Deciphering the geologic memory of a Permian conglomerate of the Southern Alps by pebble PT...

ORIGINAL PAPER

Deciphering the geologic memory of a Permian conglomerateof the Southern Alps by pebble P–T estimates

M. Iole Spalla Æ Davide Zanoni Æ Guido Gosso ÆMichele Zucali

Received: 18 July 2006 / Accepted: 30 August 2007! Springer-Verlag 2007

Abstract The volcano–clastic sequence of Trompia Val-ley, which caps the Tre Valli Bresciane Variscan basement

(TVB), comprises the Dosso dei Galli Conglomerate

(DGC), the oldest deposit containing up to metre-sizedmetamorphic pebbles. This Lower Permian formation of the

Trompia Basin was fed by the erosion products of the

Variscan chain. We used microstructural and mineralchemical data on metamorphic pebbles of the DGC to infer

a quantitative tectono-thermal evolution of the eroded pre-

Permian basement and to compare them with those of TVBand the surrounding Southalpine basement units (tectono-

metamorphic units = TMUs). Metapelitic and metaintru-

sive pebbles record a polyphase metamorphism with twometamorphic re-equilibrations: the first under epidote

amphibolite facies (M1, Tmax!PTmax) and the second under

greenschist facies (M2) conditions. Rock types and meta-morphic data largely match those of TVB basement unit.

The structural and metamorphic records in the pebbles are

pre-Permian, and the conglomerate matrix is non-meta-morphic. The DGC deposition age (283 ± 1–280.5 ± 2 Ma)

constrains the minimal exhumation age of its basementsource. The lack of staurolite bearing assemblages in

metamorphic pebbles suggests that the DGC basement

source was already exhumed to shallow structural levels(greenschist facies conditions) before the thermal equili-

bration consequent upon continental crust thickening

induced by the Variscan collision.

Keywords Southern Alps " Lower Permian

conglomerates " Variscan exhumation "Tectono-metamorphic units

Introduction

Where the structural and metamorphic evolutions of crys-talline basements are well deciphered it is possible to trace

the boundary of basement units on the ground of detailed

form surface maps (Hobbs et al. 1976) and well-refinedP–T–d–t paths (tectono-metamorphic units = TMUs of

Spalla et al. 2005). In such an outline, where basement units

are precisely contoured, the microstructural analysis ofmetamorphic pebbles of conglomerates, that cap metamor-

phic sequences, becomes a powerful tool to discriminateamong different kinds of basement sources constituting

separate units. In addition, basins containing late orogenic

conglomerates are repositories of the basement rocksexposed at the surface at the times of deposition, but not

necessarily preserved from erosion, and therefore presently

unexposed in the metamorphic basement. We combinedmicrostructural analysis with petrological investigations on

pebbles from different Permian conglomerates of the Cen-

tral Southern Alps to reconstruct their metamorphicevolutions and compare them to those of metamorphic rocks

from different basement units of the Southern Alps. We

investigated the Dosso dei Galli, Monte Aga and Val Ve-dello (Collio Formation) and Ponteranica conglomerate

pebbles (Fig. 1a) and report the microstructural features and

M. I. Spalla " D. Zanoni " G. Gosso " M. ZucaliDipartimento di Scienze della Terra ‘‘A. Desio’’,Universita di Milano, Via Mangiagalli 34,20133 Milano, Italy

M. I. Spalla " G. GossoC.N.R, I.D.P.A, Via Mangiagalli 34, 20133 Milano, Italy

D. Zanoni (&)Department of Geology, University of New Brunswick,Fredericton, NB, Canada E3B 5A3e-mail: [email protected]

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Int J Earth Sci (Geol Rundsch)

DOI 10.1007/s00531-007-0241-8

metamorphic evolutions recorded in the Dosso dei Galli

Conglomerate (DGC) pebbles for comparison to those of thefive types of basement units (TMUs) recently identified in

the Central Southern Alps (Giobbi Origoni and Gregnanin

1983; Spalla and Gosso 1999).The DGC represents, after the Basal Conglomerate, the

oldest (between 283 ± 1 and 280.5 ± 2 Ma), thickest and

widest conglomerate deriving from erosion of the Variscanbelt in the Lower Permian lithostratigraphic sequence of

the Trompia Basin (Cassinis 1966a) (Fig. 1b).Mineral abbreviations in the text, photomicrograph and

figure captions are as in Kretz (1983), except for white

mica (Wm), phengite (Phe) and eastonite (East).

Geological setting

The Southalpine domain is a portion of the Adria plate,

which represents part of the Alpine orogenic lid (Laubscher1983), extending South of the Insubric-Tonale line from

the Dinarides to the Western Alps. Lying between the

Como Lake and the Adamello Massif and between theInsubric-Tonale line and the Po plain, the Central Southern

Alps (Fig. 1a) consist of pre-Alpine metamorphic rocks and

Permian–Mesozoic cover sequences.The pre-Alpine basement of the Southern Alps com-

prises micaschists and gneisses (Edolo Schist and

Morbegno Gneisses of the literature: Salomon 1901; Cor-nelius 1916), with interlayered metagranitoids, metabasics,

marbles and quartzites. Locally the Early Palaeozoic age

has been inferred from sedimentary protoliths (Mottanaet al. 1985; Gansser and Pantic 1988). Metaintrusives like

Palone di Sopressa, (Beltrami et al. 1971; Bonsignore et al.

1971), Monte Fioraro gneisses (Siletto et al. 1993; Colomboet al. 1994) and Gneiss Chiari derive from Ordovician

granitoids (Colombo et al. 1994; Bergomi 2004).

The dominant metamorphic imprints characterisingadjacent basement portions vary from epidote–amphibolite,

or amphibolite, to greenschist facies conditions (Spalla and

Gosso 1999) and generally predate the deposition ofPermian–Mesozoic sequences and Alpine thrusting (De

Sitter and De Sitter-Koomans 1949; Bocchio et al. 1980;

Milano et al. 1988), excluding some basement portionscropping out along Lake Como that show Permian–Triassic

metamorphic ages (Diella et al. 1992). The mineral

assemblages corresponding to the different dominantmetamorphic imprints in micaschists and gneisses are

respectively: BtI, GrtI, Wm, Cld; BtII, GrtII, St, ± Ky,

WmII; WmIII, Chl, Ab, ± Ep, ± green BtIII, ± GrtIII.Five types of basement units (Giobbi Origoni and

Gregnanin 1983; Spalla and Gosso 1999) have been rec-

ognized in the Southalpine basement of the Orobic Alps(TMUs in Figs. 1a, 2): type I comprises continental crust

exhumed after continental collision and thickening at the

end of Variscan convergence (Diella et al. 1992; Gossoet al. 1997); type II preserves part of the P–T prograde

trajectory interpreted as the record of Palaeozoic subduc-

tion (Spalla et al. 1999), followed by an evolution similarto that characterising type I; type III represents the supra-

crustal Variscan units tectonically coupled under

greenschist facies conditions to type II and III units; typeIV metamorphic evolution has been interpreted as conse-

quent to the exhumation of deep-seated Variscan crustduring the Permian–Mesozoic rifting (e.g. Spalla et al.

2000). The Tre Valli Bresciane (TVB) Massif, which

consists of continental crust that escaped the Barrovianmetamorphic imprint (Giobbi Origoni and Gregnanin

1983) related to the Variscan collision, is here interpreted

as basement unit type (V); this latter kind of unit is dis-tinguished by a sequence of two syn-metamorphic

deformation phases developed under epidote–amphibolite

and greenschist facies conditions.Capping the crystalline basement, the Permian–Triassic

sedimentary sequence of the Central Southern Alps is the

result of two tectono-sedimentary cycles (Italian IGCP 203Group 1986; Cassinis et al. 1988; Massari 1988; Massari

et al. 1994). They are separated by a stratigraphic uncon-

formity and connected to a depositional and erosional gapcovering 14–27 Ma (Cassinis and Neri 1999), that marks

the boundary between Lower and Upper Permian (Cassinis

and Doubinger 1991, 1992; Barth and Mohr 1994; Cassinisand Perotti 1997). The first cycle is composed of a volcano–

clastic sequence that infills subsiding basins (Fig. 3) and the

second consists of fluvial and shallow-water sedimentshomogeneously distributed even on the structural highs

(Assereto et al. 1973; Cassinis and Peyronel Pagliani 1976;

Cassinis et al. 1980, 1988). Basins related to first cycledeposits have formed under dextral strike-slip tectonics,

which was accompanied by progressive thinning of the

Variscan lithosphere, that affected all Central SouthernEurope during the Lower Permian (Arthaud and Matte

1977; Ziegler 1988) and provided enough space to host

thick continental detrital sequences. Today, in the CentralSouthern Alps, Permian stratigraphic sequences are well

preserved in the Trompia Valley and along the Orobic

anticlines, where roughly coeval Lower Permian con-glomerates belong to the Collio Formation, Ponteranica and

Dosso dei Galli Conglomerate (Casati and Gnaccolini 1967;

Cadel et al. 1996; Cassinis and Neri 1999; Sciunnach 2001).

