Large mafic eruptions at Alban Hills Volcanic District (Central Italy): Chronostratigraphy, petrography and eruptive behavior

Journal of Volcanology and Geothermal Research 179 (2009) 217–232

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Large mafic eruptions at Alban Hills Volcanic District (Central Italy):Chronostratigraphy, petrography and eruptive behavior

Fabrizio Marra a,⁎, Daniel B. Karner b, Carmela Freda a, Mario Gaeta a,c, Paul Renne d,e

a Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italyb Department of Geology, Sonoma State University, 1801 East Cotati Avenue, Rohnert Park, CA 94985, USAc Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale A. Moro 5, 00185 Rome, Italyd Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USAd Department of Earth and Planetary Sciences, University of California, Berkeley, CA 94720, USA

⁎ Corresponding author. Tel.: +39 0651860420; fax: +E-mail address: [email protected] (F. Marra).

0377-0273/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.jvolgeores.2008.11.009

a b s t r a c t
a r t i c l e i n f o
Article history:
Despite its ultra-potassic, Received 1 April 2008Accepted 6 November 2008Available online 21 November 2008
Keywords:Alban Hills40Ar/39Ar geochronologyexplosive eruptionsK-alkaline magmaspyroclastic-flow depositsvolcanotectonics

basic geochemistry (40≤SiO2≤50 wt.%), the Alban Hills Volcanic District wascharacterized by a highly explosive phase of activity, the Tuscolano–Artemisio phase, which emplaced verylarge volumes (several tens of km3 each cycle) of pyroclastic-flow deposits, mafic in composition(SiO2≤45 wt.%) in the time span 600–350 ka. In contrast to the abundance of pyroclastic-flow deposits,very scarce basal Plinian deposits and, more in general, fallout deposits are associated to these products.While some of the pyroclastic-flow deposits have been described in previous literature, no specific work onthe Tuscolano–Artemisio phase of activity has been published so far. In particular, very little is known on theproducts of the early stages, as well as of the final, post-caldera activity of each eruptive cycle. Here wepresent a comprehensive stratigraphic and geochronologic study of the Tuscolano–Artemisio phase ofactivity, along with new textural and petrographic data. We describe the detailed stratigraphy andpetrography of five reference sections, where the most complete suites of products of the eruptive cycles,comprising the initial through the final stages, are exposed. We assess the geochronology of these sections bymeans of 18 new 40Ar/39Ar age determinations, integrating them with 16 previously performed, aimed todescribe the eruptive behavior of the Alban Hills Volcanic District during this phase of activity, and to assessthe recurrence time and the duration of the dormancies.The overall explosive activity appears to be strictly clustered in five eruptive cycles, fairly regularly spaced intime and separated by very long dormancies, in the order of several ten of kyr, during which no volumetricallyappreciable eruption occurred, as the lack of deposits dated to this time-interval testify.We propose a volcano-tectonic model that explains this peculiar eruptive behavior, unparalleled in the other coeval volcanic districtsof the Tyrrhenian margin of Italy, as related to the local transpressive tectonic regime.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

The Alban Hills Volcanic District (hereafter AHVD) is part of theRoman Province (Washington, 1906), a NW-striking chain of potassicto ultra-potassic volcanic districts that developed along the Tyrrhe-nian Sea margin of Italy since middle Pleistocene time (Fig. 1). Theproximity of this volcanic district to Rome and the fact that it cannotbe considered extinct (Funiciello et al., 2003; Marra et al., 2003; Marraand Karner 2005; Freda et al., 2006; Giaccio et al., 2007) have providedthe motivation for detailed studies of the AVHD. However, nocomprehensive papers specifically devoted to the whole pre-calderaactivity, characterized by the emplacement of mafic pyroclastic flowdeposits (“Tuscolano–Artemisio Phase”, De Rita et al., 1988) have beenpublished so far. Here we provide a detailed lithostratigraphy and

39 0651860507.

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chronostratigraphy of the Tuscolano–Artemisio (hereafter T–A) phaseof activity that enables us to identify the eruptive phases of eachmajorcycle, to discuss the eruptive behavior, and propose a volcano-tectonicmodel for the AHVD. We describe five type localities where theproducts of the T–A phase of activity are exposed, and the stratigraphyof a 173 m-deep borehole (CA-1, Mariucci et al., in press) that crossedthe whole volcanic succession. We present 18 new 40Ar/39Ar agedeterminations and integrate 16 previously determined 40Ar/39Ar ages(Karner and Renne, 1998; Karner et al., 2001; Marra et al., 2003) toestimate the duration of dormancy periods during this phase ofactivity. Moreover, we provide new textural and geochemical data onall eruptive units from the T–A phase.

2. State of the art

A summary of the stratigraphic and petrographic features of theproducts of the investigated phase of activity at the AHVD is reported

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Fig. 1. Simplified geologic map of the Alban Hills Volcanic District (modified from Fornaseri et al., 1963; Marra et al., 2003; Giaccio et al., 2006; Giordano et al., 2006), focused on theproducts of the T–A phase of activity. The location of the five reference sections proposed in this paper (black rectangles) is shown.

218 F. Marra et al. / Journal of Volcanology and Geothermal Research 179 (2009) 217–232

in Table 1, along with a historical overview on their nomenclature.The activity, stratigraphy, geochronology, and petrology of the AHVDwere described in many papers, starting with the comprehensivepaper of Fornaseri et al. (1963), which also provided a geologic map at

1:100,000 scale. A 1:50,000 scale geologic map of the AVHD wasproduced by De Rita et al. (1988), which provided a new stratigraphicsubdivision of the volcanic units into three main phases: 1) an earlyexplosive T–A phase followed by caldera collapse; 2) an intermediate

Table 1Lithostratigraphy of the eruptive cycles from the T–A phase

Eruptive cycle Deposit Age(ka)

Previous name Features

1 Tufo Pisolitico di Trigoria (TPT) 561±22) ● Tor de' Cenci Unit3) Two geochronologically undistinguished deposits with animmature paleosol in between: yellow to gray, composite,prevalently fine-grained, indurate pyroclastic ash-flow, withaccretionary lapilli, mm-sized gray and yellow scoria clasts,abundant fresh Lc and Cpx; hydromagmatic character6).

1a TPT-b ● 1st TA Pyroclastic Flow4)

1b TPT-e ● Tufo Cineritico Grigio5)

● Tufo Pisolitico Grigio5)

2a Tufo del Palatino (TP) 530±22) ● Palatino Unit3) Two geochronologically undistinguished deposits separatedby a paleosol: dark gray, massive, indurate pyroclastic ash-flow,with dark gray and orange mm-sized scoria clasts, Lc, and Cpx.

2b Tufo di Acque Albule (TAA) 527±21) ● 1st TA Pyroclastic Flow4)

● Tufo Granulare Grigio5)

● Peperino Grigio5)

3 Ash-fall succession (AF) 517±11) – Alternating layers of well sorted, poorly vesiculated, gray andreddish scoria, and massive yellow ash.500±31)

4 Vallerano Lava Flow1) 457±52) ● Acquacetosa Lava flow5) Leucititic lava flow.Pozzolane Rosse (PR) 456±32) ● 2nd TA Pyroclastic Flow4) Loose, massive pyroclastic ash-flow deposit with mm- to dm-sized,

reddish to gray scoria clasts, abundant fresh Lc, and lava,holocrystalline, and termo-metamorphosed sedimentary lithics.

● Pozzolane Rosse, or di San Paolo5)

5 Fioranello Lava Flow Leucititic lava flow.Pozzolane Nere (PN) 407±22) ● 3rd TA Pyroclastic Flow4) Loose, massive pyroclastic ash-flow deposit with mm-

to dm-sized, brown to gray scoria clasts, abundant Lc,and lava and holocrystalline lithics.

● Pozzolane Nere o di Tre Fontane5)

6 Villa Senni Eruptive Sequence7): 365±41) –

–

–

● 3rd TA Pyroclastic Flow4) TL: massive, strongly zeolitized pyroclasatic ash-flow deposit,light orange-pale yellow in color, with altered Lc, mm- tocm-sized dark gray scoria clasts, and sedimentary and lava lithics.

