290

North China craton was mainly formed during the Mesoarchaean and Neoarcliaean ages. The widely exposed rocks in tlie North China were strongly metamorphosed and deformed by Neoarchaean event.

References

UNESCO-IUGS-IGCP-368 (1 998) International Seminar on Precambi ian Crust in Eastern and Central India. October 29-30, 1998. Bhubanesliwar, Geological Survey of India, Calcutta, 24913

Geological Survey of lndia (19%) Guide Book I’rc~-Seniinar I : d d Workshop i n pal I\ o f Wc,\l kmg<il, I3111ai <ind Oiksa, India.

October 22-28, 1998. Geological Survey of India, Calcutta, 16p.

Lu, S., Yang, C., Zhu, S. and Mei, 13. (1996) T105 Precambrian Continental Crustal Profile from Eastern Hebei to Jixian, Tianjin, Beijing. Geological Publishing House, 1996.

Liu, D., Nutman, A.P., Compston, W., Wu,J. and Shen, Q.(1992) Remnants o f >3800 Ma crust in the Chinese part of the Sino- Korean craton. Geology, v.20, pp. 339-342.

Jalin, B.M., Auvray, B., Cornichet, J., Bai, Y., Shen, Q. and Liu, D. (1987) 3.5 Ga old amphibolites from eastern Hebei Province, China: Field occurrence, Petrology, Sin-Nd isochroii age and REE geochemistry. I’recamb. lics.,v. 04,

34p.

pp.011-046.

CORRESPONDENCE

Post Tectonic Granites in Sri Lanka

W.I. Starin Fernando1, Shigeru Iizumi * and R. Kitagawa

I Depilrt~i1~17t of Gmlogy, Fnciilty of Sciei~cc, Hiroslii~7in University, Ni~oslzinin 739, Japan Depnrtriier7 t c?f Geology , Fnciilty of Scicnce, Slziimme Uiziverszfy, Mntsue 690, lapa12

Granite is the most coinnion intrusive rock in Sri Lanka. These granites are generally coarse-grained, K-Feldspar rich and pink and were emplaced as oval shaped plutons or sheets. According to the Geological Map of Sri Lanka (1982) (1:506880) there are five main granitic plutons present in Sri Lanka, namely (i) Ambagaispitiya Granite, (ii) Tonigala Granite (two concordant sheet like bodies A and B), (iii) Arangala Granite, (iv) Kotadeniya Granite and (v) Balangoda Granite. Crawford and Oliver (1969) reported an age of 1059 Ma and 1096 Ma (assuming initial Sr isotope ratio of 0.7000) for Ambagaspitiya Granite and 964 Ma for Tonigala Granite in their reconnaissance geochronological studies of Sri Lankan rocks (recalculated with =1.42*10-1Y-’). These old ages are incompatible with recently published zircon evaporation ages of 550-665 Ma (Kroner et al., 1991) for tlie metamorphosed country rocks. So far no detailed petrographical and geochronological data have been reported for these granites. For the present study, all the granites except the Balangoda Granite were studied in detail, especially to gather reliable ages by Rb-Sr method, and to understand the geochemical characteristics. The granites have distinct petrographic and geochemical characteristics which distinguish them among one another. Ambagaspitiya Granite contains low CaO, TiO,, FeO and high Na,O and

K,O, compared to Arangala Granite at the SiO, range of 60- 70% Kotadeniya Granite contains high TiO,, FeO, CaO and low Na,O compared to Tonigala Granite at SiO, 70-75%. Arangala Granite, Ambagaspitiya Granite and Tonigala Granite-B show characteristics similar to A- type granite while Tonigala Granite-Aexhibits I-type characteristics and Kotadeniya Granite shows S-type characteristics. Rb-Sr and Sin-Nd isotopic studies show that whole rock isochrons are not reliable for age determination in post-tectonic granites in Sri Lanka, due to a high degree of contamination by country rocks. Whole rock-mineral isochrons yield solidification ages of 501+10 Ma and 520k3 Ma for the Ambagaspitiya Granite, 496f6 Ma for Tonigala Granite-A, 464k28 Ma for Tonigala Granite-B and 533L-6 Ma for the Kotadeniya Granite. A reference isochron age of 606 Ma is also given for Arangala Granite. Trace element characteristics resemble those of volcanic arc granite, except for the Arangala Granite, which are typical of a within plate setting. Petrochemically the granites show similarities with alkali granites from neighboring south India (Santosh et. al., 1988). The four granites are believed to have been derived from partial melting of high Rb/Sr and low Sm/ Nd sources.

