Volcanic Ashes from Frosolone (Isernia, Italy): Mineralogical Characterisation and Fission-Track Dating

Geologica Romana 37 (2003-2004), 97-108

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]

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 clinopyroxenes, K-feldspars and glass, as well as an attempt at a geochronological dating by fission-track method on glass, 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 a prevalent “orthoclasic” composition is found for the feldspars. The glass presents prevalently trachytic and/or trachy-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 and structural appearance of the host-layering yellowish material would all seem to indicate that the Frosolone ashes are referable to the explosive products of the Phlegrean Fields (Campanian activity), in particular to the Neapolitan Yellow Tuff emission, thus ruling out all earlier Campanian activities and also other probable provenances from neighbouring volcanic complexes.

KEY WORDS: volcanic ashes, clinopyroxene, k-feldspar, glass, fission-track dating, Phlegrean activity, Frosolone, South- Central Apennines.

INTRODUCTION prevalently in Central and Southern Italy. Volcanic pale- osoils are often used as chronostratigraphic pedomark- The present work examines the results of a mineralog- ers. Frezzotti & Narcisi (1996) identified two important ical and dating study carried out on volcanic ash deposits pedomarkers in Central Italy, both referable to Phlegrean found at Colle dell’Orso, near Frosolone (Isernia), activity (Campanian Ignimbrite and Neapolitan Yellow Molise, in the South-Central Apennines. Tuff); Giraudi (1989) reported a review of the lake levels The finding of volcanic ashes in the Frosolone area and climate for the last 30,000 years in the Fucino area concerns some current issues of general interest, namely: (Abruzzo, Central Italy), while Narcisi (1996) investigat- 1) the Quaternary stratigraphy of Central Italy, 2) the ed fourteen tephra layers, all belonging to Campanian characterisation and areal spread of volcanic soils in the activity, in the Vulture district (Lago Grande di South-Central Apennines, and 3) recent issues on the Monticchio, Basilicata, Southern Italy). Other similar probable intrapenninic volcanism. studies have also been carried out (Arnoldus- The area under study is situated on the boundary Huyzendveld et al., 1985; Mizota & van Reeuwijk, 1989; between the Central and South-East Apennines, and rel- Frezzotti & Giraudi, 1989, 1990, 1992; Frezzotti & atively borders on the volcanic complexes mostly distrib- Narcisi, 1989) because the regions of this area were uted west, south-west and south of the Frosolone area. widely affected during the Quaternary by the re-fall of From north to south, these are the Alban Hills (Volcan products deriving from the explosive activity of the Latiale) and Roccamonfina complexes, both in the Latial neighbouring Roman and Campanian volcanic districts. region, the Phlegrean Fields complex (Campanian The present paper firstly aims to notify the finding of a region) , and the smaller Mount Vulture complex in the volcanic ash deposit in the Frosolone area – this kind of Basilicata region. Moreover, there should also be indica- deposit never being reported here before – and also to tions on probable intrapenninic activity immediately to report a simultaneous combination of mineralogical and north of Frosolone, in the Abruzzo region (Barbieri et al., dating results which are useful in order to explain the ori- 1996; Bosi et al., 1991). Therefore, the material found in gin and source of the volcanic ashes found at Frosolone. the studied area could actually be attributed to the activ- The chemistry of the primary mineral phases (clinopy- ity of one of these neighbouring complexes, but it would roxenes and feldspars) and the results of fission-track also be reasonable to hypothesize a possible local intrap- dating on glass, carried out in this work, significantly peninic activity, albeit of a minor scale. To date, howev- contributes to solving this problem. er, there is no evidence of occurrences of autochthonous volcanic material in the Molisean area and so the Frosolone volcanic ashes must be attributed to explosive GEOLOGY AND GEOMORPHOLOGY eruptions of neighbouring volcanic activity. OF THE AREA The literature frequently presents works investigating the origin and provenance of volcanic products found The area of Colle dell’Orso (1393 m a.s.l.) is located in the municipality of Frosolone, in the province of 98 Geologica Romana 37 (2003-2004), 97-108 GIANFAGNA et al.

al. (1997), De Corso et al. (1998), Di Luzio et al. (1999) and Scrocca & Tozzi (1999). As regards local geology, the Frosolone hillock may be ascribed to the “falda molisana”, composed of a pelagic succession sedimented between the upper Trias and the Miocene (Patacca et al., 1992).

