Per. Mineral. (2002), 71, 1,49-63

PERIODICO di MINERALOGIAestablished in 1930

http://go.to/permin

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGY

and applied topics on Environment, Archaeometry and Cultural Heritage

Volcaniclastic layers in upper Triassic-Jurassic deep-sea sedimentsfrom the Lucanian Apennine, southern Italy:

mineralogy, geochemistry, and palaeotectonic implications

PAOLA DI LE01*, SALVATORE Ivo GIAN02 and MARCELLO SCHIATTARELLA3

I Istituto di Ricerca sulle Argille, C.N.R., Tito Scalo (PZ), Italy.2 Dipartimento di Strutture, Geotecnica e Geologia Applicata all'Ingegneria, Universita della Basilicata, Potenza, Italy

3 Dipartimento di Scienze Geologiche, Universita della Basilicata, Potenza, Italy.

Submitted, July 2001 - Accepted, December 2001

ABSTRACT. - In complex mountain chains suchas the southern Apennines, analysis of ancientvolcanics may provide new information on reliablepalaeotectonic scenarios. In the lower part of theScisti silicei Formation (upper Triassic-Jurassic,Lagonegro units) outcropping in the Agri Valley,Lucanian Apennine, volcaniclastic layers werefound interbedded with pelagic sediments, showingfacies changes over a narrow area. These layers areprevalently composed of quartz and feldspar.Compared with the UCC (Upper Continental Crust),the volcaniclastic beds are depleted in both large-ionlithophile (Rb, Sr) and high field strength (Ti, Nb,Hf, Zr, Th, RE) elements, and are strongly enrichedin first-row transition elements (Mn, Ni, Cu) and CeoTheir volcaniclastic origin is clearly evidenced bythe discrimination diagram based on first-rowtransition elements V, Cr and Ni and immobileelements Zr and Ti. The source rocks of thesevolcaniclastic layers were probably of dacitic­rhyolitic to rhyolitic composition, as suggested bychemical evidence. The chemical analyses ofunaltered plagioclase, indicating albitic composition,are consistent with such a source. The distributionand concentration of REE are typical of andesiticmagmas. The REE chondrite-normalized patterns ofthe sampled layers are similar to reported REE

* Corresponding author, E-mail: pdileo@ira.pz.cnr.it* Present address: I.M.A.A. - C.N.R., 85050 Tito Scalo

(Potenza), Italy

patterns of arc-related rhyolitic ignimbrites. The Ti­Y-Zr ternary plot and Y vs Nb and Y+Nb vs Rbdiscrimination diagrams proposed for felsics alsoindicate the affinity of the volcaniclastic layers witharc-related rhyolitic tephra in subacqueousenvironments. This suggests a more complexpalaeogeographic scenario than the classic view,according to which the Lagonegro deep-sea basinwas generated in an extensional regime and borderedby normal faults during the entire Mesozoic tectono­sedimentary evolution. A continental transform zoneable to produce local transpressional structures andto emulate the magmatic context of collisionalmargins may be envisaged as partly responsible forthe basin evolution in a general context ofextensional tectonics. At shallower structural level,the transform fault system may have controlled theTriassic-Jurassic facies pattern of the Lagonegrobasin whereas, at depth, such anisotropy probablyinfluenced the crustal and magmatic evolution ofthis part of the Adria palaeomargin.

RIASSUNTO. - In orogeni complessi e con altivalori di raccorciamento come I' Appenninomeridionale, l'analisi di vulcaniti 0 vulcanoclastitiantiche puo permettere di fare luce sui possibiliscenari paleogeografici e paleotettonici. Nellaporzione basale della Formazione degli Scisti silicei(Trias superiore - Giurassico) affiorante in alta Vald' Agri (Appennino lucano) sono stati rinvenutidiversi livelli vulcanoclastici intercalati a pelagiti,

