R£:-Imcu-...:n UELLA !'oclt;TA 1TALl .... N.... DI MlNER.-\LOG1A E l'ETROLOGIA, 1985, Vu!. 40, pp. 73·106

Geochronology of volcanic rocks from Latium (Italy)

MARIO FORNASERl

Istituto di Geochirnica dell'Universita, Citta Universit:nia, Piazza Aldo Moro, 0018.5 ROffiaCentro di Studio per la Geocronologia e la Geochimica delle Formazioni Recenti del CN.R.

ABSTRACT. - The age determination data forvolcanic rocks from Latium (haly) are reviewed.This paper reports the geochronological data obtainedchefly by the Ar-K t~hnique, but also by Rb-Sr,""'rh, "C and fission tI"1lcks methods.

The Latium region comprises rocks belonging tothe acidic volcanic groups of Tolfa, Ceriti and Man.ziana districlS and to Mt. Cimino group, havingstrong magmatic affinity with the Tuscan magmaticprovince and the rocks of the Roman ComagmaticRegion. lbe last one encompasses the Vulsinian,Vicoan, Sabatinian volcanoes, the Alban Hills andrhe volcanoes of the Valle del Sacco, often referredto as Mts. Ernid volcanoes, all distinguished by apronounced alkaline'-potassic character. .

To the Latium region belong also the' volcanicislands of Ponza, Palmarola, Zannone, Ventotene,and S. Stefano (the Ponza archipelago), locatedat the matgin of the Tyrrhenian trench, themagmatic activity of which evolved from an addic10 an alkaline-potassic character_

More than 180 age determinations lire availableat present and the general framework of thegeochronology of the volcanic activity in Latiumis fairly well established, even if contradictoryresults are still present.

The volcanic activity in the Tolfa, Cerili andManziana districts covered a time span from 4.2to 1.8 Ma and the present data allow to assigntheir magmatic activity to the upper Pliocenerather to the lower Pliocene as stated before.Wide range variations are shown by the ages ofignimbrites and lavas belonging to the same sectorand the age measurements are not always inagreement with the field evidence. Problems con­fl(~cted with inheritance and loss of radiogenicargon make it difficult to enter in chronologicaldetail.

The activity of Mt. Cimino volcano can belocated pr~isely enough from 1.27 to 0.94 Ma.

The oldest known pt<xlucts of Vulsinian activityappear fairly old, reaching 860 ka, but the mostfrequently dated samples cluster in the range from130 to 500 ka. The lower limit of 5.5 ka needsfurther confirmation,

The dating of rocks from Vico volcano indicateage values ranging from 820 to 95 ka. To theignimbtite C, often referred 10 as «tufo rosso ascorie nere"., one of the typical prooucts of thevicoan activity, an age of 420 ka may be reasonablyassigned.

A few reliable age measurements arc availablefor the Sabatini volcanoes, rather uniformely scat·tert-d between 607 and 85 ka. The "tufo rosso ascorie nere,. from the sabatian region, which isthe analogue of the ignimbrite C from Vico has afirmly established age of 442 + 7 ka. This formationcan be considered an impor-tant marker not onlyfor the tephrochronology but also, more generally,for the Quaternary deposits in Latium.

Taking into account all data in the literaturethe oldest known product of the Alban Hills showan age of 706 ka, but more recent measurementsindicate for these pt<xluclS a mol'C recent age{4oo kal. One of the pyroclastic unilS of the AlbanHills, the «Tufo di Villa Senni» has been datedwith extremely good accuracy lit 338 ± 8 ka andit is 10 be considered as the best reference termfor the Alban Hills chronology, A lower age limitfor the activity of the Alban Hills volcano is notyet precisely established, even there are dearsignes of a recent age (40-50,000 years B.P.) forthe last phrcalO·magmatic explosions. •

The whole chronology of the Alban Hills needsfurther consideration owing the many inconsistenciesin the published data.

The K·Ar ages obtained for the Hercinian districtrange from 700 ± 20 to 80 ± 40 ka and do notpose particular problems,

Contradictory results are also preSent regardingthe activity in the Ponza Archipelago, whichdeveloped from 4.7 to less than 0.15 Ma. On thewhole to the acidic volcanism in this group aPliocene age can be assigned. During the lowerPleistocene, stllrting from 1.75 Ma a pronouncedbreak in the volcanic activity occurred, whichstarted again at about 1.1·1.2 Ma with theemplacement of products of alkaline-potassic af.finity_

The data reported represent the results of almosl20 years of study in dating the young volrnnicsof Lalium. Starting from the first measurementsmany improvements have been achieved but at thesame time it turned uut that the general situationis far to be satisfactory due to the persistent dif·ficulties in dating young volcanic rocks.

The causes of the discl'Cpancies are discussedand the need of further work is stressed.

Key Ulord$; Italy, Latium, volcanic rocks, gea·chronology, radiometric age.

M, FORNASERI

RI"SSUN'TO. _ Vengono p2SS8te in rassegna ledatazioni delle roctt vulC2niche del Lnio. QucstOlavoro riporta i dlni onenuti prevalentemente conil melodo polassio-arxon, ma anche con i rnetodidd robidio-montio, dd C2rbonio-l4 e delle lraccedi fissione,

La regione lazi:l.le comprendc le f'OC'tt appane.nani .i &NPpi vulC2nici acidi dei complessi Tolfe.tano, Cerile e Manzi.te ed al groppo cid M. ·G­mino, nenli una pronuncialll affiniti magmalialcon la proviocia Tosama e le I'lJn"t: deJla Provinci.Comagmatica Romana. Quesl'ultima comprende ivulcani Vulsini, il vulamo di Vico, i vulcani Sal»·tini, i Calli Albani ed i vulCllfli ddla Valle ddSICCO (o Valle Latin.), spesso denominali aochevulcani degli Ernici, lUui catlllleriuati <la unoSpiCClIO catlltlere alcalino-polassico.

All. regione lui.le .ppartengono anche le isoledell'ardpelago Pontino: Ponza, Palm.rol., unnone,Ventotene, Santo Stefano, situate al margine dellafossa tirrenica, la cui atlivit3 si ~ andata evolvendoda un carattere addo a tipi alcalino-polassid.

Pill tli 180 determinationi di eta sono oggi dispo­nibili e si pue ritenere che il quadro gcnerllle dellllgeocronologia dell'atdvitii. ...ulcanica nel Lazio siaabbaSlantll bene st,bilito aoche se sussistono ancondati conltaddittori e molti pun!i oscuri.

L'altivitA vu1cllnica nei complessi Tolfelaoo, Ce­rite e Mantiate copre un imervallo di lempo da4,2 a 1,8 Ma cd in base ai dati allua.li ~ possibileamibuire quesla attivila .1 Plioa:ne superiore piut­tosto che .1 Pliocenc: inferiore come riu~nuto inpreccdenza. Notevoli variuioni si OS5en!'OO fn leeu. delle I....e e delle ignimbriti ,oche .p~nencnti

allo Slesso 5euore e le misure di eti non sonosernpre in ICCOroo con le osservuioni di eampagna.Diveni problemi connessi con eccessi di .rgonradiocenico 0 • Sllm"S$ive perdite rendono difficilese non impossibile una ricostnWane crooolosica diden.glio.

L',ui ...ili vuleaniC2 nel gruppo del M. Gmioopub esscre localinata .bbastarua bene fn. 1,27 e0,94 Ma.

I prodoni piu vecchi deIJ"ltivitA dei vukanivulsini sembrano piuttosto anlichi, essendo Slateregistn.te eli fino a 860 ka, ma i campioni pillfrequentemenle dalali si addensano fra i '00 cd i130 ka. JI limite inferiore di " ka ne<:essita diulteriore conferma.

Le datuioni dei prodotti del vu1cano di Vicoindicano valori di elll compresi fra 820 e 9' kil.Alia ignimbrile C, nOlil anche come «tufo rossa asoorie nere., uno dei prodotti tipid dell'attivillldi Vieo, si pull ragionevolmeme assegnare un'elAdi 420 ka.

Poche determinui;::;:li attcndibili di eti si hannoper i vulcani S.batini: le ela sono dimibuite piut­IOStO uniformemente fra 607 e 8' k•. 11 «tufoI'O$SO a S(.'Orie ne~. dell. rq;ione sabuil, chc ~

r.nalOJO della ignimbrite C di Vico, ha un'ela,molto ben dcfinil', di 442 + 7 k•. Quest. forma·tione pub C$5e~ considen.ra- un imponante livelloguid. non solo per la tdrocrooologi. m., piugenenlmenle, per I. cronoqi. dei dcpositi qU.ler­nari del T...uio.

Se si prendono in considcrazione lutti i d,desi5lenli in lelleratura per I. rq;ione wlcanio.

dei Colli Albani, le prime grandi effusioni Iilvichedovrebbero avere un'ela di 706 kil, ma misurepiu recenli indicano per questi prodoni un'eli piugiavane: (400 k.). Una delle formazi.oni pirodastichcdei Calli Albani, il «Iufo di Villa Senni .. ~ Sllltadatalll con grandc accuraloza e la sua eta diJJ8 ± 8 k. C05tiluisce un imporl.missimo lermine:di riferimento per la crooologia di questo wlcano.Un limite inferio~ per l'attiviti: vuk:miCll nc:i ColliAlbani non si ~ .neon porulO stabili~ con preci­sione. Vi sono pero chi.ri indizi di ;,m'ed abba­Stanxa recenle (~.'O.OOO anni B.P.) per le ultimeeruzioni frealomagtnltiche. Tuna la crooologi. deI­I'aniviti nei Calli Albani de"e essere revisKmataa causa dell. 5Cana alleooibilita di moIli datisinon pubblicati.

Le eta K·Ar Ollenute per i vulcani dcgli Ernicisono compresc fn 700 ± 20 e 80 ± 40 ka e nonpongono problemi di inlerprelazione.

Qualche dis.1CCOrdo si pue osserv.re andle perquamo riguarda le datazioni delle rocce dell'arci­pelago Pomino. Ndl'insieme si pue condudc:re cheil vukanismo acido in questo gruppo sia di etApliocenica. con initio intorno a 4,7 Ma. Dur.nteil Pleistoccne inferiore, a partire da l,n Ma si ~verificata una nelta interrutione nell'auiviti vu1c.·nica, che ha ripreso a dra l,l·I,2 Ma con lamcssa in posto di prodoui ad affinili alcalino­porassica.