6a Tufo Lionato (TL), ● Tufo Rosso Litoide5)

6b Pozzolanelle (PZ) –

● 4th TA Pyroclastic Flow4) PZ: unconsolidated to consolidated, gray to reddish-brown andorange, massive block and scoria flow deposit, with cm- todm-sized black to purple scoria clasts, up to cm-sized, fresh Lc andtypical Lc-Cpx holocrystalline clasts (Italite, Washington, 1920; 1927).

● Tufo di Giulianello5)

● Tufo di Villa Senni5)

1)This work; 2)Karner et al. (2001); 3)Marra and Rosa (1995); 4)De Rita et al. (1988); 5)Fornaseri et al. (1963); 6)Palladino et al. (2001); 7)Freda et al. (1997).

219F. Marra et al. / Journal of Volcanology and Geothermal Research 179 (2009) 217–232

strombolian and effusive Monte delle Faete phase; and 3) a lateHydromagmatic phase. Revision of the general stratigraphy in De Ritaet al. (1988) was proposed in Marra and Rosa (1995) and was includedin De Rita et al. (1995). A multidisciplinary book (AA.VV, 1995)summarized the volcanology of the AVHD and used it for volcanichazard evaluation.

High-precision 40Ar/39Ar geochronologic studies of the eruptiveepisodes in the AHVD were conducted by Karner and Renne (1998),Karner et al. (2001), and Marra et al. (2003). Karner et al. (2001)included a detailed review of the complicated informal lithostrati-graphic nomenclature that had developed for the AVHD, and proposeda formalized lithostratigraphy for the AHVD. These new age data alsoenabled Marra et al. (2003) and Freda et al. (2006) to suggest aneruptive history that was more complex than that previouslyproposed. Another revised nomenclature, along with a thoroughstratigraphic study, accompanied by a general description of thestructural and petrologic framework, was provided in Giordano et al.(2006), though some interpretations therein conflicted with the 40Ar/39Ar age studies and with the formalized stratigraphy introduced byKarner et al. (2001).

The products of the AHVD showa composition ranging from tephri-phonolite to tephrite and K-foidite with notable absence of plagioclase(Trigila et al., 1995; Marra et al., 2003; Freda et al., 2006; Gaeta et al.,2006). Even the most differentiated products are characterized by lowSiO2 content and display a modal composition consisting of clinopyr-oxene and leucite crystals. The uniqueAlbanHills liquid line of descendis explained (Gaeta et al., 2006) as a consequence of interactionbetween magmas and carbonatic host rock of the substratum.

In spite of the mafic composition of its product, the T–A phase ischaracterized by highly explosive behavior, with lack of significanteffusive activity. In the case of the first eruptive cycle (Tufo Pisolitico diTrigoria, referred as Trigoria–Tor de Cenci eruptive cycle, Palladinoet al., 2001), which shows marked hydromagmatic features, this hasbeen explained as the combined effect of rapid decompression of themagma chamber, due to possible tectonic trigger, and of shallow

magma–water interaction. In contrast, the explosive behavior of thelater, dry eruptive cycles is probably a consequence of abundant CO2

within the volatile fraction (Freda et al., 1997). Another distinctivefeature of the products of the T–A phase of activity, despite the highexplosive behavior, is the paucity of basal Plinian fall deposits, whichare characterized by low degree of vesiculation and substantialabsence of pumice clasts. This fact may also be interpreted as due tothe presence of considerable CO2 in the eruptive mixture, preventingthe rise of a buoyant, convective plume, owing to the density contrastwith the surrounding atmosphere (Freda et al., 1997).

3. Geochronology data

The 40Ar/39Ar ages reported in this paper (Table 2), obtained duringthe years 1993–2005 at the Berkeley Geochronology Center, consist ofnew and previously published data (Karner and Renne, 1998; Karneret al., 2001; Marra et al., 2003). Every effort wasmade during this timeto conduct these measurements following well-accepted procedures.Nonetheless, over this twelve-year period, analytical methods (pre-paration, analysis and data reduction), laboratory equipment, neutronflux monitors (standards), and ages for those standards, have variedmoderately. We have taken efforts to minimize the systematic effects(e.g. revision of the ages of the standards) that have occurred duringthis time interval, but analytical errors associated with individualexperiments cannot be minimized when combining data fromdifferent experimental runs. Consequently, some of our age estimates,determined by combining data from different experiments, havegreater distributions than onewould expect for a single population. Tobe conservative, we choose to include all ages that were internallyconsistent (less than 2σ from the error-weighted mean value) tocalculate the age of each eruptive unit.

For the new 40Ar/39Ar analyses, we used facilities and proceduressimilar to those described by Karner and Renne (1998). Neutron fluxmonitors (standards) used for these experiments were Fish CanyonTuff sanidine (28.02 Ma, Renne et al., 1998) or Alder Creek Tuff

Table 240Ar/39Ar ages

Lab. n. Sample Locality Unit Age (ka)±2σ

Tufo di Fosso CollerasoPyroclastic-flow deposit

1) 11406 (AH20-C4) Cave TFC-a 608±1Monti Sabatini First Air-Fall deposits

2) 11378 (AH20-C6) Cave SF-0 (MSVD) 587±33) 33154 (FoMFC1) Malafede – 591±6

Tufo Pisolitico di TrigoriaPyroclastic-flow deposit

10809 Trigoria TPT-b2 561±2a

3 0188 Magliana TPT-b2 560±11b

Late Pyroclastic-flow deposit4) 11373 (AH20-C3) Cave TPT-h 555±1

Tufo del PalatinoPyroclastic-flow deposit

5) 33811 (AH22-C9) Colleferro TP-b″ 539±811088 Trigoria TP-b″ 532±4a

6) 33809 (AH20-C12) Cave TP-b″ 531±810803 Via Flaminia TP-b″ 530±2a

Tufo di Bagni AlbuleBasal scoria-fall deposit

7) 33813 (AH23-c2bis) Via Tiburtina TAA-a 530±12Pyroclastic-flow deposit

8) 33803 (AH22-C13) Colleferro TAA-a 537±109) 11389 (AH20-C13) Cave TAA-b 527±2

Alban Hills air-fall sequenceScoria-fall deposit

10) 11390 (AH20-C15) Cave AF-a 517±1Scoria-fall deposit

11) 11404 (AH20-C17) Cave AF-d 500±3Scoria-fall deposit

12) 34408 (AH23-AFc) Via Tiburtina AF-c 506±10Pozzolane Rosse

Vallerano Lava Flow30179 Vallerano VL 457±5b

Pyroclastic-flow deposit30178 Vallerano PR-b 456±3b

Final scoria-fall deposits13) 11377 (AH21-C1) Zagarolo PR-e″ 442±3 ka

Conglomerato GialloReworked deposit

14) 33807 (CG) San Paolo CG 437±32(youngest)

Pozzolane NerePyroclastic-flow deposit

30173 Via M. Tiburtini PN-d' 407±2b

Final scoria-fall deposits15) 11396 (AH21-C3) Zagarolo PN-e 411±216) 34403 (AH21/2-C7) Zagarolo PN-g 410±11

Villa SenniTufo Lionato pyroclastic-flow deposit

17) 33160 (fiammae in LFL) Fioranello VS-e 365±47593 Campidoglio VS-e 360±30b

Italite clasts in the Pozzolanelle pyroclastic-flow deposit18) 33165 (Ca28) Fioranello VS-f 366±7

Peri-calderic hydromagmatic activityPrata Porci Crater surge-flow deposit

11354 Prata Porci VS-g 373±6c

Valle Marciana Crater scoria-fall deposit11349 Valle Marciana VS-g 370±2c

Pantano Secco Crater scoria-fall deposit11336 Pantano Secco VS-g 365±3c

Final scoria-fall activity of the peri-caldera scoria conesScoria-fall deposit

11335 Nemi VS-i 364±3c

Re-worked scoria-fall deposit11090 Cat. San Sebastiano VS-i 358±1a

Final lava flows11325 Monte Ferrari VS-i 357±9c

11073 Monte Castellaccio VS-i 351±4a

Samples dated for this work are numbered 1–18.a From Karner et al. (2001).b From Karner and Renne (1998), previously re-calculated in Karner et al. (2001).c From Marra et al. (2003).