50 P. OJ LEO, S.L GIANO and M. SCHJATIARELLA

che mostrano decisi cambiamenti di facies in ambitigeografici piuttosto ristretti. Tali livelli sonocostituiti in prevalenza da quarzo e feldspati. I livellivulcanoclastici risultano, se confrontati con la UCC(Upper Continental Crust), impoveriti sia in Rb e Srche in elementi ad alta forza di campo (Ti, Nb, Hf,Zr, Th e Terre Rare) mentre sono arricchiti in alcunimetalli di transizione (Mn, Ni, Cu) nonche in CeoL'origine vulcanoclastica dei livelli e chiaramenteevidenziata nel diagramma di discriminazione basatosui metalli di transizione V, Cr, Ni e sugli elementiimmobili Zr e Ti. I dati chimici suggeriscono che levulcaniti dal cui smantellamento derivano i livellivulcanoclastici avevano, con molta probabilita,composizione da dacitico-riolitica a riolitica. Leanalisi chimiche di plagioclasi inalterati rinvenutinei livelli vulcanoclastici mostrano unacomposizione albitica in accordo con l'ipotesi di unaroccia sorgente acida. Anche la distribuzione deglielementi delle Terre Rare etipica di magmi di naturaandesitica. I pattern delle Terre Rare, normalizzatiaIle condriti, mostrano forti similitudini con i patterndi ignimbriti di composizione riolitica associate adambiente di arco vulcanico. Diversi diagrammi didiscriminazione, su base geochimica, dell'ambientegeotettonico (Ti-Y-Zr, Y vs Nb, Y+Nb vs Rb) hannoforni to indicazioni relative all' appartenenza deilivelli vulcanoclastici a contesti sinorogenici di arcovulcanico. Poiche la storia geologica della regionepermette di escludere una tale ipotesi, unitamente alretaggio dell' orogenesi ercinica, troppo lontana neltempo, gli autori propongono che la messa in postadelle vulcaniti acide - il cui smantellamento erosivoha poi fomito il materiale costituente i livelli studiati- sia stata dominata da una zona di trasferimentotettonico in ambiente continentale con localicondizioni transpressive legate a flessi, riseghe 0

disposizioni en echelon delle faglie principali. Isistemi transpressivi sono infatti spes so intrusi damagmi acidi a causa del gradiente vertic ale dipressione lungo le zone di taglio orizzontale e dellalora capacita di spingere verso l' alto le massemagmatiche granitiche in sovrappressione. Talicondizioni avrebbero permesso di emulare i caratterimagrnatici di margini collisionali nel piu generalecontesto estensionale del paleomargine mesozoicoapulo-africano. Una simile anisotropia potrebbeinoltre essere responsabile della peculiaredistribuzione delle facies altotriassiche­infragiurassiche del bacino di Lagonegro.

KEY WORDS: mineralogy, geochemistry,palaeo tectonics, volcaniclastic horizons,Mesozoic, Lagonegro basin, southern Apennines(Italy).

INTRODUCTION AND REGIONAL OUTLINES

The pre-existence of major palaeotectonicstructures affecting the Tethyan passivemargins is often invoked to explain somepresent-day features of the peri-Mediterraneanorogenic belts, although this is not always easyto prove in complex structural patterns. As amatter of fact, the restoration of the Mesozoictectonic setting of the south-Apennines segmentof the African passive palaeomargin isproblematic, since the chain is strongly affectedby Neogene contractional and Quaternarystrike-slip and extensional tectonics. Theoccurrence of ancient volcanic layers insedimentary successions is one of the mostuseful keys to better definition of thepalaeotectonic picture and the geodynamicpalaeoenvironment of a greatly shortenedwedge like the southern Apennines. In thelower part of the Scisti silicei Formation (upperTriassic-Jurassic, Lagonegro units) outcroppingnear the village of Paterno in the high valley ofthe Agri River, Lucanian Apennine, we foundfor the first time three volcaniclastic layersinterbedded with pelagic sediments, whichwere examined by means of geological,mineralogical and geochemical approaches.Recently, Di Girolamo et al. (2000) foundupper Triassic andesitic tuffites in the shallow­water carbonates of the Calabrian CoastalRange (southern Italy). According to theseauthors, the geological and petrologicalfeatures of these rocks suggest a geodynamicscenario related to the consumption of palaeo­Tethyan lithosphere by subduction. TheRhaetian age assigned by the authors to thesefelsic tuffites is close to the age of thevolcaniclastic rocks examined in this paper (seenext section).

The southern Apennines are a NE-vergingfold-and-thrust belt, mainly derived from thedeformation of the African-Apulian passivemargin (<<Adriatic microplate» or westernAdria, sensu D' Argenio et al., 1980). Thepalaeomargin included the Lagonegro basin,generated by continental rifting since middleTriassic times (Scandone, 1975; Wood, 1981).

