SMP09-01

会場:201A

時間:5 月 27 日 09:00-09:30

Extreme metamorphism, geodynamic regimes and supercontinent cycles
Extreme metamorphism, geodynamic regimes and supercontinent cycles
BROWN, Michael1∗
BROWN, Michael1∗
11

Geology, University of Maryland, USA
Geology, University of Maryland, USA

This work is part of a larger project to use the geological record of magmatism and metamorphism to develop hypotheses
about geodynamics that may be tested using numerical models. Apparent thermal gradients of metamorphism, as recorded byclose-to-peak mineral assemblages, retrieved from rocks equilibrated at high P, P–T or T, for which the timing is obtained fromvarious chronometers, may be used be interrogate the rock record to assess secular change in the apparent thermal gradientsof metamorphism. One-sided subduction creates asymmetry in the thermal structure of convergent plate margins, with lower
dT/dP in the subduction zone and higher dT/dP in the orogenic hinterland. During collisional orogenesis these distinct thermal
environments are imprinted in the rock record as contrasting types of metamorphism. Proterozoic orogens record eclogite–HP
granulite (E-HPG) metamorphism, with gradients of 350–750C/GPa, and granulite–UHT metamorphism, with gradients of 750–
1500C/GPa. By contrast, Phanerozoic orogens register UHP metamorphism with strikingly lower gradients of 150–350C/GPa,and UHP metamorphism is the defining feature of Phanerozoic collisional orogenesis in Eurasia. What is the change in geodynamics recorded by these data?
For contemporary conditions, geodynamic modeling of collisional orogenesis shows that slab breakoff occurs at depths >300
km; strong lower crust results in coupled collision with UHP metamorphism, whereas weak lower crust results in decoupled collision with only E-HPG metamorphism. Increasing the ambient mantle temperature by 80–100C leads to shallow slab breakoff(<200 km) and unconventional modes of collision, viz a truncated hot collision regime (strong lower crust) and a two-sided
hot collision regime (weak lower crust). Inverting these data, as ambient mantle temperature declined to <100C warmer thanthe present day the change to deeper slab breakoff generated a colder environment and enabled stronger crust?mantle couplingthat allowed subduction of continental rocks to mantle depths. Thus, the appearance of UHP metamorphism is inferred to be aconsequence of secular decrease in ambient mantle temperature. By contrast, granulite facies and UHT metamorphism in central
East Gondwana likely represents deep crust metamorphosed under a large, moderately thick orogenic plateau that formed as aresult of Ediacaran collision and hinterland thickening, with radiogenic heating generating peak metamorphic temperatures in
the Cambrian. It may be no coincidence that Gondwana could have been located over the African LLSVP at the dawn of thePhanerozoic or that the first UHP belts had a subduction polarity broadly towards the core of East Gondwana.
The Ediacaran–Cambrian witnessed a change in the style of continental breakup and aggregation. In a Hoffman breakup,
continental lithosphere fragments, disperses and reassembles by elimination of the complementary superocean (e.g. the process
by which the Gondwanan elements of Rodinia were transformed into Gondwana). By contrast, in a Wilson cycle sensu stricto,
continental lithosphere simply fragments and reassembles along the same (internal) contacts, closing an internally generated
ocean basin (e.g. the transformation from Pannotia to Pangea). Internally generated ocean basins were opened and closed asymmetrically by rifting of ribbon terranes from the northern margin of Gondwana and their accretion to Laurentia, Baltica andSiberia forming the Caledonides, Variscides and Altaides, as reflected in the Cambrian to Triassic record of metamorphism. Thechange was also registered by multiple geochemical indices, such as epsilon Hf(t) and 87Sr/86Sr, with complex temporal records
characterized by short wavelength variations that reflect the overlapping opening and closure of several major oceans (Iapetus,
Rheic, Paleotethys and Neotethys). This pattern is superimposed on a simpler long wavelength variation that is temporally related
to the supercontinent cycle.