Fig. 2. Probabilistic distribution of ages determined on 48 xenocrystals picked from four samples of hydromagmatic products of the AHVD, previously analyzed (Marra et al., 2003).

Fig. 3. Stratigraphic column of the Cave reference section (AH20).



sanidine (1.194 Ma, Renne et al., 1998). Leucite phenocrysts were usedfor all age analyses, but sanidine phenocrysts were used for twosamples from the Monti Sabatini Volcanic District (MSVD) that wehave dated in this work. The complete 40Ar/39Ar age data for theseanalyses are in the Appendix. A summary is reported in Table 2.

Typically, six or more single-crystal total-fusion analyses wereperformed on each sample. However, some of the samplescontained crystals that were too small to yield high-precisionsingle-crystal ages. In these cases, multiple-crystal total-fusionanalyses were made. Additional analyses were made on samplessuspected of contamination by xenocrysts, in order to improve thechance of resolving the youngest age population. Ages are calculatedfrom the error-weighted mean 40Ar/39Ar ratios (Renne et al., 1996).The best age estimate for each sample is the error-weighted meanage from the youngest statistically-consistent population of crystals,whereby the individual measurements were required to be within2σ of the error-weighted mean value for inclusion in the best agecalculation. This criterion was used to identify and eliminatexenocrysts or alteration from the best age estimate. Ages arereported with 2σ analytical precision in Table 2 and in the text. Alsoincluded in Table 2 are the re-assessed ages of 16 volcanic samplesdated previously.

Finally, in this work we have selected and analyzed the crystalpopulations from four samples (AH-8, AH-10, AH-4, AH-1G) of asmany hydromagmatic centers of the Alban Hills (Prata Porci, ValleMarciana, Nemi, Ariccia) dated previously, which contained evident

Fig. 4. Detail location map (image from Google Eart

xenocrystic contaminants older than 600 ka (Marra et al., 2003). Weinterpret these xenocrysts entrained in the deposits during thehydromagmatic eruptions as deriving from the volcanic substrate.We have selected 48 ages on as many single crystals and plottedthem in the histogram of Fig. 2 where a significant peak around820 ka is evidenced, along with two minor peaks at around 710 kaand 910 ka.

4. Reference sections for the Tuscolano–Artemisio phase

Figs. 3–8 describe single and composite sections at five localitiesthat we propose as type sections for the T–A phase volcanic deposits.These sections, correlated in Fig. 9, enable us to study vertical andlateral variations of the T–A phase extrusive deposits. As mentionedabove, samples of 18 pyroclastic units from these and other localitieswere newly dated, and these data were combined with 16 previouslydetermined dates, some of which have been re-calculated and/or re-evaluated in this paper, in order to provide the most completegeochronology for the proposed type sections (Table 2; Karner andRenne, 1998; Karner et al., 2001; Marra et al., 2003). While some ofthese ages reported previously are from other sections, we havecorrelated them with high degree of confidence with the proposedtype sections based on age, petrology and stratigraphic constraintsprovided by the main pyroclastic flow deposits.

Moreover, we compared these reference sections to the strati-graphy of a recently drilled borehole in Santa Maria delle Mole (CA1,

h®) and pictures of the Cave reference section.

Fig. 5. Stratigraphic column, detail location map (image from Google Earth®) and pictures of the Colleferro reference section (AH22).


Mariucci et al., in press), which, being located near the eastern rimof the T–A caldera (see Fig. 1), provided the most proximalstratigraphic record of the T–A phase of activity recovered so far.Indeed, in the first 172.7 m, the borehole crossed the completevolcanic succession pertaining to this phase of activity. For adetailed description of the stratigraphy of the volcanic units weaddress the readers to Fig. 5 in Mariucci et al. (in press). The mostrelevant observation inferred from the CA1 stratigraphy is theabsence of previously unrecognized volcanic units, notwithstandingthe proximity to the vent area of the drilling site. Only the productsof the main five eruptive cycles, along with a scoria-fall layercorrelated to the 517 ka deposit of the Ash-fall succession, are foundin the CA1 cores. Another relevant observation is the paucity of thebasal fallout and surge deposits of the initial, Plinian phases of eacheruptive cycle. These never exceed a few decimetres in thickness,with the exception of the Villa Senni Eruptive Sequence reaching upto 2 m.

5. Microtextural and chemical features

Juvenile scoria clasts occurring in the pyroclastic flow unitsforming the T–A phase deposits are described in details in Fornaseriet al. (1963), Trigila et al. (1995), Freda et al. (1997), and Palladinoet al. (2001). However, scoria clasts occurring in pyroclastic flowsare not suitable for chemo-stratigraphy purposes (with the onlyexception of the vitrophyric scoria clasts occurring in the Tufo Lionatopyroclastic flow; cfr. Freda et al., 1997; Gaeta, 1998). Alban Hills K-foiditic magmas, indeed, are characterized by very high liquidustemperature at low pressure (Freda et al., in press) and this impliesthat erupted magmas crystallize forming a cryptocrystalline ground-mass. Moreover, the scarce remaining glass is easily zeolitized due to

its peculiar chemical composition (i.e., comparable to the zeolitecomposition) and because of the abundant volatiles available duringmagmas cooling.

In order to overcome this difficulty, we focused on scoriaclasts occurring in “fast-quenched” fall deposits hoping to findjuvenile fragments characterized by fresh glass. We sampled andexamined by petrographic microscope the fall deposits described inthe stratigraphy section. Among all the analyzed samples weselected samples suitable for geochemical purposes. The selectedglassy scoria clasts (reported in bold in the stratigraphy sections ofFigs. 3–8) are from millimetre to centimetre in size, from scarcelyvesiculated to vesiculated, and porphyritic (15–60 vol.%) withabundant leucite and sparse clinopyroxene, magnetite, and chaba-zite (the latter occurs in vesicles). Leucite typically shows a euhedralicositetrahedral habit; crystals showing skeletal star-like habits arealso present.

Most of the analyzed glasses (Table 3) plot in the K-foiditic fieldof the Total Alkali Silica diagram (Fig. 10). In particular two maingroups can be identified: Group I, characterized by SiO2b45 wt.%,Na2O+K2Ob12 wt.%, and MgON3 wt.%; and Group II, characterizedby SiO2N45 wt.%, Na2O+K2ON12 wt.%, and MgOb3 wt.%. Group Iincludes Tufo Pisolitico di Trigoria (561 ka), Ash-Fall sequence(500 ka), and Pozzolane Rosse (457 ka); Group II includes Tufo diAcque Albule (530 ka), Pozzolane Nere (407 ka), and Tufo Lionato(366 ka). As shown by the age of the eruption successions, the twogroups do not represent a temporal evolution of magma composi-tion, but they occur in scoria clasts characterized by differentmicrotextural features. Group I glasses occur in scoria clastscharacterized by a high amount of leucite phenocrysts showingskeletal star-like habits while the Group II glasses occur in aphyricand/or low porphyritic (rare leucite) scoria clasts. Microtextural



http://dx.doi.org/doi:10.1016/j.lithos.2007.08.008

Fig. 6. Stratigraphic column, detail location map (image from Google Earth®) and pictures of the Via Tiburtina reference section (AH23).


differences are also present in the glass-free scoria clasts of the flowdepositional units. In particular, the scoria clasts of Pozzolane Rosseflow unit (associated with fallout scoria glassy of Group I) are

characterized by microcrysts of leucite showing a star-like habit. Incontrast, scoria clasts of Pozzonale Nere and Pozzolanelle flow units(associated with fallout scoria glassy of Group II) show euhedral

Fig. 7. Stratigraphic column, detail location map (image from Google Earth®) and pictures of the Zagarolo reference section (AH21).


leucite microcrysts and are characterized by the presence ofmagnetite, apatite and mica in the mineralogical assemblages.

