First report of garnet–corundum rocks from southern India: Implications for prograde high-pressure (eclogite-facies?) metamorphism Makoto Shimpo a , Toshiaki Tsunogae b,c, * , M. Santosh d a College of Natural Sciences, University of Tsukuba, Ibaraki 305-8571, Japan b Graduate School of Life and Environmental Sciences (Earth Evolution Sciences), University of Tsukuba, Ibaraki 305-8572, Japan c Department of Geology, University of Johannesburg, P. O. Box 524, Auckland Park 2006, Republic of South Africa d Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan Received 27 April 2005; received in revised form 21 November 2005; accepted 29 November 2005 Available online 17 January 2006 Editor: E. Boyle Abstract We report here for the first time the occurrence of garnet and corundum in Mg–Al-rich rocks at Sevitturangampatti (Namakkal district) in the Palghat-Cauvery Shear Zone System (PCSS), southern India. The rocks contain several rare mineral assemblages such as garnet–corundum–sillimanite–cordierite–sapphirine–spinel–Mg-rich staurolite, garnet–corundum–sodic gedrite–cordierite– sillimanite/kyanite, garnet–Mg-rich staurolite–sillimanite/kyanite, sodic gedrite–Mg-rich staurolite–corundum–sapphirine, biotite– corundum–sapphirine and sodic gedrite–sapphirine–spinel–cordierite. Both garnet and corundum in these rocks occur as coarse- grained (1 mm to 10 cm) porphyroblasts in the matrix of sillimanite, cordierite and gedrite. Kyanite is common as inclusions in garnet, but matrix aluminosilicates are mainly sillimanite. The presence of rare garnet + corundum, which has so far been reported from kimberlite xenoliths, aluminous eclogites and ultrahigh-pressure metamorphic rocks as well as in high-pressure experiments, suggests that the assemblage is an indicator of an unusually high-pressure event, which has not been recorded in previous studies from southern India. Phase analysis of quartz- absent MAS system also suggests high-pressure stability of the assemblage. The inference of high pressure metamorphism is also supported by the presence of Mg-rich [Mg/(Fe + Mg) = 0.51] staurolite, which has been reported from high-pressure rocks, included from cores of coarse-grained garnet and gedrite. Porphyroblastic occurrence of garnet + corundum as well as staurolite and kyanite inclusions suggests that the area underwent prograde high-pressure metamorphism, probably in the eclogite field. The rocks subsequently underwent continuous heating at 940 to 990 8C, suggesting ultrahigh-temperature (UHT) metamorphism along a clockwise trajectory. Sapphirine + cordierite and spinel + cordierite symplectites between garnet and sillimanite suggest near isothermal decompression after the peak event. Evidence for such high-pressure metamorphism followed by UHT conditions along a single clockwise exhumation path is the first report from southern India. The Namakkal area occurs within the central domain of the PCSS, which defines the probable collisional suture between the Archean craton in the north and the Proterozoic Madurai Block in the south. We propose that the rocks reported in this study testify to metamorphism at mantle depths (eclogite-facies) and their subsequent heating at extreme thermal conditions during collisional orogenic event and subsequent rapid exhumation associated with the final suturing of the 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.11.042 * Corresponding author. Graduate School of Life and Environmental Sciences (Earth Evolution Sciences), University of Tsukuba, Ibaraki 305- 8572, Japan. Tel.: +81 29 853 5239; fax: +81 29 851 9764. E-mail addresses: tsunogae@arsia.geo.tsukuba.ac.jp (T. Tsunogae), santosh@cc.kochi-u.ac.jp (M. Santosh). Earth and Planetary Science Letters 242 (2006) 111– 129 www.elsevier.com/locate/epsl