Discovery and geological significance of high-pressure mafic granulites in the Pingdu–Anqiu area of the Jiaobei Terrane, the Jiao–Liao–Ji Belt, the North China Craton

Abstract

High-pressure (HP) mafic granulites in the Pingdu–Anqiu area of the Jiaobei Terrane, in the southwestern segment of the Jiao–Liao–Ji Belt of the North China Craton, occur as irregular lenses or deformed dike swarms within the Paleoproterozoic graphite-bearing paragneisses and marbles. Petrographic examination has revealed three distinct metamorphic mineral assemblages: a peak HP granulite-facies assemblage (M1) that consists of garnet+clinopyroxene+plagioclase+quartz±amphibole±Fe–Ti oxides, a post-peak decompression assemblage (M2) that is characterized by symplectites of orthopyroxene+clinopyroxene+plagioclase±amphibole±Fe–Ti oxides, and a late cooling assemblage (M3) represented by symplectites of amphibole+plagioclase+Fe–Ti oxides. Pseudosection modeling using THERMOCALC in the NCFMASHTO system and conventional thermobarometers constrained the P–T conditions of the M1, M2 and M3 assemblages to P=1.28–1.44GPa and T=757–805°C, P=0.50–0.80GPa and T=780–840°C, and P=0.55–0.73GPa and T=665–730°C, respectively. An integrated study involving laser Raman spectroscopy and scanning electron microscope analysis of mineral inclusions, cathodoluminescence imaging, and in-situ U–Pb dating of zircons showed that the protolith ages of the HP mafic granulites are mainly 2200–2000Ma and that the timing of the peak HP granulite-facies metamorphism ranges from 1950 to 1900Ma, as recorded by the cores of metamorphic zircons. The medium- to low-pressure amphibolite- to granulite-facies retrogression occurred mainly at 1900–1800Ma, as recorded by the rims of some zircon grains as well as zircon grains that contain inclusions of clinopyroxene+orthopyroxene+plagioclase+amphibole+sphene. Comprehensive petrographic, mineralogical, and geochronological investigations of the HP mafic granulites defined a clockwise P–T–t path involving near-isothermal decompression and near-isobaric cooling, which further suggests that the Jiaobei Terrane had underwent initial crustal thickening during 1950–1900Ma, followed by relatively rapid exhumation, cooling, and retrogression in the period 1900–1800Ma. This P–T–t path was probably generated by the Paleoproterozoic collisional orogenesis of the North China Craton.