The metamorphic basement of the Permiansedimentary sequence of Trompia Valley

The DGC (Dosso dei Galli Conglomerate) belongs to thePermian sequence of the Trompia Valley, which lies


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Fig. 1 a Tectonic sketch map of the Orobic Alps and types ofmetamorphic evolution recorded in the basement. 1 Penninic andAustroalpine Domain, 2 Po Plain, 3 Adamello Pluton, 4 post-Permiansedimentary cover, 5 Permian-Upper Carboniferous sediments, 6Variscan metagranitoids, 7 Permian-Carboniferous granitoids, 8 TypeI TMUs, 9 Type II TMUs, 10 Type III TMUs, 11 Type IV TMU, 12Type V TMU (Tre Valli Bresciane Massif), 13 main faults andthrusts, 14 trajectories of main Alpine fold axial planes (synform orantiform). Main tectonic lines: I.L. Insubric Line, G.L. Gallinera Line,M.L. Musso Line, V.G.L. Val Grande Line. VBB Val Biandinobasement. NEOB-A North East Orobic Basement, TVB Tre ValliBresciane Massif. POC Ponteranica conglomerate, AVC Monte Agaand Val Vedello conglomerates. Inset A Helvetic Domain, B Penninic

Domain, C Austroalpine Domain, D Southalpine Domain, E Apen-nines. Redrawn after Spalla and Gosso (1999) and Spalla et al. (2006).b Sketch map of Southalpine Permian deposits and metamorphicbasement between the high Trompia Valley, Camonica Valley andSouth Giudicarie Line; redrawn after Giobbi Origoni and Gregnanin(1983) and Cassinis (1999). 1 Adamello Pluton, 2 Upper Permiansediments (Verrucano Formations and, only in the north-easternmostpart, Val Daone Conglomerate), 3 Lower Permian sediments (BasalConglomerate, Collio Formation and Dosso dei Galli Conglomerate),4 rhyolitic and rhyodacitic Permian volcanics, 5 Tre Valli Brescianebasement: a mostly phyllites, b chloritoid-bearing phyllites, cepidote–albite gneisses, 6 faults and tectonic lines. The rectanglelocates the sampled area


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unconformably over a portion of the Southalpine meta-morphic basement known as the Tre Valli Bresciane (TVB)

Massif (Giobbi Origoni and Gregnanin 1983) that crops out

between the Camonica Valley and the Trompia Line(Fig. 1b). The TVB basement, involved in the south-

verging thrusts with the Permian–Triassic cover (Fig. 1),

consists of low-grade metapelites (phyllites and micas-chists) and albite–epidote–bearing gneisses, the latter

displaying a deformed porphyritic texture with andesitic

and granodioritic or tonalitic bulk chemical composition(Giobbi Origoni and Gregnanin 1983). The basement rocks

record two deformation phases (D1 and D2) generating S1

and S2 foliations of Variscan age (Giobbi Origoni andGregnanin 1983) and are intruded by Permian granitoids

(De Capitani et al. 1994).

D2 is the dominant deformation imprint recorded inmetapelites; Wm and Chl mark S2 foliations, but also

overgrow D2 fabrics. S2 microlithons preserve S1 folded

relics. Chl crosscuts the S2 foliation and locally formscoronas around Grt and replaces Bt. Grt shows different

kinematic relationships in all lithotypes (Giobbi Origoni

and Gregnanin 1983): (a) syn-kinematic, with respect toS1, often replaced by Bt or Chl, flattened and rotated

during the last deformation phase; and (b) post-kinematicwith respect to S2 foliation, fine grained and mainly dis-

tributed in the mica and Chl-rich layer. Bt, grown during or

post-D2 deformation, is very rare in metapelites but

widespread within the albite–epidote gneisses. Ab formspoikiloblasts that are syn-to-post-kinematic with respect to

S2 foliation and includes Qtz, Wm, fine-grained Grt, Cld,

Chl, Ap, Tur, Ilm, zircon, and graphite.Cld occurs in Al2O3-rich horizons of metapelites and

coexists locally with, and is included in, Grt (Giobbi

Origoni and Gregnanin 1983). It never occurs with Bt andis in equilibrium with Ab and pre-to-post-kinematic, with

respect to S2 foliation.

Cld-bearing assemblages have been described in otherportions of the Central Southalpine basement, i.e. in the

Domaso Cortafo Zone (di Paola et al. 2001), close to the

Como Lake, and in the North East Orobic Basement oftype A (NEOB-A of Spalla et al. 1999) in the eastern

portion. Here Cld is associated with Bt, Grt and Wm, has a

shape fabric parallel to the early foliation and is over-printed by the assemblage GrtII, BtII, WmII and St;

greenschist facies minerals mark the last syn-metamorphic

fabric. In these sequences of structural and metamorphicre-equilibrations characterising type II unit (type II TMU in

Fig. 2), the Cld-bearing assemblage developed under epi-

dote–amphibolite facies conditions and was followed bytwo syn-kinematic parageneses that formed under

amphibolite and greenschist facies conditions.The sequence of metamorphic assemblages deciphered

in the TVB basement indicates an early metamorphic

re-equilibration stage under conditions up to epidote–

Fig. 2 P–T–d–t paths (quantitative with solid and qualitative withdashed lines) of the Orobic basement units. Aluminium silicate (And,Ky, Sil) triple point is from Holdaway (1971). Circled D1, D2 and D3correspond to PT conditions for the assemblages formed during thedevelopment of the first, second and third generation of structures; thetype of TMUs is indicated in parentheses. a P–T–d–t path of the ValVedello basement, b P–T–d–t path of the Monte Muggio zone, c P–T–d–t path of the North-Eastern Orobic Basement (NEOB-A), d P–T–d–

t path of the Domaso Cortafo Zone, e qualitative P–T–d–t path of theNorth-Eastern Orobic Basement close to the Insubric Line (NEOB-B), f qualitative P–T–d–t path of North-Eastern Orobic Basementclose to Gallinera Line (NEOB-C), g P–T–d–t path of the DervioOlgiasca Zone, h qualitative P–T–d–t path here inferred after Giobbi-Origoni and Gregnanin (1983) for the TVB Massif. Redrawn fromSpalla and Gosso (1999) and di Paola et al. (2001)


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amphibolite facies, followed by a greenschist retrogression

(Giobbi Origoni and Gregnanin 1983). The distinctivecharacter of this kind of metamorphic evolution (basement

unit of type V) is the lack of a Barrovian record

(amphibolite facies imprint) in the qualitative P–T–d–t(Fig. 2) at the transition from epidote–amphibolite to

greenschist facies.

Lithostratigraphy of the Permian sequencein the Trompia Basin

The Trompia Southalpine Basin developed as an asym-

metrical half-graben delimited by crystalline structuralhighs and Permian fault zones acting either as feeders of

important igneous activity (Giudicarie paleo-shear zone) or

as normal faults uprising the basement with respect to thePermian sedimentary cover (Trompia Valley paleo-line)

(Ori et al. 1986; Cassinis and Perotti 1993; Cassinis and

Neri 1999). According to the same Authors the TrompiaBasin was separated from the Orobic Basin by a crystalline

structural high. The paleo-shear zones bounding the highly

subsiding Trompia pull-apart basin suggest a strike-slip to

extensional tectonic regime during Lower Permian times

(Cassinis and Perotti 1993; Perotti and Siletto 1996; Ber-toluzza and Perotti 1997). Such tectonic lines have been

interpreted as Hercynian faults that were subsequently

reactivated by Permian, Triassic and Jurassic tectonics(Castellarin 1972; Bosellini 1973; Cassinis and Castellarin

1988).

Two Permian sedimentary cycles are recognisable in theTrompia Valley, where cover sequences truncate the syn-

metamorphic folds and foliations characterising the pre-

Permian basement. The early cycle, Lower Permian(Fig. 4), consists of volcano-sedimentary deposits (Basal

Ignimbrite, Collio Formation, Dosso dei Galli Conglom-

erate and Auccia Volcanics) and the second, UpperPermian, consists of fluvial conglomerates (Verrucano

Formation and Val Daone Conglomerate). Deposits of the

former are localised in elongate intermontane basins iso-lated by crystalline structural highs, whereas the latter

forms a wide on-lap covering the first cycle deposits and

crystalline structural highs (Cassinis et al. 1980, 1988).The early Lower Permian products are rhyolitic ig-

nimbrites (Basal Ignimbrites: Peyronel Pagliani 1965;

Cassinis and Neri 1990), unconformably lying on the

Fig. 3 Permian sequence of Western, Eastern Orobic Basin andTrompia Basin redrawn after Cassinis and Neri (1999). 1 Strati-graphic gaps, 2 Servino Formation (SE), 3 Verrucano Lombardo (VL),4 conglomeratic facies, 5 Collio formation, 6 Volcanics, 7 BasalConglomerate, 8 Radiometric ages (Ma): a volcanics, b intrusives.DGC Dosso dei Galli Conglomerate, AVC Monte Aga and ValVedello conglomerates, PF Ponteranica Formation, AV AucciaVolcanics. Radiometric data from: Ganna volcano–plutonic

association (Bakos et al. 1990; Cassinis and Neri 1999); Montorfanoand Mottarone granites (Pinarelli et al. 1988); Val Biandino Pluton(Thoni et al. 1992); volcanics in the Collio Formation of the OrobicAlps (Cadel 1986; Cadel et al. 1987); Torgola, Navazze and Val diRango granitoids (De Capitani et al. 1994); volcanics in the CollioFormation of the Trompia Basin and Auccia Volcanics (Schalteggerand Brack 1999); Monte Croce granitoids (Rottura et al. 1997);Athesian porphyric platform (Cassinis and Neri 1999)


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Variscan basement. At the bottom they include rare con-

tinental detrital deposits of probable Stephanian age (BasalConglomerate: Cassinis 1966a). Volcanics are overlain by

the continental deposits of the Collio Formation, including

a medium-to-fine-grained alluvial–lacustrine sequence(mostly clay), with vegetal remains, tetrapod footprints

(e.g. Remy and Remy 1978; Conti et al. 1997) and minor

volcanics (Breitkreuz et al. 2001). Fluvial sedimentationcontrolled by marginal fan activations (sandstones and

conglomerates) follows the alluvial–lacustrine sequence.