We also sampled and analyzed the two main lava flows emplacedduring the T–A phase of activity: the Vallerano and the Fioranellolava flows. The Vallerano lava flow was sampled in a large quarrylocated in the southwestern sector of the District. The Fioranellolava flow samples come from three different levels (44, 43, and 41 mdeep) of the CA-1 borehole (Mariucci et al., in press). Both lava flowsare dark grey, fine grained, and nearly aphanitic; the rarephenocrysts (b10 vol.%) are leucite and clinopyroxene in theVallerano lava flow, and only leucite in the Fioranello lava flow. Inboth lavas dominant phases in the groundmass are leucite,clinopyroxene, and Ti-magnetite associated with phlogopite, nephe-line and calcite. Calcite crystals usually occur either as interstitialcrystals or in lens concentrated in the middle portion of the lavaflow. In the upper portion of both lava flows, calcite crystals areabsent and vesiculae occur. Rare olivine xenocrysts with Cr-spinelsinclusions are present in the very upper portion of the Valleranolava flow.

6. Eruptive history

6.1. The pre-Tuscolano–Artemisio Eruptive Phase

The locations of the four volcanic centers in which we acciden-tally sampled xenocrysts (Fig. 1) account for a sufficiently spatiallydistributed sampling zone; therefore, we assume that the agesreported in Fig. 2 may be regarded as roughly representative of awidespread range of activity. Based on these data, we infer that anearly activity, characterized by eruption cycles with a return periodof about 100 kyr, occurred at the AHVD since ca. 900 ka.

The age of 608±1 ka achieved on the oldest unit cropping out inCave (TFC-a, Fig. 3) is in good agreement with the abovementionedearly cyclicity. Moreover, geochemical features of this unit (Gaetaet al., 2006) confirm that this deposit can be considered the oldestproduct sampled so far at the AHVD. In contrast, according to its age of587±3 ka which corresponds to that of the earliest Monti Sabatiniproducts (FAD, 582±2 ka, Karner et al., 2001; Morlupo trachytic lavaflow, 587±7 ka, Cioni et al., 1993), and based on the presence of


2 In Cave (and possibly in Colleferro) only thin layers (SF-1, TT-b, Fig. 4) of ashbearing idiosyncratic white pumice and/or sanidine crystals are correlated to the

Fig. 8. Stratigraphic column, detail location map (image from Google Earth®) and pictures of the Valmontone reference section (AH24).


abundant yellow pumice clasts containing sanidine phenocrysts, wecorrelate the overlying scoria-fall layer (SF-0, Fig. 3) to the activity ofthe Monti Sabatini Volcanic District (MSVD).

6.2. Early Tuscolano–Artemisio Eruptive Phase: Tufo Pisolitico di Trigoria(TPT) and Tufo del Palatino–Tufo di Acque Albule1 (TP, TAA) eruptive cycles

Isotopic constraints, combined with petro-stratigraphic observa-tions at Cave (Figs. 3 and 4) and Colleferro (Fig. 5) locality, allowed usto recognize the occurrence of four major pyroclastic-flow formingeruptions and group them in two eruption cycles at 561±1 (TPT) and530–527±2 ka (TP and TAA, respectively). However, the observation inthe CA1 bore cores of a paleosol between two pyroclastic flow depositsreferred to the TPT (TPT-b2 and TPT-e in Fig. 3), combined with thepresence of tree-moulds within the deposit (TPT-d in Fig. 3), indicatesthat the first T–A eruption cycle was characterized by two sub-cyclesseparated by a stasis.

Based on the age of 555±1 ka for the uppermost pyroclasticdeposit (TPT-h, Fig. 3) dated in Cave, we assume a minimum durationof 6±3 kyr for the TPT eruption cycle. However, the lack of otherfallout deposits at the upwind location of Cave suggests that nosignificant activity occurred before the following eruption cycle;therefore, we estimate in about 25 kyr the duration of the dormancyseparating it from the following eruption cycle of TP and TAA. Thedeposits of the successive eruption cycle (TP and TAA) are character-ized by scarce fallout and surge deposits, followed by large, massivepyroclastic-flow deposits (Figs. 3–5). Similarly to the two pyroclastic-

1 The modern and Latin names of the type locality (Bagni di Tivoli and Acque Albule,respectively), were mixed and this unit was misnamed in “Tufo di Bagni Albule” inKarner et al. (2001). Here we adopt the more correct name of Tufo di Acque Albule.

flow deposits of the TPT eruption cycle, no isotopically appreciabletime span occurs between the eruptions of the TP and TAA. Therefore,we assume a maximum lag of few kyrs between the two eruptions,and we consider them as two events of the same, second eruptivecycle of the Early T–A phase of activity.

6.3. Ash-fall succession

The thick succession of stratified pyroclastic-fall deposits croppingout at the AHVD, comprised between the TAA (527±2 ka) and thePozzolane Rosse (456±3 ka) pyroclastic-flow deposits, is, at leastpartially, coeval with the “Tufi Terrosi con Pomici Bianche” recognizedin the area of Rome and attributed to the activity of the MSVD (Karneret al., 2001).2

Based on the large associated error, the age obtained for the Ash-fall deposits AF-c in Via Tiburtina (506±10 ka, Fig. 6) should beconsidered undistinguishable from that of the oldest AF-a deposit(517±1 ka). However, the occurrence of theMonti Sabatini Fall-A layer(Sottili et al., 2004) dated at 503±6 ka (re-calculated from Karner andRenne,1998) between AF-a and AF-c in Cave as well as in Via Tiburtina(Figs. 3 and 6), combined with the age of 500±3 ka on AF-d in Cave,allows us to discard the age with the largest associated error, and toestimate a dormancy of 17±4 kyr between these eruptions.

MSVD activity. On the other hand, the products of the MSVD represent the largestportion of the Ash-fall succession at the western locality of Via Tiburtina, which iscloser and downwind to their vent area, whereas it is upwind with respect to the T-Acaldera. The occurrence of these deposits, on which several age determinations wereperformed (Karner and Renne, 1998; Karner et al., 2001, and unpublished data), is hereused to give further time constraints to the coeval AHVD activity.

Fig. 9. Correlation of the five reference sections shown in Figs. 3–8.


Based on the 40Ar/39Ar age of 517±1 ka, the basal productsattributed to the AHVD (F-a, AH20-C15, Figs. 3 and 4) of the Ash-fallsuccession in Cave might be regarded as the coda from the previouseruption cycle of TP-TAA, allowing to infer a duration of approximately13±3 kyr for this cycle. On the other hand, scoria clasts of sampleAH20-C15 and those of the following layer (F-c, sample AH20-C17,Figs. 3 and 4) have similar microtextural features (are bothcharacterized by Group I microtexture), suggesting that the wholeAsh-fall succession should be considered a homogeneous, althoughdiscontinuous, phase of activity. However, the lack of analyzableglasses in the AH20-C15 sample prevents a chemical correlation.Chemical analyses of glass occurring in the overlying fallout deposit(F-c, sample AH20-C17, Figs. 3 and 4) yielded a composition compa-rable to that of the following PR eruption succession (Table 3). Basedon its age of 500±3 ka, we interpret the upper portion (AF-c throughAF-g, Figs. 3 and 4) of the Ash-fall succession to represent a new andpreviously unknown, although minor in volume and energy, eruptioncycle at the AHVD.

6.4. Late Tuscolano–Artemisio Eruptive Phase

6.4.1. The Pozzolane Rosse (PR) eruptive cycleThe eruptive cycle of PR starts with the Vallerano effusive eruption

(VL, Marra, 1999, 2001) whose 40Ar/39Ar age (457±5 ka) is indis-tinguishable from that of the PR pyroclastic-flow (456±3 ka) deposit.

The fallout deposits associated with the early stages of the PReruption succession are largelymissing in thewestern, upwind area, anddo not exceed 50 cm in thickness in the investigated eastern sections.However, the larger grain size of the scoria clasts,with respect to those ofthe previous and later eruption cycles, provided for a better preservationof the original glass. The PR eruption is closed by a relatively thinsuccession of air-fall deposits, with a narrow dispersion area around theNE axis. The topmost of these scoria-fall layers cropping out at theZagarolooutcrop#1 (Fig. 7) yielded a 40Ar/39Ar ageof 442±2ka, allowingus to infer a total duration of about 15 kyr for the PR eruption cycle.

