A Better Picture of the Mantle Section of the Nain Ophiolite Inferred from Detrital Chromian Spinels

Abstract

Detrital chromian spinels from adjacent sediment (recent sands and a Tertiary sandstone) were used to obtain a general view of the lithological and petrological characteristics of the Nain ophiolitic melange, Iran. They display a wide chemical range in terms of Cr# (=Cr/(Cr + Al) atomic ratio), from 0.10 to 0.92. Except for some grains (11% of the total), the spinels show low TiO2 contents (<0.25 wt%), typical for the spinels of mantle peridotites. Relatively high-Ti spinels (TiO2 content 10.25 wt%, up to 1.26 wt%) are relatively low in Mg# (=Mg/(Mg + Fe2+) atomic ratio), from 0.15 to 0.65, and were possibly derived from mantle peridotite that reacted with impregnating melt (e. g., dunite or plagioclase-bearing peridotites). The high-Cr# (0.55-0.92) spinels, which are dominant in the Tertiary sandstone, are chemically homogenous and clearly different from high-Cr# altered spinels (ferritchromite and Cr-magnetite), which are formed by low-T alteration, and are low in Mg# and high in Fe3+. The higher abundance of the high-Cr# spinels in the Tertiary sandstone than in the recent sands indicates higher abundance of refractory lithologies in the Paleo-Nain ophiolite than the present one. The refractory lithologies for the source of the high-Cr# spinels have not been reported from the Nain melange, which implies that the refractory dunite and harzburgite were effectively sheared to provide the melange matrix and then eroded during later stages of emplacement. This indicates uplift and intermittent protrusion of a vertical slice of heterogeneous upper mantle in the Nain since the late Cretaceous. This is in good agreement with the geological situation of the Nain melange located on a fossil transcurrent fault (Nain-Baft fault zone), which was involved in the opening of the Nain-Baft basin along an active margin of the central-east Iranian microplate (the Sanandaj-Sirjan block). The initial presence of the high-Cr# spinels implies an origin from a spreading center above a subduction zone (e. g., back-arc basin) for the Nain melange.