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

摘要

Amphibole gabbroic rocks with heteradcumulate and isotropic fabric constitute centimeter to hectometerlarge fragments in the Early Callovian to the Late Valanginian ophiolite mélange of the Mt Medvednica located at theSW tips of the Zagorje-Mid-Transdanubian Shear Zone. Normalized multielement concentration patterns have strongTa-Nb anomaly [(Nb/La)_(MORBN) = 0.28-0.72] for all rocks, while normalized REE concentration of isotropic gabbrosshow patterns transitional between mid-ocean ridge and island arc magmatic rocks [(La/Lu)_(MORBN)=0.92-1.12]. LowTi, Cr and Na content of clinopyroxene from the isotropic gabbros ( < 0.98 wt. % TiO_2, < 0.94 wt. % Cr_2O_3, <0.39 wt. %Na_2O) combined with high Ca-plagioclase (up to An93) and crystallization of after augite-diopside withtschermakite-magnesiohornblende as intercumulus oikocrystal bring in the evidence of formation in a suprasubductionsetting. 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 whileHFSE 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 Medvednicagabbroic 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 connec-tion between the Repno and Maliak-Dinaric oceanic domains cannot be positively postulated on the basis of the presenteddata.