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A suspect Pan-African suture in the Lützow-Holm Bay-Sri Lanka sector of East Gondwana
Gondwana Newsletter no. 5
it is clear that these rocks or their protoliths represent pre-Brasiliano continental crust, unlike the case for Group I. Most samples of Group II are from the DFB, so it is concluded that this belt corresponds to a reworking of an older crust at around 600 Ma, as is often described for Brasiliano rocks in Brazil (e.g. eastern Brazil) and also for Pan-African rocks (e.g. in the Sahara desert). REFERENCES Chemale F.,Jr. Babinski, M., Hartmann, L.A., Van Schmus, W.R. and Wildner, W. 1994a. Evolucao rectonica do Bloco Sao Gabriel, RS. In: Congresso Brasileiro de Geologiu38, ~~232-233. Camboriu, SC. Resumos Expandidos, SBG 1. Chemale F.,Jr. Walraven, F. Hartmann, L.A., Silva, L.C. and Gresse, P. 1994b. Pb/Pb zircon dating of the Dom Feliciano Belt, Southern Brazil. In: Abstracts Conference on Proterozoic Crustal and Metallogenic Evolution. Sp. Windkoek, South Africa. Machado, N., Koppe, J.C. and Hartmann, L.A. 1990. A late Proterozoic U-Pb age for the Boscoroca belt.Journal South American Earth Sciences 3,87-90.
A suspect Pan-African suture in the Liitzow-Holm Bay-Sri Lanka sector of East Gondwana M. YOSHIDA Department of Geosciences, Faculty of Science, Osaka City
University, Osaka 558, Japan In some of his recent papers (Yoshida, 1994,1995), there has been some criticism of some of the previous studies on East Antarctica (Shiraishi et al., 1994) and Sri Lanka (Baur et al., 1991) which concluded, based mostly on zircon SHRIMP data, that the major metamorphism in these areas took place only once during the ca 500-650 Ma Pan-African period. These studies bear the common problem that the same authors in their previous publications conceived that the areas suffered an earlier (cu 1.1 Ga) and a subsequent (0.5 Ga) metamorphic event, but later concluded a single Pan-African metamorphic episode based on SHRIMP results. These studies suffer from lack of data and discussions on the polymetamorphic histories of the terranes. In this correspondence, the author wishes to introduce the problem and demonstrate that these areas did suffer earlier tectonothermal events, in addition to the Pan-African thermal reworking. Baur et al. (1991) and H&l et al. (1994) concluded that the granulite terrain of Sri Lanka suffered only one major granulite facies event at cu 550 Ma, based on their extensive SHRIMP data. H&l et al. (1994) discussed, in some detail, previous geochronological
V
data which pointed to older metamorphic ages. However, there have been detailed structural and petrological studies stressing the polymetamorphic and polyorogenic history of this terrain (Yoshida et al., 1990). Baur et al. (1991) and H&l et al. (1994) gave little or no discussion on these, nor did they comment upon the previous zircon SHRIMP study by Kriiner et al. (1987), who provided compelling evidence for a Pb loss event at cu 1100 Ma, considering it to be a major granulite metamorphism. It may be pointed out that H&l et al. (1994) clearly documented that the metasediments are intruded by granitic orthogneisses of cu 1.0 and 1.9 Ga and therefore the deposition of the precursors of the metasediments are older than cu 1.9 Ga. From this it is easier to imagine tectonothermal events related to the intrusion of the precursors of the orthogneisses, than to imagine a single metamorphic event over several hundred millions of years without any other tectonothermal event. In the Liitzow-Holm Bay area of East Antarctica, Shiraishi et al. (1994) presented compelling evidence for an extensive crystallization and Pb loss event at cu 500-550 Ma in zircons