P61_copertina_R_OK C August 20-28,2004 -

Field Trip Guide Book - P61 Post-Congress Leaders: J.Reinhardt,F. Rossetti EXTENSION AND POST-OROGENIC OROGENIC WEDGEFORMATION, BREAK-UP TOSUBDUCTION, A RECORDFROMCONTINENTAL (SOUTHERN , ITALY): AND ISOLADELGIGLIO 32 Volume n°6-fromP55toPW06 GEOLOGICAL CONGRESS nd INTERNATIONAL P61 21-06-2004, 10:45:33 The scientific content of this guide is under the total responsibility of the Authors

Published by: APAT – Italian Agency for the Environmental Protection and Technical Services - Via Vitaliano Brancati, 48 - 00144 Roma - Italy

Series Editors: Luca Guerrieri, Irene Rischia and Leonello Serva (APAT, Roma)

English Desk-copy Editors: Paul Mazza (Università di Firenze), Jessica Ann Thonn (Università di Firenze), Nathalie Marléne Adams (Università di Firenze), Miriam Friedman (Università di Firenze), Kate Eadie (Freelance indipendent professional)

Field Trip Committee: Leonello Serva (APAT, Roma), Alessandro Michetti (Università dell’Insubria, Como), Giulio Pavia (Università di Torino), Raffaele Pignone (Servizio Geologico Regione Emilia-Romagna, Bologna) and Riccardo Polino (CNR, Torino)

Acknowledgments: The 32nd IGC Organizing Committee is grateful to Roberto Pompili and Elisa Brustia (APAT, Roma) for their collaboration in editing.

Graphic project: Full snc - Firenze

Layout and press: Lito Terrazzi srl - Firenze

P61_copertina_R_OK D 27-05-2004, 10:30:44 Volume n° 6 - from P55 to PW06

32nd INTERNATIONAL GEOLOGICAL CONGRESS

MONTE ARGENTARIO AND (SOUTHERN TUSCANY, ITALY): A RECORD FROM CONTINENTAL BREAK-UP TO SUBDUCTION, OROGENIC WEDGE FORMATION, AND POST-OROGENIC EXTENSION

AUTHORS: J. Reinhardt (University of Natal - South Africa) F. Rossetti (Università di Roma Tre - Italy)

Florence - Italy August 20-28, 2004

Post-Congress P61

P61_R_OK A 27-05-2004, 10:36:31 Front Cover: Extensional shear zone in layered marble at Cala Moresca. Asymmetrical shear bands and sigmoidal structures indicate sinistral sense of shear (top of the east)

P61_R_OK B 27-05-2004, 10:36:33 MONTE ARGENTARIO AND ISOLA DEL GIGLIO (SOUTHERN TUSCANY, ITALY): A RECORD FROM CONTINENTAL BREAK-UP TO SUBDUCTION,

OROGENIC WEDGE FORMATION,AND POST-OROGENIC EXTENSION P61

Leaders: J. Reinhardt, F. Rossetti

Introduction Apennine region. More generally, they provide Blueschist-facies mineral assemblages have been valuable insight into the compression-extension known for a long time from metabasites of the Monte dynamics of orogens, and how this is refl ected in the Argentario promontory and from two islands of rock record. the , Giglio and . The presence of blue amphiboles had been recorded more The fi eld trip visits two locations, Monte Argentario than a hundred years ago, even though the implications and Isola del Giglio. These areas played a pivotal role in terms of pressure-temperature conditions were in the more recent re-investigations of the tectono- obviously recognised only much later (e.g., Ricci, metamorphic history of the Northern Apennines 1972). As the classic Ligurian ophiolites on the Italian - - region, not least by mainland do not contain such assemblages, the high- being a geological link between Alpine Corsica pressure rocks had been considered a curiosity. to the west and the main Northern Apennines belt to the east. The focus of the excursion will be on Since then, a much improved understanding of petrological and structural aspects of a variety of mineral assemblages and mineral reactions as well rock types, including Triassic continental to marine as a systematic approach in characterising the rocks, Jurassic ophiolites, and Tertiary intrusives. metamorphism of continental-derived sedimentary The particular signifi cance of mineral assemblages, rocks has contributed to establish a more compre- prograde and retrograde, and of related structures hensive picture of orogenic regional metamorphism in will be emphasized, and their contribution to the the Northern Apennines. Findings of ferro-magnesian understanding of the larger-scale crustal dynamics carpholite in particular (Theye et al., 1997; Jolivet will be discussed. et al., 1998; Rossetti, 1999) demonstrated that high- pressure, low-temperature metamorphism is by no Logistical note: General access to the areas by road means a matter of rare local occurrences, but, apart is good as these are popular tourist destinations. The from metabasites, is rather widespread in continental- unfortunate consequence of this popularity and of derived units such as the Triassic Verrucano Group. the exceptional beauty of these places is that access to specifi c sites has become increasingly diffi cult Furthermore, detailed structural and geochrono- in recent years due to housing development and logical studies have paved the path for larger-scale the establishment of numerous luxurious private correlations and tectonic modelling of the Northern holiday mansions along the coast, particularly at Tyrrhenian region. It was recognised that the the Monte Argentario. The fact that Giglio Island is high-pressure units have a protracted history of a nature reserve will hopefully prevent this type of deformation (prograde and retrograde) and that some development, not just for the benefi t of geological of the prominent structures seen in outcrop relate to fi eld work. exhumation and crustal stretching rather than the preceding subduction and crustal thickening events Literature references: see end of guide (Jolivet et al., 1998). It was also recognised that the extension-related structures can be traced into the Map reference: young felsic intrusives such as the Giglio monzo- Gianniello, G., Lazzarotto, A. and Mazzanti, R. granite (Rossetti et al., 1999). (1964). Carta geologica del Monte Argentario e del Promontorio del Franco (Isola del Giglio) 1: 25000 Thus, since the fi rst extensive fi eld study carried out (supplement in: Lazzarotto, A., Mazzanti, R. and P55 to PW06 n° 6 - from Volume on the Monte Argentario and the island of Giglio by Mazzoncini, F. (1964). Geologia del Promontorio Lazzarotto et al. (1964) - which is still an essential Argentario () e del Promontorio del Franco basis for any new studies - the Cainozoic tectono- (Isola del Giglio - Grosseto). Boll. Soc. Geol. Ital. 83 metamorphic history has become much more (2), 1-124) transparent. The present data of these and related areas add important constraints on the reconstruction of the The following section gives an overview on the geodynamic evolution of the Tyrrhenian-Northern geological evolution of the region and presents

