Coexistence of compositionally heterogeneous chromitites in the Antalya-Isparta ophiolitic suite, SW Turkey: A record of sequential magmatic processes in the sub-arc lithospheric mantle


UYSAL İ. , Akmaz R. M. , Saka S. , Kapsiotis A.

LITHOS, cilt.248, ss.160-174, 2016 (SCI İndekslerine Giren Dergi) identifier identifier

Özet

The Antalya-Isparta region in southwestern Turkey is well known for its large ophiolitic peridotite exposures, which host various chromite orebodies. These are small-sized, massive to disseminated in texture chromitites occurring in the form of lenses or veinlets, and commonly surrounded by dunite envelopes of variable thickness. Chromitite seams from the Antalya mantle suite are both high- and intermediate-Cr varieties (Cr# = 0.56-0.83), whereas chromitites in the Isparta mantle sequence are exclusively Cr-rich (Cr# = 0.75-0.85). In situ minor and trace element abundances obtained by LA-ICP-MS analyses of unaltered Cr-spinel from the Cr-rich chromitites are comparable to those reported in Cr-spinel of chromitites from typical fore-arc peridotite complexes. However, minor and trace element concentrations in Cr-spinel from intermediate chromitites are dissimilar to those acquired from Cr-spinels of chromitites from well-known back-arc basin-derived ultramafic massifs. Calculation of parental magma compositions indicates that both types of chromitites share a common parentage with progressively fractionating arc-related melts. The studied chromitites are characterized by a systematic enrichment in IPGE [Os, Ir, and Ru (41-317 ppb)] with respect to PPGE [Rh, Pt, and Pd (3-49 ppb)], resulting in negatively-sloping chondrite-normalized PGE patterns that are less fractionated in intermediate chromitites. Their noble mineral assemblage is vastly dominated by tiny (>= 10 pm) euhedral laurite crystals, followed by subsidiary irarsite and trivial amounts of Os-Ir alloy grains. PGM grains are not encountered in the intermediate chromitites, potentially due to crystallization resulting from PGE-poor melt. Laurite is Os-poor and exhibits a narrow range of Os-for-Ru substitution [Ru/(Ru + Os) = 0.75-0.99]. However, the concomitance of laurite and millerite in the Cr-rich chromitites of the mutual Antalya-Isparta mantle suite is in favour of their precipitation from an Os-depleted melt, characterized by local and rapid variations of fS(2) prior to or coevally with Cr-spinel crystallization. Moreover, the presence of amphibole inclusions in Cr-spinel indicates that the melt triggered chromitite genesis potentially involved a hydrous component. Overall, data suggest that investigated orebodies were produced by a successively fractionating arc-derived melt that generated compositionally distinct chromitites at two different pseudo-stratigraphic levels within the Antalya-Isparta arc-type mantle suite. (C) 2016 Elsevier B.V. All rights reserved.