- Email: [email protected]
Probable Neoproterozoic Tectonism in the Larsemann Hills and Adjacent Areas, East Antarctica
641
Simmat, R. and Raith, M.M. (1998) EPMA monazite dating of metamorphic events in the Eastern Ghats Belt of India. GRG Misc. Publ. 8, pp. 126-128. Smethurst, M.A., Khramov, A.N. and Torsvik, T.H. (1998)The Neoproteorozoic and Palaeozoic palaeomagnetic data for the Siberian Platform: From Rodinia to Pangea. Earth Sci. Rev., v. 43, pp. 1-24. Tessensohn, F. (1997) Shackleton Range, Ross Orogen and SWEAT hypothesis. In: Ricci, C.A. (Ed.), The Antarctic Region: Geological Evolution a n d Processes, Terra Antarctica Pub., Siena, pp. 5-12. Unrug, R. (1996) The assembly of Gondwanaland. Episodes, v. 19, pp. 11-20. Valentine, J.W. and Moores, E.M. (1970) Plate tectonic regulation of faunal diversity and sea level: A model. Nature, v. 228, pp. 657-659. VanSchmus, W.R.,McKenna,L.W.,Gonzales,D.A., Fetter,A.H. and Rowell, A.J. (1997) U-Pb geochronology of parts of the Pensacola, Thiel, and Queen Maud Mountains, Antarctica. In: Ricci, C.A. (Ed), The Antarctica: Geological Evolution and Processes, Terra Antarctica Publ., Siena, pp. 187-200.
Veevers, J.J., Walter, M.R. and Scheibner, E. (1997) Neoproterozoic tectonics of Australia-Antarctica and Laurentia and the 560 Ma birth of the Pacific Ocean reflect the 400 m.y. Pangean supercycle. J. Geol., v. 105, pp. 225-242. Worseley, T.R., Nance, D. and Moody, J. (1984)Global tectonics and eustasy for the past 2 billion years. Marine Geol., v. 58, pp. 474-400. Yoshida, M., Bindu, R.S., Kagami, H., Rajesham, T., Santosh, M. and Shirahata, H. (1996)Geochronologic constraints of granulite terrains of South India and their implications for the Precambrian assembly of Gondwana. J. SE Asian Earth Sci., v. 14, pp.137-147. Yoshida, M., Santosh, M. and Dissanayake, C.B. (1998) Recent progress in the study of Proterozoic events in East Gondwana (IGCP-368 during 1997). Gondwana News Letter Nos. 10/11 (in Gondwana Res., v. l),pp. 414-415. Young, G.M. (1995) Are Neoproterozoic glacial deposites preserved on the margin of Laurentia related to the amalgamation of two supercontinents? Geology, v. 23, pp. 153-156.
Gondrvnnn Research, V. 2, No. 4, pp. 641-642. 0 1999 International Associntion for Gondzvana Research, Japan. ISSN: 1342-937X
GR
Goizdwnizn Research
Probable Neoproterozoic Tectonism in the Larsemann Hills and Adjacent Areas, East Antarctica Y. Zhaol, X. Liu2, B. Song1 and P. Liu' 'Institute of Geomeclzanics, CAGS, Beijing 100037, Chinn 21nstittite of Geology, CAS, Beijing 100029, China
For a Precambrian craton, composed of composite terranes, or blocks, and belts which experienced complex polyphase deformation and metamorphism, the final tectonothermal event is more significant than the earlier ones in the light of plate tectonics, because the final tectonothermal event often results from amalgamation of the craton. Although, generally, a working hypothesis of polyphase deformation and metamorphism is widely applied for high-grade terranes, discernment of its final tectonothermal episode is vital to understand its geological history. In recent years, an increasing body of evidence has revealed important early Paleozoic ("Pan-African", ca.550500 Ma) orogenics in East Antarctica which has revised the
previous notion that the East Antarctic Shield formed at ca.1000 Ma. The East Antarctic terranes, or blocks, and other Gondwana blocks assembled in early Paleozoic to form Gondwana supercontinent has been accepted as a current working hypothesis. Therefore, the poorly understood gap of Neoproterozoic geological history of East Antarctica is an important topic to be addressed. In this brief note, we discuss the probable Neoproterozoic tectonism in the Larsemann Hills and adjacent areas of East Antarctica. Outcropping on a number of icefree small peninsulas and offshore islands, rocks in the Larsemann Hills of Prydz Bay in East Antarctica are dominantly pelitic, psammitic and felsic paragneisses migmatized and metamorphosed to
