Mineral Composition, Textures and Deformation in Late Archaean Banded Iron-Formation Rich in Magnesioriebeckite and Aegirine, Bababudan, Karnataka, Southern India

Assemblages rich in magnesioriebeckite, aegirine and magnesian siderite (var. pistomesite) characterise parts of the Banded Iron-Formation low in the Mulaingiri Formation of Bababudan, Electron microprobe analyses show that aegirine contains up to 53.29 wt% Si0₂, 0.17 wt% A1₂0₃, 32.20 wt% TotFeO, 0.23 wt% MgO and 14.09 wt% Na₂0; and magnesioriebeckite contains up to 55.57 wt% Si0₂, 0.07 wt% Al103, 29.16 wt% TotFeO, 8.53 wt% MgO and 7.37 wt% Na₂0. These abundances are broadly in accord with previously published results from wet analytical techniques.

A diagenetic or very low-grade metamorphic texture of randomly oriented needles of magnesioriebeckite and granoblastic-polygonal quartz in chert mesobands is transitional into a texture dominated by long prisms of magnesioriebeckite intergrown with elongate grains of quartz which forms an LS tectonic fabric. Aegirine and magnesian siderite have locally replaced magnesioriebeckite in this fabric. The carbonate also forms small ring structures, interpreted as deformation phenomena, in parts of some mesobands.

The pre- and syntectonic textures of magnesioriebeckite are regarded as the products of percolating solutions rich in Na⁺ and Mg⁺⁺ interacting with the host during diagenesis, very low-grade burial metamorphism and progressive deformation. The tectonic LS fabric appears to have been initiated early during deformation with textural modifications taking place during buckle shortening and layer-parallel shearing.