Ultra-high temperature metamorphism recorded in Fe-rich olivine-bearing migmatite from the Khondalite Belt, North China Craton

We document the first occurrence of Fe-rich olivine-bearing migmatitic metapelite in the Khondalite Belt, North China Craton. Petrological analyses revealed two exotic assemblages of orthopyroxene+spinel+olivine and orthopyroxene+spinel+cordierite. Phase relation modelling suggests that these assemblages are diagnostic of ultra-high temperature (UHT) metamorphism in the Fe-rich system, with temperatures from 1,000 to 1,050 degrees C at 0.6GPa. U-Th-Pb SIMS analyses on zircon reveal a similar age of c. 1.92Ga for the olivine-bearing migmatite and an adjacent gabbronoritic intrusion that is therefore identified as the heat source for the UHT metamorphism. These results, coupled with additional analysis of the famous Tuguiwula sapphirine-bearing granulite, lead to a re-appraisal of the P-T path shape and heat source for the UHT metamorphism. We suggest that UHT metamorphism, dated between 1.92 and 1.88Ga, across the whole Khondalite belt, proceeded from a clockwise P-T evolution with an initial near-isobaric heating path at similar to 0.6-0.8 GPa, and a maximum temperature of 1,050 degrees C followed by a cooling path with minor decompression to similar to 0.5GPa. Considering our results and previous works, we propose that the orogenic crust underwent partial melting at temperature reaching 850 degrees C and depth of similar to 20 to similar to 30km during a period of c. 30Ma, between 1.93 and 1.90Ga. During this time span, the partially molten crust was continuously intruded by mafic magma pulses responsible for local greater heat supply and UHT metamorphism above 1,000 degrees C. We propose that the UHT metamorphism in the Khondalite belt is not related to an extensional post-collisional event, but is rather syn-orogenic and associated with mafic magma supplies.