We remark that the dormancies, as testified by the absence of anydeposit in all the investigated sections (and more in general in all the

3 Although resulting from very homogeneous populations of crystals, yielding acombined age of ca. 366 ka, these ages were previously suspected to reflect xenocrysticcontamination, based on similar results obtained for other hydromagmatic centers (i.e.Ariccia, Castiglione, see Marra et al., 2003) for which the age of ∼370 ka was evidentlystratigraphically inconsistent. However, field work performed for this study allowed usto solve most of the problematic stratigraphic interpretations reported in Marra et al.(2003). In particular, no stratigraphic inconsistency exists between the age of 373±6 ka(re-calculated from Marra et al., 2003) yielded by the hydromagmatic deposits of PrataPorci Crater and their stratigraphic position above a lava flow that was previouslyattributed (Marra et al., 2003) to the Faete phase of activity (308–250 ka). Fieldevidence, also in agreement with the 1:100,000 Geologic Map of the Alban Hills inFornaseri et al. (1963), allowed us to attribute this lava flow to the final scoria-coneactivity of the Villa Senni eruption cycle, which followed the emplacement of the twomajor pyroclastic flows since 365 (±4) ka (see also Fig. 1). The re-evaluation of the ageconcordance for the Prata Porci deposits is in good agreement with the ages of 365±3 ka and 370±2 ka (re-calculated from Marra et al., 2003), obtained on the products ofthe nearby centers of Pantano Secco and Valle Marciana.

Table 3Compositions (wt.%) of interstitial glass and lava flow bulk determined by WDS-MP and XRF analysis, respectively

Referencesection

Trigoria AH23 AH20 Vallerano AH20 AH21 CA1 AH24 AH24 AH24

Sample F1a c2b c17 VLF PRa c1 FLF 10b sc1-a sc1-b

Eruptive TPT TAA AF PR PR PR PN PN VS VS

Cycle 561 ka 530 ka 500 ka 457 ka 457 ka 441 ka 407 a 366 ka 366 ka

Lithotype Scoriaclast

Scoriaclast

Scoriaclast

Lavaflow

Scoriaclast

Scoriaclast

Lavaflow

Scoriaclast

Scoriaclast

Scoriaclast

σ (4)b σ (8) σ (5) σ (3) σ (6) σ (3) σ (3) σ (11) σ (18) σ (12)SiO2 40.98 0.32 44.07 0.55 40.46 0.75 45.29 0.53 40.41 0.16 41.26 0.15 47.95 1.31 44.42 0.35 44.09 0.28 46.87 0.33TiO2 1.36 0.10 0.74 0.12 1.35 0.21 0.93 0.04 1.33 0.04 1.42 0.05 0.92 0.05 0.82 0.08 0.90 0.05 0.69 0.03Al2O3 15.62 0.11 18.46 0.26 10.97 0.32 15.85 0.08 12.14 0.05 12.08 0.10 16.93 0.56 17.81 0.12 17.87 0.17 19.38 0.19MgO 3.10 0.13 1.35 0.07 6.02 0.64 4.62 0.31 6.19 0.17 6.34 0.07 4.17 0.76 2.08 0.08 1.91 0.05 1.51 0.04MnO 0.27 0.10 0.29 0.04 0.28 0.03 0.17 0.01 0.26 0.03 0.19 0.05 0.15 0.01 0.30 0.07 0.28 0.05 0.23 0.04CaO 12.28 0.05 9.83 0.53 16.14 1.14 11.05 1.02 14.77 0.30 16.02 0.19 8.74 0.77 9.81 0.34 9.96 0.19 7.77 0.18FeO 10.63 0.14 7.52 0.24 12.15 0.70 8.48 0.26 11.21 0.13 11.16 0.03 7.81 0.42 8.10 0.33 8.62 0.33 6.66 0.23SrO 0.52 0.12 0.50 0.06 0.33 0.04 nd – 0.27 0.04 0.15 0.07 nd – 0.66 0.03 0.42 0.10 0.26 0.07BaO 0.42 0.12 0.57 0.07 0.37 0.06 nd – 0.39 0.06 0.39 0.10 nd – 0.46 0.04 0.32 0.05 0.32 0.06Na2O 4.66 0.03 5.29 0.30 3.46 0.34 1.95 0.02 3.38 0.08 2.81 0.06 1.13 0.48 7.19 0.34 5.61 0.16 4.92 0.19K2O 6.02 0.05 8.61 0.30 5.11 0.47 8.46 0.65 4.83 0.36 4.13 0.02 9.15 0.17 6.14 0.37 6.73 0.29 9.50 0.26P2O5 0.64 0.07 0.20 0.03 1.14 0.14 0.94 0.05 1.08 0.12 1.05 0.06 0.71 0.06 0.33 0.06 0.24 0.05 0.21 0.03F 0.56 0.10 0.83 0.14 0.72 0.13 nd – 0.59 0.11 0.45 0.18 nd – 0.82 0.10 0.65 0.14 0.48 0.14SO3 0.80 0.18 0.82 0.10 0.18 0.03 nd – 0.08 0.02 0.01 0.01 nd – 0.31 0.05 0.48 0.12 0.52 0.09Total 97.86 99.09 98.68 97.75 96.94 97.46 97.67 99.24 98.08 99.34

nd = not determined.a After Palladino et al. (2001).b 1-sigma standard deviation, number in parenthesis represents the analyses numbers.


AHVD area, Karner et al., 2001), separating the PR cycle from theprevious and the following eruptive cycle are ca. 40 kyr and ca. 35 kyr,respectively.

6.4.2. The Pozzolane Nere (PN) eruptive cycleA lava flow (Fioranello lava flow, FL), showing the same

petrographic features of the VL and the same stratigraphic relation-ship with the main pyroclastic-flow deposit of the PN as the VL doeswith the PR, crops out in the Fioranello locality (Fig. 1) and has beenrecovered in the CA-1 borehole. In Fioranello the lava flow overlies thevolcaniclastic deposits of the Conglomerato Giallo (CG; 437±32 ka,Table 2). The CG is the local equivalent of the San Paolo Formation(Karner andMarra,1998, 2003) and constitutes the sedimentary fillingof paleovalleys that aggraded during the high stand of the MarineIsotopic Stage 11, dated at around 406 ka (Bassinot et al., 1994).Therefore, the emission of lava flows shortly before the climacticexplosive phase should be considered a characteristic feature of thelater T–A eruptive cycles.

The deposits at the base and at the top of the scoria-fall suc-cession that close the PN eruption have been dated in Zagarolo(samples AH21-C3, AH21-C7, yielding 411±2 and 410±11 ka,respectively, Fig. 7), allowing us to estimate a maximum durationof this eruption cycle in ca. 10 kyr (based on a youngest possible ageof 399 ka for the upper fallout layer), and the dormancy separating itfrom the following VS eruption succession in ca. 30 kyr, at the least.

6.4.3. The Villa Senni (VS) eruptive cycleThis eruption succession is the last of the large, explosive volcanic

cycles of theT–Aphaseand its products arewell exposed throughout theAHVD area. The eruption started with a sustained eruptive column orfountain characterizing only the early stages, leading to the emplace-ment of small-volume pyroclastic-surge and scoria-fall deposits (Fredaet al., 1997). At the investigated sections of Zagarolo and Valmontone,these deposits start with a discontinuous fallout layer (VS-a', Figs. 7 and8) at the base of a massive ash deposit (VS-b, Figs. 7 and 8) containingzeolitized scoria clasts, which displays very similar features to the layersobserved below the PR pyroclastic-flow deposit in Cave. The absence ofpaleosols among all the units described above suggests that no

significant time elapsed from the early phase to the climax of the VillaSenni eruption cycle. The overall reverse grading of the fallout depositindicates increasing mass eruption rate, resulting in the climax of theeruption, duringwhich two large pyroclasticflows, Tufo Lionato (TL) andPozzolanelle (PZ), were generated by either low-altitude columncollapse or boiling over (Freda et al., 1997).

Age of the lower flow unit has been re-assessed for this work to365±4 ka, based on juvenile scoria-clast samples collected in thelocality of Fioranello (Fig. 1). Caldera collapse followed the emplace-ment of the two pyroclastic flows of the VS succession; afterwards, theeruption cycle was closed by the activity of a large number of scoriacones forming a ring along the peri-caldera fracture system. Samplesof these strombolian products, and their abovementioned reworkeddeposits, yielded ages of 364±3 and 358±1 ka, respectively (re-calculated from Marra et al., 2003; Karner et al., 2001).