The upper part of the Collio Formation is heteropic withthe DGC, marking the alluvial fans progradation towards

the basin centre (Cassinis 1966a; Ori et al. 1986). The

Auccia Volcanics (ignimbrites and layered tuffs) seal thefirst cycle deposits capping the DGC (Cassinis 1966b,

1968; Peyronel Pagliani and Clerici Risari 1973) (Fig. 4).

The distribution of lacustrine and fluvial deposits,together with paleocurrent orientations, make location of

the sediments source-area along the present day southern

margin of the Trompia Basin possible (Ori et al. 1986;Perotti and Siletto 1996; Cassinis and Perotti 1997). In

Fig. 4 the detrital transport directions are oriented with

respect to Permian coordinates (Ori et al. 1986), whichaccording to the interpretation of Cassinis and Perotti

(1993) underwent a counterclockwise rotation of about 60"with respect to the stable Europe since Permian times.Mean U–Pb zircon ages of 283 ± 1 and 280.5 ± 2 Ma have

been calculated in the Basal Ignimbrite and the Auccia

Volcanics, respectively (Schaltegger and Brack 1999), thusconstraining the age of DGC (Fig. 4); palinological and

icnological data (e.g. Cassinis and Doubinger 1992; Conti

et al. 1997) are consistent with the latter U–Pb ages.The DGC is made up of coarse conglomerates and dark

red sandstones, frequently interlayered with siltstones

(Cassinis 1969), and is divided into two members: thelower is sandy and forms a coarsening upward sequence,

and the upper consists of chaotic conglomerate with int-

erbedded minor coarse sandstones (Assereto and Casati

1965; Cassinis 1966a; Ori et al. 1986). In the eastern sector

of the basin the conglomerate is thinner than in the westernpart (Cassinis and Perotti 1993) and is essentially made of

volcanic pebbles (Boni 1955; Cassinis et al. 1975; Cassinis

1983), while in the western part, along the Caffaro andCamonica Valley ridge, decimetre-sized metamorphic

pebbles occur very frequently.

Basement pebble microstructural analysis

Sampling strategy

To explore the original variety of metamorphic rocks out-cropping at Lower Permian times and that acted as

basement source of the DGC, we took about 400 samples

after a wide area survey on conglomerate outcrops using asreference the geological map of Cassinis (1988). We

selected 165 of these samples for detailed mesoscopic

observation and 88 were thin-sectioned for the micro-scalestudy.

The pebbles mainly consist of metapelites (micaschists

and paragneisses) and minor gneisses, metagranitoids andquartzites. Attention was focused on reconstructing the

structural and metamorphic histories to be compared with

that deciphered in the Southalpine basement units. We thusproceeded to select: (1) pebbles large enough to perform

microstructural analysis and to prevent the weathering or

diagenetic alteration of the conglomerate; (2) conglomeratesequences with poor lithologic sorting so as to offer the

widest variety of metamorphic rocks subjected to Lower

Permian erosion in the areas nearby; and (3) a significantvariety of mineralogical assemblages and chemical com-

positions in order to individuate the most sensitive-to-slight

metamorphic variations, to determine the maximum num-ber of metamorphic re-equilibration steps with their

relative chronology and, hence, to infer quantitative P–Tpaths.

Fig. 4 Section across lowercycle deposits in the TrompiaValley Basin. Arrows indicatedetrital transport directionsoriented with respect to Permiancoordinates. Redrawn from Oriet al. (1986)


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Methodological approach to infer the metamorphic

evolution of basement pebbles

The record of tectonic events (rock memory) influencing

the evolution of a metamorphic basement is strongly con-

trolled by partitioning of deformation and metamorphismin space. Such incomplete rock memory makes correlation

of fabric and metamorphic re-equilibration sequences dif-

ficult, even with favourable exposure. This heterogeneousrecord creates a textural puzzle with coronitic fabric

domains (mineral growth without a new oriented fabric)

juxtaposed to domains with tectonitic (mineral growth witha new linear or planar fabric) or mylonitic fabrics (mineral

growth with a new, fully obliterative planar fabric). The

textural puzzle can be imprinted during a single deforma-tion stage. Moreover domains with coeval structures may

be adjacent to domains with superposed structures.

Microstructural analysis of pebbles does not attributeregional significance to superposed structures among dif-

ferent samples, because every pebble is a fragmented

basement chip of unknown position with respect to thenatural mosaic of structural domains representing the full

deformation history. Furthermore, contemporaneous

metamorphic assemblages may mark different fabrics indifferent pebbles, as a function of the erosional sampling of

the textural puzzle developed over the full tectono-meta-

morphic evolution of the source basement. Microstructuralanalysis (Vernon 1976; Bell et al. 1986; Passchier and

Trouw 1996) was used to unravel the relative chronology

of mineral assemblages, marking superposed fabrics ineach pebble, and consequently to infer the successive

metamorphic imprints (M1, M2…). The comparison of

metamorphic evolutions, occurring systematically in peb-ble groups, with those of the rocks of the surrounding

basement units, makes the individuation of potential pebble

sources possible.The microstructural analyses allowed to group pebbles

in which a fabric sequence of the same type, as superposed

foliations (S1, S2, S3…), crenulations (D1, D2, D3…) orcoronitic fabrics (C1, C2, C3…), is marked by a sequence

of mineral assemblages characteristic of equivalent meta-

morphic conditions. On this ground a reconstruction offabric and mineral assemblage relative chronology in the

pebbly rocks is here proposed. At the end of the description

of each group of rocks the metamorphic assemblagesdeveloped during each successive re-equilibration stage

(M1, M2…) are summarized.

Microstructural and mineralogical characters

The DGC pebbles were sampled mainly along the ridge

between the Camonica and Caffaro (Bagolino) Valleys

south of the Croce dei Domini Pass (Fig. 1b), where they

are dm-sized and are made of both volcanic and meta-morphic rocks.

Hereafter the summary of the main microstructural

characters for metapelites and metaintrusives (illustrated inFigs. 5, 6, 7) is reported to allow the immediate individu-

ation of the main fabric characters and of the mineral

assemblages sequence. Details of the microstructuraldescriptions are supplied in the Appendix, with the ana-

lytical observations on relationships between fabricelements and mineral assemblages.

Metapelites

Metapelites consist of micaschists, paragneisses and fine-grained mylonitic gneisses and are mainly composed of

Wm, Qtz, Ab, Chl, Grt, opaque minerals, minor Rt, Ap,

Tur, Cld, and rare Ep (generally Czo) Bt and Ttn. Fourtypes of metapelites have been distinguished on the base of

the number of superposed planar fabrics and of the mineral

associations that form the fabrics.Type I contains two foliations (S1 and S2) marked by

Wm ± Bt. S1 is preserved only in the micro-lithons of S2,

in which it is tightly crenulated (Fig. 5a); S2 foliation isseldom crenulated by a D3 crenulation. Qtz, WmI, Grt, ±

BtI, Rt grew during S1 development; S2 is marked by Qtz,

WmII, Grt, ± BtII, Ilm, Pl. The mineral association,eithercoronitic (C3) or tectonitic (here called D3 because it

consists of a crenulation, without a new differentiated

foliation), Qtz, Chl, Ab, WmIII, Tur overgrew S2. Ana-lytical details are illustrated in Fig. 5a–e and described in

Appendix (Metapelites—Type I).

Type II shows a S3 differentiated foliation with filmsmarked by Qtz, Wm, Chl ± green Bt, in addition to S1 and

S2 Wm ± Bt bearing foliations recorded by type I metap-

elites. Syn-S1 and syn-S2 mineral assemblages are thesame as in type I; the syn-S3 mineral assemblage is Qtz,

WmIII, Chl, ± green BtIII, ± Tur, ± Ep, Ab. Due to the

very similar mineral–fabric relationships between types Iand II during S1 and S2, only analytical details on the

microstructural character of each mineral peculiar for type

II metapelites are illustrated in Fig. 5f–h and described inAppendix (Metapelites—Type II).

Type III rocks, by contrast with type I and II metapelites,

record a S1 mylonitic foliation associated with the growthof Qtz, WmII, ± Ab, opaque minerals, ± green BtII, Chl.

WmI, Grt, ± BtI are preserved as pre-S1 relics. Analytical

details are illustrated in Fig. 6a and b and described inAppendix (Metapelites—Type III).

