Can we extract ultrahigh-temperature conditions from Fe-rich metapelites? An example from the Khondalite Belt, North China Craton

In this study, garnet–sillimanite gneisses at Hongshaba in the eastern segment of the Khondalite Belt, North China Craton (NCC) are interpreted to have experienced ultrahigh-temperature (UHT) metamorphism (980–1040 °C) followed by post-T_max cooling at pressures of 8–9 kbar to the solidus (810–830 °C), consistent with rare sapphirine-bearing assemblages in surrounding regions. This interpretation is mainly based on the combination of P–T fields and garnet X_gr (=Ca/(Ca + Mg + Fe²⁺)) isopleths on the pseudosection of three garnet–sillimanite gneiss samples. Spinel tends to be enclosed in the outer margins of garnet, commonly closely associated with quartz. We interpret this to reflect the partial break down of garnet along the prograde path during heating with decompression followed by new garnet growth during cooling along an overall clockwise P–T evolution. Although Fe-rich UHT metapelites tend to contain neither diagnostic mineral assemblages nor orthopyroxene from which to extract T via Al-in-orthopyroxene thermometry, isopleths of Ca in garnet may aid in retrieving UHT conditions from these compositions. This is attributed to Ca diffusion in garnet being much slower than Fe and Mg diffusion, leading to little change in Ca contents during post-T_max cooling. LA-ICP-MS U-Pb dating of metamorphic zircon in one garnet–sillimanite gneiss sample yields a mean ²⁰⁷Pb/²⁰⁶Pb age of ca. 1.91 Ga, which is interpreted to record the timing of cooling of the UHT rocks to the solidus. This UHT metamorphism is interpreted to have been generated by mantle-derived magma during a tectonic extension from ca. 1.95 to 1.92 Ga within a post-orogenic setting.