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ABSTRACT - This paper reports the occurrence of air-fall tephra in the “Colle dell’Orso” area, near Frosolone(Isernia), in the Molisean South-Central Apennines. A mineralogical and compositional investigation on clinopy-roxenes, K-feldspars and glass, as well as an attempt at a geochronological dating by fission-track method onglass, were carried out to establish the source of the volcanic material found in this area.
The collected data show a compositional range of diopside to ferroan-diopside for the clinopyroxene, and aprevalent “orthoclasic” composition is found for the feldspars. The glass presents prevalently trachytic and/or tra-chy-andesitic (latitic) compositions. Fission-track dating on glass gives an age of 10,800 (±5,500) years.
The glass, clinopyroxene and feldspar compositions, fission-track age determination, and the morphological andstructural appearance of the host-layering yellowish material would all seem to indicate that the Frosolone ashesare referable to the explosive products of the Phlegrean Fields (Campanian activity), in particular to the NeapolitanYellow Tuff emission, thus ruling out all earlier Campanian activities and also other probable provenances fromneighbouring volcanic complexes.
KEY WORDS: volcanic ashes, clinopyroxene, k-feldspar, glass, fission-track dating, Phlegrean activity, Frosolone, South-Central Apennines.
VOLCANIC ASHES FROM FROSOLONE (ISERNIA, ITALY): MINERALOGICAL CHARACTERISATION AND FISSION-TRACK DATING
Antonio Gianfagna* & Giulio Bigazzi°
*Dipartimento di Scienze della Terra, Università degli Studi di Roma “La Sapienza”, P.le A. Moro, 5 - 00185 Roma°Istituto di Geoscienze e Georisorse del CNR di Pisa, Via G. Moruzzi, 1 - 56124 Pisa
* e-mail: [email protected]
Geologica Romana 37 (2003-2004), 97-108
INTRODUCTION
The present work examines the results of a mineralog-ical and dating study carried out on volcanic ash depositsfound at Colle dell’Orso, near Frosolone (Isernia),Molise, in the South-Central Apennines.
The finding of volcanic ashes in the Frosolone areaconcerns some current issues of general interest, namely:1) the Quaternary stratigraphy of Central Italy, 2) thecharacterisation and areal spread of volcanic soils in theSouth-Central Apennines, and 3) recent issues on theprobable intrapenninic volcanism.
The area under study is situated on the boundarybetween the Central and South-East Apennines, and rel-atively borders on the volcanic complexes mostly distrib-uted west, south-west and south of the Frosolone area.From north to south, these are the Alban Hills (VolcanLatiale) and Roccamonfina complexes, both in the Latialregion, the Phlegrean Fields complex (Campanianregion) , and the smaller Mount Vulture complex in theBasilicata region. Moreover, there should also be indica-tions on probable intrapenninic activity immediately tonorth of Frosolone, in the Abruzzo region (Barbieri et al.,1996; Bosi et al., 1991). Therefore, the material found inthe studied area could actually be attributed to the activ-ity of one of these neighbouring complexes, but it wouldalso be reasonable to hypothesize a possible local intrap-peninic activity, albeit of a minor scale. To date, howev-er, there is no evidence of occurrences of autochthonousvolcanic material in the Molisean area and so theFrosolone volcanic ashes must be attributed to explosiveeruptions of neighbouring volcanic activity.
The literature frequently presents works investigatingthe origin and provenance of volcanic products found
prevalently in Central and Southern Italy. Volcanic pale-osoils are often used as chronostratigraphic pedomark-ers. Frezzotti & Narcisi (1996) identified two importantpedomarkers in Central Italy, both referable to Phlegreanactivity (Campanian Ignimbrite and Neapolitan YellowTuff); Giraudi (1989) reported a review of the lake levelsand climate for the last 30,000 years in the Fucino area(Abruzzo, Central Italy), while Narcisi (1996) investigat-ed fourteen tephra layers, all belonging to Campanianactivity, in the Vulture district (Lago Grande diMonticchio, Basilicata, Southern Italy). Other similarstudies have also been carried out (Arnoldus-Huyzendveld et al., 1985; Mizota & van Reeuwijk, 1989;Frezzotti & Giraudi, 1989, 1990, 1992; Frezzotti &Narcisi, 1989) because the regions of this area werewidely affected during the Quaternary by the re-fall ofproducts deriving from the explosive activity of theneighbouring Roman and Campanian volcanic districts.
The present paper firstly aims to notify the finding of avolcanic ash deposit in the Frosolone area – this kind ofdeposit never being reported here before – and also toreport a simultaneous combination of mineralogical anddating results which are useful in order to explain the ori-gin and source of the volcanic ashes found at Frosolone.The chemistry of the primary mineral phases (clinopy-roxenes and feldspars) and the results of fission-trackdating on glass, carried out in this work, significantlycontributes to solving this problem.
GEOLOGY AND GEOMORPHOLOGYOF THE AREA
The area of Colle dell’Orso (1393 m a.s.l.) is locatedin the municipality of Frosolone, in the province of
Isernia (Fig. 1). The landscape is predominantly moun-tainous (1200-1400 m a.s.l.) and is composed of a seriesof calcareous-dolomitic and selciferous reliefs character-ized by rounded summits and slopes with a regular pro-file, abundant hillock pastures mostly located on thesummital plateaux, on the subplateau levels or on thebottom of basins and small valleys, the latter relating tothe local karst phenomena.