Type IV records a relict foliation (S1), preserved as

rootless folds, marked by Wm and a S2 foliation underlainby Wm and Chl. S2 is folded by a successive crenulation


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(D3); syn-D3 small Chl and Wm flakes crosscut S2. S1 is

associated with Qtz, WmI, Cld, GrtI, Rt and Ab growth,while S2 foliation development is contemporaneous with

Qtz, ± GrtII, ChlI, WmII and Ilm assemblage. Syn-D3

mineral association is Qtz, WmIII (Pg), ChlII, Tur. Ana-lytical details are illustrated in Fig. 6c–e and described in

Appendix (Metapelites—Type IV).

Figure 8 synthesizes the structural correlations inferredon the ground of the metamorphic significance of the

mineral assemblages marking fabrics in metapelitic peb-

bles. S1 and S2 fabrics, preserved in type I and type IImetapelites, are interpreted as equivalent. These fabrics are

also correlated with S1 preserved in type IV metapelites

and most likely correspond to the pre-S1 minerals in typeIII metapelites. S2 foliation and D3 fabrics in type IV

metapelites should be coeval with the development of S1 in

type III metapelites, S2 in type II and the D3/C3 mineralassemblage in type I metapelites.

This synthesis indicates two subsequent metamorphic

re-equilibrations in metapelites: M1 characterised by: Qtz,Wm, ± Bt, Grt, Rt/Ilm, ± Pl and ± Cld assemblages,

indicating epidote–amphibolite facies conditions; and M2

characterized by Qtz, Wm, ± Ab, Chl, ± Ep, ± Tur and ±green Bt bearing assemblages, indicating greenschist facies

Fig. 5 Microstructures frommetapelites. a Relict foliation(S1), marked by WmI,preserved between films ofpervasive foliation (S2), markedby WmII; crossed polars,b WmIII partially replacing Grtrim; plane polarised light,c curved internal foliation in Pl,marked by opaque minerals andcontinuous with the external S2foliation, suggests a syn-kinematic growth with respectto Se (=S2); crossed polars,d BtI porphyroclast, partiallyreplaced by Chl, preservedbetween films of pervasivefoliation (S2), marked by WmII;crossed polars, e Chl rosettasovergrowing S2; backscatteredSEM image, f Pl enclosing Grtand two superposed internalfoliations (S1 and S2); crossedpolars, g Si (=internal foliation)and Se (=external foliation)pattern indicates that Plunderwent two stages of growthsyn- and post-kinematic withrespect to Se (=S2); crossedpolars, h Chl crosscutting aWmII aggregate in which singlegrains are parallel to S2;backscattered (BSE) SEMimage


123

Fig. 7 Microstructures frommetaintrusives (froma–d = albite–epidote gneisses;e and f = leucocraticmetagranitoids). a BtI partiallyreplaced by KfsII and Ttn; BSE-SEM image; b Ttn rim betweenBt and Rt grains; planepolarised light; c zoned PlII,with an Ab-rich rim, has patchyextinction and Grt inclusion inthe core; crossed polars; d EpIwith straight grain boundariestowards WmII and PlII; crossedpolars; e Kfs porphyroclastenclosing Wm and Qtz; crossedpolars; f Grt partially replacedby Chl; plane polarised light

Fig. 6 a Sigmoid-shaped Plpoikiloblast (contoured by thinwhite line) with Qtz inclusions,has SPO parallel to s foliationand WmII marks c foliation;crossed polars. b Chl replacesincipiently micro-boudinagedBt along the neck; planepolarised light; c foliation (S1)marked by WmI, Cld and Grt;plane polarised light; d micro-boudinaged Cld reorientedparallel to S2 with Chl fillingmicroboudin necks; planepolarised light; e Chl replacesCld along the rim andcleavages; BSE-SEM image


123

conditions. As in TVB basement, note that Bt and Cld do

not coexist in the same assemblage. During M1 two dif-ferentiated foliations developed, whereas M2 is

accompanied by two deformations, one of which generates

a differentiated foliation. Microstructural analysis thussuggests that during polyphase metamorphism at least four

groups of deformation structures were recorded in the

source basement of the pebbles.

Metaintrusives

Metaintrusive pebbles consist of albite–epidote-bearinggneisses, with protoliths of intermediate composition,

leucocratic metagranitoids and meta-aplites.

The albite–epidote-bearing gneisses are made of Qtz,Wm, Pl, Bt, Chl, Grt, Ep and minor Kfs, Rt, Ilm, Ttn and Tur.

Mainly WmII, EpI and Qtz elongated lenses mark an S1

spaced foliation. Rarely deformed WmI and BtI, with dis-solved rims and shape-orientation oblique to S1, could

predate it. S1 is associated with the growth of Qtz, WmII,

BtII, EpI, PlII, Grt. BtI, WmI, PlI, Rt and ± KfsI predate S1,while WmIII, Chl, Ab, EpII, KfsII postdate it. Analytical

details are illustrated in Fig. 7a–d and described in Appen-

dix (Metaintrusives—Albite–epidote-bearing gneisses).

All these microstructural observations allow inferring

two successive metamorphic re-equilibrations (Fig. 8). M1:the paragenesis Qtz, Wm, Pl, Bt, Ep, Rt, Grt, ± Kfs, ± Ilm

suggests epidote–amphibolite facies conditions; and M2:

Qtz, Ab, Wm, Chl, Ep, Ttn, Kfs, Tur, sagenite, ± Ilmdeveloped under greenschist facies conditions. In the

albite–epidote bearing gneisses M1 is coeval with the

development of a penetrative foliation, whereas M2greenschist re-equilibration is coronitic.

Leucocratic metagranitoids and meta-aplites contain anincipient foliation (S1) underlain by Wm and Qtz ribbons.

In metagranitoids the most abundant minerals are Qtz and

Ab, with minor Kfs, Wm, Chl, Grt and green Bt. Mineralspredating S1 are Qtz, scanty red-BtI, WmI and Kfs. S1 is

marked by Qtz, WmII, Ab, ± Grt, ± green-BtII, while Chl

and WmIII postdate S1. Analytical details are illustrated inFig. 7e and f and described in Appendix (Metaintrusives—

Leucocratic metagranitoids and meta-aplites).

Meta-aplites do not contain Grt and Chl. S1 foliation isunderlain by large Wm and Ab SPO. While Wm may be

decussate, relict Wm shows strong undulose extinction,

with irregular and lobate rims, and is oblique to S1. Ab ismore abundant than Qtz, shows weak internal deformation

and is poikilitic, including Qtz trails parallel to external S1

and rare, highly deformed Wm grains.

Fig. 8 Schematic representation of fabric and assemblage evolutionin metamorphic pebbles of DGC. The correlation of deformation-metamorphism relationships between metamorphic pebbles allows

inferring the sequence of M1 and M2 metamorphic imprints (see‘‘Discussion’’). Relative chronology of superposed fabrics andrelative mineral assemblages is referred to single rock types


123

The microstructural characters of leucocratic metagra-nitoids make definition of the subsequent metamorphic

re-equilibrations possible. M1: the early assemblage Qtz,

Wm, ± Grt, ± green-Bt, Ab, Rt is compatible with epi-dote–amphibolite facies conditions; M2: growth of Qtz,

Chl, Ab, Wm, Fe-oxides, Ilm indicates a greenschist facies

re-equilibration. During M1 a faint S1 foliation developed.The summary of Fig. 8 shows the correlation between

‘‘metamorphic imprints’’ in the different groups of

metapelites and metaintrusives on the base of the relativechronology of mineral assemblages, marking superposed

fabrics in each pebble and the successive metamorphic

imprints (M1, M2…) inferred consequently. As an exampleit is shown that the mineral assemblages pre- and syn-S1 in

metaintrusives and metapelites indicate that the metamor-

phic facies was the same in both cases. In particular, Fig. 8shows that syn-S1 and S2 mineral assemblages in types I

and II metapelites and syn-S1 in type IV metapelites are

compatible (in terms of P–T conditions) with those of syn-S1 in metaintrusives. Chemical compositions of mineral

phases will allow the estimation of the physical conditions

of metamorphism during M1 and M2 stages in the differentpebble groups supplying a quantitative control on the real

match of the specific P–T conditions of each re-equilibra-

tion stage.

DGC pebbles compared to TVB and other CentralSouthalpine basement rocks

A comparison of structural and metamorphic evolutionsinferred in DGC pebbles and in TVB (Giobbi Origoni and

Gregnanin 1983) is shown in detail in Table 1. Most of the

DGC pebbles consist of the more common lithotypes(metapelites and albite–epidote-bearing gneisses) of the

TVB; some pebbles consist of leucocratic metagranitoids

and meta-aplites, which are unknown in the TVB. Gener-ally the sequence of metamorphic re-equilibration in the

pebbles is consistent with that of the basement described by

Giobbi Origoni and Gregnanin (1983) and the only dif-ference is the presence in the pebbles of two superposed

fabrics formed under epidote amphibolite facies conditions,

while in the basement only one fabric was found (seeTable 1 for details).

In spite of these microstructural differences, the same

assemblage sequence can be envisaged in similar bulkcompositions for the DGC source basement and the TVB

basement (Tables 1, 2). The first group of assemblages

indicate epidote–amphibolite facies conditions, represent-ing the Tmax!PTmax

imprint; the assemblages of the last

metamorphic re-equilibration correspond to greenschist

facies conditions.

