Syenite Emplacement during Accretion of the Late Archaean Dharwar Batholith, South India: SHRIMP U/Pb Age and Structure of the Koppal Pluton, Karnataka

The predominantly syenitic Koppal pluton was emplaccd in the western flank of the Dharwar batholith which forms the eastern half of the Dharwar craton. Previous work has shown that the batholith accreted against an older continental foreland reprcsented by the western half of the craton during Late Archacan oblique convergence. A sample of syenitc has yielded a SHRIMP U/Pb zircon age of 2528±9 Ma which is interpreted as the timc of emplacement at a late stage in the accretion of the batholith. Its age and composition make the Koppal pluton a unique feature of the Dharwar craton. Melt commingling of syenitc with other intcrrnediate to basic magmas is indicated by commmon mafic enclaves, disrupted mafic sheets and larger netveined bodies rich in hornblende and pyroxene. The arc setting of the Koppal pluton is consistent with syenitic plutons in other Late Archaean terrains.

The pluton has an elliptical outcrop trending NE-SW with an upright, asymmetrical funnel shape indicated by its magmatic-state LS fabrics and diffuse compositional banding. Banding is a function of variations in microclinc, hornblende and clinopyroxene abundances. Thc pluton was ernplaced broadly parallel to diffuse magmatic banding in the host granites which lies oblique to trends of the steep NW-SE belts of plutonic rocks that characterise the Dharwar batholith as a whole. This oblique orientation of magmatic banding has been interpreted previously as the result of emplacement when magmatic pressure exceeded regional compressive stress during sinistral displacement on steep shear zones trending NW-SE. Emplacement of the Koppal pluton is interpreted in a similar way.