engleski

Amphibole gabbroic rocks from the Mt. Medvednica ophiolite mélange (NW Croatia): geochemistry and tectonic setting

Amphibole gabbroic rocks with heteradcumulate and isotropic fabric constitute centimetre to hectometre large fragments in the Early Callovian to the Late Valanginian ophiolite mélange of the Mt. Medvednica located at SW tips of the Zagorje-Mid-Transdanubian shear Zone. Normalized multielement concentration patterns have strong Ta-Nb anomaly [(Nb/La)MORBN = 0.28 - 0.72] for all rocks while normalised REE concentration of isotropic gabbros show patterns transitional between middle ocean ridge and island arc magmatic rocks [(La/Lu)MORBN = 0.92 &#8211; 1.12]. Low Ti, Cr and Na content of clinopyroxene from the isotropic gabbros (<0.98 wt.% TiO2, <0.94 wt.% Cr2O3, <0.39 wt.% Na2O) combined with high Ca-plagioclase (up to An93) and crystallisation of plagioclase after augite-diopside with tschermakite-magnesiohornblende as intercumulus oikocrystal bring in the evidence of formation in a suprasubduction setting. The rocks were severe albitised and uralitised in greenschist facies on the sea floor and altered in prehnite-pumpellyite facies during emplacement. Due to the alterations the LILE may have been selectively enriched while HFSE and REE retained magmatic ratios. Overall geochemical data and age determination of gabbropegmatite (161.1 Ma) are liable for geotectonic constraints and advocate for an proto-arc&#8211; immature intra-oceanic island arc source of the Mt. Medvednica gabbroic fragments. It was suggested that the Mt. Medvednica gabbroic rocks represent remnants of an extinct intraoceanic arc system formed in the Repno domain of Neotethyan oceanic realm. Petrogenetic and geotectonic connection between Repno and Maliak-Dinaric oceanic domains can not be positively postulated on account of presented data.

ophiolite mélange; amphibole gabbros; island arc; Zagorje-Mid-Transdanubian shear Zone; Mt. Medvednica; Croatia

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