Table 1 Summary and comparison of microstructural features in metapelites and metaintrusives from TVB basement and DGC pebbles inwhich main differences are highlighted

Relative foliation chronology has regional significance in TVB basement and local in the pebbles. GS Greenschist facies, EA epidote–amphibolite facies


123

Unlike the metamorphic evolution recorded by DGC

pebbles and TVB basement, in type II basement units (typeII TMUs in Fig. 2) the Cld bearing paragenesis is followed

by a metamorphic re-equilibration under amphibolite facies

conditions characterised by Tmax!PTmaxSt-bearing assem-

blages and predating the greenschist facies re-equilibration

(Table 2). Cld-bearing paragenesis is lacking in the type I

and type IV basement units, where St-bearing assemblagesare the oldest metamorphic record. In type IV unit,

St-bearing assemblages predate the Bt–Sil-bearing Tmax!PTmax

assemblage. In summary, the sequence of metamor-phic imprints recorded by DGC pebbles and TVB basement

is peculiar and clearly distinct since it escapes the Barro-

vian imprint, a well-known feature in the metamorphic

outline of the Central Southern Alps. Since the latest

structural and metamorphic re-equilibration affecting thepebbles is not imprinted on the conglomerate matrix, its

minimal age can be situated at 280.5 ± 2 Ma (Fig. 4;

Schaltegger and Brack 1999).

Mineral chemistry

The major-element composition of the mineralogical pha-

ses (624 analyses) from DGC pebbles was obtained with anautomated WDS and EDS SEM-microprobe system; the

analytical conditions are specified in Table 3 caption. Due

to the lack of chemical data for TVB minerals, mineral

Table 2 Assemblage sequences in each lithostratigraphic (columns) and types of tectono-metamorphic (rows) units of the Southern Alpsbasement

TMUs Tectono-metamorphic units, LSUs litho-stratigraphic units


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Table 3 Selected minerals micro-chemical analyses

The SEM-microprobe system of the ‘‘A. Desio’’ Earth Science Department of Milan University was used. The accelerating voltage was 15 kVand the sample current 190 pA; natural silicates were used as standard. Matrix corrections were calculated using a ZAF procedure. Theproportional formulas were elaborated with MinTab (Rock and Carroll 1990) and Hyper-Form (de Bjerg et al. 1992) programs. Stoichiometricratios of elements based on 22 oxygens for Bt and Wm, 12 oxygens for Cld and Grt, 8 oxygens for feldspars and 13 oxygens for Ep. The contentof trivalent iron was calculated for Grt and Ep, whereas for all other Fe-bearing minerals it was assumed to be Fe2+. The activities of end-members and their standard deviation, calculated with AX, are also indicated


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compositions from pebbles are compared to those from the

NEOB-A metapelites and metagranitoids (type II basementunit) (Spalla et al. 1999) and from the Camonica Valley

and Monte Fioraro metagranitoids (Siletto 1991; Colombo

et al. 1994; Pigazzini 2003). In the literature the onlyavailable chemical compositions on minerals from Vari-

scan Cld-bearing assemblages of Southern Alps come from

the NEOB-A metapelites, whereas those on mineral phasesof metagranitoids come from the Camonica Valley, Monte

Fioraro and NEOB-A; for this reason we compare our datawith those coming from these rocks.

Metapelites: Si4+ content of white mica from metapelitic

pebbles varies from 5.94 to 6.90 a.p.f.u. (Fig. 9a); Wm inthe M2 assemblage shows the lowest Si4+ values and only

in Cld-bearing metapelites it reaches higher Pg (Na/Na +

K + Ca) (0.81–0.96) contents, even with respect to that ofthe Wm from NEOB-A St schists. Wm in M1 assemblages

has Si4+ contents ranging between 6.42 and 6.90 a.p.f.u.,

higher than in Wm from NEOB-A schists.

Garnet exhibits core-rim Alm, Prp increases and Sps

and Grs decrease in all metamorphic pebbles (Fig. 9b). Grt(M1) from metapelitic pebbles is rich in Alm (0.58–0.88)

and poor in Prp (0.01–0.14), in Sps (0.01–0.14) and in

Grs + Adr (0.01–0.19). As a rule in Cld-bearing metape-litic pebbles, Grt is richer in Alm and Prp and lower in

Grs, Adr and Sps than in other metapelitic pebbles and

shows a higher Prp content than the Grt from NEOB-ACld schists.

Plagioclase in metapelitic pebbles (Ab = 87–99%)shows a slight Ca decrease towards the rim. The richest Ca

portions (syn-M1) usually enclose small garnets and Rt.

The most albitic Pl (syn-M2) is usually in equilibrium withclusters of Chl. Feldspar composition is equivalent to that

of syn-D2 Pl in NEOB-A St schists, while pre-D2 Pl of

NEOB-A schists has a higher anorthite content than Pl inmetapelitic pebbles.

Chloritoid displays a slight Fetot increase and Si

decrease towards rims and higher values of Mg content

Fig. 9 Mineral compositions: a Wm from DGC pebbles of metap-elites are more phengitic than those from NEOB-A schists; Wm fromDGC pebbles of metaintrusives has higher Phe-content than that fromNEOB-A Camonica Valley and Monte Fioraro metagranitoids; b Grtfrom pebbles of metaintrusives compared to Grt from NEOB-A,Camonica Valley and Monte Fioraro metagranitoids, and from

metapelitic pebbles compared to NEOB-A Cld schists; grey arrowsindicate core/rim compositional zoning; c Cld from metapeliticpebbles shows a higher XMg with respect to Cld from NEOB-Aschists; d Bt from pebbles of albite–epidote-bearing gneissescompared to Bt from NEOB-A, Camonica Valley and Monte Fiorarometagranitoids


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(XMg = 0.18–0.22) with respect to the Cld from NEOB-A

schists. Altot (3.40–3.96 a.p.f.u.) and Zn (up to 0.013a.p.f.u.) contents are comparable to those in NEOB-A Cld

(Fig. 9c). Post-S2 chlorite (M2) (XFe = 0.43–0.63; Si4+ =

5.23–5.81 a.p.f.u.) in Cld-bearing metapelites has higherMg content with respect to Chl syn-S2 (M2) and to post-S2

Chl in type I and II metapelites.

Metaintrusives: white mica in leucocratic metagranitoids(Si4+ = 6.19–6.87 a.p.f.u.) and in albite–epidote-bearing

gneisses (Si4+ = 6.09–6.89 a.p.f.u.) shows the highest Si4+

values in the M1 assemblage (Fig. 9a). These Wm show

higher Si4+ and Fe + Mg contents than Wm from basement

metaintrusives.Biotite composition (Fig. 9d) in the albite–epidote-

bearing gneisses (Ti = 0.14–0.25 a.p.f.u.; Fetot = 2.08–2.71

a.p.u.f.; AlVI = 0.43–0.95 a.p.f.u.) fits with that of NEOB-A Bt from metagranitoids while showing lower Fe contents

than Bt from Monte Fioraro (2.97–3.34 a.p.f.u.) and

Camonica Valley metagranitoids (2.93–3.26 a.p.f.u.).Garnets from albite–epidote-bearing gneisses shows the

highest Grs + Adr amount (0.37–0.46) and the lowest Alm

(0.38–0.61), Prp (up to 0.02) and Sps (0.01–0.17) contentswith respect to other types of metamorphic pebbles

(Fig. 9b). The high Ca concentration in this Grt is con-

trolled by the bulk composition of albite–epidote gneisses(metatonalites? of Giobbi Origoni and Gregnanin 1983),

but its igneous origin can be ruled out (Miller et al. 1988;

Le Goff and Ballevre 1990). In leucocratic metagranitoidsGrt has lower contents of Sps (0.03–0.11), Prp (0.03–0.07)

and Alm (0.55–0.67) and higher Grs + Adr (0.24–0.32)

than metapelitic pebbles. With respect to basement meta-intrusives, Grt from albite–epidote-bearing gneisses shows

the highest Grs values, while Grt composition of leuco-

cratic gneisses is comparable to that of Grt from CamonicaValley orthogneisses.

There are two types of feldspar in the albite–epidote

gneisses: a probably igneous Pl, with Ab exsolutionlamellae, and Ab-rich porphyroblasts. The former has

lower Ab contents (75–80%). The composition of this kind

of Pl is comparable to that of NEOB-A metagranitoids andMonte Fioraro Complex. Ab-rich Pl (from high Na oligo-

clase to Ab) shows concentric zoning and is in equilibrium

with small grained Grt, Ep and Bt (syn-M1). In leucocraticmetagranitoids feldspar contains less than 10% of An. Pl

(An = 0.09–5.87%; Or up to 8.67%) contains rare Grt, in

textural equilibrium, and has lower An contents than inbasement metagranitoids. In Grt-poor pebbles, feldspars

have a peculiar composition: Kfs (Or = 88–96%) and

Na-feldspar (Ab = 71–89%; An = 2–5%; Or = 10–20%).In the leucocratic metagranitoids, chlorite rimming Grt has

higher XFe than matrix Chl and generally lower Si4+ con-

tent than in metapelites.

