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

摘要

The Jining Group occurs as the eastern segment of the Khondalite Belt, North China Craton andisdominated by a series of granulite facies rocks involving normal' pelitic granulites recording peak temperatures of similar to 850 degrees C and ultrahigh-temperature (UHT) pelitic granulites recording peak temperatures of 950-1100 degrees 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 similar to 870-890 degrees C/7-8kbar and >840 degrees C/>7.5kbar 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 degrees C/8-9kbar 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 Pb-207/Pb-206 ages from 1.89 to 1.79Ga for sample J1210, and from 1.94 to 1.80Ga 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.1.95Ga as suggested by the pre-T-max decompression P-T path.