642
granulite or upper amphibolite grade. Mafic granulites are minor and a majority of them are scattered as isolated lenses, narrow layers and blocks in migmatitic paragneisses. No solid evidence exists for them to be conclusively inferred as mafic dykes metamorphosed to high-grade. Ultramafic granulites are rare but were reported from the southwestern Stones Peninsula. Recognition of the importance of the early Paleozoic thermal event first occurred during studies conducted on the high-grade rocks in the Larsemann Hills and neighbouring areas by several workers, and eventually led to the modification of the earlier notion of ca.1000 Ma tectonothermal event to early Palaeozoic event, during which the final assembly of the Gondwana supercontinent occurred. But what happened in Proterozoic, can the working hypothesis of polyphase deformation and metamorphism be applied for the high-grade terrane of the Larsemann Hills? Generally, prior to a collision of two continental blocks, there must have been significant accretion of the blocks, evidence for which can be preserved in their collisional mountain belts. Recognition of accretionary tectonism is therefore, a key to understand the geological history immediately prior to the collision. However, it is very difficult to investigate this because most of the rocks exposed in East Antarctica are highly metamorphosed and intensely deformed, and much of the original informa tion relating to their protoliths has been masked by later highgrade metamorphism and deformation. Despite these, the crystallization ages of detrital zircons from paragneisses of the Larsemann Hills and adjacent areas can constrain their depositional age in Neoproterozoic, which is backed by N d model ages (TDM)obtained in studies by other workers.
Neoproterozoic206Pb /23sUages ranging 800-900 Ma, close to the concordia curve, have been reported from zircon crystals recovered from an tindeformed lamprophyre dyke on Hop Island in the Rauer Islands. Two of them with a crystallizationage at 2455k32 Ma, identical to that of Vestfold Hills Block, probably derived from a Vestfold Hills Blocklike crust at depth, which seems to support a thrust model that the whole Rauer Islands area was thrusted eastward over the Vestfold Hills Block. The rest of the ages ranging between 1000-500 Ma can be interpreted to represent the crystallization ages of xenocrysts picked up from the wall rocks of the 1000 Ma felsic orthogneiss on Hop Island and the under 1y ing F1a g - Is1a nd - li k e p a r a g neis s, which underwent regional intense orogenesis around 500 Ma. Therefore, the Flag-Island-like paragneiss could yield crystallization ages for detrital zircon grains between 800900 Ma. Recent U-Pb data are generally in agreement with previous Pb-Pb data. Four zircon crystals yielded apparent 206Pb/238U ages from C I I . 850 to 870 Ma. The results appear to confirm that there are Neoproterozoic metasediments in these areas. Further isotopic (Nd, Sr) and geochemical data for the paragneisses (Zhao unpublished data, 1993) indicate that their protoliths have characters of recycled sediments, and can be Neoyroterozoic in age. Mafic and ultramafic granulite lenses are scattered in the paragneisses, which is similar to slices of peeled and dismembered oceanic crust sandwiched tec tonically by melange of an accretionary complex. The geochemical features of the lenses of the area indicate affinities to oceanic basalt or/and back-arc mafic volcanics. The boron-and magnesium-rich gneisses occurring in the paragneisses of the Larsemann Hills suggest an origin associated with accretionary complex. Hence, an accretionary process was probably the dominant tec tonism during Neoproterozoic in the area.