Volcaniclastic layers in upper Triassic-Jurassic deep-sea sediments ... 51

The Lagonegro successions identify a 40-km­wide belt limited by Meso-Cenozoic shallow­water carbonates (Fig. I). The generalstratigraphy may be summarised as follows(Pescatore et al., 1999): i) Triassic toOligocene pre-orogenic successions of theLagonegro basin; ii) upper Oligocene-lowerMiocene siliciclastic successions of theNumidian basin; iii) Langhian to Tortoniansyntectonic successions of the Irpinian basin.

The most ancient rocks of the Lagonegropre-orogenic successions are made up ofshallow-water siliciclastic sediments,organogenic limestones and, towards the top,siliciclastic deposits, testifying progressivedeepening of the basin (Monte Facito Fm,lower-middle Triassic). The overlying pelagicsuccession is characterised by predominantcarbonate sedimentation up to the late Triassic(Caleari con selee Fm, cherty limestone), laterreplaced by Jurassic siliceous sedimentation(Seisti silieei Fm, mostly radiolarite and chert).

During Cretaceous times, turbiditicsedimentation took place (Galestri Fm,siliceous marl and shale), followed bycalcareous-clastic gravity flows interbeddedwith reddish marl and shale (<<Flysch Rosso»Auet.), indicating a substantial increase intectonic activity along platform-basinboundaries, which produced uplift and erosionof shelf margins between the late Cretaceousand the Oligocene.

The chain was accreted from late Oligoceneto Pleistocene times, and is composed of aMesozoic-Cenozoic sedimentary cover fromquite different palaeogeographic domains (i.e.,Ligurian oceanic crust and western passivemargin of the Adriatic plate), and of theNeogene-Pleistocene piggyback basin andforedeep deposits of the active margin. Theaverage trend of the chain axis is about NI50°,corresponding to the strike of the main thrustsand coaxial normal faults. Shortening of up to100% is estimated for the southern Apennine

TYRRHENIAN SEA

oI

30kmI

Fig. 1 - Geological sketch-map of southern Apennines. 1. Plio-Quaternary clastics and Quaternary position of volcaniclasticlayers from Scisti silicei Fm; 2. Miocene syntectonic deposits; 3. Cretaceous to Oligocene ophiolite-bearing internal units(Ligurian units); 4. Meso-Cenozoic shallow-water carbonates of the Apenninic platform; 5. Lower-middle Triassic to upperMiocene shallow-water and deep-sea successions of the Lagonegro basin; 6. Meso-Cenozoic shallow-water carbonates ofthe Apulian platform; 7. Thrust front of the chain; 8. Volcanoes. In the frame: the high valley of the Agri River.

52 P. DI LEO, S.l. GIANO and M. SCHIATTARELLA

wedge, excluding the internal deformation ofthe Ligurian ophiolite-bearing units(Schiattarella et al., 1997). The belt is alsoaffected by Plio-Quaternary strike-slip faults,mainly oriented along NI20o±10° and N50-60°trends (Schiattarella, 1998).

GEOLOGY OF AGRI VALLEY

AND DESCRIPTION OF SAMPLING SITE

The high valley of the river Agri is a wideNW-SE trending intermontane basin located inthe Lucanian Apennines (Fig. 1). It formedduring Quaternary times along the axial zone ofthe chain after the major Miocene-Plioceneshortening and is filled with mid-Pleistocenealluvial deposits. Tectonics has stronglycontrolled the shape, morphology andsedimentary evolution of the basin up to thepresent, as testified by earthquakes such as thatof 1857 and the occurrence of recentpalaeosoils involved in faulting (Giano et al.,2000). On the basis of recent structural studies(Giano et al., 1997; Schiattarella et al., 1998),the valley appears to be a basin more complexthan an extensional graben (Ortolani et al.,1992).

The pre-Quaternary bedrock (Fig. 2) iscomposed of Mesozoic-Cenozoic shallow­water and slope carbonates (Monte Marzano ­Monti della Maddalena Unit, Bonardi et al.,1988), prevalently outcropping along thewestern side of the basin, thrust over coevalpelagic successions (Lagonegro units,Scandone, 1967) which outcrop mainly alongthe eastern flank of the valley. The southernand eastern parts of the high valley areoccupied by Tertiary siliciclastic sediments(Albidona Fm and Gorgoglione Flysch,Carbone et al., 1991).