The geological arrangement of this Apenninic sector isvery complex and developed over a long time periodbetween the upper Trias and the Quaternary, duringwhich rocks belonging to different sedimentary environ-ments were deposited; moreover, the occurrence of tec-tonic phases has led to the present structural chain. Arecent paper by Tozzi et al. (1999) reports the finalresults of a geological-structural survey carried out on abroad area of the Molise region, comprising the entireMontagnola di Frosolone, characterised by the transi-tional sequences of the Frosolone Unit. This shows sed-imentological and lithological features of a typical tran-sitional environment, where a continuous sedimentationof detritic limestones took place from the Dogger to theMiddle Miocene.
Other previous general geological descriptions of thearea are also reported in papers by Pescatore (1963,1965), Cocco (1971), Sgrosso (1986), Naso et al. (1989),Patacca et al. (1992) and, more recently, by Corrado et
al. (1997), De Corso et al. (1998), Di Luzio et al. (1999)and Scrocca & Tozzi (1999).
As regards local geology, the Frosolone hillock maybe ascribed to the “falda molisana”, composed of apelagic succession sedimented between the upper Triasand the Miocene (Patacca et al., 1992).
RESULTS
Profile description
Some transect profiles were carried out in the Colledell’Orso area, near Frosolone (Fig. 1), along an align-ment including summit, slope and valley profiles. Of theseries of profiles performed, the valley one in particular(Fig. 2) turned out to be more interesting for its strati-graphic sequences, which are characterised by yellowishlayers with abundant glassy matrix. The other two pro-files, summit and slope, showed substantial differenceswith the valley profile since they did not present yellowlayers and contained no glass. Therefore, the presentwork will only consider the valley profile investigation,while a future separate work on the mineralogy of theother two profiles will be performed in detail in order tohave an overview of the depositional situation of theFrosolone area.
The studied valley profile (Fig. 2) presents a yellow-ochre level, from ca. 130 to 200 cm depth, that is clear-ly different from the other upper and lower sequences,which are instead characterized by a brown-blackishcolour. The yellowish level shows evident layering with
GIANFAGNA et al.98 Geologica Romana 37 (2003-2004), 97-108
Fig. 1 - Location of investigated area near Frosolone (modified afterFrezzotti and Narcisi, 1996).
Fig. 2 - Sampled levels of the valley profile. Note the layered sectionca. 70 cm thick, from yellow to whitish in colour.
variable colours ranging from yellow-ochre in theuppermost part, progressively shading to pale-yellowand to white in the lower part. It rests directly on a sed-iment of coarse material mainly composed of a thinclayey matrix of brownish or dark brown colour, withlarge siliceous splinters and Fe-Mn nodules of up to 260cm in depth, and is overlapped by a very dark brown,almost black, deposit reaching the profile surface, com-posed of a very fine clayey material containing volcanicglass, clinopyroxenes, feldspars, quartz, and, subordi-nately, also other mineralogical phases (magnetite,biotite, apatite, Fe-Mn-oxides).
Experimental
Materials and analytical techniques
On the basis of morpho-structural characteristics andcolour differences, 6 samplings were carried out at dif-ferent depths in the valley profile (Fig. 2). The samples,named “Fro” and used in the present work, are the fol-lowing:
Fro1 (10-30 cm), Fro2 (40-50 cm), Fro3 (130-140cm), Fro4 (145-155 cm), Fro5 (170-195 cm) and Fro6(250-260 cm).
Grains of clinopyroxene (light green and dark greenprismatic crystals), K-feldspar and glass (yellowish,brownish and black in colour; fibrous, vesicular andglobular in shape) were collected from the sandy gran-ulometric fraction (>0.25 mm) of the six loose samplesunder examination. The selected grains were separatedinto three aliquots, for SEM (ZEISS mod. DSM 940A),EMPA (CAMECA SX50 electron microprobe, equippedwith 5 WDS spectrometers at 15 kV, 15 nA, 100 cps,with a Link EDS system controlled by Specta software)and geochronological determinations (Fission-Trackdating method), respectively.
The fission-track analysis of the volcanic glass wascarried out in the Istituto di Geoscienze e Georisorse ofthe CNR of Pisa. The population dating method wasused (Gleadow, 1981).
Mineral characterisation
The primary mineralogical assemblage is representedby prevalent clinopyroxenes and feldspars, while brownmica and magnetite are present in all levels subordinate-ly (Tab. 1). Quartz, albite, apatite and other accessoryphases are also present. Glass is abundantly present as fardown as 195 cm in depth, while it is completely absentin the Fro6 sample (250-260 cm), where many abundantblack Fe-Mn nodules, 0.5 to 2 mm in size, appear.
Since this study aims at establishing the volcanic dis-trict of possible origin of the volcanic material found atFrosolone, the research focused on the mineralogicalphases that could yield useful data: clinopyroxenes, K-feldspars and glass. The significant compositional vari-ability of these primary phases is a good indicator of theprobable origin of the host volcanic material.
The clinopyroxenes, always present, are less than 1mm in size and display a prevalently prismatic and elon-gated habit, with varying colour from light-green, dark-green up to dark brown and black. They are often foundaltered at their surface and compositional zonations arefrequently observed in thin section.
The feldspars, always present, are generally splintery,thereby implying a derivation from larger crystals; theyare generally larger than the clinopyroxenes (2 to 3mm). Little altered at the surface, the feldspar crystalspresent a notable transparency and are colourless. Inthin section they do not present any peculiar composi-tional zonation and, in some cases, they show a narrowcontact with glass.
