Geochemistry, Lu–Hf garnet ages, and P–T conditions of blueschists from the Meliatic and Fatric nappes, Western Carpathians: Indicators of Neotethyan subduction

Abstract

Basaltic rocks from ophiolitic mélanges provide information on geodynamic setting, origin, and later tectonometamorphic conditions. This paper resolves the P–T conditions and timing of high-pressure metamorphism in an accretionary wedge that formed during the Late Jurassic closure of the Neotethyan Meliata Basin. Blueschist-facies metabasites of the Meliatic Bôrka Nappe and the Albian conglomerate pebbles of the Fatric Klape Nappe contain rare assemblages of garnet in association with glaucophane, phengite, rutile, and epidote (±albite). Here, we compare a Lu–Hf garnet age from the Meliatic Bôrka Nappe in the southern margin of the Inner Western Carpathians (IWC) with a garnet age from inferred Meliatic blueschists of the Klape Nappe overlying the IWC northern margin. The Hačava type locality of the Bôrka Nappe hosts calc-alkaline type metabasite (∼VAB-C; εNd₍₂₄₅₎ = +0.9) embedded within Middle to Upper Triassic marbles of the Neotethyan Meliata Basin northern continental margin. The investigated Klape Nappe island arc tholeiite (∼VAB-T; εNd₍₂₄₀₎ = +5.9) blueschist pebble, and associated deepwater metasedimentary rock pebbles were found in a conglomerate layer of unmetamorphosed Albian flysch. The garnet ages of 153.95±0.69 Ma and 152.1±1.5 Ma correspond to closure of the Meliata Basin during southward intraoceanic and continental margin subduction. This was followed by the exhumation of HP blocks in serpentinite mélange and the formation of an accretionary wedge with included anchimetamorphosed Jurassic flysch. The P–T conditions of the blueschists were constrained by Perple_X modelling to be 520°C and 1.55 GPa for the Bôrka Nappe, and 490–510°C and 1.68–1.72 GPa for a pebble from the Klape Nappe conglomerate. The similarity of garnet dates and metamorphic conditions between the two samples suggest that the blueschists formed during the Late Jurassic Meliatic subduction. The north-vergent Meliatic nappes are the inferred source of the Albian flysch conglomerates deposited in the foreland Fatric Basin. This material was subsequently transported by the Fatric Klape Nappe to the IWC orogenic front during the Turonian, following the closure of the Fatric Basin.