A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

Abstract

To decipher the petrogenesis of chromitites from the MohoTransition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of $500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1^50 mm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr þAl) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, f O2 , of $3 log units below the quartz^fayalite^magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 9508C at subsolidus conditions, were overgrown by more oxidized host chromite (f O2 % QFM) in association with microcrystal inclusions of silicates (plagioclase An 86 , clinopyroxene, and pargasite) that were formed between 950 and 10508C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to $0·18C a À1). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional

Podiform chromitites from the Variscan ophiolite serpentinites of Lower Silesia (SW Poland) - petrologic and tectonic setting implications

“…The sil i cate phase in clu sions (crys tallised from the melt) oc cur typ i cally in this type of chro mite (Borisova et al, 2012). Sparse sil i cate in clu sions oc cur ring in the GJSM and BBM chro mites were re ported by Delura (2012a).…”

“…Rollison and Adetunji, 2013), as well as the mech a nism of chromite crystallisation (cf. Borisova et al, 2012, González-Jiménez et al, 2014 are still the mat ters of dis cus sions.…”

Chromite in the Mantle Section of the Oman Ophiolite: Implications for the Tectonic Evolution of the Oman Ophiolite

“…The inclusions contain hydrous silicate phases -mostly Na-phlogopite, pargasite and chlorite indicating that the melt from which they crystallised was hydrous. Homogenisation experiments suggest that the melts were primitive basalts with ca 47 wt % SiO 2 , 10-12 wt % MgO, high FeO (11 wt %) and low CaO (7.6 wt %) (Schiano et al, 1997, Borisova et al, 2012.…”

“…Chromitites in the Maqsad area are located in the Moho transition zone, a 300m thick dunitic zone located below the Moho, representing an area of intense melt flux through the mantle. Melt inclusions were first described from this area by Schiano et al (1997) and more recently by Borisova et al (2012). The inclusions contain hydrous silicate phases -mostly Na-phlogopite, pargasite and chlorite indicating that the melt from which they crystallised was hydrous.…”

Secondary fluorescence effects in microbeam analysis and their impacts on geospeedometry and geothermometry

“…Llovet and Galán, 2003;Wade and Wood, 2012). For example, measurements of Cr concentrations in chromite-hosted mineral or glass inclusions with diameters up to tens of m (Schiano et al, 1998;Spandler et al, 2005;Borisova et al, 2012, Husen et al, 2016 are likely affected by the secondary fluorescence from the chromite host, but this effect has…”