The glass, absent in the Fro6 sample, has differentmorphologies in the various levels and varies fromfibrous-elongated to bubble-vesicular and filamentousup to solid-massive and globular (obsidian type) (Fig. 3),with very diversified colours from pale-brown throughdark-brown to black. Generally, the bubbles and vesiclesare filled with soft clay material of allophanic type (X-ray analysis), similar to the material mainly making upthe matrix of the whole sediment.
VOLCANIC ASHES FROM FROSOLONE (ISERNIA, ... 99Geologica Romana 37 (2003-2004), 97-108
Level Colour Mineralogical assemblages*Depth (cm)
Fro1 Brown Abundant microcrystals of quartz and feldspar; scarse fibrous and obsidianic glass; green and black(10-30) clinopyroxenes; black mica and magnetite.
Fro2 Dark brown Fibrous glass; very small green and black clinopyroxenes cristals; feldspar; quartz, black mica; (40-50) magnetite.
Fro3 Beige-yellowish Abundant fibrous glass; little obsidian; large green clinopyroxenes crystals; abundant feldspar and(130-140) little quartz; black mica; magnetite.
Fro4 Yellow-ochre Abundant fibrous glass and obsidian; feldspar; quartz; black mica; little microcrystaline clinopyroxene;(145-155) very little magnetite.
Fro5 Grey-yellowish Little fibrous glass; abundant fresh vesicular glass; abundant clinopyroxenes, feldspar; quartz, little(175-195) and withish black mica; magnetite.
Fro6 Brownish Abundant clear feldspar; quartz; microcrystals of green clinopyroxenes; grey-yellow mica; magnetite; (250-260) many black manganesipherous sferules; very splintered fragments of silicoclasts of different colours.
* = granulometric fraction > 0.25 mm (sandy)
Tab. 1 - Mineralogical assemblage of the six examined samples from Frosolone.
GIANFAGNA et al.100 Geologica Romana 37 (2003-2004), 97-108
Compositional data
Clinopyroxenes. Tab. 2 reportsthe averaged representative chemi-cal analyses of the clinopyroxenecores of the six analysed samples.However, a complete analyticaldata set is disposable for consulta-tion from A.G..
In each sample, at least two meancompositions were found, rangingfrom diopside to ferroan-diopside(nomenclature according to Rock,1990). Three different representa-tive compositions were identifiedfor sample Fro6. From the crystalchemical formula, calculated on thebasis of 6 oxygens, the percentagesof the Wo-En-Fs (Ca-Mg-ΣFe+Mn)end-members were calculated in
Fro1 Fro2 Fro3 Fro4 Fro5 Fro6analyses 2 4 2 4 8 2 2 5 5 5 3 8 2
SiO2 52.37 50.57 53.21 50.82 53.50 49.27 53.92 49.36 53.40 50.52 53.80 51.82 50.28TiO2 0.46 0.77 0.44 0.77 0.27 0.66 0.23 1.02 0.26 0.76 0.38 0.51 0.57Al2O3 2.58 3.58 1.39 3.14 1.63 4.90 1.33 4.05 1.97 3.14 1.61 3.29 3.57Cr2O3 0.32 0.07 0.13 0.02 0.39 0.09 0.62 0.08 0.60 0.06 0.60 0.15 0.01(*)Fe2O3 0.38 1.86 0.14 2.34 0.86 2.97 1.54 4.72 1.42 3.21 1.21 2.32 3.79(*)FeO 3.82 6.28 2.98 7.02 2.46 4.55 1.63 5.34 1.96 5.92 1.74 3.05 4.75MnO 0.17 0.33 0.07 0.58 0.18 0.19 0.00 0.51 0.10 0.59 0.10 0.15 0.34MgO 16.59 13.58 17.23 12.86 16.77 12.95 17.24 11.81 16.57 12.38 16.71 15.04 12.88CaO 22.48 22.18 23.37 22.37 23.60 23.19 23.90 23.19 24.06 23.13 24.12 23.90 23.54Na2O 0.15 0.42 0.07 0.49 0.30 0.42 0.41 0.71 0.36 0.62 0.44 0.40 0.53Total 99.32 99.64 99.03 100.41 99.96 99.19 100.82 100.79 100.70 100.33 100.71 100.63 100.26
Unit formula (oxygens = 6):
Si 1.927 1.888 1.956 1.894 1.951 1.846 1.948 1.840 1.937 1.886 1.952 1.897 1.872Ti 0.013 0.022 0.012 0.022 0.007 0.019 0.006 0.029 0.007 0.021 0.010 0.014 0.016IVAl 0.074 0.113 0.044 0.106 0.049 0.155 0.052 0.160 0.063 0.114 0.048 0.104 0.129VIAl 0.039 0.045 0.016 0.032 0.021 0.062 0.005 0.018 0.021 0.024 0.020 0.037 0.028Cr 0.009 0.002 0.004 0.000 0.011 0.003 0.018 0.002 0.014 0.002 0.017 0.005 0.000Fe3+ 0.011 0.052 0.004 0.066 0.024 0.084 0.042 0.133 0.040 0.090 0.022 0.059 0.107Fe2+ 0.118 0.197 0.092 0.219 0.075 0.143 0.049 0.166 0.061 0.185 0.061 0.088 0.148Mn 0.005 0.010 0.002 0.018 0.006 0.006 0.000 0.016 0.003 0.019 0.003 0.003 0.011Mg 0.910 0.756 0.944 0.715 0.911 0.723 0.929 0.656 0.892 0.689 0.903 0.833 0.715Ca 0.887 0.887 0.921 0.894 0.923 0.931 0.925 0.927 0.933 0.925 0.930 0.936 0.929Na 0.010 0.030 0.005 0.035 0.021 0.031 0.029 0.052 0.025 0.045 0.031 0.027 0.038Total 4.000 4.000 4.001 4.001 3.999 4.003 4.002 4.000 3.997 4.000 3.998 4.001 3.991
End-members Wo-En-Fs (%) :Ca 45.94 46.67 46.91 46.75 47.61 49.35 48.05 48.63 47.55 48.84 48.46 48.79 48.68Mg 47.16 39.75 48.10 37.38 47.02 38.32 41.14 37.20 47.76 34.56 47.05 43.40 37.46ΣFe+Mn 6.90 13.59 4.99 15.87 5.37 12.33 10.81 14.16 4.67 16.60 4.50 7.81 13.86
(*) calculated according to Papike et al. (1974);
Tab. 2 - Average compositions of the clinopyroxene cores from Frosolone.