3 - P61

P61_R_OK 3 27-05-2004, 10:37:05 P61_R_OK 4

Volume n° 6 - from P55 to PW06

P61- P61 Leaders: J.Reinhardt,F. Rossetti geodynamic model. a synthesisofdatainformregional-scale previously thickened orogeniczonesleadingto of orogens,theback-arcdomainshiftsinto zones progressively migratetowards external parts Tuscan region; seealsobelow). As subduction overprint (typically10-20Ma for the Tyrrhenian- HP agesonlymarginally olderthantheirextensional high-pressure (HP)metamorphiccomplexes show (Figure 1).Insuchback-arcdomains,remnantsof extension intheback-arcdomainofsubductionzones more thanatransientstateoflate Alpine tectonics)is Mediterranean (beingessentiallyasnapshotofnothing A commonfeatureontheneo-tectonicmapof processes astheorogenevolves. inversions between,compressionalandextensional these terrainscanprovide cluestothecouplingof,and conjunction withthelateCainozoictectonichistoryof zones ofcompressionversus zonesofextension in today insomeofthebelts,presentdistribution of region. As lateralcrustalmovements arestillactive history ofmany Alpine beltsintheMediterranean extensional processescharacterisestheCainozoic The overprinting oftectonicallythickened crustby references. (1998). Pleaseconsultthesepublicationsforfurther by Rossettietal.(2002)andthepaperofJolivet etal. This summaryistoalarge extent basedonthereview Regional geologicalsetting Figure 1-Simplifi ed tectonicmapoftheMediterraneanarea(fromJolivet etal.,1998).Box shows areaofFigure 2. are thoseoriginallydeeplyburied andlaterexhumed. in termsofreworking bythelateextensional processes The tectono-stratigraphicunitsmoststronglyaffected both brittleandductile. indicate eastward-dipping extensional shearzones, velocity of1.5-2cm/a.Structuralandseismicdata as magmaticcentresshiftedeastwards atanaverage further east). The centres ofextensional basinsaswell Ma inCorsicato2 Tuscany, andeven younger (Figure 3).Magmaticbodieshave beendatedfrom14 magmatism andintheonsetofbasinsedimentation There isadistinctwest-to-eastyoungingtrendinfelsic magmatic activity. of spacedcrustalshearzones,sedimentarybasinsand crustal thinningarecharacterisedbythedevelopment from theLateMioceneonwards. Extensionand of large partsofthetectono-stratigraphicsequence, the previously thickened crustalsectionandomission compressional architectureledtoextensive thinningof (Faccenna etal.,2001).Extensionalreworking ofthe and eastward migrating Apennine compressionalbelt orogenic processeslocatedatthebackofgrowing (Figure 2)developed asaconsequenceof Tertiary post- Accordingly, thepresent Tyrrhenian-Apennine system of previously deeplyburied partsoftheseorogens. detachments maycontribute totherapidexhumation controlled, andthedevelopment ofmajorextensional crustal thinningwillbetoalarge extent tectonically post-orogenic extension. Insuchcircumstances, 27-05-2004, 10:37:08 MONTE ARGENTARIO AND ISOLA DEL GIGLIO (SOUTHERN TUSCANY, ITALY): A RECORD FROM CONTINENTAL BREAK-UP TO SUBDUCTION,

OROGENIC WEDGE FORMATION, AND POST-OROGENIC EXTENSION P61 Volume n° 6 - from P55 to PW06 n° 6 - from Volume

Figure 2 - Tectonic map of the northern Tyrrhenian area with occurrences of high-pressure, low-temperature minerals (from Rossetti et al., 2002, after Jolivet et al., 1998). Arrows show the direction of stretching and sense of shear within the major extensional shear zones (hanging-wall movement).

5 - P61

P61_R_OK 5 27-05-2004, 10:37:10 P61_R_OK 6

Volume n° 6 - from P55 to PW06

P61- P61 Leaders: J.Reinhardt,F. Rossetti with someoceanicrockspreservinganeven earlier facies overprint onearlyhigh-pressureassemblages, are characterisedbyagreenschisttosub-greenschist to oceanic-derived rocks. Their metamorphichistories consists mainlyof Triassic andJurassiccontinental- On the Tuscan side,thesemetamorphiccomplexes from crustalthickening tosubsequentextension. Tyrrhenian region, andinparticularforthetransition the geodynamicprocessesshaping Apennines- units provide crucialdataforthereconstructionof Leoni). The structural-metamorphic recordofthese LT rocks(suchasMonticiano-RoccastradaandMonti Argentario as well asmainlandoccurrencesofHP- Corsica, Gorgona Island,westernGiglio,Monte include theeclogiteandblueschistterrainof Alpine These remnantsofearlysubductionandaccretion orogen predatingtheopeningof Tyrrhenian Sea. Tuscany, andrepresenttheinteriorpartoforiginal distributed betweennorth-easternCorsicaandwestern Occurrences ofhigh-pressuremetamorphicrocksare the transectshown inFigure 1(fromRossettietal.,2002). Figure 3-(a)Crustal-scalecross-sectionand(b)age-distancediagramofmagmaticactivitysyn-riftdepositsalong shear inrocksofbulk non-coaxialdeformation,in east-west oriented(Figure2). The dominantsenseof retrograde stretchinglineationsareonaverage roughly lineations arepreserved, but stronglyoverprinted. The localities areexhumation-related. Earlyfoliationsand dominant planarandlinearstructuresseeninmany stage ofdeepburial isexpressed bythe fact that the The signifi pressures decreasefromwesttoeast(Figure4). migration ofthesubductionsuture,whilepeak to about20Main Tuscany), indicatinganeastward progressively younger(fromabout60MainCorsica be observed: theageofHPmetamorphismbecomes Southern Tuscany, two trendsinmetamorphismcan Fe-Mg-carpholite. FromnortheasternCorsicato thus favouring thepreservation oftheHP/LT mineral paths generallyremainonthelow-temperature side, of localitiesareshown inFigure4.Decompression Peak P-Tconditionsandexhumation pathsforarange record ofsyn-riftingsub-seafl cance oftectonicprocesses following the or metamorphism. oor 27-05-2004, 10:37:13 MONTE ARGENTARIO AND ISOLA DEL GIGLIO (SOUTHERN TUSCANY, ITALY): A RECORD FROM CONTINENTAL BREAK-UP TO SUBDUCTION,

