An ensialic volcanic arc along the northwestern edge of Palaeotethys-Insights from the Mid-Triassic volcano-sedimentary succession of Ivanscica Mt. (northwestern Croatia)

摘要

This work aims to unveil the origin, geodynamic significance, and diagenetic history of pyroclastites and associated chert documented within the Upper Anisian volcano-sedimentary succession of the Ivanscica Mt. in Central Europe. An abundance of pyroclastic material points to polyphase volcanic activity and deep-water sedimentation along the rim of an oceanic realm. Radiolarian-based dating revealed Illyrian age of chert intercalated with pyroclastites. The latter are largely vitro-crystalloclastic and were emplaced as airborne tuff. The crystalloclasts of sanidine, plagioclase, altered pyroxene, and amphibole are principal tuff constituents merged in the altered glassy matrix consisted of palagonite, clay minerals, and calcite. Variations in alteration assemblages reflect an extensive in situ eogenesis in an open hydrologic system that gradually evolved toward a restricted fluid percolation environment. Mineralogy, chemistry, and occurrence of andesitic tuff clearly denote an explosive volcanic activity formed at the top of suprasubduction zone. Such scenario presumes a complex genesis outlined in following steps: (a) partial melting and dehydration of down-going Palaeotethyan slab which gave rise to the subduction-related magmatism {LILE, Th, and LREE [(La/Lu)(cn) = 6.51-9.42] enrichment; negative anomalies of Nb-Ta, P, and Ti [e.g., (Nb/La)(n) = 0.31-0.44]}; (b) magma contamination via interaction with continental crust during magma uplift along tectonically weakened zones of upper crust [positive Pb spikes, negative epsilon(Nd) (-4.18 to -4.44), and Sm-147/Nd-144 <= 0.113175]. This is in favour of geodynamic evolution that hypothesizes the existence of an active, ensialic, and mature volcanic arc developed along the southern active continental margins of Euramerica (Laurussia) during Late Anisian subduction of Palaeotethyan lithosphere.