Le rocce erciniche nella Zona brianzonese ligure

Transcription

1 Le rocce erciniche nella Zona brianzonese ligure Matteo Maino Dipartimento di Scienze della Terra e dell Ambiente, Università degli Studi di Pavia

2 Pre-alpine basements of the Ligurian Alps Working group: L. Gaggero, G. Dallagiovanna, S. Seno, G. Oggiano, A. Decarlis, L. Casini, M. Maino

3 Pre-Alpine basements of the Alps From Von Raumer et al., 1998; Raumer et al., 2003; Maino et al., 2012

4 Pre-Alpine rocks of the Ligurian Alps Maino et al., 2012

5 Alpine structure Seno et al., 2005

6 Alpine structure Maino et al., under rev.

7 Briançonnais units Meso-Cenozoic cover Post-Variscan volcanosedimentary succession Variscan Basement

8 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

9 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

10 Pre-Variscan basement Complex I: Two pre-alpine schistosities Complex II: One pre-alpine schistosity

11 P-T-t metamorphic paths D3 305 ± 6 Ma; Giacomini et al., Ma; Del Moro et al., Ma; Barbieri et al., 2003 D Ma; Maino et al., in prep., Giacomini et al., 2007, Del Moro et al., ±7, 380±9, 374±8 Ma, Maino et al., in prep., Giacomini et al., 2007 D1 490<D1<475 Ma; Gaggero et al., 2004

12 The Briançonnais basements Nucetto Alpine deformation Barbassiria Alpine HP Visone unit

13 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.

14 Palaeozoic history - Back arc-related magmatism (basalts, intrusives, sediments) : Ma; - Intra-continental volcanism referred to the roll-back after subduction of oceanic crust below northern Gondwana. D1: Ma Von Raumer & Stampfli, 2008

15 Palaeozoic history LIGURIA Complex II (intrusives): Ma Nucetto granites: 447 ± 3 Ma Thermal pertubation 420 Ma SARDINIA data-point error ellipses are 2!" 0, Pb/238U 0,071 0, , Concordia Age = ±11 Ma (95% confidence, decay-const. errs included) MSWD (of concordance) = 13, Probability (of concordance) = Iacopini et al., 2007; Gaggero et al., ,065 0,50 0,51 0,52 0,53 207Pb/235U 0,54 0,55 0,56

16 Palaeozoic history Oggiano et al., 2010 Von Raumer & Stampfli, 2008

17 Variscan orogeny Chronological constraints D2a Zircon U/Pb data (eo-variscan HP phases) [Giacomini et al., 2007, Maino et al., in prep.] D2b Ar/Ar data and zircon U/Pb dating (amphibolite facies metamorphism) [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) Ma [Del Moro et al., 1981, Giacomini et al., 2007]

18 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

19 Alpine structure Barbassiria massif Rhyolitic dikes Orthogneiss protolith: rhyolites+arenites and pelites Maino et al., 2012

20 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

21 U-Pb dating Barbassiria massif Dikes Maino et al., 2012

22 U-Pb dating - Barbassiria massif Orthogneiss Maino et al., 2012

23 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?

24 Late Paleozoic tectonics Von Raumer et al., 2009 Matte, Ma strike-slip tectonics

25 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

26 A solution: I Carta Geologica della Sardegna 1:200000, Servizio Geologico Nazionale 2001! Ms! Kfs! Sil! 0.1 mm! P~ GPa T: C" Buzzi et al., 2008

27 A solution: II Tectonic burial Dallagiovanna et al., 2009 Von Raumer et al., 2009

28 The post-variscan succession Stra%graphic columns of the Ligurian Briançonnais units Dallagiovanna et al., 2009

29 The post-variscan succession Fluviallacustrine deposits Early rhyolitic ignimbrite calc-alkaline characterisation (Cortesogno et al.,1998)

30 The post-variscan succession Andesite dikes, flows, tuffs calc-alkaline characterisation (Cortesogno et al.,1998; Buzzi and Gaggero, 2007) Fluviallacustrine deposits

31 Melogno Porphyroids A,B,C - Lithozones: rhyolitic- rhyodacitic ignimbrites calc-alkaline characterisation The post-variscan succession

32 Melogno Porphyroids - D Lithozone alkaline characterisation with high K2O content The post-variscan succession

33 Geochronology Early Ignimbrite Main volcanic episode Dallagiovanna et al., 2009 Late volcanic episode

34 Geochronology ü Calc-alkaline magmatism and continental deposition are set between ca Ma (early granodiorites at 294 Ma) ü Alkaline magmatism (D-Lithozone) is set ca. 258 Ma ü Gap of ca. 15 Ma in sedimentary deposition and volcanic activity

35 NNW-strike slip faults and ENE normal faults (present-day coordinates) bording pull-apart basins. Early Permian Tectonics Dallagiovanna et al., 2009

36 Permian Tectonics N Sardinia Arzachena Pluton Casini et al., 2012

37 Permian Tectonics Dallagiovanna et al., 2009 Decarlis et al., under review

38 Grazie