I dali sinon discussi nppresent.no il risuhllOdi ora 20 .nni di rittrChe imese allo studio crono­Iogico delle f'OC'tt vuleaniche dc:I Lazio. A panirewile prime misure molti progressi sono slari rnliz­uti ma allo Slesso lempo molte discrepanze sonoemc:rsc C051 cbe I. silUuione genenle e lungidall'esscre soddisfacenle a causa delle persislentidifficolti inc:rmli Ill. dalazione di roccc vulcanichegiovani.

Vengooo discusse akune delle ClIUSC di talid~parue co viene sotto!il1('l.ta la necessiti di uherioriricen:be.

PlUoI~ tbillw: Itali., Luio, rocce vulcaniche,geocronologia, eta ndiomenict..

Inlroducli(m

Volcanic rocks play ~ considerable role inthe geology of Latium. Of the 17184 Km2

of his area 4922 Km2 are i,e. 28.6 % arecovered by volcanic products.

The volcanic acrivity developed from thelate Pliocene 'up to some .50,000 years agoand is related to lhe Quaternary tensivetectonics which followed the plicative tec­tonics of the early Neogene, leading to thewell known complex series of Apennineoverthrusts. For an exhaustive discussionconcerning the slructural evolution and lhepetrogenesis reference should be made 10

the work of Loc.ARDI el al. (1976).Two. distinct series can be recognized

among the Lttium volcanites. 1be first one

GEOCHRONOLOGY OF VOLCANIC ROCKS FROM LATIUM (ITALY)

TERN_/~- ...

/ ', '\ I \,\ .... / RIET1 ,----. \

\ ,,,,,/

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FiB. I. - The volcanic BrouPS of u{ium.

75

encompasses the acidic volcanoes of Tolfa.Ceriti and Manziate districts and of Mc Ci­mino; the second the Vulsinian, Vicoao,Sabatinian volcanoes, the volcanic region ofthe Alban Hills (often referred to as « LatianVolcano.) and the volcan~ of the ValIedel Sacco (also called Vulcani della ValleLatina or Mts Ernici volcanoes).

The 6rst series has a strong petrographicaffinity with the Tuscan magmatic proviott,the anatectic nature of which has been widelyrecognized. The second is characterized by apronounced albline-potassic character and islinked to the Campania alkaline magmatic

activity (Roccamon6na, Phlegraean Fields,and Vesuvius).

Also the islands of the « Ardpelago Pon­tino.: Palmarola, Ponza, Ventotene. SantoStefano, belong to the Latium region. Theseislands are located at the margins ofthe Tyrrhenian trench and their magmaticactivity, developing from the Pliocene upto late Quaternary, evolved from an acidicto an alkaline potassic character.

BARBERI et al. (1967) pointed out thatthe 87Sr/86Sr ratios measured in the productsof the Pontine islands fall in a very narrowrange and cluster around an average value

76 M. FORNASERI

•TAR9lJlNlA

TOLF.....

C<on' •1VoN2Io&......

I1.RWXIO

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Fis. 2. - Tolfa, Cerili and M.lUiana diuriro. Samples loc.tion and 1ocaI.ity fillip.

of 0.7073. thus suggesting a single magmaticsource. On the other hand, the above ratiois significantly lower than the value normallyobserved in materials of crustal origin, buthigher as compared with typical values ofuncontaminated basalts.

Two possible models are then consideredfor the origin of the rocks of the Pontineislands: either they derive from a conta­minated basaltic magma or from partialmelting of deep crust layers in which theinitial uSe/8lISc ratio is lower in comparisonwith the uppermost surlicial zones.

Some 180 age measurements are availablefor the different volcanic districts and we cansay that nonc of the most important volcanicunits was overlooked and that the frameworkof the geochronology of the volcanic activityin Latium is fairly well eslablished even if

some contradictory results are present, somepoints are still open to discussion and some­times a more detailed work is needed.

The acidic volcanic group

Tal/a, Ceriti and Manziana district!This district comprises the Tolfa and the

Cerile massifs and the minor volcanic nucleiof the Manziana sector such as Mt. Calvaria,Mt. San Vilo and others, and together withsome of the producls of Ponza, representsthe mool ancien! occurrence of volcanic actiovity in Lalium.

The rocks of these massifs, althoughshowing topographically separated identitiesare linked by similar structural and petro­graphical features and are therefore referableto a single genelic model.

GEOCHRONOLOGY OF VOLCANIC ROCKS FROM LATlUM (ITALY) 77

Detailed research work by NEGRETTI(1963,1965 a, 1965 b), LAURO et al. (1965),NEGRETTI et al. (1966), LAURO and NE­GRETTl (1969), FERRINI et al. (1970) buildup an exhaustive geological and petro­graphical image of this volcanic complex.

As for the geochronology, after a first K-Ardating by EVERNDEN and CURTIS (1965) ona sanidine from the To1£a massif and asecond one by BIGAZZI et al. (1979) onbiotite crystals from the lava of Mt. Cuccodome (Cerveteri, Cerite district) a group of20 K-Ar datings were obtained by LoMBARDIet al. (1974) of which eleven concern theTo1£a sector, four the Cerite and five theManziana sector.

The results concerning lavas, ignimbritesand hypoabyssalites are presented in t::lble 1.From the televant data the following con­clusions may be drawn.

a) Wide range variations are shown bythe ages of the ignimbrites and lavas be­longing to the same sector. In the To1£amassif, including some minor outcrops, theage of the lavas ranges from 2.4 to 3.7 Maand the age of the ignimbrites from 2.1 to2.7 Ma. The age of 2.3 Ma determined byEVERNDEN and CURTIS (1965) approachesthe lower limit found for the lavas andfalls within the general age limits of theTolfa products.

Fot the Cerite massif the age limits are2.4 and 4.1 Ma for the lavas and 2.4-4.0 Mafor the ignimbrites. Such variations areinterpreted by the authors as due to inheri­tance or to loss of radiogenic argon andpossibly to the so called «phenocryst effect».

b) The hypoabyssalites appear to havebeen emplaced during the first period ofactivity of the To1£a volcanism, even if theyfit within the general variation limits of age.

c) While field evidence shows that thelavas are older than the ignimbrites, K·Ardating do not allow to recognize a chrono­logical order. At any rate these events musthave occurred in a narrow interval of time.

d) The magmatic activity in the To1£a,Cerite and Manziana sectors may be regardedas contemporary, belonging to the upperPliocene and not to the lower Pliocene asstated by FAZZINI et a!. (1972). Only a

biotite from a subvolcanite inclusion foundin the chaotic tuffs of 'the Tol£a area (Mt.Sassetto) showed an age of 6.4 ± 1 Ma.

On the whole the present data do notgive a clear image of the chronology of thevolcanic activity of the To1£a, Cerite andManziana districts. Problems connected withinheritance and loss of radiogenic argonmake it difficult to enter in chronologicaldetail and need to be faced with adequatetechniques.

Fig. 3. - Tolfa, Ceriti and Manziana districts:frequency distribution of measured ages.

The Cimino volcano

The geology and petrology of Mt. Ciminovolcano are well known after the works ofSABATlNl (1912), MATTIAS and VENTRIGLIA(1970), PUXEDDU (1971), MICHEWCCINI etal. (1971) including an up to date biblio­graphy.

The geologic units forming the Ciminovolcano are fairly simple and may be sum­marized as follows (PUXEDDU, 1971):

a) plateau ignimbrite of quarrzlatitic com­position, often referred to as «peperinotipico» auct.;

b) lava domes of quarrzlatitic compo­sition, often referred to as «peperino dellealture» auct.;

c) lava flows of latitic composition;

d) lava Row of olivinlatitic composition,often referred to as « radial lava Rows» or«ciminites» (WASHINGTON, 1906).

For the above-mentioned products a groupof ten age estimations is available, enoughfor locating the activity of the Cimino vol­cano chronologically, even if some problemsare still open (table 6).

As for the quarrz-Iatitic ignimbrite (pe­perino tipico lIuct.) the four measurementsavailable indicate ages varying from 1.19 ±0.05 Ma to 1.3\ ± 0.008 Ma with an average

78

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M. FORNASERl

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cltu-lti•

ONTEIl'OSI

Fig. 4. _ Cimino and Vioo volcanic groups: localities map.

value of 1.27 Ma. Five measurement~ onlava domes from Mt. Cimino range from0.975 ± 0.0.5 to Ll8 Ma with an averagevalue of 1.066 Ma.

The only dating existing iD the literaturefor the olivinlatitic lava flows (radial lavas,ciminites aucI.), of 0.94 ± 0.2 Ma, althoughaffected by a 100 large error, is consistentwith the geological setting of these rocks.

The latitic ignimbrite (pcperino tipicoauc!.) thus appears somewhat older (1.27 ±

0.08 Ma) than the age of the lava domes(1.066 ± 0.08 Ma). It is worth nOling,however, that at the lime of the measu­rements the «resolution. of these datingswas probably nOl shll1'p enough to settle thecontroversy about the priority of both for­mations (MITTEMPERCHER., TEDESCO, 1963;VENTRIGLIA, 1963; PUXEDDU, 1971); as aresult more &Iailed measurements are re­quired, taking also in account that, in moslcases, tongues of domt: lavas are ~n to flow

GEQCHRONOLQGY OF VOLCANIC ROCKS FROM LATIUM (ITALY) 79

ovt:r the already consolidates ignimbritt:fPUXEDDU. 1971) but in Other cases theignimbrite covers are likely to be youngerthan some domes IPUXEDDU, 1971).

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IQ 00

Fig. S. _ Cimino volcanic group: frequencydimibution of measured ago.

The alkaline potll8sic volcanic group

Th~ Vuhinian vokanoes

Along with the most recent geologicaland petrographical works on the vulsinianarea (ScHNEIDER. 1965; MARINELLl andMITTEMPERGHER. 1966; NAPPI. 1969; TRI­GILl.. 1966. 1969; TRlGlLA et al.. 1971;VAREKAMP. 1980; METZELTIN and VEZZOLI.1983) over 50 radiometric datings on lavasand pyroclastic products have been performedin the lasl few years.

To frame the results of these datings intoa definite and comprehensive pattern for theevolution of the volcanism in the Vulsinianarea is nOt an easy task, since most of thepublished data are affected by major errorsand because some of the recently proposedevolutionary patterns are based on radio­metric data, which in turn call for furtherverification.

We can assume as a~rtained that thevolcanoes of Bolsena and Latera developedindipendently (METZELTIN and VEZZOLl,1983) and that the activity of the Bolsenacycle started before the beginning of theactivity of Latera (NAPPI. 1969; METZELTINand VEZZOLl. 1983). That being slated theradiometric data concerning the products of

the Bolsena lIctivity (or at least not referableto the acLivity of Latera) are presented se­parately in tables 2, 3 in which they aresimply displayed in chronological order,distinguishing betw~n the lavas (table 2)and the ignimbrites (table 3). Commentswill follow concerning their consistency withthe geological situation. Table 4 conrains thedata for the activity of Latera brocken downaccording to the respective volcanic phasesfollowing the evolmionary models proposedby NAPPI (1969), SPARKS (1975). andMETZELTIN and VEZ7..oLl (1983).

