Petrology, geochemistry and tectono-magmatic affinity of gabbroic olistoliths from the ophiolite mélange in the NW Dinaric-Vardar ophiolite zone (Mts. Kalnik and Ivanščica, North Croatia)

Abstract

Mafic intrusive rocks are subordinately represented fragments of the oceanic crust in the ophiolite mélange exposedat Mts. Kalnik and Ivanščica located in the NW Dinaric-Vardar ophiolite zone. This ophiolite mélange occurs in thenorthern area of the Kalnik Unit and represents the SW surface boundary of the Zagorje-Mid-Transdanubian ShearZone. The mélange, except for mafic intrusive rocks, consists of a chaotic mixture of various extrusive rocks formedin different tectonic settings of the Repno Oceanic Domain (ROD). The ROD was the segment of Neo-Tethys thatconnects the Meliata-Maliak and Dinaric-Vardar oceanic systems. Previous study of mafic extrusive sequences suggestedan 80 Ma period of tectono-magmatic evolution of the ROD from intra-continental rifting during the Anisian,to the formation of proto-arc crust during the Callovian-Oxfordian. The domain exposes ophiolitic rocks in four mélangeareas. Isotropic gabbroic rocks that are abundant in two northern areas (Mts. Kalnik and Ivanščica), can be discriminatedinto three distinct geochemical groups: (A) N-MORB-type gabbro [(Th/Nb)n = 0.99–1.10; (Nb/La)n =0.95–0.99], (B) IAT-type amphibole gabbro with clear supra-subduction characteristics [(Th/Nb)n = 6.04–8.16; (Nb/La)n = 0.32–0.42] and (C) BABB-type amphibole-bearing gabbro [(Th/Nb)n = 2.88–4.02; (Nb/La)n = 0.58–0.69].Representative gabbro samples of each geochemical group were dated by the Ar-Ar and/or the K-Ar dating method.The Early Jurassic N-MORB-type gabbros (geochemical group A), ~185 Ma old, signifies a peculiar stage of Palaeo-Tethyan slab break-off. The Late Jurassic IAT-type gabbros (geochemical group B), ~147 Ma old, is the vestige of anascent intra-oceanic arc, whilst the Early Cretaceous BABB-type gabbros (geochemical group C), ~100 Ma old,provides evidence of magmatism in the back-arc marginal basin. The analyzed gabbroic rocks enable refinement andcompletion of the geodynamic evolution of the ROD, from the opening of an ensialic back-arc basin during the Ladinianand a continuous spreading event until the Bajocian. Intra-oceanic convergence was initiated in the Bathonian,with the formation of a nascent island-arc during the Tithonian, leading to formation of a Cretaceous ensimatic backarcmarginal basin. There are many lines of evidence that correlate the geodynamic evolution of the ROD with theAlbanide-Hellenide Neo-Tethyan oceanic segment.

Keywords

• petrology and geochemistry, N-MORB, IAT, BABB, gabbro, ophiolite mélange, Dinaric-Vardar ophiolite zone, Mts. Kalnik and Ivanščica, Croatia