Goridzunrin Rcscnrclz, V. 2,No. 4, 1999
Simmat, R. and Raith, M.M. (1998) EPMA monazite dating of metamorphic events in the Eastern Ghats Belt of India. GRG Misc. Publ. 8, pp. 126-128. Smethurst, M.A., Khramov, A.N. and Torsvik, T.H. (1998)The Neoproteorozoic and Palaeozoic palaeomagnetic data for the Siberian Platform: From Rodinia to Pangea. Earth Sci. Rev., v. 43, pp. 1-24. Tessensohn, F. (1997) Shackleton Range, Ross Orogen and SWEAT hypothesis. In: Ricci, C.A. (Ed.), The Antarctic Region: Geological Evolution a n d Processes, Terra Antarctica Pub., Siena, pp. 5-12. Unrug, R. (1996) The assembly of Gondwanaland. Episodes, v. 19, pp. 11-20. Valentine, J.W. and Moores, E.M. (1970) Plate tectonic regulation of faunal diversity and sea level: A model. Nature, v. 228, pp. 657-659. VanSchmus, W.R.,McKenna,L.W.,Gonzales,D.A., Fetter,A.H. and Rowell, A.J. (1997) U-Pb geochronology of parts of the Pensacola, Thiel, and Queen Maud Mountains, Antarctica. In: Ricci, C.A. (Ed), The Antarctica: Geological Evolution and Processes, Terra Antarctica Publ., Siena, pp. 187-200.
Veevers, J.J., Walter, M.R. and Scheibner, E. (1997) Neoproterozoic tectonics of Australia-Antarctica and Laurentia and the 560 Ma birth of the Pacific Ocean reflect the 400 m.y. Pangean supercycle. J. Geol., v. 105, pp. 225-242. Worseley, T.R., Nance, D. and Moody, J. (1984)Global tectonics and eustasy for the past 2 billion years. Marine Geol., v. 58, pp. 474-400. Yoshida, M., Bindu, R.S., Kagami, H., Rajesham, T., Santosh, M. and Shirahata, H. (1996)Geochronologic constraints of granulite terrains of South India and their implications for the Precambrian assembly of Gondwana. J. SE Asian Earth Sci., v. 14, pp.137-147. Yoshida, M., Santosh, M. and Dissanayake, C.B. (1998) Recent progress in the study of Proterozoic events in East Gondwana (IGCP-368 during 1997). Gondwana News Letter Nos. 10/11 (in Gondwana Res., v. l),pp. 414-415. Young, G.M. (1995) Are Neoproterozoic glacial deposites preserved on the margin of Laurentia related to the amalgamation of two supercontinents? Geology, v. 23, pp. 153-156.
Gondrvnnn Research, V. 2, No. 4, pp. 641-642. 0 1999 International Associntion for Gondzvana Research, Japan. ISSN: 1342-937X
GR
Goizdwnizn Research
Probable Neoproterozoic Tectonism in the Larsemann Hills and Adjacent Areas, East Antarctica Y. Zhaol, X. Liu2, B. Song1 and P. Liu' 'Institute of Geomeclzanics, CAGS, Beijing 100037, Chinn 21nstittite of Geology, CAS, Beijing 100029, China
For a Precambrian craton, composed of composite terranes, or blocks, and belts which experienced complex polyphase deformation and metamorphism, the final tectonothermal event is more significant than the earlier ones in the light of plate tectonics, because the final tectonothermal event often results from amalgamation of the craton. Although, generally, a working hypothesis of polyphase deformation and metamorphism is widely applied for high-grade terranes, discernment of its final tectonothermal episode is vital to understand its geological history. In recent years, an increasing body of evidence has revealed important early Paleozoic ("Pan-African", ca.550500 Ma) orogenics in East Antarctica which has revised the
previous notion that the East Antarctic Shield formed at ca.1000 Ma. The East Antarctic terranes, or blocks, and other Gondwana blocks assembled in early Paleozoic to form Gondwana supercontinent has been accepted as a current working hypothesis. Therefore, the poorly understood gap of Neoproterozoic geological history of East Antarctica is an important topic to be addressed. In this brief note, we discuss the probable Neoproterozoic tectonism in the Larsemann Hills and adjacent areas of East Antarctica. Outcropping on a number of icefree small peninsulas and offshore islands, rocks in the Larsemann Hills of Prydz Bay in East Antarctica are dominantly pelitic, psammitic and felsic paragneisses migmatized and metamorphosed to