OROGENIC WEDGE FORMATION, AND POST-OROGENIC EXTENSION P61

Figure 4 - P-T exhumation paths for metapelites in the northern Tyrrhenian region (from Rossetti et al., 2002;

after Jolivet et al., 1998, and references therein). The insert shows an age-distance diagram for the main metamorphic P55 to PW06 n° 6 - from Volume events along the transect shown in Figure 1 (after Brunet et al., 2000).

shear zones and on extensional detachments is top-to the mode of closure of the Ligurian ocean. Earlier the-east. models suggested a fl ip of subduction zones, mainly to explain the westward verging structures of Alpine A critical point in the tectonic interpretation of the Corsica and the eastward verging structures of the stratigraphic, structural and metamorphic record is Apennines, and to account for the fact that remnants

7 - P61

P61_R_OK 7 27-05-2004, 10:37:15 P61_R_OK 8

Volume n° 6 - from P55 to PW06

P61- P61 Leaders: J.Reinhardt,F. Rossetti distribution ofhigh-pressurerocks inthe Tyrrhenian- structural-metamorphic evolution, thepresent to bebettersuitedexplain thecontinuityof However, asimplerplate-tectonicmodelappears and fi the nappepilecreatedduringwestward subduction Ligurides) werethenobductedandincorporatedin ophiolites oftheNorthern Apennines (i.e.,the of the Western Alps. The non-HP-metamorphic subduction), thuscorrelatingwiththeHPcomplexes formed duringtheearlier Alpine event (i.e.,eastward assumed thattheHP-metamorphiccomplexes of the Tyrrhenian Sea (cf.Figure5).Itwas further of theLigurianoceaniccrustarefoundonbothsides and reconstructionoftheearly-stageorogenicwedge between CorsicaandNorthern Apennines (fromJolivet etal.,1998). Figure 5-Diagrammaticstratigraphic columnsoftheCorsica-Apenninesareawithtectonictransportdirectionsindicated, nal closureoftheLigurian-Piemonteseocean. Corsica continentalmargin. As thetwo continental verging orogenicwedgeformedattheSardinia- During continent-to-oceanorogenesis,adouble- European continentalmargin (Figure6B). with underthrustingofthe Adria margin beneaththe Piemontese oceanicbasinhadclosed,continued started withtheoceaniccrust,and,onceLigurian- (i.e., Corsica;Figure6A). The subductionprocess originally placedattheEuropeancontinentalmargin since thePalaeocene, withthesubductionsuture model proposescontinuouswestward subduction Cretaceous totheLower Pleistocene(Figs.5,6). This magmatism, andthesedimentaryrecordfromLate Apennine region, therecordoflateCainozoicfelsic 21-06-2004, 10:46:38 MONTE ARGENTARIO AND ISOLA DEL GIGLIO (SOUTHERN TUSCANY, ITALY): A RECORD FROM CONTINENTAL BREAK-UP TO SUBDUCTION,

OROGENIC WEDGE FORMATION, AND POST-OROGENIC EXTENSION P61 Volume n° 6 - from P55 to PW06 n° 6 - from Volume

Figure 6 - Schematic three-stage 2-D geodynamic model summarising the Tertiary tectonic evolution of the region between Sardinia-Corsica and the Adria continental margin (from Rossetti et al., 2002; modifi ed after Jolivet et al., 1998). The position of the deep-seated metamorphic rocks exposed in the interior of the Northern Apennines is indicated. (A) Formation of the thick-skinned, doubly-vergent orogenic wedge during the consumption of the Ligurian- Piemontese oceanic domain beneath the European margin. (B) Underthrusting of cold Adria continental margin; syn-orogenic extension and exhumation in the interior domain of the orogen. (C) Post-orogenic back-arc extension caused by the progressive eastward retreat of the Apennine subduction boundary; crustal thinning and magmatism.

9 - P61

P61_R_OK 9 21-06-2004, 10:46:43 P61_R_OK 10

Volume n° 6 - from P55 to PW06 P61- P61 Leaders: J.Reinhardt,F. Rossetti relaxation setinwhichcausedlocalpartialmelting. the internalpartsofback-arcdomain,thermal and subsidenceoftheperi-Tyrrhenian basins.In 6C). This periodischaracterisedbycrustalthinning pile fromMid-toLateMioceneonwards (Figure migrating compressionalfrontaffected theorogenic The onsetofback-arcextension behindtheexternally fi structural-metamorphic signatures,asweseeitinthe leading tojuxtapositionofunitswithvery different and non-metamorphicunitsintheorogenicwedge, and tectonicmixingofthevarious metamorphic during active convergence resultedinreworking relatively coldexhumation paths.Exhumation relaxation intheoverlying rocks,thuspromoting margin). Continuedunderthrustingprevented thermal was stillactive inmoreexternal domains(Adria exhuming HP-LT units,whileHP-metamorphism started tocollapsewiththeeffect ofprogressively margins collided,theoverthickened orogenicwedge Occurrences ofhigh-pressuremineralsareindicated. Figure 7-(a)StructuralmapoftheMonte Argentario and(b)cross-section(fromJolivet etal.(1998). eld today. the Apennines. are now widelyspreadbetweeneasternCorsicaand originating fromthedeeppartsoforogenicwedge opening ofthe Tyrrhenian Sea,therockcomplexes the double-verging orogenandduetothesubsequent transport inoppositedirectionsduringformationof post-orogenic back-arcextension. Duetotectonic orogenic wedgecollapse),andthesecondbeing process, thefi The exhumation ofHProcksisthereforeatwo-stage brittle extensional faults andshearzones. magmatism werecontrolledbyeast-dippingductileto Tuscan MagmaticProvince. Crustalthinningand provided thesourceforigneousrocksof Such anatecticmelts,mixed withsub-crustalmagmas, Please beaware thatsomeoutcropsareonprivate General note Field itinerary rst stagebeingsyn-orogenic(i.e., 27-05-2004, 10:37:25 MONTE ARGENTARIO AND ISOLA DEL GIGLIO (SOUTHERN TUSCANY, ITALY): A RECORD FROM CONTINENTAL BREAK-UP TO SUBDUCTION,

