High pressure-temperature studies on an olivine tholeiite and a tholeiitic picrite from the pavagarh region, Gujarat, India

Abstract

Experimental studies have been performed on an olivine tholeiite and tholeiitic picrite at pressure and temperature ranges of 20-40 kb and 1200-1300°C. The lower and upper limits of basalt-eclogite transition zone for tholeiitic picrite are 23 kb and 31.67 kb at 1200°C, and 24.67 kb and 33.67 kb at 1300°C, whereas for olivine tholeiite, these are 27 kb and 32.33 kb at 1200°C, and 28.70 kb and 33.70 kb at 1300°C. While the assemblages for both samples below the transition region are Pl+Px+Mt, they are Pl+Gt+Px+Mt within it. The eclogite field has Gt+Px+Mt. The ratio of garnet to plagioclase increases from the transition zone to the eclogite field and with the disappearance of plagioclase, the percentage of garnet increases to 30 in the eclogite field. Comparison of our results with previous studies on basalt-eclogite transition shows that the transition zone found by us occurs at higher pressure-temperature conditions. Seismic studies of the region below the Deccan Traps show an increase in velocity (1-4%) at depth. It is suggested that after partial melting, during ascent of the basaltic liquid, a significant portion of it crystallizes within the upper mantle as pockets of eclogite. As eclogite is more dense than peridotite, their presence should cause a similar increase in the seismic velocity below the Deccan area.