Kyanite-bearing tonalites from Cape Hinode, East Antarctica: with special reference to those occurring close to calc-silicate blocks

Abstract

  The Cape Hinode area in East Antarctica exposes Mid-Proterozoic tonalites, which are similar in chemical composition to high-SiO₂ adakites and Archean tonalite-trondhjemite-granodiorite. They contain enclaves of ultrabasic-intermediate and calc-silicate metamorphic rocks of various dimensions, as well as xenocrystic garnet and kyanite. Pinite and carbonate minerals surrounding kyanite inclusions in plagioclase are alteration products after coronas of anorthite and spinel with or without corundum, alkali feldspar, rutile and magnetite. Further observations reveal that thin tonalitic veins cutting calc-silicate enclaves carry kyanite fringed by similar anorthite coronas. Such kyanite sometimes encloses quartz inclusions. The thin tonalitic veins are characterized by the occurrence of dark green clinopyroxene as the main mafic mineral, low aluminum saturation index (0.75) in spite of the occurrence of kyanite, and a non-adakitic geochemistry.
  The calc-silicate enclaves in tonalites are composed mainly of calcic plagioclase, Ca-garnet and/or clinopyroxene. The symplectitic intergrowth of anorthite and Ca-garnet is chemically close to epidote, and may be produced by successive melt-involving and epidote-consuming reactions. These chemical and textural features of kyanite and other minerals occurring in and around kyanite, thin tonalitic veins, calc-silicate enclaves, and tonalites indicate the significance of metamorphic and magmatic epidotes during partial melting of eclogites of young and hot slab origin.