Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

V. N. Vasudev ¹, Brian Chadwick ², Allen P. Nutman ³, G. V. Hegde ⁴

Affiliations
1 120/45(A) III Block, Thyagrajanagar, Bangalore 560 028, IN
2 Earth Resources Centre, University, Exeter EX44QE, GB
3 Research School of Earth Sciences, Australian National University, Canberra ACT0200, AU
4 Department of Mines and Geology, Government of Karnataka, 16/3-5 S. P. Complex, Lalbagh Road, Bangalore 560 027, IN

Abstract

The Palkanmardi conglomerate is one of many polymict conglomerates interbedded with greywackes in the NE of the Hutti schist belt. These conglomerates are up to a few metres thick, unsorted and include rounded to sub-angular clasts of granodiorite, granite, vein quartz and rnetabasalt in a matrix of coarse-to medium-grained greywacke. Cross-bedding, convolute bedding and grading are well preserved in the interbedded greywackes in spite of deformation and homfelsing during Late Archaean regional high temperature - low pressure metamorphism. These primary structures and lack of sorting in the conglomerates are consistent with deposition as turbidites and debrites in an unstable submarine environment. This new interpretation contrasts with previous views that the Palkanmardi conglomerate is autoclastic, pyroclastic-detrital or glacio-fluvial. The conglomerate-greywacke sequence occurs low in the lithostratigraphy and is overlain by metabsalts.

A clast of granodiorite in the conglomerate bed at Palkanmardi village has yielded a SHRIMP weighted mean ²⁰⁷Pb/²⁰⁶Pb zircon age of 2576±12 Ma which is interpreted as the magmatic age of the erosional provenance. Moreover, SHRIMP zircon geochronology using a sample from the steep elongate wedge of granodiorite that extends for at least 150 km SE of the schist belt has yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2561±24 Ma and a concordia upper intercept age of 2580±31 Ma. These ages are indistinguishable within their errors and are interpreted as the age of magmatic crystallisation. Combined with the low stratigraphic position of the Palkanmardi conglomerate, the age data imply that basin development (volcanism and sedimentation) in the Hutti belt was not only rapid, but began very late in the Archaean history of this part of the Dharwar craton.

Keywords

Late Archaean, Hutti Schist Belt, SHRIMP Ages, Intra-Arc Basin, Dharwar Craton.

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Brian Chadwick ¹, G. V. Hegde ², Allen P. Nutman ³, V. N. Vasudev ⁴

Affiliations
1 12 Pennsylvania Closc, Exeter EX4 6DJ, GB
2 Department of Mines and Geology, Govt. of Karnataka, Khanija Bhavana, Race Course Road, Bangalore 560 001, IN
3 Research School of Earth Sciences, Australian National University, Canberra ACT 0200, AU
4 Australian-Indian Resources Pvt. Ltd., 120/45(A) 111 Block, Thyagarajanagar, Bangalore 560 028, IN

Abstract

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.

Keywords

Late Archaean, Syenitc, SHRIMP U/Pb Age, Koppal, Dharwar Craton.

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