- Email: [email protected]
Field Occurrence of High- and Ultrahigh-Temperature Metamorphic Rocks and Related Igneous Rocks from the Kontum Massif, Central Vietnam
Gondwana Research (Gondwana Newsletter Section) K 4, No. 2, p p . 236-241. 02001 lnternational Association for Gondwana Research, Japan.
GNL
CORRESPONDENCE
Field Occurrence of High- and Ultrahigh-Temperature Metamorphic Rocks and Related Igneous Rocks from the Konturn Massif, Central Vietnam T. Tsunogael, Y. Osanai2,M. Owada3,T. Toyoshima4,T. Hokada5 and T.V. Long6 Faculty of Education, Shimane University, Nishikawatsu 1060, Matsue 690-8504, Japan Department of Earth Sciences, Okayama University, Tsushima-naka,3-ckome, Okayama 700-8530,Japan Graduate School of Science and Technology,Niigata University, Ikaraski, Niigata 950-2181, Japan Department of Earth Sciences, Yamaguchi University, Yoskida 1677-1, Yamagucki 753-8512,Japan National Institute of Polar Research, Kaga 1-ckome, Itabaski-ku, Tokyo 173-8515, Japan South Vietnam Mapping Union, 200 Ly Chink Thang, Ho Chi Mink, Vietnam
Introduction The Kontum Massif of central Vietnam is the largest continuous metamorphic and igneous complex of the Indochina craton. Previous work, carried out by Vietnamese and French geologists, has shown that the metamorphic rocks of the Kontum Massif can be subdivided into two groups on the basis of metamorphic grade: the Kan Nack Complex and the Ngoc Linh Complex (e.g., D G W , 1998a-f; and references therein). The Kan Nack and Ngoc Linh Complexes have been traditionally regarded as Archean and Proterozoic metamorphic complexes respectively, and are characterized by the presence of granulite-facies mineral assemblages in the former and amphibolite facies assemblages in the latter. This outdated classification has been reconsidered owing to recent geochronological data. Nam (1998) obtained K-Ar age of 241 to 255 Ma for biotite and hornblende in cordierite-sillimanitegneiss and amphibolite from the Kan Nack Complex. SHRIMP zircon ages of ca. 254 Ma from the complex is also similar to the K-Ar age (Nam et al., 2000). These ages, which correspond to the time of Triassic Indosinian orogeny, are significantly younger than available Pb isochron age of 2300 Ma from the Ngoc Linh Complex and K-Ar age of 1650-1810 Ma from the Kan Nack Complex reported by Hutchison (1989) as a personal communication with Y.G. Gatinsky. The Triassic age is regarded to represent the time of cooling. Available petrological and geochemical studies in conjunction with geochronological data are therefore not enough for
evaluation of the thermal and tectonic history of the Kontum Massif. In April 2000, we made a field geological survey covering the Kontum Massif, supported by IGCP-368. The main objective was to study the metamorphic and magmatic history of the Kontum Massif during amalgamation processes of continents in East Asia. This paper briefly introduces the results from our field survey and presents some new petrological data on hightemperature and ultrahigh-temperature rnetamorphlsm and related magmatism of the Kontum Massif.
