Genesis and tectonic implications of podiform chromitites in the metamorphosed ultramafic massif of Dobromirtsi (Bulgaria)

José María González-Jiménez*, Marek Locmelis, Elena Belousova, William L. Griffin, Fernando Gervilla, Thomas N. Kerestedjian, Suzanne Y. O'Reilly, Norman J. Pearson, Ivanina Sergeeva

*Corresponding author for this work

Research output: Contribution to journalArticle

53 Citations (Scopus)

Abstract

Podiform chromitites occur in the meta-dunite horizon of the Dobromirtsi ultramafic massif, in the Central Rhodope metamorphic core complex (southern Bulgaria). Although these were deformed and metamorphosed together with their host rocks several chromite cores from massive chromitite samples still preserve compositions unaffected by metamorphism - at least in terms of major elements. In situ laser ablation ICP-MS analyses of these chromite cores show a bimodality in the composition of chromite between high-Cr and high-Al {Cr#=[Cr/(Cr+Al)], atomic ratio=0.74-0.55} with Mg# between 0.69 and 0.60 [Mg#=Mg/(Mg+Fe2+), atomic ratio]. These compositions of chromite and the whole rock enrichment of Os-Ir-Ru relative to Pt-Pd in the chromitites resemble that reported for podiform chromitites in ophiolites elsewhere.Magmatic platinum-group minerals (PGM), including laurite (RuS2), Os-Ir alloys and sulfarsenides (irarsite, IrAsS), exhibit a wide range of 187Os/188Os (from 0.1097 to 0.1272) whereas 187Re/188Os is nearly zero. The distribution of the Os model ages calculated for these PGM ranges between 0.13 and 2.6Ga, with age peaks at 0.25, 0.4, 0.7, 2.1 and 2.6Ga. Two xenocrystic zircons were identified in the chromitites and yield concordant U-Pb age of 2257±80Ma (2σ) and 1952±82Ma (2σ). The oldest zircon exhibits a depleted-mantle Hf model age (TDM) of 2.77Ga and a "crustal" Hf model age (TC DM) of 3.1Ga, whereas the youngest has a TDM of 2.21Ga and a TC DM of 2.35Ga.We interpret the coexistence of both types of chromitites in a horizon of meta-dunites as the result of their precipitation from fractionating island arc tholeiite melts. The first reactions produced dunite; later the batches of magma were isolated from reaction with the peridotite wall-rocks and progressively fractionated in a network of dunite channels developed in the upper mantle beneath the back-arc spreading centre. The Os and Hf model ages suggest that the ultramafic protolith of the Dobromirtsi ophiolite was derived from an original Archean mantle at least 3. Ga old, which underlay a continental crust and was reworked in the Paleo-proterozoic and possibly even younger times.

Original languageEnglish
Pages (from-to)555-574
Number of pages20
JournalGondwana Research
Volume27
Issue number2
DOIs
Publication statusPublished - 1 Feb 2015

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