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

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

Published: Aug 2008

Pages: 277 - 293


Abstract: Amphibole gabbroic rocks with heteradcumulate and isotropic fabric constitute centimeter to hectometer large fragments in the Early Callovian to the Late Valanginian ophiolite melange of the Mt Medvednica located at the 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 normalized REE concentration of isotropic gabbros show patterns transitional between mid-ocean ridge and island arc magmatic rocks [(La/Lu) MORBN=0.92–1.12]. 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 crystallization of plagioclase after augite-diopside with tschermakite-magnesiohornblende as intercumulus oikocrystal bring in the evidence of formation in a suprasubduction setting. The rocks were severely albitized and uralitized 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 to geotectonic constraints and advocate a proto-arc–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 intra-oceanic arc system formed in the Repno domain of the Neotethyan oceanic realm. A petrogenetic and geotectonic connection between the Repno and Maliak-Dinaric oceanic domains cannot be positively postulated on the basis of the presented data.

Keywords: Croatia, Zagorje-Mid-Transdanubian Shear Zone, Mt Medvednica, island arc, amphibole gabbros, ophiolite melange

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