RESULTS Profile description Some transect profiles were carried out in the Colle dell’Orso area, near Frosolone (Fig. 1), along an align- ment including summit, slope and valley profiles. Of the series 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 yellowish layers with abundant glassy matrix. The other two profiles, summit and slope, showed substantial differences with the valley profile since they did not present yellow layers and contained no glass. Therefore, the present work will only consider the valley profile investigation, while a future separate work on the mineralogy of the other two profiles will be performed in detail in order to have an overview of the depositional situation of the Frosolone area. The studied valley profile (Fig. 2) presents a yellow- ochre level, from ca. 130 to 200 cm depth, that is clear- Fig. 1 - Location of investigated area near Frosolone (modified after ly different from the other upper and lower sequences, Frezzotti and Narcisi, 1996). which are instead characterized by a brown-blackish colour. The yellowish level shows evident layering with Isernia (Fig. 1). The landscape is predominantly moun- tainous (1200-1400 m a.s.l.) and is composed of a series of calcareous-dolomitic and selciferous reliefs characterized by rounded summits and slopes with a regular profile, abundant hillock pastures mostly located on the summital plateaux, on the subplateau levels or on the bottom of basins and small valleys, the latter relating to the local karst phenomena. The geological arrangement of this Apenninic sector is very complex and developed over a long time period between the upper Trias and the Quaternary, during which rocks belonging to different sedimentary environ- ments were deposited; moreover, the occurrence of tec- tonic phases has led to the present structural chain. A recent paper by Tozzi et al. (1999) reports the final results of a geological-structural survey carried out on a broad area of the Molise region, comprising the entire Montagnola 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 sedimentation of detritic limestones took place from the Dogger to the Middle Miocene. Other previous general geological descriptions of the area are also reported in papers by Pescatore (1963, 1965), Cocco (1971), Sgrosso (1986), Naso et al. (1989), Fig. 2 - Sampled levels of the valley profile. Note the layered section Patacca et al. (1992) and, more recently, by Corrado et ca. 70 cm thick, from yellow to whitish in colour. VOLCANIC ASHES FROM FROSOLONE (ISERNIA, ... Geologica Romana 37 (2003-2004), 97-108 99

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. variable colours ranging from yellow-ochre in the Mineral characterisation uppermost part, progressively shading to pale-yellow The primary mineralogical assemblage is represented and to white in the lower part. It rests directly on a sed- by prevalent clinopyroxenes and feldspars, while brown iment of coarse material mainly composed of a thin mica and magnetite are present in all levels subordinate- clayey matrix of brownish or dark brown colour, with ly (Tab. 1). Quartz, albite, apatite and other accessory large siliceous splinters and Fe-Mn nodules of up to 260 phases are also present. Glass is abundantly present as far cm in depth, and is overlapped by a very dark brown, down as 195 cm in depth, while it is completely absent almost black, deposit reaching the profile surface, com- in the Fro6 sample (250-260 cm), where many abundant posed of a very fine clayey material containing volcanic black Fe-Mn nodules, 0.5 to 2 mm in size, appear. glass, clinopyroxenes, feldspars, quartz, and, subordi- Since this study aims at establishing the volcanic dis- nately, also other mineralogical phases (magnetite, trict of possible origin of the volcanic material found at biotite, apatite, Fe-Mn-oxides). Frosolone, the research focused on the mineralogical phases that could yield useful data: clinopyroxenes, K- Experimental feldspars and glass. The significant compositional vari- Materials and analytical techniques ability of these primary phases is a good indicator of the On the basis of morpho-structural characteristics and probable origin of the host volcanic material. colour differences, 6 samplings were carried out at dif- The clinopyroxenes, always present, are less than 1 ferent depths in the valley profile (Fig. 2). The samples, mm in size and display a prevalently prismatic and elon- named “Fro” and used in the present work, are the fol- gated habit, with varying colour from light-green, dark- lowing: green up to dark brown and black. They are often found Fro1 (10-30 cm), Fro2 (40-50 cm), Fro3 (130-140 altered at their surface and compositional zonations are cm), Fro4 (145-155 cm), Fro5 (170-195 cm) and Fro6 frequently observed in thin section. (250-260 cm). The feldspars, always present, are generally splintery, Grains of clinopyroxene (light green and dark green thereby implying a derivation from larger crystals; they prismatic crystals), K-feldspar and glass (yellowish, are generally larger than the clinopyroxenes (2 to 3 brownish and black in colour; fibrous, vesicular and mm). Little altered at the surface, the feldspar crystals globular in shape) were collected from the sandy gran- present a notable transparency and are colourless. In ulometric fraction (>0.25 mm) of the six loose samples thin section they do not present any peculiar composi- under examination. The selected grains were separated tional zonation and, in some cases, they show a narrow into three aliquots, for SEM (ZEISS mod. DSM 940A), contact with glass. EMPA (CAMECA SX50 electron microprobe, equipped The glass, absent in the Fro6 sample, has different with 5 WDS spectrometers at 15 kV, 15 nA, 100 cps, morphologies in the various levels and varies from with a Link EDS system controlled by Specta software) fibrous-elongated to bubble-vesicular and filamentous and geochronological determinations (Fission-Track up to solid-massive and globular (obsidian type) (Fig. 3), dating method), respectively. with very diversified colours from pale-brown through The fission-track analysis of the volcanic glass was dark-brown to black. Generally, the bubbles and vesicles carried out in the Istituto di Geoscienze e Georisorse of are filled with soft clay material of allophanic type (X- the CNR of Pisa. The population dating method was ray analysis), similar to the material mainly making up used (Gleadow, 1981). the matrix of the whole sediment. 100 Geologica Romana 37 (2003-2004), 97-108 GIANFAGNA et al.