As regards the activity of Bolsena volcanoa few comments may be appropriate.

a) The first products of [he activity ofBolsenll are fairly old. The dating obtainedby NICOLET'fl et a!. (1969) on the lava flowof Torre Alfina (820 ± 40 ka) matches wellwith a dating recently performed by VILLA(1984) on a phlogopite of a sample from adeep well drilled at Litera (862 ± 20 ka).The Torre Alfina lava flow was alreadyconsidered coeval with the Radicofani rockdated by EVERNDEN and CURTIS (196') at880 ka (VAR.EKAMP. 1980).

b) The youngest lava flow of the Bol.sena volcano was dated at 40 ka. However,the latter figure, calculated by NICOLET71 eral. (1979) proved to be unacceptable aspointed out by METZELTIN and VEZZOLIf 1983) because it is inconsistent with thegeological record and not in agreement withthe data first obtained by EVERNDEN andCURTlS (1965) of 328 ka, further confirmedat 3JJ ± 8 ka by METZELTIN and VEZZOLI(1983). The activity of Bolsena volcano isthus likely to cover a lime extent between860 and 150 ka.

c) As to [he Vulci lava flow there isdisagreement between the value obtained byNICOLETTI et al. (1981) of 180±40 ka andthat estimated by METZELTIN and VEZZOLI(Ioc. cit.) of 322 ± 6 ka. The same holdsfor the Monte Calvo flow, dated at 410 ±150 ka (with large error) by NICOLETrI eta!. (1981) and at 316± 8 ka by METZELTINand VEZZOLl. It should be noted that thevalues determined by the latter authors withrefined techniques, affected by a minor error.are highly reproducible and are thus atpresent the best available data·

80 M. FORNASERI

T~re is also disagreement between thevalue obtained by NICOLETTI et 0.1. (1969)for a lava flow of Buonviaggio, m:ar Porano(660 ± 30 ka) and the measurement ofEVERNDEN and CURns (431 ka).

01 A, 31 and 30 A) ydded isochrone (6g. 7)ages of 550 ka and this is the value recom­mended by the authors for the ignimbrirecomplex oulcropping near La Rocca alongthe Trapon2o stream.

Fig. 6. - TIle vulsinian vokanocs: localities map.

~ basal ignimbrites show ages rangingfrom 880 ± 40 ka to 400 ± 20 ka (NICo­LElTl et al., 1981). 10 the opinion of theseauthors, the youngest ages detennined (400­450 ka) are less reliable. Three samples

The ignimbrite oureropping along the Ar­rone River dated at 880 ± 40 ka may wellbe the product of an older activity.

As for the Latent activity the availabledatings suggest ages ranging from 310 ± 10

GEQCHItONOl.OGY OF VOl.CA.-m; kOCKS FROM l.ATlUM (ITAU') 81

[055 ± 22 ka (Nlcou-:rn Cl 011.,1979) withtwo highly :lCcuratc values estimatL-o by~'IETl.l:.l.T1N and Vt:zZOl.l (278 ± 8; 261 ±6.5 ka) (or the lower tr>lchytic ignimbriteIG-unill; an age of 190 ± 20 ka for theupper Irachytic ignimbrite (E-unit) and agroup of dating obtained by diff~r~nt authorsand with different methods, ranging from183 ± 4 to 117 ± 28 ka for the «complexPitigliano volcanite ». The value &termincdby NtCOLE'IT1 et al. (1970) of 55 ± 20 kafar 11 member of this complex neros furthervalidation representing one of the youngestages recordl.:d for the Latera activity and forrhe ovcrall Vulsinian activity.

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•••

VS30 I<

vs 31

vs 31 I<

Fig 8. _ 1be vulsinian volcanoes: frequencydistrlbutlon of measured ages.

Fig. 7. _ The vulsinian volc.noes. lsochrone fromsamples VS) I A, VS)() A llIld VS) 1.

1000010'.

•

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•

-

-

-

1-

are reported ond the data for the similar« tufo rosso a scorie nere ~ belonging to theSabatini district are reported in tab. 8 anddiscussed in the next chapter.

In spite of its importance only two K-Armeasuremems are available for the vicoanignimbrite. One of them (table 7, sampleno. 9) is affected by a large error (520 ±120 ka); the second shows a more reliableage of 420 ± 40 ka. This is clearly not enoughto assess precisely the age of the ignimbrite C.

Among the Others pyroc.lastic units fromVico only a «black pozzolan» was considered.

,

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,.

Tbe V ico volcano

Geology and petrology of the Vim volcanowere throughly investigated and refcrenceshould be ffillde to the works of WASHINGTON(1906), SABATlNI (1912), LocAItOt (1965),MA'ITIAS and VENTkIGLlA (1970) in whicha complete bibliography is reported.

For lhe Vico volcano the geochronologicaldata are frankly scarce and need moredetailed work and refinement. Only ~ven

age measurements have been performed onlavas and pyroclastic rocks.

One of the most important pyroclasticformations in the Vico district is the « tuforossa a scorie nere» auct. (red tuff withblack scoriae), which was interpteted b}'LocAllOI (1965) as an ignimbrite sheet(ignimbrite C) extending 10 a maximumdistance of 25 km from its emission cemerand covering an are-d of about 1250 km::.

South east and south west the ignimbriteC is in comact with the analogous formationfrom the Sabatian district. For this reasonthe ignimbrite C from Vim and its analoguebelonging to the Sabatian district were oftenconsidered as one single pyroclastk unit at·tributed to the Vico volcano. According toLocARDI (1965) however the ignimbrite Coccurs always as a 6lling in the erosionvalleys cut in the Sabatian formation. Thequestion however is far to be settled, asALVAIlEZ et al. (1975) by determining theelevation of the upper surface of the TufaRosso concluded that all this unit is of Vicoorigin.

To avoid misunderstandings in table 7 onl}'the age dala for the ignimbritc C from Vico

M. fOKNASt:1I1

Its age of 620± 220 ka, allhough :Iffectetlby such a large error is nOI in conflict withthe age obtained for Ihe ignimbrite C sincethe pozzolan underlies lhe ignimbrile C andmay well represem one of the lerms of thei~nimbrite B of LocARDl (1965).

The remaining darings available indicarean aClivity of the Vico volcano ranging from820 to 95 ka. Ir is worth noting that rheupper Hmir is affected by a vcry large errorand needs further veri6cation ahhough ir isconsisreOl with rhe srraligraphic position ofrhe lava considered (NlCOLETTl, 1969). Thelower limit (95 ka) r~uhs from a measure­mem of EVERNDEN and CURTIS (1965)performed on a leueile-rieh lava from Ve­tralla.

Fig. 9. - 1be Vico yok..no: fn:tjl)eIKY Ji$uihuliunof masured aaa.

The Sabatin; volcanoes

The aucntion of Ihe geochronologists inthe Sabatian disrrict was focused more onpyroclastic units than On lavas. This is partlydue to the nature of the Sabatian district,which is characterized by a large numberof eruptions with prevlliling explosive actiovity, as compared with the emission of lavaflows.

Tables 8, 9 display the dating publishedup to this date, making reference to thesuccession of the pyroclastic units, asestablished bv MATTlAS and VENTRIGLlA(1970). In tdble 8 the only twO availabledatings for lavas arc also shown taking intoaccount their stratigraphic position in re­larion to the pyroclastic units.

Of particular interest is Ihe resulr obtainedby VILLA (private communication) from asanidinite block embedded in the rrachyticlava flow from Morlupo, representing oneof the oldest effusive units of the Sabatiandistrict, for which two vlllues resp. of 514 ±9 and 607 ± 15 ka were obtained.

Onc of the effusive units of p:micularsignificance is the leucitilic lava flow ofMonte Cucco, on the left side of Fosso dellaMola in Cerveteri area, dated by B1GAZZI elal. (1973) at ·HO ± 20 ka. This lava flowis superimposeJ on a brown eanhy IUff and;l white pumice luff refew.-d 10 ItS «lUfistrati6C'ilti varicolori di Sacrofano (Sacrofanomanycolored IUffs)>> (') and underlying adiatomitic layer, which in lurn underliesthe «tufo rossa a scorie nere» auel, (~the chapter dealing with the Vico \'olcano:md below), rhe age of which is fairly wellestablished at -142 ka· The daling of thelava flow of Monte Cucco enabled the authorsto establish the age of the diiltomitic depositand 10 stare the age of the Tarquinian periodof marine sedimenliltion prt."Cisely.

Moreover table 8 exhibilS the age data forthe ... tufo rosso a scorie nere » from the Sa·batian region which, as alIe1tdy mentioned.is to be considered liS the analog of theignimbrite C from Vico.

The c tufo rossa II scorie nere I> is animportant marker not only for the tephro­chronology, but also for the chronology oflhe Quaternary deposils in Lttium (AMBlto­SETTI et al., 1969; BoNAOQNNA and BrGAZZr,1969, 1970; AMBROSET1'r et aI., 1972),The age of this formation may be regardedas one of the best known Ihrough Ihe nu·merous measuremcnls of [VEItNOEN :mdCURTIS (1965) including replicate measure·ments on the same s'lmple llnd on differcntsamples from dilferelll localities a~ well, 10

which one recent measurement by VILLA(1984) should be addl.-d,

In view of the d,lIa ltvailllble an age of442 ± 7 ka may be assigned to the «tuforosso a scoric nere" from the Sab"lian re·gion, resulting from lhe lIver,tge of 8measurements, 'lfler nonmt!izing all valuesto the new decay constllnts by STEIGEIl. andJAGER (1977).

Although the published dala concerningthe ignimbrite C from Vico <Ire still scanty,the age of the « tufo rossa II scorie nere I>

from the Sabatian region appears to be veryclose to that of the ignimbritc C.

In the Sabatian district the «Iufo IUS-

It I Nor to be mislllken for lne "lUfo giallo JiSacrofano .. (Socrof"no yeUou' tulfl, u,hich i~ anupper unit.

