Deformation fabric and microstructures in ophiolitic chromitites and host ultramafics, Sultanate of Oman

Abstract

f-larzburgite and dunite from the Oman ophiolite display a strong olivine fabric and microstructures typical for high temperature mantle deformation. The syntectonic, magmatically formed, chromitites occurring throughout the mantle sequence vary from almost undeformed (resembling chromitite from stratiform complexes) to highly deformed. Usually the interstitial silicates in the deformed ore possess a fabric similar to those of the host rocks, whereas a weak or poikilitic fabric is observed on relatively undeformed deposits. Chromite grains are elongated and flattened, defining strong foliations and lineations, in disseminated ore, and pull-apart-like structures perpendicular to the lineation form in the coarser massive ore. No preferred crystallographic orientation of chromite has been detected with the Xray-texture goniometer, and it is proposed that chromite mainly deforms by passive rotation in the weaker silicate matrix, cataclasis and diffusion creep. crostructures typiques de d6formations mantelliques de haute temp&ature. Les chromitites, d'origine magmatique syntectonique, qui se trouvent le long de la s6quence mantellique varient entre des termes presque non d4form& (ressemblant aux chromitites des complexes stratiformes) jusqu'~. des termes tr6s d6form6s. Les silicates interstitiels du mineral d6form6 pr&entent fr4quemment une fabrique semblable ~t celle des min&aux de ia roche h6te alors que l'on observe une fabrique peu d&elopp& ou bien une texture poecilitique dans ies minerals des gisements relativement peu d&form&. Les cristaux de chromite sont allong4s et aplatis, d6finissant une foliation et une iin6ation tr6s marqu&s dans ies minerals diss6min&; des structures d'&i-rement (pull-apart) perpendiculaires ~ la lin6ation apparaissent dans les minerals plus massifs. Aucune orientation cristallographique pr4ferentielle n'a pu &re mise en &idence au goniom&re de texture; on propose donc que la chromite se d~forme principalement par rotation passive dans une matrice silicat& plus fragile, par cataclase et par fluage-diffusion.

Fluid-present deformation aids chemical modification of chromite: Insights from chromites from Golyamo Kamenyane, SE Bulgaria

“…Recrystallization processes in chromite have been described only from a few occurrences. Similar inclusion-free recrystallized chromite grains showing a change in chemistry have been described from the Fiskenaesset deposit of West Greenland (Ghisler, 1976), the Oman ophiolite complex (Christiansen, 1985), and the Sittampundi anorthosites complex in India (Ghosh and Konar, 2012). Ghisler (1976) presented evidence for recrystallization of chromite in the most deformed parts of a pre-orogenic stratiform complex.…”

“…The often irregular recrystallized grains are devoid of silicate inclusions and show a change in chemistry from the magmatic chromite. The Oman samples also show a progressive change from euhedral chromite grains with abundant inclusions to grains with few inclusions (Christiansen, 1985).…”

“…In deformed ophiolitic chromitites, large grains are commonly perfect without bending of the lattice or internal substructures (e.g. subgrains), and no crystallographic preferred orientation of chromite has been detected using the X-ray goniometer (Christiansen, 1985). These features have been interpreted to indicate that chromite mainly deforms by passive rotation in the weaker silicate matrix, and by cataclasis and/or diffusion creep (Christiansen, 1985).…”

“…Frost and Ashby, 1982). Deformation mechanisms such as plastic deformation including diffusion creep (Christiansen, 1985;Ozawa, 1989), dissolution followed by precipitation in the presence of melt, and/or dislocation creep (Secher, 1981) are expected to play a role in the modification of chromite microstructures, but have so far commonly been overlooked due to the analytical problem noted above.…”

Grain-scale plastic deformation of chromite from podiform chromitite of the Naga-Manipur ophiolite belt, India: Implication to mantle dynamics

“…Cataclasis has long been regarded as the only deformational behaviour in chromite (Doukhan et al, 1979;Ramdohr, 1969); however, the rheological conditions to address this failure in high-temperature mantle environment were constrained much later by numerical experiments (Holtzman, 2000). Studies on pole-figures of deformed ophiolitic chromites obtained utilising X-ray goniometer also could not reveal any subgrains in the respective samples (Christiansen, 1985). As a result, dislocation creep was not thought to be a dominant mechanism; instead, diffusional creep which normally does not result in a crystallographic fabric was considered as the principal mechanism in the deformation of chromites (Ozawa, 1989).…”