Geochemistry and P-T-t evolution of Paleoproterozoic metapelites of the Zhengjiapo BIF-type iron mine, Jiaobei terrane, North China Craton.

• Metapelites of the Fenzishan Group were formed in a back-arc basin of an active continental margin. • Three age groups of detrital zircon: ∼ 3019–2816 Ma, 2761–2610 Ma and ∼ 2261–2128 Ma. • The minimum depositional age of metapelite precursor is ∼ 2261 – 2128 Ma. • Apatite U-Pb age ∼ 1923 – 1893 Ma reflects to the post peak collisional metamorphism. • Metapelites record the UHT metamorphism, showing an anticlockwise P-T path. Metapelites from the Fenzishan Group of the Zhengjiapo banded iron formation (BIF) iron mine studied have undergone high-pressure and ultrahigh-temperature (UHT) metamorphism. The rocks occur in the Changyi metallogenic belt that belongs to the Jiaobei terrane, Jiao-Liao-Ji Belt, North China Craton. P-T estimates using pseudosection modeling, mineral compositio isopleths and two-feldspar geothermometry of observed mesoperthite and antiperthite intergrowth textures of Bt-Grt-Mag-Amp gneisses, three different metamorphic stages were obtained: A prograde stage M1 at ∼ 600–610 °C/5.5–6.1 kbar, a peak metamorphic stage M2 at ∼ 899–1062 °C/8.6–9.15 kbar, an early retrograde stage M2́at 780–792 °C/8.6–9.15 kbar and a retrograde decompression stage M3 at 466–493 °C/4.5–5.8 kbar. Thus an anticlockwise trajectory can be defined indicating that granulite facies UHT-metamorphic peak conditions have been reached, followed by fast cooling during rapid exhumation. Geochemical characteristics of the metapelites indicate that the Fenzishan Group of the Zhengjiapo BIF iron mine was formed in a back-arc basin system of an active continental margin in a convergent/collisional geological setting.. Magmatic U-Pb ages (LA-ICP-MS) of detrital zircon from metapelites are ∼ 3096 – 2816 Ma and 2761 – 2610 Ma, indicating a source of the Archean basement which delivered clastic material for pelitic protolith. A zircon age of ∼ 2261 – 2128 Ma constrains the minimum depositional age of the metapelitic precursor, while an apatite U-Pb age of ∼ 1923 – 1893 Ma dating demonstrates a metamorphic age of ∼ 1923 – 1893 Ma, is indicated to represent a late Paleoproterozoic post peak metamorphic event that corresponds to the collision of the Jiao-Liao-Ji orogenic belt. [ABSTRACT FROM AUTHOR]