Formation of discordant chromitite at the initiation of sub-arc mantle processes: Observations from the northern Oman ophiolite

Abstract

Discordant chromitite, one of the structural types of ophiolitic chromitite, and surrounding peridotites, were examined in the mantle section of the northern Oman ophiolite along Wadi Hilti, to constrain its origin. Spinels from chromitite and adjacent peridotites show high Cr# (= Cr/(Cr + Al) atomic ratio), around 0.65 to 0.75, indicating an arc -related feature. Clinopyroxenes from adjacent harzburgites show low HREE contents, indicating a highly depleted character. The calculated melt composition in equilibrium with clinopyroxene in the dunite envelope indicates that the chromitite and surrounding peridotites were derived from arc -related magma, such as boninite. The arc -related magma was probably generated by the flux melting of depleted mantle harzburgite on the subducted slab, in terms of incompatible element enrichment in clinopyroxene from dunite and harzburgite. The highly depleted harzburgite surrounding discordant dunite and chromitite shows transitional changes in spinel and REE chemistry toward the surrounding oceanic harzburgites. This feature indicates that pre -existing oceanic harzburgite was partially modified by arc -related magma. Discordant chromitite, dunite, and highly depleted harzburgites are probably formed at the supra -subduction zone environment just before ophiolite obduction. Such arc -related features in mantle peridotite may be produced at a young sub -arc mantle.

Platinum-Group Minerals and Other Accessory Phases in Chromite Deposits of the Alapaevsk Ophiolite, Central Urals, Russia

“…In contrast, the majority of the supra-Moho chromitites are Al-rich and characterized by a variable content of TiO 2 (0.15 wt % to 0.8 wt %). Confirming the previous work by [8,[14][15][16][17]21], the composition of the Alapevsky chromite indicates the coexistence of the Al-rich and Cr-rich chromitites, a common feature of many ophiolitic belts worldwide [22,[36][37][38][39][40][41][42][43][44][45][46][47][48][49][50].…”

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

“…Averaged XCr values of chromite from the stratiform ore body are relatively moderate (49.0-59.9), similar to those of the disseminated chromite from the surrounding DTZ and contrasting with those of other ore bodies and their host dunites elsewhere in Oman, especially in the northern massifs of the ophiolite where XCr can reach much higher values (Ahmed and Arai, 2002;Augé, 1987;Miura et al, 2014;Rollinson, 2008;Rollinson and Adetunji, 2015). Evenly, the TiO 2 content of the stratiform chromite is typically high (0.25-0.54 wt%), a characteristic shared by most chromitites and scattered chromites in dunites from the Maqsad area and contrasting with widespread lower values observed in almost all other massifs of the Oman ophiolite.…”

Chromite Composition and Accessory Minerals in Chromitites from Sulawesi, Indonesia: Their Genetic Significance

“…However, the presence of both Al-rich and Cr-rich chromitites within one single ophiolite complex has been documented worldwide [26,27,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56]. This coexistence can be explained with the following models as summarized in [52]: (1) intrusions of different melts derived from differently depleted mantle sources (i.e.,: MORB versus boninite), during the geodynamic evolution of the oceanic lithosphere from the MOR towards the SSZ; (2) mixing of several magmas produced by multi-stage melting of a partially re-fertilized and fluid-metasomatized residual source, in the SSZ; (3) the reaction of a single intrusion of a boninitic parental melt with country-rock peridotites characterized by variable residual composition; (4) fractional precipitation during differentiation of a single batch of magma with initial high-Cr boninitic composition, in the SSZ; and (5) bimodal distribution and vertical zoning with the Cr-rich chromitite located in the deep mantle section and the Al-rich chromitite occurring higher in the succession, close or above the Moho transition.…”

“…This is related to the percolation of melts produced in the subduction environment through variably depleted peridotites in the mantle wedge. According to [55,56], based on their relationships with the host peridotite, concordant and discordant chromitites have been identified in the Oman ophiolite. However, due to the strong alteration that affects the Sulawesi chromitites, it was not possible to recognize in the field their relation with the host serpentinized peridotite.…”