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New evidence of the youngest oceanic crust in the Zagorje - Mid-Transdanubian Zone based on the example of Lower Cretaceous metamorphic sole from the Kalnik Mt. (North Croatia)

Amphibolite beneath mantle lherzolite in tectonic slices is found inserted in the upper Lower to Upper Cretaceous sedimentary succession from Kalnik Mt. in north Croatia (Ignjatić 2007, Šegvić et al. 2005, 2014). The Kalnik Mt. is a southwestern part of the narrow Zagorje – Mid-Transdanubian Zone (ZMTDZ) bounded by Zagreb-Zemplin lineament at the south and Periadriatic-Balaton lineament at the north (Pamić and Tomljenović 1998). Amphibolites provide the petrological, geochemical and chronological data necessary to constrain the late history of Repno Oceanic Domain (ROD) that connects Meliata-Maliac and Dinaric-Vardar oceanic systems. Primary amphibolite parageneses consists of pargasite-magnesio-hornblende and oligoclase-bytownite (common amphibolite) ± almandine-grossulare rich garnet ± clinopyroxene (garnet-clinopyroxene amphibolite and clinopyroxene amphibolite, respectively). Minimum equilibration temperature for amphibole-plagioclase pairs was estimated to 520°C at pressure of 0.53 GPa ; the highest metamorphic grade of 670°C at 0.71 GPa was recorded in garnet-clinopyroxene amphibolite. Mineral compositions and thermobarometric studies of the amphibolites reflect metamorphic conditions typical of metamorphic sole formation. Retrograde paragenesis consists of albite, actinolite, epidote-coisite-clinocoisite, titanite and chlorite, and corresponds to greenshist facies. Protolith REE patterns match modern oceanridge basalts [(La/Lu)cn = 0.51-0.82], whilst the normalised multielement diagram shows slight negative HFSE anomalies [(Nb/La)pmn = 0.44-0.90], diagnostic for magmatic rocks from suprasubduction zones. Overall geochemical data suggest back-arc basin (BAB) tholeiitic mafic extrusives (lack of Eu anomaly) and IAT-like cumulates (Eu positive anomaly) to be amphibolite precursors. It is suggested that protoliths of Kalnik Mt. metamorphic sole represent an oceanic crust formed at the ridge of a back-arc marginal basin. K-Ar dating on amphibole separate yielded the Lower Cretaceous age (118 ± 8 Ma) that reflects the isotopic equilibration related to the dynamothermal metamorphism caused by near ridge tectonic displacement. The existence of one or more back-arc basins placed westwards of the Tiszia-Dacia continental Mega-Unit that have been producing oceanic crust during Upper Cretaceous to Early Palaeocene is, thus, put forward. This defines the analysed amphibolites as the youngest oceanic crust of ROD, nowadays structured within the Zagorje – Mid-Transdanubian Zone , the triple junction area of the European continental margin, Tisza-Dacia Mega-Unit and the Internal Dinarides.

amphibolites; petrology; geochemistry; back-arc basin; island-arc; Lower Cretaceous; Zagorje-Mid-Transdanubian Zone Kalnik Mt. (N Croatia)

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