Pages: 511 - 530
Abstract: Ultramafic cumulates of Medvednica Mts form mega-blocks in the ophiolite melange from the Zagorje-Mid-Transdanubian segment of the Sava Zone. The blocks consist of chromite-olivine cumulates and pyroxenites. The rocks crystallized in an open system under low pressure. Early crystallization of Al-Ti-chromite and Ti-edenite/pargasite indicate high oxygen fugacity hydrous parental magma. The Medvednica Mts ultramafic cumulates are analogous to plutonic rocks from recent immature island arcs. Subsolidus reactions indicate rapid exhumation with the final equilibration resembling prehnite-pumpellyite facies. The Medvednica Mts ultramafic cumulate sequence shows common features with the analogous sequence of the Bükk Mts ophiolites from NE Hungary and differ from analogues from the Vardar/Dinaric ophiolite belts. Mantle peridotites lacking overlying crustal members crop out in the Medvednica Mts and Kalnik Mts, and were cored in drill-holes. The peridotites range in composition from lherzolite via transitional harzburgite to depleted harzburgite. Lherzolite resembles abyssal peridotite and together with transitional harzburgites are akin to the peridotites from the South Sandwichs and Mariana arc–back-arc system. Depleted harzburgites are typical of fore-arc peridotites and are similar to the harzburgites of the Inner Dinaride Ophiolite Belt. Assuming that ultramafic rocks from the Zagorje-Mid-Transdanubian Zone were generated in a single oceanic domain, the South Sandwich arc-basin system appears as the most similar analogue. If it holds true, then these ultramafic rocks must have been derived from the Meliata-Maliac ocean domain and not from the Dinaric/Vardar strands as was previously thought.
Keywords: Croatia, Zagorje-Mid-Transdanubian Zone, Medvednica Mts, geochemistry, petrology, ultramafic rocks, ophiolite melangeDownload PDF document