from some metasupracrustals and orthogneisses. Shiraishi et al. (1994) considered that zircon cores which gave older ages, including one at cu 1000 Ma, were mostly exogenic. This is interpreted to be due to the small number of detections of cu 1000 Ma ages as well as to the lack of evidence indicating polymetamorphism. From these interpretations they concluded that rocks of the LiitzowHolm Bay area suffered only one regional gram&e facies metamorphism at cu 500-550 Ma and that the sedimentation of the rocks of this area is younger than cu 1000 Ma. They concluded that these areas form a Pan-African suture along with Sri Lanka and possibly with the Trivandrum block of the southernmost Peninsular India. However, the Liitzow-Holm Bay area has been regarded to have a long crustal history and to be polymetamorphic/polyorogenic from various aspects (Yoshida, 1994). There have been numerous data to support the polymetamorphism (Yoshida, 1978; Hiroi et al., 1991). For example, overprinting of the amphibolite facies minerals, associated with later folding structures over the granulite facies minerals, which are associated with earlier structures, have been well documented throughout the Liitzow-Holm Bay coast (Yoshida, 1978). There have also been some geochronological data supporting the polymetamorphism of cu 500 Ma and 1100 Ma and a less well constrained one at cu 1900 Ma (summarised in Yoshida, 1994). For example, Shibata et al. (1985, 1986) gave well constrained Rb-Sr whole rock and mineral isochron data in which the mineral isochron ages plot at cu 500 Ma, while the whole rock isochron ages cluster at cu 700 Ma. Also, Fanning et al. (1991) concluded that the area suffered earlier granulite facies and later amphibolite facies
VI
Gondwana Newsletter no. 5
metamorphic events based on SHRIMP data. Surprisingly, this conclusion does not appear in Shiraishi et al. (1994), who also neglect discussions on structural, petrological and previous geochronological data. They do not give any reason for the reversal from their own previous conclusions with almost the same set of data. As to the granulites of Sri Lanka and the Trivandrum Block of southern India, there are extensive data and discussions on the aspect that the PanAfrican events represent rejuvenation, and that the precursors of granulites are Mesoproterozoic or older (Yoshida and Santosh, 1994; Harris et al., 1994 and references therein). Shiraishi et al. (1994) appeared to have paid little or no attention to these in previous studies. Zircon SHRIMP studies are effective in unraveling the tectonothermal record of Precambrian terranes with a polymetamorphic history. However, as they do not always provide a correct or complete picture of all the tectonothermal events suffered by a terrane (Kinny et al., 1993; Farming and McCullock, 1990; Friend and Nutman, 1992; KrSner et al., 1994). The principal signature of the Pb-U-Th concordiadiscordia method is incapable of providing information on tectonothermal events which occurred between the earlier crystallization and the last Pb loss events in cases where the latest Pb loss event is extensive. Conditions of crystallization or Pb loss in zircons are also important factors to be considered. During a metamorphic event, lack of fluid, deformation and/or mineral reactions may be unable to crystallize zircons or an extensive Pb loss and, instead, fluids in low grade metamorphism may favour these (Kinny et al., 1993). Loss of Pb from zircons or domains of zircon may also be reflected by the strain of the rocks and/or zircon grains and therefore the response of zircons to tectonothermal events may differ from grain to grain (KrGner et al., 1994). The requirement of a minimum of 