Epidote (Fe3+ = 0.41–0.58 a.p.u.f.; Al = 2.50–2.68

a.p.f.u.) from albite–epidote gneisses shows Fe3+ content

variation: the Ep (syn-M2) richest in Fe3+ is in texturalequilibrium with Ab porphyroblasts and those richest in Al

shows rational rims with Grt and Bt (syn-M1).

Fig. 11 P–T path inferred for albite–epidote-bearing gneiss pebblesof DGC. Univariant equilibria have been calculated with THERMO-CALC (Holland and Powell 1998), taking into account end-membersactivity for the successive mineral assemblages marking superposedfabrics. Thick dashed lines indicate the Si4+ isopleths in Wm and thindashed lines delimitate the T-range estimated with the Pl-Msindependent thermometer (see the text for details)

Fig. 10 P–T path inferred for type IV DGC metapelitic pebbles.Univariant equilibria have been calculated with THERMOCALC(Holland and Powell 1998), taking into account end-members activityfor the successive mineral assemblages marking superposed fabrics.Vertical dashed lines delimitate the T-range estimated with indepen-dent thermometers (Grt/Cld; Grt/Phe; see the text for details)


123

In leucocratic metagranitoids, Ep is scanty and hashigher Fe3+ (0.43–0.90 a.p.f.u.) and lower Al (1.99–2.64

a.p.f.u.) contents with respect to albite–epidote-bearing

gneisses. Locally, it contains REE (La £ 0.17 a.p.u.f.,Ce £ 0.54 a.p.u.f. and Nd £ 0.08 a.p.f.u.).

P–T estimates and metamorphic evolution

Microstructural analysis demonstrates that a single pebblemay preserve up to three mineralogical and structural

re-equilibration stages. The individuation of the timing of

mineral growth and evidence of textural equilibriumbetween mineral phases facilitate the selection of mineral-

ogical sites suitable for P–T estimates of M1 and M2

re-equilibration stages. The microstructural analysisallowed the reconstruction of mineral assemblages succes-

sion (Fig. 8), and the transition between mineral

assemblages allows discriminating the univariant equilibriacrossed during the metamorphic re-equilibration from M1 to

M2 (Figs. 10, 11). The inferred P–T paths for each bulk

composition of DGC pebbles can be compared to the P–Tpaths of the different units known in the Southalpine base-

ment. P–T estimates (Table 4) were carried out using well-

calibrated independent thermometers based on Fe–Mg andNa–K exchange reactions, net transfer reactions and the Si4+

content of white mica barometers. In Table 4 the number of

estimates (n) is reported plus or minus the standard devia-tion for each used thermo-barometer. Univariant equilibria

were also calculated with THERMOCALC (Holland andPowell 1998 and ref. therein), taking into account the end-

member activity of mineral phases marking the superposed

fabric elements and belonging to successive parageneses.

Metapelites

The Cld Grt-bearing assemblage in type IV metapelites and

the lack of Bt and St indicate, in accordance with the

petrogenetic grid of Spear and Cheney (1989) (KFMASHchemical system) that the M1 T-field should range between

500 and 570"C. This T-field matches thermometrical esti-

mates inferred by Fe–Mg exchange between Cld and Grt(Perchuk 1991), yielding T = 547 ± 32"C; Fe–Mg

exchange between Phe and Grt (using the calibrations for

Ca-poor compositional systems as in the case of metapel-ites; Green and Hellman 1982) indicates T = 568 ± 19"C

or T = 558 ± 22"C (Wu et al. 2002), the latter evaluating

the Wm Fe3+ content with the AX program (Holland andPowell 1998). These temperatures (Table 4) broadly agree

with the calculated petrogenetic grid (the end-member

activity of different mineral phases are listed in Table 3) inwhich the reaction curve Fe–Cld + Qtz = Alm + Fe–St +

H2O nearly corresponds to the 580"C isotherm and the

Ann + Fe–Cld + Qtz = Alm + Ms + H2O equilibriumranges between 525 and 550"C (Fig. 10).

The Fe–Mg exchange between Grt and Phe for M1 in

type I and II metapelites indicates T = 529 ± 19 "C (Greenand Hellman 1982) and T = 490 ± 22"C (Wu et al. 2002).

The Na–K exchange between Pl and Wm indicates a

T = 522 ± 45"C (Green and Usdansky 1986). In addition,according to the location of the univariant equilibrium

Chl + Ms = Alm + Ann on the petrogenetic grid of Bucher

and Frey (2002), T is higher than 530"C in type I and IImetapelites during M1.

In the petrogenetic grid calculated with THERMO-

CALC (Holland and Powell 1998) the P-values for M1 P–Tfield in type IV metapelites are limited at P ‡ 0.8 GPa by

the intersection between Ann + Fe–Cld + Qtz = Alm +Ms + H2O and Fe–Cld + Rt = Fe–St + Ilm + Qtz + H2O

equilibria. For the upper limit, P-values are constrained at

P £ 1.35 GPa by the Alm + Rt = Ilm + Ky + Qtz equi-librium; these P-estimates fit well with P = 1.1 ± 0.3 GPa

determined using the GPMQ (Grt, Pl, Ms, Qtz) geo-

barometer (Hodges and Crowley 1985) in type I and IImetapelites (Table 4).

The P and T conditions during M2 can be constrained by

the reaction Fe–Chl + Pg + Qtz = Alm + Ab + H2O,

Table 4 Thermo-barometrical estimates for DGC metamorphic pebbles are shown with standard deviation and the number (n) of estimates


123

which fixes the upper T limit at T £ 520"C in type IV

metapelites (Fig. 10), while M2 in type III metapelites

should be limited at T £ 425"C by the equilibrium Chl +Kfs = Ann + Ms, according to Bucher and Frey (2002).

The Si4+ content of Wm decreases from 3.3 a.p.f.u. (syn-

M1) to 3.1 a.p.f.u. (syn-M2), suggesting that during M2cooling P decreases at *0.3 GPa.

Metaintrusives

In the albite–epidote gneisses, M1 temperature was esti-mated with the thermometer based on Na–K exchange

between Pl and Ms (Green and Usdansky 1986) and yiel-

ded T = 528 ± 35"C; the Ti content in Bt (Schreurs 1985)suggests 500 £ T £ 650"C. The Si4+ content in syn-M1

Wm coexisting with Bt and Kfs (Massonne and Schreyer

1987) is compatible with P = 1.1 ± 0.25 GPa for the

inferred T-interval. The petrogenetic grid (Fig. 11), calcu-

lated with THERMOCALC (Holland and Powell 1998),shows that the appearance of Ms + Chl + Qtz (reaction:

Ms + Fe–Chl + Qtz = Alm + Bt + H2O) constrains the

M1 temperature at T ‡ 510"C, while the appearance ofPl + Chl + Qtz (reaction: An + Fe–Chl + Qtz = Alm +

Czo + H2O) limits minimal pressure during M1 at

P ‡ 0.6 GPa, in agreement with the P-values inferred byWm compositions. The late reaction Ab + Mg-Chl +

Qtz = Pg + Prp + H2O occurring in albite–epidote gneis-

ses indicates the retrogradation path during M2 up toT £ 470"C.

In leucocratic metagranitoids, the Na–K exchange

between Pl and Ms (Green and Usdansky 1986) isT = 551 ± 18"C and the Si4+ content in syn-M1 Wm

(Massonne and Schreyer 1987) indicates P = 1.0 ± 0.2

Fig. 12 Comparison of thermo-barometrical estimates inferred forthe different types of pebbles sampled in DGC. The P–T fields areplotted on metamorphic facies grid by Peacock (1993). In the insetcomparison of P-T path inferred for DGC metamorphic pebbles, P–T–d–t path inferred for NEOB-A TMU (Spalla et al. 1999) and toqualitative metamorphic evolution of TVB TMU (Giobbi Origoni and

Gregnanin 1983) from Southalpine metamorphic basement. In blackgeotherms after Cloos (1993), calculated for: 1 high T/P & 60"C/kmnear spreading ridge or active arc volcanoes, 2 normal geothermalgradients, &25"C/km of old ([25 Ma) plate interiors, 3 highP/T, & 10"C/km ‘‘warm’’ subduction zones, 4 &6"C/km ‘‘cold’’subduction zones


123

GPa. The Grs content in the albite–epidote-bearing gneisses

and in leucocratic metagranitoids (Fig. 9b), as well as thediffusion of Ab-rich Pl blastesis, indicates epidote

amphibolite facies conditions (Le Goff and Ballevre 1990).

The occurrence of Ms + Chl instead of Grt + Bt during M2indicates T £ 500"C, and the content of Si4+ in syn-M2 Wm

(Massonne and Schreyer 1987) yields P = 0.5 ± 0.1 GPa.

In summary in Fig. 12 P–T paths, inferred for the typeIV metapelites and for albite–epidote gneisses (also shown

in Figs. 10, 11) are respectively represented on the P–Tfields of metamorphic facies (Peacock 1993). The esti-

mated P–T conditions for the M1 metamorphic imprint

recorded in type I and II metapelites and in the leucocraticmetagranitoids (Table 4) are also shown to facilitate

comparison of the P–T estimates inferred in the different

DGC pebbles chemical systems. The wide superpositionbetween P–T conditions, derived from different lithotypes,

suggests that the DGC pebbles derive from a basement

portion that homogeneously recorded the same metamor-phic evolution (i.e. a single basement unit, or TMU) in

which the M1 metamorphic imprint is characterised by P–Tvalues corresponding to the upper boundary of the epidoteamphibolite facies field, towards the eclogite facies field.

