Fe–Mg–Al–Ti–Zn oxide assemblage in granulites of the Bunger Hills, East Antarctica: evidence of ultrahigh-temperature metamorphism

Abdrakhmanov I. A., Gulbin Yu. L., Gembitskaya I. M.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 4. P. 38-76

https://doi.org/10.31857/S086960552104002X

Language: Russian

Abstract

Composition and relations between oxides (minerals of the spinel supergroup, corundum, ilmenite, rutile) and silicates (garnet, sillimanite, orthopyroxene, cordierite) from metapelitic granulites of the Bunger Hills are studied. It is shown that aluminous spinels (X_Mg 0.17–0.52) from the matrix and inclusions in garnet are poor in chromium and divided into two groups by Zn contents: one which is depleted in this element (ZnO < 3 wt %) and zinc-bearing spinel (ZnO 7–20 wt %) which is assigned to intermediate members of the hercynite–gahnite series. Intergrowths of aluminous spinels (including Zn-deficient ones) and quartz are observed. Besides individuals in the matrix, aluminous spinels form plate-like inclusions of zinc-bearing hercynite and gahnite with up to 30 wt % ZnO in ilmenite and rutile. Aluminous spinel grains are associated with subparallel corundum-magnetite-ilmenite intergrowths derived from the high-temperature solid solution (Ti-Mg_SS) as well as ilmenite with lamellae of rutile possibly produced as a result of breakdown of the high-temperature Fe–Ti phase (armalcolite). The most intriguing feature of the studied granulites is flame-like exsolutions of titaniferous magnetite and ulvöspinel (X_Usp 0.55–0.78) occurred in intergrowth with ilmenite, corundum, and needle-like rutile. The calculated temperatures of the Usp–Ilm equilibrium vary between 950 and 1250 °С. Ulvöspinel coexists with garnet and orthopyroxene containing 7 to 9 wt % Al₂O₃. Temperatures estimated using Grt-Opx thermobarometry are in range of 930–1010 °С. The clockwise Р–Т path defined by mineral thermobarometry and isochemical phase diagram modeling shows an increase in temperature and pressure up to peak values of T > 1000 °С, P ~ 10 kbar, followed by isothermal decompression to 6–7 kbar, and then isobaric cooling.

Keywords: granulite, UHT metamorphism, Fe–Mg–Zn aluminous spinels, magnetite, corundum-ulvöspinel-ilmenite intergrowths, orthopyroxene, mineral thermobarometry, Bunger Hills, East Antarctica