The volcaniclastic layers come from thelower part of the Scisti silicei Fm (Lagonegrounits) outcropping in the Agri Valley close tothe village of Paterno, in the footwall of themain thrust which brought the shallow-watercarbonates on to the pelagic successions of theLagonegro basin. The basal member of that

formation marks the passage from upperTriassic to Jurassic (Miconnet, 1988) andshows facies and thickness changes over anarrow area. The layers are interbedded withmarly pelagic sediments belonging to theArmizzone facies (Scandone, 1972), whereas amore proximal succession outcrops a fewkilometres north of the sampling site tPignola­Abriola facies).

The lowest volcaniclastic layer, grey to lightbrown in colour, is about 12-15 em thick. It is afine-depleted and poorly cemented coarse­grained sandy deposit (medium ash). Its grainsize suggests a relatively proximal source. Theintermediate thin layer (3-4 em thick) is formedof incoherent dark ash. The highest layer isslightly thicker than the intermediate one and iscomposed of dark brown silty sand (fine ash).The detailed stratigraphy of the volcanic­bearing succession is sketched in Fig. 3.

METHODS AND TECHNIQUES

The mineralogy of bulk samples and the <2urn grain-size fraction were determined byXRD (Siemens D5000, cs«, radiation,graphite secondary monochromator, samplespinner) and distribution of mineralogicalcomponents was evaluated according toLaviano (1986). To estimate illite crystallinity,a known amount of the <2 urn grain-sizefraction was crushed in a hand mortar and thentransferred to a plastic container for ultrasonictreatment for 2-3 minutes. After settling, thesuspension was decanted, pipetted, and dried atroom temperature on glass slides to produce athin-layer, well-oriented aggregate with aparticle density of at least 3 mg/cm-. The illitecrystallinity index (IC), defined by the width ofthe first-order basal reflection (10 A) at half­height above the background (Kubler, 1967),was measured on both air-dried and ethylen­glycol solvated slides, using analyticaltechniques and grain-size recommendations aswell as calibration with interlaboratorystandards, as suggested by Warr and Rice(1994).

Volcaniclastic layers in upper Triassic-Jurassic deep-sea sediments ... 53

o1

NORTH

(l)Skm1

CD QUATERNARY DEPOSITS

f·:·.::··:·/··:·.J MIOCENE SILICICLASTICS

~ ALB/DONA FORMATION

I$B~ LIGURIAN OCEANIC UNITS

CARBONATE PLATFORM

LAGONEGRO UNITS

~~ FAULT

Fig. 2 - Geological map of Agri high valley.

Major elements, .Ba, Rb,Sr, Y, Zr,Nb,V,Cr, Ni, Co, Cu, Zn, Hf and Pb were determinedby X-ray fluorescence spectrometry (XRF).Estimated precision for trace element analysesare better than 5%, except for those elementsfound in quantities lower than 10 ppm (10­15%). La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, Th, Uand Sc were determined by InstrumentalNeutron Activation (INA), their precisionbeing better than 5%, with the exception of Yband Lu (better than 7%).

Plagioclase analyses were performed using aJEOL JXA-8600 electron microprobe equipped

with 4,...WD spectrometers (Vaggelli et al.,1999).

MINERALOGY AND GEOCHEMISTRY

The mineralogical and chemical compositionof the layers interbedded with sediments fromthe Scisti silicei Fm sampled in the Agri Valleynear Paterno revealed a volcanogenic nature.X-ray diffraction patterns show the presence oflarge amounts of primary minerals such as

54 P. DI LEO,S.1. GIANO and M. SCHIATTARELLA

f Oii"'••-"'--.-"-.'"I••••••••• .,;;

[~:·:~::.:3

VOLCANICLASTIC LEVELS

MASSIVEMARLYCLAY

LAMINATED SILTYCLAY

CARBON·RICH SILTY CLAY

Pav3

/1

..................J~ - -.- ---..- -.:;.~.

i · -,,::.: -.. "'.:"' -.. ;[ - -..- - ].

o

50 cm (-:::::::::::::::::::.",

- - - --

~ j«0::::>""')

IIIIIII

(,) I

~ ~a:t­o:Wa.a.:::>

"SCISTI SILICEI" Fm- CHERTY UPPER MEMBER

E§.j "SCISTISILICEI" Fm - MARLYBASALMEMBER

F·::.;.l "CALCARl CONSELCE" Fm- CHERTY LIMESTONE

Fig. 3 - Measured stratigraphic section bearing volcaniclastic layers found in basal part of Scisti silicei Fm. Left:stratigraphic section showing part of Lagonegro deep-sea succession.

