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9. Alpine stacking of two Variscan granite blocks recognised from mineral stabilities, age and structural data (Western Carpathians)

Author

Broska, Igor, Petrik, Igor, Yi, Keewook, Majka, Jaroslaw, Barnes, Christopher J., Vojtko, Rastislav, Madaras, Jan, Kurylo, Sergii, Kubis, Michal, Broska, Igor, Petrik, Igor, Yi, Keewook, Majka, Jaroslaw, Barnes, Christopher J., Vojtko, Rastislav, Madaras, Jan, Kurylo, Sergii, and Kubis, Michal

Abstract

This study presents petrological, geochemical, geochronological and structural data that serve to identify two different Alpine tectonic units formed from the Variscan-aged Tribeč-Zobor granitic rocks. The proposed model for juxtaposition of two granite blocks in the Tribeč-Zobor basement during the Alpine orogeny is based on the degree of granite alteration, particularly related to differences in monazite stabilities of the S-type granitic rocks along the studied NW- SE profile through the Tribeč-Zobor mountain range. Along the cross-section, the Variscan crystalline basement in the mountain crest is composed of hydrothermally altered and metamorphosed S-type monzogranite (zircon age: 355.2 ± 1.2 Ma). Below this monzogranite there are hypidiomorphic non-altered granitic rocks with ages spanning from 357.8 ± 0.66 Ma to 351.2 ± 0.7 Ma for northeast S-type and 358.2 ± 0.85 Ma to 347.9 ± 0.94 Ma for I-type on southwest part of profile. U-Th-Pb dating revealed both low- and high-Y types of monazite in the S-type granites. Low-Y monazite from both the structurally lower unaltered, and the structurally upper, metamorphosed S-type granitic rocks provides Tournaisian Variscan ages in the range of 354–349 Ma. The high-Y monazite from the same rocks records Visean ages in the range of 348–335 Ma, likely indicating later-staged thermal rock overprinting. Monazites from non-altered S-type granodiorites are stable due to rapid emplacement in the upper crustal position. In contrast, the monazites in the overlying metamorphosed S-type granites are retrogressed to allanite as a result of slower, nearly isobaric cooling of the granites emplaced at deeper crustal levels. Thus, monazite behaviour indicate two different Variscan granite blocks, similar in age but with different emplacement depths. The conditions for metamorphism (450–500 °C, 750–800 MPa) of the structurally upper granites are inferred from phase equilibrium thermodynamic modelling, occurring during the Alpine orogeny.

Published
2024
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14. Ultrahigh-temperature Metamorphism in the Bakhuis Granulite Belt (Surinam)

Author

de Roever, Emond, Beunk, Frank, Yi, Keewook, Donkert, Rinse-Jan, van de Steeg, Wouter, Uunk, Bertram, Davies, Gareth, Brouwer, Fraukje, de Roever, Emond, Beunk, Frank, Yi, Keewook, Donkert, Rinse-Jan, van de Steeg, Wouter, Uunk, Bertram, Davies, Gareth, and Brouwer, Fraukje

Published
2022

23. Late Cretaceous Alkaline Magmas Of The Eastern Pontides Orogenic Belt (Ne Turkey): A Review With New Geological, Geochemical And Geochronological Data

Author

Eyuboglu, Yener, Dudas, Francis O., Zhu, Di-Cheng, Santosh, M., Liu, Ze, Chatterjee, Nilanjan, and Yi, Keewook

