Discovery of pelitic high-pressure granulite from Manjinggou of the Huai’an Complex, North China Craton: Metamorphic P–T evolution and geological implications

Abstract

The first report of Paleoproterozoic mafic high-pressure (HP) granulite terranes in Manjinggou area of the Huai’an Complex, North China Craton (NCC) since 1992 has attracted broad attention from geologists, and accordingly various Early Precambrian continent-continent collision or micro-blocks amalgamation hypotheses were suggested. ​However, one important issue is still controversial whether the pelitic granulites and associated mafic HP granulites underwent similar metamorphic history. Here we report newly recognized pelitic HP granulites in Manjinggou area of the Huai’an Complex to provide direct evidence that both pelitic and mafic HP granulites suffered similar metamorphic history. The metamorphic peak mineral assemblage of the pelitic granulites is characterized by Grt-Ky-Kfs-Bt-Rt-Qz-Liq±Ms, and the subsequent medium-pressure (MP) granulite facies retrogression is characterized by Grt-Sil-Kfs-Pl-Bt-Rt-Liq-Qz. Pseudosection approaches were undertaken in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O (NCKFMASHTO) system to account for textural development, mineral composition and P–T evolution of the pelitic granulites. The estimated peak and retrograde granulite facies conditions are 11.5–15kbar, 810–860°C and ∼9.5kbar, ∼850°C, respectively, comparable with those of associated mafic HP granulites. Consistently, field geological observations and available geochronological data also indicate their similar deformation and metamorphic history from HP granulite facies stage (∼1.96–1.90Ga) to retrograde stages (∼1.88–1.80Ga). This finding changes previous views that the pelitic rocks and associated mafic granulites are different slabs with individual metamorphic history. The khondalite series from the Huai’an Complex and adjacent Ji’ning Complex were probably experienced HP granulite facies metamorphism, but previously derived MP granulite facies conditions might be attributed to mineral re-equilibrium due to long dwell time under MP granulite facies. Additionally, these granulite facies rocks record high apparent geothermal gradients and slow exhumation rates relative to HP–UHP rocks in the Phanerozoic continental collisional orogens. It seems to indicate that the thermal regimes and tectonic mechanisms of Paleoproterozoic probably differ from those of Phanerozoic eons, and these HT–HP granulite terranes from the NCC were likely formed in a hot and slow cooling orogen.