Volcaniclastic layers in upper Triassic-Jurassic deep-sea sediments ... 55

:E Phy(37%vvt)

\

qz-(55 % wt)

2 5 10 15 20 25 30 35

Fig. 4 - XRD patterns of bulk samples. Most abundant mineralogical phase observed in all samples is quartz. Plagioclaseand small amounts of alkali feldspar also occur.

quartz and plagioclase (Table 1, Fig. 4).Observed clay minerals are largely dominatedby diagenetic illite (average IC =1.34; Kubler,1967; Warr and Rice, 1994), with only a littlekaolinite (Fig. 5).

The mineralogical and chemical compositionobserved in the volcaniclastic layers from theScisti silicei Fm reveal both primary charactersand diagenetic modifications. Although therewere considerable variations in local andregional conditions, estimates of temperatures

of - 100-130 °C and a burial depth of 4-5 Kmare considered to be the probable conditions forthe basal portion of the Scisti silicei Fm wherethe volcaniclastic layers were found, asevaluated by Di Leo et al. (2002) using illitecrystallinity, illite contents in liS mixed layersand white mica polytypes. The abovetemperatures are sufficient to have causedrecrystallisation of original clay minerals andformation of diagenetic illite (Hower et al.,1976) and accounts for the lack in smectite

56 P. DI LEO,S.1. GIANO and M. SCHIATTARELLA

Qz =quartz; S Phy =clay minerals; PI =plagioclase; K-f =K-feldspar.

Fig. 5 - XRD patterns of air-dried oriented amounts of the< 2 /lm fraction. Most abundant clay mineral is illite (I).Kaolinite (K) also occurs, although in small amounts.

100

o Pav3• Pav1a• Pav1bo Pav2• Pav4ao Pav4b

10

Si02/AI203

.",Biogenic silica-rich shalesfrom Scisti silicei Fm (Oi Leoet al., 2002)

PAAS /0 0,iii 0

100 I

90

80

N

0 70Ci5

60

50

401

Fig. 6 - Si02 vs Si02/Al203 diagram (modified after DiLeo et aI., 2002).

the range 60-82 wt % and MnO in the range0.94-2.87 wt %. The high Si02 observed insome samples may be a consequence of passiveenrichment in this element during sedimentationand/or diagenesis (Fig. 6). Compared with theUCC (Upper Continental Crust), the volcaniclasticbeds from the Agri high valley are depleted inboth large-ion lithophile (Rb, Sr) and high fieldstrength (Ti, Nb, Hf, Zr, Th, RE).. elements, .andare strongly enriched in first-row transitionelements (Mn, Ni, Cu) and Ceo

Geochemical comparisons of these sampleswith sediments belonging to the basal memberof the Scisti silicei Fm (Di Leo et al., 2002)give insights into their nature and origin. Thevolcaniclastic nature of the layers in the lowerpart of Scisti silicei Fm is clearly evidenced bythe discrimination diagram betweentransformed volcaniclastic and «normal»sediments (Andreozzi et al., 1996), based onfirst-row transition elements V, Cr and Ni and

10 15 20 25 30 352

TABLE 1

Mineralogical composition (wt %)ofbulk samples.

Qz L:Phy PI K-f

Pav3 55 37 6 2PavIa 81 9 9 1Pavlb 81 9 9 1Pav2 50 22 27 1Pav4a 83 5 10 2Pav4b 55 40 5 1

usually expected as a product of earlydiagenetic volcanic glass alteration.

The chemical composition and someelemental ratios of the volcaniclastic layers areshown in Table 2. Overall, the samples havesmall amounts of Na20+K20 (0.56-1.3 wt %)and A120 3 (3.51-12.58 wt %) as well as Si02 in

Volcaniclastic layers in upper Triassic-Jurassic deep-sea sediments ... 57

TABLE 2

Chemical composition ojvolcaniclastic beds. Major elements, Ba, Rb, Sr, Y; Zr, Nb, V, Cr, Ni, Co,Cu. Zn, Ga, HI, Pb and S determined by X-ray fluorescence spectrometry. Estimated precision for

trace element determinations better than 5%, except[or elements in quantities lower than 10 ppm (10-15%). La, Ce, Nd, Sm. Eu, Tb, Yb, Lu, Th, U and Sc determined by instrumental neutron activation.