Moreover, the re-evaluation3 of the stratigraphic consistence ofthree 40Ar/39Ar ages previously determined (Marra et al., 2003)enables us to associate the hydromagmatic activity of three maarslocated at the foot of the northern slope of the caldera rim — PrataPorci (373±6 ka), Pantano Secco (365±3 ka) and Valle Marciana (370±2 ka) — to the final stages (VS-g) of the Villa Senni eruption cycle.

Fig. 10. TAS diagram.


Small-volume lava flows and dikes were emplaced at the very latestage (VS-i) of the eruption cycle: ages of those dated span 351±4–357±9 ka (Karner et al., 2001; Marra et al., 2003), suggesting that thepost-caldera activity ended about 15 kyr after the emplacement ofthe pyroclastic flows. No other significant eruptive episode tookplace until the start of the successive cycle of activity (Monte delleFaete phase) around 310 ka (Marra et al., 2003), therefore a newdormancy in the order of 40 kyr occurred.

7. Discussion

7.1. Eruptive behavior

Based on the similarity of eruptive behavior, petrochemicalfeatures, and erupted volumes, we can confidently extend to theformer eruptive cycles the observations on the post-caldera activity ofthe Villa Senni eruptive cycle described in the previous paragraph. Dueto the regionalwesterlywinds, the fallout deposits of the final stages ofthe Villa Senni eruptive cycle show a strong asymmetrical, eastwarddistribution, with lack of deposition to the west of the caldera(Fornaseri et al., 1963; De Rita et al., 1988). Similarly, ourwork, coupledwith observations of the lack of these products in thewestern sector ofthe volcanic area (Karner et al., 2001), outlines that the older falloutdeposits crop out only in the north-eastern sector of the area. Thisdistribution supports the inference that they are the equivalentproducts, in terms of the eruptive mechanism and energy, of thoseemplaced during the final stages of the previous eruptive cycles. Theages of these deposits allow us to assess the durations of the eruptivecycles andof the dormancies for the overall T–Aphase. Inparticular,weconfirmprevious inferences (Karner et al., 2001,Marra et al., 2004) thatrelative dormancies of several tens of kyr, characterized by the absenceof large-volume eruptions, separated the five eruptive cycles, and thatthe climactic explosive phases were followed by a relatively short codaof peri-caldera activity, on the order of 10–15 kyr.

The CA1 borehole stratigraphy (Mariucci et al., in press), testifyingthe absence of volumetrically significant deposits between eachexplosive cycle even at relatively small distance from the vent area,supports this interpretation.

Data presented in this paper confirm the overall recurrence periodof 45.5 kyr for the climactic eruptive phases (extended to the latereruption cycles for which a dramatic reduction of the erupted volume

occurs but not a change in their return period) determined by Marraet al. (2004). However, a slightly higher frequency of ca. 30 kyr mightbe considered for the time span 560–500 ka with respect to thefollowing period (Fig. 11).

The change in the eruption frequency that occurs at 500 ka, andthe consequent expansion of the dormancy period between theeruption cycles, is coupled with a change of the eruptive mechanism,as evidenced by the marked hydromagmatic features of the earlydeposits with respect to the later ones. De Rita et al. (1988) alreadyreported a distinction between “wet” and “dry” pyroclastic flowdeposits to distinguish between the “First T–A Pyroclastic Flow”

(including TPT, TP, and T-AA, Marra and Rosa, 1995; Karner et al.,2001) and the “Second T–A Pyroclastic Flow” (corresponding to PR,Marra and Rosa, 1995). Palladino et al. (2001) explained thehydromagmatic features of TPT as the combined effect of rapiddecompression of the magma chamber, due to possible tectonictrigger, and shallow magma–water interaction. In contrast, theexplosive behavior of the younger, dry eruptive cycles was inter-preted as a consequence of abundant CO2 within the volatile fraction(Freda et al., 1997).

7.2. Eruptive model

A direct linkage between the tectonic regime and the eruptivebehavior at the AHVD has been widely proposed in the relatedliterature (e.g. Marra et al., 2004 and reference therein). In particular,Marra (2001) suggested that different eruptive mechanisms (i.e.explosive vs. effusive) might be explained by the alternating forcing ofthe two, competitive stress-fields acting in the AHVD area: theextensional and the local stress fields, associated to regional and localtectonic regimes, respectively. Here we attempt to better define therole of these alternating regimes to possibly explain the overalleruptive behavior of the AHVD, rather than linking them to oneparticular eruptive mechanism.

The geometry of the structural lineaments (faults and fractures)associated with the transcurrent regime in the Roman area (Faccennaet al., 1994) allowed Marra (2001) to recognize its transpressivecharacter, generated under a NE-striking, horizontal σ1 (Fig. 12a). Thisimplies a maximum compression perpendicular to the strike of theNW–SE trending faults and fractures associated with the regionalextensional regime. Therefore the transcurrent regime should be


Fig. 11. Eruptive history of the AHVD, modified from Marra et al. (2004). Each eruptive cycle is represented by a bar whose height is indicative, on logarithmic scale, of the eruptedvolume, followed by an exponentially decreasing coda of activity. Duration of the coda is assessed based on ages here determined on the products of the final stages of activity for eachcycle. Inter-event times, separating the start of each eruptive cycle, are also shown. Cv=σ /μ, (where μ is the recurrence time and σ is the standard deviation) expresses the periodicityindex, with Cv=0 signifying a perfect periodicity and CvN1 an exponential distribution (see Marra et al., 2004 for statistic detail). The probabilistic distribution (cut at 8%) of agesdetermined on xenocrystals representative of the pre-T–A products (Palaeo-activity), reported in Fig. 2, is added in this figure to document the activity in the time span 900–650 ka.The ordinate axis here is not proportional to the (unknown) erupted volume, but represents (in percent) the number of crystals yielding the correspondent age with respect to thetotal number of dated crystals.


interpreted as preventing magma rise, in particular by using regionalNW–SE pathways. In this light, Marra (2001) tentatively explained theVallerano lava eruption as triggered by decompression associatedwith block rotation along the main N–S oriented, strike–slip faults inthe volcanic area (see Fig. 12b).

Moreover, the Fioranello lava flow, erupted in the early stages ofthe PN eruptive sequence, suggests that effusive emissions in earlystages of large explosive cycles might be a characteristic feature of theeruptive behavior after 500 ka at the AHVD. Although no lava flow isdocumented yet to precede the climactic phase of the VS eruptionsequence, the Tuscolo lava flow cropping out along the caldera rimunderlies the deposit of the VS eruption without evidence of paleosoland has a 87Sr/86Sr ratio (Ferrara et al., 1985; Voltaggio and Barbieri,1995) consistent with the age4 of this eruption (Gaeta et al., 2006). Wealso stress that the following Faete phase eruptive cycle (308–250 ka,Marra et al., 2003) was characterized by the early emplacement oflarge lava flows (298–277 ka), followed by the activity of a central,strombolian edifice (268–250 ka). We combine the tectonic inter-pretations on the early lava flow emission with the change infrequency, causing longer dormancies to separate the large explosiveeruptions, occurring after 500 ka, to infer that the transpressiveregime became dominant in this time span. Therefore, we propose thefollowing eruptive model for the AHVD.

The regional extensional regime acting on the Tyrrhenian Seamargin of Italy since Messinian time should be regarded as the drivingmechanism for the development of the Roman Magmatic Province(Serri et al., 1993). As discussed in Marra et al. (2004), it is likely thatthis driving mechanism acted upon re-charging of the magmachambers feeding the different volcanic districts along the Tyrrhenianmargin, either ruling the timing of accumulation of magma at the topof the mantle and weakening of the overlying crust, or the cyclinginjection of convective plumes into the mantel wedge.