30 micron area for zircon analysis by SHRIMP method also puts certain limitations for the interpretation of the results. The single zircon evaporation method, which involves stepwise dissolution of single zircons as well as the U-Th-PbCHIME method (Suzuki and Adachi, 1991), are also considered important, since these methods detect Pb loss events in smaller zircon crystals of only a few microns across. In conclusion, granulites of Liitzow-Holm Bay and Sri Lanka suffered polymetamorphism during cu 500550 Ma, 1100 Ma, and some parts possibly even earlier. Precursors of the granulites are mostly Middle Proterozoic or earlier, and therefore these areas do not form a suture belt during the Pan-African of 1.0 to 0.5 Ga as defined by Shiraishi ef al. (1994). It is pointed out that the importance of structural and
petrological studies, as well as data from other geochronological techniques in detecting polymetamorphism, should not be neglected while interpreting SHRIMP results. Discarding such information could lead to serious error while evaluating the tectonothermal history of polymetamorphic terranes. REFERENCES Baur, N., KrBner, A., Liew, T.C., Todt, W., Williams, I.S. and Hofmann, A.W. 1991. U-Pb isotopic systematics of zircons from prograde and retrogade transition zones in high grade orthogneisses, Sri Lanka. Journal Geology 99, 527-545. Fanning, C.M. and McCulloch, M.T. 1990. A comparison of U-Pb isotopic systematics in early Archaean zircons using isotope dilution, thermal ionization mass spectrometry andion microprobe. In: Extended abstracts 3rd Archaean Symposium. ~~15-17. Perth. Fanning, C.M., Hiroi, Y., Motoyoshi, Y., Shiraishi, K., Nakai, Y., Ellis, D.J. and Black, L. 1991. Evolution of the Liitzow-Holm Complex, East Antarctica geological, petrological and geochemical constraints. In: Abstracts 6th International Symposium Antarctic Geoscience. 139~. Tokyo. Friend, C.R.L. and Nutman, A.P. 1992. Response of zircon U-Pb isotopes and whole rock geochemistry to CO2 fluidinduced granulite facies metamorphism, Kabbaldurga, Karnataka, South India. Contributions Mineralogy Petrology 111,299-310. Harris, N.B.W., Santosh, M. and Taylor, P.N. 1994. Crustal evolution in South India: constraints from Nd-isotopes. ]ournal Geology 102,139-150. Hiroi, Y., Shiraishi, K. and Motoyoshi, K. 1991. Late Proterozoic paired metamorphic complexes in East Antarctica with special reference to the tectonic significance of ultramafic rocks. In: Geological Evolution of Antarctxa (Edited by Thomson, M. R. A., Crame, J. A. and Thompson, J. W.) pp 83-87. Cambridge Univeersity Press. Hiilzl, S., Kohler, H., Krbner, A., Jaeckel, P. and Liew, T.C. 1991. Geochronology of Sri lankan basement. In: The crstalline crust of Sri Lanka, Part 1 (Edited by Kr6ner, A) pp 237-257. Holzl, S., Hoffman, A.W., Todt, W. and Kohler, H. 1994. UPb geochronology of the Sri Lankan basement. Precambrian Research 66,123-149. Kinny, P.D., Black, L.P. and Sheraton, J.W. 1993. Zircon ages and distribution of Archaean and Proterozoic rocks in the Rauer islands. Antarctic Science 5,193-206. Kraner, A., Williams, I.S., Compston, W., Baur, N., Vitanage, P.W. and Perera, L.R.K. 1987. Zircon ion microbe dating of high grade rocks in Sri Lanka. Journal Geology 95, 775-791. Krijner, A., Jaeckal, P. and Williams, I.S. 1994. Pb-loss patterns in zircons from high grade metamorphic terrains as revealed by different dating methods: U-Pb and Pb-Pb ages for ignous and metamorphic zircons from northern Sri Lanka. Precambrian Research 66,151-181. Shibata, K., Yanai, K. and Shiraishi, K. 1986. Rb-Sr whole rock ages of metamorphic rocks from Eastern Queen Maud Land, East Antarctica. Memoirs National lnsfitute Polar Research 37,X4-171.