OROGENIC WEDGE FORMATION, AND POST-OROGENIC EXTENSION P61

property for which an explicit permission to enter is This refl ects the transition from maximum tectonic required. Hammering and sampling may be restricted thickening (orogenic wedge) to fast decompression in such cases. Sampling is not permitted in general (extensional low-angle shear zones and faults). on the Island of Giglio as it is a nature reserve. We will try to get a special permission, though, for the Itinerary for day 1: Drive from Florence to members of the fi eld trip. and Monte Argentario. Arrive at late in the morning. Drive on Via Panoramica to Forte DAY 1 Filippo on La Rocca. Continue on Via Panoramica South-eastern Monte Argentario to a lunch stop at the Bar Le Viste (Stop 1.1) for a geological introduction. Drive southwest on Via General overview and program of day 1: Panoramica to examine road cuts near Fortino Stella • Introduction to the geological setting and tectonic (Stop 1.2). Continue along Via Panoramica towards evolution of the Northern Apennines Torre dell Avoltore. Take downhill footpath from Via • Verrucano outcrops: petrological evidence for HP/ Panoramica to outcrops on hillside (Stop 1.3). LT metamorphism; structural overprinting during late- and post-orogenic processes Stop 1.1: Bar Le Viste, Via Panoramica, opposite the small Two geological units, Verrucano and Calcare island of Isolotto. Cavernoso, dominate the geological map of the central General introduction to the geological setting and and eastern part of the Monte Argentario Promontory tectonic evolution of the Northern Apennines. (Figure 7; Lazzarotto et al. 1964). In addition, there Geological signifi cance of the Monte Argentario are minor occurrences of Palaeozoic metasediments. - Giglio region in the Northern Apennines tectono- The term “Verrucano” describes a terrestrial to metamorphic framework. coastal-marine facies which is characteristic for the base of the Alpine sedimentary cycle. In the Northern Stop 1.2: Apennines, the Verrucano has a Mid-Triassic age. Road cuttings on Via Panoramica, southeast of The Calcare Cavernoso is essentially a “pitted” Fortino Stella calcarous rock of grey colour that lacks primary The outcrops show meta-conglomerates (“anageniti”) bedding structures. Its origin has been a matter of and psammo-pelitic rocks typical of the Mid-Triassic debate - sedimentary (representing Late Triassic Verrucano Group. The strata are steeply dipping,

evaporites) versus tectonic - and perhaps both is true. forming part of a large F2 fold structure. The S2 axial

Its tectonic signifi cance lies in the fact that it forms planar cleavage is at a low angle to S0 in the pelitic the detachment zone of the Tuscan Nappe, which layers, but is strongly refracted as it cuts across more overrides the Tuscan “basement” units, including the competent psammitic beds. Typical metamorphic Verrucano (Figure 5). mineral assemblages of this locality are muscovite- Whereas the rocks of the Tuscan Nappe are very pyrophyllite-paragonite-quartz ± chlorite ± sudoite low-grade metamorphic to non-metamorphic, in Al-rich rich phyllites, locally with chloritoid, and the Verrucano Group rocks contain low-grade muscovite-quartz ± chlorite ± sudoite ± chloritoid metamorphic minerals such as pyrophyllite, sudoite, in Al-poorer varieties (Costantini et al., 1995). As in chloritoid, and Fe-Mg-carpholite (Theye et al., 1997; so many other localities, the high-pressure indicator Jolivet et al., 1998). In conjunction with observations mineral carpholite had escaped everyone’s notice on reaction textures, these minerals have been used until recent years. However, in this particular outcrop, to characterise metamorphism and constrain P-T carpholite occurs only locally as remains. Retrograde paths. The discovery of ferro-magnesian carpholite in breakdown products now replace much of it. 2+ P55 to PW06 n° 6 - from Volume particular (XMg = Mg/(Mg+Mn+Fe ) = 0.36 - 0.70) demonstrated that the Verrucano Group of the east- Stop 1.3: central Monte Argentario had experienced HP-LT Hillside outcrops below Via Panoramica, west of conditions similar to those derived from metabasites Torre dell’ Avoltore in the west (Theye et al., 1997). The P-T path shows Exposed are psammopelitic rocks of the Verrucano peak conditions of at least 8 kbar and about 350°C, Group. Colourless to green carpholite fi bres can be followed by isothermal decompression to less than observed in Verrucano beds as well as in quartz 5 kbar at about 350°C, before major cooling set in. veins. The fi bres defi ne a distinct mineral lineation

11 - P61

P61_R_OK 11 27-05-2004, 10:37:29 P61_R_OK 12

Volume n° 6 - from P55 to PW06 P61- P61 Leaders: J.Reinhardt,F. Rossetti portions ofthenorthern Apennine orogenicwedge the post-orogenicprocessesthataffected theinternal unique sitetostudythelinkbetweensyn-and (Lazzarotto etal.1964)(Figure8). The islandisa the smallFrancoPromontoryonwesternside sedimentary rocksofJurassicto Triassic ageform monzogranite intrusions.Metamorphicand The larger partofGiglioIslandconsistsPliocene • hinges. can beseeninoutcrop-scaleD in transpositionoftheearlycarpholitefabric. As tight foldswithanS L • General overview andprogram ofday2: Isola delGiglio Rossetti etal.,1999),withthestoplocalitiesindicated. Figure 8-GeologicalmapofGiglioIsland(afterLazzarotto etal.,1964; Westerman etal., 1993; carpholite fi 1 Giglio MonzograniteIntrusion:fromlatemagmatic . SubsequentD fabrics tosolid-statedeformation exhumation-related fabrics overprinting ofpeak-metamorphicfabrics by and subsequentretrogressionin Verrucano rocks; High-pressure, low-temperature metamorphism bres arefoldedaroundtheF 2 overprinting, characterisedby 2 axialplanarfoliationresulted DAY 2 2 foldstructures,

