Unraveling Magmatic and Orogenic Provenance in Modern Sand: The Back-Arc Side of the Apennine Thrust Belt, Italy

JOURNAL OF SEDIMENTARY RESEARCH, VOL. 72, NO. 1, JANUARY, 2002, P. 2–17Copyright q 2002, SEPM (Society for Sedimentary Geology) 1527-1404/02/072-2/$03.00

UNRAVELING MAGMATIC AND OROGENIC PROVENANCE IN MODERN SAND: THE BACK-ARC SIDE OFTHE APENNINE THRUST BELT, ITALY

EDUARDO GARZANTI1,2, STEFANIA CANCLINI1, FERDINANDO MORETTI FOGGIA1, AND NICOLA PETRELLA1

1 Dipartimento di Scienze Geologiche e Geotecnologie, Universita di Milano-Bicocca, Piazza della Scienza 4, 20126 Milano, Italy2 Centro Studi Geodinamica Alpina e Quaternaria, C.N.R., Via Mangiagalli 34, 20133 Milano, Italy

e-mail: [email protected]

ABSTRACT: Modern river and beach sands carried to and depositedalong the coasts of the Tyrrhenian Sea are derived from multiple mag-matic-arc and orogenic sources, including ophiolitic sequences and tur-bidites issued from the Ligurian Ocean, carbonate-platform to pelagicsedimentary and metasedimentary successions originally depositedonto the Adria continental margin, and overlying foredeep clasticwedges.

‘‘Undissected arc’’ feldspatholithic signatures characterize limitedareas (Capraia Island, Tarquinia and Napoli gulfs). Contrasting fea-tures, with respect to Circum-Pacific suites (low to very low P/F ratio),include sanidine as the most abundant detrital feldspar and commonleucite and leucite-bearing lithic fragments, reflecting the potassic toultrapotassic character of Neogene–Quaternary magmatism. Green au-gite, associated with oxyhornblende and either abundant hypersthene(Tuscan magmatic province) or minor olivine and spinel (Roman mag-matic province) is the dominant dense mineral. ‘‘Dissected arc’’ ar-kosic signatures reflect unroofing of Miocene monzogranitic stocks inthe Tuscan archipelago.

Sands from ophiolitic sequences and remnant-ocean sediments(‘‘subduction-complex provenance’’) are characterized by common cel-lular serpentinite with few chert grains, basaltic to diabase and gab-broic grains, and by abundant shale to slate lithic grains, respectively.Dense mineral suites are dominated by either diallage or chrome spinel.

Sedimentaclastic sands ranging in composition from lithic to quartz-ofelspathic are widespread (‘‘thrust-belt provenance’’). Richest in lith-ic grains is detritus from Mesozoic platform (dominant carbonates withabundant dolostone) to pelagic (dominant limestone with abundantchert) sedimentary successions. Metamorphiclastic sands with poly-crystalline quartz, phyllite, and quartz–mica or marble lithic grainsfrom denuded core complexes are found locally (Apuane Alps). Rela-tively quartzose sands with abundant feldspars and garnet-rich dense-mineral assemblages are recycled from foredeep turbidites derived, di-rectly or indirectly, chiefly from the Alps. This signature is foreign tothe Apennine accretionary wedge, where continental basement is notstructurally involved. We calculate that one-third of the sand grainsdeposited along Tyrrhenian shores today are ultimately derived,through long-distance multistage transport and repeated recycling,from the Alps.

INTRODUCTION

Provenance models of Dickinson and co-workers in the late Seventiesto early Eighties (Dickinson and Suczek 1979; Dickinson 1985) representeda major breakthrough in clastic petrology. Combining insight and simplic-ity, they proved successful in discriminating detrital signatures of contrast-ing plate-tectonic settings at continental scale (Ingersoll 1990), includingpassive margins (‘‘continental block provenance’’) and basins associatedwith subduction of either oceanic or continental lithosphere (‘‘magmaticarc’’ versus ‘‘recycled orogen’’ provenance).

It is well known, however, that complex natural scenarios may result inincongruous petrofacies, for instance because of long-distance transfer ofdetritus from one geodynamic setting to another (Potter 1984; Velbel 1985;Dickinson 1988; Johnsson 1993). Moreover, markedly different types of

orogenic belts exist in terms of both types and volumes of rocks involved(e.g., low-relief accretionary wedges versus high-relief collision ranges;Doglioni 1992), and are consequently identified by distinct detrital signa-tures. In arc settings, where Dickinson’s models have been tested to a greatextent (Dickinson 1982; Marsaglia and Ingersoll 1992), magmatic activityand detrital record may also vary, in response to subduction geometry andtectonic regime (Doglioni et al. 1999).

Such complexities are fully recorded by sediment fill of Mediterraneanfore-arc to back-arc basins, where feldspatholithic volcano–plutonic detritusis only locally dominant, being commonly overwhelmed by sedimentary,metamorphic, or ophiolitic detritus from adjacent outer-arc ridges (Calabria,Le Pera and Critelli 1997; Hellenic arc, Saccani 1987; Cyprus, Garzanti etal. 2000). A similar situation is expected to occur in the Tyrrhenian Sea,an inter-arc basin opened in the rear of the Apennine thrust belt associatedwith steep westward subduction of Adriatic continental lithosphere andprobably oceanic Ionian lithosphere (Malinverno and Ryan 1986; Doglioniet al. 1998).

The main purpose of our study, which focuses on petrographic and min-eralogical composition of modern Tyrrhenian sands from easternmost Li-guria to northernmost Calabria (Fig. 1), is to test and implement Dickin-son’s models in a particularly complex tectonic setting, attempting a quan-titative description of volcanic and plutonic versus ophiolitic, sedimentary,metasedimentary, and recycled detrital components. The diagnostic signa-tures of detritus shed from low-relief Apennine-type accretionary wedgeswith respect to high-relief Alpine-type collision orogens or Pacific-typemagmatic arcs are investigated.

In order to cope adequately with such an arduous task, newly collecteddata on both framework-grain and dense-mineral assemblages have beenintegrated with existing high-quality analyses (Gandolfi and Paganelli1975a, 1975b, 1975c, 1984) to form a complete data base on Tyrrhenianbeach and river sands. This paper is conceived as complementary to re-search carried out with similar aims and methods in the adjacent coastalstretches of Calabria (Le Pera and Critelli 1997) and Liguria (Garzanti etal. 1998). Such detailed actualistic provenance studies are believed to rep-resent a fundamental step to upgrade petrologic models, which are, in turn,a powerful tool for unraveling the evolution of ancient thrust belts andassociated sedimentary basins.

METHODS

Sampling.—Mostly between spring 1997 and summer 1998, 110 beach,74 river, and 2 lake samples were collected along the ; 1000-km-longTyrrhenian coast from Liguria to Calabria and in the hinterland. Beachsand was sampled from the storm berm, taking care to collect representativesamples particularly in beaches exposed to strong winds, which concentratedistinct grain populations according to size, density, and form.

Framework Composition.—According to the Gazzi–Dickinson point-counting method (Ingersoll et al. 1984), 300 points were counted on 186samples. A classification scheme including over 80 categories of graintypes (including accessory, intrabasinal, and artificial components) has beendevised with the aim of recording full quantitative information on coarse-grained rock fragments. Traditional QFR parameters can easily be recal-culated from the data set obtained (Decker and Helmold 1985; Suttner andBasu 1985).

3UNRAVELING MAGMATIC AND OROGENIC PROVENANCE IN MODERN TYRRHENIAN SAND

FIG. 1.—Geographic and geologic sketch maps (after Consiglio Nazionale delle Ricerche 1990) of the Tyrrhenian side of the Apennine thrust belt. Locations of analyzedmodern sand samples are shown. FI 5 Firenze; GR 5 Grosseto; LT 5 Latina; LI 5 Livorno; MC 5 Massa–Carrara; NA 5 Napoli; PE 5 Perugia; PI 5 Pisa; SA 5Salerno; SP 5 Spezia.

Dense Minerals.—We supplemented the excellent data sets containedin Gandolfi and Paganelli (1975a, 1975b, 1975c, 1984; 135 samples) byanalysis of 76 samples from areas not covered by these authors (e.g., Tus-can archipelago, southern Italy), from major drainage basins (i.e., Arno,Ombrone, Tevere, Garigliano, Volturno, Sele), or chosen for methodolog-ical control. For each sample 200 to 400 transparent dense minerals werecounted. Dense minerals were concentrated with sodium metatungstate(density 2.9 g/cm3), using the 63–250 micron size fraction treated withhydrogen peroxide, oxalic, and chloridric (Tuscan samples) or acetic acids(other samples) to eliminate organic matter, iron oxides, and carbonates,respectively.

Key Indices.—In order to discriminate petrographic provinces and sub-provinces of modern sands (i.e., geographic areas in which consistent de-trital modes and dense-mineral assemblages document similar provenance),canonical parameters and ternary plots (QtFL, QmFLt, QmPK, QpLvm-Lsm, LmLvLs; Dickinson 1985; Ingersoll et al. 1993) are convenientlysupplemented, specifically as lithic grains are concerned, by an extendedspectrum of eight key indices, calculated according to the Gazzi–Dickinsonmethod (Table 1). Carbonate lithic fragments include marble grains, whichcould not be consistently distinguished from recrystallized sparites. Thevery few fine-grained plutonic lithic fragments (aplite, granophyre) werereapportioned to quartz and feldspars in the conventional proportion 1: 2.Further information on source rocks is provided by secondary ratio param-eters, calculated according to the traditional QFR method (only the classicP/F ratio is calculated according to the Gazzi–Dickinson method). For asynthetic description of transparent dense-mineral suites, ten key indices

have been used (Table 1). Confidence intervals for the means in triangularspace were calculated according to a statistically rigorous method devisedspecifically for unit-sum constrained data (Weltje 1998).