Gondwana Newsletter no. 5 Shibata, K., Yanai, K. and Shiraishi, K. 1995. Rb-Sr mineral isochron ages of metamorphic rocks around Syow station and from Yamato mountains, East Antarctica. Memoirs National lnstitute Polar Research 43, X4-171. Shiraishi, K., Ellis, D.J., Hiroi, Y., Fanning, CM., Motoyoshi, Y. and Nakai, Y. 1994. Cambrian erogenic belt in East Antarctica and Sri Lark implications for Gondwana assembly. Journal Geology 102,4765. Suzuki, K. and Ada&i, M. 1991. Shibata, K., Yanai, K. and Shiraishi, K. 1986. Precambrian provenance and Silurian metamorphism of Tsubonosawa paragenesis in south Kitakami terrain, northeast Japan revealed by the chemical Th-U total isochron ages of monazite, zicon and xenotime. Memoirs National institute Polar Research 43, 133-149. Yoshida, M. 1978. Tectonics and petrology of chamockites around Liitzow Holmbukta, East Antarctica. Journnl Geosciences Osaka City University 21,65-152. Yoshida, M. 1994. Tectonothermal history and tectonics of Liitzow Hahn Bay area, East Antarctica: a reinterpretation. Journal Geological Society Sri Lanka 5,81-93. Yoshida, M. 1995a. Was the Liitzow-Holm-Bay-Sri Lanka Pan-African Suture? A critique to recent zircon SHRIMP studies. Abstracts Centennal Geocongress Johannesburg, South Africa. Yoshida, M., 1995b. Cambrian erogenic belt in East Antarctica and Sri Lanka: implications for Gondwana reassembly- a discussion.Journal Geology 103,467468. Yoshida, M. and Santosh, M. 1994. A tectonic perspective of incipient chamockites in East Gondwana. Precambrian Research 66,379-392. Yoshida, M., Kehelpannala,K.V.W., Hiroi and Vitanag P.W. 1990. Sequence of deformation and metamorphism of gram&es of Sri Lanka. Journal Geoscience Osuka City University 33,69-107.
PROJECTS Thermotectonic evolution of East Gondwana: granulites of southern Madagascar DJORDJE
GRUJIC
ETH Zentrum, Geological Institut, Sonneggstrasse 8092 Zurich, Switzerland
5,
The proposed project comprises both field work and laboratory studies in order to improve our understanding of: i) plate tectonic evolution of the region (late PanAfrican orogeny and the breakup of Gondwana); ii) uplift history of a gram&e terrane; iii) processes of continental breakup; and iv) deformation in the lower continental crust in general, deformation mechanisms and deformation history of gram&e grade rocks. Preliminary studies were undertaken in southern
VII
Madagascar during two field seasons: in 1993 and 1994. Regional geology The aim of this part of the study is to establish the regional kinematic framework during the late PanAfrican orogeny. The main interest lies in several large shear zones and the emplacement of the Anosy granitic massif in southern Madagascar. During field work in November 1993, emphasis was placed on large-scale fold interference patterns and on the Ampanihy regional shear zone. In 1994, more data were collected from the same shear zone and additional mapping was done on two other large shear zones; the N-S trending Bereketa shear zone and the NW-SE trending Bongolava-Ranotsara shear zone. The contact between the country rock and the Anosy granitic massif was studied in order to establish the emplacement mechanism of the granite intrusion. Results of our field observations suggest a different structural setting than the one proposed in literature. Radiometric dating using fission track analysis Preliminary results have provided useful information. Two samples of zircons yielded ages of 265 and 320*25 Ma. Two samples of apatite yielded practically the same age of XX&23 Ma. The geological implications of these data are still uncertain and require additional measurements. Zircon ages might record initiation of ‘Karoo’ rifting during the Late Palaeozoic and possibly reactivation of the N-S trending shear zones (the two samples are on opposite sides of the Ampanihy shear zone). U-Pb dating of xenotime and titanite, and Sm-Nd dating of garnet, both in progress, should provide additional information on the uplift history of