Field Observations Field observations and sample collection were done from the so-called ‘Archean” Kan Nack Complex and “Proterozoic” Ngoc Linh Complex. We also made a geological survey of Hai Van Pass Granite Complex, which is located about 15 km northwest of Danang (Fig. 1). The Kan Nack Complex occurs in the southeastern part of the Kontum Massif (Figs. 1 and 2a). Our field traverse followed the Ba River, north of Kan Nack, and around the area north of Qui Nhon. The complex is composed of four stratigraphic units: Kon Cot, Xa Lam Co, Dak Lo, and Kim Son Formations. They are composed mainly of pelitic and granitic gneisses with minor mafic and calcareous gneisses. Field occurrence of garnet-cordierite gneiss of the Kon Cot Formation is shown in Figs. 2b and 2c. In the area north of Kan Nack, garnet in pelitic gneiss is rimmed by bluish cordierite, suggesting the decompressionprocess
237
Fig. 1. Location and simplified geological map of the Kontum Massif, central Vietnam. Gorzdwaria Rrsearih, V. 4, No. 2, 2001
238
Fig. 2. I;icld photographs ol metamorphic and igneous rocks of the Knn Nack Complcx in thc Kontum Massif, central Vietnam. (a) A view of highgradc metamorphic rocks of Kaii Nack Complex around the arca north of Qui Nhon. (11) Outcrop of garnet-cordierite gneiss of the Dak Lo Formation and garnet-cordieritc-silliiiianitc granitc of the Plei Man I
after peak metamorphism (Fig. 2c). Evidence for ultrahigh-temperature metamorphism were recorded for the first time from the pelitic gneisses of the area (Osanai
e t al., 2000a, b ) . Mafic a n d calcareous gneisses occur as lenses or layers enclosed in pelitic and granitic gneisses (Fig. 2d). The mafic gneiss is occasionally Gorrdiuilrlu 12rwurch, V 4, No 2, 2001
239
garnetiferous. Garuct porphyrohlasts up to 4 cm in size occur in the amphibolites of the area north of Qui Nhon (Fig. 2e). The Kan Nack Complex is intruded by several igneous plutons. The Plei Man Ko Complex is one of the most
widely exposed igneous bodies in Kan Nack area. It is composed of g r a n i t e g r o u p ( g a r n e t g r a n i t e a n d orthopyroxene granite) and gabbro-norite group (Owada et al., 2000). The field relationship between garnet granite and gabbro-norite group is shown in Fig. 2f.
Fig. 3. Field photographs of metamorphic and igneous rocks of the Ngoc Linh Complex in the Kontum Massif and Hai Van Pass Granite Complex, central Vietnam. (a) Migmatitic texture of biotite gneiss and leucocratic gneiss of the Son Re Formation, west of Quang Ngai (locality 041003). (b) Layered structure of biotite-sillimanite gneiss and lcucocratic gneiss of the Tac Po Formation, north o i Kon Tum (locality 041401). (c) Layered metagabbro of the Tac Po Formation, north of Koii Tun (locality 041301). (d) Orthopyroxene-bearing leucocratic pod in orthopyroxene-biotite pelitic gneiss of the Tac Po Formation, north of Kon Tun1 (locality 041401). The texture suggests an evidence of partial melting at granulite-facies condition. (e) Porphyritic gneissose granite of Silurian Dien Binh Complex, north of Kon Turn (locality 041303). (f) Cordierite-garnet-biotite tonalite of Ilai Van Pass Granite Complex, approximately 15 ltm northwest of Danang (locality 042201) which contains large cordierite crystals of up to 4 cm in diameter. Gonduiana J
240
The Ngoc Linh Complex is mainly distributed in the northern and western part of the Kontum Massif (Fig. 1). The following description is based on field survey in the areas around Quang Ngai and Kon Turn. The complex is mainly composed of Son Re and Tac Po Formations. Biotite gneiss and hornblende-biotite gneiss are the major lithological formations. Most of them suffered significant effects of partial melting and migmatisation (Fig. 3a). Pelitic and mafic gneisses are also present, although not abundant (Figs. 3b and 3c). Euhedral orthopyroxenebearing leucocratic pods occur in orthopyroxene-biotite pelitic gneiss in the area north of Kon Tum ( Fig. 3d). The texture is suggestive of partial melting at granulite-facies conditions. Although the Ngoc Linh Complex has been regarded as an amphibolite-facies complex, our new petrological data indicate that maximum temperature of the complex is probably higher than 800°C. The Ngoc Linh Complex is intruded by several magmatic bodies ranging in age from Proterozoic to Triassic. Tu Mo Rong Complex (migmatitic gneissose granite) and Phu My Complex (gabbroic amphibolite) are major Proterozoic igneous rocks in the area. Fig. 3e shows a porphyritic gneissose granite of Silurian Dien Binh Complex. It is characterized by an oriented distribution of large K-feldspar phenocrysts owing to deformation in the early Triassic. The Hai Van Pass Granite Complex is composed mainly of tonalite and biotite gneiss (leucocratic and melanocratic) bearing cordierite. Cordierite grains in the tonalitic gneiss range upto 4cm in size (Fig. 30. Although the complex is located about 70 km north of the area, the magmatic activity in this region could be associated with that of the Kontum Massif.
