Soc. Itlljillnll di MinerlllogtG e Petrologill _ BENJ)1CONTJ, Vol. XXXIl (1). 1916 ; 1l1l . 147-JSZ
W ALTER ALVAREZ·, MASSIMO NICOLETII··, CLAUDlO PETRUCCIANI··
POTASSIUM.ARGON AGES ON PYROCLASTIC ROCKS FROM lliE PLEISTOCENE SABATINI VOLCANIC DISTRICT, NORTH OF ROME
ABSTRACT. - Potassium.argon age determinations have beed carried out on two volcanic formations in the Pldstocem: Sabatini Volcanic District north of Rome. Mudflows near the base of the" Tufi Stratificati Varioolori de' La Storta» (a stratified tuff sequence) gave several ages clustered at about 0.49 m.y., indicating that deposition of this unit began very shortly after emplacement of the "Tufo Rosso a Soorie Nere », a Widespread ignimbrite that has previously been dated at about 0.5 m.y. The "Tufo GiaUo di Sacrofa no .. , a yellow ignimbrite that filled valleys eroded through the two units mentioned above, gave an age of 0.}7±0.07 m.y. These (Wo new dates are in excellent agreement with observations on the volcanic stratigraphy in this area. RIASSUNTO. - Sono stati datati 001 metodo K·Ar due prodotti del vulcanesimo sabatino nella wna delJa Valle del Treia. I valori ottenuti sono di 0,49 m.a. per akune oolate di fango alia base dei" Tufi Stratificati Varicolori de' La Storta .. , e di 0,37±O,07 m.a. per il "Tufo Giallo di Sacrofano », un'ignimbrite che ha tiempito un sistema di valli erose dopo la deposizione dei "Tufi Stratificati Varicolori de' La Stotta ». Questi valori sono in ottimo acoordo con osservazioni dettagliate swla stratigrafia vukanica in questa wna.
Introduction
We report here the results of a geochronologicalstudy of two volcanic units in the area Districts of interfingering between the Vico and Sauatini Districts - the central two of the four districts that make up the Pleistocene Roman Volcanic Province. As a general rule K/ Ar dating of rocks as young as Pleistocene has proven very difficult because of the low levels of radiogenic argon, but in the case of the Roman Volcanic Province· this difficulty is at least partially offset by the high potash content of the rocks. Previous KjAr age determ!nations on rocks of the Vico and Sabatini Districts have been published by EVERNDEN et al. (1964), EVERNDEN and CURTIS (1965), NICOLETII (1969), BONAOONNA and BIGAZZI (1969), AMBROSETII et al. (1969), BONADON NA and BIGAZZI et al. (1970), fission track ages have been reported by BIG AZZI et al. (1973) .
• Lamont.Doherty Geological Observatory of Columbia University, Palisades, New York 10964, U.S.A .• Contribution n. 2326 . •• Istituto di Geochimica dcl.l'Universid. di Roma. Centro Srudi per_ la Geochimica Applicata alia Stratigrafia Recente. 148 W. ALVAREZ, M. NICOLETTI, C. PETkUCCI,,-NI
The stratigraphy is punctuated by three ignimbrite units, each about 30 m thick. The middle ignimbrite unit is the ( Tufo Rosso a Scatie Nerc) (MATTIAS most important of the Vieo ash flow tuffs. As mentioned above, this unit has ~n and VENTRIGLlA, 1970 = f: Ignimbrite C" i.ocAR.ol, 1%5; BERTIN I et al., 1971), the dated at about 05 m.y . (0.43 m.y., EVERNDEN and CURTlS, 1%5; 0.52±O.l2 m.y., NJCOl.EITI, 1969).
