Le rocce erciniche nella Zona brianzonese ligure Matteo Maino Dipartimento di Scienze della Terra e dell Ambiente, Università degli Studi di Pavia
Pre-alpine basements of the Ligurian Alps Working group: L. Gaggero, G. Dallagiovanna, S. Seno, G. Oggiano, A. Decarlis, L. Casini, M. Maino
Pre-Alpine basements of the Alps From Von Raumer et al., 1998; Raumer et al., 2003; Maino et al., 2012
Pre-Alpine rocks of the Ligurian Alps Maino et al., 2012
Alpine structure Seno et al., 2005
Alpine structure Maino et al., under rev.
Briançonnais units Meso-Cenozoic cover Post-Variscan volcanosedimentary succession Variscan Basement
Pre-Variscan basement Complex I: Eclogite, Amphibolite (tholeiitic basalts) Meta-intrusives, volcanites and paragneisses (granodiorites, granites, rhyolites and sediments) Complex II: Meta-monzogranites and granodiorites Olivine-Clinopyroxene metagabbros Cortesogno et al., 1998, Gaggero et al., 2004
Pre-Variscan basement (Pre-piedmont) Cortesogno et al., 1997; Gaggero et al., 2004 Meta-volcanites I : 507 ± 5 Ma Meta-intrusives I: 494 ± 3 Ma Meta-volcanites I: 478 ± 4 Ma Metagranites II: 473 ± 2 Ma 468 ± 2 Ma 460 ± 3 Ma 462 ± 1 Ma Cortesogno et al., 1998, Gaggero et al., 2004, 2010, 2012
Pre-Variscan basement Complex I: Two pre-alpine schistosities Complex II: One pre-alpine schistosity
P-T-t metamorphic paths D3 305 ± 6 Ma; Giacomini et al., 2007 327-297 Ma; Del Moro et al., 1981 313-302 Ma; Barbieri et al., 2003 D2 348-320 Ma; Maino et al., in prep., Giacomini et al., 2007, Del Moro et al., 1981 392±7, 380±9, 374±8 Ma, Maino et al., in prep., Giacomini et al., 2007 D1 490<D1<475 Ma; Gaggero et al., 2004
The Briançonnais basements Nucetto Alpine deformation Barbassiria Alpine HP Visone unit
Briançonnais basement (Nucetto massif) Nucetto Metagranites II 447 ± 3 Ma, >20 Ma younger than the Pre-piedmont Orto II? Variscan metamorphism Maino et al., in prep.
Palaeozoic history - Back arc-related magmatism (basalts, intrusives, sediments) : 510-490 Ma; - Intra-continental volcanism referred to the roll-back after subduction of oceanic crust below northern Gondwana. D1: 490-475 Ma Von Raumer & Stampfli, 2008
Palaeozoic history LIGURIA Complex II (intrusives): 475-458 Ma Nucetto granites: 447 ± 3 Ma Thermal pertubation 420 Ma SARDINIA data-point error ellipses are 2!" 0,073 450 206Pb/238U 0,071 0,069 430 440 0,067 420 Concordia Age = 424.7 ±11 Ma (95% confidence, decay-const. errs included) MSWD (of concordance) = 13, Probability (of concordance) = 0.000 Iacopini et al., 2007; Gaggero et al., 2012 0,065 0,50 0,51 0,52 0,53 207Pb/235U 0,54 0,55 0,56
Palaeozoic history Oggiano et al., 2010 Von Raumer & Stampfli, 2008
Variscan orogeny Chronological constraints D2a Zircon U/Pb data (eo-variscan HP phases) 390-350 [Giacomini et al., 2007, Maino et al., in prep.] D2b Ar/Ar data and zircon U/Pb dating (amphibolite facies metamorphism) 340-320 [Del Moro et al., 1981, Giacomini et al., 2007, Maino et al., in prep.] D3 Ar/Ar data and U/Pb on zircon rims (greenschist facies metamorphism) 320-300 Ma [Del Moro et al., 1981, Giacomini et al., 2007]
Briançonnais basement Barbassiria massif Available data (Cortesogno et al., 1998) I) pre-alpine amphibolite facies metamorphism II) Rb-Sr dating of white mica indicate an age of ~250 Ma for the effusive protolith (Del Moro et al., 1981) III) Metarhyolites above 286 Ma (U-Pb dating) volcanics Barbassiria
Alpine structure Barbassiria massif Rhyolitic dikes Orthogneiss protolith: rhyolites+arenites and pelites Maino et al., 2012
Petrography Barbassiria massif Alpine garnet Biotite-muscovite and quartz-feldspar layers Igneous K- feldspar and plagioclase Lepidoblastic muscovite and alpine phengite Pha1 Variscan muscovite dicordant on the Alpine schistosity
U-Pb dating Barbassiria massif Dikes Maino et al., 2012
U-Pb dating - Barbassiria massif Orthogneiss Maino et al., 2012
Open questions Barbassiria massif Age of the rhyolitic protolith is 320 Ma à but this is the same age of the amphibolite facies Variscan metamorphism! à The 260 ± 3 Ma age of the dikes overlaps the Rb/Sr 257 ± 4 Ma (Del Moro et al. 1981) à When the amphibolite facies metamorphism developed within the Barbassiria massif?
Late Paleozoic tectonics Von Raumer et al., 2009 Matte, 2001 320-300 Ma strike-slip tectonics
A solution: I Late orogenic extension and thermal doming Casini & Oggiano, 2008 In the Barbassiria massif, the presence of muscovite+k-feldspar paragenesis exludes HT contact metamorphism
A solution: I Carta Geologica della Sardegna 1:200000, Servizio Geologico Nazionale 2001! Ms! Kfs! Sil! 0.1 mm! P~0.15-0.19 GPa T: 605-630 C" Buzzi et al., 2008
A solution: II Tectonic burial Dallagiovanna et al., 2009 Von Raumer et al., 2009
The post-variscan succession Stra%graphic columns of the Ligurian Briançonnais units Dallagiovanna et al., 2009
The post-variscan succession Fluviallacustrine deposits Early rhyolitic ignimbrite calc-alkaline characterisation (Cortesogno et al.,1998)
The post-variscan succession Andesite dikes, flows, tuffs calc-alkaline characterisation (Cortesogno et al.,1998; Buzzi and Gaggero, 2007) Fluviallacustrine deposits
Melogno Porphyroids A,B,C - Lithozones: rhyolitic- rhyodacitic ignimbrites calc-alkaline characterisation The post-variscan succession
Melogno Porphyroids - D Lithozone alkaline characterisation with high K2O content The post-variscan succession
Geochronology Early Ignimbrite Main volcanic episode Dallagiovanna et al., 2009 Late volcanic episode
Geochronology ü Calc-alkaline magmatism and continental deposition are set between ca. 286-273 Ma (early granodiorites at 294 Ma) 258 258 273 ü Alkaline magmatism (D-Lithozone) is set ca. 258 Ma 272 294 ü Gap of ca. 15 Ma in sedimentary deposition and volcanic activity 273 278 286
NNW-strike slip faults and ENE normal faults (present-day coordinates) bording pull-apart basins. Early Permian Tectonics Dallagiovanna et al., 2009
Permian Tectonics N Sardinia Arzachena Pluton Casini et al., 2012
Permian Tectonics Dallagiovanna et al., 2009 Decarlis et al., under review
Grazie