GEOCHRONOLOGY Of' VOLCANIC ROCKS FROM LATtUM (ITALY) 83

so .. underlies the c tufi stratificati varko­lori de La Storta.. (La Storta stratifiedmanycolored ruffs). The later ones show awide spectrum of ages not very significantowing the large associated errors and ofteninconsistent with their stratigraphic position.When considering its heterogeneous natureand its complex depositional mechanism(MA'ITIAS and VENTRJGLlA, 1970) it isconceivable that this unit may reasonablyrepresent the product of the deposition ofolder, rc=worked material.

DONNA and BIGAZZI. 1969) may wc=1I rc=­present one of the final events of the depo­sition of the complex unit of the c tufi deLa Storta.. the age of which may be corn·prised berwec=n 442 ±7 (agc= of the c tuforosso a scoric= oere .. ) and 225 ± 60 ka.Obviously. further study is needed to removeany residual uncertainty.

The ages established with diffc=rc=nt me­thocls by ALVUEZ et al. (1976) and byCoNFORTO c=t al. (1977) for the c tufo giallodi Sacrohno .. (J70 ± 70 and 200 ± 100 ka

Fig. 10. - S.blUini volanic dimict: loca1itics map.

The volcanic level interbedded in thediatomitic deposit of Riano, for which twoquite concordant ages are reported. obtainedby different authors and with dilIerentmc=thods (AMBROSE'ITI et al., 1969; BoNA-

resp.) are aHectc=d with large errors. Forthe same unit VILLA (1984 b) determined anage of 288 ± 6 kat which can be consideredthe best value availablc=. owing to the 5maUassociated error. In any case the time interval

M. t'OIl.NAScKl

between the deposition of the upper levelsof the « tu6 strad6cati varicolori de La Star­ta» and of the « tufo giallo di Sacrofano»appears to be rather small.

It turns out again that increasingly precisemeasurements are needed to better establishthe chronological relationship between thetwo above-mentioned units.

As for the « tufo di Bracciano » which, inseries, like the « tufo giallo di Sacrofano »,overlies the «tu6 de La Storta », researchwork by BoNADONNA and BIGAZZI (1970) inthe area of Monte Cucco (Cerveteri) showedthe existence of four levels; three of themwere dated with the fission track techniqueon biotite crystals, yielding ages of 177 ±30; 127 ± 13 and 90 ± 18 ka, respectively.These data were used by the authors toestablish the correlation between erosivephases and intratyrrhenian regressions of theQuaternary in the surroundings of Rome.

,r-

I- f-I.---i500 1000>10".

Fig. 11. - Sabatini vulcanic dis!rkl: frequencydistribution of measured ages.

An ejected block from the « tufo di Brac­dano », considered as a fragment of a skarnconnected with an underlying plutonic body,was dated by VILLA (private communica­tion) with the potassium-argon method at334 ± 48 ka, a value which is obviouslyin line with the data previously reported.

For the « tufo di Bracciano » which in thestratigraphic sequence established by MAT­T1AS and VENTRIGLIA (1970) appears tooverlap the above described « tufo di Brac­ciano», VILLA (1984 b) obtained a consistentage of 85 ± 9 ka.

Summing up, for the Sabatini volcanoesfew reliable datings are available and morework has to be carried out la improveprevious data and to examine the pyrodastic

units not yet studied from the chronologicalviewpoint, such as the «tufo giatlo dellaVia Tiberina », which represents also animportant unit, llOd others.

The Albafl Hills

Even if the existing literature contains aconsiderable amount of data, the presentstate of the Alban Hills geochronology isfar to be satisfactorv.

To frame the existing data into a singleevolutionary pattern reference will be madeto the chronostratigraphic sequence proposedby FORNASERI, SCHERILLO and VENTRIGLIA(1963) of which use was made in drawingup table 10.

For the lower terms of the series, the«6rst outcropping effusions» two datingsare available, substantially concordant, byEVERNDEN and CURTIS (1965), at 706 kaand by GASPARtNl and ADAMS (1969) at680 ± 50 ka, and two obtained more recentlyby VILLA (1984). The last ones, of 400 ±ID and 460 ± 6 ka, with which a very smallerror is associated would rejuvenate the ageof those which were considered so far as theoldest lava flows of the Alban Hills, byalmost 300 ka.

In the central area of this district, super­imposed on the above mentioned first largelava effusions there is a significant pyroclasticcomplex referred to as the « complex of thelower tuffs» (complesso dei tufi inferiori).This complex includes the following terms,from top to bottom, and omitting details,for which reference should be made to thepreviously 'mentioned work by FORNASERIet al. (I 963),

a) Tufo di Villa Sennib) Upper pozzolanas (grey pozzolanas)c) Tufo Iitoide della Campagna Romana,

often referred to as «tufo Iionato dacostruzione »

d) Middle pozzolanas or pozzolane neree) Yellow conglomeratef) Lover pozzolanas often referred to as

«red pozzolanas» or «pozzolanas ofSan Paolo ».

According the most recent papers (BER­NARDI et aI., 1982; PECCERILLO et aI., 1984)the first activity phase of the Alban Hills

GEOCHRONOLOGY OF VOLCANIC ROCKS FROM LATIUM (ITALY)

Fig. 12. ~ Alban Hills: localities map.

85

was terminated by a big explosive episode,giving rise to the deposition of the VillaSenni tuff, which is interpreted as due toempting of the magma chamber and suh­sequent collapse of the caldeta. Followingthis model the products of the activity ofthe Alban Hills are often referred to as«pre-caldera)~ and «post-caldera» dependingon their location with tespect to the « VillaSenni» tuff.

The « Villa Senni» tuff is also the bestknown formation from the geochronological

standpoint. For this formation, a wide agespectrum, extending from 620 ± 200 to160 ± 30 ka can be inferred from the lite·rature (MASI et al., 1976; TADDEUCCI, 1969;BIGAZZI et al., 1977; RADICATI DI BROZOLOet al., 1981; VILLA, 1984). Fortunately tomake order in such a dispute an extremelyaccurate methodological study by RADICATTDI BROZOLO et al., carried out with 4.0Ar_39Arand with Rh-Sr methods was published in1981, whose results may be summarized asfollows:

86 M. FORNASEIU

').-::::-'-..../····1

~===-v'~

: lOl'vw,

ANAGNI

Fig. 13. - Anagni and FOfllana Ranuecio archacoqical site: locality map.

a) six leucite samples yielded de6ned·"Ar·:JllAr ages with an avernge value of338 ± 8 ka (Stt table 11).

b) Rh-Sr measurements on leudte, bio­fite and pyroxene allowed the determinationof internal isochrone ages ranging from 380 ±20 to 330 ± 20 ka, in good agreement with[ht: previous ones.

c) The biorite ~oAr_aIlAr ages are in threecases in agreement with the cagenetic leueites,although less preeise, but in the remainingcases two discordant ages were obtained,suggesting that the biodtes must ~ con­siderro with cauLion for .oAr·!'Ar and K-Ardating of Quaternary rocks.

Nonetheless the study by RAOICATI et al.allows 10 establish with certainty t~ ageof the« Tufo cli Villa Sc-nni. at 338±8 ka;given its eXlension over an area of about1500 Km:! this IUff repre2nts one of the

mayor markers of the Alban Hills volnnicdistricl.

Out the previous datings of the sameformation, only twO of the four obtainedwith" the fission track technique are inagreement with the above value. Other valuesare to be considered as aberrant (tOO youngor too old) and arc to be rejected.

The situation for the lower terms in thecomplex of the « lower tuffs» is far frombeing satisfactory. The only chronologicalinformation we have comes from a study onthe archaeological site of Fontana Ranuccio,Anagni basin, conducted with the K/Armethod by RADICATI DI BROZOLO and VILLA(in BlDDITTU et al., 1979). The stratigraphicprofile of this site (fig. 14) includes differentpyroclaslic uniu, among them the .. tufoIitoide della Campagna Romana. (3) is theupper limit of the Ranuccio site series,which includes, proceeding dovnwards, thearchaeological layer (6), a thin layer of leucite

GEOCHRONOLOGY Of VOLCANIC ROCKS FROM LATlUM (lTALY) Si

crystals (12), a thick layer of pozzolana (13),and a tuffitic Iimnosol (14).

The « tufo litoide» (3) was estimated tobe slightly younger than 366 ±0.4 ka, thisbeing the age of a lapiIli tephra layer (4)found immediately below it.

The age of the thick pozzolana layer (13)should be comprised between 528 ± 6 ka,which is the age of 14 and 487 ± 7.5 ka,the age of 12· Incidentally, the age of thepalaeosol atchaeological layer (6) withindustry of the lower Palaeolithic wasmeasured at 458±5.7 ka.

5

II

L'A' B25m ~.: .- ._.. _523.000.!l. 14

Fig. 14. - Fontana Ranuccio site section.Fig. 14. - 1: .. Terra rossa,. paleosol • 2; POOt:).genized layer. 3; .. Tufo litoide:,. - 4: Lapillitephra . 5: Solifluclion and cryoturbatOO horiwn .6: Palaeosol archaeological layer. 7; Tephra fromErnici Volcanoes with BUXUJ and Zelkovl1 floraand Cydortomll elegllnJ . 8; Clayey Iimnosol .9: Iron duticrust . 10; Tufitic current bedded sands. 11; Palaeosol - 12; Thin layer of leucile crystals ­13; .. Poz7.01ana ,. - 14; Tuliitic limnosol.

'(From SEGRE and ASCENZI, 1984).

Now, while the age of the «mfo litoide»is consistent with the expected age of thisformation, which in the central area of theAlban Hills underlies the «mfo di Villa

Senni », some problems arise as regards thethick pozzolana layer (13) which is, likethe «tufo litoide» (3) a product of the AlbanHills (BJDDlTTU et al., 979). The age esti·mated for this pozzolana layer (between 428and 487 ka) is consistent with the expectedage if one admits the value of 706 ±680 kafor the oldest lava flows (EVERNDEN andCURTIS, 1965; GASPARINI and ADAMS,1969) on which the pozzolana is super·imposed in the Rome area (Acquacerosa,Vallerano e[c). However this estimate is nolonger in agreement with the expected ageif one accepts for these lavas the new valuesof VILLA (1984) (400 ± ID ka) and BER­NARDl et a!. (1982) (460 ± 6 ka). Whetherthe pozzolana layer of Fome Ranuccio is tobe considered as the same overlying the lavaflows of Acquacetosa and VaIIerano or re­presents an older pyrodastic unit is still anopen is~ue, taking into account that theFome Ranuccio series lies some 30 km away,eastward and on the opposite side of thevolcanic complex.