Discussion and conclusions

The comparison of the P–T paths inferred from DGCpebbles with the P–T–d–t paths of type II basement units

(type II TMUs as NEOB-A of Spalla et al. 1999) and with

qualitative metamorphic evolution of type V basementunits (TVB: Giobbi Origoni and Gregnanin 1983), both

containing metapelites with Cld–Grt assemblages, is fun-

damental to infer the type of source basement unit(Figs. 12, 13). As noted supra, the correspondence of P–Tevolutions inferred in the different rock types occurring in

the DGC pebbles indicates that the source basementbelonged to a single basement unit. In the inset of Fig. 12

the pebble P–T evolution is in good agreement with the

qualitative P–T evolution of TVB unit and sensibly differsfrom the P–T path of NEOB-A unit. The assemblage

sequences in the different rock types from TVB basement

and DGC pebbles (compare Tables 1, 2 with Fig. 8) indi-cates that, after the epidote–amphibolite facies imprint

(Cld–Grt-bearing assemblages), the source basement of

DGC pebbles, as well as the TVB basement, underwent agreenschist facies imprint, escaping the higher T/Pre-equilibration under amphibolite facies conditions cha-

racterising the NEOB-A basement (St–Grt-bearingassemblages).

The available P–T patterns indicate the TVB basement

as the most likely source for the DGC metamorphic peb-bles (Figs. 12, 13), while NEOB-A cannot be regarded as

basement source of the DGC, even if the D1a stage is also

characterised by Cld–Grt-assemblages (Table 2) like M1 inmetapelitic pebbles. Actually the P–T evolution of NEOB-

A basement unit is different from that of DGC pebbles

(inset in Fig. 12) and no St–Grt-assemblage, postdating theCld–Grt-assemblage, was ever found in DGC pebbles. In

addition Grt and Cld in pebbles have a higher Mg content

with respect to those from NEOB-A (Fig. 9c).The lack of metamorphic fabrics in the conglomerate

matrix comparable to that observed in the pebbles and thelithologic and metamorphic affinities between pebbles and

TVB basement indicate a pre-Permian age for the meta-

morphic evolution recorded in the DGC pebbles. Sincedeposition age of DCG conglomerate is constrained by

U–Pb ages on zircons between 283 ± 1 and 280.5 ± 2 Ma

(see ‘‘Lithostratigraphy of the Permian sequence in theTrompia Basin’’), we interpret this time interval as the

minimal age for the exhumation of its source basement.

Such a metamorphic pre-Permian history suggests that thepebbles recorded a Variscan metamorphic history, since

radiometric mineral data (Rb–Sr on white mica) on the

Cld-bearing basement rocks yield 364 ± 15 Ma (Del Moro,in Riklin 1983), interpreted as the age of epidote–

amphibolite facies metamorphic conditions (Siletto et al.

1993; Marotta and Spalla 2007 and references therein).The complete variety of TVB basement rocks occurs in

DGC pebbles with the sole exception of the leucocratic

metagranitoids, which were observed only in the DGCpebbles but never reported in the TVB basement rocks

(Boni et al. 1972; Giobbi Origoni and Gregnanin 1983;

Cassinis et al. 1988). Such metagranitoids may representeither (1) remains of differentiated metaintrusives in the

Permian TVB basement similar to those of the northern-

most Central Southalpine basement (Fig. 13a), todaycompletely eroded; or (2) pebbles deriving from Cima

Fraitina and Palone di Sopressa metagranitoids via a

detrital transport of at least 60 km (Carminati et al. 1997and references therein). These two types form the elon-

gated body of Variscan metagranitoids located South-West

of Edolo in Fig. 13 and belong to NEOB-A basement unit.Since the metamorphic evolution of the leucocratic

metagranitoids matches that of the other DGC pebbles, we

can envisage that this lithotype is now completely erodedin the present TVB basement and cannot derive from

NEOB-A metagranitoids.

The good lithologic match of pebbles and TVB base-ment rocks supports the interpretation, based on

metamorphic ground, that the source of DGC was the

TVB basement portion already exposed to erosion duringEarly Permian. Therefore, the eroded crystalline rocks

were not subjected to a long-distance detrital transport, nor

did the Alpine convergence cause a wide displacementalong thrust surfaces between basement and sedimentary


123

cover in this part of the south-verging Alpine thrust sys-

tems. The individuation of the TVB unit as the source

basement of DGC metamorphic pebbles is in goodagreement with the reconstruction of transport dynamics

within the depositional system, which locates the sediment

source-area along the present day southern margin of theTrompia Basin (Ori et al. 1986; Perotti and Siletto 1996;

Cassinis and Perotti 1997) that is constituted by TVB

metamorphic basement. As already pointed out, the cor-responding P–T evolutions of the pebbles mean that they

derive from a basement constituted by a single basement

unit, suggesting that the parts of the Permian source areanow buried under the Alpine thrusts (e.g. Carminati et al.

1997) may belong to the same unit of the TVB basement

(Fig. 13b, f).

Thermo-barometrical estimates in metapelites from

DGC pebbles indicate that the Tmax!PTmaxfor M1 assem-

blages is at a lower T/P ratio than the thermal gradient of astable continental crust and of young, warm subduction

geotherms (inset in Fig. 12). The P/T and T/Depth ratios

are respectively £2.2 · 10–3 GPa "C–1 and £ 12"C km–1,showing that the thermal regime under which the basement,

representing the pebbles source, had been buried at depth

was compatible with an active subduction of oceanic lith-osphere during Variscan convergence, as suggested by the

location of M1 P–T conditions between the geotherms

of ‘‘warm’’ and ‘‘cold’’ subduction zones in Fig. 12. Thegreenschist imprint recorded during the Variscan exhu-

mation is characterised by a higher thermal regime, with

P/T £ 1.1 · 10–3 GPa "C–1 and T/Depth £ 36"C km–1,

Fig. 13 a Tectonic sketch mapof the Central Southern Alpswith highlighting of the TMUscontaining a Cld-bearingassemblage in theirmetamorphic record with theirP–T paths. 1 Penninic andAustroalpine Domain, 2 PoPlain, 3 Adamello Pluton,4 Post-Permian sedimentarycover, 5a Permian-UpperCarboniferous sediments alongOrobic anticlines, b Permian-Upper Carboniferous sedimentsof the Trompia Basin,6 Variscan metagranitoids,7 Permian-Carboniferousgranitoids, 8 Type II TMUs,9 Type V TMUs, 10 main faultsand thrusts, 11 trajectories ofmain Alpine fold axial planes(synform or antiform). Maintectonic lines: I.L. InsubricLine, G.L. Gallinera Line, M.L.Musso Line, V.G.L. Val GrandeLine. b CROP Central Alpsseismic section (A–A0), inwhich the Permian deposits andbasement portions containingCld assemblages are indicated;modified from Carminati et al.(1997). c–e P–T paths ofNEOB-A (c) and Tre ValliBresciane (d) basements and ofDosso dei Galli metamorphicpebbles (e). f Permianpaleogeography redrawn fromCassinis and Perotti (1993);DGC occurs in Trompia basinsas indicated by the asterisk


123

lying between the geothermal gradient of plate interiors

and volcanic arcs and compatible with a continental col-lision (e.g. England and Thompson 1984; Cloos 1993). The

lack of re-equilibration under amphibolite facies conditions

occurring at *330 Ma and recorded in other units of theCentral Southalpine basement (e.g. NEOB-A) suggests that

the DGC source basement had already been exhumed at

shallower structural levels (P £ 0.5 GPa of the greenschistfacies re-equilibration) during the thermal relaxation con-

sequent to the Variscan collision, unlike the CentralSouthalpine basement units of type I and II which were still

buried at that time (Spalla and Gosso 1999).

In absence of more detailed exhumation-rate determi-nations, an evidently non-linear average rate of 0.35–

0.56 km Ma–1 may be envisaged for DGC basement source

since the age of the epidote amphibolite facies metamor-phic equilibration in TVB basement is known

(364 ± 15 Ma) and corresponds to M1 in pebbles, as well

as the age of the DGC deposition is known (283 ± 1–280.5 ± 2 Ma). This rough estimate may be slightly refined

by separating the uplift trajectory in two portions: (1) the

early part of the path running from the baric peak Cld-bearing assemblage (at 364 ± 15 Ma) and the greenschist

re-equilibration, which we may interpret as contempora-

neous with collision on the base of its thermal regime(*330 Ma in accordance with literature data and inter-

pretations synthesised in Fig. 2, Table 2); (2) the second

part of the path running from the greenschist retro-grada-tion to the DGC deposition (*280 Ma). In this case the

former exhumation path might have been characterised by

a (roughly estimated) rate of 1 km Ma–1 and the latter bythe slower rate of 0.2 km Ma–1.

As a general conclusion it should be noted that, in

determining the parent basement rocks of conglomerates,there are cases in which it is not sufficient to individuate

single detrital mineral phases or single mineralogical

assemblages. Where non-diagnostic minerals with partic-ular geochemical signature are present (e.g. Zack et al.

