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 ultrama c 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​.

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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. 3E) have no average carbonatite (Woolley and Kempe, 1989; Le Bas, 1999) but con- Eu and almost no Ce anomalies, and HREEs are only slightly fractionated sistent with cumulative carbonatites (e.g., Yang et al., 2011; Fig. 2). In over LREEs as is characteristic for zircons in carbonatites (Belousova et

the studied area, marbles within paragneisses display lower Sr contents al., 2002). VM1 zircons have an εHf(187 Ma) of 0.8 to −5.2 (average value of 189–546 ppm and REE contents one to two orders of magnitude lower −1.2 ± 0.8, MSWD [mean square of weighted deviates] = 1.1, probability than in intrusive carbonate rocks (Fig. 2). of fit = 0.35, 176Hf/177Hf = 0.28259–0.28268; Fig. 3G; Item DR7). VF33

zircons show two groups of distinct Hf composition. In group 1, εHf(195 Ma) INTRUSIVE CARBONATE ROCK ZIRCONS is 3.5 to −0.8 (average value 1.3 ± 1.2, MSWD = 0.95, prob. = 0.489, 176 177 Zircons from samples VM1 and VF33 are rounded to slightly elongated, Hf/ Hf = 0.28263–0.28275), whereas in group 2, εHf(195 Ma) is −5.3 to 50–150 μm long, and homogeneous to faintly zoned in cathodolumines- −9.5 (average value −7.7 ± 2.4, MSWD = 0.77, prob. = 0.46, 176Hf/177Hf = cence (Fig. 3A; Item DR4). A few grains in VF33 display oscillatory-zoned 0.28238–0.28250). The Permian oscillatory-zoned zircon in sample VF33 cores and homogeneous or faintly zoned rims (Item DR4). In the homoge- has lower Th/U and Zr/Hf ratios of 0.01 and 35, and is rich in Hf and poor 176 177 neous zircons of both samples, calcite inclusions were identified by Raman in Th (Fig. 3F). Its εHf(287 Ma) of −15 ( Hf/ Hf = 0.28217) is similar to εHf(t) spectrometry (Fig. 3B; Item DR5), testifying for crystallization from a in zircons from paragneisses of the Ivrea zone (Fig. 3G; Ewing et al., 2014). calcite-saturated environment. In sample VM1, 30 spots were analyzed by laser ablation–inductively coupled plasma–mass spectrometry on 30 RIFT-RELATED JURASSIC CARBONATITES homogeneous to faintly zoned grains. Nineteen (19) spots are concordant The Early Jurassic ages of the intrusive carbonate rocks exclude that and give an age of 187 ± 2.4 Ma (Fig. 3C), interpreted as intrusion age. they formed through partial melting of crustal carbonates during the granu- litic thermal peak, which occurred during the Permian (Ewing et al., 2015). 1 GSA Data Repository item 2019068, Item DR1 (analytical methods), Item DR2 (photos of field area and samples), Item DR3 (bulk rock geochemistry), Item Their bulk geochemistry, zircon composition, calcite inclusions, and ages DR4 (cathodoluminescence images of carbonatite zircons), Item DR5 (inclusions suggest that these intrusive carbonate rocks are Jurassic carbonatites, the in zircons), Item DR6 (zircon U-Pb-Th isotope data), Item DR7 (zircon Hf-Lu- first ones reported so far in the Alps. The telltale association with alka- Yb isotope data), Item DR8 (zircon trace element composition), Item DR9 (major line intrusions supports this interpretation. The alkaline magmatism in element bulk rock compositon of alkaline intrusions from the Ivrea zone and Serie dei Laghi), and Item DR10 (LA-ICP-MS trace element bulk rock composi- the Ivrea zone is mostly Late Triassic to Early Jurassic (Schaltegger et tion of alkaline intrusions from the Ivrea Zone), is available online at http://www​ al., 2015), but was already present in the late Permian (Fiorentini et al., .geosociety​.org​/datarepository​/2019/, or on request from [email protected] 2018), testifying for a protracted period of extension. Extension started

