Goiidivnrin Resennrcli, V 2, No 3, p p . 471-472. 0 1999 Oiteur~atro~irrl Assocrnt/o~ifor Gondivana IZesenrc 11, Japan. ISSN: 1,342-937X GR

Gondwana Research

The Significance of the Palghat-Cauvery Shear Zone in Southern India for Correlations between

South-West India and Eastern Madagascar

Nigel Harris

Depnrtmei?t of Earth Sciences, Open University, Milfoiz Keyies, MK7 6AA, UK

jMnntiscript received Arigust 11, 1998; nccepted November 11, 1998)

Reconstructions of East Gondwana suggest that the well- documented Bongolava-Ranotsara shear zone of Madagascar should be correlated to the east with a major shear system in South India (Windley et al., 1994; De Wit et al., 1995). The Palghat-Cauvery shear zone (PCSZ), together with subsidiary diverging shear belts such as the Moyar- Bhavani structurc, form a major post-Archaean tectonic boundary delimiting the Archaean granite-greenstone belt of the Karnataka (Dharwar) Craton to the north and the Proterozoic mobile belts of southernmost India and Sri Lanka to the south (Harris et al., 1994). It also marks the northern limit of Pan-African influence in South India; to the north of the PCSZ Rb-Sr biotite ages indicate sub- amphibolite facies cooling at -2400 Ma, whereas to the south cooling occurred at around 480 Ma implying that contrasting thermal histories across thc PCSZ persisted as late as the Ordovician (Harris et al., 1994 and references therein).

The Karnataka Craton underwent crustal growth from -3600 Ma to -2500 Ma (Harris et al., 1994) with the youngest crustal growth and granulite formation occurring near its southern margin, coeval with the intrusion of the major North-South trending Closepet granite (251344 Ma; Friend and Nutman, 1991). Massive charnockites are exposed near the southern margin of the craton (BR Hills) and well- documented examples of incipient charnockite occur further north (Kabbaldurga; Janardhan et al., 1979) straddling the regional orthopyroxene isograd where their arrested nature is probably controlled by local reaction kinetics.The PCSZ, marking the southern margin of this craton, is a complex structure comprising of anastomosing belts of reworked gneisses with strong planar fabrics. This shear zone encloses relatively undeformed blocks and lenses of granulites and anorthosite-gabbro-ultramafic complexes (Sittampundi). Fabrics suggest a dextral sense of movement (Drury and Holt, 1980). The largest granulite block is the Nilgiri Massif

underlain by late Archaean massive charnockites with no evidence for a pre-2600 Ma crustal history (Harris et al., 1994). Several lines of evidence suggest a meta-igneous origin for these charnockites that crystallised in the presence of a mantle-derived carbonic fluid at depths in excess of 30 km (Touret and Hansteen, 1988).

South of the PCSZ, a Late Archaean-Palaeoproterozoic mobile belt (Harris et al., 1994) is characterised by model Nd ages of 3000-2100 Ma, comparable with Nd model ages from southern Madagascar (Paquette et al., 1994). Metasedimentary basins in southernmost India include Neoproterozoic material that can be correlated with similar lithologies in Sri Lanka and East Antarctica (Harris et al., 1996). The mobile belt is dissected into several tectonic blocks and incorporates high-grade metasedimentary belts and massive charnockite complexes which underwent several tectonothermal events, including granite magmatism at -2400 Ma (Koddaikanal; Bartlett et al., 1998), upper-amphibolite facies metamorphism at -1800 Ma (Kerala Khondalite Belt), and widespread charnockite formation at -550 Ma coeval with emplacement of alkali granites and syenites (Choudhary et al., 1992). Incipient charnockites of this age (eg Ponmudi, Mannantala) result from channelised carbonic fluid flow through amphibolite- facies assemblages that lie close to the orthopyroxene stability field in T-aH,O space (Santosh et al., 1990). In some cases these fluids can be linked to decarbonation of proximal calc-silicate lithologies (Nuliyam; Jackson and Santosh, 1992).