Estimated precision (except[or Yb and Lu) better than 5%; that jor Yb and Lu better than 7%.

Pav3 PavIa Pavlb Pav2 Pav4a Pav4b

s.o, 60.68 72.38 75.47 63.78 81.48 62.29'no, 0.3 0.28 0.23 0.35 0.35 0.37Alz03 11.22 3.51 4.67 9.4 5.62 12.58FeZ03 6.56 7.6 5.93 7.02 4.03 4.49MnO 2.72 4.52 3.45 2.87 1.46 0.94MgO 0.78 0.1 0.21 0.63 0.87 1.13CaO 0.18 0.15 0.18 0.32 0.28 0.38NazO 0.03 0.44 0.4 0.32 0.58 0.39KzO 1.04 0.12 0.19 0.85 0.33 0.91PzOs 0.11 0.14 0.09 0.12 0.12 0.11L.O.I. 16.38 10.77 9.2 14.35 4.89 16.41Ba 346 375 448 454 213 250Rb 77 23 25 64 24 79Sr 41 33 31 43 45 59Y 34 17 20 37 35 57Zr 132 164 104 168 102 116Nb 16 10 8 15 5 16V 69 45 43 67 44 67Cr 54 30 37 52 28 55Ni 66 33 84 67 37 59Sc 11 9 6 10 5 11Co 17 18 10 14 6 7Cu 70 98 127 102 11 27Zn 69 57 68 45 47 62Hf 1.1 2.1 0.9 1.7 1.1 1.7Th 3.7 3.5 2.1 4.4 2.3 5.3U 1.4 2 1.6 1.1 0.8 1.1Pb 170 36 30 30 11 7La 14 11.2 8.9 12.8 13.3 16.1Ce 163 143 95 99 51 63Nd 23 14 10 16 15 15Sm 7.8 4.5 3 4.5 3.9 3.4Eu 1.7 1 0.8 1.1 0.9 0.9Tb 1.2 0.6 0 0.7 0 0.5Yb 2.4 1.5 1.2 1.8 1.3 1.7Lu 0.34 0.2 0.18 0.25 0.19 0.26

Ti/AI 0.03 0.08 0.05 0.04 0.06 0.03(LalYb)ch 3.9 5 5 4.8 6.9 6.4Eu/Eu* 0.67 0.71 0.75 0.83Ce/Ce* 4.7 5.6 4.8 3.4 1.7 1.8

(La/Ybj.j, = (LaILach)/(YblYbch); Eu/Eu* = EUchN(Smch • Gd ch)' Gdch calculated as (2/3Tbch + 1/3 Smch); Ce/Ce* =(3Ce/Cech)/(2La/Lach+ Nd/Nd.j). Condrite values from Taylor and McLennan (1985).

58 P. Dr LEO, S.1. GrANo and M. SCHrATTARELLA

immobile elements Zr and Ti (Fig. 7).Geochemical discrimination betweenvolcaniclastic and terrigenous sediments isparticularly effective if «immobile» elementsand transition metal data are combined: theformer are mainly controlled by primarydifferences, the latter are more stronglyinfluenced by diagenetic reactions whichmodified original distributions. Discriminationbetween terrigenous sediments andvolcaniclastic sediments may be possible usingtransition elements such as V, Fe, Cr, Zn andNi (Andreozzi et al., 1996), which areparticularly sensitive to variations in redoxconditions (V, Fe, Cr), affinity for organicmatter (V, Fe, Ni, Zn) and adsorption processeson clay minerals (Zn, Ni, Cr, V). They aremobilized during diagenetic transformation,such as degradation or modification of organicmatter, formation of redox fronts (Pearce andJarvis, 1995, and references therein), and claymineral recrystallisation (Hower et al., 1976).

For these reasons, they are suitable forhighlighting differences between sediments ofdifferent origins, especially when combinedwith immobile elements. Fig. 7 shows thegenetic differences, mostly indicated by theZr/Ti0 2 ratio (which may help to definemagmatic source compositions for lithified ashlayers) and post-depositional transformations.