キーワード: ultrahigh-temperature metamorphism, eclogite–high-pressure granulite metamorphism, ultrahigh-pressure metamorphism, geodynamic regimes, supercontinent cycles, Hoffman breakups and Wilson cycles
Keywords: ultrahigh-temperature metamorphism, eclogite–high-pressure granulite metamorphism, ultrahigh-pressure metamor-

1/2

SMP09-01

会場:201A

時間:5 月 27 日 09:00-09:30

phism, geodynamic regimes, supercontinent cycles, Hoffman breakups and Wilson cycles

2/2SMP09-02

会場:201A

時間:5 月 27 日 09:30-09:45

アジア大陸形成期の変成テクトニクス
Tectono-metamorphic evolution during Asian continental growth
小山内 康人 1∗ ; 馬場 壮太郎 1 ; 大和田 正明 2 ; 豊島 剛志 1 ; サティシュクマール エム 3 ; 吉本 紋 1 ;チャルシリ パンニャ 4 ; セレーネン ジャルガラン 5
OSANAI, Yasuhito1∗ ; BABA, Sotaro1 ; OWADA, Masaaki2 ; TOYOSHIMA, Tsuyoshi1 ; SATISH-KUMAR, M3 ;YOSHIMOTO, Aya1 ; CHARUSIRI, Punya4 ; SEREENEN, Jargalan5
1

九州大学, 2 山口大学, 3 新潟大学, 4 チュラロンコーン大学, 5 モンゴル科技大学
Kyushu University, 2 Yamaguchi University, 3 Niigata University, 4 Chulalongkorn University, 5 Mongolian University of Scienceand Technology

1

カンブリア紀以降の微小大陸衝突集合過程で起こった衝突型変成作用の解析から，アジア大陸の形成・進化過程を検討する．
キーワード: アジア大陸, 大陸衝突, 衝突型変成作用, 極限変成作用, U-Pb ジルコン年代
Keywords: Asian continent, micro-continent collision, collision metamorphism, extreme metamorphism, zircon U-Pb age

1/1

SMP09-03

会場:201A

時間:5 月 27 日 09:45-10:00

ベトナム北部・レッドリバー剪断帯に分布する変成ボーキサイト
Metamorphosed bauxites from the Red River Shear zone, northern Vietnam: inferencesand geological significance中野 伸彦 1∗ ; 小山内 康人 1 ; Nam Nguyen Van2 ; Tri Tran Van3
NAKANO, Nobuhiko1∗ ; OSANAI, Yasuhito1 ; NAM, Nguyen van2 ; TRI, Tran van3
1

九州大学比較社会文化研究院地球変動講座, 2 Vietnam Institute of Geosciences and Mineral Resources, 3 Department of
Geology and Minerals, Vietnam
1
Division of Earth Sciences, Faculty of Social and Cultural Studies, Kyushu University, 2 Vietnam Institute of Geosciences andMineral Resources, 3 Department of Geology and Minerals, Vietnam

Aluminous metamorphic rocks from the Red River Shear zone in northern Vietnam were investigated in this study. The
shear zone has abundant pelitic rocks that have been formed under sillimanite-grade metamorphic condition at the Paleogene
period. The aluminous rocks are mainly divided into high-grade (garnet-sillimanite-corundum, garnet-sillimanite-spinel, garnetcorundum-spinel rocks) and low-grade (kyanite-corundum-hematite-phengite rock) rocks and both types show similar whole rockchemical composition with basaltic to andesitic bauxites. Detailed petrographical observations, the high-grade metamorphosed
bauxites have experienced prograde stage more than 2.0 GPa at 800 C (based on the assemblage of staurolite + kyanite + rutile+ siderite in garnet). The low-grade bauxite preserves the peak condition at 500 C and pressure more than 1.2 GPa (based on
the assemblage of hematite + corundum + rutile + phengite + allanite + chloritoid + kyanite in the matrix). Because bauxite
commonly formed at the surface on the continent, such high-pressure metamorphic conditions from the metamorphosed bauxites
suggest continent-continent collision and its subduction.
Zircon grains in garnet-sillimanite-corundum rock show several U-Pb ages from 265 Ma to 36 Ma may due to the loss of Pb
during shearing and/or thermal event at the Paleogene time. However, dark luminescent zircon cores show concordia age of
257 +/- 8 Ma. The zircon cores contain CO2 rich fluid inclusions and the density is similar to other CO2 rich fluid inclusions
trapped in garnet, corundum, and staurolite. Therefore, we conclude that the high-pressure metamorphism has occurred at theLate Permian that should have strong relation to continental collision between the Indochina and South China cratons.
Keywords: metamorphosed bauxite, continental collision, Red River Shear zone, Vietnam