From the Tuscolano·Artemisio period,different lava flows were dated. One of themis the well known lava flow of Capo di Bove(WASHINGTON, 1925; FORNASERl et al.,1963) for which GASPARINI and AOAMSobtained 520 ± 40 ka on leueite separatesand 500 ± 50 ka on the whole rock.Nevertheless these concordant values areinconsistent with the stratigraphic positionof the lava flow which overlaps the VillaSenni tuff of 338 ± 8 ka. For the same lavaflow [he dating of NrCOLETTI (360 ± 40 ka)is more acceptable, but the best availablevalue was obtained by BERNARDI et al. (1982)at 292 ± 6 ka.

For the Divino Amore lava flow EVERNDENand CURTIS (1965) calculated 268 ka andmore recently BERNAROI et aI. (1982) 175±2 ka. For the Saponara lava flow, BERNARDlet al. give a consistent age of 260 ± 5 ka.The age obtained by the same authors fora small lava outcrop east of Saponara posessome problems since this small lava flowunderlies the « tufo Iitoide» and, accordingto its stratigraphic position it should be« pre-caldera ».

The large Java effusion of Laghetto (AI-12)which represents a « post caldera» eruption,shows an age (365 ± 5 ka) slightly olderthan the expected one.

88 M. FORNASERI

Even more serious problems are raisedby the two lava flows which belong tothe Tuscolano-Artemisio rim and which,according to FORNASERl et al. (1963) a~

la be considered • post caldera ». One ofthe2 lavas (AI-9) was dated by BERNARDI

et al. (1982) at 477 ± 6 ka; the other(Al-38) at .524 ± 9 n. Both ages are olderthan t~ obtained by the same authors andby VILLA (l984) for the lava £lows of Valle­raneUo (460 ±6 ka) and Acquacetosa (4oo ±10 ka) underlying nOt only the «tufo diVilla Senni» but also the lower ponolanaformation, and consequently being «Pltcaldera» products.

If one considers the ages of Valleranel10and of Acquacetosa lava flows and of « lUfodi Villa Senni » as well established, the agesof AI·9 and of Al-38 samples are difficultto explain because they would represent theoldest lava products of the Alban Hills.Furt~r 6eld work is needed to verify thestl'lltigl'llphic position of both AI-9 and AI-38lavas and to give a satisfactory interpretationto such unexpected old ages C:).

Among t~ products of the lasr pe.riod ofthe centl'lll activiry the ages measured rangefrom 331 ± 7 (rhe lowesr of the: 7 lava flowoutcropping at the north-east wall of the:cl'llter of Nemi) to 240 ± 3 ka for the lavaflow of Arcioni (Roaa di Papa) (BERNARDIet al., 1982) which is the youngest of theprOOucts from the last period of the centralactivity.

Coming back to the lava flow of DivinoAmore, until yet considered as a product ofthe Tuscolano-Artemisio period (FORNASERIet al., 1983), its younger age (In: ka) risesthe question whether this flow belongs tothe Tuscolano-Artemisi<? activity or representsrather the products of a more rttent excen­tric vent. It is worth underlining that theDivino Amore flow represents the youngestlava prOOucts known far for the Alban Hills.

The last activity in this volcanic regionhad a strong explosive phreato-magmaticchal'llcter. giving rise to the well knowncraters of Albano, Nemi and others, sur-

rounded by typical explosion products. Oneof them is the well known «peperino .. ofAlbano.

Many attempts were made to estimatethe age of this eruption.

a) The zircon found in the sands fromthe Nettuno linoral with the fusion tracktechnique revealed an age of 59 ± 9 ka(BJGAZZI and FERRAIl.A, 1971); such sandsare reasonably believed to have been fed hythe «peperino .. of Albano (BIGAZZI andFERRARA, 1971). The same zircons (CERltAtet al., 1965) gave 43 ± 5 ka with the~3l)Th method while TADDEUCCI (1969)obtained an age of 67.5 ka, still workingwith disequilibrium methods on mineralsof the «peperino» groundmass. These re­ported data are to be considered as an upperlimit, since the afore mentioned mineralscan be older and not necessarily coeval withthe explosion.

hI Unhurned wood samples, embeddedin the «peperino's .. layers were dated al29.7 ± 0.4 ka with the lie method byDE VRIES (1958). Pro£. DE VRlES (privatecommunication) pointed out that this datehad to be verified hut this did not unfortu­nately happen owing his untimely death.Further unbumed wood samples collected inthe following years were repeatedly dlitedin Rome (ALESSIO et al., 1966; CoRTESl,1984) but the obtained ages turned outalways higher than the local limit of themethod 07-40 kal.

.., ,-,,-

,

r--

FIg. 15. - AJ1»n Hilb: frequency diSlfibulionof~ 118e$.

to, Attordill8 FUNICIELLO (prinle communicalion)lhe AI-9 J.v. flow .ppcan underlyill8 the • IUfodi Villa Senni ... Thi.\ rouId explain its unapcncdold age.

'" 1000010'.

(;EOCHIWNOLOGV OF VOLCANIC ROCKS FROM LAT!UM (ITALV)

Fi~. 16. _ The lkrnkan disnicl: lo.:alititcs map.

It is apparent Ihat there are clear signsof ,I recent age for this eruption and furtherattempt are being made to obtain new infor­mation, At present it m,ly be reasonablyassumed that the eruption of Albano rookplace about 40-50,000 years RP,

Th" H",nicolI district

The volcanoes of the valley of Sacco orValle Latina, often referred to as Hernicanvolcanoes, are located about 50 Km east­southeast of the Alban Hills and 70 Kmnortheast of the Auruncan district (Rocca­monfina), the latter one belonging to theNeapolitan area.

This district, well known after the studiesby VIOLA (1896, 1889), WASHINGTON (1906),$CHERILLO (1937), ACCORDI et al. (1967),FRANCO and DE GENNARO (1971), ANGE·LUCCI et al. (1974) was recently studied byBASILQNE and CIVETTA (1975), CIVETTA(1974) as regards the geochronology and by

Cl VE"fTA et al. (1981) in its geochcmiCal'ISpectS.

The district involves different eruptivecenters, located near Patrica, Giuliano diRoma, Colle Castellone, Pofi, Roana andothers; ,their products may be consideredas leudte tephrites and leucitites.

The K/At ages obtained, shown in table14, range between 700 ± 20 and 80 ± 40 kaand do not pose particular problems atpresent.

""•

'00 1000' 10'.

Fig. 17. - The Hcrniclln Jistrict: fr~llenLY dislri­bution of measured ages.

90 M. FORNASERI

A discussion concerning the chronologicalrelation between the volcanic activity of theHernian disukt and of the Alban Hills ishard 10 undertake at present, owing the bigproblems still open concerning the chrono­logy of the Alban Hills.

I t may be considered as ascertained thatboth volcanic complexes were contempora­neously active for a long period of ti~.

The Ponza archipelago

The Ponza archipelago consists of twodifferent groups of islands: the eastern groupincludes the islands of Ventotene and SantaSrefanoj the western group the islands ofPonza, Palmarola and Zannone.

The geology of the Pontine islands wasoutlined in different works by DoELTER

(1876), SABATlNI (1893, 1896, 1898), SEGRE0954, 1956, 1960) and in a more recentpaper by CAKMASSI et a1. (in press). Vol·canology and petrology were studied indetail by BARBERI et al. (1967), METRICHand SANTACROCE (in press).

Age determinations have been performedby BARBERI et al. (1967) and more recently

by SANTACROCE et a!. (1983) and SAVELt.I(1984).

The results obtained for the eastern group(Ventotene and Santo Stefano) are displayedin table 15. According 10 BARBEIU et al.(1967) a quaternary volcanic activity pro­duced trachybasaltic lava Rows at Ventoteneand phonolithic ones at Santo Stefano. Theages measured range from 1.2 Ma for SantoStefano to 1.7 Ma for Ventotene. Newdatings by SANTACROCE et al. (in press) forthe Punta dell'Arco lava Rows (Ventotene)covering the entire pre-caldera sequence gaveages ranging from 0.81 to 0.48 Ma, in pooragreement with the previous value of 1.7 Maobtained for the base of the same sequence.KtAr measurements on the Romanello lavaflow and on pumices from different pyro·clastic units representing the products of apost-caldera activity indicate 0.2 Ma as thelower age limit.

Table 15 also displays the age deter­minations for the volcanires of the westernislands (Ponza, Palmarola). According 10

BARBERl et al. (1967), the volcanic activityin the western group started during the PHo­cene (5 Ma) with the emplacement of

ISOLA 01 PONZA

D

I, ZANNONE'-

~,6J,..tSl~

I, VEN10TENE

•

..~'P,

\., PAlMAROlA

\

rig. 18. - lnc Ponza archipelago: 100001i1ie$ IIUIp.

GF.OCHRONOLOCY OF VOLCANIC ROCKS FROM LATIUM (ITALY) 91

rhyolitic volcanites. Approximately at theQuatcrnary-Pliocene boundary (1.9 Mal se­veral dykes intruded the brecciated lavas.A little later (1.7·1.6 Ma), an intrusion of asodalite lava dome took place at Palmarola.The volcanic products, although of diffe~nt

age and composition have a common magma­tic origin as shown by the concordant"1Sr/MlSr initial ratio of 0.7073 (mean value).

On the basis of the new datings bySAVELLI (1984) however, the above modelwas somehow modified in that the «rhyo­litic dikes. display a K/Ar age substantiallysimilar to that of the other acidic rocksand therefore they are not to be consideredas being separated by a significant inrervalof time.

This result is in agreement with themodel proposed by CARMASSI et al. for thevolcanic evolution of the island of Ponza ina shallow submarine environment.

The acidic volcanism of the Ponza archi­pelago is thus of pliocenic age. According toSA VELL! (1984) during the lower Pleisto­cene, starting from 1.75 Ma, a pronouncedbreak in the volcanic activity occurred, notonly in the Poma archipelago but also inthe emerged volcanic a~as surrounding theThyrrhenian Sea. The volcanic activity startedaJZ.ain at about 1.1-1.2 Ma with the em­placement of products of alkaline potassicaffinity (Monte La Guardia trachytic lavaflow, having an age of 1.1-1.2 Ma, accordingto BARBERI et al.. 1967).

"

Fill. 19. The Ponza archipdallO: frequencydistribution of measured ages.

Dating volcanic material interhedded insedimentary (ormationll

A number of age measurements have beenperformed on volcanic levels interbedded inNeogene and Quaternary sediments, mostlyaimed to define a chronostratigraphic scale.

These data, together with the al~ady

reported and discussed age measurements on

volcanic rocks of Latium, have been widelyused in discussing the Pliocene-Pleistoccnelimit in Latium. This problem however isout of the scope of this work and will notbe discussed he~.

In table 17 the K/AI and fission trackresults obtained up today are reponed.

