http://www.sciencedirect.com/science/article/pii/S1674987112000060

The Khondalite Belt within the Inner Mongolia Suture Zone（IMSZ） in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature（UHT） metamorphism.Here we report new spinel-bearing metapelitic granulites from a new locality at Xumayao within the southern domain of the IMSZ.Petrological studies and thermodynamic modeling of the spinel+quartz-bearing assemblage shows that these rocks experienced extreme metamorphism at UHT conditions.Spinel occurs in two textural settings:（1） high X_Zn（Zn/（Mg+Fe^Ⅱ+Zn）=0.071-0.232） spinel with perthitic K-feld-spar. sillimanite and quartz in the rock matrix;and（2） low X_Zn（0.045—0.070） spinel as inclusions within garnet porphyroblasts in association with quartz and sillimanite. Our phase equilibria modeling indicates two main stages during the metamorphic evolution of these rocks:（1） near-isobaric cooling from 975℃to 875℃around 8 kbar.represented by the formation of garnet porphyroblasts from spinel and quartz;and（2）cooling and decompression from 850℃.8 kbar to below 750℃.6.5 kbar,represented by the break-down of garnet.The spinel+quartz assemblage is considered to have been stable at peak metamorphisni.formed through the break-down of cordierite.indicating a near isothermal compression process.Our study confirms the regional extent of UHT metamorphisni within the IMSZ associated with the Paleoproterozoic subduction-collision process.