Origin and significance of the Permian high-K calc-alkaline magmatism in the central-eastern Southern Alps, Italy

The Atesina Volcanic District, the Monte Luco volcanics, and the Cima d'Ast a, Bressanone-Chiusa, Ivigna, Monte Croce and Monte Sabion intrusions, in t he central-eastern Southern Alps, form a wide calc-alkaline association of Permian age (ca. 280-260 Ma). The magmatism originated during a period of p ost-orogenic extensional/transtensional faulting which controlled the magma ascent and emplacement. The magmatic products are represented by a continu um spectrum of rock types ranging from basaltic andesites to rhyolites, and from gabbros to monzogranites, with preponderance of the acidic terms. The y constitute a metaluminous to weakly peraluminous series showing mineralog ical, petrographic and chemical characteristics distinctive of the high-K c alc-alkaline suites. In the MORB-normalized trace element diagrams, the mos t primitive volcanic and plutonic rocks (basaltic andesites and gabbros wit h Mg No. = 66 to 70; Ni = 25 to 83 ppm; Cr = 248 to 679 ppm) show LILE and LREE enriched patterns with troughs at Nb-Ta and Ti, a distinctive feature of subduction-related magmas. Field, petrographic, geochemical and isotopic evidence (initial Sr-87/Sr-86 ratios from 0.7057 to 0.7114; epsilon(Nd) va lues from -2.7 to -7.4; partial derivative(18)O values between 7.6 and 9.5 parts per thousand) support a hybrid nature for both volcanic and plutonic rocks, originating through complex interactions between mantle-derived magm as and crustal materials. Only the scanty andalusite-cordierite and orthopy roxene-cordierite bearing peraluminous granites in the Cima d'Asta and Bres sanone-Chiusa Sr-87/Sr-86 = 0.7143-0.7167; initial epsilon(Nd) values intru sive complexes can be interpreted as purely crustal melts (initial between -7.9 and -9.6, close to average composition of the granulitic metasedimenta ry crust from the Ivrea Zone in the western Southern Alps). Although the Pe rmian magmatism shows geochemical characteristics similar to those of are-r elated suites, palaeogeographic restorations, and geological and tectonic e vidence, seem not to support any spatial and/or temporal connection with su bduction processes. The magmatism is post-collisional and post-orogenic, an d originated in a regime of lithospheric extension and attenuation affectin g the whole domain of the European Hercynian belt. A change in the converge nce direction between Gondwana and Laurasia, combined with the effects of g ravitational collapse of the Hercynian chain, could have been the driving m echanism for lithosphere extension and thinning, as well as for upwelling o f hot asthenosphere that caused thermal perturbation and magma generation. In the above context, the calc-alkaline affinity and the orogenic-like sign ature of the Permian magmatism might result from extensive contamination of basaltic magmas, likely derived from enriched lithospheric mantle source(s ), with felsic crustal melts. (C) 1998 Elsevier Science B.V. All rights res erved.