High-pressure/ultrahigh-temperature mafic and felsic granulites associated with magnetite deposits from the Jiaobei terrane, Jiao-Liao-Ji Belt, North China Craton

Abstract

Garnet-bearing mafic and felsic granulites associated with magnetite deposits that have undergone high-pressure (HP) and ultrahigh-temperature (UHT) metamorphism occur in the Yangjiazhai area of the Jiaobei terrane within the Jiao-Liao-Ji Belt, North China Craton. In mafic granulite, the early prograde assemblage (M1) is preserved as inclusions in the core of garnet. Matrix minerals and the garnet mantle define the peak metamorphic assemblage (M2). Near-isothermal decompression (M3) is recorded by the garnet rim and symplectitic minerals. The final cooling process is defined by the late growth of amphibole (M4). Abundant magnetite occurring in some mafic granulite samples may be inherited from the protolith. The felsic granulite records the peak metamorphic stage (M2) defined by the matrix minerals and the garnet mantle, and the retrograde stage defined by the symplectite and the garnet rim (M3). P-T estimations using pseudosection techniques and two-feldspar thermometry yield ∼8–9.8 kbar/710–828 °C for the prograde stage M1, ∼14.5–15 kbar/1040–1073 °C for the peak P-T stage M2, 9.1–10 kbar/965–1015 °C for the post-peak stage M3 and 6.6–9.4 kbar/820–853 °C for the cooling stage M4 as recorded by the mafic granulite. P-T conditions of ∼14–16.5 kbar/985–1077 °C for the peak P-T stage M2 and ∼7.8–9 kbar/889–902 °C for the post-peak stage M3 were derived from the felsic granulite. Thus a clockwise trajectory can be defined by constraining peak-metamorphic high-pressure/ultrahigh-temperature (HP-UHT) conditions. Zircon U-Pb dating and REE characteristics show a contrasting information. Oscillatory zoned zircon domains yield an age of ∼2150 Ma with a steep HREE characteristic, which is interpreted to indicate a magmatic origin of the mafic granulite. The peak HP-UHT stage obtained from metamorphic zircon domains in felsic and mafic granulites is constrained by an age of ∼1950–1900 Ma, which possibly corresponds to a collisional event shortly after subduction. The decompression occurred at ∼1880 Ma following the subsequent cooling at ∼1820 Ma indicating a rapid exhumation of the HP-UHT granulite followed by slow cooling.