Fro1 Fro2 Fro3 Fro4 Fro5 Fro6 analyses 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 50.28 TiO2 0.46 0.77 0.44 0.77 0.27 0.66 0.23 1.02 0.26 0.76 0.38 0.51 0.57 Al2O3 2.58 3.58 1.39 3.14 1.63 4.90 1.33 4.05 1.97 3.14 1.61 3.29 3.57 Cr2O3 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 4.75 MnO 0.17 0.33 0.07 0.58 0.18 0.19 0.00 0.51 0.10 0.59 0.10 0.15 0.34 MgO 16.59 13.58 17.23 12.86 16.77 12.95 17.24 11.81 16.57 12.38 16.71 15.04 12.88 CaO 22.48 22.18 23.37 22.37 23.60 23.19 23.90 23.19 24.06 23.13 24.12 23.90 23.54 Na2O 0.15 0.42 0.07 0.49 0.30 0.42 0.41 0.71 0.36 0.62 0.44 0.40 0.53 Total 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 1.872 Ti 0.013 0.022 0.012 0.022 0.007 0.019 0.006 0.029 0.007 0.021 0.010 0.014 0.016 IVAl 0.074 0.113 0.044 0.106 0.049 0.155 0.052 0.160 0.063 0.114 0.048 0.104 0.129 VIAl 0.039 0.045 0.016 0.032 0.021 0.062 0.005 0.018 0.021 0.024 0.020 0.037 0.028 Cr 0.009 0.002 0.004 0.000 0.011 0.003 0.018 0.002 0.014 0.002 0.017 0.005 0.000 Fe3+ 0.011 0.052 0.004 0.066 0.024 0.084 0.042 0.133 0.040 0.090 0.022 0.059 0.107 Fe2+ 0.118 0.197 0.092 0.219 0.075 0.143 0.049 0.166 0.061 0.185 0.061 0.088 0.148 Mn 0.005 0.010 0.002 0.018 0.006 0.006 0.000 0.016 0.003 0.019 0.003 0.003 0.011 Mg 0.910 0.756 0.944 0.715 0.911 0.723 0.929 0.656 0.892 0.689 0.903 0.833 0.715 Ca 0.887 0.887 0.921 0.894 0.923 0.931 0.925 0.927 0.933 0.925 0.930 0.936 0.929 Na 0.010 0.030 0.005 0.035 0.021 0.031 0.029 0.052 0.025 0.045 0.031 0.027 0.038 Total 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 48.68 Mg 47.16 39.75 48.10 37.38 47.02 38.32 41.14 37.20 47.76 34.56 47.05 43.40 37.46 ΣFe+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.

Compositional data Clinopyroxenes. Tab. 2 reports the averaged representative chemical analyses of the clinopyroxene cores of the six analysed samples. However, a complete analytical data set is disposable for consulta- tion from A.G.. In each sample, at least two mean compositions were found, ranging from diopside to ferroan-diopside (nomenclature according to Rock, 1990). Three different representative compositions were identified for sample Fro6. From the crystal chemical formula, calculated on the basis of 6 oxygens, the percentages of the Wo-En-Fs (Ca-Mg-ΣFe+Mn) end-members were calculated in

Fig. 3 - SEM microphotographs of the fresh glass from Frosolone with different morphologies: a) prismatic elongated vescicules; b) rounded and ovoidal small bubbles; c) waved bundles; d) very elongated parallel and fibrous vescicules. VOLCANIC ASHES FROM FROSOLONE (ISERNIA, ... Geologica Romana 37 (2003-2004), 97-108 101

