Relative timing of folding and metamorphism in the ruby mine area of the Harts Range, central Australia

The tectonothermal history of a predominantly supracrustal sequence within the eastern Arunta Inlier has been deduced from detailed mapping in the Harts Range ruby mine area. The Irindina supracrustal assemblage comprises metapelites, with uncommon interlayered marble and quartzite, and layered amphibolites. The Entire anorthosite was intruded into this sequence. It is reasoned that the amphibolites were submarine basalts and as a consequence their high water contents facilitated early in situ partial melting resulting in tonalite interlayers. New mineral growth, during prograde upper amphibolite facies metamorphism, produced pervasive S₁ foliations and L₁ mineral lineations. Fabric development accompanying later fold phases was limited because it involved recrystallization of existing stable minerals rather than the growth of new ones. F₂ and F₃ folds plunge gently to the north, parallel to L₁. Major recumbent isoclinal F₂ folds cause repetition of the lithological sequence. F₃ folding was less intense than F₂ and postdated retrograde amphibolite facies metamorphism that formed chlorite in ultramafic boudins. Open upright F₄ and F₅ folds refold earlier structures. The ruby deposits resulted from local metasomatic alteration of anorthosite folded into ultramafic boudins by F₁. S₁ hornblende in ultramafics grew from a reaction between silica‐deficient minerals and silica derived from anorthosite. Ruby formed in more highly altered anorthosite because alumina was concentrated by this process.

Key words:

• Proterozoic
• structure
• metamorphism
• fabric development
• anorthosites
• basaltic amphibolites
• ruby deposits
• metasomatism
• Arunta Inlier