Epitaxial quartz inclusions in corundum from a sapphirine–garnet boudin, Bamble Sector, SE Norway: SiO₂–Al₂O₃ miscibility at high P–T dry granulite facies conditions

Abstract

An Al-rich, SiO 2 -deficient sapphirine-garnet-bearing rock occurs as a metapelitic boudin within granulite facies Proterozoic charnockitic gneisses and migmatites on the island of Hisøy, Bamble Sector, SE Norway. The boudin is made up of peraluminous sapphirine, garnet, corundum, spinel, orthopyroxene, sillimanite, cordierite, staurolite and biotite in a variety of assemblages. Thermobarometric calculations based on coexisting sapphirine-spinel, garnet-corundum-spinel-sillimanite, sapphirine-orthopyroxene, and garnet-orthopyroxene indicate peak-metamorphic conditions near to 930°C at 10 kbar. Corundum occurs as single 200 to 3000 micron sized skeletal crystal intergrowths in cores of optically continuous pristine garnet porphyroblasts. Quartz occurs as 5-60 micron-sized euhedral to lobate inclusions in the corundum where it is in direct contact with the corundum with no evidence of a reaction texture. Some crystal inclusions exhibit growth zoning, which indicates that textural equilibrium was achieved. Electron Back-Scatter Diffraction (EBSD) studies reveal that the quartz inclusions share a common c-axis with the host corundum crystal. The origin of the quartz inclusions in corundum is enigmatic as recent experimental studies have confirmed the instability of quartz-corundum over geologically realistic P-T ranges. The combined EBSD and textural observations suggest the presence of a former silica-bearing proto-corundum, which underwent exsolution during post-peak-metamorphic uplift and cooling. Exsolution of quartz in corundum is probably confined to fluid-absent conditions where phase transitions by coupled dissolution-precipitation mechanisms are prevented.

Metasomatism of gabbro - mineral replacement and element mobilization during the Sveconorwegian metamorphic event

“…Amphibolite to granulite facies conditions have been documented for the Sveconorwegian event in the Bamble sector (e.g. Nijland & Maijer, 1993; Knudsen, 1996; Kihle et al. , 2010).…”

“…, 1997; Pawley, 1998). Sapphirine is generally considered to crystallize at very high temperatures (Harley, 1989; Kihle et al. , 2010), but experiments have shown that in Al‐rich system with excess water it can also form at 600–760 °C for pressures up to 0.7 GPa (Seifert, 1974).…”

“…The Bamble sector of the Precambrian crust of South Norway has been viewed as one of the classical high‐grade metamorphic terranes (e.g. Touret, 1971; Knudsen, 1996; Kihle et al. , 2010).…”

“…Cordierite‐ and sapphirine‐bearing rocks have also been used to argue for high‐ T metamorphism (e.g. Visser & Senior, 1990; Kihle et al. , 2010).…”

“…1) is considered as one of the classical examples of high‐grade metamorphic terranes (e.g. Touret, 1971; Knudsen, 1996; Kihle et al. , 2010) recording the transition from amphibolite to granulite facies metamorphism.…”