Abstract

The Eastern Pontides Orogenic Belt (EPOB) in northeastern Turkey is a continental arc that is part of the Himalayan-Alpine orogeny. Late Cretaceous magmatism in this belt is dominated by high-K calc-alkaline, shoshonitic, and alkaline rocks that occur over an area of -600 km (E-W) and -200 km (N-S). We present new geochemical data (whole rock major and trace element analyses, mineral compositions, U-Pb zircon and titanite ages, zircon trace element data, and Sr-Nd-Pb isotopic analyses) for 153 samples, with compiled data for an additional 265 samples. Magmatism is diachronous across the arc. In the north, Cycle-I (92-84 Ma) commenced with tholeiitic and calc-alkaline basalts and andesites, and continued with felsic extrusive rocks. Cycle-II (84-70 Ma) includes mainly intermediate and felsic extrusive rocks. Over time, the low-K granitoids progressively give way to calc-alkaline granitoids in the north. In the south, Cycle-I (84-78 Ma) includes high-K trachyandesites, trachytes and felsic extrusive rocks. Cycle-II (78-65 Ma) comprises leucitites, trachybasalts, and other K-rich rocks. Shoshonitic granitoids in the south show ages between 80 and 65 Ma. Clinopyroxene compositions in trachyandesites and leucitites indicate initial crystallization at -38 km depth and 1120 degrees C. Amphibole joins the phase assemblage at -8 km depth and T < 950 degrees C. All samples have LILE and LREE enrichments and negative Nb, Ta, and Ti, and positive Pb anomalies, consistent with their subduction-related origin. Isotopic data indicate mixing between magmas from heterogeneous, enriched subcontinental mantle sources and those from a range of crustal sources. Mantle-derived and crustally-derived magmas are clearly separated by (Gd/Yb)N. Chemical zoning of the EPOB, from tholeiitic and calc-alkaline rocks to high-K calc-alkaline, shoshonitic, and alkaline rocks, from north to south, and the timing of magmatism younging southward, are consistent with a model of southward subduction in the Late Cretaceous. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Published
2021

24. Parageneses and Th-U distributions among allanite, monazite, and xenotime in Barrovian-type metapelites, Imjingang belt, central Korea

Author

Kim, Yoonsup, Yi, Keewook, and Cho, Moonsup

Subjects
Crystallization -- Observations, Monazite -- Properties, Thorium -- Properties, Earth sciences
Abstract

The paragenetic relationships and Th-U distributions among allanite, monazite, and xenotime were investigated in a progressive sequence of garnet- to kyanite-zone metapelites of the Imjingang belt, Korea. Allanite is predominant in the garnet and staurolite zones, whereas monazite and xenotime predominate in the kyanite zone. Epidote grains in the lower garnet zone are commonly zoned, from allanite (core) to relatively Y-rich, rare-earth-element (REE)-epidote to clinozoisite (rim), although both REE-epidote and clinozoisite disappear in higher-grade metapelites. Moreover, allanite and REE-epidote often contain minute inclusions of thorium silicate. The isogradic distributions and similarity of REE patterns between allanite and monazite suggest that the latter has grown at the expense of the former. In addition, the discontinuous Th zoning in monazite is apparently inherited from heterogeneous Th distribution and thorium silicate inclusions in allanite. Thus, thorium silicate possibly provided the additional Th and U necessary for the monazite formation. Paragenetic relationships of allanite and monazite inclusions within various index minerals suggest that at amphibolite-facies conditions allanite is stable at higher pressures than monazite. Xenotime grains in the staurolite zone are rarely produced by the breakdown of clinozoisite and REE-epidote, whereas those in the kyanite zone are grown primarily at the expense of garnet. Incorporation of Th and U into monazite and xenotime is governed mainly by the brabantite and thorite substitutions, respectively. Taken together, our results suggest that the allanite-to-monazite transformation is primarily responsible for the distributions of REEs, Th, and U among metapelitic phases, and that the xenotime formation was facilitated by the contribution from major silicates, particularly garnet. Keywords: Allanite, monazite, xenotime, Th and U distributions, lmjingang metapelites

Published
2009

26. Neoarchean magmatism in the southern Scott and Raggatt Mountains, Napier Complex, east Antarctica

Author

Król, Piotr, Kusiak, Monika A., Dunkley, Daniel J., Wilde, Simon A., Yi, Keewook, Whitehouse, Martin J., Lee, Shinae, and Harley, Simon L.

