Slab Contamination of Asthenospheric Melts Triggered the Crystallization of Chromite in the Formation of Podiform Chromite Deposits

English Abstract:

Podiform chromitites in the mantle sections of ophiolites typically contain a diversity of unusual minerals and mineral inclusions, reflecting a variety of origins. Among these, plagioclase, amphibole and zircon may represent a mineral assemblage of amphibolite, whereas coesite, kyanite and garnet represents an eclogitic assemblage. These two assemblages were present in the magmas from which chromite crystallized. Multiphase mineral inclusions demonstrate that podiform chromitites form from hydrous mafic magmas in suprasubduction zone environments. Highly reduced and ultrahigh pressure minerals including diamonds are reported in literature both in podiform chromitites and host peridotites of ophiolites. Some of these minerals in association with host peridotites may have been brought by the uprising asthenosphere at mid-oceanic ridges due to the mantle convection. Thus, the formation of podiform chromite deposits may have involved two stages: an early stage in spreading centres and a later stage in newly created intra-oceanic subduction zones. The continued subduction carries crustal materials to deep levels where they are metamorphosed under greenschist, amphibolite and ecologite facies conditions. Tearing and breakoff of the subducted slab, possibly along the transitional contact between amphibolites and eclogites, create a slab window through which the underlying asthenosphere rises and melts to generate Cr-rich mafic magmas. These upward-migrating magmas pass through the subduction zone and assimilate the subducted slab. As a result of slab contamination, these magmas become more siliceous, more oxidized and more hydrous, rapidly triggering chromite crystallization. Minute grains of chromite are suspended in the upward-moving magmas as they migrate through the overlying metasomatized mantle wedge. Such chromite-bearing magmas eventually deposit chromite in magma conduits in the uppermost mantle close to the Moho where the upward flow changes from vertical to subhorizontal and velocity is greatly reduced. Some minerals of subducted slabs are preserved because they are encapsulated in chromite grains where they are protected from the SSZ melts. Some of these SSZ mantle wedges are emplaced on land to become podiform chromitite-bearing ophiolites.