Protoliths of the basement rocks in the Orobic area (Central Alps)

The studied portion of the Orobic crystalline basement (Southern Alps) is located south of Morbegno and close to the eastern shore of Lake Como (Dervio Olgiasca Zone: DOZ). It is traditionally divided from east to west in three units: Scisti di Edolo, Filladi di Ambria and Gneiss di Morbegno. It mainly consists of pelitic to psammitic metasiliciclastic rocks with minor metagranitoids (e.g. Monte Fioraro complex, Monte Legnone, Gneiss Chiari del Corno Stella or Gneiss Chiari) and rare amphibolites, quartzites and marbles. The Variscan metamorphism is under amphibolite facies conditions in the central western part and under prevailing greenschist facies towards the east. The metasiliciclastic rocks are classified as shales and a few as litharenites. The shales are relatively more homogeneous than litharenites. The DOZ shales have a more limited range in silica and in many minor and trace elements than those close to the Morbegno area. In general, positive correlation of Th-Ce and Th-U, decrease in Eu/Eu (super *) and increase in Sigma REE with increasing clay fraction suggest retention of zircon in the fine grained fraction and dominantly residual character of the sediments. Some samples have REE parameters and trace element ratios suggesting an intermediate to felsic volcanic input in the protolith. Shales have REE patterns consistent with those of NASC and PAAS, more enriched than those of litharenites. All the metagranitoids are peraluminous with typical patterns of calcalkaline rocks; the Gneiss Chiari are enriched in Rb and U and depleted in Ba, Th, HFSE and REE with respect to the others. The REE patterns exhibit negative Eu anomaly and variable enrichments of LREE over HREE; the Gneiss Chiari show limited fractionation and relatively flat HREE patterns, probably due to the presence of small quantities of HREE-bearing minerals. The geochemical features of Gneiss Chiari are consistent with those of Himalayan leucogranites. The presented whole geochemical composition of the Orobic basement resembles an upper continental crust, with a significant contribution of felsic to intermediate volcanic rocks in the sedimentary protolith. The tectonic setting of the original sedimentary rocks seems to be transitional between active and passive continental margin.