[S-MP25] Supercontinents and Crustal Evolution

[SMP25-11] Diffusion-controlled reactions from the Archean Sittampundi Layered Complex, India

*Sudarsan Veni¹、Sajeev K¹、Windley B. F.² (1.Centre for Earth Sciences, Indian Institute of Science, Bangalore, India、2.School of Geography, The University of Leicester, Leicester, UK)

キーワード：Chromite、Metasomatism、Archean Layered Complex

The Archean chromite-layered anorthosite of the Sittampundi Complex, Tamil Nadu, India is bordered by a layer containing peraluminous corundum, spinel, sapphirine, sillimanite, staurolite and högbomite all set in a matrix of plagioclase. A similar sapphirine-bearing assemblage has prompted many workers to infer ultrahigh-temperature (UHT) metamorphism from the same location. However, our detailed petrological investigations demonstrate that diffusion-controlled metasomatic reactions were responsible for formation of the high-temperature sapphirine, spinel, corundum etc. There are two types of spinel: Fe-Mg (XMg= 0.4) and Cr-rich (Cr₂O₃ 12wt%) in the same sample; nevertheless, chromite is not present in this layer. Fe-Mg-spinel is associated with Ti-bearing högbomite (TiO₂ ~5wt%), which are indistinguishable under transmitted light. Sapphirine (7:9:3 polytype) occurs along the grain boundaries of Fe-rich staurolite (XFe= 0.64), sapphirine and staurolite are mostly along the dihedral grain boundaries of plagioclase (XAn=0.92), and corundum forms a thin film along plagioclase-gedrite grain boundaries. Ca-rich plagioclase inclusions are in gedrite, sapphirine, and cordierite, suggesting an early-stage formation of plagioclase. Ca-amphibole forms inclusions in cordierite that has up to ~2wt% Cr₂O₃. Also, sapphirine, corundum, gedrite, sillimanite, högbomite and staurolite have significant contents of Cr₂O₃, indicating mobility of chromium derived from nearby chromitite layers in the anorthosite. The textures and mineral chemistry indicate a diffusion-driven alteration of an anorthosite layer close to chromitite. Similar unusual mineral assemblages are reported in metasomatic sapphirine-bearing rocks of the Fiskenaesset complex, Greenland, and in the Limpopo belt, South Africa. These assemblages are indicative of cm-scale metasomatic reactions between two distinct lithologies driven by a significant chemical potential gradient.