Session 2D — Lectures by Fellows/Associates

Renee M. Borges, IISc, Bengaluru

Origin of ruby in chromiferous anorthosites, from the Sittampundi Layered Complex, South India View Presentation

The origin of colourless and pink corundum (ruby) is a subject of significant interest to petrologists and gemologists. Studies have shown that economically viable ruby is associated with alluvial deposits and hence genesis of this gem mineral remains the subject of considerable debate. Corundum / ruby is rarely found in basic-ultrabasic / anorthositic rocks. A number of diverse petrogenetic models have been proposed to explain the formation of corundum in basic-ultrabasic rocks and anorthosites (extremely rare). These are (1) as a liquidus phase during magmatic crystallization; (2) as a product of high- to ultra-high pressure metamorphism; (3) through anatexis (melting) of anorthositic rocks; (4) as a result of metasomatism i.e. by desilification of rocks. The ~2.9 Ga old metamorphosed layered anorthosite complex of Sittampundi (SLC) developed ruby (with up to 2.2 wt% Cr₂O₃) in the anorthosite rocks (with unusually Ca-Al-rich feldspars, with >96 mole % anorthite end-member), proximal to chromitite layers. Textural features and numerically computed phase diagrams in the systems NCASH (Na₂O-CaO-Al₂O₃-SiO₂-H₂O) and CASH suggest that incongruent melting of the Ca-Al-rich feldspars in the anorthosite rocks requires metamorphic temperature in excess of 1000C at 9 kbar, due to the presence of Na (albite content in feldspar) and H₂O. Integrating all the geological features it is proposed that vapour (H₂O) assisted partial melting of the anorthosite rocks of the SLC at >1000°C and at ~9 kbar, formed the ruby and corundum. The Cr₂O₃ that renders the pink color of the ruby was presumably derived from the adjoining chromite band during partial melting. This study offers a new petrogenetic model for the origin of ruby in nature.