Petrogenesis of calc-alkaline and shoshonitic post-collisional Oligocene volcanics of the Cover Series of the Sesia Zone, Western Italian Alps

High-K calc-alkaline and shoshonitic Oligocene volcanics of the Cover Series of the Sesia Zone occur within a volcano-sedimentary unit located in the internal part of the Sesia Zone in proximity of the Canavese Line. High-K calc-alkaline rocks are basaltic andesites to andesites (K2O from 2.0 to 2.6 wt%) with high alumina contents (19.2-20.3 wt% for basaltic andesites). These rocks are enriched in incompatible trace elements of low ionic potential (with Ba ranging from 920 to 1320 ppm, Rb/Sr = 0.11-0.17). High-field-strength elements (HFSE) are within the typical range of orogenic andesites, with Zr ranging from 150 to 180 ppm. Rare Earth Element patterns for high-K intermediate rocks are enriched in LREE (La/Yb ranging 15.2-16.0) and show a small Eu negative anomaly. Shoshonitic rocks are trachyandesites to trachydacites, with K2O ranging from 4.6 to 5.5 wt%. Alumina contents are lower than those of the high-K calc-alkaline rocks ranging from 14.8 to 16.8 wt%. When compared to calc-alkaline andesites,shoshonitic rocks show higher contents in incompatible elements (with Ba higher than 2500 ppm; Rb/Sr = 0.30-0.35; La/Yb = 26.2-28.8; Zr = 380-450 ppm). Thermobarometric data indicate that andesitic s.l. lavas of the Cover Series of the Sesia Zone started to crystallize their phenocrystic phases at pressures between 3.6-4.8 kbar, for temperatures ranging from 1,080 to 1,150 °C, for water contents of at least 2-2.7 wt%. Shoshonitic lavas crystallized at 2-2.6 kbar for temperatures of about 900 °C, and approaching water-saturated conditions. REE patterns and spiderdiagrams, together with Pearce element ratios, indicate comagmatism between the lavas and the rocks of the Valle del Cervo Pluton. In particular there is a close genetic link between the shoshonitic and the high-K calc-alkaline volcanic rocks. Geochemical modelling shows that both suites may derive from a common “parental calc-alkaline basalt” with variable enrichments in terms of incompatible elements and transition metals. This basalt has suffered a different degree of fractionation in the genesis of balsaltic andesites (with F = 0.29) and monzonites of the Valle del Cervo Pluton (F= 0.19). High-K calc-alkaline andesites may be obtained by fractional crystallization of basaltic andesites, whereas shoshonitic lavas result from fractional crystallization of a monzonitic parent. This suggests that these lavas represent a differentiated top of a magma chamber, whose remnants are now represented by the Valle del Cervo Pluton. This is also supported by comparison of the Sr-Nd isotopic signatures of both intrusive and volcanic rock suites (the latter ranging from 0.7094 to 0.71175 in measured 87Sr/86Sr, and 0.51120-0.51228 in measured 143Nd/144Nd). Calculated isotopic parameters(αSm/Nd= 0.46-0.52,δSm/Nd= 0.23 to 0.26 calculated at 2 b.y., and present εNd = –7 to –8.5) indicate that parental basaltic magmas to the high-K calc-alkaline and shoshonitic rocks were derived by low degree of partial melting (5-7%) of a moderately enriched “eclogitic” metasomatised mantle-source (i.e., garnet pyroxenite). This is consistent with the presence of ultrapotassic dykes in this sector of Western Alps (showing high values in radiogenic Sr-Nd isotope ratios). Finally, we point out that the effusive and intrusive rocks of the investigated area represent a volcano-plutonic complex within the “Tertiary Periadriatic Province”.