Subjects
Geochemistry and Petrology, Geology
Abstract

The Napier Complex of Enderby Land and Kemp Land is a unique part of the Eastern Antarctic Shield, as it preserves history of crustal growth extending from the Eo- to Neoarchean. It is composed mainly of enderbitic and charnockitic gneisses and granulites metamorphosed at ca. 2.5 Ga, and locally at ca. 2.8 Ga, under high- to ultra-high temperature conditions. These events have been well-characterized in the central and western parts of the complex; however, the south-southwestern region has been little studied. Samples from the southern Scott and Raggatt Mountains were selected for zircon U-Pb dating by Secondary Ion Mass Spectrometry and whole-rock geochemical analysis. Tonalitic gneiss from the Raggatt Mountains and trondhjemitic gneiss from the southern Scott Mountains have protolith ages of 2711 ± 5 Ma and 2726 ± 6 Ma, respectively. In contrast, tonalitic samples from the southern Scott Mountains have protolith ages of ca. 2539 ± 5, 2533 ± 5 and 2522 ± 9 Ma. The younger generation of magmatism overlaps the timing of metamorphic zircon growth between 2540 and 2440 Ma observed in this study. The geochemistry of ca. 2720 Ma gneisses indicates that magmatism was generated by medium-pressure melting (>1.5 GPa) of basaltic crust, whereas ca. 2530 Ma magmas were generated at lower pressures (

Published
2022
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27. Eoarchean contrasting ultra-high-pressure to low-pressure metamorphisms (<250 to >1000 °C/GPa) explained by tectonic plate convergence in deep time

Author

Nutman, Allen Phillip, Bennett, Vickie C., Friend, Clark, Yi, Keewook, Nutman, Allen Phillip, Bennett, Vickie C., Friend, Clark, and Yi, Keewook

Abstract

© 2020 Elsevier B.V. Greenland's Itsaq Gneiss Complex (IGC) shows Eoarchean (>3600 Ma) 250–400 °C/GPa (low T/P – high pressure) and ≥1000 °C/GPa (high T/P) metamorphic regimes, demonstrating a similarity of contrasting metamorphic T/P regimes from the Phanerozoic back to the start of Earth's rock record. Low T/P metamorphism produced: (i) Deep crustal eclogitised mafic rocks which upon partial melting formed the tonalites dominating the IGC; (ii) ~550 °C ≥ 2.6 GPa conditions (≤250 °C/GPa) demonstrated by an olivine + antigorite + titano-chondrodite/titano-clinohumite relict assemblage within mantle slivers showing geochemical and crystallographic features of a suprasubduction environment, that were exhumed into the crust by 3712 Ma; (iii) rare vestiges of 3658 Ma high-pressure (garnet + clinopyroxene) granulite; and (iv) Barrovian-style kyanite + staurolite assemblages. High T/P metamorphism is shown by 3669 Ma crustal melts equilibrated with orthopyroxene. This was coeval to the youngest juvenile tonalitic crust in the complex (latter derived by anatexis under low T/P conditions), and a 3670–3570 Ma history of deep crust migmatisation under low pressure, garnet-free conditions. Structural geology of the IGC indicates its low T/P regimes coincide with crustal imbrication by compression of arc-like tholeiites, boninite-like lavas, andesites, felsic-intermediate volcano-sedimentary rocks and chemical sedimentary rocks, whereas post-3660 Ma high T/P metamorphism was marked by late-orogenic extension/exhumation and deep crustal flow with mafic underplating and partial melting generating granites. Thus the diversity of Earth's earliest-recorded geodynamic settings resembles more those of modern geodynamics, than the lithological and structural relationships expected from theoretical non-uniformitarian scenarios like drip tectonics in a stagnant lid regime. The recognition of an ultra-high-pressure ≤250 °C/GPa metamorphic regime at >3700 Ma in the IGC removes the last arg

Published
2020

29. The Final Pulse Of The Early Cenozoic Adakitic Activity In The Eastern Pontides Orogenic Belt (Ne Turkey): An Integrated Study On The Nature Of Transition From Adakitic To Non-Adakitic Magmatism In A Slab Window Setting

Author

Eyuboglu, Yener, Dudas, Francis O., Santosh, M., Eroglu-Gumruk, Tugba, Akbulut, Kubra, Yi, Keewook, and Chatterjee, Nilanjan

