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

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 similar to 600-610 degrees C/5.5-6.1 kbar, a peak metamorphic stage M2 at similar to 899-1062 degrees C/8.6-9.15 kbar, an early retrograde stage M2at 780-792 degrees C/8.6-9.15 kbar and a retrograde decompression stage M3 at 466-493 degrees 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 similar to 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 similar to 2261 - 2128 Ma constrains the minimum depositional age of the metapelitic precursor, while an apatite U-Pb age of similar to 1923 - 1893 Ma dating demonstrates a metamorphic age of similar to 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.