According to Palladino et al. (2001), the eruptive behavior of theearly T–A phase of activity (561–516 ka) is consistent with a regionalextensional tectonic forcing. The volcanic activity is characterized bysynchronous start (561 ka) of the large explosive eruptions at thenearby Monti Sabatini Volcanic District (Karner et al., 2001), by higher

4 Gaeta et al. (2006) have shown that the 87Sr/86Sr ratio of the Colli Albani productshas a linear, decreasing trend in time, ranging from0.7112 (Tufo di Fosso Colleraso, 608±1 ka, this work) to 0.7095 (late Albano activity, 36±1 ka, Freda et al., 2006).

frequency (multiple pyroclastic-flow deposits and shorter dorman-cies), and by evidence of rapidmagma decompression.We remark thatthe eruption cycle at around 530 ka, and its continuation until 517 ka, isthe only one that does not fit in the average 45 kyr cyclicity of theoverall RomanMagmatic Province evidenced inMarra et al. (2004).Weinterpret the dramatic, although temporary, reduction of eruptedvolume during the following cycle at 500 ka (moderate, strombolianactivity), and the successive enlargement of the rest periods betweenthe sudden releasing of huge volumes of explosive products, asevidence of a dominant transpressive regime affecting the regionabove themagma chamber in the time span 517–250 ka. Transpressivestress temporarily sealed the major northwest-striking normal faultsin the Alban Hills areas, and thus prevented magma from reaching thesurface. This caused themagma to remain in themagma chamber for along time, enhancing carbonate assimilation from the host rocks(Gaeta et al., 2006) and producing an increase of the amount ofvolatiles and thus resulting in a highly explosive behavior. Thismechanism is also in agreement with the proposed model to explainthe rather constant composition of the Alban Hills through time,whereby this is a consequence of interaction with the carbonate hostrock of the substratum (Gaeta et al., 2006).

We remark that in other similar contexts along the Tyrrhenian Seamargin of Italy, where the magmatic chambers are hosted in carbonaterocks (e.g. Vesuvius, Barberi and Leoni, 1980), shorter recurrence timesanddifferent eruptive styles are observed (Bonasia et al.,1985; Scandoneet al., 1993). We interpret these differences as due to the particular localtectonic regime acting in the area of the AHVD.

According to the interpretation that the transpressive regime is akinematic effect due to the presence of a NS crustal disengagementzone crossing through the Tyrrhenian Sea margin (Faccenna et al.,1996; Marra, 2001), a decoupling of the stress between an upper crustzone, characterized by transpression, and a lower crust zone with aprevalent extensional regime is here hypothesized. This assumptionexplains why during the time interval 517–250 ka, as well as in thefollowing new hydromagmatic cycle since 200 ka (Marra et al., 2003),volumes and timing of rise of magma feeding the magmatic chamberremains ruled by the deeper, extensional stress field, as testified by thevery regular return periods and their fit into the general regional trend(Marra et al., 2004). The transpressive regime, in our opinion, causesthe eruptive activity to be almost absent between each climactic phase,and influences the eruptive mechanisms, according to the model

Fig. 12. a) The transpressive stress in the Alban Hills region is originated by the presence of a series of N–S right-lateral fault segments affecting the Quaternary terrains along themargin of the Central Apennine mountain chain (Faccenna et al., 1994). The geometry of these fault segments originates a restraining bend, and the NE–SW trending tensor of themaximum stress (σ1) associated to this local tectonic regime seals the pre-existing NW–SE extensional faults (re-drawn after Marra, 2001). b) Schematic block-diagram of the areabordered by the rectangle in Fig. 11a, showing an interpretation of the interaction at depth between the surface faults and the magma chamber of the Alban Hills. The horizontalrotation of crustal blocks bordered by the right-lateral N–S faults is hypothesized (Marra, 2001) to induce localized decompression, tapping peripheral portions of the magmareservoir at depth, and causing the emission of lava flows preceding the explosive eruption.


proposed by Marra (2001) for the emission of the Vallerano lava flow(that we extend here to the following Fioranello and Faete lava flows).

The cessation of the predominance of the transpressive regime andthe re-establishment of the extensional stress field in the upperportion of the crust in this area are likely to have occurred since 70 ka.Indeed, another change in the eruptive style, leading to the newvolcanic phase that occurred with incremented activity (both involumes and frequency, Freda et al., 2006; Giaccio et al., 2007), isobserved at the Albano Maar at this time (Fig. 11).

8. Conclusions

The T–A phase of activity at the AHVD spanned the interval 608±1–351±4 ka. During this time span, five large eruptive cycles occurred,with climactic phases at 561±1 ka (Tufo Pisolitico di Trigoria EruptiveSequence), 530±2 ka (Tufo del Palatino+Tufo di Acque Albule Eruptive

Sequence), 456±3 ka (Pozzolane Rosse Eruptive Sequence), 407±2 ka(Pozzolane Nere Eruptive Sequence), and 365±4 ka (Villa SenniEruptive Sequence). Moreover, two smaller, strombolian eruptivephases occurred at 517±1 ka and 500±3 ka. Despite the homogeneousand constant basic signature of the erupted deposits, all clustered inthe K-foiditic field of the TAS-diagram, the main five eruptive cycleswere highly explosive and caused the emplacement of large volumes(on the order of several ten of km3) of deposits. All these eruptivesequences were characterized by an early sustained column orfountain, causing the emplacement of relatively thin layers of poorlyvesiculated fallout deposits, and scarce surge deposits at the base of themain pyroclastic-flow deposits. Indeed, most of the erupted magma(probably N90%) emplaced during the climax of each eruptive phase inthe form of one single, or two distinct, large, massive pyroclastic-flowdeposits. The eruption sequences were then closed by a much lessenergetic strombolian phase of activity that occurred from numerous


peri-calderic scoria cones, and lasted several (5 to 15) kyr. In contrast,very long (30 to 50 kyr), nearly absolute dormancies separated eachone of the eruptive cycles, as testified by the lack of any eruptiveproduct between those we dated at the top and at the base of eacheruptive sequence.

A direct relation with the changes in the local stress field, dueto the superposition of two contrasting tectonic regimes, isproposed in order to explain the eruptive behavior of the AHVD.In particular, the long dormancies and the scarcity of intermediatevolcanic activity among the major eruptive cycles are attributed tothe prevailing of a local transpressive regime over the regionalextensional regime.

Acknowledgements

We are grateful to Gianluca Sottili for field assistance and expertiseon the products of Monti Sabatini Volcanic District.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jvolgeores.2008.11.009.

References

AA.VV, 1995. In: Trigila, R. (Ed.), The Volcano of the Alban Hills. Università degli Studi diRoma “La Sapienza”, Rome, Italy. 283 pp.

Barberi, F., Leoni, L., 1980. Metamorphic carbonate ejecta from vesuvius plinianeruptions: evidence of the occurrence of shallow magma chambers. Bulletin ofVolcanology 43/1, 107–120. doi:10.1007/BF02597615.

Bassinot, F.C., Labeyrie, L.D., Vincent, E., Quidelleur, X., Shackleton, N.J., Lancelot, Y., 1994.The astronomical theory of climate and the age of the Brunhes–Matuyamamagnetic reversal. Earth Planet. Sci. Lett. 126, 91–108.

Bonasia, V., Del Pezzo, E., Pingue, F., Scandone, R., Scarpa, R.,1985. Eruptive history, seismicactivity, and ground deformations at Mt. Vesuvius, Italy. Annales Geophisicae 3 (3),395–406.

Cioni, R., Laurenzi, M.A., Sbrana, A., Villa, I.M., 1993. Chronostratigraphy of the initialactivity in the Sabatini Volcanic complex (Italy). Bollettino della Societa GeologicaItaliana 112, 251–263.

De Rita, D., Funiciello, R., Parotto, M., 1988. Geological Map of the Colli Albani VolcanicComplex. Progetto Finalizzato Geodinamica C.N.R., Rome, Italy.

De Rita, D., Faccenna, C., Funiciello, R., Rosa, C., 1995. Stratigraphy and volcano-tectonics.In: Trigila, R. (Ed.), The Volcano of the Alban Hills. Università degli Studi di Roma “LaSapienza”, Rome Italy, pp. 33–71.

Faccenna, C., Funiciello, R., Mattei, M., 1994. Late Pleistocene N–S shear zones along theLatium Tyrrhenian margin: structural characters and volcanological implications.Bollettino di Geofisica Teorica Applicata 36, 507–522.

Faccenna, C., Davy, P., Brun, J.P., Funiciello, R., Giardini, D., Mattei, M., Nalpas, T., 1996.The dynamics of back-arc extension: an experimental approach to the opening ofthe Tyrrhenian Sea. Geophysical Journal International 126, 781–795.