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/3/199/4650284/199.pdf by guest on 23 September 2021 0.035 U VM1 VF33 VM1 oceanic island basalt series. We propose that melting of amphibole-rich 238 / 200 mantle domains generates sodic magmas enriched in LREEs over HREEs

0.031 Pb

206 and Nb but with variable Th and U contents, consistent with the results of melting experiments on amphibole-rich ultramafic sources (Pilet et al., calcite 0.027 160 2008). The degree of melting and amphibole/clinopyroxene ratios in the B 20μm Age: 187 ± 2.4 Ma 0.023 C MSWD = 6.4 source control the amount of highly incompatible elements (Pilet et al., 0.12 0.16 0.20 0.24 0.28 A 30μm 207Pb/235U 2008). Interactions of such melts with shallower mantle increases their 5 10 zircons from Ivrea metapelites 0.032 U VF33 Al content by dissolution of Al-rich orthopyroxene and precipitation of

238 200

(Kunz et al., 2018) / 103 olivine (Pilet et al., 2008). Fractionation of olivine, clinopyroxene, amphi- Pb

206 bole, and Ti-oxides then further increases Al and alkalis but decreases Ti. 10 0.030 Amphibole fractionation yields the flat to spoon-shaped MREE-HREE 10-1 rcon/chondrite carbonatitic zircons Age: 192 ± 2.5 Ma patterns of the more-evolved melts (Blundy and Wood, 2003). The late zi D 180 MSWD = 1.00 10-3 (Belousova et al., 2002) E 0.028 fractionation of plagioclase would favor a strong alkali enrichment, pro- La Ce PrNd SmEu Gd Tb Dy Ho ErTmYb Lu 0.17 0.19 0.21 0.23 0.25 0.27 207Pb/235U ducing residual melts sufficiently rich in alkalis to hit the carbonatite- +5 16000 zircons from chromitite silicate miscibility gap. In the total alkali–silica (TAS) diagram (Fig. 4), Ivrea metapelites 0 layers ppm ) alkali gabbros, alkali diorites, plagioclasites, and syenites follow dif- 12000 -5 VF33 Hf ( gr1 Ivrea ferentiation trends that lead through alkali enrichment to immiscibility 8000 carbonatitic εHf(t) -10 metapelites zircons with carbonatite melts (Schmidt and Weidendorfer, 2018). Plagioclasite 4000 -15 VF33 Permian core gr2 enclaves are similar in composition to plagioclasite dikes and fit the frac- 0 F Th/U -20 G Age (Ma) 10-3 10-2 10-1 110 180 200 220 240 260280 300 tionation path of the syenites, which may represent the evolving alkaline silicate melts just before or at immiscibility. Figure 3. Data from sampled zircons, Ivrea zone, European Southern Alps. A: Cathodoluminescence image of faintly zoned zircon (sample VM1). B: Calcite inclusion within zircon (sample VF33). C,D: Concor- MAGMA SOURCE

dia diagrams of carbonatite zircons. Ages are calculated as weighted In the studied carbonatites, the positive to slightly negative εHf(t) of mean and errors at 95% confidence level. Values along the Concordia the sample VM1 zircons and the sample VF33 group 1 zircons points to are ages in Ma. Ellipses are plotted with 2σ error; colored ellipses are a metasomatic mantle source of the parent magma, whereas moderate calculated average ages. MSWD—mean square of weighted deviates. E,F: Trace-element composition of zircons. Red is sample VM1; blue, incorporation of paragneiss during magma emplacement is suggested by homogeneous grains in sample VF33; yellow, mixed core and rim in the lower εHf(t) of group 2 zircons and Permian oscillatory-zoned core in VF33; black, Permian core in VF33. G: versus age in zircon (gr1, εHf(t) sample VF33. Zircons with age and εHf(t) akin to those of the VM1 zir- gr2—zircon groups 1 and 2). Ivrea metapelite zircons are from Ewing et cons and VF33 group 1 zircons occur in metasomatic chromitite layers al. (2014); zircons from Ivrea chromitite layers within mantle peridotite are after Zanetti et al. (2016) and Malitch et al. (2017). within amphibole-phlogopite mantle peridotites of the Ivrea zone (Fig. 3G; Zanetti et al., 2016; Malitch et al., 2017), indicating that the studied with the collapse of the Variscan belt in the late Carboniferous and, after carbonatites isotopically match parts of the mantle. Available bulk Sr-Nd the Permian magmatic event, progressed in the Mesozoic with the con- isotope data for the Ivrea alkaline intrusions present a large scatter (Stähle tinental rifting related to the Pangea breakup. Rifting was characterized et al., 2001; Garuti et al., 2001) fitting the spread in mantle rocks (Voshage by pulses of deep-seated magmatism, fluid influx, and heating due to et al., 1987). This suggests that these magmas arise from heterogeneously lithospheric extension and low-degree mantle melting (Ewing et al., 2015), metasomatized mantle domains. Their variable isotopic signatures reflect which strongly modified the geochemical and rheological structure of the the complex compositional structure of the source, irrespective of the age(s)