Abstract

The Eastern Pontides Orogenic Belt, one of the best examples of a fossil continental arc in the Alpine-Himalayan system, is characterized by adakitic magmatism during the Early Cenozoic. Popular models correlate the adakitic magmatism to syn- or post-collisional processes occurring after the collision between the Eastern Pontides Orogenic Belt and the Tauride Platform at the end of Late Mesozoic and/or beginning of the Cenozoic. We present new geological, petrological and chronological data from andesites and felsic tuffs exposed in the Bayburt area, in the southern part of the Eastern Pontides Orogenic Belt, and discuss the nature of the transition from adakitic to non-adakitic activities in a continental arc. Major, trace and rare earth element concentrations of both andesites and felsic tuffs clearly suggest that they are related to arc magmatism in a continental arc with adakitic composition. The isotopic compositions are permissive of mixing between a component similar to depleted mantle and a second component that is either mafic lower crust or subducted oceanic crust. Ar-39/Ar-40 hornblende and U/Pb zircon dating indicate that this adakitic magmatism in the Bayburt area ended by about 47 Ma, and transformed into non-adakitic, granitoid arc magmatism in the area immediately north of Bayburt in the Lutetian (similar to 46 Ma). Based on our new results in conjunction with available data, we propose that the beginning of northward rollback of a south-directed subducting slab, and simultaneous opening of a slab window related to ridge subduction, triggered both adakitic magmatism for approximately a 10 Myr period between 57.6 and 47 Ma and arc-parallel extension that caused the opening of the Early Cenozoic sedimentary basins. We also suggest that the shallow marine environment, in which Nummulite-bearing sandy limestones accumulated in the Early Cenozoic, was transformed into a saline-lake environment during the pyroclastic activity that produced the studied felsic tuffs at similar to 47 Ma.

Published
2018

31. Eocene Granitoids Of Northern Turkey: Polybaric Magmatism In An Evolving Arc-Slab Window System

Author

Eyuboglu, Yener, Dudas, Francis O., Thorkelson, Derek, Zhu, Di-Cheng, Liu, Ze, Chatterjee, Nilanjan, Yi, Keewook, and Santosh, M.

Abstract

The Eastern Pontides Orogenic Belt offers critical clues on the origin of Early Cenozoic continental arc magmatism in the Alpine-Himalayan system. Systematic geological, geochemical and chronological studies indicate that there are three subgroups among the Early Cenozoic intrusions in the Eastern Pontides Oro genic Belt - Late Paleocene-Early Eocene adakitic intermediate-felsic intrusions, Eocene mafic intrusions, and Eocene non-adakitic granitoid intrusions. Here we focus on the petrology and geodynamic setting of the Eocene non-adakitic granitoid intrusions that are well exposed in a belt between the Thanetian-Ypresian adakitic intrusions in the south and the Lutetian gabbroic intrusions in the north. We also present data on enclaves and surrounding Eocene volcanics. The studied intrusions can be grouped into two main categories, based on their field and petrographical characteristics: granodiorite and monzodiorite-dominated and syenite-dominated bodies. They can be further subdivided into four groups of differing K2O content: low-K2O (cevrepinar, Kaletas, Saricicek and Uzengili), mixed (Sorkunlu, Kozluk and Tamdere), and high-K2O (Dolek, Mesebasi, cakirbag, and Arslandede) rocks are granodioritic and monzodioritic, whereas shoshonitic (Kosedak, Meydanli and Bademli) bodies are syenitic. Zircon U-Pb age determinations reveal that these granitoids were emplaced into crustal rocks of the Eastern Pontides Orogenic Belt between 47 and 42 Ma, in Lutetian time, simultaneously with the gabbroic intrusions in the north. Mineral compositions and P-T calculations are consistent with the interpretation that crustal melting or magma storage started at mid-crustal depth (similar to 25 km), with a magma system that subsequently extended to shallow levels (

Published
2017

33. Where Are The Remnants Of A Jurassic Ocean In The Eastern Mediterranean Region?

Author

Eyuboglu, Yener, Dudas, Francis O., Santosh, M., Xiao, Yilin, Yi, Keewook, Chatterjee, Nilanjan, Wu, Fu-Yuan, and Bektas, Osman