the lower continental crust in southern Madagascar. The most recent geochronological studies by other workers indicate a 560-580 Ma age for the peak of Pan-African metamorphism and synchronous magmatism. The latest Pan-African activity in southern Madagascar is represented by the formation of zircon-bearing calcite veins at 520 Ma. These data provide precise time constraints for high-temperature, low-pressure granulite facies metamorphism in the Mozambique Belt. By the further dating of minerals with various closure temperatures, the uplift-cooling history of the basement of southern Madagascar could be determined. In this study, it is proposed to use the following systems: apatite, zircon and titan& fission tracks, titanite U/Pb, xenotime U/Pb, garnet Sm/Nd, biotite Ar/Ar and white mica Rb/Sr and Ar/Ar. The regional geology of Madagascar is still largely unknown and requires more field work. The main interest lies in the relation between the shear zones and the folding, the relation between the N-S
it is clear that these rocks or their protoliths represent pre-Brasiliano continental crust, unlike the case for Group I. Most samples of Group II are from the DFB, so it is concluded that this belt corresponds to a reworking of an older crust at around 600 Ma, as is often described for Brasiliano rocks in Brazil (e.g. eastern Brazil) and also for Pan-African rocks (e.g. in the Sahara desert). REFERENCES Chemale F.,Jr. Babinski, M., Hartmann, L.A., Van Schmus, W.R. and Wildner, W. 1994a. Evolucao rectonica do Bloco Sao Gabriel, RS. In: Congresso Brasileiro de Geologiu38, ~~232-233. Camboriu, SC. Resumos Expandidos, SBG 1. Chemale F.,Jr. Walraven, F. Hartmann, L.A., Silva, L.C. and Gresse, P. 1994b. Pb/Pb zircon dating of the Dom Feliciano Belt, Southern Brazil. In: Abstracts Conference on Proterozoic Crustal and Metallogenic Evolution. Sp. Windkoek, South Africa. Machado, N., Koppe, J.C. and Hartmann, L.A. 1990. A late Proterozoic U-Pb age for the Boscoroca belt.Journal South American Earth Sciences 3,87-90.
A suspect Pan-African suture in the Liitzow-Holm Bay-Sri Lanka sector of East Gondwana M. YOSHIDA Department of Geosciences, Faculty of Science, Osaka City
University, Osaka 558, Japan In some of his recent papers (Yoshida, 1994,1995), there has been some criticism of some of the previous studies on East Antarctica (Shiraishi et al., 1994) and Sri Lanka (Baur et al., 1991) which concluded, based mostly on zircon SHRIMP data, that the major metamorphism in these areas took place only once during the ca 500-650 Ma Pan-African period. These studies bear the common problem that the same authors in their previous publications conceived that the areas suffered an earlier (cu 1.1 Ga) and a subsequent (0.5 Ga) metamorphic event, but later concluded a single Pan-African metamorphic episode based on SHRIMP results. These studies suffer from lack of data and discussions on the polymetamorphic histories of the terranes. In this correspondence, the author wishes to introduce the problem and demonstrate that these areas did suffer earlier tectonothermal events, in addition to the Pan-African thermal reworking. Baur et al. (1991) and H&l et al. (1994) concluded that the granulite terrain of Sri Lanka suffered only one major granulite facies event at cu 550 Ma, based on their extensive SHRIMP data. H&l et al. (1994) discussed, in some detail, previous geochronological
V
