Jurassic carbonatite and alkaline magmatism in the Ivrea zone (European Alps) related to the breakup of Pangea

We report on pipe-like bodies and dikes of carbonate rocks related to sodic alkaline intrusions and amphibole mantle peridotites in the Ivrea zone (European Southern Alps). The carbonate rocks have bulk trace-element concentrations typical of low–rare earth element carbonatites interpreted as cumulates of carbonatite melts. Faintly zoned zircons from these carbonate rocks contain calcite inclusions and have trace-element compositions akin to those of carbonatite zircons. Laser ablation–inductively coupled plasma–mass spectrometry U-Pb zircon dating yields concordant ages of 187 ± 2.4 and 192 ± 2.5 Ma, coeval with sodic alkaline magmatism in the Ivrea zone. Cross-cutting relations, ages, as well as bulk and zircon geochemistry indicate that the carbonate rocks are carbonatites, the first ones reported from the Alps. Carbonatites and alkaline intrusions are comagmatic and were emplaced in the nascent passive margin of Adria during the Early Jurassic breakup of Pangea. Extension caused partial melting of amphibole-rich mantle domains, yielding sodic alkaline magmas whose fractionation led to carbonatite-silicate melt immiscibility. Similar occurrences in other rifts suggest that small-scale, sodic and CO2-rich alkaline magmatism is a typical result of extension and decompressiondriven reactivation of amphibole-bearing lithospheric mantle during passive continental breakup and the evolution of magma-poor rifts. INTRODUCTION The common occurrence of carbonatites, alkaline intrusions, and metasomatized mantle peridotites in passive continental margins (e.g., Bailey, 1974; Natarajana et al., 1994; Azer et al., 2010) suggests that (1) a metasomatized mantle source is a prerequisite to generate rift-related alkaline and carbonatite melts (Foley, 1992; Pilet et al., 2008), (2) reactivation of metasomatic mantle domains and magma production are tectonically controlled, and (3) alkaline and carbonate melts are cogenetic and share common mantle sources. In this regard, carbonatite-silicate liquid immiscibility after strong fractionation of alkaline silicate melts has been invoked to produce carbonatites (Lee and Wyllie, 1997). We present field, petrological, geochemical, and geochronological data on intrusive carbonate rocks and alkaline intrusions spatially related to amphibole-rich mantle peridotites from the Ivrea zone (European Southern Alps). The Ivrea zone exposes a piece of lower crust that was located in the margin of Adria during Triassic– Jurassic Pangea breakup (Decarlis et al., 2017) and offers the opportunity to study magmatic processes that acted in both the lithospheric mantle and the lower continental crust during passive rifting. This study aims to understand the origin and evolution of CO2-rich alkaline intrusions in such a passive rift setting, where the reactivation of heterogeneously metasomatized amphibole-bearing mantle domains can be observed in the field. GEOLOGICAL SETTING The Ivrea zone and Serie dei Laghi (Fig. 1) show a complete continental crustal section. In the lower crust, Permian gabbros intruded paragneisses, marbles, and amphibolites; at shallower levels, Permian granitoids intruded gneisses, whereas acidic volcanic rocks extruded at the surface (Quick et al., 2009). Lenses of mantle peridotite occur within the gabbros. Peridotites exhibit amphibole-, apatite-, carbonate-, or phlogopite-rich domains, pyroxenites, hornblendites, and gabbro dikes, and dunite, wehrlite, and chromitite bands (e.g., Zanetti et al., 1999). Geochronological data (e.g., Grieco et al., 2001) and the spread in Sr and Nd bulk isotope data (Voshage et al., 1987) suggest that the mantle experienced a multistage history of melt and fluid percolation from the Devonian to the Early Jurassic, with the involvement of both mantle and crustal components (Grieco et al., 2001; Locmelis et al., 2016). Alkaline ultramafic pipes (Garuti et al., 2001) intruded the lower crust in the Permian–Early Triassic (Locmelis et al., 2016; Fiorentini et al., 2018). Their origin is attributed to the melting of mantle domains metasomatized through the Variscan subduction and reactivated during the collapse of the Variscan belt (Locmelis et al., 2016). A suite of Late Triassic–Early Jurassic sodic and alumina-rich alkaline intrusions, ranging from calcite-clinopyroxene–bearing hornblendites (Stähle et al., 2001), CITATION: Galli, A. , et al., 2019, Jurassic carbonatite and alkaline magmatism in the Ivrea zone (European Alps) related to the breakup of Pangea: Geology, v. 47, p. 199–202, https:// doi . org /10 . 1130 /G45678 . 1 . Manuscript received 12 June 2018 Revised manuscript received 13 December 2018 Manuscript accepted 14 December 2018 https:// doi . org /10 . 1130 /G45678 . 1 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 18 January 2019 Sesia zone (Austroalpine) Penninic units