2004), it is necessary to individuate a sequence of mineral

assemblages so as to restrict the thermal connotation of thepotential source basement, provide the surrounding base-

ment units (or tectono-metamorphic units) are well

individuated and contoured.

Acknowledgments C. Malinverno provided thin sections and A.Rizzi the technical assistance at the SEM-EDS. Funding of M.I.U.R.project ‘Structural markers of divergent and convergent tectonics inthe crustal infrastructure of the Central-Western Alps’ (COFIN 2005)are acknowledged. Elisabetta Spreafico shared fieldwork with D. Z.;Fabrizio Felletti and Lucia Angiolini are thanked for fruitful discus-sions. Ivan Mercolli and an anonymous reviewer comments andsuggestions greatly improved the earlier version; we are honestlygrateful to one of the reviewers who raised several meaningfulquestions that reconciled us with a more effective expression delivery.

Appendix: Analytical description of the pebblesmicrostructure

Metapelites

Type I Wm, with undulose extinction (WmI), marks the

micro-folded S1. Slightly deformed grains of WmII, with

lattice and shape-preferred orientation (LPO and SPO),underlay the foliation S2 (Fig. 5a). In the isoclinal micro-

fold hinges of S1, WmII shows a decussate structure; very

thin grains of new mica (WmIII) rim WmII and Grt(Fig. 5b).

Pl poikiloblasts contain Grt, Rt with Ilm rims, opaque

minerals, Wm and Qtz; Pl shows Carlsbad twinning,zonation growth and undulose extinction. They contain

curved or quite straight internal foliations marked by

opaque minerals and are continuous with respect to theexternal one, suggesting their syn-kinematic growth with

respect to S2 foliation (Fig. 5c). Ab seldom grew in mi-

crofold hinges of D3 crenulation.Grt usually occurs as small grains scattered on S2,

enclosed in Pl or as large porphyroblasts wrapped by S2

foliation. Large Grt porphyroblasts are fractured, skeletaland enclose Qtz, Rt and minor Ep. Fine-grained Grt shows

rational rims within enclosed Pl and WmII, suggesting

competing growth between these phases. All these Grtmicrostructural traits suggest a pre-to-syn-kinematic

growth of this mineral with respect to S2.Bt is scarce and often poorly preserved. It may show

SPO with [001] oblique to S2 (Fig. 5d) foliation (BtI);

single grains have either a lenticular shape and dissolutionrims, or rational rims with WmII and SPO marking S2

(BtII).

Chl replaces Grt and Bt, with minor Ti-oxides; Grtenclosed in Pl is partially preserved. Chl forms rosettas

(Fig. 5e) overprinting folded S2, is associated with Ab,

shows rational rims with Tur and frequently a reddishcolour due to oxidation.

Rt shows sharp-grain boundaries with WmI and is

generally aligned parallel to pervasive foliation. Ilm marksS2 foliation with SPO and rims Rt. Ap is rare and shows

sharp rims with WmII; WmIII infills its fractures.

Type II SPO and LPO of Wm (WmIII) mark a locallydifferentiated S3 foliation, which is in places of crenulation

cleavage type.

Pl contains two generations of internal S1 and S2 foli-ations (Fig. 5f). Fine-grained Grt enclosed in Pl is wrapped

by S2; it is deflected but continuous with S2 in the matrix.

In the marginal part of Pl porphyroblasts internal foliationis parallel and continuous with external S2 (Fig. 5g). Ab

has an internal foliation parallel and continuous with folded

S2 in the matrix foliation (D3 crenulation). These


123

microstructures suggest that Pl grew syn-S2 and Ab grew

after D3 microfolding of S2.Chl replaces Grt and Bt and marks S3 (Fig. 5h) together

with WmIII and minor green Bt.

Tur euhedral crystals contain a folded internal foliationcontinuous with S2 marked by opaque minerals.

Type III Wm rarely occurs as grains (Wm I) with high

internal deformation and at a high angle with respect to S1mylonitic foliation; it generally shows an SPO parallel to

the mylonitic foliation and a slightly undulose extinction(WmII).

Skeletal albitic Pl in these rocks is more abundant than

in type I and II metapelites. Ab poikiloblasts encloseabundant Qtz and the minor opaque minerals Grt and Rt.

Qtz inclusions mark a deflected internal foliation continu-

ous with the external (S1) mylonitic foliation. Sigmoidallyshaped poikiloblasts are aligned along s planes in rare s-c

structures (Fig. 6a).

Grt relics are widely replaced by Chl and opaque min-erals, with fractures filled by WmII and Ab. Relic brown

BtI rarely occurs; it is incipiently micro-boudinaged

(Fig. 6b) and displays SPO at a high angle to S1 myloniticfoliation; new green BtII marks the mylonitic foliation. Chl

widely replaces brown BtI and occupies, rarely with Kfs,

the necks of red-Bt microboudinsType IV Wm with LPO and SPO marks the S2 pervasive

foliation (WmII) and folded relic of S1 (WmI). Randomly

oriented, fine-grained Wm with dissolved rims is concen-trated in S2 microlithons and results from WmI

recrystallisation; an additional generation of Na-rich

WmIII crosscuts WmII and S2.Cld displays an SPO either parallel to S1 (Fig. 6c) or

parallel or at a high angle with S2. ChlI, with SPO parallel

to S2, fills necks of microboudinaged Cld and rims it(Fig. 6d, e). Cld grains show slight internal deformation,

rare twinning and rational rims with WmI and GrtI; these

microstructural traits suggest that Cld predates S2.Ab shows rational grain boundaries with GrtI in micr-

olithons and locally occurs in the mica-rich domain

wrapped by S2 foliation.Grt occurs in two grain-sizes: larger GrtI grains are

wrapped by S2, micro-fractured in S2 microlithons and in

places rimmed by coronas of small GrtII. GrtI cores arerich of very fine-grained inclusions of Qtz, Rt and scanty

Ttn. Grt is replaced by ChlI at the rims or in radial fractures

and the latter flows along S2; these microstructures indicatethat GrtI predates S2 and that GrtII is syn-kinematic with

respect to the early stages of D2.

Rt is rimmed by Ilm and occurs also in fine grained-Wmaggregates. Ilm has SPO roughly parallel to S2.

Partially replacing Cld and Grt, ChlI is either aligned in

S2, develops along [001] of WmI marking D2 microfold

hinges or fills veins crosscutting the S2 foliation at a high

angle. ChlII flakes crosscut WmII, with SPO sub-parallel toWmIII.

Rare Tur occurs as euhedral crystals overgrowing S2.

Metaintrusives

Albite–epidote-bearing gneisses Large flakes of WmII with

SPO and LPO mark S1. Scarce, highly deformed crystalsare oblique to S1 (WmI) and show lobate rims suggesting

dissolution. Fine-grained WmIII replaces PlI. Bt with SPO

marks S1 (BtII) and shows rational contacts with WmII.Locally, highly deformed grains with dissolved rims are

transversal to S1 (BtI). Bt is replaced by Chl and KfsII, or

by Chl and Ttn, and/or contains Rt needles (Fig. 7a, b).Pre-S1 PlI is wrapped by S1 films, shows undulose

extinction and Qtz inclusions; it does not contain either Grt

or EpI. PlII is poikilitic, enclosing Qtz, fine-grained Grt,EpI and rare Rt with Ilm rims. PlII and the enclosed Grt

and EpI show rational rims, suggesting competing growth

between these phases. PlII is gently deformed and showsgrowth twinning and zoning (Fig. 7c) with rims of Ab. Grt

also occurs in the matrix, is partially replaced by Chl

locally and shows sharp contacts with EpI and with BtIImarking S1, suggesting a syn-S1 growth.

Zo and Czo (EpI) mark S1 SPO and straight grain

boundaries with BtII and WmII (Fig. 7d) and locally arereplaced by WmIII and Chl. Showing rational rims with

Chl, EpII (pistacite) occurs in small crystal aggregates

postdating S1. Chl flakes crosscut S1.Leucocratic metagranitoids and meta-aplites Wm SPO

and LPO, quite scarce with respect to albite–epidote-

bearing gneisses, mark S1 (WmII). Relict grains transversalto S1 show strong internal deformation (WmI). Fine-

grained WmIII partially replaces Kfs. Poikilitic Ab

includes Wm, Qtz and rare Grt, has growth and deforma-tion twinning, or deformation lamellae, and displays an

SPO parallel to S1. Aggregates of Qtz and fine-grained Ab

crystals rimmed by new grains occur locally, with unduloseextinction and interlobate inequigranular structure. Kfs

porphyroclasts SPO marks S1 and is parallel to WmII; it

shows growth twinning (Fig. 7e). Grt has rational grainboundaries with Ab, suggesting textural equilibrium, and is

frequently replaced by Chl (Fig. 7f), minor opaque min-

erals and WmIII.Brown Bt is very scarce and often completely replaced

by Chl and Fe-oxides; together with Kfs, it may represent

igneous relics. Green Bt is particularly abundant in well-foliated rocks, where S1 is marked with WmII, and gen-

erally shows weak undulose extinction. Ilm rims Rt, and

randomly oriented Chl cuts across S1.


123

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