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Jurassic Carbonatite and Alkaline Magmatism in the Ivrea Zone (European Alps) Related to the Breakup of Pangea A

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Jurassic Carbonatite and Alkaline Magmatism in the Ivrea Zone (European Alps) Related to the Breakup of Pangea A https:// doi .org /10 .1130 /G45678 .1 Manuscript received 12 June 2018 Revised manuscript received 13 December 2018 Manuscript accepted 14 December 2018 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 18 January 2019 Jurassic carbonatite and alkaline magmatism in the Ivrea zone (European Alps) related to the breakup of Pangea A. Galli, D. Grassi, G. Sartori, O. Gianola, J.-P. Burg, and M.W. Schmidt Department of Earth Sciences, ETH Zurich, Sonnegstrasse 5, 8092 Zurich, Switzerland ABSTRACT GEOLOGICAL SETTING We report on pipe-like bodies and dikes of carbonate rocks related The Ivrea zone and Serie dei Laghi (Fig. 1) show a complete conti- to sodic alkaline intrusions and amphibole mantle peridotites in the nental crustal section. In the lower crust, Permian gabbros intruded parag- Ivrea zone (European Southern Alps). The carbonate rocks have bulk neisses, marbles, and amphibolites; at shallower levels, Permian granitoids trace-element concentrations typical of low–rare earth element car- intruded gneisses, whereas acidic volcanic rocks extruded at the surface bonatites interpreted as cumulates of carbonatite melts. Faintly zoned (Quick et al., 2009). Lenses of mantle peridotite occur within the gabbros. zircons from these carbonate rocks contain calcite inclusions and have Peridotites exhibit amphibole-, apatite-, carbonate-, or phlogopite-rich trace-element compositions akin to those of carbonatite zircons. Laser domains, pyroxenites, hornblendites, and gabbro dikes, and dunite, weh- ablation–inductively coupled plasma–mass spectrometry U-Pb zircon rlite, and chromitite bands (e.g., Zanetti et al., 1999). Geochronological dating yields concordant ages of 187 ± 2.4 and 192 ± 2.5 Ma, coeval data (e.g., Grieco et al., 2001) and the spread in Sr and Nd bulk isotope with sodic alkaline magmatism in the Ivrea zone. Cross-cutting rela- data (Voshage et al., 1987) suggest that the mantle experienced a multi- tions, ages, as well as bulk and zircon geochemistry indicate that the stage history of melt and fluid percolation from the Devonian to the Early carbonate rocks are carbonatites, the first ones reported from the Jurassic, with the involvement of both mantle and crustal components Alps. Carbonatites and alkaline intrusions are comagmatic and were (Grieco et al., 2001; Locmelis et al., 2016). emplaced in the nascent passive margin of Adria during the Early Alkaline ultramafic pipes (Garuti et al., 2001) intruded the lower Jurassic breakup of Pangea. Extension caused partial melting of crust in the Permian–Early Triassic (Locmelis et al., 2016; Fiorentini amphibole-rich mantle domains, yielding sodic alkaline magmas whose et al., 2018). Their origin is attributed to the melting of mantle domains fractionation led to carbonatite-silicate melt immiscibility. Similar metasomatized through the Variscan subduction and reactivated during occurrences in other rifts suggest that small-scale, sodic and CO2-rich the collapse of the Variscan belt (Locmelis et al., 2016). A suite of Late alkaline magmatism is a typical result of extension and decompression- Triassic–Early Jurassic sodic and alumina-rich alkaline intrusions, ranging driven reactivation of amphibole-bearing lithospheric mantle during from calcite-clinopyroxene–bearing hornblendites (Stähle et al., 2001), passive continental breakup and the evolution of magma-poor rifts. 8°10‘E INTRODUCTION CH The common occurrence of carbonatites, alkaline intrusions, and meta- Penninic units SC16 somatized mantle peridotites in passive continental margins (e.g., Bailey, Italy BC 1974; Natarajana et al., 1994; Azer et al., 2010) suggests that (1) a meta- 44°N VF somatized mantle source is a prerequisite to generate rift-related alkaline Luino and carbonatite melts (Foley, 1992; Pilet et al., 2008), (2) reactivation 08°E Pogallo Line Serie dei Laghi of metasomatic mantle domains and magma production are tectonically N CA1 Permian-Mesozoic controlled, and (3) alkaline and carbonate melts are cogenetic and share ST2 volcanic rocks common mantle sources. In this regard, carbonatite-silicate liquid immisci- Permian plutonic rocks VM Gneisses and schists bility after strong fractionation of alkaline silicate melts has been invoked to 45°50‘N Ivrea Zone produce carbonatites (Lee and Wyllie, 1997). We present field, petrological, Migmatitic metasediments geochemical, and geochronological data on intrusive carbonate rocks and Permian gabbros alkaline intrusions spatially related to amphibole-rich mantle peridotites Alkaline ultramac pipes Insubric Line Peridotites from the Ivrea zone (European Southern Alps). The Ivrea zone exposes a Sesia zone piece of lower crust that was located in the margin of Adria during Triassic– (Austroalpine) Carbonatites Jurassic Pangea breakup (Decarlis et al., 2017) and offers the opportunity 10 km Marbles within paragneiss to study magmatic processes that acted in both the lithospheric mantle and the lower continental crust during passive rifting. This study aims to Figure 1. Geological map of Ivrea zone and Serie dei Laghi, European understand the origin and evolution of CO -rich alkaline intrusions in such Souther nAlps, with sample locations. Modified after Zingg (1983). 