Information about the magmatic sources ofvolcaniclastic layers from the Scisti silicei Fmcomes from the classification diagram ofWinchester and Floyd (1976), based on low­mobility element ratios (Fig. 8). The studiedsamples fall on the subalkaline side of thediagram, in the dacite-rhyodacite and rhyolitefields. The chemical composition of unalteredplagioclase, showing albitic composition, areconsistent with a rhyolitic magmatic source(Table 3).

Of the trace elements, Rare Earth Elements(REE) are the least mobile during alteration,reflecting the original chemical composition ofvolcaniclastic sediments and volcanic ash

Volcaniclastic sediments

0.1No

i=I::::N

0.01

Terrigenous sediments

NotN

0.1

0.01

RhyodaciteDacite• • Trachy-Andesite

Alkaline Basalt• volcaniclastic levelsfromScistisiliceiFmo shalesfromScistisiliceiFm (OiLeoet aI.,2002)

Subalkaline Basalt

0.1

100010 100

(V+Ni+Cr)/AI203

0.001 +-~~~,.,.,-~~~-r-rrr-~~~1

Fig. 7 - Discrimination diagram between transformedvolcaniclastic and «normal» sediments (from Andreozzi etal., 1996). Non-volcaniclastic sediments used forcomparison are shales from basal member of Scisti siliceiFm (Pignola section, Di Leo et al., 2002).

NbN

Fig. 8 - Classification diagram of volcaniclastic layersfrom Scisti silicei Fm (Winchester and Floyd, 1976).

Volcaniclastic layers in upper Triassic-Jurassic deep-sea sediments ... 59

TABLE 3 (McLennan et al., 1980). The REE chondrite-

Electron microprobe analyses ofplagioclase normalized patterns (Taylor and McLennan,

from volcaniclastic layers. 1985) of the sampled layers (Fig. 9) showLREE/HREE fractionation (Lan/Yb n = 5.3 ±

Pay 2 Pay 2 Pay3 Pay3 Pay4a1.1), negative Eu anomaly (Eu/Eu* = 0.74 ±0.07) and positive Ce anomaly (Ce/Ce* = 3.66± 1.64). The distribution and concentration of

Si02 69.54 68.97 69.54 69.33 73.03 REE are typical of andesitic magma. The REETi02 0.04 0.02 0.03 0.06 0.02 patterns are similar to those reported for arc-

A120 3 20.11 20.35 21 20.81 21.16 related rhyolitic ignimbrites (Ikeda et al., 1980,Leo et al., 1980; Masuda, 1980) and altered ash

FeO 0.07 0.1 0.2 0.09 0.03 layers from the Indonesian volcanic arcMoO 0.06 0.04 0.03 0.11 0.05 (Martin-Barajas and Lallier-Verges, 1993).

MgO 0 0 0 0 0 A peculiar feature of the REE patterns from

CaO 0 0.03 0.04 0.03 0.02the studied layers is the positive Ce anomaly.Although REE are reported to be affected

Na20 10.92 10.39 9.6 8.92 8.36 during diagenesis in the sea bottom (De Baar etK20 0 0 0.01 0.01 0.02 al., 1987), this modification mainly occurs in

reducing marine conditions. Therefore, a

Total 100.75 99.93 100.52 99.38 102.68positive Ce anomaly may suggest the effects ofa hydrogenous phase (Fleet, 1984). Amorphous

(/)

2'C'"CCo

...co.........(/)(])

a..Ero(/)

1000

100

10

oPav3-PavtaoPav1b·Pav2APav4at,Pav4b

00 Sumatra rhyolitic ignimbritesrm Altered ash layers from central Indian Basin

I I

La CeI I I

Nd SmEuI

TbI I

Yb Lu

Fig. 9 - Chondrite-normalized REE distribution pattern of volcaniclastic layers from Scisti silicei Fm. REE pattern fieldsare rhyolitic ignimbrites from Sumatra (Ikeda et al., 1980; Leo et al., 1980; Masuda, 1980) and altered ash layers fromcentral Indian Basin (Martin-Barajas and Lallier-Verges, 1993).

60 P. 01 LEO, S.1. GlANO and M. SCHIATIARELLA

Fe-Mn oxy-hydroxides and other authigenicminerals are considered responsible for positiveCe anomalies in pelagic environments (Piper,1974; Toyoda and Masuda, 1991), as Mn andCe have a similar oxidation mechanism andvery similar oxidation kinetics in sea-water.Mn-oxides, as consequence of both associatedbacteria mediation and surface chemicalproperties, act as scavenging agents for Ce4+

and determine its fractionation with respect toother REE (see Fig. 9).

