Titanium-bearing sapphirine in a partially melted aluminous granulite xenolith, Vestfold Hills, Antarctica: geological and mineralogical implications
Harley, Simon L.; Christy, Andrew G.
European Journal of Mineralogy Volume 7 Number 3 (1995), p. 637 - 654
published: May 19, 1995
manuscript accepted: Dec 20, 1994
manuscript received: Apr 13, 1993
ArtNo. ESP147050703016, Price: 29.00 €
Abstract A new compositional variety of sapphirine containing up to 1.5 wt% TiO2 and close to the synthetic 2MgO:2Al2O3.SiO2 (2:2:1) end-member, has been formed through partial melting of an aluminous and magnesian sapphirine-enstatite-spinel granulite xenolith entrained in a noritic intrusive in the northern Vestfold Hills, East Antarctica. This sapphirine is 1Tc polytype and preserves internal twinning and stacking faults similar to other high-temperature sapphirines. The 2:2:1 sapphirine occurs in feldspathic areas and stringers throughout the xenolith and formed at temperatures in the range 1100-1180°C. Phase relations involving sapphirine, enstatite, spinel and cordierite constrain the pressure of contact melting, and therefore the emplacement depths of the ca. 2240 Ma post-metamorphic norite, to be near 3.5 kbar (8-11 km). The Vestfold Hills underwent substantial uplift following the ca. 7-8 kbar late-Archaean (ca. 2500-2480 Ma) metamorphism, rather than experiencing prolonged deep-crustal burial and slow exhumation. The xenolith also contains rare and localised textural domains within resorbed rutile in which a contrasting peraluminous sapphirine occurs coexisting with corundum. This demonstrates that both peraluminous and Si-rich sapphirines are stable to high temperatures at low pressures, contrary to previous suggestions that sapphirine is necessarily peraluminous under these conditions.