Some constraints on geochemical features in the Triassic mantle of the easternmost austroalpine-southalpine domain: evidence from the Karawanken pluton (Carinthia, Austria)

The Karawanken pluton, near Eisenkappel (Carinthia, Austria), is composed of closely alternating, E/W-running, essentially granitic and dioritic bands, with minor gabbro, monzonite, and hybrid rocks, and is cut by diabase dykes. This pluton, of Triassic age, is a shallow-emplaced intrusion, the rocks of which belong to a series of alkaline affinity and, despite local evidence of mixing and mingling of magmas, the bulk of the pluton in the examined area evolved mainly by fractional crystallization. The shift in the initial Sr isotope ratio from gabbro (0.70313) to monzonite (0.70525) and steady values from monzonite to granite (0.70473) suggest a predominant assimilation and fractional crystallization (AFC) process in the mafic stage, followed by a dominant fractional crystallization (FC) process when residual liquids became felsic. The geochemical characteristics of the Karawanken pluton point to a mantle source enriched both in LILE and HFSE, whereas the coeval magmas of the nearby Dolomites (NE Italy), emplaced in the same extensional±transtensional geodynamic framework, derive from a mantle modified by preceding (Variscan) subductive processes. These geochemical differences in the Triassic mantle of this part of the Eastern Alps may involve the different Paleozoic geodynamic evolution of these two sectors, e.g., the fact that they belong to two different microplates linked in the Middle Carboniferous. In any case, both the magmas of the Karawanken pluton and of the Dolomites clearly fit the mid-Triassic transtensional±extensional tectonism which acted in the Southalpine±Austroalpine and Dinaric domains during the initial stages of Mesozoic rifting.