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

Abstract

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 CO 2-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.

Mantle-derived Corundum-bearing Felsic Dykes May Survive Only within the Lower (refractory/inert) Crust: Evidence from Zircon Geochemistry and Geochronology (Ivrea-Verbano Zone, Southern Alps, Italy)

“…These authors reported TIMS zircon ages at ~200 Ma from lower crustal pyroxenites and dunites that they interpreted as a distal and deep expression of the Central Atlantic Magmatic Province. A Jurassic age of emplacement (LA-ICP-MS concordant age of 187 ±2.4 and 192 ±2.4 Ma, see Figure 2) has been recently reported also for carbonatite bodies and dykes by Galli et al, [59] that they linked to sodic CO2-bearing melts derived from an amphibole-rich mantle source. In particular, Hf values indicates a moderate crustal assimilation during emplacement of carbonatitic magma [59].…”

“…By comparing zircons from dykes studied in this work and from other Triassic-Jurassic intrusive bodies of IVZ it is evident that studied zircon grains are characterised by M-HREE contents significantly higher than those documented in zircons from leucocratic dykes, locally corundumbearing, in the Finero Complex (northern IVZ; [14,22]) and carbonatites (central IVZ; [59]). The zircons in both felsic dykes are characterized by pronounced negative Eu anomalies that can be absent in the other local Mesozoic intrusions (Figure 9b).…”

“…According to recent geochronological investigations episodic magmatism producing mafic/ultramafic rocks, alkaline dykes/pegmatites and carbonatites (with evidence of crustal assimilation) continued up to Jurassic, likely related discrete episodes of thinning and exhumation (Figure 13; [14,22,57,59,60,62,148]). However, further investigations are needed in order to place more detailed constraints on the mantle-crust interplay ruling the rifting of the continental before the opening of the Alpine Tethys.…”

The evolution of the Sesia Zone (Western Alps) from Carboniferous to Cretaceous: insights from zircon and allanite geochronology

“…Vavra et al 1996 ; Vavra and Schaltegger 1999 ). In the Ivrea Zone, sporadic intrusions are dated in the Triassic (granitic dikes at 251 ± 2 Ma, Wright and Shervais 1980 ; the Finero Mafic Complex at 232 ± 3 Ma, Zanetti et al 2013 ; and syenite dikes 225 ± 13 Ma, Stähle et al 1990 ), alkaline pegmatoids at the Triassic-Jurassic boundary (210–190 Ma, Oppizzi and Schaltegger 1999 ; Schaltegger et al 2008 ; 2011 ) and carbonate rocks related to sodic alkaline intrusions and amphibole mantle peridotites in the early Jurassic (187 ± 2.4 and 192 ± 2.5 Ma, Galli et al 2019 ). This further supports a high geothermal gradient and circulation of fluids in the middle to lower crust of the Sesia and Ivrea zones between ~ 250 and 190 Ma, also enhanced by the intense deformation affecting these domains in the Triassic and the Jurassic (e.g.…”

Triassic magmatism in the European Southern Alps as an early phase of Pangea break-up

“…Cassinis & Zezza, 1982;Pamić, 1984;Vaccaro et al 1991;Laurenzi et al 1994;Visonà & Zanferrari, 2000;Stähle et al 2001;Liberi et al 2011;Zanetti et al 2013;Beltràn-Triviño et al 2016;Storck et al 2019), with minor pulses at about 215-220 Ma (e.g. Stähle et al 2001;Cassinis et al 2008;Casetta et al 2019) and 192-187 Ma (Galli et al 2019). However, U-Pb zircon ages of the deep crustal intrusions from Finero (e.g.…”