Relative contribution of crust and mantle to flood basalt magmatism, Mahabaleshwar area, Deccan Traps

Abstract

The 1200 m section of flat-lying basalts in the Mahabaleshwar area is divided into three formations on the basis of the trace elements Sr, Ba, Rb, Zr and Nb. The lowermost unit, the Poladpur Formation, is characterized by high Ba, Rb, and Zr/Nb, and low Sr. These features are accompanied by high K and Si, high and variable 87 Sr/ 86 Sr initial ratios (0.7043 - 0.7196), and low and variable e N d values (+ 2.6 to -17.4). The formation is interpreted as having developed by contamination of the overlying Ambenali magma-type with ancient granitic crust, with simultaneous fractionation of a gabbroic mineral assemblage. The more basic members of the formation are found towards the base of the succession and are more contaminated than the upper flows. The succeeding Ambenali Formation, characterized by the Ambenali magma type, has low Ba, Rb, Sr and Zr/Nb, and low and rather uniform 87 Sr/ 86 Sr initial ratios (0.7038-0.7043) coupled with high and relatively uniform e N d (+4.7 to +6.4). It is interpreted as being essentially uncontaminated and derived from a mantle source with a history of slight trace-element enrichment relative to m.o.r.b.-source. The uppermost group of flows, the Mahabaleshwar Formation, is, like the Poladpur, enriched in Ba, Rb, K and Si relative to the Ambenali, but has lower Zr/Nb and higher Sr. 87 Sr/ 86 Sr initial ratios (0.7040-0.7056) are slightly higher than in the Ambenali, and e N d lies in the range +7.1 to -3.0. In this formation Sr correlates positively with the other incompatible elements and with 87 Sr/ 86 Sr initial ratios. This is in strong contrast to the relations observed in the Poladpur, and we believe that the behaviour of Sr may be a simple pointer to the distinction between mantle and crustal contributions. Assuming that late-stage crystal fractionation processes can be allowed for, if Sr correlates positively with elements such as K, Rb and Ba then mantle enrichment processes are clearly implied. Conversely, as for example in the Poladpur, if the correlation is negative, crustal contamination is suspected because Sr is unlikely to behave as an incompatible element in most crustal derived melts or fluids because of buffering by residual plagioclase. Furthermore, the relative uniformity of the Mahabaleshwar Formation, the position on the Sr and Nd isotope diagram close to the ‘mantle array’, the fact that in terms of both incompatible element concentrations and isotopes the rocks are similar to tholeiites from oceanic islands such as Hawaii and Kerguelen, are all factors that reinforce the conclusion that these are mantle derived magmas which have suffered insignificant crustal contamination. They are, however, derived from a mantle which is trace-element enriched relative to the Ambenali source. Thus in the succession as a whole the crustal contribution appears to be small. Maximum amounts of contamination in the Poladpur Formation are difficult to determine but the average amount is probably in the region of 6-12 percentage mass. The whole sequence therefore contains a crustal contribution of about 2-3%.