The provenance of the above mentionedvolcanic materials remains still an openquestion which deserves further attention inthe furure.

Concluding remarkll

The data here reported represent the resultof almost twenty years of activity in dating theyoung VOlc~lOic rocks of Latium with varioustechniques, among them the most widelyused was the K·Ar method. From the firstage measurements a significant improvementhas been achieved due also to the availabilityof more sophisticated instruments andtechniques, resulting in more precise data,as shown by their small 115sociated error.

Nevertheless the general situation con­cerning the geochronology of volcaniC! ofLatium is still far to be satisfactory, due tothe persistent difficulties in dating veryyoung volcanic rocks.

Many points a~ still open to discussion.Among them there a~ analytical problemsconnected with measurements near the limitsof the method. Moreover an accuratesampling. i.e. an accurate selection of sampleswith a precise location of the sampling pointis of fundamental importance, and sometimesthe significance of an age measurement ishard to evaluate and to be compared withthe results obtained in different laboratoriesowing to the lack of information on samplinglocalities and location. This remark is notso obvious because we have to take intoaccount the variability which is likely tooccur even in the same formation.

As a matter of fact one of the possibleperturbing factor in dating young volcanicsis the often invoked presence of excess argon.Such presence is now to be considered morethan a suspect after the 4°Ar/u Ar stepwiseheathing experiments carried out by VILLA(19&") on twO lcucites from the Alban Hills(VS3 and AL43). revealing their contrastingbehaviour with respect to AI geochemistry.

AL 43 is the first documented case of ~oAr

92 M. FORNASEKI

excess in a leucite. VII.LA (198') considersas an attractive hypothesis the derivation ofthe excess argon from the man de.

The experiment of VILLA indicates alsothat such excess argon is nOI homogeneouslydistributed in the Alban Hills products and,indirectly, that the results of the conven­tional K-Ar dating should be considered withcaution. In this respect the accuracy in medescription of the sample location SttffiS tobe appropriate and it is worth noting thatan high analytical precision, even if desirable,is not necessarily a warranty that the ap­parem K-Ar age be a real age.

Taking this into account it seems advisableto recommend whenever possible, the use ofthe llIlAr.~oAr dating technique this one beingthe specific method by which me eventualp~ce of excess argon can be direcrlyrevealed.

At present, to keep under control t~

conven!ional K-Ar dates, a comparison withdiR'crent methOOs is advisable. RADICATI etal. (I 98 I) showw that t~ Rb-Sr methOOcan Ix used as a cross cha:k criterion forrocks as young as l50 ka. An allempt torevive the K-Ca technique for high potas­sium minerals (leucite, kaliophilite, kalsilire)should also Ix taken into consideration.

Similarly fission track dating technique andalso the use of the 23O'fh disequilibriummelhod deserve particular attention as pos­sible alternative methods in estimating theage of Ihe very young volcanic rocks.

A. + 1, 5.305 x 10 HI , '~uK 0.01l9 atom%

SCHNEIDER, 1965,

.I, 0.583 X 101l'a l

.I. 4.737 X 10-10 .'1. + A, .'5.32 X IO- IU " ,

EVERNDEN and CURTIS, 1965:

.I. ~ 0.584 X lO+IO , '

.I, = 4.72 X 10.10 ,-'A. + A, = 5.304 X 10-10 a+ 1

BERNARDI et al., 1982; BtODI1IU et al.,1981; METZELTlN and VEZZOLI, 1983; NI­COUrTr el at., 1979, 1981; RAOICATI etal., 1981; VILLA, 1984 and private comm.;VAREKAMP, 1979, 1980:

..1... 0..'581 x 10- 10 a-I

..1., 4.962 x 10-10 a-I

A. + A, '5 ..'543 X 10.10 a-I

iOK 0.0167 atom%

Constants used by Rb-S, dating

RAOICATI et al., 1981:

..1.. 1.42 x 10. 11 a-I

Acknowlcdg",..nll. - This work was carried ourin t~ frame of the research programme of IheCentra di Studio per la Gcocronologia e la Geo­chimka dc:lle Formuioni Re<:enli of the CN.R.

The author thanks prof. S. METZELTlN, prof.L. VF.ZZOll and Dr. r. VILLA who kindly allowed10 make use of their unpublished data and prof.R. FUNICIP.LL() (or helpful discussion.

Constants used by the K-Ar dating

ALVAREZ el al., 1976; BARBERI et al.,1967; BAslLoNE and CIVETTA, 1975; BI­GAZZl Cl al., 1979; CIVETTA, 1984 lInd pri­vate comm.; DISCENDENTl cl aI., 1978; GA­SPARINI and AnAMs, 1969; LoMBAROl etaI., 1974; MASI et 1I1., 1976; NICOLETTI.196'5:

.4.. = 0..'58.'5

..1., = 4.72x 10 III ax 10 I" a

Abbreviations used in the Tables

Anc AnalcimeAp ApatiteAug AugiteBi BiotiteFT Fission tracksFS Feldspar separateGM Groundmassl.L Leucite0/ OlivinePhi PhlogopirePx PyroxeneSa SanidineTit TilanileWR Whole rockZir Zircon

GEOCHRONOLOGY OF VOLCANIC ROCKS FROM. LATIUM (ITALY)

TABLE 1

Tolla, Ceri/e, Manzia/e acidic volcanic group

93

~4(lu-rad'_u ArrO<l.I/. ~ w.~~ LOCALIT'Y

~-~, . ..

~

• 10.... • 100 •

,"1• e-at_I. o-Tu 1.tIo. .. ~ 7.1. 0.6677 2•. 3 2•• : 0.2 ~

1 "- ......f Tolf. - ~ 10.~7 " ••• D_co dlTolf. LI .....I'" ... ~ '.03 1.0717 0'.3 ••• : 0.1 ~

s'"" <loll. S,..... -" I.Uta '" .. .. ~.Ol 0.590' 13.0 3.0 1 0 •• ~

. . " " " '" .. ~ ,.n 0.lIOt1 ~ .. 2.9: 0.2 "~.~ .. 110110<1•• Liparlt. '" ... ~ .." 1.239 " .. 3.1:0.1 ~

L.o Tolf.cd. -" I.U ... '" .. ~ '.M 0.1.10 ~., 2 •• : 0.2 ~

L.o _ .......10Llparl" n, .. ~ '.M ,.- ~.. 2.1 : 0.1 ~

,-10 ...11. , ... lI. _Ok porp/Q'f'J' (2) ... ~ '.M ..- q .• 3.5: 0.1 ~

L.o ~1. _Ok~f'J'(2J on ~ •. 7. 1.1177 U 3.2 : 0.2 ~

.' ~Icclo L1por.q. I ... '" ... ~ 1o,'7 1."3 "., 3.3: 0.1 ~

• ' __n"• 1..... l.IoU .. '" ... .. '.M ..- 1•• 7 ... : ••• ~

........ c:..e"" Tr"""",'" .. 3.93 ,.~ " 3.71°. 12 s.n_... _to1'TOdVUc U ...I ... .. ~ 1.11 0.71" 1~.3 2 •• : 0.2 ~,

.' L.o 0I>0nI1. -...:, I.U ... U ~ ..~ ..- q .• '.' : 0.2 ~,0 L.o Golerooce Lt_I... '" .. D ..~ ,.= ~.. 2.5: 0.2 ~

.'_le LI .....I .. '" 0" D .." 1.3512 el.O '.0: 0.2 ~

M'. ,. Sele,te """ollte 0 .. D '.I? 0.7'19 10.' 3.2 : O.~ "!_..

Ilon'er_ """ollte 0 ~ D ..'1 0.8U5 ~.. 2.3: 0.1 .", " ,,",Icelo_.

leU ... .. D '.M ..- n., 3.8:0.1 ~

• ,. <:elverlo L.otlte '" 0 " ~ '.M ,.- R.' 12.2:0.1) ~

,. <:el ......lo L.oUte '" 0 ~ ~ •.51 ,.- ~.. 11 .• :0.1) ~

(1) ignimbrites; (2) hyflOlibyMlllite5; (3) inclusions.

:sr;;:g~t>r;;~~ll::S:S ~ ~:ll:g :g ~ ~~IO:S:Sll:• • • ~ :S:l:Slii~· -.§-l!

ji

'122~iiR2lil

§": i' ~' ~' ~' ~' i' ~' ~' ~51 i .... "! .... .. ........

•., .j .,., ., .,., a .,., .,., .,~~~~~~~ aa~R~

,;

•"~

,.,I

•.,I

i ~ I': 22

~' ;';';";';III ;; ';::

•·,, la SI SI i la., ., ., ., .,HUi

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Iil ;! :g la ll! & 8 ~ ~ ~ ~ ~~ ~~ ~ ~ ~ ~";::::""'~2:!::!:::::';":';";'; ~"::e:e:!! ~ :e:e ...... :!: .. :!:

.1"j~lj.JjjjS lli;;;!li" li ,,;;; j

,.'l! ';0 I ,J if!'" ~ 2~- ~:s"!~"!~

..!;, K .;,; <> <> <>

, ,, , 'l1 - - € ;,ill l ,;: ~ 3 :l :l ;:

lliiil

• i f::,; ..: ..

!"Ii ' I 6•

j 33.!llf

,},j

~ ,. ,. -2 lliiS:J

~ !i lG 1!.; ';N ,; ,;

1~la1ll2111...;~~~~.;.;

E

,I

.. o2lilo ... , !:I I I'!,.; ~ 2£ ri ~ ~Il g ~ ~ ~ .

I j IS j

. ... -...- ....-~ :l - - ..

"i'l-l ..'I.. ::.

i i i i !

~~ ~ ; ~~';":ci .. oo

,},j

,o

i,.-iE

1j

!

:Ul:llnliliil~:2

,I I .! U.. ,.; ,.; ,.;,.; ,.;,.;

;g!

,i

~~ ~:g :e~~;i";

!!::: Ill!! 'I "ii1::: )Il;;; ;;; Ei00 ~~ ,; ..

i I

,, ,, ,, ,

r;; 8;.:.; ~i ;:

i

,i

l~ i ..cici ci .;,;~ ~

1iI ll; I? IIIti ~ ~ ~

! i !

l •

••

, ,~ ~

!

I

" . , ...fl~l:l~J2o:

l:l~:Slil:::.;.;"':.;';~

, . ~

~ ~ ~~ ~ :• • •

••

!i

I!.

i:~

.~:sCl

•••.~

...,.

!~

"".~.=

N ~" ." ~~J

"""~~

>

;;:

,~

I, .1i ,di

, !,L •'i t Jl ~"" .. .3 "I..