lithospheric mantle. The age of the studied carbonatites indicates that they and mechanism(s) of metasomatism. The generation of sodic CO2-bearing marked the last stage of rift magmatism on the nascent margin of Adria, melts from isotopically variable amphibole-rich peridotites during passive before the opening of the Tethys Ocean and the formation of oceanic crust rifting explains the relatively low trace-element concentrations and hetero- in the Late Jurassic (Rubatto et al., 1998). geneous isotopic signature. The Ivrea zone is also famous for isotopically The Ivrea carbonatites present absolute bulk trace-element concentra- enriched phlogopite peridotites related to crustal metasomatism (Voshage tions lower than those of typical carbonatites (Woolley and Kempe, 1989) et al., 1987). These do not constitute a suitable source for the sodic melts, but in the range of those of low-REE carbonatites interpreted as cumulates (Yang et al., 2011). In fact, the almost monomineralic alkali-poor calcite rocks do not represent true carbonatite melt compositions, which should 18 phonolite minimum Enclave in carbonatite VF 16 contain 5–20 wt% bulk Na2O + K2O (Schmidt and Weidendorfer, 2018), t alkali feldspar Nepheline syenite ten maximum O (wt%) x p

14 2 but rather wall cumulates formed during the early stages of carbonatite e ga Plagioclasite dike K m u lity granite + m i 12 axi cib minimum Alkaline gabbro/diorite crystallization stacked in the lower crust. During the Early Jurassic, the O is 2 m f m o Gabbro associated with VF country rocks had a regional temperature <600 °C (Ewing et al., 2015), 10 Na

Hornblendite cumulus which contrasts with the shallow-crust or surface environment of most 8 alkaline carbonatites and led to a comparatively slower cooling and more favor- 6 Hornblendite dike able conditions for cumulate formation. 4 subalkaline Enclave in carbonatite VM Alkaline ultrama c pipe 2 SiO (wt%) Alkaline dike Serie dei Laghi LINK TO ALKALINE MAGMATISM 0 2 Nearly 95% of the Late Triassic–Early Jurassic alkaline intrusions in 30 40 50 60 65