2 fold tectonic andsolutionbrecciasoftheCalcareCavernoso dolomites andbeddedlimestonesinassociationwith almost 150meters.Upper Triassic massive andstratifi of thecentralpartpromontorywithathickness (Oph) andLiguriancalcschists.UnitMunderliesmost by fragmentsofhigh-pressuremetamorphicbasicrocks to theuppermostpartof Verrucano sequence, and composed ofmetapelites(Vc)atleastinpartbelonging complex. UnitMisatectonicmelangedominantly and constitutesthelowermost unitofthewholenappe extensively alongthewesternpartofpromontory with athicknessofaround100meters,cropsout belonging tothe Tuscan Verrucano Group. This unit, (predominately quartz-richphyllitesandschists), V consistsofFe-Mg-carpholite-bearingmetasediments stratigraphic unitscanberecognized(Figure8).Unit Monte Argentario, thestackingofdifferent tectono- At theFrancoPromontory, similartothenearby thickened orogeniccrust(Rossettietal.,1999). the lateextensional attenuationofthepreviously thermal relaxationprocessesthataccompanied Pliocene intrusives, whichinturnareproductsof orogenic wedgeareinfact intectoniccontactwith rocks representingtheexhumed deeprootofthe in the Tyrrhenian region. Fe-Mg-carpholite-bearing 27-05-2004, 10:37:30 ed MONTE ARGENTARIO AND ISOLA DEL GIGLIO (SOUTHERN TUSCANY, ITALY): A RECORD FROM CONTINENTAL BREAK-UP TO SUBDUCTION,

OROGENIC WEDGE FORMATION, AND POST-OROGENIC EXTENSION P61 Volume n° 6 - from P55 to PW06 n° 6 - from Volume

Figure 9 - (a) Fe-carpholite - quartz composite fi brous association in Verrucano metapelites. (b) Thin section of Verrucano metapelite from Giglio Island showing the blueschist peak equilibrium association of Fe-carpholite, quartz and phengite.

13 - P61

P61_R_OK 13 27-05-2004, 10:37:32 P61_R_OK 14

Volume n° 6 - from P55 to PW06 P61- P61 Leaders: J.Reinhardt,F. Rossetti lighthouse atPuntaFenaio(Stop2.3). road toGiglioPorto. Walk 2kmonfootpathtothe Drive towards thenortherntipofislandon pavements around Torre delCampese(Stop2.2). reach Stop2.1.BacktoCampeseexamine granite km acrosstheFrancoPromontorynappepileto the nappepileofFrancoPromontory. Walk 1.5 along thecontactbetweenmonzograniteand Campese onthewestcoast,andfurthersouthwards side oftheislandacrossGigliomonzograniteto to GiglioPorto.Drive fromGiglioPortoontheeastern Itinerary for day2: shear features. including progressive ductiletobrittletop-to-the-east of solid-statestructuresalongtherimGMI, plutonic rim.Stops2and3illustratethedevelopment of theisland,anareathatmayrepresentoriginal structures aremostlylocalisedinthenorth-westernpart and Papini 1992)(Figure11). Transitions tosolid-state and itmaybeconsideredthecoreofpluton(Burrelli facies haspreserved themagmatic,concentricstructures textured (ArenellaFacies, ARF; Figure8). The latter inner zonethatisporphyriticandhomogeneously and locallylayered(PietrabonaFacies, PBF)andan is characterisedbyanouterzonethatstronglyfoliated Ma (Westerman etal.,1993). The mainintrusive body with anaverage Rb/Srradiometriccoolingageof5.0 S-type peraluminousandsub-alkalinecomposition, two different monzograniticintrusionsofanoverall The GiglioMonzograniteIntrusion(GMI)comprises top ofthenappecomplex (T2)(Figure1). between thetwo high-pressreunits(T1)andalsoatthe at Caladell’ Allume, constituteunit T. This unitisfound Formation, croppingoutalongtheGulfofCampeseand of theearly Apennine orogenichistory. The fi and metamorphicepisodes,whicharecharacteristic illustrates thesuccessionoftwo maindeformation The Mugnibeachlocalityisaspectacularoutcropthat Mugni beach,Promontorio delFranco Stop 2.1: minor ankerite. Late,staticformationofgreenbiotite of Fe-Mg-carpholite,phengite,chlorite,quartzand Verrucano rocksexposed attheMugnibeachconsist Petrography shearing. low-grade greenschistfacies duringeastward-directed retrogressive stagewithafi episode isrelatedtotheHP/LT stage,thesecondto FerryfromPortoSantoStefano nal re-equilibrationinthe rst The S the mainstructuralfeaturein Verrucano rocks. a penetrative L-Stectonicfabric, whichconstitutes the retrogradegreenschistmetamorphism,resultedin Carpholite compositionsrangefromX early event inthemetamorphichistory. formation ofthecarpholite-bearingsegregations isan and unduloseextinction inquartz,indicatethatthe quartz +phengiteaggregates, sub-grainformation relationships, suchasfoldingofthecarpholite+ parallel tothefoliationplanes(Figure9). Textural commonly observed onquartz-richsegregations lying is locallyobserved. Fibrouscarpholitecrystalsare phengite-substitution inwhitemicas(Si wt%. Peakpressureestimatesobtainedfromthe 0.41, withalow fl The D to mesoscopic,eastward asymmetricF strain-slip crenulationcleavage andisaxialplanar retrograde greenschistD the earlierD conclusive structuraldataareavailable to reconstruct green biotiteinsomesamplesrefl + quartzstabilityfi cooling duringdecom-pressionwithinthekaolinite Replacement ofcarpholitebylatekaolinitesuggests the P-Tgradientmusthave remainedrelatively cool. producing carpholitebreakdown reaction,andhence, the exhumation pathdidnotcrossthechloritoid- kaolinite. The absenceofchloritoidindicatesthat chlorite). Furtheralterationofcarpholiteproduced grained aggregates ofphyllosilicates(whitemica+ Carpholite phenoblastsarelocallyreplacedbyfi between 250°and350°C(Rossettietal.,1999). values rangingfrom10to12kbarfortemperatures p.f.u.) inequilibriumwithcarpholiteindicatepressure mica. S by aretrogressive assemblageofchlorite+white reworking causedbytheD defi HP/LT (Fe-Mg)-carpholitebearingassemblages Structures the celadonitesubstitutioninmuscovite islower. Argentario samples wherechloritoidispresentand reconstructed by Theye etal.(1997)fortheMonte defi The derived (pre-intrusive) peakP-Tconditions increase toabout400°C. by thenearbyplutonandashort-lived temperature ne arelicS ne ahigherP/Tmetamorphicratiothanthose 2 2 event, fromsyn-topost-kinematicrelative to schistosityisalmostfl 2 schistosityisassociatedwithapervasive 1 deformationepisode. 1 uorine contentbetween0and0.13 schistositythatoccurswithina eld. Finally, thepresenceofstatic 2 fabric. The extensive 2 event meansthatno at-lying anddefi ects alateinfl 4+ Mg =3.19-3.24 = 0.37to 27-05-2004, 10:37:35 2 folds uence ned ne- MONTE ARGENTARIO AND ISOLA DEL GIGLIO (SOUTHERN TUSCANY, ITALY): A RECORD FROM CONTINENTAL BREAK-UP TO SUBDUCTION,