Unraveling Mixed Provenance.—In order to unravel mixed-sourcecompositions we used a simple, empirical forward approach (rather thanan inverse modeling strategy, as in Weltje 1995). With a complete petro-graphic and mineralogical data base at our disposal, we first identified sev-eral ideal primary sources (i.e., various magmatic, oceanic, continental-margin, and foredeep units) and assessed composition of detritus shed bythese end members. Samples from pocket beaches or minor rivers withgeologically homogeneous, small drainage basins are particularly useful inthis regard (first-order sampling scale of Ingersoll et al. 1993). We nextevaluated, by means of a computer spreadsheet, ‘‘best fit’’ contributionsfrom each primary source to major fluvial and deltaic systems (second-order to third-order sampling scales). In order to verify outcome sensitivityon assumed end-member sources, several independent trials were carriedout according to a range of different, geologically reasonable assumptions.More robust estimates were achieved by averaging the results obtained. Forthe sake of simplicity, we hypothesized that recycling of arenaceous unitsproduces framework-grain and dense-mineral suites identical to the source,which seems not too unreasonable in Mediterranean-type settings (Cavazzaet al. 1993; Garzanti et al. 1998), and we did not consider compositionalmodifications due to weathering, transport, or depositional processes. Denseminerals, being supplied by various source rocks in concentrations that maydiffer by two orders of magnitude, were treated separately. A distinct, com-

4 E. GARZANTI ET AL.

TABLE 1—Recalculated key indices for framework composition and dense-mineralsuites.

Key Indices–Framework Composition (QFL%)

Gazzi-Dickinson method

QFLvLcLpLchLmLo

quartzfeldspars and feldspathoidsaphanitic volcanic and subvolcanic lithic fragments (including diabase)carbonate lithic fragments (including marble grains)aphanitic terrigenous lithic fragments (shale, siltstone)chert lithic fragments (including cherty shale)aphanitic metamorphic lithic fragmentsaphanitic serpentine lithic fragments

Ls 5 Lc 1 Lp 1 Lch 5 total aphanitic sedimentary lithic fragmentsL 5 Lv 1 Ls 1 Lm 1 Lo 5 total aphanitic lithic fragments (crystal size , 63 microns)Q 1 F 1 L 5 total main extrabasinal framework grains (not including micas and dense minerals)

Ratio parameters–Framework Composition (%)

Gazzi–Dickinson method

P/F plagioclase (not including chessboard-albite)/total feldspars and feldspathoids

Traditional methodQp/QVm/VCd/CMb/MMp1 /MpSc/S

polycrystalline quartz (not including chert)/total quartzmicrolitic and lathwork to diabase rock fragments/total volcanic rock fragmentsdolostone rock fragments/total carbonate rock fragmentsmetabasite rock fragments/total metamorphic rock fragmentsslate grains/total metapelite rock fragmentscellular serpentinite grains/total serpentine rock fragments

Key Indices–Dense Mineral Suites (HM%)

ZTRT&ACPXOPX

ultrastable minerals (zircon, tourmaline, rutile)titanium minerals (sphene, anatase, brookite) and other mineralsamphibolesclinopyroxenesorthopyroxenes

OSLgMGtHgM

olivinespinellow-grade metamorphic minerals (epidotes, chloritoid, prehnite, pumpellyite)garnethigh-grade metamorphic minerals (staurolite, andalusite, kyanite, sillimanite)

PX 5 CPX 1 OPX 5 total pyroxenesOS 5 O 1 S 5 olivine and spinelMM 5 LgM 1 Gt 1 HgM 5 total metamorphic mineralsHM 5 ZTR 1 T& 1 A 1 PX 1 OS 1 MM 5 total transparent dense minerals (density . 2.9 g/cm3)

plementary set of provenance estimates was thus calculated, which provedto be essential in testing the overall consistency of the results obtained.

THE APENNINE THRUST BELT

Relief.—The Apennine low-relief belt is characterized by a frontal activeaccretionary wedge below sea level. The main elevated ridge, with a steeplysloping northeastern flank in regional contraction and a gently slopingsouthwestern flank in a state of regional collapse, is the result of uplift andextension (Doglioni 1994; Cavinato and DeCelles 1999). Mountains barelyreach 2000 m in Tuscany, Campania, and Lucania, and only carbonatepeaks of the Abruzzi Apennines approach 2500 m.

Drainage.—The few major rivers display markedly angular coursesclosely following the NW–SE-trending grabens developed since the latestMiocene (Mazzanti and Trevisan 1978). Larger rivers include the Tevere(405 km; 17156 km2), the Arno (241 km; 8221 km2), the Volturno (175km; 5558 km2), the Garigliano (158 km; 4992 km2), the Ombrone (161km; 3496 km2) and the Sele (64 km; 3480 km2). Water discharge is irreg-ular, with prolonged summer droughts and recurrent flooding events, par-ticularly in autumn. Mean annual discharge reaches 230 m3/s for the Tevereand 123 m3/s for the Garigliano; maximum annual values . 2000 m3/sare recorded for the Tevere and Sele, with peak discharge as high as 3500m3/s and 4100 m3/s calculated for the Ombrone and Arno rivers duringthe catastrophic flood of November 1966 (Tacconi 1994). The abundanceof easily erodible, unstable mudrocks and ‘‘chaotic complexes’’ all alongthe Apennines favors mass-flow processes and accelerated erosion, leadingto high suspended sediment discharge (5 million tons/year for the Tevere,2 million tons/year for the Arno and Ombrone; Bellotti 2000).

Climate.—The Tyrrhenian coast has temperate-warm Mediterranean cli-mates, with average temperatures from 6–108C in January to 22–288C inJuly. Precipitation, markedly concentrated in autumn and minor in summer,is 800–1000 mm/year in Campania and ; 600 mm/year in Latium, steadilyincreasing northward in Tuscany to 1100 mm/year along the Versilia coast.In the hinterland, average annual temperatures decrease with altitude to lessthan 108C in the Apennines, whereas rainfall increases to 1800–2000 mm/year in both northern Tuscany (Magra and Serchio basins) and Campania(Sele Basin), reaching 2700 mm/year in Lagonegro and as high as 3200mm/year in the Apuane Alps, where peaks of 477 mm in 15 hours wererecorded during the catastrophic event of 19 June 1996 (Rapetti and Rapetti1996). Snowfall in winter may be abundant, but glaciers are lacking. Stron-gest winds, reaching 20 knots, are from the south and subordinately fromthe west; wave heights are mostly # 4 m. Tidal ranges are low (40–60cm), and river deltas are typically wave-dominated (Bellotti et al. 1995;Bellotti et al. 1999).

Geological Outline

The Apennines are an arcuate, low-relief thrust belt accreted during steepwestward subduction of the Adria microplate since the Oligocene, whenophiolitic sequences and remnant-ocean sediments of the Liguride unitswere thrust upon the Tuscan domain, representing the outer continentalmargin of Adria (Bally et al. 1986). Apennine subduction nucleated at thefront of the Alpine retro-arc belt (Doglioni et al. 1998). Since Miocenetimes, because of eastward rollback of the subduction hinge (Malinvernoand Ryan 1986), progressively more external parts of the Adria marginexperienced rapid tectonic subsidence and turbiditic foredeep sedimenta-tion, shortly followed by incorporation within the growing accretionarywedge (Ricci Lucchi 1986).

On the back-arc side, an eastward-propagating extensional wave asso-ciated with subduction-related magmatism led to progressive boudinage ofthe former double-vergent Alpine stack of oceanic nappes (Doglioni et al.1998), followed by rift-shoulder uplift and denudation of metamorphic corecomplexes since the late Miocene (e.g., Apuane Alps; Abbate et al. 1994a;Carmignani et al. 1994) and finally to spreading of the Tyrrhenian inter-arc basin in the Pliocene (Kastens et al. 1988). During ongoing extensionalsubsidence, syn rift clastic wedges were deposited in intra-arc half grabensbeginning in late Tortonian times (Patacca et al. 1990). These basins pro-gressively widened and decreased in altitude with time, passing from al-luvial-fan to fluviatile, lacustrine, and finally marine clastic sedimentationlocally associated with lava flows and ignimbrites (Mariani and Prato 1988;Bossio et al. 1998; Cavinato and DeCelles 1999).

The Apennine belt includes ophiolitic sequences and remnant-ocean sed-iments offscraped during oceanic subduction in Late Cretaceous to Eocenetimes (‘‘alpine subduction complex’’). Continental-margin successions con-sisting largely of pelagic to platform carbonates were accreted along withthe overlying foredeep turbidites during westward continental subductionbeginning in the Oligocene (‘‘apenninic accretionary prism’’).

Liguride Oceanic Units.—Remnants of the Liguria–Piedmont Ocean,opened in the mid-Jurassic between Europe and Adria–Africa, represent thehighest structural units of the Apennines. The Internal Liguride ophioliticsequences, including serpentinized mantle peridotites associated with lim-ited gabbros and discontinuous basaltic flows, or directly overlain byophiolitic breccias, radiolarites, calpionellid limestones, and shales, are dis-tinguished from ophiolite slices, olistoliths, or olistostromes incorporatedwithin the External Liguride units (Bortolotti 1983; Abbate et al. 1994b).

Ophiolitic sequences are best exposed in the Cecina basin (Lazzarotto andMazzanti 1976; Serri 1980). Upper Cretaceous to Eocene deep-water mu-drocks and arenaceous to calcareous turbidites crop out in central Tuscanyto northern Latium and Elba Island (Sagri 1969; Aiello et al. 1977; Fontana


TABLE 2—Reference data on detrital modes and dense minerals for selected Apenninic clastic wedges, including Liguride remnant-ocean turbidites of Cretaceous–Paleogene age and main foredeep to wedge-top Tertiary turbidites of Oligo–Miocene age.