N e w Results 1.From mineral thermobarometric studies, we obtained ultrahigh-temperature metamorphic conditions of T= 1000-1050°C on garnet-orthopyroxene-sillimanitequartz assemblage in pelitic gneiss of the Dak Lo Formation, north of Kan Nack (Osanai et al., 2000a, b). This is considerably higher than the previously reported values (80O-85O0C; DGW, 1998d). 21, CHIME dating of monazite from cordierite-sillimanite gneisqelded an age of 248+16 Ma (Osanai et al., 2000a, b) which is consistent with K-Ar and SHRIMP zircon ages of Nam (1998) and Nam et al. (2000). 3. At least three different magmatic events were identified from the Plei Man KO Complex on the basis of petrographical and geochemical data (Owada et al., 2000). The interaction texture of two melt phases of garnet granite and gabbro-norite group (Fig. 2f) may indicate that felsic and basic magmatism took place at
the same stage. Owada et al. (2000) also proposed a model of partial melting of crustal materials (such as the pelitic gneiss) to form the garnet granite. 4. Metamorphic temperatures of the Kan Nack Complex from the area north of Qui Nhon were estimated to be about 800°C from garnet-bearing amphibolites (Tsunogae et al., 2000). This estimate is lower than the maximum temperature estimated from the complex around Kan Nack area (Osanai et al., 2000b). Our ongoing studies are expected to provide critical information on the metamorphic and magmatic history of the Kontum Massif and its role in the evolution of East Asia.
References Department of Geology and Minerals of Vietnam (DGMV) ( 1 9 8 a ) Geology and mineral resources of Dak To sheet (D48-XII). Geology and mineral resources map of Vietnam (1: 200,000), Hanoi. Department of Geology and Minerals of Vietnam (DGMV) (1998b) Geology and mineral resources of Kon lbm sheet (D-48-XVIII).Geology and mineral resources map of Vietnam (1: 200,000), Hanoi. Department of Geology and Minerals of Vietnam (DGMV) (1998~)Geology and mineral resources of Quang Ngai sheet (D-49-VII).Geology and mineral resources map of Vietnam (1: 200,000), Hanoi. Department of Geology and Minerals of Vietnam (DGMV) (1998d) Geology and mineral resources of Mang Den - Bong Son sheet (D-49-XII1, D49-XIV). Geology and mineral resources map of Vietnam (1: 200,000), Hanoi. Department of Geology and Minerals of Vietnam (DGMV) (1998e) Geology and mineral resources of An Khe sheet (D49-XIX). Geology and mineral resources map ofVietnam (1: 200,000), Hanoi. Department of Geology and Minerals of Vietnam (D,GMV) (19980 Geology and mineral resources of Qui Nhon sheet (D-49-XX). Geology and mineral resources map of Vietnam (1:200,000), Hanoi. Hutchison, C.S. (1989) Geological evolution of south-east Asia. Oxford Sci. Publ., 36813. Nam, T.N. (1998) Thermotectonic events from early Proterozoic to Miocene in the Indochina craton: implication of K-Ar ages in Vietnam. J. Asian Earth Sci., v. 16, pp. 475-484. Nam, T.N., Toriumi, M., Itaya, T., Okada, T., Long, T.V.,and Kiem, D.D. (2000) Tectonothermal events in Indochinese crystalline basement: evidence from geochronological data. WPGM 2000, Abstr. v., p.190. Osanai, Y., Owada, M., Tsunogae, T., Toyoshima, T., Hokada, T. and Long, T.V. (2000a) High- and ultrahigh temperature metamorphic rocks from the Kontum Massif in central Vietnam. The 107th Annual Meeting of the Geol. SOC.Japan, Abstr. v., p. 300. Osanai, Y., Owada, M., Tsunogae, T., Toyoshima, T., Hokada, T., and Long, T.V. (2000b) UHT pelitic granulites from Kontum Massif, central Vietnam. 1999 Annual Meeting of the Japanese Association of Mineralogists, Petrologists, and Economic Geologists, Abstr. v., p. 36. Gondwana Research, V. 4, No. 2,2001
241
Owada, M., Osanai, Y., Tsunogae, T., Toyoshima, T., Hokada, T, and Long, T.V. (2000) Geochemistry of Plei Man KO Complex in Kontum Massif, central Vietnam. 1999 Annual Meeting of the Japanese Association of Mineralogists, Petrologists, and Economic Geologists, Abstr. v., p. 63.
