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Geochronologic data evaluation critical for the Proterozoic
tectonics of East Gondwana
Masaru Yoshida
Department of Geology, Trichandra Campus, Tribhuvan University, Kathmandu, Nepal,
and
Gondwana Institute for Geology and Environment, Hashimoto, Japan
East Gondwana as well as Rodinia is on fire (e.g., Yoshida et al., 2003), due mainly to
arguments based on geochronologic data from Antarctica. The ca 1000 Ma Circum
East Antarctic Orogen (CEAO) defined by Yoshida (1992, 1995) as having assembled
East Gondwana has been negated, due in one hand to the distinct age difference within
the orogen and in another to the development of two Pan-African orogens crossing the
CEAO; thus the Pan-African assembly of East Gondwana has been put forward,
resulting in the necessity of drastic re-consideration of the classical Rodinia
configuration.
In the present lecture, the above criticism will be assessed in the following points.
1. There is not much distinct difference in ages between the western and central
CEAO, with regard to zircon ages and Sm-Nd mineral-whole rock isochron ages.
There are some difference in age between the eastern sector and remaining sectors
of CEAO. However, as a large-scale orogen such as fringing East Antarctic continent,
which might have been the result of conversion of various plates at various sectors,
there is no necessity of precise synchronism of major orogenic phases.
2. Several obvious ca 1000 Ma rocks occur within the two Pan-African orogens in East
Antarctica, which supports an idea that the Pan-African orogens are the
intracratonic rather than intercratonic.
3. No or rare detection of ca 1000 Ma zircon ages as well as of Sm-Nd mineral-rock
ages could be met with under strong deformational, high-temperature and fluidus
metamorphism on rocks which initially had geochronologic components of ca 1000
Ma. Thus, even though the lack or rarelity of detections of ca 1000 Ma ages can be
met with, it does not guarantee the lack of ca 1000 Ma events within the
Pan-African orogen in East Antarctica.
Several examples of dating by U-Pb, Pb-Pb, Sm-Nd and Rb-Sr systematics in rocks and
minerals from the CEAO and Pan-African orogens in East Antarctica will be discussed.
References
Fitzsimons, I.C.W., 2000. Grenville age basement provinces in East Antarctica:
Evidence for three separate collisional orogens. Geology, 28, 879-882.
Fitzsimons, I.C.W., 2003, Proterozoic basement provinces of southern and southwestern
Australia, and their correlation with Antarctica. In Yoshida, M., Windley, B.F. &
Dasgupta, S. (eds), Proterozoic East Gondwana: Supercontinent Assembly and
Breakup. Geol. Soc., London, Special Pub. 206, 93-130.
Yoshida, M., 1992. Late Proterozoic to early Palaeozoic events in East Gondwanan
crustal fragments.
Abstracts, 29th International Geological Congress, Kyoto,
v.2/3, 265.
Yoshida, M., 1995a, Assembly of East Gondwanaland during the Mesoproterozoic
and its rejuvenation during the Pan-African period.
In: Yoshida, M. &
Santosh, M. (eds.), India and Antarctica during the Precambrian, Geological
Society of India, Memoir 34, 25-45.
Yoshida, M., Jacobs, J., Santosh, M. and Rajesh, H.M., 2003, Role of Pan-African events in
the Circum-East Antarctic Orogen of East Gondwana: a critical overview. In Yoshida,
M., Windley, B.F. & Dasgupta, S. (eds), Proterozoic East Gondwana: Supercontinent
Assembly and Breakup. Geol. Soc., London, Special Pub. 206, 57-75.