Formation of chromitites and ferrogabbros in ultramafic and mafic members of the Variscan Ślęża ophiolite (SW Poland)

Abstract

Ultramafic and mafic rocks of the Variscan Ślęża ophiolite (Bohemian Massif, Sudetes, SW Poland) contain abundant Cr and Fe-Ti mineralizations. The occurrence of chromitites is related with serpentinites of refractory harzburgitic protoliths. Chromite I from chromitites has variable Cr and Mg numbers (Cr# = 0.41–0.58, Mg# = 0.62–0.83) and low TiO2 contents (<0.20 wt%). Chromite II surrounds chromite I or fills fissures in it and is enriched in Cr and depleted in Al relative to chromite I (Cr# = 0.54–0.68). Chromite I grains contain spherical grains of diopside, phlogopite, pargasite, pyrite and chalcopyrite, occurring as mono- or polyphase solid inclusions. Metagabbros with isotropic fabric contain locally up to 14.6 vol% of magnetite and ilmenite. Magnetite has variable TiO2, Al2O3 and V2O3 contents (0.19–9.82 wt%, 0.13–1.67 wt% and 1.08–1.61 wt%, respectively). It forms xenomorphic grains with trellis or sandwich exsolutions of ilmenite, as well as aggregates with individual ilmenite grains.The moderate Cr# of chromite I points to its origin from a MORB-type melt. The occurrence of hydrous inclusions and nodular textures indicates chromitite formation and alteration under low pressure conditions. Primary sulfide, phlogopite and pargasite inclusions suggest that the parental melt was moderately hydrous and sulphur-rich. Water in the magmatic system originated supposedly from interaction of primary MORB melt with hydrothermal fluids and a significant influence of fluids on chromite composition is expressed by its very low Ti content.Metagabbros containing abundant Fe-Ti oxide mineralization may be classified as ferrogabbros. High V and Ti contents in magnetite indicate a magmatic origin. Its xenomorphic and intragrain mode of occurrence suggest crystallization between pre-existing gabbroic minerals like pyroxene and plagioclase. Ilmenite exsolutions indicate its growth at the expense of magnetite under oxidizing conditions. Occurrence of chromitites in harzburgites and ferrogabbros related to isotropic gabbros may suggest various spreading conditions during formation of the mafic and ultramafic members of the Ślęża ophiolite.