Abstract

The subduction polarity of Tethyan oceanic lithosphere during Jurassic is a controversial topic in relation to the geodynamic evolution of the Alpine-Himalayan system. We present new geological, geochemical and zircon U-Pb data from four different regions of the Eastern Pontides Orogenic Belt, a key area of the Alpine-Himalayan system. We discuss the origin of the magmatism and also the existence of an ocean in the eastern Mediterranean region during the Jurassic period. Jurassic intrusions, predominantly gabbro, tonalite and minor diorite, are well exposed in the southern and axial zones of the orogenic belt. Thermobarometry indicates that high-pressure (6-10 kb) crystallization of these intrusions occurred at temperatures of 1183-1250 degrees C. Zircon U-Pb dating from 10 samples show ages between 195 and 165 Ma, indicating that magmatism occurred between Sinemurian and Callovian time. We characterize the intrusions from electron microprobe, zircon geochronology, and whole rock and Sr, Nd, and Pb isotopes. Our data show that the studied intrusions are broadly tholeiitic, except for two calc-alkaline bodies, and formed in an arc-related setting with minimal involvement of older crust or sediment. The most widely accepted model proposes that the ultramafic-mafic rocks exposed between the Pontide arc and the Tauride belt are remnants of a Jurassic Penrose-type and/or suprasubduction zone ophiolite. However, new zircon U-Pb age data from mafic lithologies cutting the Kop ultramafic massif do not support this model and clearly indicate that the ultramafic lithologies are Paleozoic or older in age and are not remnants of a Jurassic ocean that known as "Northern Branch of Neotehtys". (C) 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Published
2016

36. Mesoarchaean collision of Kapisilik terrane 3070Ma juvenile arc rocks and >3600Ma Isukasia terrane continental crust (Greenland)

Author

Nutman, Allen P, Bennett, Victoria, Friend, Clark, Yi, Keewook, Lee, Seung Ryeol, Nutman, Allen P, Bennett, Victoria, Friend, Clark, Yi, Keewook, and Lee, Seung Ryeol

Abstract

The Mesoarchaean Kapisilik and Eoarchaean Isukasia terranes in the Nuuk region of southern West Greenland were tectonically juxtaposed in the Archaean. The north of the Isukasia terrane is distal from the Kapisilik terrane and has only rare growth of ~2690Ma metamorphic zircon and no 2980-2950Ma metamorphic zircon. The southern part of the Isukasia terrane lies between two ~2690Ma shear zones, and has locally preserved high pressure granulite facies assemblages and widespread growth of 2980-2950Ma metamorphic zircon and also sporadic growth of ~2690Ma metamorphic zircon. Within this southern part of the Isukasia terrane there is a folded klippe of mylonitised Mesoarchaean detrital meta-sedimentary rocks (carrying >3600 and ~3070Ma detrital zircons), mafic and ultramafic rocks, with ~2970Ma metamorphic zircon overgrowths. South of the Isukasia terrane is the Kapisilik terrane, containing ~3070Ma arc-related volcanic rocks, gabbro-anorthosites and meta-tonalites, intruded by 2970-2960Ma granites. Zircons of an Ivisârtoq supracrustal belt ~3075Ma intermediate volcanic rock have initial e{open}Hf values of +2 to +5 thus are juvenile crustal additions. ~3070Ma tonalites along the northern edge of the Kapisilik terrane have whole rock positive initial e{open}Nd values and thus are also juvenile crustal additions. In contrast, igneous zircons in 2960Ma granites intruded into juvenile ~3075Ma supracrustal rocks of the Kapisilik terrane have initial e{open}Hf values of -5 to -10, and must have involved the partial melting of >3600Ma Isukasia terrane rocks.The integrated structural and zircon U-Th-Pb-Hf isotopic data show that at 2980-2950. Ma the Kapisilik terrane juvenile arc components collided with, and over-rid, the Isukasia terrane. The southern edge of the Isukasia terrane came to lie in the deep crust under the Ivisârtoq supracrustal belt and melted at 2970-2960. Ma to produce granites. These granites derived from ancient crust rose into the upper crust, where they in

Published
2015

37. Mesoarchaean collision of Kapisilik terrane 3070Ma juvenile arc rocks and >3600Ma Isukasia terrane continental crust (Greenland)

