engleski

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

At the NW inflexion of the Sava-Vardar Suture Zone ophiolite mélanges, 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 mélange, 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 mélange, 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-Maliak 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 subducting slab leading to 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 mélange of the accretionary wedge formed in the proto-arc–fore-arc region. The Mt. Samoborska Gora ophiolite mélange 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.

Triassic-Jurassic ; Croatia ; Samoborska Gora Mt. ; proto-arc extension ; back-arc spreading ; alkali within-plate basalts ; tholeiitic basalts ; ophiolite mélange

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