able compositions; those of horizons Fro4, Fro5 and Fro6 show a broader compositional range with respect to the other samples; Fro1 and Fro2 are practi- cally homogeneous. The highest Or content (~98%) is in sample Fro4, while the lowest (~45%) is found in sample Fro6. Obviously, these varying values of Or correspond to variable values of Ab, which range from ~ Ab2 to ~ Ab52. The content of An varies from 0 to ~ 3, on average keeping to very low values. Fig. 5 gives a better compositional representation of the feldspars of Frosolone and clearly shows how samples Fro1 and Fro2 are concentrated around composi- Fig. 4 - Representative compositions of the clinopyroxenes from Frosolone plotted in the tions of ~ Or Ab An , while sample Ca-Mg-ΣFe graph (Wo-En-Fs end-members), correlated with NYT (Orsi et al., 1995) and 80 17 3 CI (Civetta et al., 1997) clinopyroxenes. Di = Diopside; Fe-Di = Ferroan-Diopside. Fro3 – although relatively homogeneous Nomenclature according to Rock (1990). – tends to differ from the previous samples for its low An content (from 0.3 to 2.4). Samples Fro4, Fro5 and Fro6 show order to have a graphic representation of the composi- a greater chemical variability (Or Ab for Fro4 to tions (Fig. 4). This graph clearly indicates only diopsidic 98 2 Or Ab An for Fro6) with respect to the previous and ferroan-diopsidic (Fe-Di) compositions for all the 45 52 3 samples. The feldspars with greatest variability are those analysed clinopyroxenes. The same graph also shows the of sample Fro6 (Or Ab to Or Ab An ), always clinopyroxene compositions reported by Orsi et al. 86 14 45 52 3 close to the Ab-An side of the compositional triangle. On (1995), for the Neapolitan Yellow Tuff (NYT), and by the other hand, apart the sample Fro6, the feldspars of Civetta et al. (1997), for the Campanian Ignimbrite (CI). samples Fro4 and Fro5 have a lower point scatter and The CI clinopyroxenes fall within a mostly narrow and reach a peak value of Ab for sample Fro4 and Ab for central area of the ferroan-diopside field, while the NYT 27 37 sample Fro5. clinopyroxenes spread over the same field, but always As a comparison, the graph of Fig. 5 also plots the nearest to the investigated Frosolone clinopyroxenes, feldspar compositions reported by Orsi et al. (1995) for which are distributed into two distinct groups, like the the NeapolitanYellow Tuff and by Civetta et al. (1997) NYT clinopyroxenes. The CI clinopyroxenes are con- for Campanian Ignimbrite. The representative points of centrated in an area in between these two groups where, the above-mentioned compositions fall perfectly within a in part, the Fro1 and Fro2 clinopyroxenes also fall. linear trend also including the feldspars of the Fro1 and Feldspars. Tab. 3 reports the representative analyses Fro2 samples. While the latter two samples perfectly and percentages (%) of the Or-Ab-An end-members for match the NYT feldspar compositions (Orsi et al., 1995), the feldspars of the six horizons. those of Civetta et al. (1997) show a more scattered trend, Like the clinopyroxenes, the feldspars also show vari- parallel to the feldspars of Fro6, Fro5 and Fro4, moving, respectively, from bottom up in the relative graph. Consequently, we have two compositional trends: one that is poorer in anorthitic content but has a greater compositional scatter, shown by samples Fro3, Fro4, Fro5 and Fro6, and the other with more homogeneous compositions but with higher anorthite con- tents, as in samples Fro1 and Fro2, and the NYT and CI. Although the latter sample shows a greater An content, it also shows greater compositional scatter as with the feldspars of the other trend. Glass. Tab. 4 reports the representative analyses of the volcanic glasses found until to about 200 cm in depth. Sample Fro1 display a large compositional vari- Fig. 5 - Representative compositions of the K-feldspars from Frosolone plotted in the Or-Ab- ability (three different compositions), An graph and correlated with NYT (Orsi et al., 1995) and CI (Civetta et al., 1997) K-feldspars. whereas the other samples show greater 102 Geologica Romana 37 (2003-2004), 97-108 GIANFAGNA et al. 00.46 0 11.876 11.944 11.788 .876 19.972 19.956 19.864 Fro1 Fro2 Fro3 Fro4 Fro5 Fro6 0.03 0.14 0.07 0.03 0.07 0.120.00 0.070.00 0.05 0.04 0.00 0.00 0.06 0.00 0.00 0.04 0.05 0.00 0.04 0.00 0.00 0.07 0.05 0.00 0.00 0.02 0.04 0.25 0.10 0.00 