1/1

SMP09-04

会場:201A

時間:5 月 27 日 10:00-10:15

領家変成帯における高温作用の継続時間推定とミグマタイト中でのジルコン/ザクロ
石間の REE 分配
Duration of low-P/T type metamorphism and zircon/garnet REE partitioning in migmatites,
Ryoke metamorphic belt, Japan
河上 哲生 1∗ ; 堀江 憲路 2 ; 外田 智千 2 ; 服部 健太郎 1 ; 平田 岳史 1
KAWAKAMI, Tetsuo1∗ ; HORIE, Kenji2 ; HOKADA, Tomokazu2 ; HATTORI, Kentaro1 ; HIRATA, Takafumi1
11

京都大学理学研究科, 2 国立極地研究所
Graduate School of Science, Kyoto University, 2 NIPR

Duration of low-pressure/high-temperature type, upper amphibolite to granulite facies Ryoke metamorphism is estimated by
SHRIMP zircon U-Pb dating. Zircon in migmatites (up to ca. 800o C, ca. 0.5 GPa) has inherited core and metamorphic rim, andbetween them is a dark-cathodoluminescence (dark-CL) annulus with melt (glass) inclusions (Kawakami et al., 2013). The rim
shows variation in age from ca. 95.5 Ma to 88.7 Ma, suggesting the duration of hypersolidus high-temperature condition for ca.
7 Myr. Zircon rims with old ages tend to show steeply positive HREE patterns whereas the young zircon rims tend to show lesssteeply positive HREE patterns. Ti content in zircon rims are low, ranging from 1.23 to 2.25 ppm. This small variation in Ti
content may imply narrow temperature range of zircon rim formation.
Garnet in the same sample has zoning in trace elements, and REE and Y contents are high in the core and lower towards therim, suggesting prograde growth of this garnet. Garnet shows steeply positive HREE patterns at the core whereas it becomes lesssteep at the rim. Zircon grains with dark-CL annulus with um-sized inclusions are also included in the garnet, and judging fromthe mixed analysis of thin zircon rim and core, these inclusion zircon also has rims with steeply positive HREE patterns. Thissuggests that the inclusion zircons also have the rim that grew during the prograde, melt-present stage of the Ryoke metamorphism, but has the steeply positive HREE patterns even though they likely grew simultaneously with garnet in the presence ofmelt.
Systematic change of REE patterns in garnet and in zircon rim described above suggests that availability of HREE decreased
as zircon grew, because of the simultaneous growth of garnet. In spite of this, DREE (garnet/zircon) does not show flat pattern nor
approach unity for HREE (e.g., Rubatto, 2002), showing different trend from the cases in the UHT rocks. Possible controlling
factors of DREE (garnet/zircon) include (i) difference in pressure and temperature conditions, (ii) difference in garnet composition, especially in grossular content (Taylor et al., 2014), (iii) effect of self-diffusion of garnet trace element compositional
profiles under UHT conditions (Buick et al. 2006), and (iv) different timing of zircon and garnet growth (Buick et al. 2006).
Among the possible controlling factors of the difference of DREE (zircon/garnet) from UHT examples discussed above, (ii) canbe neglected because garnet in this study has low grossular content (0.02-0.03) but showing the steep HREE pattern. Factor(iii) is not likely because our garnet preserves zoning in trace elements and even the high-Y annulus is preserved. Factor (iv)
is also not likely from the above-mentioned observations that support simultaneous growth of zircon and garnet. Therefore, thetemperature condition could be the most likely factor to control DREE (garnet/zircon), although an example of Whitehouse andPlatt (2003) who reported the flat DREE (garnet/zircon) pattern near unity does not fit this interpretation. Our example suggeststhat it can be misleading to judge timing of ’normal’ granulite facies metamorphism solely from the DREE (garnet/zircon) patternon the assumption that DREE (garnet/zircon) becomes unity when garnet and zircon coexisted.
キーワード: 高温変成作用, 部分溶融, ジルコン, ザクロ石, 希土類元素
Keywords: high temperature metamorphism, partial melting, zircon, garnet, rare earth elements

1/1

SMP09-05

会場:201A

時間:5 月 27 日 11:00-11:30

East Antarctica and supercontinent configuration: the Dronning Maud Land perspective
East Antarctica and supercontinent configuration: the Dronning Maud Land perspective
JACOBS, Joachim1∗ ; LAEUFER, Andreas3 ; ELBURG, Marlina4 ; JOKAT, Wilfried5
JACOBS, Joachim1∗ ; LAEUFER, Andreas3 ; ELBURG, Marlina4 ; JOKAT, Wilfried5
1

