Petrochronology of oxidized granulites from southern Peru

Sapphirine-quartz granulites from the Cocachacra region of the Arequipa Massif in southern Peru record early Neoproterozoic ultrahigh-temperature metamorphism. Phase equilibrium modelling and zircon petrochronology are used to quantify timing and pressure-temperature (P-T) conditions of metamorphism. Modelling of three magnetite-bearing sapphirine-quartz samples indicates peak temperatures of >950 degrees C at similar to 0.7 GPa and a clockwise P-T evolution. Elevated concentrations of Al in orthopyroxene are also consistent with ultrahigh-temperature conditions. Neoblastic zircon records ages of c. 1.0-0.9 Ga that are interpreted to record protracted ultrahigh-temperature metamorphism. Th/U ratios of zircon of up to 100 reflect U-depleted whole-rock compositions. Concentrations of heavy rare earth elements in zircon do not show systematic trends with U-Pb age but do correlate with variable whole-rock compositions. Very large positive Ce anomalies in zircon from two samples probably relate to strongly oxidizing conditions during neoblastic zircon crystallization. Low concentrations of Ti-in-zircon (<10 ppm) are interpreted to result from reduced titania activities due to the strongly oxidized nature of the granulites and the sequestration of titanium-rich minerals away from the reaction volume. Whole-rock compositions and oxidation state have a strong influence on the trace element composition of metamorphic zircon, which has implications for interpreting the geological significance of ages retrieved from zircon in oxidized metamorphic rocks.