the Ivrea zone are sodic (on average Na2O/K2O ~5; Item DR9). Horn- Figure 4. Total alkali–silica (TAS) diagram with composition of alkaline blendites and alkali gabbros are rich in Ti and Nb, and show variable Th rocks from Ivrea zone, European Southern Alps. Gray arrow shows frac- and U contents and an enrichment of LREEs over HREEs. More-evolved tionation path following Schmidt and Weidendorfer (2018). Note that melting of various mantle volumes yields slightly differing primitive alkali diorites, plagioclasites, and syenites are rich in Al, poor in Ti, dis- melts and most intrusions are slightly cumulative, thus fractionation play steep LREE patterns, but flat to spoon-shaped middle REE (MREE)– paths are approximate. Bulk-rock compositions and references are HREE patterns and one order of magnitude less enrichment than typical in Item DR9 (see footnote 1). VF—Val Fiorina; VM—Val Mastallone.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/3/199/4650284/199.pdf by guest on 23 September 2021 Grieco, G., Ferrario, A., Von Quadt, A., Köppel, V., and Mathez, E.A., 2001, The but their local involvement would contribute to the variable Na2O/K2O ratios (40–0.4) and isotopic signature of the alkaline melts. Finally, the Zircon-bearing chromitites of the phlogopite peridotite of Finero (Ivrea Zone, Southern Alps): Evidence and geochronology of a metasomatized mantle slab: small-scale alkaline magmatism in the Ivrea zone occurs over a distance Journal of Petrology, v. 42, p. 89–101, https://doi​ .org​ /10​ .1093​ /petrology​ /42​ .1​ .89​ . of ~80 km, hence cannot stem from a single mantle volume but from local Kunz, B.E., Regis, D., and Engi, M., 2018, Zircon ages in granulite facies rocks: mantle regions, in all likelihood the most enriched ones. Parent magmas are Decoupling from geochemistry above 850 °C?: Contributions to Mineralogy hence expected to be slightly heterogeneous in geochemical and isotopic and Petrology, v. 173, 26, https://​doi​.org​/10​.1007​/s00410​-018​-1454​-5. compositions, rendering the entity of all intrusions not strictly comagmatic Le Bas, M.J., 1999, Sövite and alvikite: Two chemically distinct calciocarbonatites C1 and C2: South African Journal of Geology, v. 102, p. 109–121. and enabling some differences in the fractionation path. Lee, W.J., and Wyllie, P.J., 1997, Liquid immiscibility between nephelinite and carbonatite from 1.0 to 2.5 GPa compared with mantle melt compositions: CONCLUSIONS Contributions to Mineralogy and Petrology, v. 127, p. 1–16, https://​doi​.org​ The association of carbonatites, sodic alkaline intrusions, and amphi- /10.1007​/s004100050261. Locmelis, M., Fiorentini, M.L., Rushmer, T., Arevalo, R., Jr., Adam, J., and Denyszyn, bole peridotites is a common feature of magma-poor passive continental S.W., 2016, Sulfur and metal fertilization of the lower continental crust: Lithos, margins. We propose that low-degree partial melting of amphibole-bearing v. 244, p. 74–93, https://​doi​.org​/10​.1016​/j​.lithos​.2015​.11​.028. lithospheric mantle domains during extension generated small-scale sodic, Malitch, K.N., Belousova, E.A., Griffin, W.L., Badanina, I.Yu., Knauf, V.V., O’Reilly, S.Y., and Pearson, N.J., 2017, Laurite and zircon from the Finero chromitites CO2-rich alkaline magmatism in the passive margin of Adria during Trias- sic–Jurassic Pangea breakup. Fractionation of these melts resulted in sodic (Italy): New insights into evolution of the subcontinental mantle: Ore Geology Reviews, v. 90, p. 210–225, https://​doi​.org​/10​.1016​/j​.oregeorev​.2017​.06​.027. alkaline and carbonatite intrusions. This magmatism contrasts in style McDonough, W.F., and Sun, S.