0.11 0.23 0.00 0.00 0.00 0.11 0.02 0.06 0.08 0.07 0.11 0.00 0.01 0.04 0.10 0.06 0.02 0.09 0.07 0.15 0.00 0.17 0.01 0.29 0.00 0.10 0.00 0.21 0.23 0.13 0.06 0.21 0.13 0.34 0.12 0.00 0.00 0.23 0.00 0.52 0.31 64.51 62.80 64.0919.04 19.31 18.83 64.65 64.71 18.93 63.78 19.05 64.66 65.04 17.88 19.06 66.15 19.15 65.48 64.75 64.46 17.89 17.76 64.99 64.52 18.89 18.79 18.88 18.46 65.13 64.56 64.94 65.71 18.08 19.05 18.96 18.63 64.79 64.90 64.89 65.28 18.57 64.90 18.75 66.35 18.59 18.98 19.24 19.62 3 3 5 O 1.76 1.95 2.15 1.90 2.08 0.27 2.29 2.27 0.19 0.77 1.28 1.82 2.32 2.87 0.60 2.22 3.55 4.11 1.49 2.38 2.62 3.92 4.58 5.54 2 2 O O 2 O 13.46 12.26 12.99 13.47 13.19 15.31 11.44 13.30 13.81 15.46 13.51 13.19 13.08 11.75 15.39 13.06 10.91 10.63 13.83 12.83 12.08 10.79 9.03 7.40 Total 99.72 99.40 99.05 100.05 99.84 98.39 100.04 100.73 98.94 100.31 98.95 99.87 100.00 98.15 100.50 99.68 99.49 99.77 99.55 99.71 98.78 99.83 99.24 1 2 2 O 2 2 Na TiO Al FeOMnOMgO 0.19CaO 0.00 0.21K 0.11 0.00 0.18 0.05 0.57 0.00 0.57 0.03 0.26 0.50P 0.00 0.08SrO 0.04 0.00 0.07 0.50 0.65 0.00 0.47 0.00 1.03 0.31 0.05 0.09 0.20 0.00 0.21 0.09 0.02 0.07 0.48 0.60 0.26 0.28 0.48 0.14 0.00 0.06 0.18 0.06 0.23 0.00 0.21 0.00 0.02 0.13 0.08 0.26 0.00 0.06 0.06 0.18 0.00 0.00 0.03 0.00 0.01 0.11 0.13 0.00 0.43 0.00 0.06 0.18 0.35 0.10 0.00 0.25 0.00 0.35 0.06 0.00 0.04 0.00 0.01 0.00 0.05 0.02 0.25 0.22 0.30 0.04 0.00 0.02 0.26 0.01 0.00 0.00 0.04 0.00 0.01 0.30 0.00 0.04 0.04 0.06 0.21 0.02 0.00 0.00 0.16 0.33 0.02 0.00 0.00 0.01 0.23 0.16 0.00 0.61 0.58 0.00 0.00 0.00 0.00 0.00 0.00 Unit formula (oxygens = 32) SiTiAl 11.865Fe 11.725 11.868Mn 0.003 0.019Mg 4.129 11.875 4.248 0.009Ca 0.029 11.875 0.000 4.1110.033K 0.000 11.973 0.031 0.027 11.809Na 0.004 11.854 0.0000.015 0.009 0.113Ba 4.098 12.172 12.059 0.009 0.115 11.952 4.120Cr 0.040 11.864 3.157 11.891 11.960 0.016 0.000 0.099 0.012 0.627P 2.919 0.010 0.000 3.955 0.704 0.011 0.003 0.005Sr 3.069 4.104 12.04811.99111.857 0.019 0.101 11.857 0.772 0.145 4.115 0.097Total 0.000 0.159 0.000 0.008 0.091 0.014 0.008 0.030 0.007 0.000 3.157 0.085 0.000 11.932 3.880 11.882 0.000 0.008 11.96 0.005 19.957 0.676 3.087Or 3.855 0.059 0.000 0.000 19.941 0.000 0.072 0.019 4.112 0.018 0.741 0.011 0.012 19.986Ab 0.044 0.009 0.000 0.040 0.119 4.076 0.008 0.028 0.008 0.021 0.095 3.665 0.003An 0.007 4.072 0.000 0.032 0.008 0.099 4.032 2.670 81.03 19.984 0.063 0.006 0.020 0.038 3.094 19.968 0.813 0.008 0.000 0.000 78.70 16.09 0.028 0.000 0.009 0.802 0.008 0.016 0.000 0.006 0.000 0.000 0.026 0.014 18.08 77.94 compositions of the K-feldspar cores from Frosolone. 0.012Average 0.015 3 - Tab. 0.000 3.244 19.913 0.009 4.004 0.010 0.000 2.89 19.819 0.033 3.924 20.028 0.000 0.032 3.632 19.51 0.068 0.004 4.124 3.184 0.032 0.005 0.276 0.000 0.026 3.22 4.082 0.066 0.000 19.500 0.000 0.027 0.460 3.096 0.000 80.26 19.969 0.038 0.000 19.792 0.009 0.004 0.052 19.892 0.648 0.016 0.000 0.016 3.061 78.81 2.56 0.001 0.000 17.12 0.004 19.971 0.000 19.900 0.006 2.776 0.000 0.069 0.823 0.000 0.000 4.030 0.017 0.072 18.86 0.004 1.032 0.004 4.064 0.008 0.016 0.002 0.007 0.008 0.008 0.046 96.90 4.040 0.012 0.002 0.009 0.004 0.000 19.60019.92619.960 0.040 0.000 19.920 0.000 0.012 79.64 2.62 4.072 0.050 3.376 0.002 0.000 0.020 0.004 0.000 3.619 2.61 77.59 0.016 3.992 0.000 3.062 0.000 0.072 0.212 0.000 0.044 0.000 0.789 2.33 4.120 20.04 2.542 0.004 0.024 0.002 19.864 19.912 0.052 1.259 0.004 0.000 20.04 19 0.012 0.012 97.95 0.000 0.008 0.032 0.002 0.000 0.013 0.012 0.000 92.57 0.000 0.000 0.000 0.004 0.000 0.032 87.10 0.008 0.49 3.248 0.002 2.05 0.532 0.000 2.996 0.044 82.50 0.000 0.32 0.044 0.846 2.840 0.008 0.006 7.03 78.32 0.936 0.024 2.37 0.008 0.120 0.010 72.93 2.504 12.57 0.001 0.036 1.384 0.005 0.020 2.464 16.88 1.448 0.018 0.003 0.008 2.092 21.00 1.612 0.008 0.00 0.000 27.07 0.020 94.26 0.004 0.032 0.41 79.31 0.000 0.034 0.000 66.01 0.000 0.000 0.020 0.33 62.90 0.000 0.000 0.036 0.000 0.63 5.50 0.036 0.000 20.34 32.61 0.080 0.000 85.96 0.68 36.90 0.000 77.85 0.00 74.63 63.68 14.04 54.71 21.84 45.37 24.53 0.24 35.20 0.35 42.15 1.38 51.62 0.20 0.00 0.31 0.84 1.12 3.14 3.02 BaOCr 0.04 1.98 0.19 0.17 0.11 0.00 0.35 0.15 0.00 0.05 0.05 0.98 0.10 0.03 0.11 0.16 0.07 0.05 0.12 0.06 0.09 0.00 0.00 0.00 analyses 2SiO 3 2 2 2 2 3 4 2 3 1 2 2 3 4 2 3 3 2 1 3 2 3 2 VOLCANIC ASHES FROM FROSOLONE (ISERNIA, ... Geologica Romana 37 (2003-2004), 97-108 103