REF N Q F Lv Lc Lp Lch Lm Lo Total Qp/Q P/F Vm/V Cd/C Sc/S REF N ZTR T& A PX OS LgM Gt HgM Total

FOREDEEP TURBIDITESMacignoModinoCervarolaMarnoso-ArenaceaLaga

32211

7626154163

5448434855

2727343722

31102

007

1112

10111

00212

13211016

13100

100100100100100

282735

815

4427457139

838665——

—05

4439

83

20——

9

988

25

144630

14

2894

7

666

0

001

2

321

0

000

32

208

20

46

437255

0

03

13

100

100100100

OTHER MIOCENE UNITSMancianoLatium–AbruzziAgnone–CastelvetereAlbanellaS. MauroPollica

415555

225034156342

575446566759

291942262020

602176

114313

617

2000

030000

024

135

11

150000

100100100100100100

814

2123

353249733823

———

0—

0

—462132

——————

88

1010

277

2618

46

6985

45

11

12

00

10

00

00

00

3110

11

5164

2913

814

00

100100

100100

LIGURIDE TURBIDITESM. MorelloPietraforteTolfaMarina di CampoGhiaieto

64

77

4940

98

6738

6257

173

2735

112

21

523

10

010

10

12

10

813

67

00

00

100100

100100

2516

2232

2489

3135

—4

——

5288

——

——

——

88

137

7256

54

00

1016

00

00

1325

00

100100

Data sources (REF): 1 5 Chiocchini and Cipriani (1992); 2 5 Andreozzi and Di Giulio (1994); Di Giulio (1999); 3 5 Costa et al. (1991); Costa et al. (1992); Costa et al. (1997); 4 5 Fontana (1980, 1991); 5 5 Critelli(1987); Critelli and Le Pera (1990, 1995b); Critelli et al. (1994); 6 5 Ponzana (1993); 7 5 Aiello et al. (1977); 8 5 Civitelli et al. (1979); Civitelli et al. (1991); Civitelli and Corda (1982); 9 5 Valloni et al. (1991); 105 Cippitelli (1968). N 5 number of samples. Indices are explained in Table 1.

1991). In central Italy, Liguride ophiolitic sequences are exposed outside thestudy area close to the Lucania–Calabria boundary, whereas the oceanic or-igin of shaly allochthons in the Cilento area is uncertain (Critelli 1993).

Subliguride Units.—Widely exposed in the Apennines are deep-watersediments largely of Paleogene age, and commonly with chaotic aspect. Inthe northern Apennines, shaly to calcareous turbidites are found tectonicallysandwiched between the Macigno foredeep turbidites and the overlyingLiguride thrust sheets (Canetolo Complex; Abbate and Sagri 1970; Bettelli1985). These sediments may partly have been deposited in the oldest fore-land basin formed in front of the Alpine retro-arc thrust belt. In the centralApennines, varicolored turbiditic mudrocks occur at various structural lev-els and are interpreted as allochthonous sheets of either oceanic or conti-nental-margin origin (Mostardini and Merlini 1986; Marsella et al. 1995).

Adria Continental-Margin Units.—The few metamorphic rocks in-volved in the Apennines are discontinuously exposed along an arcuate beltfrom the Apuane Alps to southernmost Tuscany and eastern Elba. Theyinclude mostly low-grade phyllites, metapsammites, and porphyroids large-ly of Paleozoic age; in the deepest structural units polymetamorphic gneiss-es and garnet-bearing micaschists with andalusite porphyroblasts documenta thermal event around 285 Ma (Pandeli et al. 1994). Metamorphic growthof chloritoid, epidote, and locally kyanite took place in Mesozoic to Oli-gocene sediments during nappe stacking around 27 Ma (Franceschelli etal. 1986; Kligfield et al. 1986).

The Tuscan to Umbria Apennines consist of sedimentary thrust sheetsdetached along incompetent Upper Triassic evaporites. Upper Triassic–Lower Jurassic shallow-water dolostones and limestones are overlain bypelagic cherty limestones to radiolarites, and next by Cretaceous–Paleogenedeep-water varicolored mudrocks or cherty marlstones (Fazzuoli et al.1994). These basinal successions are replaced southward by the Latium–Abruzzi Mesozoic carbonate platform, representing the backbone of thecentral Apennines (Accordi et al. 1988). In the southern Apennines, theCampania–Lucania Mesozoic platform carbonates tectonically overlie pe-lagic successions of the Lagonegro basin, represented by Upper Triassic toJurassic cherty limestones and radiolarites, passing upward to Cretaceous–Paleogene varicolored clays (Miconnet 1988).

Foredeep Turbidites.—Widely exposed all along the Apennines are tur-biditic synorogenic sediments deposited in Oligo–Miocene times in wedge-top to main-foredeep depozones. Main foredeep fills display ages youngingfrom late Oligocene–early Miocene in the west (Macigno and Cervarolaformations) to middle–late Miocene (Marnoso–Arenacea Formation) andfinally latest Miocene–Pliocene in the east (Laga Formation). Their com-

position is quartzofeldspathic with common metamorphic and sedimentarylithic fragments (Gandolfi et al. 1983; Valloni and Zuffa 1984; Gandolfiand Paganelli 1993). Dense-mineral suites are dominated by garnet asso-ciated with epidote (Macigno and Cervarola formations) or staurolite andkyanite (Marnoso–Arenacea and Laga formations), documenting direct toindirect provenance chiefly from the Alps (Table 2).

Several other turbidite to shallow-marine clastic sediments were depos-ited in foredeep and wedge-top depozones perched upon Liguride thrustsheets in the Tuscan Apennines (e.g., Fontana 1980), and upon Mesozoiccarbonate platforms or deep-water mudrock successions in the central (La-tium–Abruzzi turbidites; Bellotti et al. 1984) to southern Apennines (Irpi-nian turbidites, Cilento Group; Pescatore 1988; Amore et al. 1988). Theirquartzolithic to quartzofeldspathic composition (Chiocchini and Cipriani1992; Critelli and Le Pera 1995a), with dense-mineral assemblages domi-nated by garnet and including either epidote or higher-grade metamorphicminerals, indicates ultimate supply largely from Alpine basement nappes.Depleted suites of the Cilento Group consist of ultrastable minerals withsubordinate garnet (Cippitelli 1968).

Magmatic Suites.—Subduction beneath the Apennines and opening of theTyrrhenian back-arc basin have been associated with extensive magmatismsince the late Oligocene. Geochemical and isotopic signatures suggest dom-inant contribution of Adriatic continental lithosphere for the felsic volcanicand plutonic rocks of the Tuscan magmatic province (Serri et al. 1993). Thesilica-saturated to strongly undersaturated potassic and ultrapotassic lavas ofthe Roman magmatic province were largely derived from refractory to fertilemantle sources heterogeneously hybridized and enriched in large-ion litho-phile elements by subduction of upper-crustal rocks, including variousamounts of carbonate sediments (Peccerillo 1985). Character of volcanismsouth of the 418 N lithospheric discontinuity, including the Phlegrean fieldsand Vesuvius centers, indicates mantle sources enriched during subductionof the probably oceanic Ionian lithosphere (Serri 1990).

Onset of magmatism was recorded by Oligo–Miocene calc-alkaline ba-salts to rhyolites in southern France and Sardinia, followed in the mid-Miocene by the Sisco lamproites in Corsica. Three subsequent stages re-corded discontinuous eastward migration of magmatic activity, associatedwith Tyrrhenian extension and deposition of syn rift sedimentary succes-sions (Serri et al. 1993). Monzogranites intruded at high crustal levels andassociated with aplitic dike swarms (W Elba, Montecristo; Poli 1992),along with high-K andesitic lavas (Capraia), characterized the Tuscan ar-chipelago at late Miocene times (7.5 to 6 Ma), followed by emplacementof monzo-syenogranitic stocks (Campiglia, Gavorrano, Giglio; Westerman


FIG. 2.—Petrologic provinces and key provenance terranes for both framework grains and dense minerals in modern Tyrrhenian sands. Indices are explained in Table 1.Main directions of longshore sand transport as independently assessed by petrographic and mineralogical analysis are shown, along with occurrences of allochems andartificial grains.

→

FIG. 3.—Diagnostic grains in Tyrrhenian sands. Thrust-belt source areas: A) marble (Lc) and phyllite (Lm) lithic fragments from metamorphosed continental-marginsuccessions (Versilia River); B) cellular serpentinite grains (Lo) from Liguride ophiolites (Cecina delta); C) radiolarian chert grains (Lch) from pelagic continental-marginsuccessions (Bianco River); D) quartz (Q), feldspars (F), and siltstone lithic fragments (Lp) from foredeep turbidites (Arno delta). Magmatic-arc source areas, Tuscanmagmatic province: E) microlitic lithic fragments (Lv) from high-K calc-alkaline lavas (Capraia beach); F) arkosic sand (P 5 plagioclase; K 5 K-feldspar) frommonzogranite stocks (W Elba beach). Roman magmatic province: G) sanidine (s) and sanidine-bearing lithic fragments (Fiora River); H) leucitite lithic grain (le 5 leucite;Vesuvius beach). Photos A, D, and F: crossed polars. Scale bar 250 mm long.

et al. 1993) and rhyolites (S Vincenzo, Capraia, Roccastrada) to trachy-dacites (Tolfa) in the Pliocene (5 to 2 Ma). Eventually, potassic (trachytes,latites) to ultrapotassic (leucite tephrites to phonolites and leucitites) lavasbecame dominant in the Quaternary (Roman magmatic province; mostly0.6 to 0.1 Ma). Trachydacites occur at Mt. Amiata, and strongly undersat-urated products locally characterize the Umbria Apennines farther to theeast (Peccerillo 1998).

THE ONSHORE PETROLOGIC PROVINCES

The Tuscan Superprovince

This superprovince, extending from easternmost Liguria to the Argen-tario promontory, corresponds to the arcuate belt where Tuscan continental-margin successions and Macigno turbidites are exposed, tectonically over-lain by Liguride units (Fig. 1). Modern sands, derived from multiple oro-genic sources, range in composition from lithic to quartzofeldspathic. Sev-eral petrographic provinces, characterized by alternating predominance ofrecycled foredeep (Pisa, Follonica) to Liguride oceanic detritus (Massa-Carrara, Cecina, Grosseto), are recognized (Gandolfi and Paganelli 1977,1984). Neogene–Quaternary volcano-plutonic rocks supply significant de-

tritus in the Tuscan archipelago, whereas the Mt. Amiata volcanic centerprovides hypersthene but few volcanic lithic fragments to the Grossetoprovince (Fig. 2).

Cinque Terre–Spezia Province.—Significant streams are lacking andfew pocket beaches occur along the high rocky coast of easternmost Li-guria. Detritus from Mesozoic–Paleogene Tuscan successions is mixed invarious proportions with detritus from Liguride units or Macigno turbidites,and composition ranges from predominantly terrigenous or calcareous lithicfragments to relatively quartzose. Varied dense-mineral assemblages in-clude diallage, amphiboles, and spinel from Liguride units, along with ep-idotes, garnet, minor andalusite, and chloritoid from Tuscan successionsand Macigno turbidites.

