High P-T phase relation of magnesian (Mg0.7Fe0.3) staurolite compositon in the system FeO-MgO-Al2O3-SiO2-H2O: Implications for prograde high-pressure history of ultrahigh-temperature metamorphic rocks

Abstract

High-pressure and high-temperature phase relation on Mg 0.74 Fe 0.26 -staurolite composition with excess water in the system FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O was experimentally determined at 12-19 kbar and 850-1050 °C, with a focus on the lower-pressure stability limit of magnesian staurolite. The experimental results show that the stability field of staurolite in the system shifts to higher temperatures as compared to that of pure-Mg staurolite. At 950 °C, the lower-pressure stability limit is located between 15 and 14 kbar. This study demonstrates that staurolite breaks down with isothermal decompression by a series of reactions at 950 °C: St → Opx + Crn + Melt and St + Opx → Spr + Melt. The Mg' [=Mg/(Fe+Mg)] of staurolite in the former reaction is 0.7. When staurolite coexists with orthopyroxene in run products, the staurolite Mg' is decreased to 0.6-0.5. Moderate-Mg staurolite (Mg' = ∼0.5) in natural occurrences has been reported as inclusion minerals in poikiloblastic garnets within ultrahigh-temperature (UHT) metamorphic rocks from major collisional orogenic belts in southern India and southern Africa. The experimental data presented in our study suggest the possibility that the staurolite-bearing UHT metamorphic rocks had experienced high-pressure metamorphism and/or UHT extreme metamorphism during the prograde metamorphic stage.