Microstructures of the Zunhua 2.50 Ga Podiform Chromite, North China Craton and Implications for the Deformation and Rheology of the Archean Oceanic Lithospheric Mantle

This chapter focuses on microstructures of the Zunhua 2.50 Ga podiform chromite, North China craton and implications for the deformation and rheology of the archean oceanic lithospheric mantle. Podiform chromite deposits are distinctive discontinuous concentrations of chromite commonly enveloped by dunite, typically characterized by nodular and orbicular chromite textures, and they may be very irregular in shape, size, and distribution within the host rocks. Podiform chromite deposits are generally associated with harzburgites and only reported in ophiolites and oceanic lithosphere. The North China craton is divided into two major blocks, the Western Block and the Eastern Block, separated by the Neoarchean Central Orogenic Belt. The Zunhua structural belt contains various tectonic slices, including tonalite-trondhjemite-granodiorite gneiss, mafic plutonic rocks, supracrustal sequences and granites, all complexly intercalated with each other, intruded by ca. 2.56–2.5 Ga tonalitic gneiss, then 2.5–2.4 Ga granites. In the inner parts of massive chromite bodies, nodular and orbicular chromites show sedimentary-style bedding with size-graded layers and dense cumulate layers that form massive ores.