Author

Nutman, Allen Phillip, Bennett, Vickie C, Friend, Clark R. L, Yi, Keewook, Lee, Seung Ryeol, Nutman, Allen Phillip, Bennett, Vickie C, Friend, Clark R. L, Yi, Keewook, and Lee, Seung Ryeol

Abstract

The Mesoarchaean Kapisilik and Eoarchaean Isukasia terranes in the Nuuk region of southern West Greenland were tectonically juxtaposed in the Archaean. The north of the Isukasia terrane is distal from the Kapisilik terrane and has only rare growth of ~2690Ma metamorphic zircon and no 2980-2950Ma metamorphic zircon. The southern part of the Isukasia terrane lies between two ~2690Ma shear zones, and has locally preserved high pressure granulite facies assemblages and widespread growth of 2980-2950Ma metamorphic zircon and also sporadic growth of ~2690Ma metamorphic zircon. Within this southern part of the Isukasia terrane there is a folded klippe of mylonitised Mesoarchaean detrital meta-sedimentary rocks (carrying >3600 and ~3070Ma detrital zircons), mafic and ultramafic rocks, with ~2970Ma metamorphic zircon overgrowths. South of the Isukasia terrane is the Kapisilik terrane, containing ~3070Ma arc-related volcanic rocks, gabbro-anorthosites and meta-tonalites, intruded by 2970-2960Ma granites. Zircons of an Ivisârtoq supracrustal belt ~3075Ma intermediate volcanic rock have initial e{open}Hf values of +2 to +5 thus are juvenile crustal additions. ~3070Ma tonalites along the northern edge of the Kapisilik terrane have whole rock positive initial e{open}Nd values and thus are also juvenile crustal additions. In contrast, igneous zircons in 2960Ma granites intruded into juvenile ~3075Ma supracrustal rocks of the Kapisilik terrane have initial e{open}Hf values of -5 to -10, and must have involved the partial melting of >3600Ma Isukasia terrane rocks.The integrated structural and zircon U-Th-Pb-Hf isotopic data show that at 2980-2950. Ma the Kapisilik terrane juvenile arc components collided with, and over-rid, the Isukasia terrane. The southern edge of the Isukasia terrane came to lie in the deep crust under the Ivisârtoq supracrustal belt and melted at 2970-2960. Ma to produce granites. These granites derived from ancient crust rose into the upper crust, where they in

Published
2015

38. Discovery Of Miocene Adakitic Dacite From The Eastern Pontides Belt (Ne Turkey) And A Revised Geodynamic Model For The Late Cenozoic Evolution Of The Eastern Mediterranean Region

Author

Eyuboglu, Yener, Santosh, M., Yi, Keewook, Bektas, Osman, and Kwon, Sanghoon

Abstract

The Cenozoic magmatic record within the ca. 500 km long eastern Pontides orogen, located within the Alpine metallogenic belt, is critical to evaluate the tectonic history and geodynamic evolution of the eastern Mediterranean region. In this paper we report for the first time late Miocene adakitic rocks from the southeastern part of the eastern Pontides belt and present results from geochemical and Sr-Nd isotopic studies as well as zircon U-Pb geochronology. The Tavdagi dacite that we investigate in this study is exposed as round or ellipsoidal shaped bodies, sills, and dikes in the southeastern part of the belt Zircons in the dacite show euhedral crystal morphology with oscillatory zoning and high Th/U values (up to 1.69) typical of magmatic origin. Zircon LA-ICPMS analysis yielded a weighted mean Pb-206/U-238 age of 7.86 +/- 0.15 Ma. SHRIMP analyses of zircons with typical magmatic zoning from another sample yielded a weighted mean Pb-206/U-238 age of 8.79 +/- 0.19 Ma. Both ages are identical and constrain the timing of dacitic magmatism as late Miocene. The Miocene Tavdagi dacite shows adakitic affinity with high SiO2 (68.95-71.41 wt.%), Al2O3 (14.88-16.02 wt.%), Na2O (327-4.12 wt.%), Sr (331.4-462.1 ppm), Sr/Y (85-103.7), La-N/Yb-N (34.3-50.9) and low Y (3.2-5 ppm) values. Their initial Nd-143/Nd-144 (0.512723-0.512736) and Sr-87/Sr-86 (0.70484-0.70494) ratios are, respectively, lower and higher than those of normal oceanic crust. The geological, geochemical and isotopic data suggest that the adakitic magmatism was generated by partial melting of the mafic lower crust in the southeastern part of the eastern Pontide belt during the late Miocene. Based on the results presented in this study and a synthesis of the geological and tectonic information on the region, we propose that the entire northern edge of the eastern Pontides-Lesser Caucasus-Elbruz magmatic arc was an active continental margin during the Cenozoic. We identify a migration of the Cenozoic magmatism towards north over time resulting from the roll-back of the southward subducted Tethys oceanic lithosphere. Slab break-off during Pliocene is proposed to have triggered asthenospheric upwelling and partial melting of the subduction-modified mantle wedge which generated the alkaline magmatic rocks exposed in the northern part of the magmatic arc. (C) 2012 Elsevier B.V. All rights reserved.