2 CH—Switzerland; BC—Bocchetta di Campo; VF—Val Fiorina; VM— a passive rift setting, where the reactivation of heterogeneously metaso- Val Mastallone; SC—Alpe Scaredi; CA—Candoglia; ST—Val Strona; matized amphibole-bearing mantle domains can be observed in the field. UM—ultramafic. 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 . Geological Society of America | GEOLOGY | Volume 47 | Number 3 | www.gsapubs.org 199 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/3/199/4650284/199.pdf by guest on 23 September 2021 alkali gabbros, and diorites to plagioclasites and nepheline syenites 10000 average carbonatitesmarbles A (Schaltegger et al., 2015), occurs associated in space with amphibole carbonatite VF CA1 1000 peridotites. Their ages suggest a relation to the early Mesozoic breakup of cumulate VM ST2 te carbonatite BC SC16 Pangea (Schaltegger et al., 2015). Intrusive carbonate rocks of unknown 100 age and origin are found with amphibole peridotites and alkaline intru- hondri /c sives (Fig. 1). In the field, these discordant intrusive carbonate rocks le 10 form pipe-like bodies and dikes (Items DR2A–DR2E in the GSA Data mp sa Repository1) structurally distinct from metasedimentary marbles, which 1 are concordant with paragneisses. In this study, three intrusive carbonate 0.1 rocks from Val Mastallone, Val Fiorina, and Bocchetta di Campo, Italy La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y 10000 (referred to as VM, VF, and BC; locations in Fig. 1), and their associated average carbonatite B 1000 alkaline intrusives were investigated by geochemical analyses. Zircons le cumulate from VM and VF were U-Pb dated by laser ablation–inductively coupled nt 100 carbonatite ma plasma–mass spectrometry (analytical methods are provided in Item DR1). ve 10 ti mi INTRUSIVE CARBONATE ROCKS: FIELD RELATIONS, ri 1 /p le MINERALOGY, AND WHOLE-ROCK GEOCHEMISTRY 0.1 mp VM and VF are up to 40 × 70 m large, subcircular, steeply dipping sa 0.01 pipe-like bodies with sharp discordant magmatic contacts to the host Rb Ba Th UKTaNbLa CePb Pr Sr Nd PHfZrSmEuGdTiTbDyY HoEr TmYb Lu rocks (Items DR2A and DR2D). VM cuts across garnet-amphibole gab- Figure 2. Chondrite-normalized bulk-rock rare-earth-element (REE) bronorites, whereas VF intrudes paragneisses and an ultramafic-mafic contents (A) and primitive mantle–normalized bulk trace-element body composed of phlogopite-amphibole-carbonate–bearing peridotite contents (B) for carbonatites and marbles within paragneiss in surrounded by alkaline hornblendite and gabbro. Plagioclasite dikes are the Ivrea zone, European Southern Alps. See Figure 1 for sample associated with both carbonate rocks. BC is a 250-m-long and 20-m-thick, locations. Normalization values after McDonough and Sun (1995). Average carbonatite after Woolley and Kempe (1989) and Le Bas steeply dipping dike (Item DR2E) intrusive into a garnet-amphibole gab- (1999); low-REE cumulate carbonatite compositions after Comin- bronorite close to bodies of amphibole peridotites and dikes of alkali Chiaramonti et al. (2002), Yang et al. (2011), and Srivastava (2013). gabbro, alkali diorite, nepheline syenite, and plagioclasite (10%–25% anorthite [An10–25]; Item DR2C). All carbonate rocks are composed of In sample VF33, 20 points on 20 homogeneous to faintly zoned grains calcite with minor clinopyroxene, amphibole, apatite, sphene, allanite, and seven points in the cores of grains with oscillatory-zoned cores and and zircon (Item DR2F) and contain polymict xenoliths and enclaves. homogeneous rims were analyzed. On the homogeneous grains, 17 points VM encloses clinopyroxenites (Item DR2B). VF and BC contain xeno- are concordant; 15 of them yielded a concordant age of 192 ± 2.5 Ma (Fig. liths from the host rocks and phlogopite-amphibole–bearing plagioclasite. 3D), interpreted as intrusion age. Two points are slightly younger, probably Intrusive carbonate rocks have 0.01–0.94 wt% Na2O + K2O, 278–4368 due to the influence of inclusions, as suggested by their compositions (Item ppm Sr, and total rare-earth-element (REE) content of 83–211 ppm, are DR6). One point on the largest oscillatory-zoned core yielded a Permian age enriched in light REEs (LREEs) over heavy REEs (HREEs) (chondrite- of 287 ± 6 Ma, and six points yielded mixed core-rim ages of 201–226 Ma. normalized La/Yb ratio of 13.9–18.4) Ba, Th, U, and Sr, and are depleted Concordant homogeneous zircons from both samples have low REE, in Rb, K, Ta, Nb, P, Hf, Zr, and Ti, as typically reported for carbonatites U, Th, Y contents (Item DR8) and relatively high Th/U (0.2–1.4; Fig. 3F) (Fig. 2; Item DR3). Absolute trace-element concentrations are lower than and Zr/Hf (44–145). Chondrite-normalized REE patterns (Fig.
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