Thermal perturbation during charnockitization and granulite facies metamorphism in southern India

We have deduced the steady‐state lithospheric geotherm at c. 1 Ga in the south Indian shield area using the available data on the concentration of radioactive elements, and the P‐T conditions of Proterozoic mantle xenoliths in the south Indian kimberlites as constraints. The geotherm was adjusted back to 2.5 Ga by keeping the surface temperature constant and calculating the temperature change at the top of convecting upper mantle. The reduced or mantle heat flux, which was treated as an adjustable parameter, was 20.9–21.3 mW/m 2 at 1–2.5 Ga. Comparison of the calculated steady‐state geotherm with the available P‐T data of the Archaean (c. 2.5 Ga) charnockites and granulites from southern India suggests that the granulite facies metamorphism in this region had resulted from a major thermal perturbation, which was c. 400° C at 25 km. Seismic tomographic and gravity data essentially preclude any significant magma underplating of the granulitic crust in southern India. Previous workers have suggested that the formation of charnockites in this region was associated with copious CO 2 influx from a deep‐seated source, possibly the mantle. In this work, we have evaluated both the transient and steady‐state thermal effects of the heat convected by CO 2 outgassing from upper mantle. It is shown that the thermobarometric array of charnockites and granulites can be produced by the convective perturbation of the steady‐state geotherm, and that a flux of CO 2 of ±90 mol/m 2 yr (corresponding to Darcy velocity of ±0.30 cm/yr) for a period of ±30 Ma was needed to produce the required perturbation. This is c. 150 times the average CO 2 flux through the tectonically active area of the Earth's crust at the present time. There is, however, an uncertainty of a factor of 3 in this value. Seismic tomographic and gravity data independently suggest thickening of the crust beneath the granulite terrane compared with the adjacent Dharwar craton. This suggests thermal perturbation due to overthrusting as a major potential cause for the granulite facies metamorphism in south India. Overthrusting of a 30–35‐km‐thick thrust block was needed to produce the required thermal effect. The estimated thickness of the original crust from geobarometric and seismic tomographic data south of the orthopyroxene isograd or ‘transition zone’is compatible with the emplacement of a thrust block of this magnitude. However, the latter fails to match the estimated pre‐uplift crustal thickness at the transition zone, if it is assumed that the crust has not thinned by non‐erosional processes since the Archaean. Thus, we propose a combination of overthrusting and CO 2 fluxing from a deep‐seated source as the cause for the formation of charnockites in this zone. The required focusing of CO 2 in this case is c. 40% of that estimated in the model where CO 2 fluxing was considered to be the sole reason for thermal perturbation. This combined thrusting—CO 2 fluxing model also helps explain the development of patchy charnockites in the transition zone from amphibolite facies rocks.