Podiform chromitite genesis in an Archean juvenile forearc setting: The 2.55 Ga Zunhua chromitites, North China Craton

Abstract

Podiform chromitites are unique rock units of ophiolites and ophiolitic mélanges and represent invaluable repositories that reliably record the formation environment and evolution of their source and host rocks. However, there are relatively few podiform chromitites reported from Archean terranes. Here we document the major, minor, and trace element characteristics of podiform chromitites from the Archean Zunhua ophiolitic mélange in the North China Craton. The Zunhua ophiolitic mélange includes blocks of serpentinized peridotite (dunite and harzburgite), podiform chromitite, pyroxenite, amphibolite, metabasites (basalt and diabase), chert, and tectonic lenses of banded iron formation in a strongly sheared metapelitic matrix. The chromitites, host peridotites, and granitic dikes cutting the mélange all have ages between 2.6 and 2.5 Ga. Chondrite-normalized, whole-rock platinum group element (PGE) patterns of the Zunhua chromitites show a negative sloping PGE trend enriched in Ir and depleted in Pt, similar to typical Phanerozoic podiform chromitites. The geochemical characteristics of the calculated parental magma of the Zunhua chromitites closely resemble those of podiform chromitites in Phanerozoic ophiolites and ophiolitic mélanges but different from those of continental stratiform chromite deposits. The calculated major, minor, and trace element concentrations of the parental melt of Zunhua chromitites are similar to those of boninitic melts found in forearc settings. Geochemical characteristics and calculated parental melt composition of the chromites with the high Mg# values of their host peridotites suggest that the Zunhua podiform chromitites formed by magmatic processes including reactions between boninitic melt and harzburgite in a supra-subduction zone environment.