Although there are clear parallels between the geological histories of South India and Madagascar it is important to establish whether a southern limit to Archaean basement and whether a northern limit to the Pan-African tectonothermal event can be recognised in Madagascar. Furthermore the dextral shear sense obtained from the

472 N. HARRIS

PCSZ, and also from the Achankovil shear zone to the south (Sacks et al., 1997), can not be readily reconciled with the sinistral kinematics repor ted for the Ranotsara shear (Windley et al., 1994). If the Bongolava-Ranotsara shear zone is confirmed as the western extension of the PCSZ the Bongolava-Ranotsara shear has clearly been reactivated following continental breakup.

References

Bartlett, J.M., Dougherty-Page, J.S., Harris, N.B.W., Hawkesworth, C.J. and Santosli, M. (1998) The application o f single crystal evaporation and model Nd ages to the interpretation of polymetamorphic terrains: an example from the Proterozoic mobile belt of south India. Contrib. Mineral. Petrol., v.131, pp.181-195.

Choudhary, A.K., Harris, N.B.W., Van Calsteren, P. and Hawk es w o r t h, C . J . ( 1 9 9 2) Pan- African c h a r n oc k i t e formation in Kerala, South India. Geol. Mag., v.129,

De Wit, M., Vitali, E. and Ashwal, L. (1995) Gondwana reconstruction o f the East Africa-Madagascar-India-Sri Lanka-Antarctica fragments revisited. Centennial Geocongress Abstracts, Geol. Soc. South Africa, Johannesburg, pp.218-221.

Drury, S.A. and Holt, R.W. (1980) The tectonic framework of the south Indian craton: a reconnaissance involving LANDSAT imagery. Tectonophysics, v.65, pp.1-15.

Friend, C.R.L. and Nutman, A.P. (1991) SHRIMP U-Pb Geochronology of the Closepet Granite and Peninsular Gneiss, Karnataka, South India. J. Geol. Soc. India, v.38,

pp. 257-264.

pp.357-368.

Harris, N.B.W., Santosli, M. and Taylor, P.N. (1994) Crustal evolution in South India: Constraints from Nd isotopes. J. Geol., v. 102, pp. 139-150.

Harris, N.B.W., Bartlett, J .M. and Santosh, M. (3996) Neodymium isotope constraints on the tectonic evolution of East Gondwana. J. Southeast Asia Earth Sci., v.14, pp.

Jackson, D.H. and Santosh, M. (1992) Dehydration reaction and isotope front transport induced by CO, infiltration at Nuliyam, South India. J. Metamorphic Geol., v.10, pp.

Janardhan, AS., Newton, R.C. and Smith, J.V. (1979) Ancient crustal metamorphism at low PI 1p: charnockite formation at Kabbaldurga, south India. Nature, v.278, pp.511-517.

Paquette, J.-L., Ne'de'lec, A., Moine, B. and Rakotondrazafy, M. (1994) U-Pb single zircon evaporation and Sm-Nd isotopic study of a granulite domain in SE Madagascar. J. Geol.,

Sacks, P.E., Nanibiar, C.G. and Walters, L.J. (1997) Dextral Pan- African shear along the southwes tern edge of the Achankovil Shear Belt, south India: constraints on Gondwana reconstructions. J . Geol., v.305, pp.275-284.

Santosh, M., Harris, N.B.W., Jackson, D.H. and Mattey, D.P. (1990) Dehydration and incipient charnockite formation: a phaseequilibria and fluid inclusion study from South India. J . Geol., v. 98, pp.915-926.

Touret, J.L.R. and Hansteen, T.H. (1 988) Geotliermobarometry and fluid inclusions in a rock from the Doddabetta charnockite complex, southwest India. Rend. Soc. Ital. Mineral. Petrol., v.43, pp.65-82.

Windley, B.F., Razafiniparany, A,, Razakamanana, T. and Ackermand, D. (1994) Tectonic framework of the Precambrian of Madagascar and its Gondwana connections: a review and reappraisal. Geol. Rundschau, v.83, pp. 642-659.

119-125.

365-382.

v.102, pp.523-538.

Gorzd7onizn Resenrch, V. 2, No. 3, 1999