Geochemical characteristics of metamorphic rocks in Northern Karawanke

Obravnavane kontaktno–metamorfne kamnine iz Karavanške magmatske cone in metamorfni skrilavci iz Štalenskogorske serije predstavljajo izrinjen del paleozojske podlage, ki v Sloveniji izdanja na območju Severnih Karavank. V okviru te naloge smo petrografsko in geokemično opredelili te kamnine ter jim s pomočjo pridobljenih podatkov določili provenienco in tektonsko okolje odlaganje protolita. Rezultati optične mikroskopije so pokazali, da kontaktno–metamorfne kamnine, med katere uvrščamo filite in kordieritne skrilavce, vsebujejo največ kremena, ki mu po zastopanosti sledijo sljude, kordierit in plagioklazi. Metamorfne skrilavce Štalenskogorske serije večinoma gradi drobnozrnata osnova iz sericita v kateri so zrna plagioklazov, kremena in sljud. Izračuni CIA, CIW in PIA indeksov nakazujejo na to, da so bili protoliti kontaktno–metamorfnih kamnin in metamorfnih skrilavcev podvrženi zmernemu preperevanju, ki v obeh primerih sledi trendu prepervanja za granodiorit. Razmerja med nemobilnimi prvinami pričajo o protolitu felzičnega značaja obravnavanih kamnin. Felzično provenienco obeh litologij potrjujejo tudi strmi LREE segmenti z visokimi (La/Yb)N razmerji, nefrakcionizirane HREE in izrazito negativne Eu anomilje. Primerjava geokemičnih podatkov kordieritnih skrilavcev, filitov in metamorfnih skrilavcev s podatki o kamninah iz različnih provenienčnih tipov je pokazala, da je odložen sediment predstavljal mešanico materiala izvirajočega iz stare zgornje kontinentalne skorje in materiala izvirajočega iz mladega diferenciranega loka. Trikotna diskriminacijska diagrama La–Th–Sc in Th–Sc–Zr/100 ter primerjava slednih prvin preučevanih kamnin z vrednostmi slednih prvin iz različnih tektonskih okolij so pokazali, da so se izvorni sedimenti obravnavanih litologij odložili v prostoru v bližini kontinentalnega magmatskega loka. Glede na predhodne raziskave in pridobljene podatke so se sedimenti iz katerih so nastali filiti in kordieritni skrilavci odlagali v času od zgornjega kambrija do spodnjega ordovicija na pasivnem robu severne Gondvane. Sedimenti iz katerih so nastale kamnine Štalenskogorske serije so se glede na biostratigrafske podatke odlagali kasneje v času od spodnjega do zgornjega devona. Sedimentacijsko okolje je predstavljal poglabljajoč se zaločni bazen na območju Galatijskih terenov.The studied contact metamorphic rocks from the metamorphic belt of the Karawanke magmatic zone and the slates from Magdalensberg formation represent parts of the Paleozoic basement that were extruded and outcrop in the Northern Karawanke area in Slovenia. In this work we have characterized the rocks petrographically and geochemically and using the acquired data, determined provenance and tectonic setting of their protoliths. The results of optical microscopy revealed that contact metamorphic rocks, represented by phyllites and cordierite slates, contain the highest content of quartz, followed by micas, cordierite, and plagioclase. Slates from the Magdalensberg formation mainly consist of a fine–grained matrix of sericite with inclusions of plagioclase, quartz and micas. Calculations based on the CIA, CIW and PIA suggest that the protolith of both contact metamorphic rocks and slates underwent moderate weathering, following a weathering trend similar to that of granodiorite. The element ratios of immobile elements indicate a felsic protolith for both contact metamorphic rocks and slates. The felsic provenance of these lithologies is further supported by steep LREE segments with high (La/Yb)N ratios, unfractionated HREE, and distinct negative Eu anomalies. Comparison of the geochemical data from contact metamorphic rocks and slates with data from rocks of various provenance types revealed that the deposited material comprised a mixture derived from the old upper continental crust and material originating from a young differentiated arc. Ternary discrimination diagrams of La–Th–Sc and Th–Sc–Zr/100, as well as a comparison of trace elements in the studied rocks with values from different tectonic environments, indicate that the source sediments of contact metamorphic rocks and magdalensberg slates were deposited in an environment close to a continental magmatic arc. Based on previous research and obtained data, the source sediments of phyllites and cordierite slates were deposited from the Upper Cambrian to the Lower Ordovician on the passive margin of North Gondwana. The sediments that formed the magdalensberg slates were deposited later, based on biostratigraphic data, from the Lower to the Upper Devonian. The sedimentary environment represented a deepening back–arc basin in the area of Galatian terrains