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

Abstract

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 (K 2 O 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 K 2 O 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 87 Sr/ 86 Sr, and 0.51120-0.51228 in measured 143 Nd/ 144 Nd). Calculated isotopic parameters (α Sm/Nd = 0.46-0.52, δ Sm/Nd = 0.23 to 0.26 calculated at 2 b.y., and present ε Nd =-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" m...

Dating emplacement and evolution of the orogenic magmatism in the internal Western Alps: 1. The Miagliano Pluton

“…2) from calc-alkaline, shoshonitic to ultrapotassic (lamprophyre and lamproite; Dal Piaz et al 1979;Venturelli et al 1984;Bellini et al 1993;Peccerillo and Martinotti 2006;Owen 2008;Prelević et al 2008;Conticelli et al 2009). Subaerial volcanic rocks (Callegari et al 2004;Kapferer et al 2012b) cover a thin band of ca. 20 km on top of the Mombarone nappe (Sesia-Lanzo Zone) along the Canavese Line.…”

Structure and petrography of the southwestern margin of the Biella pluton, Western Alps

“…These magmas rose along the Periadriatic line, which is the main Alpine crustal break (Figure 1 (a)), and subsequently emplaced in the form of plutons, dykes, and volcanites (Berger, Thomsen, Ovtcharova, Kapferer, & Mercolli, 2012;Bernardelli, Castelli, & Rossetti, 2000;Bigioggero, Colombo, Del Moro, Gregnanin, Macera, & Tunesi, 1994;Callegari, Cigolini, Medeot, & D'Antonio, 2004;Carraro & Ferrara, 1968;Dolenec, 1994;Fodor et al, 2008;Kapferer, Mercolli, Berger, Ovtcharova, & Fügenschuh, 2012;Mayer, Cortiana, Dal Piaz, Deloule, De Pieri, & Jobstraibizer, 2003;Müller, Mancktelow, & Meier, 2000;Oberli, Meier, Berger, Rosenberg, & Gieré, 2004;Pamić & Palincaš, 2000;Romer, Schärer, & Steck, 1996;Romer & Siegesmund, 2003;Rosenberg, 2004;Rossetti, Agangi, Castelli, Padoan, & Ruffini, 2007;Stipp, Fügenschuh, Gromet, Stünitz, & Schmid, 2004;Van Merke de Lummen & Vander Auwera, 1990;Von Blanckenburg, Früh-Green, Diethelm, & Stille, 1992;Von Blanckenburg et al, 1998;Zanoni, 2010;Zanoni et al, 2008). The Periadriatic line nucleated due to the contrasting rheology between rocks of the Southalpine domain and rocks of the Austroalpine and Penninic domains, during Alpine nappe structural development (Figure 1(a)).…”

“…The Periadriatic line nucleated due to the contrasting rheology between rocks of the Southalpine domain and rocks of the Austroalpine and Penninic domains, during Alpine nappe structural development (Figure 1(a)). West of the western segment of the Periadriatic line (Canavese line) Oligocene igneous rocks form the Biella and Traversella km-sized plutons (Bigioggero et al, 1994;Pagliani Peyronel, 1959;Van Merke de Lummen & Vander Auwera, 1990;Zanoni, 2010;Zanoni et al, 2008;Zanoni, Spalla, & Gosso, 2010) and andesitic dykes and lava flows (Callegari et al, 2004;De Capitani, Fiorentini Potenza, Marchi, & Sella, 1979; Compagnoni, Dal Piaz, Hunziker, Gosso, Lombardo, and Williams (1977), Passchier, Urai, Van Loon, and Willimas (1981), and Spalla and Zulbati (2003); the mapped area is denoted by the grey shading.…”

Geochemical Characterization, Geochronology, and Geodynamic Implications of Grenville Rare Earths Bearing Syenites, Haut-Saint-Maurice, QC, Canada

“…The interpretation that shoshonitic syenites, associated or not with a parent mafic magma, are related to a subduction zone or to asthenospheric upwelling has been made elsewhere in the world and for different time periods [37][38][39][40]. However, in our two-stage model for the period between 1040 and 1000 Ma, we did not take into account the well-accepted continent-continent between Laurentia and probably Amazonia [41].…”