Multi-scale development of a stratiform chromite ore body at the base of the dunitic mantle-crust transition zone (Maqsad diapir, Oman ophiolite): the role of repeated melt and fluid influxes. Lithos

ROSPABÉ, M.; CEULENEER, G.; GRANIER, N.; ARAI, S.; Y, B.A.

Авторы: ROSPABÉ MATHIEU, GEORGES CEULENEER, NICOLAS GRANIER, SHOJI ARAI, BORISOVA ANASTASSIA Y.
Журнал: Lithos
Год издания: 2019
Издательство: Elsevier BV
Местоположение издательства: Netherlands
DOI: 10.1016/j.lithos.2019.105235
Аннотация: A stratiform chromite ore body crops out in the lower part of the dunitic mantle-crust transition zone (DTZ) that developed at the top of a mantle diapir in the Maqsad area in the Oman ophiolite. It is made of layers ranging in thickness from a few mm to a maximum of 3 meters, and in modal composition from massive to antinodular and disseminated ore. The ore body is about 50 m thick and its lateral extent does not exceed several hundred meters. The layering is slightly dipsping gently to the southeast, parallel to thate one of the overlying gabbroic cumulates. The chromite composition is typical of a MORB kindred - moderate XCr (100 × Cr/(Cr + Al) atomic ratio), ranging from 48 to 60, and relatively high TiO2 content, ranging from about 0.3 to 0.5 wt% -, a characteristic shared by mostall lithologies issued from the igneous activity of the Maqsad diapir. The silicate matrix is essentially made of slightly serpentinized olivine with minor clinopyroxene and rare pargasitic amphibole, orthopyroxene and garnet. This strongly contrasts with the nature of the mineral inclusions mostly made of the assemblage amphibole-orthopyroxene-mica, enclosed in the chromite grains and represented in abundance all along the ore body whatever the ore grade. The inclusionsis demonstrates the involvement of a silica- and water-rich melt and/or fluid, in addition to MORB, in the early stages of chromite crystallization. The chemical compositions of all mineral phases (chromite, silicate matrix, together with the one ofand silicate inclusions) display well defined evolutions verticalvertically along the stratiform chromitite evolutions, essentially in XCr, TiO2, Fo (100 × Mg/(Mg + Fetotal) atomic ratio), NiO and XNa (100 × Na/(Na + K) atomic ratio). At the scale of the ore body, the compositional trends are independent of the ore concentration but the major kinks in these trends are well-nicely correlated withto the few horizons levels of magmatic breccias. This shows that abrupt chemical changes can be attributed to sudden melt +/- fluids injection events followed mainly by melt-fluid-rock interaction and in a lesser extent by quieter evolution by fractional crystallization. At the thin section scale, second order chemical variations, essentially in the Mg# (100 × Mg/(Mg + Fe2+) atomic ratio) of chromite and Fo of olivine, are clearly attributable to re-equilibration between these two solid phases, possibly in the presence of an interstitial melt/fluid.
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Multi-scale development of a stratiform chromite ore body at the base of the dunitic mantle-crust transition zone (Maqsad diapir, Oman ophiolite): the role of repeated melt and fluid influxes. Lithosстатья

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