Department of Earth Science, University of Bergen, PB 7803, N-5020 Bergen, Norway, 2 Norwegian Polar Institute, Fram Centre, N-9296 Tromso, Norway, 3 Bundesanstalt fur Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D-30655 Hannover,Germany, 4 Department of Geology, University of Johannesburg, Auckland Park 2006, Johannesburg, South Africa, 5 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germnay
1
Department of Earth Science, University of Bergen, PB 7803, N-5020 Bergen, Norway, 2 Norwegian Polar Institute, Fram Centre, N-9296 Tromso, Norway, 3 Bundesanstalt fur Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D-30655 Hannover,Germany, 4 Department of Geology, University of Johannesburg, Auckland Park 2006, Johannesburg, South Africa, 5 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germnay
The geology of East Antarctica and its correlation in major supercontinents is highly speculative, since only a very small partof it is exposed. Therefore a better connection between geology and geophysics is needed in order to correlate exposed regionswith ice-covered, geophysically-defined, blocks. In Dronning Maud Land (DML), two distinct late Mesoproterozoic/early Neoproterozoic tectono-metamorphic provinces appear, separated by the major, NE-trending Forster Magnetic Anomaly and South
Orvin Shear Zone. To the west of this lineament, the Maud Belt has clear affinities with Grenville-age continent-continent mobile belts. East of the Forster Magnetic Anomaly, juvenile rocks with early Neoproterozoic age (Rayner-age) and an accretionary
character crop out. The international GEA-II expedition (2012) targeted a white spot on the geological map immediately to the
E of the Forster Magnetic Anomaly. This area allows the characterization and ground-truthing of a large and mostly ice-covered
region, the SE DML Province that had previously been interpreted as an older cratonic block. However, new SHRIMP/SIMS
zircon analyses and their geochemistry indicates that the exposed basement consists of a ca. 1000-900 Ma juvenile terrane thatis very similar to rocks in Sor Rondane. It lacks significant metamorphic overprint at the end of crust formation, but it shows
medium to high-grade overprinting between ca. 630-520 Ma, associated with significant felsic melt production, including A-typegranitoid magmatism. Therefore, the aeromagnetically distinct SE DML province does neither represent the foreland of a Late
Neoproterozoic/EarlyPaleozoic mobile belt, nor a craton, as has previously been speculated. It more likely represents the morejuvenile, westward continuation of Rayner-age crust (1000-900 Ma). To the west it abuts along the NE-trending Forster Magnetic
Anomaly. The latter is interpreted as a suture, which separates typical Grenville-age crust of the Maud Belt (ca. 1200-1030 Ma)
to the W from Rayner-age crust to the E. Therefore the larger eastern part of DML has clearly Indian affinities. Its juvenile
character with a lack of metamorphic overprint at the end of crust formation points to an accretionary history along this part of
the Indian segment of Rodinia, immediately following final Rodinia assembly.

キーワード: Dronning Maud Land, Forster Magnetic Anomaly, supercontinents, juvenile crust, early Neoproterozoic, suturezone
Keywords: Dronning Maud Land, Forster Magnetic Anomaly, supercontinents, juvenile crust, early Neoproterozoic, suture zone

1/1

SMP09-06

会場:201A

時間:5 月 27 日 11:30-11:45

Insights from zircon chronology and chemistry constraints into Neoproterozoic orogens
at Sor Rondane, East Antarctica
Insights from zircon chronology and chemistry constraints into Neoproterozoic orogens
at Sor Rondane, East Antarctica
外田 智千 1∗ ; 堀江 憲路 1 ; 小山内 康人 2 ; 豊島 剛志 3 ; 馬場 壮太郎 4 ; 中野 伸彦 2 ; 足立 達朗 2
HOKADA, Tomokazu1∗ ; HORIE, Kenji1 ; OSANAI, Yasuhito2 ; TOYOSHIMA, Tsuyoshi3 ; BABA, Sotaro4 ;NAKANO, Nobuhiko2 ; ADACHI, Tatsuro2
1
1

National Institute of Polar Research, 2 Kyushu University, 3 Niigata University, 4 University of Ryukyus
National Institute of Polar Research, 2 Kyushu University, 3 Niigata University, 4 University of Ryukyus

