Paleoproterozoic multistage metamorphic events in Jining metapelitic rocks from the Khondalite Belt in the North China Craton: Evidence from petrology, phase equilibria modelling and U–Pb geochronology

Abstract

Metapelitic rocks of the Jining Complex (sillimanite-cordierite-garnet (Sil–Crd–Grt) gneisses, sillimanite-garnet (Sil–Grt) gneisses and quartzofeldspathic rocks) are exposed in the eastern segment of the Khondalite Belt (KB) in the North China Craton (NCC). The Sil–Crd–Grt gneisses have preserved polyphase mineral assemblages and microstructural evidence of anatexis, resulting from biotite dehydration melting. Petrological observations revealed that the Sil–Crd–Grt gneisses contain three metamorphic assemblages: a peak assemblage of garnet porphyroblast and matrix biotite+sillimanite+K-feldspar+plagioclase+quartz+ilmenite+magnetite, a post-peak near-isothermal decompressional assemblage of garnet+cordierite+biotite+sillimanite+K-feldspar+plagioclase+quartz+ilmenite+magnetite, and a decompressional cooling assemblage of garnet+biotite+cordierite+K-feldspar+plagioclase+quartz+ilmenite+magnetite. A clockwise P–T path was defined involving the inferred peak stage followed by post-peak near-isothermal decompression and decompressional cooling stages, with P–T conditions of 790–825°C and 9–10kbar, 810–890°C and 6.0–6.5kbar, and 780–810°C and 4.0–5.5kbar, respectively. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA–ICP–MS) U–Pb analyses of the Sil–Crd–Grt gneisses and Sil–Grt gneisses for the detrital and metamorphic zircons yielded a protolith age of ∼2.0Ga and the late Paleoproterozoic metamorphic age of 1895–1885Ma. The results reveal that the metapelitic rocks of the Jining Complex underwent continent–continent subduction or collision in the peak metamorphic stage, followed by a post-collisional exhumation event in the post-peak decompressional stage, and a subsequent decompressional cooling stage between the Yinshan and Ordos blocks to form the Paleoproterozoic KB.