Published
2012

41. Gondwana margin evolution from zircon REE, O and Hf signatures of Western Province Gneisses, Zealandia

Author

Hiess, Joe, Yi, Keewook, Woodhead, Jonathan, Rattenbury, M, Ireland, Trevor, Hiess, Joe, Yi, Keewook, Woodhead, Jonathan, Rattenbury, M, and Ireland, Trevor

Abstract

U-Th-Pb dated zircons from Western Province paragneisses and orthogneisses were analysed for rare earth element (REE) concentrations, as well as oxygen and hafnium isotopic compositions. Experiments performed in situ using a sensitive high-resolution ion

Published
2014

42. Tracing Archaean terranes under Greenland's Icecap: U-Th-Pb-Hf isotopic study of zircons from melt-water rivers in the Isua area

Author

Yi, Keewook, Bennett, Vickie, Nutman, Allen Phillip, Lee, Seung Ryeol, Yi, Keewook, Bennett, Vickie, Nutman, Allen Phillip, and Lee, Seung Ryeol

Abstract

The Archaean gneisses of the Nuuk area (southern West Greenland) are partitioned into tectonostratigraphic terranes - blocks of arc-like crust that evolved independently until they coalesced by collisional orogeny. To 'map' terranes to the east under the Inland Ice, sand samples were taken from rivers issuing from the edge of the Icecap; three from the Isua area and one from ∼20 km to the south. Bedrock along this part of the ice front consists of ∼3820-3600 Ma amphibolite facies rocks. 40 km south of Isua (Kapisilik terrane in the Ivisaartoq area) and also from ∼10 km to the north there are Mesoarchaean amphibolite facies gneisses (3070-2950 Ma) exposed at the ice front. In the moraine fields in the Isua area there are erratic blocks of granulite facies gneisses. These were sourced from a hidden terrane to the east under the ice because no such rocks are exposed in the Isua area. The majority of the zircons from the sand samples yielded close to concordant U-Pb ages. Apart from one 2414 Ma grain, all are Archaean. The Isua sands show ∼2695, 2710 and 2730 Ma; then 2790, 2805 and 2840 Ma clusters with a few grains back to 2950 Ma and then a complex 3440-3960 Ma spectrum. Less than 1% of the grains have ages between 3000 and 3400 Ma. In the Isua sands, Neoarchaean and late Mesoarchaean zircons form >50% of the population. The sand 20 km to the south shows a similar span of zircon ages, but there is a 2960 Ma peak not seen in the Isua sands. As the Eoarchaean and late Mesoarchaean-Neoarchaean cycles progress, the spread of Th/U and ɛHf values seen in each zircon age population increases. This is interpreted as repeated addition of juvenile material to the crust, but also with increasing amounts of high temperature tectonothermal recycling of older materials within a package. However, there is no isotopic evidence for any contribution of Eoarchaean crust to the later ≥2750 Ma Archaean cycle(s), showing evolution in disparate terranes. Furthermore, no evidence for Hadean

Published
2014

45. Parageneses and Th-U distributions among allanite, monazite, and xenotime in Barroviantype metapelites, Imjingang belt, central Korea