Geologi~al setling
The Vice District comprises a single composite volcano located about 50 km north-northwest of Rome. Rim elevations reach 96S m eM. Fogliano) and the summit is marked by a caldera containing the Lago di VieD and a small, late fODe (M. Venere). The principal output of the Vico Volcano has bttn a series of four ignimbrite units (l..ocARnl, 1%5 ; MATTIAS and VENTRICLI_A. 1970; BERTINl et aI., 1971) which in most directions extend out to about 20 km from the crater. The most im portant of these ignimbrite units is the «Tufo Rosso a Scoric Nere. (MAlTIAS and VENTRICLIA, 1970) or «Ignimbrit(, C . (l...ocARDJ, 1965), fo r which sev('ral K j Ar dates of around 0.43 m.y. (EVER NDEN and CURTIS, 1965) and on(' of 0.52 ± 0.12 m.y. (NICQ LETTI, 1969) have been obtained. ALVAREZ d: al. (1975) hav(' shown that th(' « T ufo Rosso a Scari(' Nere. can be trac('d 50 km south and southeast from th(' Vico Crater, nearly to the outskirts of Rome. In addition to the four ignimbrit(' units, the Vieo Volcano erupted a large amount of pyroclastic material in th(' form of mudflows and air-fall tuffs. Lava flows are a rdatively minor component, occurring mostly near the crater; they have been studied by AMENDOUG INE and al. (1963). MAlTIAS (1966), and CUNDAltI and MAlTlAS (1974). The Sabatini District is marked by an east-west lin(' of maars 25 km northwest of Rome that link the large collapse caldera of Lago di Bracciano to the west with the Sacrofano Crater to the ('ast. The Sacrofano Crater was formed by collapse of a volcano that was probably the source of s('veral units, including one or more ignimbrites. Gen('ral studies of the Sabatini District hav(' been published by &HERn.LQ (1947), MATTIAS and VENTRICLIA (1970) and BERTINI et al. (1971); a census of the craters is given by MAlTlAS (1970) and the maars and their base surge deposits have been sludi('d by MAlTSON and ALvAltEZ (1973). In the area betwe('n the Vico Volcano and the Sabatini craters products of the two districts interfinger (MAlTIAS and VENTRICLIA, 1970). Th(' stratigraphy in this area is very complicat('d because of competition betwe('n stream erosion and volcanic deposition, which has result('d in extensive channelling on all scales (ALVAREZ, 1972, 1973). The sampl('s dated in the present study were collect('d in the area of interfingering betwttn the two districts because available matigraphic control in this area establishes the chronological order of the various units and provides a tie betwttn the volcanic ~ue nc es of the two districts. POTASSIUM-ARGON ACES ON PYROCLASTlC ROCKS ECC. 149
Stratigraphy of the sampling area (Fig. 1) Our samples were collected in the area of the 1 :25,000 map sheet c Nepi ) (143-I-SE). In this area the Treia River and its tributaries have cut canyons up to 100 m deep that expose the entire volcanic sequence together with the underlying Pliocene day and sand. In the central part of the Nepi sheet (ALVAREZ, 1972) the volcanic sequence rests on Lower Pleistocene gravels marking the bed of the Paleotiber River.
~ Tun STRATl flCATI r--I [UfO GIALLO 01 ~ m lCGLGRI DELLA STORTA L----1 SACROFAHD
Fig. l. ~ GeOlogICal map of Ma'l.Zano Romano area from Mallias e Vcntriglia (1970). rcvisc:d by Alvarez. 150 W. ALVARU, M. NICOLETTI, C. PETRUCCIANI
The lower ignimbrite (.: Tufo Giallo della Via Tiberina :.) and the upper ignimbrite (c: Tufo G iallo cli Sacrofano :.) are both products of the Sabatini District and probably of the Sacrofano Volcano (MATTIAS and VEN'TRIGLl.... , 1970; ALVARll, 1972).