, .! JH! . ,

u •

!••

Ij

, <, ,

IiI

j,• j

, '! i J

I • , , ,:l ! .'l •- ,.

'*:1.,.;.;

!•,,•:: ' ., !

!I! ;g

GEOCIIRONOLOGY OF VOLCANIC ROCKS FROM LATlUM (ITALY) 95

TABLE 3

Vulsiniol1 District - Boluna activity: basal ignimbrites

K·Ar dlling

WClU.ITT-~ -~

.ooATnd

-.Ill.00ATrwI r. .~U'. .. _.

u ., • 103

a..,u_. _~u nu ~ 12.37 L_ 30.23 ~:.2O "u_ T.......I ... phD. n .~ 0 12.la 2.0120 12.e11 QO :. «I "",,11 ... n» . 10•• L_ 30<.81 ~: 20 "nD 0 '.81 ,.- ».~ lIeO :. 20 "ro"'....II.l_t... -" ft III 0 '." L= ~.~ 610 :. 20 "-'. n" 0 11. 211 1.1200 2:3.11 720 :. .00 "n- o 10.12 3.0101 ~.M 720 :. .00 ",.~ n" . 10.001 ].l!lOl D.M em :. «I "

TABLE 4Vulsinian District· LAtera activity

~,• 10 •

,.~~,

~ MT.

.. - 0.32l& 3.'"2 145.1: 9

0.3218 1.a7 la5.3: 9

... Dopnoo O.26lI? 6.06 1511: 53 .1.laU... YIII. 12 GII 4.37

.�__t----I-'~---'-'---'-t_-------+--_t---I_--t_-'-.---.-_t--'-,_.R__+-'-"--,,-'-+-M-I

.,

.,

, '

" :22 3lI

117 : 211 311

1$<05:4

,~

,~

,~

'M

'"171.7

'R

,.,•..~.M

0.301

..~0.6....

0.6195

0.1266

1.1511

1.155.l

16.12

1l.29

10.11

•••12.05

'lux unit wl'"~I_ ... Idedn_a

.....1 nux unit"I!.he _I.,."

c.v.

"M

.:; 10

.:; 10

.:; 6.

.:; 6.

,.'"

19.11

X.X

17.13

O.lloI50

LR

I. '3

1.11631.12.5

1.0151u

11.03

10.71

11.93

10.01

l.tppu n ... unH.~I__Ie_

_ n ... _It.

...1... _le..

•

,

~ nLl.OII

LITIIOIO

g!I-'-'+~"'---+---+---+---+--t-+--+---+----+-I.. u I _..... t_hJ". vs 66 Sa 9.15 O."qa 13.66 190:. 20 67

::: E1--1I----I-------+-'--'-'-'----_t-'--'-1--'--'-t_-'--'-+---'---I-----t_--.:.-+-,:: aH ~

96 M. FQRNASERI

TABLE 5

Vulsinian District - Latera activity-DJ dating

SallPWI ." R~r.P~AS! "" UX:'~ITY~

ItlnR,AL ,.., •

" • . ... al... 146 :: ~ "Plti&l1ano ~.. .. 190:: 30 ".~~<o-phx ColHna

. ~. ...Cd<ter. .oleonlU

TABLE 6

M' Gmino acidic volcanic groupK-Ar dating

,~"~~

rad-1/• ~~ ."WC>Lln ROCX 1'YP! u,. .. • "~P." ., ...

103

•." .~.~7 " 1310 :: 80 .-

-~, quact. 'u,tie • ~ 10.0.0

• .. • ut I.,.l..,r'" ~.51 ~.' 1390 !. 7'0 .-l_clnr , ~ .." '.u " 1190 ~ ~ .-

of Vlurbo tlpieo .uet. I , .. 8.18 ,.~ V 1190 ~ ~ .-

Po.jIlonello quoru I_titleU1l6. ~ 10.97 .. ,,~ ".. CI.lr>o

,a., __!lAUDl ~ 10.59 " lISl "(..-porl""

'.~ " • .- ,~

(nor"" .....,.,1 dolle .lturo • ~ 10.44 .-.""t.) '.~ " !n~ !. 25

9\lO !. ~ ~

Ollvlne laUto

lA> PI,,"" rod •• l In. ne- • ~ ..~ ,.~ .., 9<10!.200 .-(Cl_1nl'" .\.<;t.)

TABLE 7

Vieo volcanic DistrictK-Ar dating

.~" 00.,......1/ • oo.~rood " ~.I.QCJ,UTY ~,~ ..nRUL .. ."" ,., • 10 •..,

"o<,all. L.""ltlc Iova ~" " 15.~2 , .. "p.,IQ C•••ll .... •" !l.a 2.... ~ 211

Tr..,"'," " •• 10. 7~ .-•" '.00 3-<1 :: 7~

~. "<><c_ 'GMIMS.,rJ: C

"oF L()CaRD'~~ ,. 1•• )' 2.0'. •••88 4a> :t 40

••,. UOOU •• (Tofo ,..,..0 0 .<.2rh nuo o""t.) • ~ 10.go ,.~ ' .. S20 !. 120 ....,. SorvoUI no<. po..o.......

~ .0.07 &40 !. .20 .-(IG111R8R.U 81) • '.~~ ,..

San Roe<;O Le_P",,"ollte ,~ 10.15 2.8. ,~ 700 !. ••0 ..

S.." "uU"" T......."« ~ ..pOonoI. • ~ 10.5(1 ,.~ • 820 !. .~

GEOCHkONOLOGY OF VOLCANIC ROCKS FROM LATIUM (ITALY)

TABLE 8

MI. Sabatini volcanic District

97

K-Ar dating

~

radod/• ",~U " " .~~OC~~ITT roRIIAFIOll ""TDIIAL .. rad " g •.

" -, ,.., • " ._0-

~" BACC""" 1I W. " .. Il.~ '.~ ~.' M, • "6o.ee""" IMCUJSIOII I. TOI"tI ., -

01 BRAceI""" (J.)... 8.'3 L~ 17.3 3:1-< : 48 "~

Qu..'ry.""". Turo CULLO 01 =, .. 10.:1-< L~ 10.12 370 : 70 ,".......,- &AC>l<'WAIIO (I) ~. .. 12. I 1.35 ~. -, • "Cale... rd

".. Inc."", Vole...le 10.01 ,. , ,.- ,., 2SO : 80

lBl..,.,) dl .......,_II)l2) " '-' •, 0.591 ,., 200 : 80

c...,. "" 'W, " 15.75 ,.- '3.ll9 6.0 :"XI80<1••_ STlllTIFlCAFI SAIl ID ,

.~IIICOLORI " '.00 0.952 '.M 000 :200,.!lOO :'00

~ Sl'OllTA1.593 '.~ ,

" '.00Cop<> " ...11""" = " 1.556 '.M '90 :200

(BASI) " 18.7 ,.= 23.6-0 '70 :! 50 ,

For..., I~ Pt ...... Turo 005S0 OM~ .. '" ''" "" " ~ SCOllIE M"BB ",", 12.18

" ." ".." " .=. 1<>1185 .. 12.07 " '" "

ea....... ""lIn. ("""'OIl'" of t .... .- .. I 1I.S3 " '" "" "

'/V"-'>r'1o C of 1<>1175 .. ll.'~ " ." "I.o<...dl)Ca.' dOl Cocco - .. 1l.9 " '" "0-- ..... s.... ...... ,. 7 .- .. 12.08 " '" ..So".,,,.., _.

~, .. ll.' ,... ,;s.S -, • "" ".. "o"a IIola <.0",,1 ... " 1••89 '.M ".' -..0 :! 20

Fr",,""'. ~, .. II.~ ,." "., S,. :! • "lIorl,-

Fr""",. ...._.. .. ILl 2.M .... ~, " "

(I) Scc also dating with Fission Tracks and -rh, Table 9. (2) Average value 225 ± 60 X 10" a.

TABLE 9

MI~ Sabatini volcanic District FT and """Th dating

LOC~LITT roRIIAllOll_u .00

IIOTliRlAL U~ .".m ~ ,... •

~ , ," 177 :! "_Tt cucco

(Corv...r'l ", ,

" ~ 12? :! 's ""cr~

, ," go:! ,a

_YE GBUT(> TUf'O G1ALLOSoB 7 ... .~ 200 :! 100

(iIO• ....,) ... MO. ',"" "DJ SACROF"""SOB B

">~

.~.

VOLL£DELL' IH,...RNO

VOLCAIIIC LBVBL "- " ~" " "(Ri..,.,)IN OIOTOOllTliS (oo <lo'eet'>r1

98 M. FORNASERI

TABLE 10

A/ban Hills

K·Ar dating

~ _11, ~~rad" .~'c

~" LOCALlrY ==. SAMPU; U>. " c" Rlr..,

103

•• " .TlNAI.

P~uet. " '"ph,..,..lc e,plool""o.. Covo ..,,,,,1,,,,,,,,,,. K~4<)11 ~ 11.311 " = "&&11'.-<...... ~., '.""Iu .ophclU " • ~ 17.0 ,." u., -, • "•

! ,... 17.8 2~ ~ "."""", phonol1t1c topnrlto " • ~ .5.3 ", ,." '.0 242 ~ •01_ lake "'phrltic ',""eltita " " ~ 1~.9

, .. ~., -, ,",

> ".., 101<., n_'1.1 , ~ 19.7 '"

,"I low•• ' Ion

'_phrltlc '.",cltlt. ",

" ,, ,- Divino _c. "phrltle 'o""ltlt' ~ ,~ ~ 16.07 " ,.. "; ," Divino

_.toohrHlc '.""ltit. " " " 16.2 1.13 92.' ". , ,

"•,3.25 10.5> ..• Co;><> dl .... 'e.hrHlc leu<ltlte ~ 16.10 (2) =, ..3.49 15.8

• 1.4' ,.,, C."" dl .... tophrltle 10001U.. ~ 7.05 m =, .. .., L" '.',Cop<> dl ....

"""""' ticlcyclel.o ~ 16.9() ,

~ 20.11 -, ~ "C&j>O dl .... tephr! tic '.",eltit. " ~ ~ 17.3 ,

"' '.' 292 ~ • "L.qhottQ tepll'ltlc '.""IUt.. " " ~ 17.2 ,.~ ..., ~, • "S_r. "'ph<IUe '.""ltita " ~ ~ 17.\ ,... v .• ~ , • "E••• of So""".r. "'phdUc '...,c<t>to " " ~ .5.• 1.67 37.3 213 ~

,"

1101. CoY""_ ~ 15.7 ..~ 2•.• =, ~ "," F.'co~. Tufc dl ~ 1&.9 3.~~ 42.7 ~, ~ "Mcla C.v""," Villa .. U 2.0~ 6.97 620 ~ = "!, Cia rc~tta

..~,fc....Uon ro ~ 17.2 ,." ~ 3SO! ~

, ..,,, r,..,. '0 ""t.<"",e~ ~

~~ ,., 33a ~ ., ~\Ill3_VS7 ~ " .~ 'c§ ! ! Tufo lItolda •• ~ '7.2 2.42 M.' -, • • ", I- ronta".

