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 clinopy- roxenes, 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 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 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 pro- files, 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 character- ized by rounded summits and slopes with a regular pro- file, 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.