642
granulite or upper amphibolite grade. Mafic granulites are minor and a majority of them are scattered as isolated lenses, narrow layers and blocks in migmatitic paragneisses. No solid evidence exists for them to be conclusively inferred as mafic dykes metamorphosed to high-grade. Ultramafic granulites are rare but were reported from the southwestern Stones Peninsula. Recognition of the importance of the early Paleozoic thermal event first occurred during studies conducted on the high-grade rocks in the Larsemann Hills and neighbouring areas by several workers, and eventually led to the modification of the earlier notion of ca.1000 Ma tectonothermal event to early Palaeozoic event, during which the final assembly of the Gondwana supercontinent occurred. But what happened in Proterozoic, can the working hypothesis of polyphase deformation and metamorphism be applied for the high-grade terrane of the Larsemann Hills? Generally, prior to a collision of two continental blocks, there must have been significant accretion of the blocks, evidence for which can be preserved in their collisional mountain belts. Recognition of accretionary tectonism is therefore, a key to understand the geological history immediately prior to the collision. However, it is very difficult to investigate this because most of the rocks exposed in East Antarctica are highly metamorphosed and intensely deformed, and much of the original informa tion relating to their protoliths has been masked by later highgrade metamorphism and deformation. Despite these, the crystallization ages of detrital zircons from paragneisses of the Larsemann Hills and adjacent areas can constrain their depositional age in Neoproterozoic, which is backed by N d model ages (TDM)obtained in studies by other workers.
Neoproterozoic206Pb /23sUages ranging 800-900 Ma, close to the concordia curve, have been reported from zircon crystals recovered from an tindeformed lamprophyre dyke on Hop Island in the Rauer Islands. Two of them with a crystallizationage at 2455k32 Ma, identical to that of Vestfold Hills Block, probably derived from a Vestfold Hills Blocklike crust at depth, which seems to support a thrust model that the whole Rauer Islands area was thrusted eastward over the Vestfold Hills Block. The rest of the ages ranging between 1000-500 Ma can be interpreted to represent the crystallization ages of xenocrysts picked up from the wall rocks of the 1000 Ma felsic orthogneiss on Hop Island and the under 1y ing F1a g - Is1a nd - li k e p a r a g neis s, which underwent regional intense orogenesis around 500 Ma. Therefore, the Flag-Island-like paragneiss could yield crystallization ages for detrital zircon grains between 800900 Ma. Recent U-Pb data are generally in agreement with previous Pb-Pb data. Four zircon crystals yielded apparent 206Pb/238U ages from C I I . 850 to 870 Ma. The results appear to confirm that there are Neoproterozoic metasediments in these areas. Further isotopic (Nd, Sr) and geochemical data for the paragneisses (Zhao unpublished data, 1993) indicate that their protoliths have characters of recycled sediments, and can be Neoyroterozoic in age. Mafic and ultramafic granulite lenses are scattered in the paragneisses, which is similar to slices of peeled and dismembered oceanic crust sandwiched tec tonically by melange of an accretionary complex. The geochemical features of the lenses of the area indicate affinities to oceanic basalt or/and back-arc mafic volcanics. The boron-and magnesium-rich gneisses occurring in the paragneisses of the Larsemann Hills suggest an origin associated with accretionary complex. Hence, an accretionary process was probably the dominant tec tonism during Neoproterozoic in the area.
Goridzunrin Rcscnrclz, V. 2,No. 4, 1999