post-collisional magmatic regime. The discovery of this complex marks the first known occurrence in Korea, of Fe and Nb-REE mineralization related to phoscorite-carbonatite igneous activity. (C) 2016 Elsevier B.V. All rights reserved.</P>

U-Pb SHRIMP zircon geochronology, petrogenesis, and tectonic setting of the Neoproterozoic Baekdong ultramafic rocks in the Hongseong Collision Belt, South Korea

Oh, C.W.,Seo, J.,Choi, S.G.,Rajesh, V.J.,Lee, J.H. Universitetsforlaget ; Elsevier Science Ltd 2012 Lithos Vol.128 No.-

The Hongseong area, located in the western Gyeonggi Massif, South Korea, can be correlated with the northern margin of the South China block (Yangtze Craton). This area experienced Neoproterozoic igneous activity related to subduction before the amalgamation of Rodinia. Several isolated, lenticular, and serpentinized ultramafic-mafic bodies occur in the Hongseong area. The Baekdong body, one of the largest ultramafic bodies, has been highly deformed and metamorphosed to eclogite- and granulite-facies. The petrogenesis and tectonic environment of the Baekdong rocks are assessed using the composition of unaltered cores of spinel and olivine grains, and show that these rocks represent the mantle section of a suprasubduction ophiolite. The rocks originated from oceanic lithosphere that formed during the transition from nascent back-arc to mature island arc, related to subduction roll-back. During the back-arc stage, Al-rich spinel harzburgite formed through melt-rock interaction caused by the intrusion of magma. This magma was produced in small amounts, by less than 10% of partial melting of the wedge mantle. Subsequently, during the mature island arc stage, Cr-rich spinel dunite formed through melt-rock interaction caused by the intrusion of relatively evolved magma that formed by 30-35% partial melting due to a high input of volatiles from the subducted slab and sediments. The Baekdong ultramafic rocks, together with the Bibong ultramafic rocks, indicate that a suprasubduction tectonic setting prevailed before the amalgamation of Rodinia (at 860-890Ma) in the Hongseong area, which may be an extension of the northern margin of the Yangtze Craton.

Two-stage partial melting and contrasting cooling history within the Higher Himalayan Crystalline Sequence in the far-eastern Nepal Himalaya

Imayama, T.,Takeshita, T.,Yi, K.,Cho, D.L.,Kitajima, K.,Tsutsumi, Y.,Kayama, M.,Nishido, H.,Okumura, T.,Yagi, K.,Itaya, T.,Sano, Y. Universitetsforlaget ; Elsevier Science Ltd 2012 Lithos Vol.134 No.-

The timing of partial melting and the pressure-temperature (P-T) paths in the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal has been investigated using zircon chronology, rare earth element (REE) compositions, and P-T pseudosection analysis. Zircon from migmatites formed during Himalayan thermal events displays inherited magmatic core overgrown by two generations of metamorphic rims. The new rims are distinguished on the basis of their Tertiary ages, low MREE contents, and low Th/U ratios. The inner zircon rims from Sil+Grt+Bt+Kfs+Pl+Qtz and Ky+Sil+Grt+Bt+Ms+Pl+Qtz migmatites at different structural level of the HHCS display ages of c. 33-28Ma (Early Oligocene) and c. 21-18Ma (Early Miocene): these rims are characterized by flat MREE to HREE patterns and were overgrown by partial melt through muscovite dehydration melting under the stability of garnet, which occurred at P=c. 7-10kbar and T=c. 730-780<SUP>o</SUP>C, and at P=c. 8-14kbar and T=c. 720-770<SUP>o</SUP>C, respectively. The outer zircon rims are relatively enriched in HREE with respect to the inner rims and were overgrown at c. 27-23Ma (Late Oligocene) and at c. 18-16Ma (Early Miocene) during melt crystallization accompanying breakdown of garnet at P=c. 4-7kbar and T=c. 650-725<SUP>o</SUP>C. Early Miocene Ms-Bt leucogranites with two successively overgrown zircon rims at c. 18.3+/-0.3Ma and c. 16.3+/-0.2Ma were intruded into Early Oligocene migmatite hosts. Microstructural observations and the corresponding P-T conditions associated with the two generations of zircon rims indicate that the Early Oligocene and Early Miocene migmatites show relatively isobaric and nearly isothermal P-T paths during exhumation, respectively. The inferences are consistent with higher average cooling rates for the Early Miocene (c. 30-40<SUP>o</SUP>C/My) than the Early Oligocene (c. 15-25<SUP>o</SUP>C/My) migmatites, inferred from peak-T conditions and FT (c. 6Ma for both migmatites) and U-Pb zircon ages. The P-T-t paths of the two migmatites indicate that burial of the Early Miocene migmatites has been coeval with exhumation of the Early Oligocene migmatites, implying the formation of large-scale thrust within the HHCS.

Multiple generations of mafic-ultramafic rocks from the Hongseong suture zone, western South Korea: Implications for the geodynamic evolution of NE Asia

Kwon, S.,Kim, S.W.,Santosh, M. Universitetsforlaget ; Elsevier Science Ltd 2013 Lithos Vol.160 No.-

Dismembered mafic and serpentinized ultramafic bodies within the Hongseong suture zone of the Gyeonggi massif, southwestern Korean Peninsula are tectonically correlated with those from the Chinese Qinling-Dabie-Sulu collisional belt. We report sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages, detailed mineral compositions and whole rock geochemical data from the mafic rocks including eclogite and garnetite from the Hongseong suture zone. The zircon U-Pb data show multiple protolith ages corresponding to Neoproterozoic (ca. 815-770Ma), Paleozoic (ca. 310Ma) and Middle Triassic (ca. 240Ma). The peak high-pressure and retrograde regional intermediate-pressure metamorphic events of Middle Triassic (ca. 240-230Ma) are also recorded in zircons from the mafic rocks in the Hongseong suture zone. The ages and P-T estimates presented in this study match well with those of the peak ultra-high-pressure metamorphism in the Sulu area of China. Our results from the Hongseong suture zone provide important insights into the geodynamic history of the NE Asian region, with evidence for both Neoproterozoic subduction and a Paleozoic to Triassic subduction/accretion event.