International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Geochemistry, petrology and tectonomagmatic significance of basaltic rocks from the ophiolite melange at the NW External-Internal Dinarides junction (Croatia)

Pages: 273 - 292

Authors: DAMIR SLOVENEC, BOSKO LUGOVIC, IRENA VLAHOVIC

Abstract: At the NW inflexion of the Sava-Vardar Suture Zone ophiolite melanges, known as the Kalnik Unit, form the surface of the slopes of several Pannonian inselbergs in the SW Zagorje-Mid-Transdanubian Zone. The Mt Samoborska Gora ophiolite melange, thought to be a part of the Kalnik Unit, forms a separate sector obducted directly onto Dinaric Triassic carbonate sediments. Basaltic rocks, the only magmatic rocks incorporated in the melange, include Middle–Triassic (Illyrian–Fassanian) alkali within-plate basalts and Middle Jurassic (uppermost Bathonian–Lower Callovian) tholeiitic basalts. The latter sporadically constitute composite olistoliths, and are geochemically divided into N-MORB-like (high-Ti basalts) and transitional MORB/IAT (medium-Ti basalts). These geochemically different rocks suggest crystallization at various tectonomagmatic settings, which is also indicated by the rock paragenesis and host clinopyroxene compositions. Alkali basalts reflect melts derived from an OIB-type enriched mantle source [Ti/V=62.2–82.4; (La/Lu)cn=6.4–12.8] with Nd-Sr isotope signatures close resembling the Bulk Earth [εNd(T=235 Ma)=+1.6 to +2.5]. They are recognized as pre-ophiolite continental rift basin volcanic rocks that closely predate the opening of the Repno oceanic domain (ROD) of the Meliata-Maliac oceanic system. The high-Ti and medium-Ti basalts from composite blocks derived from a similar depleted mantle source (εNd(T=165 Ma) =+6.01 vs. +6.35) succesively metasomatized by expulsion of fluids from a subducting slab leading to a more pronounced subduction signature in the latter [Ti/V=31.6–44.8 and (Nb/La)n=0.67–0.90 vs. Ti/V=21.5–33.9 and (Nb/La)n=0.32–0.49]. These composite blocks indicate crust formation in an extensional basin spreading over the still active subducting ridge. The majority of high-Ti basalts may represent the fragments of older crust formed at a spreading ridge and incorporated in the melange of the accretionary wedge formed in the proto-arc–fore-arc region. The Mt Samoborska Gora ophiolite melange represents the trailing edge of the Kalnik Unit as a discrete sector that records the shortest stage of tectonomagmatic evolution related to intraoceanic subduction in the ROD.

Keywords: Triassic-Jurassic, Croatia, Samoborska Gora Mts, proto-arc extension, back-arc spreading, alkali within-plate basalts, tholeiitic basalts, ophiolite melange

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