data which pointed to older metamorphic ages. However, there have been detailed structural and petrological studies stressing the polymetamorphic and polyorogenic history of this terrain (Yoshida et al., 1990). Baur et al. (1991) and H&l et al. (1994) gave little or no discussion on these, nor did they comment upon the previous zircon SHRIMP study by Kriiner et al. (1987), who provided compelling evidence for a Pb loss event at cu 1100 Ma, considering it to be a major granulite metamorphism. It may be pointed out that H&l et al. (1994) clearly documented that the metasediments are intruded by granitic orthogneisses of cu 1.0 and 1.9 Ga and therefore the deposition of the precursors of the metasediments are older than cu 1.9 Ga. From this it is easier to imagine tectonothermal events related to the intrusion of the precursors of the orthogneisses, than to imagine a single metamorphic event over several hundred millions of years without any other tectonothermal event. In the Liitzow-Holm Bay area of East Antarctica, Shiraishi et al. (1994) presented compelling evidence for an extensive crystallization and Pb loss event at cu 500-550 Ma in zircons from some metasupracrustals and orthogneisses. Shiraishi et al. (1994) considered that zircon cores which gave older ages, including one at cu 1000 Ma, were mostly exogenic. This is interpreted to be due to the small number of detections of cu 1000 Ma ages as well as to the lack of evidence indicating polymetamorphism. From these interpretations they concluded that rocks of the LiitzowHolm Bay area suffered only one regional gram&e facies metamorphism at cu 500-550 Ma and that the sedimentation of the rocks of this area is younger than cu 1000 Ma. They concluded that these areas form a Pan-African suture along with Sri Lanka and possibly with the Trivandrum block of the southernmost Peninsular India. However, the Liitzow-Holm Bay area has been regarded to have a long crustal history and to be polymetamorphic/polyorogenic from various aspects (Yoshida, 1994). There have been numerous data to support the polymetamorphism (Yoshida, 1978; Hiroi et al., 1991). For example, overprinting of the amphibolite facies minerals, associated with later folding structures over the granulite facies minerals, which are associated with earlier structures, have been well documented throughout the Liitzow-Holm Bay coast (Yoshida, 1978). There have also been some geochronological data supporting the polymetamorphism of cu 500 Ma and 1100 Ma and a less well constrained one at cu 1900 Ma (summarised in Yoshida, 1994). For example, Shibata et al. (1985, 1986) gave well constrained Rb-Sr whole rock and mineral isochron data in which the mineral isochron ages plot at cu 500 Ma, while the whole rock isochron ages cluster at cu 700 Ma. Also, Fanning et al. (1991) concluded that the area suffered earlier granulite facies and later amphibolite facies
VI
Gondwana Newsletter no. 5
metamorphic events based on SHRIMP data. Surprisingly, this conclusion does not appear in Shiraishi et al. (1994), who also neglect discussions on structural, petrological and previous geochronological data. They do not give any reason for the reversal from their own previous conclusions with almost the same set of data. As to the granulites of Sri Lanka and the Trivandrum Block of southern India, there are extensive data and discussions on the aspect that the PanAfrican events represent rejuvenation, and that the precursors of granulites are Mesoproterozoic or older (Yoshida and Santosh, 1994; Harris et al., 1994 and references therein). Shiraishi et al. (1994) appeared to have paid little or no attention to these in previous studies. Zircon SHRIMP studies are effective in unraveling the tectonothermal record of Precambrian terranes with a polymetamorphic history. However, as they do not always provide a correct or complete picture of all the tectonothermal events suffered by a terrane (Kinny et al., 1993; Farming and McCullock, 1990; Friend and Nutman, 1992; KrSner et al., 1994). The principal signature of the Pb-U-Th concordiadiscordia method is incapable of providing information on tectonothermal events which occurred between the earlier crystallization and the last Pb loss events in cases where the latest Pb loss event is extensive. Conditions of crystallization or Pb loss in zircons are also important factors to be considered. During a metamorphic event, lack of fluid, deformation and/or mineral reactions may be unable to crystallize zircons or an extensive Pb loss and, instead, fluids in low grade metamorphism may favour these (Kinny et al., 1993). Loss of Pb from zircons or domains of zircon may also be reflected by the strain of the rocks and/or zircon grains and therefore the response of zircons to tectonothermal events may differ from grain to grain (KrGner et al., 1994). The requirement of a minimum of 30 micron area for zircon analysis by SHRIMP method also puts certain limitations for the interpretation of the results. The single zircon evaporation method, which involves stepwise dissolution of single zircons as well as the U-Th-PbCHIME method (Suzuki and Adachi, 1991), are also considered important, since these methods detect Pb loss events in smaller zircon crystals of only a few microns across. In conclusion, granulites of Liitzow-Holm Bay and Sri Lanka suffered polymetamorphism during cu 500550 Ma, 1100 Ma, and some parts possibly even earlier. Precursors of the granulites are mostly Middle Proterozoic or earlier, and therefore these areas do not form a suture belt during the Pan-African of 1.0 to 0.5 Ga as defined by Shiraishi ef al. (1994). It is pointed out that the importance of structural and
petrological studies, as well as data from other geochronological techniques in detecting polymetamorphism, should not be neglected while interpreting SHRIMP results. Discarding such information could lead to serious error while evaluating the tectonothermal history of polymetamorphic terranes. REFERENCES Baur, N., KrBner, A., Liew, T.C., Todt, W., Williams, I.S. and Hofmann, A.W. 1991. U-Pb isotopic systematics of zircons from prograde and retrogade transition zones in high grade orthogneisses, Sri Lanka. Journal Geology 99, 527-545. Fanning, C.M. and McCulloch, M.T. 1990. A comparison of U-Pb isotopic systematics in early Archaean zircons using isotope dilution, thermal ionization mass spectrometry andion microprobe. In: Extended abstracts 3rd Archaean Symposium. ~~15-17. Perth. Fanning, C.M., Hiroi, Y., Motoyoshi, Y., Shiraishi, K., Nakai, Y., Ellis, D.J. and Black, L. 1991. Evolution of the Liitzow-Holm Complex, East Antarctica geological, petrological and geochemical constraints. In: Abstracts 6th International Symposium Antarctic Geoscience. 139~. Tokyo. Friend, C.R.L. and Nutman, A.P. 1992. Response of zircon U-Pb isotopes and whole rock geochemistry to CO2 fluidinduced granulite facies metamorphism, Kabbaldurga, Karnataka, South India. Contributions Mineralogy Petrology 111,299-310. Harris, N.B.W., Santosh, M. and Taylor, P.N. 1994. Crustal evolution in South India: constraints from Nd-isotopes. ]ournal Geology 102,139-150. Hiroi, Y., Shiraishi, K. and Motoyoshi, K. 1991. Late Proterozoic paired metamorphic complexes in East Antarctica with special reference to the tectonic significance of ultramafic rocks. In: Geological Evolution of Antarctxa (Edited by Thomson, M. R. A., Crame, J. A. and Thompson, J. W.) pp 83-87. Cambridge Univeersity Press. Hiilzl, S., Kohler, H., Krbner, A., Jaeckel, P. and Liew, T.C. 1991. Geochronology of Sri lankan basement. In: The crstalline crust of Sri Lanka, Part 1 (Edited by Kr6ner, A) pp 237-257. Holzl, S., Hoffman, A.W., Todt, W. and Kohler, H. 1994. UPb geochronology of the Sri Lankan basement. Precambrian Research 66,123-149. Kinny, P.D., Black, L.P. and Sheraton, J.W. 1993. Zircon ages and distribution of Archaean and Proterozoic rocks in the Rauer islands. Antarctic Science 5,193-206. Kraner, A., Williams, I.S., Compston, W., Baur, N., Vitanage, P.W. and Perera, L.R.K. 1987. Zircon ion microbe dating of high grade rocks in Sri Lanka. Journal Geology 95, 775-791. Krijner, A., Jaeckal, P. and Williams, I.S. 1994. Pb-loss patterns in zircons from high grade metamorphic terrains as revealed by different dating methods: U-Pb and Pb-Pb ages for ignous and metamorphic zircons from northern Sri Lanka. Precambrian Research 66,151-181. Shibata, K., Yanai, K. and Shiraishi, K. 1986. Rb-Sr whole rock ages of metamorphic rocks from Eastern Queen Maud Land, East Antarctica. Memoirs National lnsfitute Polar Research 37,X4-171.
