Ultramafic cumulate rocks represent the rarest allochthonous fragments of the Mesozoic oceanic lithosphere observed today in the Upper Jurassic to Lower Cretaceous mélange of Mt. Kalnik, located in the SW part of the Zagorje‒Mid-Transdanubian Zone (ZMTDZ). Poikilitic hetero-adcumulate ultramafic rocks of Mt. Kalnik are represented by amphibole lherzolites/harzburgites and plagioclase lherzolites. Both were formed by in-situ processes within a magma chamber following the general crystallization sequence of: Al-chromite → Mg-rich olivine → enstatite ± augite → Ca-amphibole (pargasite ± edenite ± magnesiohornblende) → Ca-plagioclase (An82.6-87.4). Cumulate minerals are spinel and olivine as well as orthopyroxene and clinopyroxene which are usually enclosed in intercumulate phases such as amphibole and/or plagioclase that render an interstitial mesostasis. Rocks’ textural characteristics, mineral crystallization order and their phase chemistry are all suggestive of low-pressure sub-solidus crystallization in an open system. The low Ti content in augite and scant HFSE abundances suggest the studied rocks may have formed from a depleted mantle source. In addition to the medium to high degree of partial melting of the source, the parental process that gave rise to the Mt. Kalnik ultramafic cumulates also included a low degree of fractional crystallization. The segregation of oxidized Al-chromite and oikocrysts of pargasite and edenite in an early crystallization stage illustrates the formation of a cumulate sequence from volatile-rich magmas. These magmas usually have a high oxidation potential and are exclusively found in intra-oceanic subduction zones, predominantly in island arcs. The overall whole-rock geochemistry [e.g. (Nb/La)n = 0.25-0.34; (Ti/Gd)n = 0.49-0.89; (Th/Nb)n = 5.29-8.63; (La/Lu)cn = 0.57-0.68] together with a record of Ca-rich plagioclase (up to An87.4) and low Ti clinopyroxene (≤0.54 wt%) corroborate the supra-subduction tholeiitic nature of the magma source. Ultramafic cumulates from the ophiolitic mélange of Mts. Kalnik and Medvednica show common genetic features and geotectonic provenance. Comparison with analogous ultramafic lithotypes of the north-eastern segment of the ZMTDZ (the Szarvaskö Complex, Hungary), the ultramafic cumulates of Mts. Kalnik and Medvednica portray some subtle differences that may indicate their distinctive geotectonic provenance. Mts. Kalnik and Medvednica ultramafic cumulates represent the vestiges of a single Upper Jurassic intra-oceanic arc system formed in the western branch of the Meliata-Maliak segment of the Neotethyan oceanic realm.
- Island arc
- Ophiolite mélange
- Ultramafic cumulates
- Zagorje-mid-transdanubian zone