Massa–Carrara Province.—The coastal sands of the Versilia rivierawere derived largely from the Magra River, draining both Liguride unitsand Macigno turbidites. The Magra River carries quartz, feldspars, carbon-ate, shale and slate grains from both remnant-ocean and foredeep turbidites;serpentinite grains are supplied mostly by the Vara River. The Frigido andVersilia rivers carry abundant marble and phyllite to micaschist fragmentsfrom the Apuane Alps (Fig. 3A). Common serpentinite grains in beachsands indicate southward sediment drift. Metasedimentary detritus from the



Apuane Alps is subordinate even at the Frigido and Versilia river mouths.Garnet and epidote recycled largely from Macigno turbidites, associatedwith pyroxenes chiefly from the Vara River, prevail in the Magra Riversand and dominate the Frigido River sand, where significant chloritoidsuggests additional supply from Tuscan metasediments. Pyroxenes aremore abundant in beach than in river sands.

Pisa Province.—Continuous sandy beaches between Viareggio and Li-vorno are fed largely by the Arno River, draining mainly foredeep turbidites(Macigno and Cervarola formations). The Arno River carries quartz andfeldspars with subordinate sedimentary and metasedimentary lithic frag-ments (Fig. 3D). The Serchio River sand is less quartzose and includeslathwork volcanic, shale, and chert grains. Dominant garnet and epidote,with few ultrastable minerals and sphene (associated with anatase or brook-ite in the Tirrenia beaches) are recycled from foredeep turbidites. Excessepidote suggests additional supply from Tuscan metasediments. The Arnodelta is undergoing rapid erosion, and beaches to the south (Calambroneprovince of Gandolfi and Paganelli 1977) are less quartzose and containsome serpentinite and diallage grains from Liguride units exposed farthersouth, hinting at northward sediment drift.

Cecina Province.—Rocky cliffs with small pocket beaches pass south-ward to continuous sandy beaches fed mostly by the Cecina River. TheCecina River carries abundant cellular serpentinite, microsparite–sparite,shale to slate, and lathwork volcanic grains from Internal Liguride ophiol-itic sequences (Fig. 3B). Sands of minor rivers are also derived mainlyfrom Liguride units. The Cecina ophioliticlastic signature, with slight de-crease in serpentine grains and progressive increase in quartz, is recognizedin beaches to ; 25 km south of the mouth and offshore to water depths. 50 m (Leoni et al. 1991), indicating southward sediment drift. Lowquartz and common shale to slate and serpentinite grains also characterizethe northern part of the province.

Diallage dominates the dense-mineral fraction of the Cecina River andbeach sands, with subordinate hornblende and mostly brown spinel. Alsominor rivers north of the Cecina carry mainly clinopyroxene with horn-blende; garnet is locally abundant. Minor rivers south of the Cecina chieflycarry spinel, along with garnet, epidote, and a few ultrastables recycledfrom both Liguride and Macigno turbidites. Similar suites characterizebeaches between the Cecina and Fine river mouths, whereas farther northspinel gradually becomes negligible and clinopyroxene again dominant.

Follonica Province.—The low-energy Follonica gulf is characterized bycontinuous sandy beaches without major rivers. Macigno turbidites, Lig-uride units, and Pliocene magmatic rocks are exposed. The Valmaggioreand Alma river sands are relatively quartzose and much closer to beachsands than the Cornia and Pecora river sands, including abundant carbonateand terrigenous lithic fragments. Relatively quartzose composition of beachsands indicates polycyclic detritus from both coastal-plain sediments andthe Macigno Formation. Carbonate grains (including calpionellid wacke-stone) and subordinate metabasite and quartz–mica lithic fragments arederived from Liguride oceanic and Tuscan continental-margin sequences.A few felsic volcanic lithic fragments are shed from Neogene igneousrocks. Beaches north of the Piombino promontory have less quartz butsimilar dense-mineral populations (Campigliese province of Gandolfi andPaganelli 1977).

Epidote, garnet, minor staurolite, ultrastable and titanium minerals aresupplied by Tuscan sequences and both foredeep and Liguride turbidites.Spinel (more abundant in beach sands), or clinopyroxene with hornblende,are derived mostly from Liguride units. Andalusite, common in the Val-maggiore sands and decreasing in beach sands away from its mouth, comesultimately from Tuscan metasediments or from contact aureoles of Neogenestocks. Close association with tourmaline suggests, in fact, recycling ofNeogene coastal-plain sediments originally derived from eastern Elba (Mar-inelli et al. 1993). Orthite and hedenbergite, probably from contact aureolesof monzo-syenogranite intrusions (Barberi et al. 1972), are locally abundantin the Campiglia area.

Grosseto Province.—Mixed detritus from Liguride oceanic to Tuscancontinental-margin units and Macigno turbidites is found both north andsouth of Punta Ala, and in the Ombrone system (Pian d’Alma, Castiglione,and Grosseto provinces of Gandolfi and Paganelli 1977). The OmbroneRiver carries quartz, feldspars, microsparite–sparite, shale to slate, andquartz–mica metamorphic grains. The few serpentine grains are suppliedby the Merse River. The Orcia River provides largely microsparite–spariteand terrigenous lithic fragments. The Gretano River sands are relativelyquartzose. The Bruna River sands are lithic. Abundant carbonate lithic frag-ments in beaches of the Ombrone delta suggest additional supply fromrocky coasts in the south. Beach sands north of the Ombrone delta aredepleted in quartz and enriched in shale to slate grains from Liguride units;quartz and feldspars from Macigno turbidites increase north of Punta Ala.

Abundant hypersthene (supplied by the Orcia River, draining Mt. Amiatavolcanic rocks; Van Bergen et al. 1983) is associated with diallage andamphiboles from Liguride units (mainly drained by the Merse River), andwith garnet, epidote, and ultrastable minerals from orogenic turbidites andTuscan metamorphic rocks. The Bruna River carries clinopyroxene, horn-blende, garnet, and epidote from both Liguride and foredeep turbidites.Beach sands north of the Ombrone delta also include clinopyroxene, horn-blende, and garnet. Common hypersthene as far as Punta Ala indicatesnorthward sediment drift.

Albinia Province.—Sandy beaches in the low-energy Talamone gulf arefed mainly by the Albegna River, draining largely Subliguride units. Bothriver and beach sands contain abundant microsparite–sparite to pelagiclimestone, shale, slate, and chert grains from deep-water successions. Phyl-lite to quartz–mica lithic fragments suggest an additional contribution fromTuscan metasediments. Dominant clinopyroxene (augite), with hornblendeor spinel, garnet, and minor staurolite are derived from volcanic and oce-anic rocks.

Tuscan Archipelago.—The small islands of the Tuscan archipelago(Elba, the largest by far, is 224 km2) emerge from an ; 60-km-wide andmainly 100–150 m deep continental shelf, facing the over 2-km-deep nar-row Corsica basin. Rocky coasts include several pocket beaches. Sand com-position reflects contrasting geology of various islands. Neogene magmaticrocks supply dominant volcanic lithic fragments, sanidine, commonlyzoned plagioclase, augite, hypersthene, and minor brown hornblende andapatite to the volcaniclastic Capraia sand (Fig. 3E), or arkosic detritus,tourmaline, and subordinate hornblende to the plutoniclastic western Elbaand Montecristo sands (Fig. 3F). The Rio Marina lithic sand includes ser-pentine, mafic plutonic, microsparite–sparite, fine-grained terrigenous, andmetapelite to metabasite grains from Internal Liguride ophiolitic sequences(Bortolotti et al. 1994). The central Elba sands include felsitic to granitoidgrains from Neogene hypabyssal to plutonic rocks, additional quartz andfeldspars recycled from Upper Cretaceous remnant-ocean turbidites, andpolycrystalline quartz and quartz–mica lithic fragments from Tuscan me-tasedimentary rocks. Epidote, amphiboles, garnet, andalusite, and enstatiteconfirm mixed supply from Liguride units, Tuscan metasediments, or con-tact aureoles (Keller and Pialli 1990; Pertusati et al. 1993). Detritus fromUpper Triassic platform carbonates dominates the Giannutri sand, includingsanidine and volcanic lithic fragments, and is significant in the largelymonzogranite-derived quartzofeldspathic Giglio sand.

The Latium–Campania Superprovince

This superprovince, extending between the Argentario and Sorrentopromontories and corresponding with Quaternary potassic to ultrapotassicvolcanic centers of the Roman magmatic province, is characterized by vol-canic detritus and augite-dominated dense-mineral suites (Fig. 2). Volcan-iclastic sands, however, occur only at its northern and southern ends (Tar-quinia and Napoli gulfs), with sedimentary detritus prevailing elsewhere.In spite of varied geology along the Apennines, sands of major river sys-tems (Tevere, Garigliano, and Volturno) display only minor compositional


differences, reflecting recycling of Miocene turbidites with similar com-position and widespread exposures of Mesozoic carbonates.

Tarquinia Province.—Continuous sandy beaches of the Tarquinia gulfare fed by several rivers draining Quaternary volcanic fields. In beachsands, tephrite–phonolite lithic fragments, sanidine, leucite, and very lowP/F ratios (commonly , 15) reflect the potassic character of the lavas (Fig.3G). The Fiora, Arrone, and Marta river sands contain a slight excess inlocally well rounded quartz grains, hinting at recycling of coastal-plainsediments. Quartz, microsparite–sparite, and terrigenous to low-grade me-tasedimentary grains from both continental-margin and deep-water succes-sions are more significant in the northern part of the gulf (Feniglia provinceof Gandolfi and Paganelli 1984). Sedimentary detritus increases also in theMignone River and beaches at the southern end of the gulf, reflectingsupply from deep-water successions of the Tolfa area. Augite dominatesthe dense-mineral fraction, with minor garnet and locally olivine or hy-persthene (Fiora River).