Fig. 3 - SEM microphotographs of the freshglass from Frosolone with different mor-phologies: a) prismatic elongated vesci-cules; b) rounded and ovoidal small bub-bles; c) waved bundles; d) very elongatedparallel and fibrous vescicules.
order to have a graphic representation of the composi-tions (Fig. 4). This graph clearly indicates only diopsidicand ferroan-diopsidic (Fe-Di) compositions for all theanalysed clinopyroxenes. The same graph also shows theclinopyroxene compositions reported by Orsi et al.(1995), for the Neapolitan Yellow Tuff (NYT), and byCivetta et al. (1997), for the Campanian Ignimbrite (CI).The CI clinopyroxenes fall within a mostly narrow andcentral area of the ferroan-diopside field, while the NYTclinopyroxenes spread over the same field, but alwaysnearest to the investigated Frosolone clinopyroxenes,which are distributed into two distinct groups, like theNYT clinopyroxenes. The CI clinopyroxenes are con-centrated in an area in between these two groups where,in part, the Fro1 and Fro2 clinopyroxenes also fall.
Feldspars. Tab. 3 reports the representative analysesand percentages (%) of the Or-Ab-An end-members forthe feldspars of the six horizons.
Like the clinopyroxenes, the feldspars also show vari-
able compositions; those of horizonsFro4, Fro5 and Fro6 show a broadercompositional range with respect to theother samples; Fro1 and Fro2 are practi-cally homogeneous. The highest Or con-tent (~98%) is in sample Fro4, while thelowest (~45%) is found in sample Fro6.Obviously, these varying values of Orcorrespond to variable values of Ab,which range from ~ Ab2 to ~ Ab52. Thecontent of An varies from 0 to ~ 3, onaverage keeping to very low values.
Fig. 5 gives a better compositional rep-resentation of the feldspars of Frosoloneand clearly shows how samples Fro1 andFro2 are concentrated around composi-tions of ~ Or80Ab17An3, while sampleFro3 – although relatively homogeneous– tends to differ from the previous sam-ples for its low An content (from 0.3 to2.4). Samples Fro4, Fro5 and Fro6 show
a greater chemical variability (Or98Ab2 for Fro4 toOr45Ab52An3 for Fro6) with respect to the previoussamples. The feldspars with greatest variability are thoseof sample Fro6 (Or86Ab14 to Or45Ab52An3), alwaysclose to the Ab-An side of the compositional triangle. Onthe other hand, apart the sample Fro6, the feldspars ofsamples Fro4 and Fro5 have a lower point scatter andreach a peak value of Ab27 for sample Fro4 and Ab37 forsample Fro5.
As a comparison, the graph of Fig. 5 also plots thefeldspar compositions reported by Orsi et al. (1995) forthe NeapolitanYellow Tuff and by Civetta et al. (1997)for Campanian Ignimbrite. The representative points ofthe above-mentioned compositions fall perfectly within alinear trend also including the feldspars of the Fro1 andFro2 samples. While the latter two samples perfectlymatch the NYT feldspar compositions (Orsi et al., 1995),those of Civetta et al. (1997) show a more scattered trend,
parallel to the feldspars of Fro6, Fro5 andFro4, moving, respectively, from bottomup in the relative graph. Consequently,we have two compositional trends: onethat is poorer in anorthitic content but hasa greater compositional scatter, shown bysamples Fro3, Fro4, Fro5 and Fro6, andthe other with more homogeneous com-positions but with higher anorthite con-tents, as in samples Fro1 and Fro2, andthe NYT and CI. Although the latter sam-ple shows a greater An content, it alsoshows greater compositional scatter aswith the feldspars of the other trend.
Glass. Tab. 4 reports the representativeanalyses of the volcanic glasses founduntil to about 200 cm in depth. SampleFro1 display a large compositional vari-ability (three different compositions),whereas the other samples show greater
VOLCANIC ASHES FROM FROSOLONE (ISERNIA, ... 101Geologica Romana 37 (2003-2004), 97-108
Fig. 4 - Representative compositions of the clinopyroxenes from Frosolone plotted in theCa-Mg-ΣFe graph (Wo-En-Fs end-members), correlated with NYT (Orsi et al., 1995) andCI (Civetta et al., 1997) clinopyroxenes. Di = Diopside; Fe-Di = Ferroan-Diopside.Nomenclature according to Rock (1990).
Fig. 5 - Representative compositions of the K-feldspars from Frosolone plotted in the Or-Ab-An graph and correlated with NYT (Orsi et al., 1995) and CI (Civetta et al., 1997) K-feldspars.
