CRUST-MANTLE INTERACTIONS IN THE ORIGIN OF THE EARLY TO MID-PROTEROZOIC BASALTIC DIKES AND SILLS IN AND AROUND CUDDAPAH BASIN, SOUTH INDIA

Early to Mid-Proterozoic basaltic dikes intrude Archean gneisses along the NW-SE and NE-SW lineaments of the South Indian shield around the intracratonic Cuddapah basin. They also trend E-W and N-S and form a system of radial dikes around a large lopolithic intrusion inside the basin, which originated from a plume-like mantle upwelling. Concentric sills of tholeiite also occur around the lopolith inside the basin. The basaltic dikes originated under a pressure of about 6 kb and temperatures between 1029 and 1187oC, as determined by clinopyroxene barometry and 2-pyroxene thermometry. Crustal contamination, metamorphic alteration and the cumulate nature of the samples have obscured fractional crystallization relations on variation diagrams. Textural evidence of finger-like intrusion and the presence of Mg-rich overgrowth rim on relict orthopyroxene indicate that magma mixing also played an important role. The P-T ranges of equilibration of the dikes are similar to those of the sills inside the basin.

The incompatible trace and rare earth element (REE) contents of these basalts are markedly similar to those of Late Archean and Proterozoic basalts worldwide and indicate mixing between a primitive magma and Archean crustal gneisses in a ratio between 85:15 and 75:25. The Rb/Nb and Ce/Nb ratios of the Cuddapah and the crustally contaminated Bushe samples of the Deccan Traps are close to the Archean gneisses, whereas, these ratios for the neighboring Late Archean Kolar basalts are closer to the average continental lithospheric mantle. The Kolar basalts have lower contents of large-ion-lithophile elements (LILE) and light-REE (LREE). Available ages indicate that the change to higher LILE and LREE contents may have occurred at around 2.9Ga. Compared to contemporary Laurentian basalts, the later Proterozoic Gondwana basalts worldwide show similar enrichments in LILE and LREE, but high field-strength elements and heavy-REE are much more depleted.

The position and shape of the Proterozoic basins of South India seem to be controlled by the major lineaments. The mantle upwelling is centered on an intersection of the NW-SE and NE-SW lineaments. The dikes are emplaced along the lineaments and represent their magmatic reactivation. Evidence of repeated magma-injection in a dike SW of the lopolith outside the basin.