Civitavecchia Province.—This mainly rocky coastal stretch includesfew pocket beaches. Microsparite–sparite to pelagic limestone, shale toslate, and commonly radiolarian chert grains from deep-water successionsof the Tolfa area are dominant; felsitic grains from the Tolfa trachydacitesoccur. Green augite from volcanic rocks is associated with amphiboles,epidote, and garnet from orogenic sources.

Roma Province.—Sands of the Tevere River and delta contain sedi-mentary (quartz, feldspars, and minor terrigenous lithic fragments fromforedeep turbidites; limestone and chert from the Umbria pelagic succes-sion) and subordinate volcanic detritus (including leucite and sanidine; P/F , 50). Sands of minor rivers are volcaniclastic. Augite is very abundant(commonly $ 35% of sand grains), with minor garnet and locally melaniteor olivine.

Latina Province.—The coast includes rocky stretches in the Anzio–Nettuno area and long sandy beaches north of the Circeo promontory,where several meter-high eolian dunes enclose coastal lakes. Sands of mostminor rivers are quartzose with negligible carbonate grains but abundantchert, suggesting recycling of coastal-plain sediments; the Moscarello Riversand is instead volcaniclastic. Beach sand is locally enriched in chert ormonocrystalline quartz and depleted in carbonate lithic fragments, sug-gesting reworking of coastal-plain to shelf sediments. Augite dominatesover garnet and melanite.

Gaeta Province.—The coast south of the Circeo promontory includeshigh cliffs where Mesozoic platform carbonates are exposed; significantrivers are absent. Beach sands include carbonate, quartz, and feldspargrains, with less volcanic detritus than in adjacent provinces (Lv # 3;augite , 10% of sand grains). Volcanic lithic fragments with microlitic tolathwork textures increase southward, along with dolostone grains; chert iscommon close to the Circeo promontory. The Vespe River carries carbon-aticlastic sand, whereas sand of the Amaseno River, largely reworkingcoastal-plain sediments, is close to beach sands. Augite dominates the densefraction; garnet and melanite are common in the Amaseno and Vespe rivers.

Minturno Province.—Sands of the Garigliano River and delta containsedimentary (carbonate lithic fragments from the Latium–Abruzzi platform,quartz and feldspars from the overlying Miocene turbidites) and subordi-nate volcanic detritus. The quartzose San Limato and feldspatholithic AuriaRiver sands are derived mainly from Neogene sandstones and Roccamon-fina volcanic rocks, respectively. Augite dominates over garnet (largelymelanite), minor epidote, spinel, kyanite, and staurolite.

Capua Province.—Sands of the Volturno River and delta consist ofquartz, feldspar, and terrigenous grains recycled mainly from Miocene tur-bidites, along with carbonate and mafic volcanic detritus. Augite is nearlythe only dense mineral, with minor hornblende and locally olivine.

Napoli Province.—Volcaniclastic detritus is dominant in the Napoligulf, where beaches are few and the Sarno delta is undergoing active ero-sion. The Sarno River carries volcanic detritus (including sanidine and leu-cite) and abundant dolostone and limestone grains from Mesozoic platform

carbonates. Beaches close to Napoli contain vitric to felsitic grains andlocally biotite from pyroclastic rocks of the Phlegrean fields. Volcanic lithicfragments from Vesuvius are dominantly mafic, with peak abundance ofleucite (Fig. 3H). Limestone and subordinate dolostone grains occur. Greenaugite, with minor green to brown and black hornblende, is associated witholivine and brown to dark brown spinel in Vesuvius sands.

The Sorrento–Policastro Superprovince

The Tyrrhenian coast from the Sorrento promontory to the Policastrogulf is framed by exposures of the Campania–Lucania Mesozoic carbonateplatform. Deep-water mudrocks overlain unconformably by Tertiary tur-bidites also are exposed widely. Quaternary volcanic rocks crop out locallyand, south of Salerno, volcanic lithic fragments are scarce but augite stilldominates dense-mineral assemblages (Fig. 2). At the southernmost end ofthe superprovince, carbonaticlastic sands include southward-increasingphyllite lithic fragments from metasedimentary rocks of the Calabrian arc(Lao province of Le Pera and Critelli 1997).

Sorrento Province.—The coast is rocky and high, with pocket beachesfed locally from Mesozoic platform carbonates and subordinate Quaternaryvolcanic rocks. Limestone grains are dominant in the west (where Creta-ceous to locally Jurassic units are exposed), and dolostone grains in theeast (where Upper Triassic dolostones crop out). Volcanic lithic fragments(including tephrite–phonolite) and dense minerals (nearly exclusive augitewith minor brown to black hornblende, spinel, and olivine) are more abun-dant in the south (# 23% of sand grains).

Salerno Province.—The continuous sandy beaches of the Salerno gulfare fed mainly by the Sele River, which drains carbonate platform to deep-water sedimentary successions and carries mainly microsparite–sparite lith-ic fragments. Commonly radiolarian chert is derived largely from the La-gonegro pelagic succession via the Bianco River (Fig. 3C), which alsocarries dolostone. Terrigenous lithic fragments are abundant in the CaloreRiver. Volcanic detritus (including leucite and sanidine) decreases south-ward. Green augite dominates over green to black hornblende, sphene, spi-nel, and diopside, suggesting supply from Quaternary volcanic and sub-ordinately oceanic rocks.

Cilento Province.—Along the high rocky coasts of the Cilento prom-ontory, quartzofeldspathic turbidites derived from unroofing of the CalabriaAlpine terrane are widely exposed (Critelli and Le Pera 1994). Larger pock-et beaches are fed by the Alento, Lambro, and Mingardo rivers, carryingrecycled quartz and feldspars but also sparite, siltstone, shale, and slate toquartz–mica lithic fragments from deep-water successions (Liguride andSicilide complexes, Cilento Group). A few volcanic, sanidine, and leucitegrains are invariably present (Solofrone River). Beach sands are enrichedin quartz and feldspars and depleted in terrigenous and slate lithic frag-ments. Beaches in the north (Agropoli subprovince) contain less quartz andmore carbonate grains (including dolostone). Green augite dominates overdiopside, hornblende, yellow-brown to brown spinel, rutile and locally ol-ivine, indicating supply from Quaternary volcanic and subordinately oce-anic rocks; depleted suites of Cilento turbidites supply few ultrastables andnegligible garnet.

Policastro Province.—Beaches in the northern Policastro gulf are fedby the Bussento and other minor rivers. The Bussento River carries abun-dant microsparite–sparite and terrigenous to slate lithic fragments derivedmainly from deep-water mudrocks. Beach sands are depleted in terrigenousto slate grains and enriched in carbonate and chert. Radiolarian chert fromthe deep-water Lagonegro succession is locally common. Few volcaniclithic fragments are present. Green augite dominates over diopside, horn-blende, spinel, and locally olivine.

Praia Province.—Mesozoic carbonate platforms are exposed along thehigh rocky coast, which includes the Noce delta. The Noce River carriesabundant carbonate grains, with shale to slate lithic fragments from thedeep-water Lagonegro succession. Dolostone and microsparite–sparite


FIG. 4.—Classic QFL plot for Tyrrhenian sands (provenance fields of Dickinson1985; modes recalculated without carbonate lithic fragments). Optimum discrimi-nation between orogenic and magmatic-arc provenance is achieved by adjusting theboundary line as indicated by numbered ticks on QF and FL legs of triangle. Oth-erwise, ophioliticlastic sands from Liguride units would plot in the magmatic-arcfield, and detritus from accreted continental-margin, Subliguride, mixed-orogenic,and foredeep-turbidite sources would straddle the boundary between magmatic-arc(MA) and recycled-orogen (RO) fields at increasing distances from the L pole. Near-ly ‘‘ideal arkoses’’ from dissected plutons of the Tuscan archipelago plot in the‘‘continental-block’’ field (CB).

grains are dominant in beach sands, containing common chert. Green augitedominates over diopside, hornblende, spinel, and locally olivine.

INTERPRETING PROVENANCE RELATIONS

Magmatic-Arc Provenance

Volcanic to locally plutonic detritus (mixed in various proportions withthrust-belt detritus; Fig. 4) reflects the relatively felsic to mafic potassic char-acter of the Neogene–Quaternary Tuscan and Roman magmatic provinces,from the Capraia Island to the Vesuvius cone and farther south (Fig. 5).

Undissected-Arc Sources.—Isolated volcanic centers in Tuscany supplypure volcaniclastic sands only at Capraia Island, where dominant microliticlithic fragments with sanidine (and lack of leucite) reflect high-K calc-alkaline sources (P/F 44; Lv 76, Vm/V 79). Volcanic detritus is minor inthe Orcia River, draining Mt. Amiata, and in the Civitavecchia province,where mainly felsitic lithic fragments are supplied by the relatively felsicproducts of the Tolfa area (Lv # 7, Vm/V 33). Dense-mineral assemblages(2–5% of the very fine to fine sand fraction) are characterized by hyper-sthene, nearly exclusive in the Orcia sands (OPX 93) or associated withaugite and minor reddish-brown hornblende at Capraia Island (OPX 41;Fig. 6). Hypersthene from Mt. Amiata is found also in the Ombrone systemdownstream of the Orcia confluence, and in the Fiora River.

The potassic volcanic fields of Latium and Campania supply abundantmicrolitic to lathwork lithic fragments, including tephrite–phonolite (Lv #69, Vm/V 60–100). Sanidine is typically the most abundant detrital feld-spar, and leucite is common. The P/F ratio is thus mostly # 45 (as low as, 15 in the Tarquinia province), contrasting markedly with classic arc-derived Circum-Pacific suites (Dickinson 1985; Marsaglia and Ingersoll1992). The ultrapotassic Vesuvius products shed abundant leucite and leu-citite lithic fragments (# 10% and # 17% of sand grains, respectively);

similar composition characterizes some minor river sands of the Roma andLatina provinces. Pyroclastic rocks around Napoli provide abundant sani-dine (17% of sand grains), felsitic lithic fragments, and minor volcanicquartz (3–4% of sand grains).