Zircon chronology combined with its mineral chemistry is key tools for evaluating the temporal evolution of metamorphic
processes. Assembly of Gondwana supercontinent has been argued in numerous studies. Generally the reported ages of Gondwana collision zones are in the range of 750-500 Ma (e.g., Meert, 2003; Jacobs et al., 2003; Grantham et al., 2003). From East
Antarctica, a number of Neoproterozoic to Cambrian high-grade metamorphic terranes are distributed, and pervasive 550-500
Ma metamorphic ages have been reported from most of these terranes (e.g., Fitzsimons, 2000; Harley, 2003; Shiraishi et al.,2003) with minor >600-550 Ma ages (Shiraishi et al., 2008; Dunkley et al., 2014; Hokada and Motoyoshi, 2006).
Sor Rondane Mountains is one such area where older c.640-600 Ma high-grade metamorphic rocks along with pervasive550-500 Ma age events have been reported (e.g., Shiraishi et al., 2008; Osanai et al., 2013; Adachi et al., 2013; Hokada et al.,2013). Hokada et al. (2013) discussed based on the zircon and monazite U-Th-Pb and REE analyses by using ion microprobe
and electron microprobe applying to garnet-biotite-sillimanite gneiss and associated multiple generations of leucocratic veins in
the central part of Sor Rondane Mountains, and suggested multiple (at least three stages of) metamorphic and fluid (intrusion ofleucocratic vein) events at >700 Ma, 640-630 Ma and 550-520 Ma with main granulite-facies event at c.637+/-6 Ma. Combinedwith the other available and newly obtained age data, temporal processes of possible geologic events recorded in Sor Rondane
Mountains and their implications for supercontinent evolution of Gondwana during c.700-550 Ma will be discussed.
Keywords: Gondwana supercontinent, zircon U-Pb age, rare earth elements, high-grade metamorphism, East Antarctica, Sor
Rondane Mountains

1/1

SMP09-07

会場:201A

時間:5 月 27 日 11:45-12:00

Sr and Nd isotopes in metacarbonate rocks as proxies for paleo-tectonic reconstruction
prior to supercontinent assembly
Sr and Nd isotopes in metacarbonate rocks as proxies for paleo-tectonic reconstruction
prior to supercontinent assembly
SATISH-KUMAR, Madhusoodhan1∗ ; OTSUJI, Naho2 ; KAMEI, Atsushi3 ; TAKAZAWA, Eiichi1 ;TSUCHIYA, Noriyoshi4 ; GRANTHAM, Geoffrey5 ; ISHIKAWA, Masahiro6 ; KAWAKAMI, Tetsuo7 ;OSANAI, Yasuhito8
SATISH-KUMAR, Madhusoodhan1∗ ; OTSUJI, Naho2 ; KAMEI, Atsushi3 ; TAKAZAWA, Eiichi1 ;TSUCHIYA, Noriyoshi4 ; GRANTHAM, Geoffrey5 ; ISHIKAWA, Masahiro6 ; KAWAKAMI, Tetsuo7 ;OSANAI, Yasuhito81Department of Geology, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan, 2 Graduate School of Science and Technology, Niigata University, Japan, 3 Department of Geosciences, Shimane University, Matsue, 690-8504, Japan,
4
Department of Geoscience and Technology, Graduate School of Engineering, Tohoku University, Japan, 5 Council for Geoscience, P/Bag X112, Pretoria, South Africa, 6 Department of Geology and Mineralogy, Kyoto University, Japan, 7 Graduate
School of Environment and Information Sciences, Yokahama National University, Japan, 8 Division of Earth Sciences, Faculty ofSocial and Cultural Studies, Kyushu University, Japan1
Department of Geology, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan, 2 Graduate School of Science and Technology, Niigata University, Japan, 3 Department of Geosciences, Shimane University, Matsue, 690-8504, Japan,
4
Department of Geoscience and Technology, Graduate School of Engineering, Tohoku University, Japan, 5 Council for Geoscience, P/Bag X112, Pretoria, South Africa, 6 Department of Geology and Mineralogy, Kyoto University, Japan, 7 Graduate
School of Environment and Information Sciences, Yokahama National University, Japan, 8 Division of Earth Sciences, Faculty ofSocial and Cultural Studies, Kyushu University, Japan
Chemically precipitated carbonate sediments directly record seawater composition that helps us to decode the Earths paleoenvironment, existence of paleo-oceans and provide valuable clues on paleo-tectonic position of continents in the Earths history.
In addition, the geochemical and isotopic composition of carbonate rocks also have a strong dependence on the depositional tectonic setting and surrounding source rock composition especially in the Precambrian era, during which biological activity wereless prominent and vegetation was virtually absent. Here we present evidence for the existence of an Oceanic Island arc systemand peripheral Oceans before the formation of Gondwana supercontinent in the Neoproterozoic. Applying a multi-element isotope geochemical approach on chemostratigraphically well-constrained metacarbonate rocks collected from several supracrustal
terrains in Gondwana, including the remote Sor Rondane Mountains in East Antarctica, we model carbonate deposition surrounding an island arc system, mid-ocean volcanic islands and shallow marine continental shelf of a yet unidentified interior
Antarctic continent, all of which accreted and amalgamated in the late Neoproterozoic to early Paleozoic to form the Gondwana
supercontinent.
We also compare the metacarbonate data with basement rocks from various neighboring Gondwana continents, wherein someregional affinities could be established. Nd model ages of cratonic basements from East Africa (Kalahari) and India (Dharwar) is
obviously different from Balchen carbonate rocks from Sor Rondane in East Antarctica, and the possible existence of a cratonic
nucleus within the East Antractic continent is speculated, surrounding which the carbonate rocks of the Balchen region might
have deposited. Thus, the Sr and Nd isotopic compositions provide important information about depositional setting of sedimentary rocks and relationship with surrounding basement and continents. Geochemical proxies, such as Nd and Sr isotopes ofmetacarbonate rocks, can yield key information not only of paleo-oceans but also about the surrounding rocks of oceanic crust(basement) and continental crust during the time of deposition, which can lead us to a better understanding of paleo-tectonic
setting of crustal fragments that assemble to form supercontinents.
キーワード: Metacarbonate rocks, Gondwana, Sr isotopes, Nd isotopes, tectonic setting
Keywords: Metacarbonate rocks, Gondwana, Sr isotopes, Nd isotopes, tectonic setting

