Evidence of the spreading culmination in the Eastern Tethyan Repno oceanic domain, assessed by the petrology and geochemistry of N-MORB extrusive rocks from the Mt. Medvednica ophiolite mélange (NW Croatia)

Abstract

Eastern Tethyan Repno oceanic domain (ROD) from Zagorje-Mid-Transdanubian Shear Zone bounds Meliata-Maliak and Dinaric-Vardar oceanic systems. The domain lacks ophiolite complexes and oceanic rocks are restricted into Early Callovian to Late Valanginian four ophiolite mélanges integrated in Kalnik Unit. Previous researchs on ophiolitic rocks from Kalnik Unit provided high-resolution tectonomagmatic evolution of crust formation at different settings from Late Anisian to Cretaceous. N-MORB-type crust with peculiar suprasubduction signatures formed on spreading ridge from Late Carnian to Late Pliensbachian and first intraoceanic subduction rocks were dated to Late Bathonian. The newly discovered basaltic rocks in coherent slices with Latest Bajocian-Early Bathonian radiolarian cherts from Mt. Medvednica ophiolite mélange sector completed the Late Pliensbachian-Late Bathonian chemostratigraphic gap in the domain. The analyzed unfractionated extrusive rocks lack any influence of a subduction related component [(Nb/La)_n = 1.16-1.34; Th/Ta = 0.99-1.05] and are akin to proper N-MORB compositions [(Nd/Lu)_N-MORB ~ 1.1; (La/Lu)_cn = 0.82-0.92] that were derived from slightly depleted mantle source [ε_{Nd(T=170 Ma)} = +6.21 to +6.27; (⁸⁷Sr/⁸⁶Sr)_i = 0.703365 to 0.703511]. The rocks are interpreted as vestiges of the middle ocean ridge crust formed during culmination stage of spreading in the ROD.