Tsunogae, T., Osanai, Y., Owada, M., Toyoshima, T., and Long, T.V. (2000) High-temperature metamorphism of metabasites from Kontum Massif, central Vietnam. 1999Annual Meeting of the Japanese Association of Mineralogists, Petrologists, and Economic Geologists, Abstr. v., p. 37.
Gondwana Research (Gondwana Newsletter Section) V 4, No. 2, p p . 241-243. 02001 International Association for Gondwana Research, Japan.
GNL
CORRESPONDENCE
Current Status on the Study of Supercontinent Tectonics M. Yoshidal, M. Santoshl and C.B. Dissanayake* Department of Geosciences, Faculty of Science, Osaka City University, Sugimoto 3-3-138, Osaka 558-8585,Japan, E-mail: [email protected] Department of Geology, Peradeniya University, Peradeniya, Sri Lanka
Studies of Gondwanaland and supercontinent tectonics have made important progress during past several years. Below given is a brief summary of development in the field of the above studies, mostly referring publications related to the IGCP-368 activity.
Rodinia and Gondwanaland Tectonics The episodic distribution of isotopic ages in continental crust at 2.7, 1.9 and 1.2 Ga were interpreted to reflect super event cycles, which lasts 600-800 Ma. In a recent postulation, supercontinental break ups during the earth’s history a t 2.3-2.1 Ga, 700-600 Ma and 160 Ma are proposed to have triggered mantle avalanche resulting in enhanced production of juvenile crust (Condie, 1999, GR2). The post-Rodinia breakup of the protoGondwanaland and its interactions with Baltica and protoEuropean terranes were also topics of recent interest (Erdtmann, 1999, GR2). Attempts were made to define the geological characteristics of the suture zone between West and East Gondwana (Muhongo, 1999, GR 2). The Neoproterozoic high P-T regional granulite terranes which form the most significant part of this megasuture are considered to be the products of crustal thickening that occurred when the Mozambique Ocean closed during the Neoproterozoic continent-continent collision of the protoGondwana fragments. The reconstruction of Rodinia was re-evaluated in the light of the SWEAT hypothesis (Dalziel,
2000, IGC) and the geologic connection between southwestern United States and East Antarctica was held as one of the strongest lines of evidence linking Laurentia and the present continents in a supercontinent which assembled at the close of the Mesoproterozoic. However, severe critics including alternatives to this model also exist and are increasing. The position of Kalahari Craton within the Rodinia ensemble has been enigmatic. Although Dalziel still prefers (2000, IGC) his revised idea (1997, GSA Bull., 109) that the Kalahari is far apart from Antarctica, Powell recently (2000, IGC) insisted that it should have juxtaposed with the western Dronning Maud Land as Dalziel initially proposed. A detailed analysis of the Early Paleozoic evolution of NW Gondwanaland was recently presented with data from southern Turkey and surrounding regions (Goncuoglu and Kozlu, 2000, GR3). The study traced the closure of a Palaeotethyan oceanic basin to the north of the periGondwana terranes. The global crustal tectonics during ca. 800 to 500 Ma and their relationship to global environmental changes has recently emerged into focus (Yoshida and Arima, 2000, Japan Contribution to IGCP) . While the existence of the Neoproterozoicsupercontinent Rodinia is still hypothetical, the proposal of the ca. 550 Ma supercontinent termed Pannotia has emerged as yet another unsolved mystery.