OROGENIC WEDGE FORMATION, AND POST-OROGENIC EXTENSION P61

Figure 10a - Second-phase vertical coaxial shortening (D2) in Verrucano metasediments reworking the early HP plano-linear fabric.

refolding the early HP/LT assemblages (Figure 10a). direction ranging from N60°E to E-W.

L2 stretching lineations trend roughly E-W and are defi ned by quartz and mica-chlorite aggregates. Stop 2.2: Similarly oriented stretching is also indicated by vein- Torre del Campese, monzogranite pavements

fi lling quartz fi bres accompanying D2 boudinage. This stop shows solid-state deformation structures that developed while the pluton cooled down. Carpholite fi bres in metapelites are boudinaged and Ductile shearing deformation is localised on

sheared along the L2 direction, with retrograde quartz- low-angle east-dipping aplitic dykes. Stretching chlorite-micas associations fi lling the gaps. Quartz directions (provided mostly by quartz and tourmaline fi bres formed after carpholite retrogression are folded aggregates) strike N120°E and the sense of shear, as with the same geometry, attesting the post-HP character deduced from the offset of pre-existing dykes and S-

of this deformation. Sections parallel to L2 stretching C relationships, is always top-to-the-east (Figs. 11,

lineations and perpendicular to the S2 foliation, 12). Locally, late cataclasites and ultracataclasites generally show a non-coaxial shear fabric. Shear developed along the C-surfaces accompanying a criteria, such as σ-type porphyroclasts, asymmetric late, eastward-directed, extensional faulting. This boudins, pressure shadows and S-C structures are is consistent with continuing shear deformation as synkinematic relative to the retrogressive greenschist temperature decreases. P55 to PW06 n° 6 - from Volume metamorphism and indicate a constant top-to-the east

sense of shear (Figure 10b). D2 fl attening features, Stop 2.3:

S2 foliation and strain-slip crenulation cleavage, are Punta del Fenaio (coastal outcrop)

re-oriented and cut along D2 shear bands. Further At Punta del Fenaio, the monzogranite is seen in primary deformation features include semi-brittle low-angle contact with its wall rocks. The monzogranite contains and brittle high-angle conjugate extensional faults, zones of low-grade mylonite overprinting the primary accommodating a general maximum extension igneous structure (Figure 11). Along the coast, formation

15 - P61

P61_R_OK 15 27-05-2004, 10:37:36 P61_R_OK 16

Volume n° 6 - from P55 to PW06 P61- P61 Leaders: J.Reinhardt,F. Rossetti The Triassic sequenceisrepresentedbypsammo- Giglio. pile oftheFrancopromontoryonisland melange displaysmoreaffi many ofwhich are notfoundfurthereast. This melange comprisingalarge variety ofrocktypes, western partofthepromontoryformsatectonic In contrasttotheeast-central Argentario, the General overview andprogram ofday3: Western Monte Argentario indicators. derived fromS-Crelationshipsandotherkinematic N120°E andthesenseofshearistop-to-the-east,as mostly marked byquartzandtourmalinecrystals,trend recrystallised quartz.StretchinglineationsonC-surfaces, are defi of S-Ctectonitescanbealsoobserved. Csurfaces • • dolomitic marble metamorphic history Ductile andbrittledeformationincalcitic- Ophiolite unit:petrologicalaspectsand ned byzonesrichinmicaandfi Figure 10b-Top-to-the-east greenschist ductileshearbandsin Verrucano metapelites.

DAY 3 nity withthetectonic ne-grained signature (Ricci&Serri,1975),andavariety of have notbeenobserved) withaMORBgeochemical (largely dykes inmetagabbros;pillow structures pyroxenite), metagabbro(Stop3.3)andmetabasalt serpentinite, locallytalc-tremoliterockand of thisunitincludeultramafi (Lazzarotto etal.,1964,Ricci,1968).Rocktypes substantial partofthewestern Argentario melange a stronglyfragmentedophioliteunitforms Similar totheFrancotectonicstackonGiglio, 1982, unpubl.theses). the calcitemarbleunit(Reinhardt,1982;Stünitz, transitional infacies betweenpelitic Verrucano and calcareous bedsandphyllites areconsideredasbeing unit” (Decandia&Lazzarotto,1980)Interlayered sequence hasbeengiven thelocalname“CalaPiatti throughout theNorthern Apennines). The marble Verrucano facies (thelatterbeingfar morecommon Hence, thismarbleisanageequivalent oftheclastic lowermost Carnian(Gelmini&Mantovani, 1980). 3.2), datedwithmicrofossilsasMid-Triassic to addition, abundant calcitemarbleispresent(Stop and gypsum(equiv. toBurano Anhydrite). In pelitic Verrucano, dolomiticmarble(“grezzoni”) c rocks(mainly 27-05-2004, 10:37:41 MONTE ARGENTARIO AND ISOLA DEL GIGLIO (SOUTHERN TUSCANY, ITALY): A RECORD FROM CONTINENTAL BREAK-UP TO SUBDUCTION,