-s., 1995, The composition of the Earth: Chemical and volume with the deeply rooted, strongly enriched alkaline magmas Geology, v. 120, p. 223–253, https://​doi​.org​/10​.1016​/0009​-2541​(94)00140​-4. typical of continental breakup driven by large-scale mantle convection Natarajana, M., Bhaskar, R., Parthasarathy, R., Kumar, A., and Gopalan, K., 1994, (e.g., the East African Rift; Baker et al., 1972). This study suggests that 2.0 Ga old pyroxenite-carbonatite complex of Hogenakal, Tamil Nadu, South India: Precambrian Research, v. 65, p. 167–181, https://​doi​.org​/10​.1016​/0301​ strong fractionation of alkaline suites may be an efficient mechanism to -9268​(94)​90104-X. produce carbonatites at relatively high lithospheric levels. Pilet, S., Baker, M.B., and Stolper, E.M., 2008, Metasomatized lithosphere and the origin of alkaline lavas: Science, v. 320, p. 916–919, https://​doi​.org​/10​.1126​ ACKNOWLEDGMENTS /science​.1156563. G. Fellin, L. Zehnder, O. Laurent, and M. Guillong are acknowledged for analytical Quick, J.E., Sinigoi, S., Peressini, G., Demarchi, G., Wooden, J.L., and Sbisà, A., support. ETH generously financed this work. C. Pin, M. Fiorentini, and four anony- 2009, Magmatic plumbing of a large Permian caldera exposed to a depth of mous reviewers are acknowledged for their constructive reviews. 25 kilometers: Geology, v. 37, p. 603–606, https://​doi​.org​/10​.1130​/G30003A​.1. Rubatto, D., Gebauer, D., and Fanning, M., 1998, Jurassic formation and Eocene REFERENCES CITED subduction of the Zermatt–Saas-Fee ophiolites: Implications for the geodynamic Azer, M.K., Stern, R.J., and Kimura, J.-I., 2010, Origin of a late Neoproterozoic (605 evolution of the Central and Western Alps: Contributions to Mineralogy and ± 13 Ma) intrusive carbonate-albitite complex in Southern Sinai, Egypt: Inter- Petrology, v. 132, p. 269–287, https://​doi​.org​/10​.1007​/s004100050421. national Journal of Earth Sciences, v. 99, p. 245–267, https://​doi​.org​/10.1007​ Schaltegger, U., Ulianov, A., Muntener, O., Ovtcharova, M., Peytcheva, I., Vonlan- /s00531​-008​-0385​-1. then, P., Vennemann, T., Antognini, M., and Girlanda, F., 2015, Megacrystic Bailey, D.K., 1974, Continental rifting and alkaline magmatism, in Sørensen, H., ed., zircon with planar fractures in miaskite-type nepheline pegmatites formed at The Alkaline Rocks: New York, John Wiley & Sons, p. 148–159. high pressures in the lower crust (Ivrea Zone, southern Alps, Switzerland): The Baker, B.H., Mohr, P.A., and Williams, L.A.J., 1972, Geology of the Eastern Rift American Mineralogist, v. 100, p. 83–94, https://doi​ .org​ /10​ .2138​ /am​ -2015-4773​ . System of Africa: Geological Society of America Special Paper 136, 67 p., Schmidt, M.W., and Weidendorfer, D., 2018, Carbonatites in oceanic hotspots: Geol- https://​doi​.org​/10​.1130​/SPE136. ogy, v. 46, p. 435–438, https://​doi​.org​/10​.1130​/G39621​.1. Belousova, E.A., Griffin, W.L., O’Reilly, S.Y., and Fisher, N.I., 2002, Igneous zir- Srivastava, R.K., 2013, Petrological and geochemical characteristics of Paleopro- con: Trace element composition as an indicator of source rock type: Contribu- terozoic ultramafic lamprophyres and carbonatites from the Chitrangi region, tions to Mineralogy and Petrology, v. 143, p. 602–622, https://​doi​.org​/10​.1007​ Mahakoshal supracrustal belt, central India: Journal of Earth System Science, /s00410​-002​-0364​-7. v. 122, p. 759–776, https://​doi​.org​/10​.1007​/s12040​-013​-0311​-1. Blundy, J., and Wood, B., 2003, Partitioning of trace elements between crystals and Stähle, V., Frenzel, G., Hess, J.C., Saupé, F., Schmidt, S.T., and Schneider, W., 2001, melts: Earth and Planetary Science Letters, v. 210, p. 383–397, https://​doi​.org​ Permian metabasalt and Triassic alkaline dykes in the northern Ivrea zone: Clues /10​.1016​/S0012​-821X​(03)00129​-8. to the post-Variscan geodynamic evolution of the Southern Alps: Schweizerische