Middle Triassic high-K calc-alkaline effusive and pyroclastic rocks from the Zagorje-Mid-Transdanubian Zone (Mt. Kuna Gora; NW Croatia): mineralogy, petrology, geochemistry and tectonomagmatic affinity

Abstract

This study uses mineralogical, petrological, geochemical, and Sr and Nd isotope data along with K-Ar ages to infer the petrogenesis and geodynamic evolution of Middle Triassic high-K calc-alkaline lavas and their associated pyroclastics of Mt. Kuna Gora in NW Croatia. Their analogue mineralogy and bulk-rock geochemistry testify to the coeval origin of both rock types. Sanidine and plagioclase accompanied by inor augite and Ti-bearing magnetite are the major phases found in a matrix of devitrified volcanic glass and plagioclase microlites. Hydrothermal anddiagenetic processes in the pyroclastics originated the formation of chlorite and white mica, and mixed-layer clay minerals, respectively. Petrography reveals the following crystallization order: spinel→clinopyroxene→plagioclase→alkali-feldspar±Fe-Ti oxides. Geochemical and isotopic data suggests that the studied rocks had a complex origin that included the contamination of subduction-generated magmas by lithospheric mantle melts. This presumes an interplay between fertile arc mantle, subducted continental crust, and depleted or ocean island basalts-like mantle. A low degree of crustal contamination stands as a last step in the formation of such “hybrid” magmas. The subducted Paleotethyan oceanic lithosphere went through processes of partial melting at depths of ~45-49km and pressures of ≤1.6GPa and fractionation that produced melts which gave rise to the studied rocks. In the model we are proposing herein such formed partial melts are related to the demise of the northward subduction of the Paleotethys oceanic lithosphere during the Early to Middle Triassic epoch, which is consistent with an active, ensialic mature volcanic arc developing along Laurussian southern active margins.

Major and rare earth element mineral chemistry of low-grade assemblages inform dynamics of hydrothermal ocean-floor metamorphism in the Dinaridic Neotethys

“…Petrographic studies of intrusive, effusive and pyroclastic rocks have previously been conducted (e.g. Slovenec et al 2011; Slovenec & Šegvić, 2021) and, as a result, only the most pertinent information is provided here. Intrusive rocks from the KU ophiolite mélange are represented by cumulate and non-cumulate medium- to coarse-grained gabbro that shows a preserved igneous fabric and heteroadcumulate poikilitic, and/or anhedral granular texture and homogenous structure (Fig.…”

“…Jamtveit et al 2016). Triassic extrusive rocks of the Western Neotethys affected by hydrothermal activity are largely composed of plagioclase, clinopyroxene, K-feldspar and multiple minor phases (Marci, 1987;Slovenec & Šegvić, 2021). With the exception of plagioclase and clinopyroxene, none of the other phases show any significant REE partitioning (Rollinson & Pease, 2021).…”

“…1). Volcanic and volcanoclastic rocks are intercalated with Middle Triassic siliciclastic and carbonate marine sediments, and consist of fine-grained and strongly altered basic, intermediate to acid volcanic and pyroclastic lithologies (Marci, 1987; Goričan et al 2005; Slovenec & Šegvić 2021). Alteration processes documented in non-ophiolite basalts/andesites and tuffs are attributed to burial metamorphism and hydrothermal activity, and account for the volcanic glass devitrification, plagioclase degradation and occurrence of low-grade assemblages of prehnite-pumpellyite facies (Marci, 1987).…”

“…2). These rocks are incorporated as slices and metre-to-kilometre-sized blocks in ophiolite mélange (Slovenec & Šegvić, 2021) known as the Kalnik Unit (KU) (Haas et al 2000) (Fig. 2).…”

“…1; e.g. Schmid et al 2008) based on the occurrence of non-ophiolite basalts/andesites and tuffs of the active continental margin (Slovenec & Šegvić, 2021), others suggest that it forms the most southern portion of the Southern Alps (Haas et al 2000; Schmid et al 2008; Fig. 1).…”