"0 ....e....""1. laJOer ., ~ 17.' 3.12 72.9 '" .'.'R""""".", IMajl1l1) e1 ...rlta ,,, .,~ 1&.4 3.07 57.6 467 ~ ,.. "; 11....,..,1 '" .' ~ 16.6 3.45 "., 5211 ~ • ",

i Aequo aceto.. "'pIlrlU<> h,,,,,'ttte~-

~ 1&.92 "' - ~

d ~<;qu. Ace"'" ~ 7.45 ,." 111.9~ , .. ~

2.05 17.6

'" Aeq"" Ac''''ea ....pheltlc le""it"a " " ~ 15.& 2.42 25.' 400 ~ " ..e~., Vdleranal10 tepllrl tie le"elUte " " ~ 15.6 2.86 ~. 460 ~ 6 "

Eaat or Tua<oolo la""IU"" " • ~ 11.0 ,." ~.O 4ll? :: II ",~ M"""ecclc

" ~ ~ 16.6 3.47 42.3 524 ~ 9 "'". ~rt..hlo)

(1) See Table 13. (2) Average of two values. y-speclr. 1810%, at abs. 16.0} %. (3) Average oftwo values: y-spectr. 7.18 %; at. al». 6.29 %. (4) See Table 11. (5) Probably from ErnlCl volcanoes.but ovc:rlying a ... pozzolana .. layer from the Alban Hills. (6/ Tulfitic limnsol underlying the or po-t­2olana .. layer.

GEOCHRONOLOGY OF VOLCANIC ROCKS FROM LATlUM (ITALY)

T,\BLF. IIAlball Hills· Tu/o di Villa Senni (1)

99

"'Ar·"'Ar dating

SA~PLE '" SAllPtE '" REF.MATERIAt !IF-F. ~ATE!lIAt

" x 103

• ",

x 10 •

" 342 .!. 16 " 336 =20~, ~,

278!. 15.. 348 !. 15 .." 343 1 " " 333 !. 22

VS 4A ~,

344 1 25 ".. 341 1 " " ..~ .. " 329 1 32 " 332 !. 17

~,

" m, "" 335 1 19

'" ".. 380 1 22 " 344 1

~" " 315 1 "Averaae value froJJ leucite ....ples

~',~., VS4B. VS5, VS6. ~,

338 .!. 8·x ,,' •

'I) Sampk'S from different outcrops.

TABLE 12Alball Hills· Tu/o di Villa Senni (J)

Rb-Sr dating

fORMATION 87 Rb/86,~ (87Sr/815 '"SAMPtE MAT• .. " sr/86

SrJ1REF.

,~ ". " '" x 103

a•Co, 1177 12.89 218.2 0.71141 , ,"

, ,,~ 13.00 216.6 0.711371 4~, 0.71026 ! 3 372 ! 2Q.. m 297.4 ,,~ 0.71029 1 4

~ ,0- m o,~ 0.71026 !. 4

0.7108.2 !. 6

" 1347 ~,' 107.00.71078 1 6

TUFO~~ 0.71031 , ' 337 1 50 "0.71032 1 5

" .. '" 350.5 '.0- 0.71038 1 7

VrttA " 22.7 482.8 0.112 . 0.71032 !. 3

SENNI 0.71095 ! 3

" '" 18.4 122.6O.71IOO! 12

~" 0.7103713 334 !. 2Q0. 71041 1 3.. ~, 278.9 '.W 0.71038! 4

•• 14.1 585.3 0.05 0. 71035 1 3

(I) Samples from different OUICropS.

100 M. FORNASERl

TABLE 13

Alban Hills"C. Fission Tnc and -rh daling.

LocALITY fO......'f!C'< -- -,. ,. ...--.. • 103

•

flnal «Uv'''' ~o--l'''' '19.7: 0.' "Pn><Ioeta of ft_135 "

,<>!>,..oUe R_13~ , 1 "

,-- !~i",0'0100'''''' R_13r.A "

,(""....,"0 R_135S "

," -- "_13~ 1] i "

,L."".. )

· ....&, '~Th 67 .': 5 MlI,to•• l

'~ThHott""" · '" 43 .': 5 •_.

· '" n ~:. 9 "M.

," n 340 :. 70

" ," n ,&0 :. 30

vUl. •'"-, ,

" n 300 :. ""," n =, "

TABLE 14Hernican District (Mt. Ernici)

K-At dating

._- -0......011• ""..._~~

='" ~~ "'",,'0' • ~.

• • ,0-' • ,0" •

~".- .~ " 13.40 '.&!oII' " """:.20 '"....,.. ." " ,.. l.vn .,400 -'- "" "

Cl~li.... -....._, " lO." 2.~"" " ~, • "T......'''. 1."""". • ... a.ale' • ~, "

c" c...... , ••0.....« 1O",,"'t. .. • .." 0._' ... ~, • "c" Co"ot. • 0.6_ •• =. • "T......"le 1.""it'" .. •.?l

o.~. •• 'iO ::: •ch ""... ll.... ......."u. '" " ".13 ' .•eo> 1'.' """'-'- ..,

'",_,,It. • .." 1.719J " oeCI:i!O

C" ..,...... t ... 1.<_1........1< .. • •••• 0.2'51 , , .010 .0:'00 "c'O Spl..... ' ... ......,.." ....." .. • 2.1. 0.111" ,. 2:'0 -'- !l0

"(d.... 1 ",<>tk_ ,. .. • .. ,. 0.1= ,., '00 -,-'"

0" ..'n....' ... ""...,.."....." ~ , • ,.OO O.IOW ,. '100 -'- -0 ",.. " ...., ....... " """ ~ '0 ..!'Of!. hi ....... ......."'e ,...,!tIt.

d ... o.?W .. ,,,, ~ ZO

". ~'Ut. ... • 5.lg ..~ '0.2 0'" :!. 20 "C,· vu.'1l O••n.- U 'XI:!. JO

",!'On) !.otic"it. '" • 5.0' 0.25'0 ,.. no :!. •..- ,..",'""""Ho , .. 0.'_ KI:!.40

"(Por!) """""' ..." ....a • ••W ..- ,.. "'o:!. 60.......,,_t.-Itle '.oolt". ,,' " ,.... 2.'- "." _:!.zo ..

1!'Or! )

Cl..., .. , • 2.~1 0.0'" ,.. :t"1O:!. .., "._- '''H•..,~.lt .. • ,.. 0.'_ ,., "o:!. eo "

& !:!: g

. .,.. 000g"L .,., 'j

: ":'" ~

,. .i ,I ;: '

! •I 1

5 III ..i i

l I. ,

.~-ll

~

Jc~

•­•~'" "-."

" ". ,~'"....

•N<o

'"

.1 jI !, :. ,. -

¥10UII1Yd

~ ": "!

!3 51 :

•••o 0 0., • t .,

••.. ..

•

,= II ,i ::: i i; .f; ;i li i

~ ~ ~

HI

I ., I G !. ;; ::: : ,I: , 1 I ,~~":oi~

.. :si ~ • ~..... ., ..

•

•

I,

,!

,,~

3!l:!::~t:!::000000"""""01

• • •lC:~'i~::i~

:!:: ;;: ilil I!' :::: I.. -:.,;..:.:.:

1,•,

! i i ~•~ i ~ ~

1 H 1

'~l!l~~!lii~I!~.

;;llllj~jllllllll

.. ~ ill 3~ .,; .,; ..

~ ~ 1Il r.::

~~~~~~~t'!·~~~~I~~.. ... .. ... ... .. '" ... .. .. .. ... .. ...

:!:: !:; : So 0 0 0 '""""" ..

to .. ! ... v.. ,.; .. .

!, I.,, II _

~ ~o 0., .,, ,o 0

ONUUS

~"

I I .:! !Ii

• •Jl ::l ~

;;:;~!lll,,; ,,; oi .,;

~ ~ ~ ~

Illl

, ~ ilil.. ..-:

§ i, . - ,...

,,., .,~

•

•;

.,

,,

,,

!

1

'I

, ..; 0

• •

t t ~ ~ :::

I II 11

lI L i I :~ji!GG~

j:I~jj

j j j

::IlIl-c'': .. 1l1!l00000000

v v v v v

• •~ oi 5

n.QUi"'~"~lll~~"

INUOJ.NilA

~ i

11111;;;;;;

lilQIl:'.~1!13. . ~ .: .. ..: ..: .,;

~

.

tIn

"

I I0'1 ':1:1 !1H I

~

~

~

~

••

i,l'•

•i}•.....J :..

.§ ~ ~~r!r!r!r!r!tr!r! r! I ~~ttl :::::::::~~::::::1:1:1:I!I!I::::::~ I

~ .. ~ ~~~ ~~ " ........ "'''' .......~ tl ~ .,...,.,., -: -: -: ~ -:

4; v~~, ..........• ~ : 0 .. V V V V V

c~

c

•N<C

'"

•~

~ "

."

" ". ,~'"....

"•~u

~~~

"o>§?~%

~o

>,5~•"

~-

o-

102 M. FORNASERI

TABLE 17

Volcanic level interbedded in sedimentary Imits

K-Ar and FT daling

"",,-,n STRATICRAPHlC POSITION u=~ """"" ""'. ~.." •Marco SI_ Quarry VolcanIc l,.."d 1nterbed<led " .-~ 4.20 • 0.2 ,V1. P.le-bare.. 1n blue -..1 ... Pliceene ~ .-~ 4.34.!. 0.8 ,~ntaNo, "- cl.y. a lass " 4.03 !: 0.93 "

alas. " 3.32 • 0.30 "a 1.." " 3.18 :.. 0.31 "

V.ll .. Rice. Volcanic "she. level a l .." " 2.13 • 0.27 "Montel"(\tondo inte..bedded 1n ....Ine glUG " 2.03. 0.26 ",- " ..ndy clays

Co..chl"no Volc..nlc level ove.. lylna3.85.:.. 0.5 4,23a l ...." "Viterbo Middle PlIce.,n" ch)'.

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(:EOCliRONOLOCY OF VOLCANIC ROCKS FROM LATlUM (ITALY) 105

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