GNL
CORRESPONDENCE
Field Occurrence of High- and Ultrahigh-Temperature Metamorphic Rocks and Related Igneous Rocks from the Konturn Massif, Central Vietnam T. Tsunogael, Y. Osanai2,M. Owada3,T. Toyoshima4,T. Hokada5 and T.V. Long6 Faculty of Education, Shimane University, Nishikawatsu 1060, Matsue 690-8504, Japan Department of Earth Sciences, Okayama University, Tsushima-naka,3-ckome, Okayama 700-8530,Japan Graduate School of Science and Technology,Niigata University, Ikaraski, Niigata 950-2181, Japan Department of Earth Sciences, Yamaguchi University, Yoskida 1677-1, Yamagucki 753-8512,Japan National Institute of Polar Research, Kaga 1-ckome, Itabaski-ku, Tokyo 173-8515, Japan South Vietnam Mapping Union, 200 Ly Chink Thang, Ho Chi Mink, Vietnam
Introduction The Kontum Massif of central Vietnam is the largest continuous metamorphic and igneous complex of the Indochina craton. Previous work, carried out by Vietnamese and French geologists, has shown that the metamorphic rocks of the Kontum Massif can be subdivided into two groups on the basis of metamorphic grade: the Kan Nack Complex and the Ngoc Linh Complex (e.g., D G W , 1998a-f; and references therein). The Kan Nack and Ngoc Linh Complexes have been traditionally regarded as Archean and Proterozoic metamorphic complexes respectively, and are characterized by the presence of granulite-facies mineral assemblages in the former and amphibolite facies assemblages in the latter. This outdated classification has been reconsidered owing to recent geochronological data. Nam (1998) obtained K-Ar age of 241 to 255 Ma for biotite and hornblende in cordierite-sillimanitegneiss and amphibolite from the Kan Nack Complex. SHRIMP zircon ages of ca. 254 Ma from the complex is also similar to the K-Ar age (Nam et al., 2000). These ages, which correspond to the time of Triassic Indosinian orogeny, are significantly younger than available Pb isochron age of 2300 Ma from the Ngoc Linh Complex and K-Ar age of 1650-1810 Ma from the Kan Nack Complex reported by Hutchison (1989) as a personal communication with Y.G. Gatinsky. The Triassic age is regarded to represent the time of cooling. Available petrological and geochemical studies in conjunction with geochronological data are therefore not enough for
evaluation of the thermal and tectonic history of the Kontum Massif. In April 2000, we made a field geological survey covering the Kontum Massif, supported by IGCP-368. The main objective was to study the metamorphic and magmatic history of the Kontum Massif during amalgamation processes of continents in East Asia. This paper briefly introduces the results from our field survey and presents some new petrological data on hightemperature and ultrahigh-temperature rnetamorphlsm and related magmatism of the Kontum Massif.
