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 tectono-magmatic affinity

This study brings the mineralogical, petrological, geochemical, Sr and Nd isotope data as well as K-Ar ages used to infer on petrogenesis and geodynamic evolution of Middle Triassic high-K calc-alkaline effusives and their associated pyroclastites of Mt. Kuna Gora in NW Croatia. Their matching mineralogy and bulk-rock geochemistry testify on coeval origin of both rock types. Sanidine and plagioclase accompanied by minor augite and Ti-bearing magnetite are major phases merged in a matrix made of devitrified volcanic glass and plagioclase microlites. Hydrothermal and eogenetic alterations documented in pyroclastites gave rise to the formation of chlorite/white mica and mixed-layer clays, respectively. Rocks’ petrography unveiled the following crystallization order: spinel → clinopyroxene → plagioclase → alkali-feldspar ± Fe-Ti oxides. Geochemical and isotopic evidences point to a complex origin of studied rocks which included the contamination of subduction-generated magmas by melts derived from lithospheric mantle. This presumes an interplay of the fertile arc mantle, old(?) continental crust, and depleted or OIB-like mantle. A low degree of crustal contamination stands as a last phase of the formation of such “hybrid” magmas. The subducted Paleotethyan oceanic lithosphere went through the processes of partial melting of mantle source placed at depths of ~45-49 km and pressures ≤ 1.6 GPa and subsequent fractionation which produced melts parental to studied rocks. Suggested model is linked to the demise of northward subduction of Paleotethys during the Early Mid-Triassic, which is in favor of an active, ensialic mature volcanic arc developed along the southern active continental margins of Laurussia.