GIANFAGNA et al.102 Geologica Romana 37 (2003-2004), 97-108F
ro1
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23
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264
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62.8
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64.6
564
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63.7
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65.0
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65.4
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64.4
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64.5
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64.5
664
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65.7
164
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65.2
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0.14
0.07
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0.07
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0.17
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419
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18.9
618
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0.19
0.21
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0.65
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0.20
0.48
0.28
0.18
0.21
0.26
0.18
0.43
0.18
0.35
0.35
0.30
0.26
0.04
0.30
0.21
0.33
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0.26
0.14
0.06
0.13
0.06
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0.06
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0.11
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0.31
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0.25
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0.57
0.57
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0.47
0.09
0.60
0.48
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0.06
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0.13
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0.05
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0.16
0.23
0.61
0.58
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13.4
612
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12.9
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915
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11.4
413
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13.8
115
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13.5
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80.
008
0.01
40.
015
0.03
30.
016
0.00
60.
004
0.00
80.
000
0.02
0A
l4.
129
4.24
84.
111
4.09
84.
120
3.95
54.
104
4.11
53.
880
3.85
54.
112
4.07
64.
072
4.03
24.
004
3.92
44.
124
4.08
24.
030
4.06
44.
040
4.07
23.
992
4.12
0Fe
0.02
90.
033
0.02
70.
040
0.01
20.
101
0.15
90.
030
0.07
20.
044
0.02
80.
032
0.03
80.
028
0.03
20.
066
0.02
70.
052
0.04
60.
040
0.00
40.
044
0.05
20.
032
Mn
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.00
70.
000
0.04
00.
021
0.00
80.
020
0.00
90.
000
0.00
00.
009
0.01
60.
000
0.00
00.
000
0.00
00.
004
0.00
00.
000
Mg
0.03
10.
015
0.00
90.
011
0.01
90.
085
0.05
90.
018
0.00
00.
063
0.00
80.
000
0.00
90.
032
0.00
00.
069
0.01
70.
002
0.00
00.
004
0.01
20.
000
0.00
80.
000
Ca
0.11
30.
115
0.09
90.
097
0.09
10.
019
0.11
90.
095
0.00
00.
016
0.01
20.
000
0.02
60.
000
0.00
80.
008
0.00
90.
050
0.00
00.
012
0.03
20.
044
0.00
80.
120
K3.
157
2.91
93.
069
3.15
73.
087
3.66
52.
670
3.09
43.
244
3.63
23.
184
3.09
63.
061
2.77
63.
376
3.61
93.
062
2.54
23.
248
2.99
62.
840
2.50
42.
464
2.09
2N
a0.
627
0.70
40.
772
0.67
60.
741
0.09
90.
813
0.80
20.
068
0.27
60.
460
0.64
80.
823
1.03
20.
072
0.21
20.
789
1.25
90.
532
0.84
60.
936
1.38
41.
448
1.61
2B
a0.
003
0.14
50.
014
0.01
20.
008
0.00
00.
026
0.01
00.
000
0.00
40.
004
0.07
20.
007
0.00
40.
000
0.00
80.
013
0.00
40.
010
0.02
00.
008
0.00
00.
004
0.00
0C
r0.
000
0.00
80.
005
0.00
70.
006
0.00
00.
004
0.00
50.
000
0.00
00.
004
0.01
20.
002
0.01
60.
000
0.00
00.
002
0.00
00.
018
0.00
80.
020
0.00
00.
000
0.00
0P
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.03
80.
000
0.01
60.
002
0.01
20.
000
0.00
20.
012
0.04
40.
006
0.02
40.
036
0.03
20.
034
0.02
00.
036
0.03
60.
080
Sr0.
000
0.01
10.
003
0.00
60.
000
0.00
00.
001
0.00
00.
000
0.00
00.
000
0.02
40.
002
0.00
00.
008
0.00
10.
005
0.00
30.
000
0.00
00.
000
0.00
00.
000
0.00
0To
tal
19.9
5719
.941
19.9
8619
.984
19.9
6819
.913
19.8
1920
.028
19.5
0019
.969
19.7
9219
.892
19.9
7119
.900
19.6
0019
.926
19.9
6019
.920
19.8
6419
.912
19.8
7619
.972
19.9
5619
.864
Or
81.0
378
.70
77.9
480
.26
78.8
196
.90
79.6
477
.59
97.9
592
.57
87.1
082
.50
78.3
272
.93
94.2
679
.31
66.0
162
.90
85.9
677
.85
74.6
363
.68
54.7
145
.37
Ab
16.0
918
.08
19.5
117
.12
18.8
62.
6120
.04
20.0
42.
057.
0312
.57
16.8
821
.00
27.0
75.
5020
.34
32.6
136
.90
14.0
421
.84
24.5
335
.20
42.1
551
.62
An
2.89
3.22
2.56
2.62
2.33
0.49
0.
322.
370.
000.
410.
330.
630.
680.
000.
240.
351.
380.
200.
000.
310.
841.
123.
143.
02
Tab.
3 -
Ave
rage
com
posi
tions
of
the
K-f
elds
par
core
s fr
om F
roso
lone
.
homogeneity in composition, for the same examinedlevel.
The glass microprobe analyses were normalized toΣ=100 in order to allow their representation and classifi-cation in the TAS (Total Alkali-Silica) compositionalgraph, according to Le Bas et al. (1986). In Table 4 arereported only the normalised values for SiO2 andNa2O+K2O oxides.
