Phase equilibria modelling and zircon age dating of pelitic granulites in Zhaojiayao,from the Jining Group of the Khondalite Belt,North China Craton

The Jining Group occurs as the eastern segment of the Khondalite Belt, North China Craton and is dominated by a series of granulite facies rocks involving ‘normal’ pelitic granulites recording peak temperatures of ~850 °C and ultrahigh‐temperature (UHT) pelitic granulites recording peak temperatures of 950–1100 °C. The P–T paths and ages of these two types of granulites are controversial. Three pelitic granulite samples in the Jining Group comprising two sillimanite–garnet gneiss samples (J1208 and J1210) and one spinel–garnet gneiss sample (J1303) were collected from Zhaojiayao, where ‘normal’ pelitic granulites occur, for determination of their metamorphic evolution and ages. Samples J1208 and J1210 are interpreted to record cooling paths from the T_max stages with P–T conditions respectively of ~870–890 °C/7–8 kbar and >840 °C/>7.5 kbar constrained from the stability fields of the observed mineral assemblages and the isopleths of plagioclase, garnet and biotite compositions in pseudosections. Sample J1303 is interpreted to record pre‐T_max decompression from the kyanite‐stability fields to the T_max stage of 950–1020 °C/8–9 kbar and a post‐T_max cooling path revealed mainly from the stability field of the observed mineral assemblage, the plagioclase zoning and the biotite composition isopleth in pseudosections. The post‐T_max cooling stage can be divided into suprasolidus and subsolidus stages. The suprasolidus cooling may not result in an equilibrium state at the solidus in a rock. Therefore, different minerals may record different P–T conditions along the cooling path; the inferred maximum temperature is commonly higher than the solidus as well as different solidi being recorded for different samples from the same outcrop but experiencing different degrees of melt loss. Plagioclase compositions, especially its zoning in plagioclase‐rich granulites, are predicted to be useful for recording the higher temperature conditions of a granulite's thermal history. The three samples studied seem to record the temperature range covering those of the ‘normal’ and UHT pelitic granulites in the Jining Group, suggesting that UHT conditions may be reached in ‘normal’ granulites without diagnostic UHT indicators. LA‐ICP‐MS zircon U–Pb data provide a continuous trend of concordant ²⁰⁷Pb/²⁰⁶Pb ages from 1.89 to 1.79 Ga for sample J1210, and from 1.94 to 1.80 Ga for sample J1303. These continuous and long age spectrums are interpreted to represent a slow cooling and exhumation process corresponding to the post‐T_max cooling P–T paths recorded by the pelitic granulites, which may have followed the exhumation of deeply buried rocks in a thickened crust region resulted from a collision event at c. 1.95 Ga as suggested by the pre‐T_max decompression P–T path.