Geochemistry and origin of Archean banded iron-formation from the Bababudan schist belt, India

Abstract

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, grunerite, biotite, ankerite, siderite, and calcite is responsible for variation in major element composition. High Al 2 O 3 , TiO 2 , K 2 O, and MgO contents characterize the shaly banded iron-formation, whereas the cherty banded iron-formation is depleted in these constituents. Zr, Hf, Rb, St, 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. Cherty banded iron-formation has depleted and less fractionated rare earth element (BEE) patterns coupled with prominent positive Eu anomalies. These features closely resemble BEE distribution in the present-day hydrothermal fluids added at midocean ridge vent sites. In contrast the elevated Epsilon 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 simultaneously and the ratio between the two is relatively constant. Nd and its relationship with Al 2 O 3 and Fe 2 O (sub 3(total)) indicates that both hydrothermal and continental flux have been significant sources to varying degrees for banded iron-formation at various stratigraphic levels. Intermittent availability of oxygen appears to have resulted in rhythmic precipitation of iron hydroxide, and thus the banding of iron- and silica-rich layers. The essential components iron and silica of the banded iron-formation were supplied through hydrothermal fluids added to ocean water in the deep-marine part of the basin and transported due to the thermo-chemocline for precipitation in the shallow shelf zone. Seasonal suspended and dissolved fluvial flux from a continental source played a significant role in generating the composition of the banded iron-formation suite. Occurrence of the banded iron-formation with underlying quartz-pebble conglomerate and quartzites and the overlying turbidites might be explained by assuming a faster rate of generation and consumption of the Archean oceanic lithosphere.