Field Observations Field observations and sample collection were done from the so-called ‘Archean” Kan Nack Complex and “Proterozoic” Ngoc Linh Complex. We also made a geological survey of Hai Van Pass Granite Complex, which is located about 15 km northwest of Danang (Fig. 1). The Kan Nack Complex occurs in the southeastern part of the Kontum Massif (Figs. 1 and 2a). Our field traverse followed the Ba River, north of Kan Nack, and around the area north of Qui Nhon. The complex is composed of four stratigraphic units: Kon Cot, Xa Lam Co, Dak Lo, and Kim Son Formations. They are composed mainly of pelitic and granitic gneisses with minor mafic and calcareous gneisses. Field occurrence of garnet-cordierite gneiss of the Kon Cot Formation is shown in Figs. 2b and 2c. In the area north of Kan Nack, garnet in pelitic gneiss is rimmed by bluish cordierite, suggesting the decompressionprocess
237
Fig. 1. Location and simplified geological map of the Kontum Massif, central Vietnam. Gorzdwaria Rrsearih, V. 4, No. 2, 2001
238
Fig. 2. I;icld photographs ol metamorphic and igneous rocks of the Knn Nack Complcx in thc Kontum Massif, central Vietnam. (a) A view of highgradc metamorphic rocks of Kaii Nack Complex around the arca north of Qui Nhon. (11) Outcrop of garnet-cordierite gneiss of the Dak Lo Formation and garnet-cordieritc-silliiiianitc granitc of the Plei Man I
after peak metamorphism (Fig. 2c). Evidence for ultrahigh-temperature metamorphism were recorded for the first time from the pelitic gneisses of the area (Osanai
e t al., 2000a, b ) . Mafic a n d calcareous gneisses occur as lenses or layers enclosed in pelitic and granitic gneisses (Fig. 2d). The mafic gneiss is occasionally Gorrdiuilrlu 12rwurch, V 4, No 2, 2001
239
garnetiferous. Garuct porphyrohlasts up to 4 cm in size occur in the amphibolites of the area north of Qui Nhon (Fig. 2e). The Kan Nack Complex is intruded by several igneous plutons. The Plei Man Ko Complex is one of the most
widely exposed igneous bodies in Kan Nack area. It is composed of g r a n i t e g r o u p ( g a r n e t g r a n i t e a n d orthopyroxene granite) and gabbro-norite group (Owada et al., 2000). The field relationship between garnet granite and gabbro-norite group is shown in Fig. 2f.
Fig. 3. Field photographs of metamorphic and igneous rocks of the Ngoc Linh Complex in the Kontum Massif and Hai Van Pass Granite Complex, central Vietnam. (a) Migmatitic texture of biotite gneiss and leucocratic gneiss of the Son Re Formation, west of Quang Ngai (locality 041003). (b) Layered structure of biotite-sillimanite gneiss and lcucocratic gneiss of the Tac Po Formation, north o i Kon Tum (locality 041401). (c) Layered metagabbro of the Tac Po Formation, north of Koii Tun (locality 041301). (d) Orthopyroxene-bearing leucocratic pod in orthopyroxene-biotite pelitic gneiss of the Tac Po Formation, north of Kon Tun1 (locality 041401). The texture suggests an evidence of partial melting at granulite-facies condition. (e) Porphyritic gneissose granite of Silurian Dien Binh Complex, north of Kon Turn (locality 041303). (f) Cordierite-garnet-biotite tonalite of Ilai Van Pass Granite Complex, approximately 15 ltm northwest of Danang (locality 042201) which contains large cordierite crystals of up to 4 cm in diameter. Gonduiana J
240
The Ngoc Linh Complex is mainly distributed in the northern and western part of the Kontum Massif (Fig. 1). The following description is based on field survey in the areas around Quang Ngai and Kon Turn. The complex is mainly composed of Son Re and Tac Po Formations. Biotite gneiss and hornblende-biotite gneiss are the major lithological formations. Most of them suffered significant effects of partial melting and migmatisation (Fig. 3a). Pelitic and mafic gneisses are also present, although not abundant (Figs. 3b and 3c). Euhedral orthopyroxenebearing leucocratic pods occur in orthopyroxene-biotite pelitic gneiss in the area north of Kon Tum ( Fig. 3d). The texture is suggestive of partial melting at granulite-facies conditions. Although the Ngoc Linh Complex has been regarded as an amphibolite-facies complex, our new petrological data indicate that maximum temperature of the complex is probably higher than 800°C. The Ngoc Linh Complex is intruded by several magmatic bodies ranging in age from Proterozoic to Triassic. Tu Mo Rong Complex (migmatitic gneissose granite) and Phu My Complex (gabbroic amphibolite) are major Proterozoic igneous rocks in the area. Fig. 3e shows a porphyritic gneissose granite of Silurian Dien Binh Complex. It is characterized by an oriented distribution of large K-feldspar phenocrysts owing to deformation in the early Triassic. The Hai Van Pass Granite Complex is composed mainly of tonalite and biotite gneiss (leucocratic and melanocratic) bearing cordierite. Cordierite grains in the tonalitic gneiss range upto 4cm in size (Fig. 30. Although the complex is located about 70 km north of the area, the magmatic activity in this region could be associated with that of the Kontum Massif.
