Late Archean high-pressure pelitic granulites in the Yinshan Block, North China Craton

Abstract

Ancient high-pressure metamorphism provides constraints for the understanding of Archean tectonic processes. Archean high-pressure metamorphic rocks, however, are rarely preserved in old cratons. Here, we report the newly discovered Archean high-pressure metapelites in the Yinshan Block of the North China Craton. The peak metamorphic stage (M2) is represented by kyanite, garnet, plagioclase, K-feldspar, quartz, rutile and melt. Kyanite was replaced by sillimanite during post-peak (M3) decompression process. Phase equilibria modelling in the MnNCKFMASHT system indicates nearly isothermal decompression with peak conditions of 840–870°C and >14–11kbar. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating of leucosome (13XWL35) and monazite U–Pb dating of kyanite-garnet-bearing metapelite (13XWL22) yield similar ages of ∼2520Ma, which represent the crystallization age of granulite-facies melt. The high-pressure metapelites are enriched in Na2O (1.8–3.0wt.%), MgO (3.4–5.7wt.%), Cr (245–421ppm) and Ni (21.0–91.9ppm) contents, suggesting that volcanic rocks are probably the main sedimentary provenance. High δ18O(WR) values (up to 12.4‰) and zircon oxygen isotopes (av. 11.4‰) of the metasediment source indicate they have undergone low-temperature alteration. The pressure-temperature-time constraints of this study provide a rare record indicating that surficial sediments have been buried to a depth of >40km at ca. 2520Ma in the Yinshan Block. Such crustal thickening, combined with available synchronous magmatism suggests that high-pressure metamorphism was triggered in a late Archean deep arc setting in the Yinshan Block of the North China Craton.