Petrology and geochemistry of Paleogene tonalites from the easternmost parts of the Periadriatic Zone

The Pohorje Mts. and Karavanke plutons occur in the easternmost parts of th e Periadriatic Zone separating the Eastern Alps from the Southern Alps and the northwesternmost Dinarides, respectively. The plutons, which are compos ed largely of tonalites with subordinate granodiorites and rare diorites ma inly of medium-K calc-alkaline affinity, are part of a series of mid-Tertia ry intrusives emplaced over similar to 700 km E-W along the Periadriatic Zo ne. Major and trace element data indicate that gabbrodiorites originated by fra ctional crystallization of a primary mafic magma, but with a significant co ntribution of continental crust. REE patterns are steep and characterized b y LREE enrichment, and a weak Eu anomaly, suggesting fractional crystalliza tion of primary mafic magma. Sr-87/Sr-86 ratios indicate that on the basis of Sr isotopic composition an upper mantle melt contribution cannot be reco gnized. The 87Sr/86Sr VS. delta(18)O diagram demonstrates that all analyzed rocks plot near the centre of the Adamello pluton field. Tonalite plutonism was genetically controlled by previously subducted Mesoz oic oceanic crust, metasomatized upper mantle wedge, and overlying continen tal crust. All analytical data obtained indicate that primary mafic magma w as of olivine tholeiite affinity. Based on element modelling data it is rea sonable to assume that a garnet peridotite, probably slightly metasomatized , was the source rock which was subjected to partial melting. The governing AFC processes resulted in crustal isotopic signatures due to crustal assim ilation and melting which produced the most acid members of the tonalite su ite.