Retrograde metamorphism of ultrahigh-temperature granulites from the khondalite belt in Inner Mongolia, North China Craton: Evidence from aluminous orthopyroxenes

Abstract

Orthopyroxene-sillimanite-sapphirine-spinel-quartz granulites have been reported from the khondalite belt in Inner Mongolia, North China Craton (NCC), which record ultrahigh-temperature (UHT) metamorphism. These rocks record metamorphic temperatures up to 1050°C and pressures up to 10kbar during the peak metamorphism, as indicated by diagnostic UHT mineral assemblages such as equilibrium orthopyroxene+sillimanite+quartz, sapphirine+quartz and low-Zn-spinel+quartz. To well constrain the retrograde P-T conditions, we investigate orthopyroxenes in different textural settings in these UHT rocks and the results reveal marked variations in alumina contents, indicating a multistage formation. The core of coarse orthopyroxene porphyroblasts and some of the grains associated with sillimanite and quartz have the highest alumina content of up to 7.61-8.05 wt%. Rims of coarse orthopyroxene porphyroblasts and those occurring as coronas surrounding garnet grains have intermediate alumina value ranging from 6.21 to 7.61wt%. Fine grained orthopyroxenes which form symplectite with cordierite have the lowest alumina content between 5.52 and 6.15wt%. Based on the XAl-XMg isopleths, orthopyroxenes in different textural associations and textures indicate formation under varying temperature and pressure conditions, ranging from 950°C, 10.5kbar to 840°C, 8.5kbar. These results are in accordance with the P-T estimates from previous studies on these rocks using Ti-in-zircon geothermometry and pseudosection analysis. The reaction textures and orthopyroxene chemistry suggest an initial stage of isobaric cooling followed by isothermal decompression, and a final stage of cooling. Combined with the available age data which suggest the timing of the UHT event as 1.92Ga, and the previous models on P-T evolution of the khondalites, our results suggest a complex exhumation of the UHT granulites within the Inner Mongolia Suture Zone associated with the final assembly of the NCC within the Columbia supercontinent during the Palaeoproterozoic. © 2010 John Wiley & Sons, Ltd..