Heterogeneous nature of the ultrabasic-basic association of the Kurtushiba Range (Western Sayan, Russia): Vendian ophiolites and Ordovician gabbroids

   Research subject. The ultramafic-mafic association of the Kurtushiba Range (the Altai–Sayan Segment of the Central Asian Mobile Belt), conventionally regarded as a single ophiolitic association composed of dunites, harzburgites, pyroxenites, gabbroic rocks, diabases, and basalts.

   Aim. To demonstrate the heterogeneous nature of this association, to substantiate the distinction of independent Ordovician “late” gabbroids that were previously included in the ophiolite assemblage, as well as to clarify their role in the overall scheme of Caledonian magmatism in the Western Sayan.

   Materials and methods. Field observations and rock sampling, petrographic study, whole-rock petrogeochemical analysis, Sm–Nd isotope geochemistry, and amphibole ⁴⁰Ar/³⁹Ar geochronology; comparison of the data obtained with those published on the Kurtushibinsky ophiolites and gabbroic rocks from adjacent areas.

   Results. The ultramafic-mafic association of the Kurtushibinsky Range was shown to comprise two complexes of different ages: Vendian suprasubduction-zone ophiolites and Ordovician syn-collisional gabbroids (amphibole ⁴⁰Ar/³⁹Ar age of 487 ± 7 Ma for the Verkhneozhinsky massif) that intruded the ophiolites and are accompanied by narrow near-contact zones of serpentinite polymict mélanges. The “late” gabbroids are characterized by an N-MORB-like geochemical affinity and display two levels of REE depletion (ΣREE = 2.9–7.1 and 24–44 ppm) with εNd(T) values of +6.8 and +8.7, respectively, indicating a juvenile DMM source with no significant subduction-related component.

   Conclusions. The ultramafic-mafic association of the Kurtushibinsky Range is heterogeneous. Thus, Vendian ophiolites are associated with Ordovician gabbroic intrusions formed by renewed partial melting of depleted mantle during the collisional stage of Caledonian evolution in the Western Sayan, most likely in a setting of syn-collisional transtensional deformation. Recognition of the “late” gabbroids as distinct from the ophiolite assemblage refines the accepted model of Caledonian magmatic evolution of the region and may provide a basis for revising other ultramafic-mafic associations conventionally regarded as exclusively ophiolitic.

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