Fig. 6a plots all the compositions of the Frosoloneanalysed glasses and, for comparison, the glasses report-ed in the literature for neighbouring volcanic areas: (AH)Alban Hills (Trigila et al., 1995), (RM) Roccamonfina(Giannetti & Luhr, 1983), (PF) Phlegrean Fields (de’Gennaro et al., 1990; Orsi et al., 1992; Melluso et al.,1995; Orsi et al., 1995; Civetta et al., 1997; de’ Gennaroet al., 1999). Glasses from Frosolone prevalently fallwithin the Trachytic and Trachy-andesitic (Latitic) fields,overlapping those from the Phlegrean Fields (lineardashed area). Fig. 6b details the PF glass compositionspresent in the literature, including CI and NYT. Finally,Fig. 6c plots only the representative analyses of theFrosolone glass. This graph shows that, while the glass ofsample Fro1 is characterised by a large compositionalvariability, the glass of the other samples have a morehomogeneous composition. In fact, sample Fro1 showsvariable compositions from Trachyte to Latite, whilesamples Fro3, Fro4 and partly Fro2, are only latitic.Sample Fro5 instead has a clear trachytic composition.
Fission-track analysis
As regards recent rocks, the problem of the experimen-tal error magnitude of ages determined with fission-trackdating is particularly serious because the uranium contentcommonly present in the rocks does not allow the accu-
mulation of an adequate number of tracks ina short time. However, this method is attimes the only one usable in dating recentvolcanic rocks. In particular, volcanic glassis of great importance because it is the onlydatable phase of many tephra (Walter,1989).
Only samples Fro1, Fro2 and Fro3 wereconsidered for fission-track analysisbecause these levels yielded promising glassgrain aliquots for dating, in view of theircharacteristics (transparency and amount).The results of the analyses are shown inTable 5. Despite the fact that a relativelylarge surface was analysed (0.586 cm2),only 2 spontaneous tracks were found forsamples Fro2 and Fro3 (counting surfaces0.254 and 0.296 cm2, respectively), whereasno tracks were observed for glasses fromsample Frol in a significantly smaller sur-face (0.036 cm2). Characteristics (such astrack densities and sensitivity to chemicaletching) of the glass fragments from sam-ples Fro1–3, that yielded polished surfaces
useful for counting, suggest that they make up a homoge-neous population. Therefore, in order to obtain a betterestimate for the age of these glasses, the age of the over-all 3 samples was computed.
This age (10,800 ± 5,500 years) is affected by a largeexperimental error (> 50%), almost totally due to the fos-sil track areal density determination.
Due to the low stability of fission tracks in glass duringgeological times, most glasses show partially rejuvenat-ed ages. The presence of partial track annealing is detect-ed by a reduction of the sizes of the spontaneous tracks incomparison with those of the “fresh” induced tracks(Storzer and Wagner, 1969). Although a spontaneous toinduced track mean size ratio, DS/DI, very close to 1,0,98 (± 0.10) had been determined (individual values DS= 5.63 ± 0.56 and DI = 5.68 ± 0.11 µm, errors are ± 1 σ),a certain amount of annealing of spontaneous tracks can-not be ruled out due to the large experimental error ofDS/DI. However, the track-size data suggest that a possi-ble rejuvenation would be negligible. In actual fact, thisresult was expected owing to the very young age of thematerial used for the investigation.
DISCUSSION AND CONCLUSIONS
General considerations
The volcanic ash deposit found near Frosolone, thesubject of the present paper, demonstrates that in lastperiod there was a fall-out of fly-ashes on the superficialgeomorphological folds of the well-known Moliseangeological area called “La Montagnola di Frosolone”.These volcanic materials, which have been casuallyfound south of the Colle dell’Orso locality (Fig. 1), donot have any correlations with the older geology of the
VOLCANIC ASHES FROM FROSOLONE (ISERNIA, ... 103Geologica Romana 37 (2003-2004), 97-108
Fro1 Fro2 Fro3 Fro4 Fro5
analyses 4 3 3 10 11 4 8
SiO2 55.00 57.63 59.33 56.70 54.02 56.24 60.02TiO2 0.63 0.51 0.41 0.54 0.65 0.57 0.48Al2O3 18.01 18.45 18.09 18.14 17.65 17.97 18.27Cr2O3 0.00 0.06 0.06 0.02 0.08 0.09 0.01*FeO(tot) 5.45 4.20 3.09 4.56 5.06 5.04 3.39MnO 0.12 0.18 0.16 0.14 0.10 0.17 0.09MgO 1.91 1.17 0.64 1.41 1.83 1.57 0.67CaO 4.89 3.54 2.62 4.00 4.67 4.42 2.57Na2O 3.04 3.23 3.71 3.29 3.09 3.11 3.88K2O 7.46 8.62 8.71 8.02 7.28 7.90 8.47BaO 0.17 0.23 0.11 0.17 0.20 0.25 0.08P2O5 0.32 0.21 0.18 0.22 0.29 0.28 0.03Cl 0.49 0.43 0.56 0.50 0.46 0.45 0.53F 0.18 0.13 0.17 0.14 0.16 0.19 0.13Total 97.62 98.55 97.65 97.83 95.45 98.07 98.63
Normalized to: Σ = 100SiO2 56.34 58.47 60.76 57.95 56.60 57.34 61.21Na2O+K2O 10.76 12.02 12.72 11.56 10.85 11.23 12.75
* FeOtot detected as FeO.
Tab. 4 - Average compositions of glass from Frosolone.