In conclusion, the volcaniclastic layers foundin the basal portion of the Scisti silicei Fmshow peculiar compositional features whichdifferentiate them considerably from othervolcaniclastic horizons observed so far in thesouthern Apennines.

y

Fig. 10 - Ti-Y -Zr ternary diagram (modified afterYamamoto et al., 1986) showing composition ofvolcaniclastic layers from Scisti silicei Fm.

PALAEOTECTONIC IMPLICAnONS

Fig. 11 Y vs Nb and Y+Nb vs Rb tectonic discrimationdiagrams for felsites (after Pearce et al., 1984, and Twistand Harmer, 1987).

1000

Ocean-ridge

100

Y+Nb

10

•••10

1000

100

Nb10

11 10 100 1000

Y

Rb

100

For more specific constraints on thepalaeotectonic setting associated with thevolcanic source of the volcaniclastic beds fromthe Scisti silicei Fm, the Ti-Y-Zr ternarydiagram (modified after Yamamoto et al.,1986) and the Y vs Nb and Y--Nb vs Rbdiscrimination diagrams proposed for granites(Pearce et al., 1984), which also proved to beuseful for felsic volcanic rocks (Twist andHarmer, 1987), were applied.

Al and Ti contents are useful parameters foridentifying the tectonic setting of volcaniclasticsediments (e.g., Hein and Scholl, 1978;Yamamoto et al., 1986). The TilAl ratios of thesampled volcaniclastic layers (Table 2),ranging from 0.03 to 0.08, are indeedconsistent with other TilAl ratios of volcanicash and bentonite layers attributed to arcvolcanism (Martin-Barajas and Lallier-Verges,1993, and references therein). In the Ti-Y-Zrternary diagram, the volcaniclastic samples plotin the area of arc-related rhyolitic tephra insubaqueous settings (Fig. 10). In both Y vs Nband Y--Nb vs Rb diagrams, the samples fall inthe field of volcanic-arc felsites (Fig. 11), incontrast with the classical view, according towhich the Lagonegro basin was generated in an

Volcaniclastic layers in upper Triassic-Jurassic deep-sea sediments ...

transfer fault

61

Fig. 12 - Block diagram showing possible Mesozoic palaeotectonic scenario of a portion of African-Apulian palaeomargin.Acronyms: CLP: Campania-Lucania Platform; LB: Lagonegro Basin; AP: Apulian Platform.

extensional regime and bordered by normalfaults during the entire Mesozoic tectono­sedimentary evolution.

On the above basis, a more complexpalaeotectonic setting of the deep-sea basinmay be hypothesized. The presence of a syn­collisional volcanic arc during the Triassic­Jurassic transition must be ruled out, since theLagonegro basin is close to the boundary ofdivergent plates and included in the Africanpassive margin. Also, the effects of theHercynian orogeny, too distant in time to havehad an appreciable influence on magmaticevolution, may be neglected. It is thereforesuggested that a different geodynamicenvironment characterised the Mesozoicscenario. A continental transform zone able toproduce local transpressional structures and toemulate the magmatic features of collisionalmargins may be envisaged as partly responsiblefor the basin evolution, in a more generalcontext of extensional tectonics (Fig. 12).Transpressional systems are in fact oftenintruded by magmas (D'Lemos et al., 1992),because of the steep pressure gradients invertical strike-slip shear zones and their abilityto force magma upwards. Such dynamicconditions cause some magma overpressuring,

which in turn expels granitic magma upwards,following the vertical pressure gradient (SaintBlanquat et al., 1998).

At a shallower structural level, the transformfault system may have controlled the Triassic­Jurassic facies pattern of the Lagonegro basinwhereas, at depth, such an anisotropy probablyinfluenced the crustal and magmatic evolutionof that part of the Adria palaeomargin.

ACKNOWLEDGEMENTS

This study was funded by the Institute ofResearch on Clays of the Italian National ResearchCouncil. Dr. Giovanni Mongelli is thanked for hishelpful comments and critical reading of an earlyversion of the paper. Professor Sandro Conticelli andthe «Centro di Studio per la Minerogenesi e laGeochimica Applicata» at the Italian NationalResearch Council in Florence are kindlyacknowledged for performing quantitative electronmicroprobe analyses.

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