Gotidillfltln Kessnircli (GolldiLlfltin Nczuslcttrr Scctioii) c! 2, No. 2, p p . 290-291. 0 7999 Iiitcrrintiotml Associntioti for Gortdroatin Rcscnrcli, Inpntz

29 I

References

Crawford, A. R. and Oliver, R. L. (1969) The precainbrian geoclironology o f Ceylon. Spec. Publ. Geol. Soc. Australia, v.2, pp.283 -306.

Geological Survey Department, Sri Lanka, Gcological Map of

Kroner,A., Cooray, P.G.and Vitanage,P.W. (3991) Lithotectonic subdivision of the Precambrian basement in Sri Lanka. In: Kroner, A. (Ed.) The crystalline crust of Sri Lanka, Part 1. Geological Survey of Sri Lanka, Professional Paper 5 pp.69-88.

Santosh, M. and Drury, S. A. (1988) Alkali granite with Pan- African affinities from Kerala, S. India. Jour. Geol., v.96,

Sri Lanka. (1982) pp.616-626.

CORRESPONDENCE

Latest Neoproterozoie Gondwana Eastern Margin

T.Watanabel, Y. Oriliaslii2 and C.M.Fanning3

lDivisiori of Earth aiid Planetary Sciences, Holcknido University, Sapporo, Inpan 2Dcpt. of Enrtlz arid Plaiietavy Scieiices, Tokyo lrzstitzife of Techtzologj, Tokcyo, Japatz

iResearcli Scliool of Earth Scierice, Azistraliaiz National Uizivenity, Canberra, ACT, Aiistralin

Rodinia Breakup and Eastern Australia

Reconstruction of the Rodinia supercontinent, especially palaeogeographical position of the south China block, has been one of the disputable problems. It is debated whether the parts of south China block were situated between Australia and Laurentia prior to the break up of Rodinia or not (c.f. Hoffman,1991; Dalziel, 1992; Li et al., 1995, among others). If Laurentia was in direct contact with Australia, radial dyke swarm activity proposed by Park (1995) was the first testament of the initiation of rifting of the supercontinent. Subsequent breakup and the timing of initiation of compressive regime along Australia and Antarctica formed constructive tectonism for passive western margin of Laurentia (Goodge, 1997). However, recent study on eastern Australia leads us to a new scenario.

Late Neoproterozoic Basaltic Magmatism in South - Central Australia

Prior to the breakup of Rodinia at ca.700Ma along the eastern Gondwana margin giant dike swarm activity derived from upwelling plumes have been recognized at 1600-1800Ma and ca.800Ma in central and south Australia (Zhao and McCulloch,l993; Zhao et a1.,1994). Recent chronological study reveals that Neoproterozoic basaltic magmatism in south - central Australia for ca. 800 Ma dyke activity is at least 40Ma before the mafic intrusion in western

North America. Precise ion-microprobe U-Th-Pb analyses of baddeleyite and zircon yield 827Ma for emplacement of the dyke swarm. Hence a hypothesis of radiating dyke swarm (Park et al., 1995) would not be accepted. They support roughly coeval magmatism in Australia and south China block. Hypothetical paleogeographic position of the south China block between Australia and Laurentia (Li et al., 1995) must be positively examined.

Neoproterozic Ages in the Tasman Fold System

The Tasman Fold System comprises four fold belts. They are the Kanmantoo, Lachlan/Thomson and New England from west to east. Crawford et al. (1997) reported ca.580Ma riftogenic greenstones in the Mt Wright area in the Kanmantoo Fold Belt. Recently Watanabe et a1.(1998) found ca.570Ma eclogite (Attunga eclogite) from the serpentinite melange in the New England Fold Belt. From the Thomson Fold Belt high temperature metamorphic rocks of Neoproterozoic age (ca.580Ma) was reported (Fergusson et al., 1998).

Eclogite in the New England Fold Belt

In 1970’s, occurrence of eclogite in the serpentinite zone was reported by Shaw and Flood (1974) at Attunga, and

Goridrvnnn Iiesenrcli (Goiidrvniin Nerrislel ter Section) V. 2, No. 2, p p . 291-293. 0 7 999 lriterrintionnl Associntioll for Gondrunnn l-k?senrcli, 1npnnrl