Paleoproterozoic ultrahigh‐temperature metamorphism in the Alxa Block, the Khondalite Belt, North China Craton: Petrology and phase equilibria of quartz‐absent corundum‐bearing pelitic granulites

Abstract

AbstractAn Al‐rich, quartz‐absent corundum‐bearing pelitic granulite is reported for the first time at the Diebusige complex in the Alxa Block in the western part of the Khondalite Belt (KB), North China Craton (NCC). A detailed petrographic study shows that a quartz‐present domain occurs in porphyroblastic garnet cores from the corundum‐bearing pelitic granulite, with a prograde metamorphic mineral assemblage (M1) of garnet + plagioclase + quartz + biotite + spinel + rutile ± melt, and a quartz‐absent domain occurs in the matrix of this rock, with a preserved peak mineral assemblage (M2) of garnet + corundum + spinel + biotite + plagioclase + K‐feldspar + rutile + melt and a retrograde metamorphic mineral assemblage (M3) of garnet + sillimanite + corundum + biotite + plagioclase + K‐feldspar ± rutile. Phase equilibrium modelling and Zr‐in‐rutile and ternary feldspar thermometry all reveal similar peak conditions of 890–940°C at 7.5–9.8 kbar, which indicate a high geothermal gradient of ~110°C/kbar and ultrahigh‐temperature (UHT) conditions. Combining this information with the detailed analysis of metamorphic zircons, we obtain the metamorphic evolution of the corundum‐bearing pelitic granulite. Specifically, the corundum‐bearing pelitic granulite experienced granulite‐facies metamorphism at ~1950 Ma. Then, this rock was slowly uplifted with heating and partial melting until it achieved UHT conditions. Finally, it cooled to the solidus at ~1830 Ma. Thus, we propose that the UHT metamorphic conditions of a corundum‐bearing pelitic granulite from the Diebusige complex of the Alxa Block may be the result of long‐term slow uplift with heating under a high geothermal gradient.