OROGENIC WEDGE FORMATION, AND POST-OROGENIC EXTENSION P61

metasedimentary rocks of oceanic provenance (see ophiolite unit. The metasediments include pelitic Stop 3.1). Even though no geochronological studies material, hematitic chert, radiolarite, calcareous beds, have been undertaken yet to determine the age of graded beds of mafi c detritus, and breccias contain- the ophiolite, it appears most likely that these are ing a mixture of sedimentary and mafi c fragments. fragments of the Ligurian oceanic crust, and hence have a Jurassic age (cf. Figure 5). Blueschist facies metamorphism is evident from Further to the units described here, there are minor mineral assemblages observed in the clastic rocks that occurrences of metasedimentary rocks (siliceous- contain gabbro-derived detritus, and from calcareous calcareous rocks, greywacke, etc.) whose age and beds containing sodic amphibole (“blue marble”), and provenance are still obscure. Nevertheless, their locally lawsonite. The range of metamorphic minerals presence emphasizes the complex tectonic character in the gabbro-derived clastic beds is largely the same of this terrain. The tectonic melange of the Western as in the metagabbros proper (cf. Stop 3.3). Argentario has a hanging-wall position with respect to the east-central complex. Calcare Cavernoso and Stop 3.2: carbonate-dominated tectonic breccia separate the Cala Moresca (coastal cliffs) two, but are also found at higher structural levels in Exposed is laminated to thick-bedded calcite marble, the west. with interlayers of dolomitic marble towards the top of the sequence. Most of the marble is relatively pure, Itinerary for day 3: Drive along Via Panoramica with rare intercalations of thin quartzitic beds. The from to Cala Grande (Stop 3.1; structural inventory includes small-scale intrafolial on private property). Continue along Via Panoramica folds, shear bands and a strong fragmentation of to the entrance of the Cala Moresca private holiday bedding due to boudinage (Figure on front cover). complex. Walk or drive down to the coast. Take Part of the calcite marble shows a strongly developed footpath along coastal cliffs to the north (Stop 3.2). macroscopic L-S tectonite fabric typical of mylonites. Back to the Via Panoramica and to the roadcuttings Related microstructures are largely obscured, though, opposite the Ristorante “Il Moresco” (Stop 3.3). due to pervasive post-deformational recrystallization Return to Florence or Rome in the afternoon. of calcite. Only quartzitic layers retain evidence for ductile shearing (dynamic recrystallization, Stop 3.1: SPO, CPO). In contrast to the calcite marble, the Cala Grande, below Via Panoramica dolomitic beds show little evidence for ductile fl ow, This area shows a variety of metasedimentary rocks but are intensely fractured instead. Where both in association with metabasites, all belonging to the are interlayered, boudinage of dolomite marble is Volume n° 6 - from P55 to PW06 n° 6 - from Volume

Figure 11 - Schematic block diagram illustrating the structural pattern of the Giglio Monzogranite Intrusion.

17 - P61

P61_R_OK 17 27-05-2004, 10:37:44 P61_R_OK 18

Volume n° 6 - from P55 to PW06 P61- P61 Leaders: J.Reinhardt,F. Rossetti fi Associated isminormetabasalt,plusphylliteand strongly defomedtoentirelyundeformedvarieties. The dominantrocktypeismetagabbro,occurringas Roadcuts on Via Panoramica, above CalaMoresca Stop 3.3: Stop 3.1). units whichdidexperience HPmetamorphism(cf. Triassic marbles,norinthemarblesofophiolite remains ofaragonitehave beenfound,neitherinthe P-T estimatesforthe Western Argentario. Sofar, no the uncertainlyrangesofthisreactionand aragonite equilibriumcurve, takingintoconsideration complex arecloseto(above orbelow) thecalcite- derived fromrocksinthe Western Argentario tectonic metamorphism ofthissequence.PeakP-Tconditions There is(asyet)nodirectevidence forhigh-pressure top-to-the-east. common. The overall senseofshearinthisoutcropis Note theoffset oftheapliticdyke. Figure 12-Semibrittleeastward-dipping extensionalfaultinmonzogranite atTorre delCampese. clinopyroxene crystals visibleinoutcrop). and mineralscanstillbeidentifi multiphase metamorphichistory. Igneoustextures this particularaspect,themetagabbrospreserve a their conspicuousblueamphiboles.However, beyond metamorphism inthisregion (Ricci,1972),dueto rocks provided the originalevidence forhigh-pressure ne-grained siliceous metasediments. The mafi ed (notablythelarge c strongly foliated. converted toblueschistsaretypicallyfi and reactionsoftening. Those rockscompletely the feedbackeffects betweendeformation,infi infi refl This variation inmineralassemblages andreactions clinopyroxene arenotuncommon. Thus, reactionrimsofblueamphiboleonigneous single sampleofpartiallyequilibratedmetagabbro. stages ofmetamorphismmaybeevident ineach to completeoverprinting. Furthermore, notall varies alotbetweensamples,fromincipientreaction equilibration atthedifferent stagesofmetamorphism been calculated(Theye etal.,1997). The extent of assemblage, P-Tconditionsof340°Cand7kbarhave Stünitz, 1982,unpubl.data).For theHP-LT mineral the LP-HTandLP-LT assemblages(Reinhardt,1982; lawsonite and,rarely, Na-pyroxene, inturnoverprint pressure assemblagescontainingNa-amphiboles, actinolite, epidote,chlorite,andpumpellyite.High- by thepresenceofmineralssuchashornblende, to sub-greenschistfacies conditionsisdocumented environment, fromamphibolitefacies conditions Hydrothermal overprinting inthe sub-seafl ects thevarying degree ofwater accessand ltration throughoutthemetamorphichistory, plus ne-grained and 27-05-2004, 10:37:50 ltration oor MONTE ARGENTARIO AND ISOLA DEL GIGLIO (SOUTHERN TUSCANY, ITALY): A RECORD FROM CONTINENTAL BREAK-UP TO SUBDUCTION,