Author

Kim, Yoonsup, Yi, Keewook, and Cho, Moonsup

Abstract

The paragenetic relationships and Th-U distributions among allanite, monazite, and xenotime were investigated in a progressive sequence of garnet- to kyanite-zone metapelites of the Imjingang belt, Korea. Allanite is predominant in the garnet and staurolite zones, whereas monazite and xenotime predominate in the kyanite zone. Epidote grains in the lower garnet zone are commonly zoned, from allanite (core) to relatively Y-rich, rare-earth-element (REE)-epidote to clinozoisite (rim), although both REE-epidote and clinozoisite disappear in higher-grade metapelites. Moreover, allanite and REEepidote often contain minute inclusions of thorium silicate. The isogradic distributions and similarity of REE patterns between allanite and monazite suggest that the latter has grown at the expense of the former. In addition, the discontinuous Th zoning in monazite is apparently inherited from heterogeneous Th distribution and thorium silicate inclusions in allanite. Thus, thorium silicate possibly provided the additional Th and U necessary for the monazite formation. Paragenetic relationships of allanite and monazite inclusions within various index minerals suggest that at amphibolite-facies conditions allanite is stable at higher pressures than monazite. Xenotime grains in the staurolite zone are rarely produced by the breakdown of clinozoisite and REE-epidote, whereas those in the kyanite zone are grown primarily at the expense of garnet. Incorporation of Th and U into monazite and xenotime is governed mainly by the brabantite and thorite substitutions, respectively. Taken together, our results suggest that the allanite-to-monazite transformation is primarily responsible for the distributions of REEs, Th, and U among metapelitic phases, and that the xenotime formation was facilitated by the contribution from major silicates, particularly garnet.

Published
2009
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46. Comparative assessment of net CO2 exchange across an urbanization gradient in Korea based on eddy covariance measurements.

Author

Hong, Je-Woo, Hong, Jinkyu, Chun, Junghwa, Lee, Yong Hee, Chang, Lim-Seok, Lee, Jae-Bum, Yi, Keewook, Park, Young-San, Byun, Young-Hwa, and Joo, Sangwon

Subjects
SUBURBS, LAND use, URBAN planning, URBANIZATION, LAND cover, CARBON cycle, PER capita
Abstract

Background: It is important to quantify changes in CO2 sources and sinks with land use and land cover change. In the last several decades, carbon sources and sinks in East Asia have been altered by intensive land cover changes due to rapid economic growth and related urbanization. To understand impact of urbanization on carbon cycle in the monsoon Asia, we analyze net CO2 exchanges for various land cover types across an urbanization gradient in Korea covering high-rise high-density residential, suburban, cropland, and subtropical forest areas. Results: Our analysis demonstrates that the urban residential and suburban areas are constant CO2 sources throughout the year (2.75 and 1.02 kg C m−2 year−1 at the urban and suburban sites), and the net CO2 emission indicate impacts of urban vegetation that responds to the seasonal progression of the monsoon. However, the total random uncertainties of measurement are much larger in the urban and suburban areas than at the nonurban sites, which can make it challenging to obtain accurate urban flux measurements. The cropland and forest sites are strong carbon sinks because of a double-cropping system and favorable climate conditions during the study period, respectively (− 0.73 and − 0.60 kg C m−2 year−1 at the cropland and forest sites, respectively). The urban area of high population density (15,000 persons km−2) shows a relatively weak CO2 emission rate per capita (0.7 t CO2 year−1 person−1), especially in winter because of a district heating system and smaller traffic volume. The suburban area shows larger net CO2 emissions per capita (4.9 t CO2 year−1 person−1) because of a high traffic volume, despite a smaller building fraction and population density (770 persons km−2). Conclusions: We show that in situ flux observation is challenging because of its larger random uncertainty and this larger uncertainty should be carefully considered in urban studies. Our findings indicate the important role of urban vegetation in the carbon balance and its interaction with the monsoon activity in East Asia. Urban planning in the monsoon Asia must consider interaction on change in the monsoon activity and urban structure and function for sustainable city in a changing climate. [ABSTRACT FROM AUTHOR]

Published
2019
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