The Nilgiri Hill Massif (NHM) is part of the precambrian granulite terrain of South India and covers an area of around 5000 km2. It is bounded by two huge shear belts:the Moyar Shear Belt in the N -it demarcates the NHM from the Archaean Dharwar Craton- and the Bhavani Shear Belt in
the S.
The NHM represents a highly granulitic terrain, reflecting P,T-conditions of 7 to 10 kb and 730o to 750o C. The most abundant rock type in the NHM is a migmatic rsp. massive Enderbite (plag + qz + hyp + gnt + bio + ilm/rut + pyrrh). Other rock types are: garnet-bearing basic granulites with relictic magmatic textures, gabbro-noritic to anorthositic granulites, and small lenses of pyroxenites (Raith et al., 1990; Srikantappa et al., 1992). Very rarly occur isolated lenses of kyanite-bearing metaquarzitic rocks.
Previous geochronological investigations on NHM-enderbites yielded Rb-Sr whole rock ages of 2.4 to 2.5 Ga (Crawford, 1969; Buhl, 1987; Peucat et al., 1989) and an zircon age (upper intercept) of 2535 Ma (Buhl, 1987).
The aim of our isotopical work is:
1. To determine the intrusion age of the NHM enderbites (Rb-Sr- and Sm-Nd-whole rock dating).
2. To clarify the postmetamorphic cooling history
(Sm-Nd-garnet- and Rb-Sr-biotite dating).
3. To reveal the origin of the enderbitic magmas
(Nd- and Sr-isotope systematic, Nd-model ages).
To achieve this we analyzed 10 whole rock samples from the NHM (8 enderbites, 2 metaquarzites), as well as
10 garnet- and 3 biotite concentrates. For comparision we investigated further an orthogneiss (whole rock, garnet, biotite) from the southern part of the Dharwar Craton.
Whole rock measurements:
The Nd-model ages of the rocks from the NHM scatter in a narrow range from 2.62 to 2.87 Ga. The orthogneis from the Dharwar Craton yields a value of 3.28 Ga. Rb-Sr- and Sm-Nd-whole rock results define regression lines with ages of 2460 ± 81 Ma (95% conf.; MSWD = 26.5) rsp. 2506 ± 70 Ma (95% conf.; MSWD = 2.64). The initial ratios are: (87Sr/86Sr)i = 0.7030 ± 0.0005 and eCHUR,i = -0.4.
Garnet- and biotite measurements:
Sm-Nd-garnet ages of 7 enderbites from the NHM form two age groups: four values range from 2361 ± 6 Ma to 2373 ± 10 Ma, the others vary from 2403 ± 6 Ma to 2436 ± 17 Ma. Garnet ages from the metaquarzites are significantly lower around 2145 Ma.
Biotite measurements are in preparation.
1. The intrusion age of the primary enderbitic magma is given by the Rb-Sr- and Sm-Nd-whole rock age.
2. The whole rock ages and the zircon age indicate a rapid timing of intrusion and granulitisation but there is a significant period between the climax of metamorphism and garnet crystallisation during postmetamorphic cooling as shown by the garnet ages.
3. The Nd- and Sr-initial ratios are in the range of a primitive, undifferentiated mantle. This points either to the existence of an undifferentiated mantle at that time or results from a mixture of DM-derived magma and archaic crust.
Buhl, D., PhD Thesis, Universität Münster, Germany (1987).
Crawford, A.R., J. Geol. Soc. India 10, 117-166 (1969).
Peucat, J.J., J. Geol. 97, 537-550 (1989).
Raith, M. et al., In Granulites and Crustal Evolution; NATO ASI Series C, 311 (1990).
Srikantappa, C. et al., J. Petrol. 33, 733-760 (1992).