OROGENIC WEDGE FORMATION, AND POST-OROGENIC EXTENSION P61

Acknowledgements cristallino stratifi cato nero dell’Argentario (Toscana We would like to thank our colleagues from Italy, meridionale). In “L’evoluzione tettonico-sedimentaria France, Germany and Switzerland (C. Faccenna, dell’Appennino settentrionale”. Libretto guida, pp. R. Funiciello, B. Goffé, L. Jolivet, M. Mattei, F. 105-107, 70° Congresso, Società Geologica Italiana, Storti, H. Stünitz, T. Theye, and others) who have 1980. contributed in many ways to the present state of knowledge on this area, and with whom we have co- Jolivet, L., Faccenna, C., Goffé, B., Mattei, M., operated for years. It is to be hoped that international Rossetti, F., Brunet, C., Storti, F., Funiciello, R., co-operation will continue and that new ideas keep Cadet, J.P., d’Agostino, N. and Parra, T. (1998). evolving from that. Mid-crustal shear zones in postorogenic extension: Example from the northern Tyrrhenian Sea. J. We would also like to thank those who have supported Geophys. Res. 103, 12123-12160. our cause by granting access to outcrops on their properties, during previous periods of fi eld work Lazzarotto, A., Mazzanti, R. and Mazzoncini, (Cala Grande and Franco in particular), and for this F. (1964). Geologia del Promontorio Argentario international fi eld trip. (Grosseto) e del Promontorio del Franco (Isola del Giglio - Grosseto). Boll. Soc. Geol. Ital. 83 (2), 1- References cited 124 Brunet, C., Monié, L., Jolivet, L. and Cadet, J.P. (2000). Migration of compression and extension Principi, G. and Treves, B. (1984). Il sistema corso- in the Tyrrhenian Sea, insights from 40Ar/39Ar ages appenninico come prisma d’accrezione. Riffl essi sul on micas along a transect from Corsica to Tuscany. problema generale del limite Alpi-Appennino. Mem. Tectonophysics 321, 127-155. Soc. Geol. Ital. 28, 529-576.

Burrelli, F. and Papini, P. (1992). Structural analysis of Ricci, C.A. (1968). Le rocce metamorfi che di natura Giglio island pluton. In “Contribution to the Geology basica e ultrabasica nelle serie a facies toscana. Studio of Italy with Special Regard to the Palaeozoic chimico e petrografi co. Atti Soc. Tosc. Sci. Nat., Mem., Basements” (L. Carmignani and P. Sassi, Eds.). IGCP Serie A 75, 1-67. 276, Newsletters vol. 15, 449-452. Ricci, C.A. (1972). Geo-petrological features of the Costantini, A., Decandia, F.A., Elter, F.M., metamorphic formations of Tuscany. Atti Soc. Tosc. Franceschelli, M., Giorgetti, G., Lazzarotto, A., Sci. Nat., Mem., Serie A 79, 267-279. Liotta, D., Memmi, I., Sandrelli, F. and Tavernelli, E. (1995). Low-temperature metamorphism in Ricci, C.A. and Serri, G. (1975). Evidenze Southern Tuscany: P-T indicators and geodynamic geochimichesulla diversa affi nità petrogenetica delle implications. Guide-book to the fi eld excursion, roche basiche comprese nelle serie a facies toscana. University of Siena. Boll. Soc. Geol. Ital. 94, 1187-1198.

Decandia, F.A. and Lazzarotto, A. (1980). Le Rossetti, F., Faccenna, C., Jolivet, L., Funiciello, R., unità tettoniche del Monte Argentario (Toscana Tecce, F. and Brunet, C. (1999). Syn- versus post- meridionale). In “L’evoluzione tettonico-sedimentaria orogenic extension: the case study of Giglio Island dell’Appennino settentrionale”. Libretto guida, pp. (northern Tyrrhenian Sea, Italy). Tectonophysics 304, 91-103. 70° Congresso, Società Geologica Italiana, 71-93. Siena 1980. Rossetti, F., Faccenna, C., Jolivet, L., Goffé, B. P55 to PW06 n° 6 - from Volume Faccenna, C., Becker, T.W., Lucente, F.P. and and Funiciello, R. (2002). Structural signature Rossetti, F. (2001). History of subduction and back- and exhumation P-T-t paths of the blueschist units arc extension in the central Mediterranean. Geophys. exposed in the interior of the Northern Apennine J. Int. 145, 1-21. chain, tectonic implications. Boll. Soc. Geol. Ital., Spec Vol. 1. 829-842. Gelmini, R. and Mantovani, M.P. (1980). Ritrovamento di fossili triassici nel calcare Theye, T., Reinhardt, J. Goffé, B., Jolivet, L. and

19 - P61

P61_R_OK 19 27-05-2004, 10:37:52 P61_R_OK 20

Volume n° 6 - from P55 to PW06 P61- P61 Leaders: J.Reinhardt,F. Rossetti reconstruction. Verrucano sequence,andsignifi high-pressure metamorphismofthemetasedimentary from theMonte Argentario (Italy):Firstevidence for Brunet, C.(1997).Ferro-andmagnesiocarpholite Eur. J. Mineral. 9,859-873. cance forP-Tpath Giglio. G. (1993). The Plioceneintrusionsoftheisland Westerman, D.,Innocenti,F., Tonarini, S.andFerrara, Mem. Soc.Geol.Ital. 49,345-363. 27-05-2004, 10:37:54 Back Cover: fi eld trip itinerary

P61_copertina_R_OK E 27-05-2004, 10:30:55 FIELD TRIP MAP INTERNATIONAL GEOLOGICAL CONGRESS nd 32

Edited by APAT

P61_copertina_R_OK B 27-05-2004, 10:31:24