Early Variscan I-type pluton in the pre-Alpine basement of the Western Alps: The ca. 360Ma Cogne diorite (NW-Italy)

The Grand St Bernard‐Briançonnais Nappe System and the Paleozoic Inheritance of the Western Alps Unraveled by Zircon U‐Pb Dating

“…The less evolved samples (SiO 2 < 60%; Figures e and f, inset), where the effects of fractional crystallization and crust contamination are not expected to mask the primary features of the magma source, show deep troughs in HFSE on PM‐normalized diagrams coupled with strong enrichment of hydrous‐fluid mobile LILE, resulting in high LILE/HFSE (e.g., Ba/Nb > 27.4), LREE/HFSE (La/Nb > 1.8), and Th/Yb (>7.65) ratios, which are typical of a suprasubduction setting (Tatsumi & Eggins, ; Wilson, ). This is also suggested by the quite primitive radiogenic isotopic signature of the Grand Nomenon metadiorites (Nd 360 from −1.2 to +0.9, 87 Sr/ 86 Sr 360 from 0.7054 to 0.7063; Guillot et al, ).…”

“…BM: Žák et al (, ) and Janousek et al (, ); LTS: Shaw et al (). Inset: PM‐ (Figure 5e) and REE (Figure 5f)‐normalized multielement variation diagrams for samples with SiO 2 < 60 wt % compared with those of Guillot et al (). In Figures 5g and 5h, Lower Permian magmatic products from Briançonnais (BR), other Penninic (PE), and Southalpine (SA) domains are plotted for comparison.…”

“…The Leverogne and Grand Nomenon units (Figure ) exposed farther east are classically referred to as the Briançonnais Internal zone (Amstutz, ; Cigolini, ; Dal Piaz & Govi, ; Debelmas et al, ; Desmons & Mercier, ; Elter, ; Polino et al, ), together with the Mont Fort nappe in the nearby Swiss Alps (Sartori et al, ; Thélin et al, ). These units show no evidence of the eclogitic relics and Variscan high‐grade metamophic assemblages that are found in the basement rocks of the Briançonnais External zone (Bertrand, Guillot, et al, ; Cigolini, ; Desmons, Compagnoni, et al, ; Guillot et al, , ). The Leverogne unit (Figure ) consists of garnet‐chloritoid micaschist, garnet‐glaucophane schists, fine‐grained gneisses with poikilolastic albite, graphite‐albite schists, massive quartzites, and abundant mafic and felsic igneous rocks with pervasive Alpine epidote‐blueschist facies metamorphism and greenschist facies retrogression (Malusà, Polino, & Martin, ; Schiavo, ).…”

“…The host rock locally preserves a pre‐Alpine epidote‐amphibolite facies foliation, crosscut by metagranitoid body and only partly obliterated by Alpine greenschist‐facies metamorphism (Malusà, Polino, & Martin, ). The metagranitoid body, hereafter referred to as Grand Nomenon pluton for the sake of simplicity, consists of massive to foliated greenschist‐facies derivatives from calc‐alkaline tonalite and granodiorite, minor quartzdiorite, cumulus gabbro, and aplitic dykes (Cigolini, ; Guillot et al, ). The intrusion age of the Grand Nomenon pluton is referred to the earliest Carboniferous based on U‐Pb conventional isotope dilution thermal ionization mass spectrometry (IDTIMS) (357 ± 24 Ma) and SHRIMP (356 ± 3 Ma) analyses of igneous zircons (Bertrand, Pidgeon, et al, ).…”

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

“…In the Belledonne massif, U–Pb dating of zircon from trondhjemites gave an age of 367 ± 17 Ma (Ménot et al 1988 ). In the internal part of the Briançonnais Zone, U–Pb dating of zircon from the Cogne diorite gave an age of 356 ± 3 (Bertrand et al 2000 ; Guillot et al 2012 ), while an older age of 371 ± 0.9 Ma was proposed by Bergomi et al ( 2017 ). In the Bohemian massif, Sm–Nd whole-rock dating of gabbroic cumulates, diabase dikes and pillowed volcanics, resulted in an age of 351 ± 16 Ma (Pin et al 1988 ).…”