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SMP09-08

会場:201A

時間:5 月 27 日 12:00-12:15

Comparison of Sr-Nd isotope data from N. Mozambique and Dronning Maud Land and
Sor Rondane, Antarctica.
Comparison of Sr-Nd isotope data from N. Mozambique and Dronning Maud Land and
Sor Rondane, Antarctica.
GRANTHAM, Geoffrey1∗ ; SATISH-KUMAR, Madhusoodhan2 ; OTSUJI, Naho3 ; BURGER, Erasmus4 ;KAWAKAMI, Tetsuo5 ; ISHIKAWA, Masahiro6 ; TSUCHIYA, Noriyosh7 ; LE ROUX, Petrus8
GRANTHAM, Geoffrey1∗ ; SATISH-KUMAR, Madhusoodhan2 ; OTSUJI, Naho3 ; BURGER, Erasmus4 ;KAWAKAMI, Tetsuo5 ; ISHIKAWA, Masahiro6 ; TSUCHIYA, Noriyosh7 ; LE ROUX, Petrus8
1

Department of Geology, University of Johannesburg, South Africa., 2 Department of Geology, Niigata University, Japan, 3 Graduate
School of Science and Technology, Niigata University, Japan, 4 Department of Geology, University of Pretoria, South Africa,
5
Department of Geology and Mineralogy, Kyoto University, Japan, 6 Graduate School of Environment and Information Sciences,
Yokahama National University, Japan, 7 Department of Geoscience and Technology, Graduate School of Engineering, Tohoku
University, Japan, 8 Department of Geological Science, University of Cape Town, South Africa
1
Department of Geology, University of Johannesburg, South Africa., 2 Department of Geology, Niigata University, Japan, 3 Graduate
School of Science and Technology, Niigata University, Japan, 4 Department of Geology, University of Pretoria, South Africa,
5
Department of Geology and Mineralogy, Kyoto University, Japan, 6 Graduate School of Environment and Information Sciences,
Yokahama National University, Japan, 7 Department of Geoscience and Technology, Graduate School of Engineering, Tohoku
University, Japan, 8 Department of Geological Science, University of Cape Town, South Africa
A recently proposed mega-nappe model for the Neoproterozoic?Cambrian-age Kuunga Orogeny involves collision between
N. and S. Gondwana. S.Gondwana is inferred to have comprised southern Africa (consisting of the Kalahari Craton and partsof adjacent metamorphic belts including the Barue and Nampula complexes of the Mozambique Belt), western Dronning MaudLand (WDML), Antarctica (consisting of the Grunehogna Craton and the Maud Belt) and Sri Lanka (consisting of the Vijayan
Complex). N. Gondwana is inferred to have comprised parts of south central Africa, Sri Lanka, Madagascar and India (consistingof the Tanzanian Craton and parts of adjacent metamorphic belts including the Xixano Complex of Cabo Degado Complex, theHighlands and Wanni Complexes, the Central Dronning Maud Land and Sor Rondane areas and Lutzo Holm Bukta areas.
Differences in published geochronological data from the metamorphic belts of the various areas are fundamental to defining
the various components of the mega-nappe model. Comparison of published and unpublished Sr and Nd radiogenic isotope data,
calculated at 500Ma from the metamorphic belt basement gneisses of the mega-nappe component areas, show broad differences
between the different areas from N and S Gondwana.
Neoproterozoic to Cambrian-age granitoids which intrude the various areas mostly mirror their host country rocks suggesting
localised anatexis without significant juvenile input. Sr and Nd isotope data from some of these intrusions suggest that they weresourced in the footwall but intrude the hanging wall components of the mega-nappe. The Sr-Nd data from the N Gondwana
correlated areas dominantly show marginally negative, less evolved ²Nd characteristics but similar, positive, wide ranged ²Srcharacteristics compared to the S. Gondwana correlated areas.
Comparison of the Sr-Nd isotopic provinces show broad similarities with various geophysical domains defined by recently
published aeromagnetic and gravity data sets from Antarctica.
The data are evaluated in terms of their implications for the meg-nappe model for the Kuunga Orogeny.
キーワード: N. and S. Gondwana, Sr and Nd isotopes, mega-nappe model, Kuunga Orogeny
Keywords: N. and S. Gondwana, Sr and Nd isotopes, mega-nappe model, Kuunga Orogeny