Quaternary volcanic rocks yield so much clinopyroxene that augite rep-resents 15% of sand grains on average and $ 80% of dense minerals inthe Latium–Campania superprovince (50–80% in the Sorrento–Policastrosuperprovince). Olivine and spinel (both commonly occurring in metamor-phic carbonate ejecta; Barberi and Leoni 1980) reach 5%, and hornblende7%, in the Vesuvius sands. Melanite (a titanian andradite formed by meta-somatic exchanges between potassic magmas and carbonate–evaporite sed-iments; Marinelli and Franceschini 1993) is particularly common in theGarigliano system, and locally reaches 15%. Orthopyroxenes are negligible.

Dissected-Arc Sources.—The monzogranitic stocks and aplite dikes ofthe Tuscan archipelago, considered as the roots of a latest Miocene arc,shed plutoniclastic sands with a few felsitic volcanic lithic fragments (Q45–51; F 43–53, P/F 28–37; Lv , 5), approaching ‘‘ideal arkose’’ com-position (Dickinson 1985). Tourmaline, associated with hornblende (Hb13–31), represents 52–57 % of transparent dense minerals (2–4% of thevery fine to fine sand fraction).

Subduction-Complex Provenance

Oceanic rocks tectonically incorporated into the Alpine subduction com-plex in Paleogene times (including Internal Liguride ophiolitic sequences,External Liguride remnant-ocean sediments, and Subliguride turbiditicmudrocks) shed detritus with diagnostic petrographic signatures and dense-mineral suites along most of the Tyrrhenian coast.

Ophiolitic Sequences.—Ophioliticlastic sands with Internal Liguridesignatures (e.g., Bracco province of Garzanti et al. 1998), including abun-dant cellular serpentinite grains (Lo 21–27, Sc/S 58–88), are best repre-sented in the Cecina system. Mafic volcanic to subvolcanic (basalt, dia-base), plutonic (gabbro, pyroxenite), and metamorphic (metadiabase, am-phibolite) grains (Lv # 8; Lm 8–11, Mb/M # 31) are few with respectto detritus from classic ophiolite complexes (Garzanti et al. 2000). Thisreflects the limited and irregular thickness of the Ligurian Ocean crust,formed in slowly-spreading ridge to transform settings (Abbate et al.1994b). Chert is minor (Lch , 5). Dense minerals (3% of sand grains inthe Cecina River) are dominated by diallage (CPX # 75) with subordinateamphiboles and other pyroxenes, or brown spinel.

Remnant-Ocean Sediments.—Detritus from Liguride turbidites isfound in most of the Tuscan superprovince, from Cinque Terre to ElbaIsland. Typical features include abundant shale to slate and microsparite–sparite lithic fragments from pelitic to calcareous turbidites (Lc # 40, Cd/C , 10; Lp # 53; Lm # 25, Mp1/Mp $ 60). Only arenaceous turbiditessupply abundant quartz and feldspars. Dense minerals (1–3% of the veryfine to fine sand fraction) include chrome spinel (reaching $ 40% in theCecina to Follonica provinces), garnet (Gt # 35), pyroxenes, amphiboles,and locally staurolite. A largely polycyclic origin from ophioliticlastic tur-bidites is inferred for spinel, a chemically stable and mechanically resistantmineral which is not abundant in first-cycle detritus from mantle rocks (Fig.6; Garzanti et al. 2000). Garnet, spinel, or staurolite occur in depleteddense-mineral suites of Liguride turbidites (Civitelli and Corda 1982; Pon-zana 1993; Fontana et al. 1994).

Other Turbiditic Mudrocks.—Detritus from Subliguride shaly to cal-careous turbidites characterizes limited tracts of the Tyrrhenian coast (Al-binia and Civitavecchia provinces) and a few minor rivers (e.g., Orcia). Italso occurs, mixed with thrust-belt detritus, in most of the Sorrento–Poli-castro superprovince (Sele system). Sedimentary lithic fragments are dom-inant (including limestone, shale to slate, and chert grains; Lc 30–44, Cd/C # 3; Lp 11–28; Lch 2–10; Lm # 27, Mp1/Mp . 75), with low quartzand feldspars (Q # 21, F # 9), and negligible serpentinite grains (Lo #1). Dense-mineral assemblages (, 2% of the very fine to fine sand fraction)


FIG. 5.—Various types of magmatic, orogenic, and mixed provenances for Tyrrhenian sands can be discriminated by using an extended set of petrographic key indices(explained in Table 1). Potassic volcanic sources shed Lv-rich sands with low P/F ratios due to abundant sanidine or leucite (both included in the K pole). Subduction-complex sources supply abundant shale to slate grains (remnant-ocean sediments), and either abundant (ophiolitic sequences) or negligible (Subliguride units) serpentinegrains. Accreted continental-margin sources mostly provide carbonate, and, locally, chert grains. Quartzose and feldspathic detritus from foredeep turbidites includes a fewmetamorphic lithic fragments. Ninety-percent confidence regions about the mean are calculated after Weltje (1998).

are of Liguride affinity, with clinopyroxene, hornblende, yellow-brown tobrown spinel, or garnet.

Thrust-Belt Provenance

Detritus from sedimentary successions of the Adria continental margin,including metasedimentary rocks and foredeep turbidites tectonically in-corporated into the Apennine accretionary prism since the Oligocene, isprominent all along the Tyrrhenian coast. Petrographic and mineralogicalsignatures are inevitably intermingled, and commonly mixed in variousproportions with subduction-complex or arc-derived detritus. Thrust-beltprovenance is used here as broadly equivalent to the ‘‘foreland-uplift prov-enance’’ of Dickinson and Suczek (1979).

Accreted Continental-Margin Units.—Low-grade metasedimentaryrocks (Apuane core; ‘‘epimetamorphic sources’’) supply polycrystallinequartz, phyllite to quartz–mica, and marble lithic fragments (Lc 32–59, Cd/C # 36; Lm 17–33, Mp1/Mp # 10; Frigido and Versilia rivers), along

with epidote or chloritoid (Gretano River) and higher-grade minerals (main-ly andalusite; Follonica to central–eastern Elba provinces).

Shallow-water carbonates (Latium–Abruzzi and Campania–Lucania plat-forms; ‘‘platform sources’’) supply both limestone and dolostone lithicfragments (Lc 72–86, Cd/C # 87). Pelagic sedimentary rocks (Umbria andLagonegro successions; ‘‘basinal sources’’) shed nearly exclusively lime-stone and chert grains (Lch 20–30 in the Nera and Bianco rivers, andlocally in the Latina province). Mesozoic successions provide negligibledense minerals.

Accreted Foredeep Turbidites.—Modern-sand composition is stronglyaffected by recycling of synorogenic Tertiary sandstones, supplying quartzand feldspars in similar proportions all along the Apennine belt, with minorterrigenous and metamorphic lithic fragments (e.g., Pisa and Cilento prov-inces). Recycled dense-mineral assemblages, although typically represent-ing only , 1% of sand grains, allow recognition of three distinct domains.In the Tuscan superprovince, garnet, epidote, minor sphene, and ultrastables


FIG. 6.—Dense minerals in Tyrrhenian sands are derived mostly from magmatic-arc, subduction-complex, and foredeep-turbidite sources. Undissected-arc volcanic sourcessupply common to dominant hypersthene (Tuscan magmatic province) or nearly exclusive green augite with minor olivine and spinel (Roman magmatic province); dissected-arc plutonic sources shed tourmaline and hornblende. Suites from oceanic units are either diallage-dominated (Internal Liguride units) or spinel-rich (External Liguride units;data for Bracco province and northern Apennine River sands after Garzanti et al. 1998). Foredeep turbidites shed garnet and epidote to the Tuscan superprovince (Macigno andCervarola formations) and garnet with minor staurolite and kyanite in the Latium–Campania superprovince (Marnoso–Arenacea and Latium–Abruzzi turbidites; data after Civitelliet al. 1979; Civitelli et al. 1991; Valloni et al. 1991). Suites of Volturno system to Sorrento–Policastro superprovince are garnet-poor. Modes in lower-left diagram recalculatedwithout pyroxenes. Indices are explained in Table 1. Ninety-nine-percent confidence regions about the mean are calculated after Weltje (1998).

are largely recycled from the Macigno and Cervarola formations, derived,in turn, from the Alps (Di Giulio 1999; Dinelli et al. 1999). In the Latium–Campania superprovince (as far south as the Garigliano system), nonvol-canic dense minerals include mostly garnet with minor staurolite and ky-anite from the Marnoso–Arenacea to Laga and Latium–Abruzzi turbidites(Fig. 6). In the Volturno province to Sorrento–Policastro superprovince,garnet is negligible.

Abundant quartz and feldspars in modern Tyrrhenian sands (and mainlygarnet among dense minerals) are not ultimately derived from the Apen-

nines, which do not involve basement rocks, but from the Alps. Beingrecycled through several sedimentary cycles from remnant-ocean and fore-deep turbidites fed long distances during successive development stages ofthe Alpine belt since the Late Cretaceous, they represent a spurious prov-enance signal, typical of high-relief collision orogens and not of low-reliefaccretionary wedges (‘‘collision-orogen provenance’’ of Dickinson andSuczek 1979). The Himalayas-derived Bengal Fan sands, tectonically in-corporated in the accretionary prism offshore Sumatra (Velbel 1985), canbe considered a larger-scale analog. Thick-skinned collision orogens such


as the Alps or the Himalayas, in fact, typically shed detrital volumes anorder of magnitude greater than their associated foreland basins (Doglioniet al. 1999). Abundant basement-derived detritus can thus be transportedto remote sedimentary basins (Graham et al. 1975) and ultimately incor-porated into low-relief accretionary prisms, typically associated with un-derfilled foredeeps (Doglioni 1994).

Unraveling Mixed Provenance

Modern Tyrrhenian sands are complex mixtures of detritus from severaltypes of magmatic-arc and orogenic sources. Quantitatively assessing theirrelative incidence in detail is an exceedingly difficult exercise, which wetentatively carried out with the main purpose of putting numerical con-straints on inferred provenances.