Ferrara, G., Laurenzi, M.A., Taylor, H.P., Tonarini, S., Turi, B., 1985. Oxygen and Strontiumisotopes studies of K-rich volcanic rocks from the Alnan Hills, Italy. Earth andPlanetary Science Letters 75, 13–28.

Fornaseri, M., Scherillo, A., Ventriglia, U., 1963. La regione vulcanica dei Colli Albani: ilVulcano Laziale. Consiglio Nazionale delle Ricerche (C.N.R.), Rome, Italy, p. 561.

Freda, C., Gaeta, M., Palladino, D.M., Trigila, R., 1997. The Villa Senni Eruption (AlbanHills, Central Italy): the role of H2O and CO2 on the magma chamber evolution andon the eruptive scenario. Journal of Volcanology and Geothermal Research 78,103–120.

Freda, C., Gaeta, M., Karner, D.B., Marra, F., Renne, P.R., Taddeucci, J., Scarlato, P.,Christensen, J.N., Dallai, L., 2006. Eruptive history and petrologic evolution of theAlbano multiple maar (Alban Hills, Central Italy). Bulletin of Volcanology 68,567–591.

Freda, C., Gaeta, M., Misiti, V., Mollo, S., Dolfi, D., Scarlato, P., in press. Magma-carbonateinteraction: an experimental study on ultrapotassic rocks from Alban hills (CentralItaly). Lithos. doi:10.1016/j.lithos.2007.08.008.

Funiciello, R., Giordano, G., De Rita, D., 2003. The AlbanoMaar lake (Colli Albani Volcano,Italy): recent volcanic activity and evidence of pre-Roman Age catastrophic laharevents. Journal of Volcanology and Geothermal Research 123, 43–61.

Gaeta, M., 1998. Petrogenetic implications of Ba-sanidine in the Lionato Tuff (ColliAlbani Volcanic District, Central Italy). Mineralogical Magazine 62, 697–701.

Gaeta, M., Freda, C., Christensen, J.N., Dallai, L., Marra, F., Karner, D.B., Scarlato, P., 2006.Evolution of the mantle source for ultrapotassic magmas of the Alban Hills VolcanicDistrict (Central Italy). Lithos 86, 330–346.

Giaccio, B., Sposato, A., Gaeta, M., Marra, F., Palladino, D.M., Taddeucci, J., Barbieri, M.,Messina, P., Rolfo, M.F., 2007. Mid-distal occurrences of the Albano Maar pyroclasticdeposits and their relevance for reassessing the eruptive scenarios of the mostrecent activity at the Colli Albani Volcanic District, Central Italy. QuaternaryInternational 171/172, 160–178. doi:10.1016/j.quaint.2006.10.013.

Giordano, G., De Benedetti, A.A., Diana, A., Diano, G., Gaudioso, F., Marasco, F., Miceli, M.,Mollo, S., Cas, R.A.F., Funiciello, R., 2006. The Colli Albani mafic caldera (Roma, Italy):stratigraphy, structure and petrology. Journal of Volcanology and GeothermalResearch 155, 49–80. doi:10.1016/j.jvolgeores.2006.02.009.

Karner, D.B., Marra, F., 1998. Correlation of fluviodeltaic aggradational sections withglacial climate history: a revision of the classical Pleistocene stratigraphy of Rome.Geological Society of America Bulletin 110, 748–758.

Karner, D.B., Marra, F., 2003. 40Ar/39Ar dating of glacial termination V and duration ofthe Stage 11 highstand, in: earth's climate and orbital eccentricity: the marineisotope Stage 11 question, American Geophysical Union. Geophysical Monograph137, 61–66.

Karner, D.B., Renne, P.R., 1998. 40Ar/39Ar geochronology of Roman Volcanic Provincetephra in the Tiber River Valley: age calibration of middle Pleistocene sea-levelchanges. Geological Society of America Bulletin 110, 740–747.

Karner, D.B., Marra, F., Renne, P.R., 2001. The history of the Monti Sabatini and AlbanHills volcanoes: groundwork for assessing volcanic-tectonic hazards for Rome.Journal of Volcanology and Geothermal Research 107, 185–219.

Mariucci, M.T., Pierdominici, S., Marra, F., Montone, P., Pizzino, L., in press. Looking into avolcanic area: an overviewon the 350 m scientific drilling at Colli Albani (Rome, Italy).Journal ofVolcanologyandGeothermal Research. doi:10.1016/j.jvolgeores.2008.04.007.

Marra, F., 1999. Low-magnitude earthquakes in Rome: structural interpretation andimplications for local stress-field. Geophysical Journal International 138, 231–243.

Marra, F., 2001. Strike–slip faulting and block rotation: a possible triggering mechanismfor lava flows in the Alban Hills? Journal of Structural Geology 1, 127–141.

Marra, F., Karner, D.B., 2005. The Albano Maar (Alban Hills Volcanic District, Italy):active or dormant volcano? Il Quaternario 18 (2), 173–185.

Marra, F., Rosa, C., 1995. Stratigrafia e assetto geologico dell'area romana. In: Funiciello,R. (Ed.), La Geologia di Roma. Il Centro Storico. Memorie Descrittive della CartaGeologica d'Italia, vol. 50. Poligrafico e Zecca dello Stato, Rome, Italy, pp. 49–118.

Marra, F., Freda, C., Scarlato, P., Taddeucci, J., Karner, D.B., Renne, P.R., Gaeta, M., Palladino,D.M., Trigila, R., Cavarretta, G., 2003. 40Ar/39Ar geochronology of the recent phase ofactivity of the Alban Hills Volcanic District (Rome, Italy): implications for seismicand volcanic hazards. Bulletin of Volcanology 65, 227–247.

Marra, F., Taddeucci, J., Freda, C., Marzocchi, W., Scarlato, P., 2004. Eruption recurrenceinterval of the Alban Hills and coupling with other volcanic districts of theTyrrhenianmargin of Italy: possible tectonic influence and implications for volcanichazard. Tectonics 23, TC4013.

Palladino, D.M., Gaeta, M., Marra, F., 2001. A large k-foiditic hydromagmatic eruptionfrom the early activity of the Alban Hills Volcanic District (Italy). Bulletin ofVolcanology 63, 345–359.

Renne, P.R., Deckart, K., Ernesto, M., Féraud, G., Piccirillo, E.M., 1996. Age of the PontaGrossa Dike Swarm (Brazil) and implications for Paraná flood volcanism. Earth andPlanetary Science Letters 144, 199–211.

Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T., De Paolo, D.J., 1998.Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating.Chemical Geology 145, 117–152.

Scandone, R., Giacomelli, L., Gasparini, P., 1993. Mount Vesuvius: 2000 years ofvolcanological observations. Journal of Volcanology and Geothermal Research 58,5–25.

Serri, G., Innocenti, F., Manetti, P., 1993. Geochemical and petrological evidence of thesubduction of delaminated Adriatic continental lithosphere in the genesis of theNeogene–Quaternary magmatism of Central Italy. Tectonophysics 223, 117–147.

Sottili, G., Palladino, D.M., Zanon, V., 2004. Plinian activity during the early eruptivehistory of the Sabatini Volcanic District, Central Italy. Journal of Volcanology andGeothermal Research 135, 361–379.

Trigila, R., Agosta, E., Currado, C., De Benedetti, A.A., Freda, C., Gaeta, M., Palladino, D.M.,Rosa, C., 1995. Petrology. In: Trigila, R. (Ed.), The Volcano of the Alban Hills.Università degli Studi di Roma “La Sapienza”, Rome, Italy, pp. 95–165.

Voltaggio, M., Barbieri, M., 1995. Geochronology. In: Trigila, R. (Ed.), The Volcano of theAlban Hills. Università degli Studi di Roma “La Sapienza”, Rome, Italy, pp. 167–192.

Washington, H.S.,1906. The RomanComagmatic Region. Carnegie Institute,Washington.Publ no. 57.


http://dx.doi.org/10.1007/BF02597615

http://dx.doi.org/10.1016/j.quaint.2006.10.013

http://dx.doi.org/10.1016/j.jvolgeores.2006.02.009

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Large mafic eruptions at Alban Hills Volcanic District (Central Italy): Chronostratigraphy, petrography and eruptive behavior