GIANFAGNA et al.104 Geologica Romana 37 (2003-2004), 97-108
area and are mostly found in the valley part of the folds.This observation leads to the hypothesis that the volcanicmaterials were first at length deposited on the hilly sur-face and were then very likely affected by valley shiftsdue to natural events such as ice or snow melts, floods,etc. Thus, the volcanic ashes of the yellow part of the val-ley profile (Fig. 2) almost surely underwent a removaland reworking with respect to the original depositionafter the eruptive event. This hypothesis should also beconfirmed by the total absence of both yellow depositsand volcanic glass in the slope and summit profiles,simultaneously carried out with the valley one.Therefore, the present work has only considered the val-ley profile, and the investigations were made to establishthe origin and the source of the unknown volcanic mate-rial.
In fact, the aim of this paper is firstly to notify, for thefirst time, the finding of a volcanic ashes deposit in theFrosolone area, and then to correlate the mineralogical,compositional, and datational results of the more signifi-cant mineralogical phases and glass investigated, in orderto trace the source of the host volcanic material.
Discussion of the experimental data
The data reported in this work refer to the study of theprimary volcanic mineral phases (clinopyroxenes andfeldspars) and coexistent glass, and they allow us tomake the following considerations.
As regards the composition of the analysed clinopyrox-enes, which range from diopside to ferroan-diopside, thegraph of Fig. 4 shows that the NYT pyroxenes have sim-ilar behaviour to Frosolone ones, while, in contrast, theCI pyroxenes fall within a more restricted area of the fer-roan-diopside field. However, this limited area alsoincludes the Fro1 and Fro2 samples, even if at a lowerlimit, and this fact leads one to think that the two samplesare more similar to those of the CI. In effect, the Fro1 andFro2 samples are the most superficial with respect to the
valley profile and, always with reference to the samegraph, we note a certain sequence related to the contentof the calcic component in the clinopyroxenes, withrespect to the sampling depth. A good correlation couldalso obviously result by taking into consideration themagmatic evolutionary process of the NYT, described indetail by Orsi et al. (1995). The authors described a three-layer magma chamber for the magmatic and evolutionaryhistory originating the Neapolitan Yellow Tuff, distin-guishing three different magma (magma1, magma2 andmagma3) on the basis of a geochemical, mineralogical,and isotopic study of the volcanic formation. Althoughnot being the object of the present work, it is, however,interesting to see a certain sequential correlation in thecompositional data reported on the clinopyroxenes.
As regards the behaviour of the analysed feldspars,also with respect to those from the CI and NYT forma-tions taken for comparison, the graph of Fig. 5 shows adouble compositional trend. While for the clinopyrox-enes there is a contemporary presence of the same sam-ple in the two well-distinguished compositional groups(Diopside – Fe-Diopside), in the case of the feldspars wehave a more clear distinction for the analysed feldsparcrystals. Samples Fro1 and Fro2, for example, find them-selves alone in the very limited group of ~ Or80Ab20composition, and have an anorthitic content that is cer-tainly higher than all the others, while as regards the par-allel trend, characterised by a lower anorthitic content,we found the feldspars of a much greater heterogeneouscomposition. This behaviour can again be justified, as forthe clinopyroxenes, by the evolutionary magmaticprocess described by Orsi et al. (1995). Moreover, thehigh compositional heterogeneity of the feldspars fromsample Fro6, which are shifted in the lower part of thegraph of Fig. 5 and are correlated to those of CI (withhigher Ab and An content), allow us to put forward thehypothesis that the volcanic material of the Fro6 samplecould belong to another activity, perhaps the CI eruption.
Sample ρS
NS
C.S.S
ρI
NI
C.S.I
p(χ2) Age ( ± 1σ)
(cm-2) (cm2) (cm-2) (cm2) (%) (a)
Fro1 _____ _____ 0.036 57,600 98 0.0017 48 _____
Fro2 7.88 2 0.254 56,100 225 0.0040 57 12,800 ± 8,800
Fro3 6.75 2 0.296 53,300 356 0.0067 61 11,000 ± 7,800
Overall 6.82 4 0.586 54,800 679 0.0124 57 10,800 ± 5,500
ρS (ρI) surface density of spontaneous (induced) tracks;
NS (NI): number of spontaneous (induced) tracks;C.S.S (C.S.I): counting surface for spontaneous (induced) tracks;p(χ2): probability to obtain calculated values in applying the p(χ2) test to confront the counts distribution of induced tracks with the Poisson distri-bution.
Parameters used for age calculation: λ = 1.55125 x 10-10 a-1; λI = 8.46 x 10-17 a-1; σ = 5.802 x 10-22 cm-2; 238U/235U = 137.88.
Samples were irradiated in the Lazy Susan position (Cd ratio: 6.5 for Au and 48 for Co) of the LENA Triga Mark II reactor (Pavia University), with
a neutron fluence of 1.75 x 1015 cm-2, referred to NRM IRMM-540 standard glass (De Corte et al., 1998).
Chemical etching for track development: 60 sec in 20% HF at 40°C. Track counting was carried out with a Leica Orthoplan microscope at 500x.
Track size measurements were performed using a Leica Microvid equipment at 1000x.
Tab. 5 - Fission-track dating of Frosolone glass.