Tuscan Superprovince.—Liguride subduction-complex sources accountfor ; 85% of both framework and dense-mineral grains in the Cecinaprovince. Subliguride signatures are dominant in the Albinia province (;75%). Subduction-complex sources account for 40–55% of detritus in theCinque Terre–Spezia, Massa–Carrara, and Grosseto provinces, and for ;20% of detritus in the Pisa province, Follonica province, and Tuscan ar-chipelago. Accreted continental-margin sources are widespread, althoughgenerally subordinate (, 20%, but ; 45% in the Cinque Terre–Speziaprovince and ; 25% in the Follonica to Grosseto provinces). Accretedforedeep-turbidite sources represent 10%–60% of detritus along the Tus-cany coast, reaching 65% of framework grains and 85% of dense mineralsin the Pisa province. Volcano–plutonic detritus, significant and locallydominant in the Tuscan archipelago, is minor in mainland Tuscany (exceptfor abundant hypersthene in the Orcia River and Ombrone system).

Latium–Campania Superprovince.—Foredeep turbidites are the mainsource of framework grains in the Tevere, Garigliano, and Volturno systems(50–60%), with significant detritus from platform to pelagic continental-mar-gin successions (25–40%) and subordinately from volcanic rocks (# 15%).Subliguride sources are dominant in the Civitavecchia province (; 65%) andsignificant for the Volturno system (; 10%). Volcanic detritus is dominantin the Napoli province (; 80%) and prevalent in the Tarquinia province (;50%), where continental-margin detritus occurs in the north (; 30% in theCapalbio subprovince) and Subliguride detritus in the south (; 50% in theMignone River and delta). Everywhere but in the small Civitavecchia prov-ince, volcanic sources account for most of the dense minerals ($ 85%), withminor supply from foredeep turbidites (5–15%).

Sorrento–Policastro Superprovince.—Volcanic rocks are the dominantsource for dense minerals (. 60%). Framework grains are instead derivedfrom platform carbonates (45–75% in the Praia to Sorrento provinces),Lagonegro pelagic successions (; 20% in the Sele system), or subduction-complex sources (; 50% in the Policastro province) and recycled-turbiditesources (; 80% in the Cilento province).

IMPACT OF COASTAL DYNAMICS AND HUMAN ACTIVITIES

Sand composition does not exclusively record tectonic setting and geo-graphic distribution of source rocks even in the Apennines, where moderateriver gradients lead to limited mechanical abrasion and Mediterranean-typeclimates induce minor chemical weathering. Sampling of both river andbeach sands allows us to: (1) assess the effect of environmental processeson petrographic and mineralogical composition (Johnsson 1993); (2) pro-vide an independent way to evaluate prevalent directions of longshore sed-iment transport (Fig. 2); (3) test the impact of artificial beach nourishmentand other anthropic activities; and (4) verify recognition of extrabasinallimestone versus intrabasinal allochems (Zuffa 1985). Correct identificationof artificial and intrabasinal grains, which is seldom straightforward, isneeded to avoid gross mistakes in provenance evaluations.

Composition of River Versus Beach Sands

Composition of major river and beach sands within the same province(second-order and third-order sampling scales; Critelli et al. 1997) compareclosely all along the Tyrrhenian coast, reflecting limited wave energy cou-pled with short residence time of detritus on the narrow continental shelvesof such a tectonically active region. The most significant difference, apartfrom calcareous allochems, concerns metasedimentary and terrigenous lith-ic fragments (Table 3).

Intrabasinal Grains.—Bioclasts (bivalves, echinoids, coralline algae,benthic foraminifers, gastropods, bryozoans, or other encrusting organisms)and rare oolitic rims represent 0.7% of grains in beach sands on average,reaching 5–7% in pocket beaches of the Tuscan archipelago or where beachrock is eroded along the shore. Remains of fresh-water mollusks, or marinebenthos recycled from coastal-plain sediments, are found sporadically inriver sands (0.2% on average). Noncarbonate intrabasinal grains (soil clasts,collophane bones) represent 0.5% of sand grains in rivers and 0.4% inbeaches.

Durability of Lithic Grains.—Slate lithic fragments, and to a lesserextent other foliated metapelite to metafelsite and shale–siltstone lithic frag-ments, are systematically depleted in Tyrrhenian beaches (Massa–Carrarato Praia provinces). They thus appear to be selectively destroyed by waveaction in high-energy coastal environments (Picard and McBride 1993).Concentration of durable grains (monocrystalline quartz, and to a lesserextent polycrystalline quartz, chert, and feldspars) at the expense of me-tasedimentary and terrigenous lithic fragments is most evident in beachsands of the Sorrento–Policastro superprovince, where it is partly ascribedto recycling of coastal dunes.

Limited durability is also indicated for volcanic lithic fragments, whichdecrease downstream in the Tevere River in Latium (Lv 18 to 6) and arenegligible in the Orcia river sand ; 40 km downstream of Mt. Amiata.Volcanic grains are not depleted in beach sands, owing to proximity ofvolcanic fields to the Latium and Campania coasts.

Serpentine and carbonate grains are equally abundant in river and beachsands. Carbonate lithic fragments, although locally depleted because ofrecycling of older coastal deposits (Latina province), or directly suppliedto beach sediments by erosion of coastal cliffs (Grosseto province), thusprove to be relatively durable in Mediterranean-type climates (Arribas etal. 2000).

Human Activities and Sand Composition

Since Etruscan and Roman times, progradation of Tyrrhenian deltas tookplace primarily as a response to population growth, agricultural develop-ment, and intense deforestation (Innocenti and Pranzini 1993). River deltasbegan to experience cusp erosion in the second half of the nineteenth cen-tury, when human activities (reclamation works, harbors, jetties, sandpits,river dams, and check dams) strongly altered natural equilibria and signif-icantly changed sedimentary budgets. The process continues today at ratesapproaching even 20 m/year in some places (Cocco and De Pippo 1988;Cipriani and Pranzini 1999).

Beach Nourishments.—Coastal erosion, affecting about half of Tyrrhe-nian beaches from Tuscany to Campania, was controlled mostly with con-struction of hard structures (seawalls, breakwaters, groins), which in mostcases have merely shifted erosional processes downdrift. In fewer casesbeach nourishment was preferred, which, although local sediment is com-monly used (e.g., Cipriani et al. 1999), may cause serious bias in actualisticprovenance studies. Good correspondence between river and beach sands,however, shows that volumes of foreign materials are not important enoughto affect compositional results significantly. Marble wastes from local quar-ries and factories, for instance, have been discharged extensively in riversand used for beach nourishment in the Massa–Carrara province (E. Pran-zini, personal communication 1999), but without detectable effects on sandcomposition. Major perturbations may occur in pocket beaches with high


TAB

LE3—

Det

rita

lmod

esan

dde

nse-

min

eral

asse

mbl

ages

ofm

oder

nTy

rrhe

nian

sand

s.

NQ

FLv

LcLp

Lch

LmLo

Tota

lQ

p/Q

P/F

Vm

/VC

d/C

Mb/

MSc

/Sm

HM

%N

ZTR

T&A

CPX

OPX

OS

LgM

Gt

HgM

Tota

l

SUPE

RPR

OV

INC

ETU

SCA

NR

iver

s

Bea

ches

30 46

29 11 31 15

19 8 20 12

2 2 5 11

18 12 20 18

13 8 10 11

2 2 2 2

13 5 7 5

4 6 4 6

100

100

50 9 44 15

42 10 42 17

46 35 38 28

5 9 4 9

3 5 6 9

68 17 60 18

0% 1% 0% 1%

34 70

9 9 7 10

4 4 3 3

9 6 11 8

23 23 31 25

8 19 9 15

6 10 5 8

17 12 16 15

23 19 17 13

1 2 1 3

100

100

LATI

UM

–CA

MPA

NIA

Riv

ers

Bea

ches

31 38

28 13 22 12

20 8 24 8

10 10 15 18

32 15 26 12

4 4 5 5

3 5 5 6

3 3 2 3

0 0 0 0

100

100

21 17 38 17

46 18 34 16

65 25 66 24

15 19 4 6

3 4 1 1

— —

7% 7% 14%

17%

30 55

1 2 1 2

1 1 1 1

2 1 2 2

72 25 85 13

1 2 1 2

1 2 1 2

5 8 2 4

16 16 8 7

0 1 0 1

100

100

SOR

REN

TO–P

OLI

CA

STR

OR

iver

s

Bea

ches

13 26

24 9 31 20

12 6 16 8

2 2 4 5

33 17 36 26

12 6 6 5

5 7 4 4

12 10 4 4

0 0 0 0

100

100

42 18 33 13

38 12 43 9

51 24 73 45

10 17 23 25

0 0 0 1

— —

1% 1% 6% 8%

9 13

3 4 1 1

2 2 1 1

9 2 10 3

81 6 80 7

0 0 0 0

3 3 6 5

1 1 0 1

1 2 1 1

0 0 0 0

100

100

MA

INPR

OV

EN

AN

CE

MA

GM

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C-A

RC

UN

DIS

SEC

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scan

mag

mat

icpr

ovin

ce1

220

762

00

00

100

—44

79—

——

2%2

0 00 0

4 126 32

68 350 0

1 00 0

0 010

0

UN

DIS

SEC

TED

AR

CR

oman

mag

mat

icpr

ovin

ceD

ISSE

CTE

DA

RC

Tusc

anM

agm

atic

Prov

ince

4 5

4 2 47 3

30 8 47 6

56 12 2 1

7 6 1 1

2 1 1 1

0 0 0 0

0 0 1 1

0 0 0 0

100

100

0 0 29 9

45 18 32 3

89 12 —

12 13 —

— —

— —

16%

13% 1% 0%

13 2

1 1 56 2

0 0 5 5

4 1 24 7

89 7 8 6

1 1 1 1

2 3 1 2

2 4 2 0

2 2 1 2

0 0 1 1

100

100

SUB

DU

CTI

ON

-CO

MPL

EXO

PHO

LITE

SEQ

UEN

CES

REM

NA

NT-

OC

EAN

SED

IMEN

TS

SUB

LIG

UR

IDE

UN

ITS

5 7 5

14 5 20 11 16 3

14 2 11 5 6 3

4 3 3 5 2 1

20 9 19 10 36 6

15 10 27 15 24 5

2 1 2 2 5 2

10 2 14 9 11 4

22 4 3 2 1 1

100

100

100

51 10 44 8 50 10

62 8 58 20 36 14

50 27 — —

0 0 4 7 0 1

14 12 2 4 4 5

69 13 — —

1% 1% 0% 0% 0% 1%

7 8 3

0 0 5 3 7 6

1 1 2 2 6 3

12 2 11 6 9 7

68 9 30 18 49 15

2 4 2 3 0 0

6 5 23 14 3 5

3 1 10 7 2 1

7 5 17 9 23 6

0 0 1 2 1 1

100

100

100

THR

UST

-BEL

TC

ON

TIN

ENTA

L-M

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GIN

UN

ITS

MET

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DIM

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RY

UN

ITS

FOR

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PTU

RB

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2 2 26

1 1 18 1 48 9

1 2 4 0 23 6

1 2 1 0 2 2

75 15 45 19 11 5

4 5 3 3 6 5

17 15 0 0 1 1

0 0 25 11 8 4

0 0 4 3 1 1

100

100

100

— 53 20 40 11

— — 40 10

— — 11 9

3 5 18 23 6 9

— 0 0 4 6

— — —

0% 0% 0% 0% 1% 2%15

— — 7 2

— — 6 21 1

— — 7 4

— — 1 1

— — 1 1

— — 36 11

— — 41 13

— — 0 010

0

Key

indi

ces

and

ratio

para

met

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(exp

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edin

Tabl

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are

show

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Sorr

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tourist value (e.g., Talamone beach, nourished with volcanic sand from thenorthern Tarquinia gulf).