Fro1 Fro2 Fro3 Fro4 Fro5 mulation of an adequate number of tracks in analyses 4 3 3 10 11 4 8 a short time. However, this method is at times the only one usable in dating recent SiO 55.00 57.63 59.33 56.70 54.02 56.24 60.02 2 volcanic rocks. In particular, volcanic glass TiO2 0.63 0.51 0.41 0.54 0.65 0.57 0.48 is of great importance because it is the only Al2O3 18.01 18.45 18.09 18.14 17.65 17.97 18.27 Cr2O3 0.00 0.06 0.06 0.02 0.08 0.09 0.01 datable phase of many tephra (Walter, *FeO(tot) 5.45 4.20 3.09 4.56 5.06 5.04 3.39 1989). MnO 0.12 0.18 0.16 0.14 0.10 0.17 0.09 Only samples Fro1, Fro2 and Fro3 were MgO 1.91 1.17 0.64 1.41 1.83 1.57 0.67 considered for fission-track analysis CaO 4.89 3.54 2.62 4.00 4.67 4.42 2.57 because these levels yielded promising glass Na2O 3.04 3.23 3.71 3.29 3.09 3.11 3.88 grain aliquots for dating, in view of their K2O 7.46 8.62 8.71 8.02 7.28 7.90 8.47 characteristics (transparency and amount). BaO 0.17 0.23 0.11 0.17 0.20 0.25 0.08 The results of the analyses are shown in P O 0.32 0.21 0.18 0.22 0.29 0.28 0.03 2 5 Table 5. Despite the fact that a relatively Cl 0.49 0.43 0.56 0.50 0.46 0.45 0.53 2 F 0.18 0.13 0.17 0.14 0.16 0.19 0.13 large surface was analysed (0.586 cm ), Total 97.62 98.55 97.65 97.83 95.45 98.07 98.63 only 2 spontaneous tracks were found for samples Fro2 and Fro3 (counting surfaces Normalized to: Σ = 100 0.254 and 0.296 cm2, respectively), whereas SiO2 56.34 58.47 60.76 57.95 56.60 57.34 61.21 no tracks were observed for glasses from Na2O+K2O 10.76 12.02 12.72 11.56 10.85 11.23 12.75 sample Frol in a significantly smaller sur- 2 * FeOtot detected as FeO. face (0.036 cm ). Characteristics (such as track densities and sensitivity to chemical Tab. 4 - Average compositions of glass from Frosolone. etching) of the glass fragments from samples Fro1–3, that yielded polished surfaces homogeneity in composition, for the same examined useful for counting, suggest that they make up a homoge- level. neous population. Therefore, in order to obtain a better The glass microprobe analyses were normalized to estimate for the age of these glasses, the age of the over- Σ=100 in order to allow their representation and classifi- all 3 samples was computed. cation in the TAS (Total Alkali-Silica) compositional This age (10,800 ± 5,500 years) is affected by a large graph, according to Le Bas et al. (1986). In Table 4 are experimental error (> 50%), almost totally due to the fos- reported only the normalised values for SiO2 and sil track areal density determination. Na2O+K2O oxides. Due to the low stability of fission tracks in glass during Fig. 6a plots all the compositions of the Frosolone geological times, most glasses show partially rejuvenat- analysed glasses and, for comparison, the glasses reported ages. The presence of partial track annealing is detected in the literature for neighbouring volcanic areas: (AH) ed by a reduction of the sizes of the spontaneous tracks in Alban Hills (Trigila et al., 1995), (RM) Roccamonfina comparison with those of the “fresh” induced tracks (Giannetti & Luhr, 1983), (PF) Phlegrean Fields (de’ (Storzer and Wagner, 1969). Although a spontaneous to Gennaro et al., 1990; Orsi et al., 1992; Melluso et al., induced track mean size ratio, DS/DI, very close to 1, 1995; Orsi et al., 1995; Civetta et al., 1997; de’ Gennaro 0,98 (± 0.10) had been determined (individual values DS et al., 1999). Glasses from Frosolone prevalently fall = 5.63 ± 0.56 and DI = 5.68 ± 0.11 µm, errors are ± 1 σ), within the Trachytic and Trachy-andesitic (Latitic) fields, a certain amount of annealing of spontaneous tracks can- overlapping those from the Phlegrean Fields (linear not be ruled out due to the large experimental error of dashed area). Fig. 6b details the PF glass compositions DS/DI. However, the track-size data suggest that a possi- present in the literature, including CI and NYT. Finally, ble rejuvenation would be negligible. In actual fact, this Fig. 6c plots only the representative analyses of the result was expected owing to the very young age of the Frosolone glass. This graph shows that, while the glass of material used for the investigation. sample Fro1 is characterised by a large compositional variability, the glass of the other samples have a more homogeneous composition. In fact, sample Fro1 shows DISCUSSION AND CONCLUSIONS variable compositions from Trachyte to Latite, while General considerations samples Fro3, Fro4 and partly Fro2, are only latitic. Sample Fro5 instead has a clear trachytic composition. The volcanic ash deposit found near Frosolone, the subject of the present paper, demonstrates that in last period there was a fall-out of fly-ashes on the superficial Fission-track analysis geomorphological folds of the well-known Molisean As regards recent rocks, the problem of the experimen- geological area called “La Montagnola di Frosolone”. tal error magnitude of ages determined with fission-track These volcanic materials, which have been casually dating is particularly serious because the uranium content found south of the Colle dell’Orso locality (Fig. 1), do commonly present in the rocks does not allow the accu- not have any correlations with the older geology of the 104 Geologica Romana 37 (2003-2004), 97-108 GIANFAGNA et al.