VOLCANIC ASHES FROM FROSOLONE (ISERNIA, ... 105Geologica Romana 37 (2003-2004), 97-108
Fig. 6 - TAS graph representa-tion for glass compositions.
a) Frosolone glass (black solidcircles) compared with thecompisitions of the glassfrom Alban Hills (AH area;Trigila et al., 1995), Rocca-monfina (RM area; Gian-netti & Luhr, 1983) andPhlegrean Fields (PF delim-ited area; various authors:see legend. Fig 6b). T-P h = Te p h r i - p h o n o l i t e ;Ph=Phonolite; T=Trachyte;L=Latite (K) (according toLe Bas et al., 1986).
b) Plot of the compositions ofthe glass from the Phlegre-an Fields (references in leg-end; graphic abbreviationsas in 6a).
c) Plot of the average compo-sitions of the glass fromFrosolone (graphic abbrevi-ations as in 6a).
a
b
c
Not finding any glass in the Fro6 sample, it was not pos-sible to date the material using the fission-track method.However, the fact that the Fro6 sample does not containglass shows that this material (already under about 200cm in depth, Fig. 2) has a different, and certainly older,origin.
The chemical and datational analyses carried out on theglass support what has been said so far. The variablecomposition from trachyte to trachy-andesite (latite) ofthe glass, as highlighted by the three TAS compositionalgraphs (Figs. 6a, 6b and 6c), demonstrates how the chem-istry of the glass contained in the volcanic ashes found atFrosolone can be referred to an evolutionary magmaticactivity. This could also be demonstrated by the differentmorphologies of the glass fragments, as seen in Fig. 3,which also shows that the glass is quite fresh. The gener-al TAS graph of Fig.6a shows that the analysed glassesfrom Frosolone can be referred to Phlegrean Field activ-ity, ruling out other origins, such as the Alban Hills prod-ucts (Trigila et al., 1995) or those of Roccamonfina(Giannetti & Luhr, 1983), the nearest volcanic districts toFrosolone. The graph of Fig. 6b highlights the composi-tional differences of the glasses of Campanian Ignimbritewith respect to those of the NYT. Even if the latter arefairly scattered in the graph, falling within the four com-positional fields (T, L, Ph and T-Ph), it is obvious that theCI and Phlegrean Field glasses (Civetta et al., 1997; de’Gennaro et al., 1999; Melluso et al., 1995) are generallyless scattered and fall mainly within the Ph-T fields. Fig.6c reports only the mean compositions of Frosolone glassand shows a fair correlation between these compositionsand those of NYT, at least as regards the Latite-Trachy-andesite evolutionary trend. The results of the fission-track dating on glass contribute to confirming what hasbeen said here and, even if flawed by a high degree ofexperimental error, they agree with the indicationsalready found in the analytical compositions concerningthe probable origin of the material under study. The ageof 10,800 (±5,500) years falls well within the period ofPhlegrean Field activity, which gave rise to theNeapolitan Yellow Tuff formation. Thus, the determinedage rules out the earlier activity that produced theCampanian Ignimbrite, for which an age of about 35,000years was established, as reported in Rosi & Sbrana(1987). On the other hand, glass is the only datable phasein these types of tephra and, in the specific case ofFrosolone, the experimental data were in any case signif-icant despite the high degree of error.
Finally, as regards the features of the investigated vol-canic material, such as structure and texture, laminar lay-ering and relative differences in colour at various depths,the section of the profile reported in Fig. 2 clearly showsthat level Fro5 (170-195 cm) is characterized by a clear-er colouring with respect to the upper one, with coloursthat go from a light yellow to grey-white and white.
These evident differences in colour could be attributed toa simple process of migration of elements and/or also toa process of pedogenesis. In fact, the dark colour of themost superficial levels may be due to the slight differencein composition of the parental material undergoing analterative process with following intense pedogenesis.However, we must also to consider the particular geolog-ical period of deposition, when strong superficial move-ments caused by natural events brought about a trans-portation and superficial resedimentation of the material,with a consequent reworking with respect to the originaldeposit. Therefore, another, but different, scientificapproach is necessary to understand the various sedimen-tary events, which have interested the global area ofFrosolone in subsequent times.
CONCLUSIONS
In view of what has been said in the present paper, wecan conclude that the features of the analysed material(structure and texture, laminar layering and relativecolour differences, variability in composition of theanalysed phases), have allowed us to attribute theFrosolone volcanic materials to a Phlegrean origin, inparticular to the products of the Neapolitan Yellow Tufferuptive period, as also shown by the dating analyses thatattribute an age of 10,800 (± 5,500) years to the investi-gated glass. This allows us to confirm the source as thelate period of Phlegrean activity (NYT), ruling out theproducts of Campanian Ignimbrite that date back toabout 35,000 years ago. However, at moment, we do notconsider it appropriate to provide further details on theexact reconstruction of the volcanic events, whichrequire another type of investigation, and also becausewe did not aim to make this sort of considerations.Instead, we wanted to stress how this type of research,focusing on mineralogical and dating determinations, canestablish the period as well as the origin and provenanceof the volcanic products of the material found in the“Montagnola di Frosolone” area. The experimental datapresented in this work could also suggest the need forfurther study of these volcanic ashes by focusing more onthe identification and attribution of the exact periods ofthe relative volcanic activities and their depositionmodality.
ACKNOWLEDGEMENTS - The authors are grateful to Dr.G. Cavarretta of the CNR (“Istituto di Geologia Ambientale eGeoingegneria”) for permission to use the electron microprobe,and to Mr. M. Serracino for his help in the analytical tests.Particular thanks are due to Prof. F. Terribile (Faculty ofAgrarian Studies, Naples), who made it possible to start thiswork. Heartfelt thanks also to Prof. M. de’ Gennaro for his crit-ical reading of the manuscript and to Proff. F. Bonadonna andF. Lisker for their invaluable advice for improvements. Thiswork was supported by MIUR and CNR grants.
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Accettato per la stampa: Marzo 2003
GIANFAGNA et alii108 Geologica Romana 37 (2003-2004), 97-108
