Depositional environment and tectonic implications of the Paleoproterozoic BIF in Changyi area,eastern North China Craton:Evidence from geochronology and geochemistry of the metamorphic wallrocks

【摘要】：The Changyi banded iron formation(BIF) in the eastern North China Craton(NCC) occurs within the Paleoproterozoic Fenzishan Group.Three types of metamorphic wallrocks interbedded with the BIF bands are identified,including plagioclase gneisses and leptynites,garnet-bearing gneisses and amphibolites.Protolith reconstruction suggests that the protoliths of the plagioclase gneisses and leptynites are mainly graywackes with minor contribution of pelitic materials,the garnet-bearing gneisses are Fe-rich pelites contaminated by elastics,and the amphibolites are tholeiitic rocks.Trace elements of La,Th,Sc and Zr of the plagioclase gneisses and leptynites and the garnet-bearing gneisses support that these meta-sedimentary rocks were probably derived from recycling of Archean rocks with felsic and mafic materials differentiated into different rock types.~(207)Pb/~(206)Pb ages of detrital zircons from the meta-sedimentary rocks concentrate at 2.7-3.0 Ga,confirming their derivation from the Archean rocks.The presence of several Paleoproterozoic detrital zircons(2240 to2246 Ma),however,also suggests minor involvement of Paleoproterozoic materials.The Archean detrital zircons have ε_(Hf)(t) values varying from — 0.7 to 7.6,which mainly fall between the 3.0 Ga and 3.3 Ga average crustal evolution lines on the age vs.ε_(Hf)(t) diagram,further illustrating that the rocks providing materials for the meta-sedimentary rocks mainly originated from partial melting of a Mesoarchean crust.This is strongly supported by their crust-like trace element distribution patterns(such as Nb,Ta,P and Ti depletion) and ancient Nd depleted mantle model ages(T_(DM) = 2.9-3.4 Ga).In addition,the remarkably high ε_(Hf)(t) values(7.5 to 9.3) of the Paleoproterozoic detrital zircons constrain the Paleoproterozoic materials to originate from a depleted mantie.The amphibolites show low SiO_2(46.5 to 52.8 wt.%) and high MgO(5.68 to 10.9 wt%) contents,crustlike trace element features and low ε_(Nd)(t) values(— 4.5 to —0.3),suggesting that these ortho-metamorphic rocks were mainly derived from subcontinental lithospheric mantle with some contamination by Archean crustal materials.Since an intra-continental environment was required for the formation of the above metamorphic rocks,these rocks not only confine the depositional environment of the Changyi BIF to be an intra-continental rift,but also support the rifting processes of the eastern NCC during Paleoproterozoic.

【作者单位】：Key Laboratory of Mineral Resources,Institute of Geology and Geophysics,Chinese Academy of Sciences State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences
【基金】：financially supported by the Major State Basic Research Development Program(2012CB416605) the Natural Science Foundation of China(41202065)
【分类号】：P588.3;P548