N e w Results 1.From mineral thermobarometric studies, we obtained ultrahigh-temperature metamorphic conditions of T= 1000-1050°C on garnet-orthopyroxene-sillimanitequartz assemblage in pelitic gneiss of the Dak Lo Formation, north of Kan Nack (Osanai et al., 2000a, b). This is considerably higher than the previously reported values (80O-85O0C; DGW, 1998d). 21, CHIME dating of monazite from cordierite-sillimanite gneisqelded an age of 248+16 Ma (Osanai et al., 2000a, b) which is consistent with K-Ar and SHRIMP zircon ages of Nam (1998) and Nam et al. (2000). 3. At least three different magmatic events were identified from the Plei Man KO Complex on the basis of petrographical and geochemical data (Owada et al., 2000). The interaction texture of two melt phases of garnet granite and gabbro-norite group (Fig. 2f) may indicate that felsic and basic magmatism took place at
the same stage. Owada et al. (2000) also proposed a model of partial melting of crustal materials (such as the pelitic gneiss) to form the garnet granite. 4. Metamorphic temperatures of the Kan Nack Complex from the area north of Qui Nhon were estimated to be about 800°C from garnet-bearing amphibolites (Tsunogae et al., 2000). This estimate is lower than the maximum temperature estimated from the complex around Kan Nack area (Osanai et al., 2000b). Our ongoing studies are expected to provide critical information on the metamorphic and magmatic history of the Kontum Massif and its role in the evolution of East Asia.
References Department of Geology and Minerals of Vietnam (DGMV) ( 1 9 8 a ) Geology and mineral resources of Dak To sheet (D48-XII). Geology and mineral resources map of Vietnam (1: 200,000), Hanoi. Department of Geology and Minerals of Vietnam (DGMV) (1998b) Geology and mineral resources of Kon lbm sheet (D-48-XVIII).Geology and mineral resources map of Vietnam (1: 200,000), Hanoi. Department of Geology and Minerals of Vietnam (DGMV) (1998~)Geology and mineral resources of Quang Ngai sheet (D-49-VII).Geology and mineral resources map of Vietnam (1: 200,000), Hanoi. Department of Geology and Minerals of Vietnam (DGMV) (1998d) Geology and mineral resources of Mang Den - Bong Son sheet (D-49-XII1, D49-XIV). Geology and mineral resources map of Vietnam (1: 200,000), Hanoi. Department of Geology and Minerals of Vietnam (DGMV) (1998e) Geology and mineral resources of An Khe sheet (D49-XIX). Geology and mineral resources map ofVietnam (1: 200,000), Hanoi. Department of Geology and Minerals of Vietnam (D,GMV) (19980 Geology and mineral resources of Qui Nhon sheet (D-49-XX). Geology and mineral resources map of Vietnam (1:200,000), Hanoi. Hutchison, C.S. (1989) Geological evolution of south-east Asia. Oxford Sci. Publ., 36813. Nam, T.N. (1998) Thermotectonic events from early Proterozoic to Miocene in the Indochina craton: implication of K-Ar ages in Vietnam. J. Asian Earth Sci., v. 16, pp. 475-484. Nam, T.N., Toriumi, M., Itaya, T., Okada, T., Long, T.V.,and Kiem, D.D. (2000) Tectonothermal events in Indochinese crystalline basement: evidence from geochronological data. WPGM 2000, Abstr. v., p.190. Osanai, Y., Owada, M., Tsunogae, T., Toyoshima, T., Hokada, T. and Long, T.V. (2000a) High- and ultrahigh temperature metamorphic rocks from the Kontum Massif in central Vietnam. The 107th Annual Meeting of the Geol. SOC.Japan, Abstr. v., p. 300. Osanai, Y., Owada, M., Tsunogae, T., Toyoshima, T., Hokada, T., and Long, T.V. (2000b) UHT pelitic granulites from Kontum Massif, central Vietnam. 1999 Annual Meeting of the Japanese Association of Mineralogists, Petrologists, and Economic Geologists, Abstr. v., p. 36. Gondwana Research, V. 4, No. 2,2001
241
Owada, M., Osanai, Y., Tsunogae, T., Toyoshima, T., Hokada, T, and Long, T.V. (2000) Geochemistry of Plei Man KO Complex in Kontum Massif, central Vietnam. 1999 Annual Meeting of the Japanese Association of Mineralogists, Petrologists, and Economic Geologists, Abstr. v., p. 63.