Sample ρ N C.S. ρ N C.S. p(χ2) Age ( ± 1σ) S S S I I I (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

ρ (ρ ) surface density of spontaneous (induced) tracks; S I 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 distribution. λ -10 -1 λ -17 -1 σ -22 -2 238 235 Parameters used for age calculation: = 1.55125 x 10 a ; I = 8.46 x 10 a ; = 5.802 x 10 cm ; U/ U = 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. area and are mostly found in the valley part of the folds. valley profile and, always with reference to the same This observation leads to the hypothesis that the volcanic graph, we note a certain sequence related to the content materials were first at length deposited on the hilly sur- of the calcic component in the clinopyroxenes, with face and were then very likely affected by valley shifts respect to the sampling depth. A good correlation could due to natural events such as ice or snow melts, floods, also obviously result by taking into consideration the etc. Thus, the volcanic ashes of the yellow part of the val- magmatic evolutionary process of the NYT, described in ley profile (Fig. 2) almost surely underwent a removal detail by Orsi et al. (1995). The authors described a three- and reworking with respect to the original deposition layer magma chamber for the magmatic and evolutionary after the eruptive event. This hypothesis should also be history originating the Neapolitan Yellow Tuff, distin- confirmed by the total absence of both yellow deposits guishing three different magma (magma1, magma2 and and volcanic glass in the slope and summit profiles, magma3) on the basis of a geochemical, mineralogical, simultaneously carried out with the valley one. and isotopic study of the volcanic formation. Although Therefore, the present work has only considered the val- not being the object of the present work, it is, however, ley profile, and the investigations were made to establish interesting to see a certain sequential correlation in the the origin and the source of the unknown volcanic mate- compositional data reported on the clinopyroxenes. rial. As regards the behaviour of the analysed feldspars, In fact, the aim of this paper is firstly to notify, for the also with respect to those from the CI and NYT forma- first time, the finding of a volcanic ashes deposit in the tions taken for comparison, the graph of Fig. 5 shows a Frosolone area, and then to correlate the mineralogical, double compositional trend. While for the clinopyrox- compositional, and datational results of the more signifi- enes there is a contemporary presence of the same sam- cant mineralogical phases and glass investigated, in order ple in the two well-distinguished compositional groups to trace the source of the host volcanic material. (Diopside – Fe-Diopside), in the case of the feldspars we have a more clear distinction for the analysed feldspar Discussion of the experimental data crystals. Samples Fro1 and Fro2, for example, find them- The data reported in this work refer to the study of the selves alone in the very limited group of ~ Or80Ab20 primary volcanic mineral phases (clinopyroxenes and composition, and have an anorthitic content that is cer- feldspars) and coexistent glass, and they allow us to tainly higher than all the others, while as regards the par- make the following considerations. allel trend, characterised by a lower anorthitic content, As regards the composition of the analysed clinopyrox- we found the feldspars of a much greater heterogeneous enes, which range from diopside to ferroan-diopside, the composition. This behaviour can again be justified, as for graph of Fig. 4 shows that the NYT pyroxenes have sim- the clinopyroxenes, by the evolutionary magmatic ilar behaviour to Frosolone ones, while, in contrast, the process described by Orsi et al. (1995). Moreover, the CI pyroxenes fall within a more restricted area of the fer- high compositional heterogeneity of the feldspars from roan-diopside field. However, this limited area also sample Fro6, which are shifted in the lower part of the includes the Fro1 and Fro2 samples, even if at a lower graph of Fig. 5 and are correlated to those of CI (with limit, and this fact leads one to think that the two samples higher Ab and An content), allow us to put forward the are more similar to those of the CI. In effect, the Fro1 and hypothesis that the volcanic material of the Fro6 sample Fro2 samples are the most superficial with respect to the could belong to another activity, perhaps the CI eruption. VOLCANIC ASHES FROM FROSOLONE (ISERNIA, ... Geologica Romana 37 (2003-2004), 97-108 105

a Fig. 6 - TAS graph representation for glass compositions.

a) Frosolone glass (black solid circles) compared with the compisitions of the glass from Alban Hills (AH area; Trigila et al., 1995), Rocca- monfina (RM area; Gian- netti & Luhr, 1983) and Phlegrean Fields (PF delim- ited area; various authors: see legend. Fig 6b). T- Ph=Tephri-phonolite; Ph=Phonolite; T=Trachyte; L=Latite (K) (according to Le Bas et al., 1986).

b) Plot of the compositions of the glass from the Phlegre- an Fields (references in legend; graphic abbreviations as in 6a).

c) Plot of the average compositions of the glass from Frosolone (graphic abbrevi- b ations as in 6a).

c 106 Geologica Romana 37 (2003-2004), 97-108 GIANFAGNA et al.