1/1

SMP09-09

会場:201A

時間:5 月 27 日 12:15-12:30

Mesoproterozoic suture between India and Madagascar
Mesoproterozoic suture between India and Madagascar
SAJEEV, Krishnan1∗ ; ISHWAR-KUMAR, C1 ; RATHEESH-KUMAR, R.t.1 ; SATISH-KUMAR, M.2
SAJEEV, Krishnan1∗ ; ISHWAR-KUMAR, C1 ; RATHEESH-KUMAR, R.t.1 ; SATISH-KUMAR, M.2
1

Centre for Earth Sciences, Indian Institute of Science, Bangalore, India, 2 Department of Geology, Faculty of Science, Niigata
University, Niigata, Japan
1
Centre for Earth Sciences, Indian Institute of Science, Bangalore, India, 2 Department of Geology, Faculty of Science, Niigata
University, Niigata, JapanThe understanding of position of southern India in various supercontinent assemblies, its amalgamation and rifting with various
continental fragments have significant implication in understanding the tectonic processes through geological time-scale. The
southern India is a mosaic of various crustal domains, which are divided by shear/suture zones, but several controversies exist
regarding the tectonic framework, shear zones and crustal blocks of southern India. The detailed structural lineament mapping ofsouthern India along with new and compiled of geological and geochronological datasets help in delineating new shear zones andredefining the Precambrian crustal blocks of southern India. Especially, in a Madagascar- India correlation point of view, recent
studies have reconstructed plate margins of India and Madagascar based on flexural isostasy along the Western Continental Margin of India (WCMI) and the Eastern Continental Margin of Madagascar. In this context, the newly proposed Mesoproterozoic
Kumta and Mercara suture zones (1400-1000 Ma) welding Archean crustal blocks in western India offer critical insights into
Precambrian continental juxtapositions and the crustal evolution of Gondwana. The textural evidences, mineral chemistry andthermodynamic modeling of quartz-phengite schist, garnet-biotite schist from the Kumta suture suggest peak metamorphic P-T
conditions of c. 18 kbar, 550o C and c. 11 kbar, 790o C respectively and garnet-biotite-kyanite-gedrite-cordierite gneiss from the
Mercara suture suggests peak metamorphic P-T conditions of c. 12.5 kbar, 825o C. The calc-silicate granulite and mafic granulite were re-equilibrated under high-pressure conditions of 15-20 kbar at a temperature of 800-900o C. The Bondla-ultramafic
complex in the northwest of Kumta suture contains shale, basalt, dolerite, gabbro, chromite bearing serpentinite, chromitite andperidotite. The chromite chemistry from ultramafics suggests the evolution in a supra-subduction zone arc setting. Towards theeast of the Kumta suture, the Sirsi shelf contains weakly deformed sedimentary rocks (limestone, shale, banded iron formations,
greywacke, sandstone and quartzite) unconformable on high-grade ca.2571 Ma gneisses of the Dharwar craton. The Karwar
block to the west of the Kumta suture is mainly composed of weakly deformed tonalite-trondhjemite-granodiorite (TTG) withenclaves of amphibolite. Whole-rock major and trace element data suggest that the TTGs (Type I, low K2O, high Na2O, Sr) were
derived from a volcanic arc, and that the TTGs (Type II, high K2 O, low Na2 O, Sr) have within-plate signatures. Amphibolites
have a chemical composition comparable to basalts to basaltic andesites with MORB signatures. The Karwar block TTGs (TypeI) are ca. 3200 Ma with ?Hf range of -0.7 to 4.4. The whole-rock ?Nd ranges from -2.4-2.1 representing juvenile crustal origin.
The Coorg block, about 100 km south of Karwar block mainly consists of highly metamorphosed lower-crustal rocks yielding
3200 Ma age with positive to negative ?Hf spread (3.3. to -3.2) indicating their source as mixture of juvenile and recycled crustal
materials. Metasedimentary rocks from the Kumta suture have ?Hf (t) values that range from -9.2-5.6, and TDM ages that range
from 2747-3546 Ma; comparable values in metasedimentary rocks from the Mercara suture range are from -18.9-4.2 and from3214-3647 Ma respectively. Synthesis of the above results suggests that the Kumta and Mercara suture zones incorporate sediments, which range in age from Paleoarchean to Mesoproterozoic, and were subjected to high-pressure metamorphism in thelate Mesoproterozoic. The protolith sediments were mainly derived from juvenile crust that was mixed with products of recycled
older continental crust. Integration of the results indicates the Mesoproterozoic Kumta-Mercara suture in western Peninsular
India interpreted as eastern extension of the Betsimisaraka suture of Madagascar.

