Geochemistry of Archaean Granites of the Indian Shield: A Review

Archaean granites and granite gneisses are known to occur in (a) the Iron Ore Province of Singhbhum-Orissa region in eastern India, (b) the Granite-Greenstone Belt of Karnataka and Andhra Pradesh, (c) the Granulite Belts of Tamil Nadu and Kerala and (d) the Bundelkhand-Rajasthan Precambrian province.

In the Iron Ore Province of Singhbhum region, the Older Metamorphic tonalite-gneiss (OMTG-c. 3200 Ma) has strong geochemical resemblance with the 'Ancient tonalite-gneiss' of Barberton. South Africa. The low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7018-703) for the total-rock Rb-Sr isochrons indicate derivation of the tonalitic melt from the mantle perhaps in a two-stage process. The magmatic units of the composite batholith of Singhbhum Granite (2900-3000 Ma) show distinctive trends of fractional crystallisation for the three successive but closely related phases. Multivariate analysis of variance of the trace-element data indicates statistically significant differences among the phases while Rsmode Factor analysis suggests operation of significantly different processes of trace-element distribution in the three phases. The geochemical relationship as well as the high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.711 ± .009 suggest that the three magmatic phases were derived by partial melting successively in the same source region of the crust. The Bahalda metasomatic unit of Singhbhum Granite shows certain abnormal trends of trace-element variations in the sequence: hornblende schist→hornblende diorite→shornblende (-epidote) granodiorite. These have been explained by assuming that during progressive granitisation, (a) ionic charge and electronegativity have been relatively ineffective and (b) certain trace-elements are preferentially incorporated in some preexisting minerals.

The Peninsular Gneiss and associated Archaean granitic bodies in the Granite Greenstone Belt of South India show wide compositional variations ranging from migmatitic and tonalitic gneiss through granodiorite to K-rich adamellite. These rocks show clusters of Rb-Sr isochrons at 3200-3300Ma, 2900-3000Ma and around 2600Ma. Enrichment of nickel and chromium in parts of these gneisses has been reported. Although a metasomatic origin of the Peninsular Gneiss has been suggested, the low initial ⁸⁷Sr/⁸⁶Sr ratios (0.701-0.704) do not support this view and instead indicate direct or multi-stage derivation of the granitic melt from the mantle. Occurrence of several abnormal U-Th enriched granite bodies within Peninsular Gneiss is of special interest.

The Bundelkhand Granite is a composite body of several granitic and gneissic units and appears to be dominantly adamellitic - monzonitic in composition (2555 ± 55 Ma), but has numerous xenoliths and rafts of migmatite and gneiss, mostly of granodiorite and tonalite composition.

It appears that in the Indian shield, the oldest gneisses and granites (> 3000 Ma) are dominantly tonalites while adamellitic granites become more and more abundant in the younger assemblages.