In this study we have dated samples from two stratigraphic units. Samples SAB-IO and SAB-I5 are fro m the Tun. Stratificati Varicolori de'La Storta. In the Teeia Valley area this unit is composed of orange or golden
Analytical technique
Potassium-argon radiometric determinations were carried out on crystals of leucite, sanidine, and biotite, separated with sta ndard enrichment methods (heavy liquids. magnetic separator). Potassium was determined by Flame Spectrophoto metry; controls have been carried out using as reference International Standards. Argon was determined by the methodology used in previous studies (NICOLEITt and PETR UCC IANI, 1974); using as reference standards Muscovite P207, Muscovite Berna 4M, and Phonolite MZ. These standards have given the following values (standards in parentheses): 805± ! (81± I); 183± O.8 (18 .7 ± 05); 75± 03 (7.4± ±02) m.y.
The results obtained are summarized III Table 1.
(1 ) In other areas these uni ts have a level contact which shows sign of erosion (Sou;· IlLW, 1940). POTASSIUM' AROON AGE S ON PYII,OCLASTIC IlOCKS ECC. 151
TA8LE 1 ExpmmmtaJ results
.oAr C.C. S. T . P . J!A!! 40", ""D:Io ~ ,H 5ompl•.o • ".Y. ~ IAft·' ""'10 <110110 oIi ",°ll'n" r< , ...... w, 50 •.-.1.... .1 ..... to . m 1Qu.....,.""", ...1.,,_ . ,.-, '0.11 ,o.u O. U!(I. OT ...... c.leo .. rood) ..""
u,,_,o 't'afI """(1Iq. . ..,~., ... N, 0'...... w, IAwoh. • . ..1 • • lO-' 'Ln . . U ! O. ' U ••o1 ... _ ...... ~ .. U .... I "...... It.... 01 c.-lo ...' _._ _'" 0."$0 • " ' ' •• 1 L ..! o.. . .. T_:I ""8_11 't'afI ...... lf$c& . .. ",#" N; _._ w, _'oo 0...... 1'-' LloO!e. , 0..-', U _0.11 eoIorl ._. n ... p .. "' ...... ••• ••• 0.1"• • ' , '.M L ' ~. ' .• ~os ..,,. lI. n o...... 01-=-...... LSOU · .....
• Loo,iI\I DiKUllllion These new dates are in excellent agre(:ment with .detailed geological observations on the volcanic stratigraphy in the Treia Valley area (ALvAREZ, 1972). As discussed above, the uneroded contact between the lowest mud How beds of the c Tun Stratificati Varicolori de' La Storta. and the underlying c Tufo a Scorie Nere. ignimbrites suggests that there should be little difference in age between these two units. Thus the five new dates clustering around 0.49 m.y. for the basal mud Rows (SAB-lO and IS) are in good agreement with the previous dating of the c Tufo Rosso a Scorie Nere. at about 0.5 m.y. A comparison of the dimensions of the modern canyons of the Treia Valley system with those of the paleovalleys that were filled by the c Tufo Giallo di Sacrofano. led us to anticipate an age for this unit of somewhat more than half that of the c Tufo RO$so a Scorie Nere •. The age obtained, 037±0.07 m.y .. is thus reasonable on the basis of stratigraphic observations. Considerably younger dates have been reported for the c Tufi Stratificati Vari· colori de' La Storta . in the Riano area, about IS km southeast of the Treia Valley area (BoNADONNA and BICAZZI, 1%9). This difference in age between the Riano area ::l.Il d the Treia Valley area is not necessarily contradictory, since a stratified tuff sequence like the c Tufi Strati ficati Varicolori de' La Storta. may represent deposition during a long interval of time. We conclude that in the T reia Valley area depbsition of beds assigned to this formation began immediately after emplacement of the c Tufo Rosso a Scorie Nere., and terminated well before emplacement of the c Tufo Giallo di SacrofaDo. ignimbrite at 037±0.07 m.y. In the Riano area, where the cTufo Giallo di Sacrofano. is not recognized. deposition of beds assigned to the c T ufi Stratificati Varicolori de' La Storta. continued until about 0125 m.y. 152 W. ,u.VARE2, M. NICOt.ETTI, C. 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