March 2007 LIP of the Month

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Did a major large igneous province (LIP) occupy a polar location in Early Proterozoic times?

H.C. Halls
Department of Chemical and Physical Sciences, University of Toronto at Mississauga, Mississauga, ON L5L 1C6 Canada; hhalls@utm.utoronto.ca

A. Kumar
National Geophysical Research Institute, Uppal Road, Hyderabad  500 007, India

R. Srinivasan
Australian Indian Resources Pty. Ltd, Bagaloare, India.

M.A. Hamilton
Jack Satterly Geochronology Laboratory, Department of Geology University of Toronto, Toronto , ON M5S 3B1, Canada

Extracted and adapted from Halls et al. (2007)

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A U-Pb baddeleyite age of 2367 ±1 Ma from a diabase dyke together with previously published age data, suggests that a major early Proterozoic dyke swarm (termed herein the Bangalore swarm) cuts across the structural grain of the Archean Dharwar craton in India (Halls et al. 2007). Paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30° with convergence to the west (Figure 1). It was originally emplaced at high latitudes, and together with the 2415 Ma Widgiemooltha dykes of the Yilgarn block of Australia, may have been a segment of a larger radiating swarm related to a long-lived mantle plume event, active for at least 50 My (Figure 2), which was situated at a polar location, probably the southern one. Although the existence of this larger radiating dyke pattern is speculative, the new results may prompt the search for other segments of this radiating pattern, that may, for example, exist in Africa and Antarctica.


Figure 1: Geological sketch map of the central Indian Peninsula showing the main rock types of the Archean Dharwar craton, dykes that have general E-W trends and their paleomagnetic sampling sites. Legend: Archean: 1- supracrustal rocks ; 2- Closepet and related granites; 3- charnockites and other high grade rocks; 4: gneisses and granite gneisses. Paleoproterozoic: 5- Sedimentary rocks of the Cuddapah Basin; 6 – basaltic dykes. 7 – Paleomagnetic sites. Inset of Peninsula India: DC- Dharwar Craton; CL: Closepet Granite; G – Godavari Rift; C- Cuddapah Basin; D – Deccan plateau basalts (Cretaceous). The locations A, B and C refer to key dykes that are discussed in Halls et al. (2007). A major Pan African (~550 Ma) shear zone is shown schematically in grey shading (after Drury and Holt 1980). 


Figure 2: South polar projection showing two positions of the Dharwar craton of India (I) with respect to the Yilgarn Craton of Australia (A), based on paleomagnetic results of this work for the 2367 Ma Dharwar dykes and those of Evans (1968) for the 2415 Ma Widgiemooltha dyke suite. Cratonic areas in pink are shown with Archean structural trends drawn as green lines and dykes as solid red lines.  Star locates hypothesized mantle plume centre based on approximate convergence of radiating dykes (consistent with either of the two positions of the Dharwar craton relative to the Yilgarn craton). Figure produced using the GMAP software of Trond Torsvik, Norwegian Geological Survey, Trondheim.

A regional change in the intensity of brown feldspar clouding in the Indian dykes suggests that the Dharwar craton was tilted northwards, in harmony with previous observations on the structure and metamorphism of the Archean rocks (Pichamuthu 1959; Halls and Zhang 1995; Halls et al. 2007). Towards the south the brown feldspar clouding becomes blacker. The change, whose precise origins remain unknown, is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at ~800 Ma, and with a shear zone and change in structural trend related to Pan-African deformation at ~550 Ma (Halls et al. 2007, see Figure 1).  The  remagnetization yields a paleomagnetic pole that falls approximately in the expected region of the Neoproterozoic apparent polar wander path for India (Torsvik et al. 2001).

References

Drury, S.A. and Holt, R.W. 1980. The tectonic framework of the South Indian craton: a reconnaissance involving LANDSAT imagery. Tectonophysics 65, T1-T15.

Evans, M.E. 1968. Magnetization of dikes: a study of the paleomagnetism of the Widgiemoolths dike suite, Western Australia. J. Geophys. Res. 73, 32361-33270.

Halls, H.C. and Zhang, B. Tectonic implications of clouded feldspar in Proterozoic mafic dykes. Geol. Soc. India Memoir 33, 65-80.

Halls, H.C., Kumar, A., Srinivasan, R. and Hamilton, M.A. 2007. Paleomagnetism and U-Geochronology of easterly trending dykes in the Dharwar craton, India: feldspar clouding, radiating dyke swarms and the position of India at 2.37 Ga. Precambrian
Research, In Press.
See Elsevier Science Direct [www.elsevier.com/locate/precamres]
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Pichamuthu, C.S. 1959. The significance of clouded feldspar in the basic dykes of  Mysore State , India. J. Geol. Soc. India 1, 68-79.

Torsvik, T.H., Carter, L.M., Ashwal, L.D.,Bhushan, S.K.,Pandit, M.K., Jamtveit, B. 2001. Rodinia refined or obscured: paleomagnetism of the Malani igneus suit (NW India). Precam. Research 108, 319-333.