キーワード: Gondwana Supercontinent, Continental Correlation, Crustal evolution, Mesoproterozoic suture
Keywords: Gondwana Supercontinent, Continental Correlation, Crustal evolution, Mesoproterozoic suture

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SMP09-10

会場:201A

時間:5 月 27 日 12:30-12:45

A comprehensive study on India-Madagascar paleo-fit in the Gondwana Supercontinent
A comprehensive study on India-Madagascar paleo-fit in the Gondwana Supercontinent
RATHEESH-KUMAR, R. T.1∗
RATHEESH-KUMAR, R. T.1∗
1
1

Centre for Earth Sciences, Indian Institute of Science, Bangalore -560012, India
Centre for Earth Sciences, Indian Institute of Science, Bangalore -560012, India

This study addresses one of the most disputed problems in the history of the earth science, which is on the paleo-fit of India andMadagascar continents in the Gondwana Super Continent Assembly. The study contributes new constraints on, and definitionsof, the reconstructed plate margins of India and Madagascar based on flexural isostasy along the western continental margin ofIndia (WCMI) and the eastern continental margin of Madagascar (ECMM). We have estimated the nature of isostasy and crustal
geometry along the two margins, and have examined their possible conjugate structure. Here we utilize elastic thickness (Te) andMoho depth data as the primary basis for the correlation of these passive margins. We employ the flexure inversion technique that
operates in spatial domain in order to estimate the spatial variation of effective elastic thickness. Gravity inversion and flexure
inversion techniques are used to estimate the configuration of the Moho/Crust-Mantle Interface that reveals regional correlations
with the Te variations. These results correlate well with the continental and oceanic segments of the Indian and African plates.
The present study has found a linear zone (˜1680 km) of anomalously low-Te (1-5 km) along the WCMI, which correlates wellwith the low-Te patterns obtained all along the ECMM. We suggest that the low-Te zones along the WCMI and ECMM represent
paleo-welding zones of lithosphere thermally and mechanically weakened by the combined effects of the Marion hotspot andlithospheric extension due to rifting. Based on the present Te results, we have produced a precise India-Madagascar assembly
during the time of their rifting (see Figure below), which is confirmed by the Moho geometry and the bathymetry of the conjugate
shelf margins, and by the matching of tectonic lineaments, lithologies and geochronological belts between India and Madagascar.
Figure Caption: Elastic thickness structure of the western margin of India and eastern margin of Madagascar (left), correlationof similar Te zones of India-Madagascar conjugate margins (centre), and the paleo-fit of India and Madagascar deduced from the
Te correlations justified by the fit of shear zones (Ratheesh Kumar et al., Gondwana Research, 2014).

キーワード: Gondwana, Paleo-fit, elastic thickness
Keywords: Gondwana, Paleo-fit, elastic thickness

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