Gondwana Newsletter no. 5 Shibata, K., Yanai, K. and Shiraishi, K. 1995. Rb-Sr mineral isochron ages of metamorphic rocks around Syow station and from Yamato mountains, East Antarctica. Memoirs National lnstitute Polar Research 43, X4-171. Shiraishi, K., Ellis, D.J., Hiroi, Y., Fanning, CM., Motoyoshi, Y. and Nakai, Y. 1994. Cambrian erogenic belt in East Antarctica and Sri Lark implications for Gondwana assembly. Journal Geology 102,4765. Suzuki, K. and Ada&i, M. 1991. Shibata, K., Yanai, K. and Shiraishi, K. 1986. Precambrian provenance and Silurian metamorphism of Tsubonosawa paragenesis in south Kitakami terrain, northeast Japan revealed by the chemical Th-U total isochron ages of monazite, zicon and xenotime. Memoirs National institute Polar Research 43, 133-149. Yoshida, M. 1978. Tectonics and petrology of chamockites around Liitzow Holmbukta, East Antarctica. Journnl Geosciences Osaka City University 21,65-152. Yoshida, M. 1994. Tectonothermal history and tectonics of Liitzow Hahn Bay area, East Antarctica: a reinterpretation. Journal Geological Society Sri Lanka 5,81-93. Yoshida, M. 1995a. Was the Liitzow-Holm-Bay-Sri Lanka Pan-African Suture? A critique to recent zircon SHRIMP studies. Abstracts Centennal Geocongress Johannesburg, South Africa. Yoshida, M., 1995b. Cambrian erogenic belt in East Antarctica and Sri Lanka: implications for Gondwana reassembly- a discussion.Journal Geology 103,467468. Yoshida, M. and Santosh, M. 1994. A tectonic perspective of incipient chamockites in East Gondwana. Precambrian Research 66,379-392. Yoshida, M., Kehelpannala,K.V.W., Hiroi and Vitanag P.W. 1990. Sequence of deformation and metamorphism of gram&es of Sri Lanka. Journal Geoscience Osuka City University 33,69-107.
PROJECTS Thermotectonic evolution of East Gondwana: granulites of southern Madagascar DJORDJE
GRUJIC
ETH Zentrum, Geological Institut, Sonneggstrasse 8092 Zurich, Switzerland
5,
The proposed project comprises both field work and laboratory studies in order to improve our understanding of: i) plate tectonic evolution of the region (late PanAfrican orogeny and the breakup of Gondwana); ii) uplift history of a gram&e terrane; iii) processes of continental breakup; and iv) deformation in the lower continental crust in general, deformation mechanisms and deformation history of gram&e grade rocks. Preliminary studies were undertaken in southern
VII
Madagascar during two field seasons: in 1993 and 1994. Regional geology The aim of this part of the study is to establish the regional kinematic framework during the late PanAfrican orogeny. The main interest lies in several large shear zones and the emplacement of the Anosy granitic massif in southern Madagascar. During field work in November 1993, emphasis was placed on large-scale fold interference patterns and on the Ampanihy regional shear zone. In 1994, more data were collected from the same shear zone and additional mapping was done on two other large shear zones; the N-S trending Bereketa shear zone and the NW-SE trending Bongolava-Ranotsara shear zone. The contact between the country rock and the Anosy granitic massif was studied in order to establish the emplacement mechanism of the granite intrusion. Results of our field observations suggest a different structural setting than the one proposed in literature. Radiometric dating using fission track analysis Preliminary results have provided useful information. Two samples of zircons yielded ages of 265 and 320*25 Ma. Two samples of apatite yielded practically the same age of XX&23 Ma. The geological implications of these data are still uncertain and require additional measurements. Zircon ages might record initiation of ‘Karoo’ rifting during the Late Palaeozoic and possibly reactivation of the N-S trending shear zones (the two samples are on opposite sides of the Ampanihy shear zone). U-Pb dating of xenotime and titanite, and Sm-Nd dating of garnet, both in progress, should provide additional information on the uplift history of the lower continental crust in southern Madagascar. The most recent geochronological studies by other workers indicate a 560-580 Ma age for the peak of Pan-African metamorphism and synchronous magmatism. The latest Pan-African activity in southern Madagascar is represented by the formation of zircon-bearing calcite veins at 520 Ma. These data provide precise time constraints for high-temperature, low-pressure granulite facies metamorphism in the Mozambique Belt. By the further dating of minerals with various closure temperatures, the uplift-cooling history of the basement of southern Madagascar could be determined. In this study, it is proposed to use the following systems: apatite, zircon and titan& fission tracks, titanite U/Pb, xenotime U/Pb, garnet Sm/Nd, biotite Ar/Ar and white mica Rb/Sr and Ar/Ar. The regional geology of Madagascar is still largely unknown and requires more field work. The main interest lies in the relation between the shear zones and the folding, the relation between the N-S