Tsunogae, T., Osanai, Y., Owada, M., Toyoshima, T., and Long, T.V. (2000) High-temperature metamorphism of metabasites from Kontum Massif, central Vietnam. 1999Annual Meeting of the Japanese Association of Mineralogists, Petrologists, and Economic Geologists, Abstr. v., p. 37.
Gondwana Research (Gondwana Newsletter Section) V 4, No. 2, p p . 241-243. 02001 International Association for Gondwana Research, Japan.
GNL
CORRESPONDENCE
Current Status on the Study of Supercontinent Tectonics M. Yoshidal, M. Santoshl and C.B. Dissanayake* Department of Geosciences, Faculty of Science, Osaka City University, Sugimoto 3-3-138, Osaka 558-8585,Japan, E-mail: [email protected] Department of Geology, Peradeniya University, Peradeniya, Sri Lanka
Studies of Gondwanaland and supercontinent tectonics have made important progress during past several years. Below given is a brief summary of development in the field of the above studies, mostly referring publications related to the IGCP-368 activity.
Rodinia and Gondwanaland Tectonics The episodic distribution of isotopic ages in continental crust at 2.7, 1.9 and 1.2 Ga were interpreted to reflect super event cycles, which lasts 600-800 Ma. In a recent postulation, supercontinental break ups during the earth’s history a t 2.3-2.1 Ga, 700-600 Ma and 160 Ma are proposed to have triggered mantle avalanche resulting in enhanced production of juvenile crust (Condie, 1999, GR2). The post-Rodinia breakup of the protoGondwanaland and its interactions with Baltica and protoEuropean terranes were also topics of recent interest (Erdtmann, 1999, GR2). Attempts were made to define the geological characteristics of the suture zone between West and East Gondwana (Muhongo, 1999, GR 2). The Neoproterozoic high P-T regional granulite terranes which form the most significant part of this megasuture are considered to be the products of crustal thickening that occurred when the Mozambique Ocean closed during the Neoproterozoic continent-continent collision of the protoGondwana fragments. The reconstruction of Rodinia was re-evaluated in the light of the SWEAT hypothesis (Dalziel,
2000, IGC) and the geologic connection between southwestern United States and East Antarctica was held as one of the strongest lines of evidence linking Laurentia and the present continents in a supercontinent which assembled at the close of the Mesoproterozoic. However, severe critics including alternatives to this model also exist and are increasing. The position of Kalahari Craton within the Rodinia ensemble has been enigmatic. Although Dalziel still prefers (2000, IGC) his revised idea (1997, GSA Bull., 109) that the Kalahari is far apart from Antarctica, Powell recently (2000, IGC) insisted that it should have juxtaposed with the western Dronning Maud Land as Dalziel initially proposed. A detailed analysis of the Early Paleozoic evolution of NW Gondwanaland was recently presented with data from southern Turkey and surrounding regions (Goncuoglu and Kozlu, 2000, GR3). The study traced the closure of a Palaeotethyan oceanic basin to the north of the periGondwana terranes. The global crustal tectonics during ca. 800 to 500 Ma and their relationship to global environmental changes has recently emerged into focus (Yoshida and Arima, 2000, Japan Contribution to IGCP) . While the existence of the Neoproterozoicsupercontinent Rodinia is still hypothetical, the proposal of the ca. 550 Ma supercontinent termed Pannotia has emerged as yet another unsolved mystery.