Not finding any glass in the Fro6 sample, it was not pos- These evident differences in colour could be attributed to sible to date the material using the fission-track method. a simple process of migration of elements and/or also to However, the fact that the Fro6 sample does not contain a process of pedogenesis. In fact, the dark colour of the glass shows that this material (already under about 200 most superficial levels may be due to the slight difference cm in depth, Fig. 2) has a different, and certainly older, in composition of the parental material undergoing an origin. alterative process with following intense pedogenesis. The chemical and datational analyses carried out on the However, we must also to consider the particular geolog- glass support what has been said so far. The variable ical period of deposition, when strong superficial move- composition from trachyte to trachy-andesite (latite) of ments caused by natural events brought about a trans- the glass, as highlighted by the three TAS compositional portation and superficial resedimentation of the material, graphs (Figs. 6a, 6b and 6c), demonstrates how the chem- with a consequent reworking with respect to the original istry of the glass contained in the volcanic ashes found at deposit. Therefore, another, but different, scientific Frosolone can be referred to an evolutionary magmatic approach is necessary to understand the various sedimen- activity. This could also be demonstrated by the different tary events, which have interested the global area of morphologies of the glass fragments, as seen in Fig. 3, Frosolone in subsequent times. which also shows that the glass is quite fresh. The general TAS graph of Fig.6a shows that the analysed glasses from Frosolone can be referred to Phlegrean Field activ- CONCLUSIONS ity, ruling out other origins, such as the Alban Hills prod- In view of what has been said in the present paper, we ucts (Trigila et al., 1995) or those of Roccamonfina can conclude that the features of the analysed material (Giannetti & Luhr, 1983), the nearest volcanic districts to (structure and texture, laminar layering and relative Frosolone. The graph of Fig. 6b highlights the composi- colour differences, variability in composition of the tional differences of the glasses of Campanian Ignimbrite analysed phases), have allowed us to attribute the with respect to those of the NYT. Even if the latter are Frosolone volcanic materials to a Phlegrean origin, in fairly scattered in the graph, falling within the four com- particular to the products of the Neapolitan Yellow Tuff positional fields (T, L, Ph and T-Ph), it is obvious that the eruptive period, as also shown by the dating analyses that CI and Phlegrean Field glasses (Civetta et al., 1997; de’ attribute an age of 10,800 (± 5,500) years to the investi- Gennaro et al., 1999; Melluso et al., 1995) are generally gated glass. This allows us to confirm the source as the less scattered and fall mainly within the Ph-T fields. Fig. late period of Phlegrean activity (NYT), ruling out the 6c reports only the mean compositions of Frosolone glass products of Campanian Ignimbrite that date back to and shows a fair correlation between these compositions about 35,000 years ago. However, at moment, we do not and those of NYT, at least as regards the Latite-Trachy- consider it appropriate to provide further details on the andesite evolutionary trend. The results of the fission- exact reconstruction of the volcanic events, which track dating on glass contribute to confirming what has require another type of investigation, and also because been said here and, even if flawed by a high degree of we did not aim to make this sort of considerations. experimental error, they agree with the indications Instead, we wanted to stress how this type of research, already found in the analytical compositions concerning focusing on mineralogical and dating determinations, can the probable origin of the material under study. The age establish the period as well as the origin and provenance of 10,800 (±5,500) years falls well within the period of of the volcanic products of the material found in the Phlegrean Field activity, which gave rise to the “Montagnola di Frosolone” area. The experimental data Neapolitan Yellow Tuff formation. Thus, the determined presented in this work could also suggest the need for age rules out the earlier activity that produced the further study of these volcanic ashes by focusing more on Campanian Ignimbrite, for which an age of about 35,000 the identification and attribution of the exact periods of years was established, as reported in Rosi & Sbrana the relative volcanic activities and their deposition (1987). On the other hand, glass is the only datable phase modality. in these types of tephra and, in the specific case of Frosolone, the experimental data were in any case signif- ACKNOWLEDGEMENTS - The authors are grateful to Dr. icant despite the high degree of error. G. Cavarretta of the CNR (“Istituto di Geologia Ambientale e Finally, as regards the features of the investigated vol- Geoingegneria”) for permission to use the electron microprobe, and to Mr. M. Serracino for his help in the analytical tests. canic material, such as structure and texture, laminar lay- Particular thanks are due to Prof. F. Terribile (Faculty of ering and relative differences in colour at various depths, Agrarian Studies, Naples), who made it possible to start this the section of the profile reported in Fig. 2 clearly shows work. Heartfelt thanks also to Prof. M. de’ Gennaro for his crit- that level Fro5 (170-195 cm) is characterized by a clear- ical reading of the manuscript and to Proff. F. Bonadonna and er colouring with respect to the upper one, with colours F. Lisker for their invaluable advice for improvements. This work was supported by MIUR and CNR grants. that go from a light yellow to grey-white and white. VOLCANIC ASHES FROM FROSOLONE (ISERNIA, ... Geologica Romana 37 (2003-2004), 97-108 107

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