Discovery of kyanite in typically cordierite/sillimanite-bearing low- to medium-pressure pelitic granulites from the Jiaobei terrain, North China Craton

Abstract

The pelitic granulites along the Paleoproterozoic Jiao-Liao-Ji belt (JLJB) are dominated by cordierite/sillimanite-bearing low- to medium-pressure (LMP) conditions. Recent studies have recognized a few typical high-pressure (HP) pelitic granulites without retrograde cordierite from the Jiaobei terrane, the southern part of the JLJB. The relationship between the dominant LMP and rare HP pelitic granulites is not well constrained, and whether the LMP types are retrograde products of the HP ones remains uncertain. This uncertainty precludes a full understanding of the original spatial distribution of HP pelitic granulites and the tectonic affinity of the JLJB. We identify in this study for the first time, relict kyanite and the mineral assemblage of kyanite + K-feldspar + rutile + garnet from typically cordierite/sillimanite-bearing LMP pelitic granulites within the Jiaobei terrane. Integrated results from petrology, pseudosection modeling, geothermobarometry, and zircon and monazite U-Pb geochronology suggest that these LMP pelitic granulites experienced an earlier HP granulite-facies metamorphism possibly at ~1.95 Ga, with minimum P-T conditions of 13–14 kbar and 860–870 °C. Subsequently, these rocks underwent near isothermal decompression to 4–7 kbar and >840 °C, followed by isobaric cooling to ~600 °C and 4–6 kbar. The decompression and cooling triggered LMP granulite-facies retrogression at ~1.85 Ga that eclipsed the earlier HP granulite-facies records. The clockwise P-T-t path defined is consistent with those reported from typical HP pelitic and mafic granulites elsewhere in the Jiaobei terrane. The results confirm that at least some portions of the typically cordierite/sillimanite-bearing LMP pelitic granulites are retrograded from the HP types. This greatly expands the spatial distribution of HP pelitic granulites and suggests a linear HP granulite-facies metamorphic belt within the Jiaobei terrane. The linear distribution of HP pelitic granulites whose protoliths are sedimentary rocks from Earth’s surface and the consistent clockwise P-T-t paths require Paleoproterozoic subduction, collision, and exhumation processes similar to those operating in modern plate tectonics. Our results from the JLJB thus provide new metamorphic constraints on the nature of Paleoproterozoic plate tectonics, and have implications for other Precambrian high-grade terranes worldwide where pelitic granulites are dominated by LMP types.