Changes Related to Coastal Erosion.—Anthropic modifications of thenatural environment affect sand composition mostly indirectly and in thelong term. Effects are therefore complex and difficult to assess. In theMassa–Carrara province, construction of the Carrara harbor in the early1920s has interrupted continuity of southward longshore transport from theMagra River to the Versilia coast; river dams and sediment extraction forbuilding purposes have reduced sediment supply further. Nonetheless, orig-inal compositional signatures can be recognized as far south as Forte deiMarmi beach, where abundant pyroxenes and serpentinite lithic fragmentsindicate supply from Internal Liguride ophiolites in past times, prior todamming of the Vara River.

Construction of hard defenses aimed at reducing the erosive power ofincoming waves also affects sand composition. In the Casalvelino beach,highly protected with breakwaters to control erosion of the Alento delta,enrichment in durable grains at the expense of terrigenous and slate lithicfragments is much less evident than in other beaches of the Cilento coast,exposed to strong southwesterly winds.

Dense minerals, concentrated by hydraulic processes in proportion totheir density, are sensitive indicators of coastal erosion (‘‘erosional plac-ers’’; Bellotti et al. 1994; Frihy et al. 1995). They reach up to 40% of sandgrains in the Tevere and Garigliano deltaic cusps, and locally as high as75% in the Latium–Campania superprovince. Maximum values of all ultra-dense species (garnet, melanite, zircon, rutile, spinel) are recorded in theNettuno beach (Gandolfi and Paganelli 1984). Opaque minerals, spinel andgarnet are concentrated with respect to other, less dense minerals in thedeltaic cusp of the Sele River and decrease exponentially with distancealong shore. A similar trend is observed for the Cecina delta.

Artificial Grains.—Direct, more easily identified anthropic effects arerelated to dumping of artificial solid particles in rivers and along the coast,beginning with mining in Etruscan and Roman times and continuing withmodern industrial activities. Artificial grains are generally minor compo-nents of Tyrrhenian sands (0.4% in rivers, reaching 2% in the Tevere sandat Roma; 0.8% in beaches), but macroscopic exceptions do occur. Mois-sanite and corindone, used as abrasives in the marble industry, represent5% of dense minerals in the Frigido River sand (Gandolfi and Paganelli1975a). Glassy slag fragments from iron-mining wastes are ; 2% of sandgrains in the Follonica Gulf and 9% in the Giglio beach. Opaque mineralsfrom mine tailings reach 46% of sand grains at Rio Marina (Elba). Fine-sand calcite spars, released at sea since 1918 by the Rosignano Solvaybicarbonate chemical plant (180,000–300,000 tons/year; Pranzini 1978;personal communication 1999), represent $ 50% of the Vada beach sand;accumulation of artificial calcite can be traced for nearly 10 km offshore,to water depths over 50 m (Leoni et al. 1991).

CONCLUSIONS

In this actualistic provenance study, we present detailed petrographic andmineralogical data on a geologically complex region, characterized by sev-eral tectonic regimes superposed during ; 100 My of convergence betweenthe Adria–Africa and European continents. All types of magmatic (‘‘un-dissected-’’ to ‘‘dissected-arc’’) and orogenic detrital signatures (‘‘subduc-tion-complex’’, ‘‘foreland-uplift’’, ‘‘collision-orogen’’ provenances ofDickinson and Suczek 1979) are recognized in modern sands depositedalong the Tyrrhenian coast, which thus represents an excellent testingground for Dickinson’s petrologic models.

Undissected-arc detritus is here characterized by distinctive features (lowto very low P/F ratios, common sanidine, leucite and tephrite–phonolitelithic fragments), not documented previously from Pacific-type arc settingsand reflecting undersaturated potassic magmatism in an intra-arc basin re-lated to steep westward subduction (Doglioni et al. 1999). Green augite isassociated with oxyhornblende and either locally dominant hypersthene

(Tuscan magmatic province) or minor olivine and spinel (Roman magmaticprovince). Dissected-arc provenance is reflected by nearly ideal arkosiccomposition, with tourmaline and hornblende.

Detrital orogenic signatures are chiefly dependent on tectonic style anddepth of decollement surfaces in the orogenic wedge, as highlighted bysharply contrasting composition of detritus from thin-skinned Apennine-type versus thick-skinned Alpine-type belts. The Alpine subduction com-plex includes ophiolite sequences and remnant-ocean sediments (‘‘subduc-tion-complex provenance’’). The former shed serpentinite lithic fragmentsand diallage, with amphiboles and enstatite; the latter supply shale to slate,microsparite–sparite lithic fragments, and locally abundant chrome spinel.The Apenninic accretionary prism includes sedimentary and minor meta-sedimentary rocks (‘‘thrust-belt provenance’’). Accreted continental-marginunits shed dominant carbonate fragments: platform successions supply bothlimestone and dolostone, pelagic successions limestone and chert. Meta-morphic core complexes provide polycrystalline quartz, metamorphic lithicfragments (phyllite, marble), and dense minerals (epidote, chloritoid, an-dalusite). Foredeep-turbidite wedges fed long-distance from the Alps rep-resent a major additional source of detritus, shedding quartz, feldspars, andfew metamorphic lithic fragments; garnet is associated with epidote andsphene in Tuscany or minor staurolite and kyanite in Latium. Such grains,ultimately shed by a thick-skinned belt (‘‘collision-orogen provenance’’),convey a foreign provenance signal in Apennine-derived sands.

The Tyrrhenian case confirms the effectiveness of Dickinson’s approachbut indicates the necessity of amendments as regards: (1) ophioliticlasticdetritus, plotting in the ‘‘magmatic-arc’’ field of QFL diagrams (unlessboundary lines are adjusted as in Fig. 4), and (2) chert-rich detritus, typicalof outer continental-margin (‘‘thrust-belt provenance’’) rather than oceanicsources (‘‘subduction-complex provenance’’). Dickinson’s models can beupgraded by superseding a purely ternary logic and by using an extendedset of key indices to describe the varied lithic types found in orogenicsediments. Successful discrimination of contrasting orogenic signatures canthus be achieved. The resolution power of provenance analysis can be fur-ther improved substantially if key complementary information from dense-mineral assemblages is taken into full account.

Detailing the geographic distribution of onshore Tyrrhenian provincesprovides a suitable basis to understand coastal dynamics, thus supplyinginformation for littoral-management plans and to assess entry points ofdetritus and dispersal pathways of deep-sea sediments. The magmatic andorogenic provenance signatures here recognized may represent a key ref-erence for interpreting sandstone petrofacies in ancient, Mediterranean-typeconvergent margins.

ACKNOWLEDGMENTS

We warmly thank the various groups of researchers who kindly offered support toour Tyrrhenian sand project. E. Pandeli, L.E. Cipriani, E. Pranzini, F. Pelliccia, andC. Bartolini (Universita di Firenze and Regione Toscana) gave invaluable informationon rock units and coasts of Tuscany; P. Bellotti, P. Tortora, and S. Evangelista (Univ-ersita di Roma) collected samples of the Ombrone system and willingly shared theirknowledge on the Tevere system; M.R. Senatore (Universita di Benevento) suppliedsamples and stratigraphic data on the Sele coastal plain; S. Critelli and E. Le Pera(IRPI-CNR) provided samples, publications, and point-counting data on sands andsandstones of northern Calabria to southern Campania. Invaluable insight on sedimen-tary processes along the Tyrrhenian coasts was obtained from numerous speakers andparticipants of Seminars of Coastal Dynamics held at Parma (1995), Lerici (1996), andSapri (1998). The paper has benefited greatly from helpful, detailed reviews by R.V.Ingersoll, S. Critelli and M. Johnsson, valuable comments and suggestions by E. Pan-deli and P. Bellotti, and enlightening discussions with C. Doglioni. A. Di Giulio andG. Gosso kindly provided point-counting data on Tertiary foredeep turbidites, andinformation on Tuscan metamorphic rocks, respectively. G. Groppelli collected theCapraia sample. G.G. Zuffa and F. Serra gave valuable advice, and A. Coltura and M.Russo generous help, for dense-mineral analysis. C. Malinverno and M. Minoli pro-duced excellent thin sections and drawings.

Two tables have been archived and are available in digital form at the WorldData Center-A for Marine Geology and Geophysics, NOAA/NGDC, 325 Broadway,


Boulder, CO 80303; (phone: 303-497-6339; fax: 303-497-6513; e-mail:[email protected]; URL:http://www.ngdc.noaa.gov/mgg/sepm/jsr/.) TableA1 is entitled Main Framework Composition and provenances of Modern TyrrhenianSands, and Table A2 is entitled Dense Mineral Suites and Provenance Types forModern Tyrrhenian Sands.

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Received 2 July 2000; accepted 18 May 2001.

Documents

Unraveling Magmatic and Orogenic Provenance in Modern Sand: The Back-Arc Side of the Apennine Thrust Belt, Italy