Abstract
Ultramafic cumulate rocks represent the rarest allochthonous fragments of the Mesozoic oceanic lithosphere observed today in the Upper Jurassic to Lower Cretaceous melange of Mt. Kalnik, located in the SW part of the Zagorje‒Mid-Transdanubian Zone (ZMTDZ). Poikilitic heteroadcumulate ultramafic rocks of Mt. Kalnik are represented by amphibole lherzolites/ harzburgite sand 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 areusually enclosed in intercumulate phases such as amphibole and/or plagioclase that render aninterstitial 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 toAn87.4) and low Ti clinopyroxene (≤0.54 wt%) corroborate the supra-subduction tholeiitic nature of the magma source. Ultramafic cumulates from the ophiolitic melange 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 Szarvasko 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.
Highlights
Their geotectonic origin is usually linked to midoceanic ridges (MOR) and supra-subduction zones (SSZ) (SUN & NESBITT, 1978; SAUNDERS et al, 1980; SERRI, 1981; SHERVAIS, 2001; PEARCE et al, 2003; PEARCE, 2008; BORTOLOTTI et al, 2013; SACCANI & TASSINARI, 2015)
Fragments of cumulate ultramafic rocks are the rarest member of the Mesozoic oceanic lithosphere documented in the ophiolitic mélange of Mt
These metre-sized olistoliths originated from deep portions of oceanic crust and predominantly account for amphibole lherzolite and, to a lesser extent, amphibole harzburgite and plagioclase lherzolite
Summary
Ophiolitic fragments of oceanic lithosphere represent an important source of information needed for a better understanding of the origin of ancient ocean crust shedding more light on the evolution of orogenic belts and ophiolitic complexes documented worldwide (COLEMAN, 1977; PARLAK et al, 1996, 2002; HUOT & MAURY, 2002; DILEK, 2003; BORTOLOTTI, et al, 2005, 2013; DILEK & FURNES, 2011, 2014; SACCANI, et al, 2017). As integral parts of an ophiolite, ultramafic cumulates emerge as slices of oceanic plates that are observed obducted and emplaced onto passive continental plate margins These cumulates occur in the form of fragmented allochthonous bodies (olistoliths), embedded in the chaotic rock mixture usually referred to as an ophiolitic mélange (e.g. WAKABAYASHI & DILEK, 2003; FESTA et al, 2010). Based on matrix palynomorph assemblages, the accretionary age of the Kalnik Unit, i.e. ophiolite mélange is defined as Early Callovian to Late Valangian (BABIĆ et al, 2002) This time interval represents a period of accumulation of lithostratigraphically diverse material in an intra-oceanic trench (SLOVENEC et al, 2011). There are arguments suggesting that the ZMTDZ was displaced by translation and rotation along the Zagreb-Zemplin lineament in a NE direction, reaching its present position during the Middle Miocene (PAMIĆ, 1997; TOMLJENOVIĆ et al, 2008; Fig. 1a-b)
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