GEOCHEMISTRY AND ORIGIN OF ARCHEAN BANDED IRON-FORMATION FROM THE BABABUDAN SCHIST BELT, INDIA

Oxide and mixed oxide-silicate-carbonate facies banded iron-formation of the Bababudan schist belt are metamorphosed to a lower amphibolite facies. A wide spectrum of minerals including quartz, magnetite, grune rite, biotite, ankerite, siderite, and calcite is responsible for vari ation in major element composition. High Al2O3, TiO2, K2O, and MgO con tents characterize the shaly banded iron-formation, whereas the cherty banded iron-formation is depleted in these constituents. Zr, Hf, Rb, Sr, Ba, U, and Th distinguish the felsic clastic flux from mafic flux, which is characterized by an abundance of Cr, Ni, Co, Sc, V, and Y. C herty banded iron-formation has depleted and less fractionated rare ea rth element (REE) patterns coupled with prominent positive Eu anomalie s. These features closely resemble REE distribution in the present-day hydrothermal fluids added at midcean ridge vent sites. In contrast th e elevated Sigma REE, fractionated patterns, and negative Eu anomalies characterize shaly banded iron-formation, which is more influenced by the supply of terrigenous debris. Both light and heavy REE increase s imultaneously and the ratio between the two is relatively constant. Nd and its relationship with Al2O3, and Fe2O3(total), indicates that bot h hydrothermal and continental flux have been significant sources to v arying degrees for banded iron-formation at various stratigraphic leve ls. Intermittent availability of oxygen appears to have resulted in rh ythmic precipitation of iron hydroxide, and thus the banding of iron- and silica-rich layers. The essential components iron and silica of th e banded iron-formation were supplied through hydrothermal fluids adde d to ocean water in the deep-marine part of the basin and transported due to the thermo-chemocline for precipitation in the shallow shelf zo ne. Seasonal suspended and dissolved fluvial flux from a continental s ource played a significant role in generating the composition of the b anded iron-formation suite. Occurrence of the banded iron-formation wi th underlying quartz-pebble conglomerate and quartzites and the overly ing turbidites might be explained by assuming a faster rate of generat ion and consumption of the Archean oceanic lithosphere.