Comin-Chiaramonti, P., De Barros Gomes, C., Castorina, F., Di Censi, P., Antonini, Mineralogische und Petrographische Mitteilungen, v. 81, p. 1–21. P., Furtado, S., Ruberti, E., and Scheibe, L.F., 2002, Geochemistry and geody- Voshage, H., Hunziker, J.C., Hofmann, A.W., and Zingg, A., 1987, A Nd and Sr namic implications of the Anitápolis and Lages alkaline-carbonatite complexes, isotopic study of the Ivrea zone, Southern Alps, N-Italy: Contributions to Min- Santa Caterina State, Brazil: Revista Brasileira de Geociencias, v. 32, p. 43–58, eralogy and Petrology, v. 97, p. 31–42, https://​doi​.org​/10​.1007​/BF00375212. https://​doi​.org​/10​.25249​/0375​-7536​.20023214358. Woolley, A.R., and Kempe, D.R.C., 1989, Carbonatites: Nomenclature, average Decarlis, A., Beltrando, M., Manatschal, G., Ferrando, S., and Carosi, R., 2017, chemical compositions, and element distribution, in Bell, K., ed., Carbonatites: Architecture of the distal Piedmont-Ligurian rifted margin in NW Italy: Hints Genesis and Evolution: London, Unwin Hyman, p. 1–14. for a flip of the rift system polarity: Tectonics, v. 36, p. 2388–2406,https:// ​doi​ Yang, K.-F., Fan, H.-R., Santosh, M., Hu, F.-F., and Wang, K.-Y., 2011, Mesopro- .org​/10​.1002​/2017TC004561. terozoic carbonatitic magmatism in the Bayan Obo deposit, Inner Mongolia, Ewing, T.A., Rubatto, D., and Hermann, J., 2014, Hafnium isotopes and Zr/Hf of North China: Constraints for the mechanism of super accumulation of rare earth rutile and zircon from lower crustal metapelites (Ivrea-Verbano Zone, Italy): elements: Ore Geology Reviews, v. 40, p. 122–131, https://​doi.org​ ​/10.1016​/j​ Implications for chemical differentiation of the crust: Earth and Planetary Sci- .oregeorev​.2011​.05​.008. ence Letters, v. 389, p. 106–118, https://​doi​.org​/10​.1016​/j​.epsl​.2013​.12​.029. Zanetti, A., Mazzuchelli, M., Rivalenti, G., and Vannucci, R., 1999, The Finero Ewing, T.A., Rubatto, D., Beltrando, M., and Hermann, J., 2015, Constraints on phlogopite-peridotite massif: An example of subduction-related metasomatism: the thermal evolution of the Adriatic margin during Jurassic continental break- Contributions to Mineralogy and Petrology, v. 134, p. 107–122, https://​doi​.org​ up: U-Pb dating of rutile from the Ivrea-Verbano Zone, Italy: Contributions to /10​.1007​/s004100050472. Mineralogy and Petrology, v. 169, https://​doi​.org​/10​.1007​/s00410​-015​-1135​-6. Zanetti, A., Giovanardi, T., Langone, A., Tiepolo, M., Wu, F.-Y., Dallai, L., and Fiorentini, M.L., LaFlamme, C., Denzsyzn, S., Mole, D., Maas, R., Locmelis, M., Mazzucchelli, M., 2016, Origin and age of zircon-bearing chromitite layers Caruso, S., and Bui, T.-H., 2018, Post-collisional alkaline magmatism as gateway from the Finero phlogopite peridotite (Ivrea-Verabno Zone, Western Alps) and for metal and sulfur enrichment of the continental lower crust: Geochimica et Cos- geodynamic consequences: Lithos, v. 262, p. 58–74, https://​doi​.org​/10​.1016​/j​ mochimica Acta, v. 223, p. 175–197, https://doi​ .org​ /10​ .1016​ /j​ .gca​ .2017​ .11​ .009​ . .lithos​.2016​.06​.015. Foley, S.F., 1992, Vein-plus-wall-rock melting mechanism in the lithosphere and the Zingg, A., 1983, The Ivrea and Strona-Ceneri Zones (southern Alps, Ticino and origin of potassic alkaline magmas: Lithos, v. 28, p. 435–453, https://​doi​.org​ N-Italy): A review: Schweizerische Mineralogische und Petrologische Mit- /10​.1016​/0024​-4937​(92)90018​-T. teilungen, v. 63, p. 361–392. Garuti, G., Bea, F., Zaccarini, F., and Montero, P., 2001, Age, geochemistry and petrogenesis of the ultramafic pipes in the Ivrea Zone, NW Italy: Journal of Petrology, v. 42, p. 433–457, https://​doi​.org​/10​.1093​/petrology​/42​.2​.433. Printed in USA

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