THERMAL PERTURBATION DURING CHARNOCKITIZATION AND GRANULITE-FACIES METAMORPHISM IN SOUTHERN INDIA

We have deduced the steady-state lithospheric geotherm at c. 1 Ga in t he south Indian shield area using the available data on the concentrat ion of radioactive elements, and the P-T conditions of Proterozoic man tle xenoliths in the south Indian kimberlites as constraints. The geot herm was adjusted back to 2.5 Ga by keeping the surface temperature co nstant and calculating the temperature change at the top of convecting upper mantle. The reduced or mantle heat flux, which was treated as a n 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 In dia suggests that the granulite facies metamorphism in this region had resulted from a major thermal perturbation, which was c. 400 degrees 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 charnock ites in this region was associated with copious CO2 influx from a deep -seated source, possibly the mantle. In this work, we have evaluated b oth the transient and steady-state thermal effects of the heat convect ed by CO2 outgassing from upper mantle. It is shown that the thermobar ometric array of charnockites and granulites can be produced by the co nvective perturbation of the steady-state geotherm, and that a flux of CO2 of greater than or equal to 90 mol/m(2) yr (corresponding to Darc y velocity of greater than or equal to 0.30 cm/yr) for a period of les s than or equal to 30 Ma was needed to produce the required perturbati on. This is c. 150 times the average CO2 flux through the tectonically active area of the Earth's crust at the present time. There is, howev er, an uncertainty of a factor of 3 in this value. Seismic tomographic and gravity data independently suggest thickening of the crust beneat h the granulite terrane compared with the adjacent Dharwar craton. Thi s suggests thermal perturbation due to overthrusting as a major potent ial cause for the granulite facies metamorphism in south India. Overth rusting of a 30-35-km-thick thrust block was needed to produce the req uired thermal effect. The estimated thickness of the original crust fr om geobarometric and seismic tomographic data south of the orthopyroxe ne isograd or 'transition zone' is compatible with the emplacement of a thrust block of this magnitude. However, the latter fails to match t he estimated pre-uplift crustal thickness at the transition zone, if i t is assumed that the crust has not thinned by non-erosional processes since the Archaean. Thus, we propose a combination of overthrusting a nd CO2 fluxing from a deep-seated source as the cause for the formatio n of charnockites in this zone. The required focusing of CO2 in this c ase is c. 40% of that estimated in the model where CO2 fluxing was con sidered to be the sole reason for thermal perturbation. This combined thrusting-CO2 fluxing model also helps explain the development of patc hy charnockites in the transition zone from amphibolite facies rocks.