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Geological and archaeological significance of volcanic artefacts from the East Adriatic Sea

Subangular or slightly rounded gravels and boulders (<15 cm) of exotic volcanic rocks occur within the nearshore carbonate gravely deposits in the secluded coves of the East Adriatic. The sites are located in the hinterland of Zadar in the cove of Ljubač, and the coves of islands of Pag, Rab and Dugi Otok. The samples are fresh, porphyritic and vesicular, without amygdules. Matrix is microcrystalline to cryptocrystalline, occasionally with brown glass. Nodules of cognate cumulus rocks are common in all samples. Some fragments embed thermally metamorphosed Eocene numulite limestone suggesting (post)-Lutetian age of these samples. In the TAS classification scheme, the volcanic samples plot in the fields of hawaiite, mugearite, latite, trachyte, basaltic andesite and dacite. In the K2O-SiO2 plot hawaiites and mugearites define high-K calc-alkaline series (K2O/Na2O = 0.40-0.52) whereas latites and trachytes form shoshonitic trend (K2O/Na2O = 1.0-1.3). Basaltic andesites and dacites do not correlate to the other two rock series. Hawaiite and mugearite contain phenocryxsts of plagioclase, clinopyroxene, olivine and ores set in the matrix of phenocrystic population and apatite. Clinopyroxene is augite/diopside (Wo42.7-47.2En33.4-46.1Fs8.1-20.3). Feldspar phenocrysts are normally zoned plagioclase with the rim of ternary feldspar (An34.8-82.2Or0.6-7.7). Matrix feldspar has similar compositions (An26.1-69.9Or1.2-15.4). Olivine phenocryst is normally zoned (Fo75.4-62.0). Opaque mineral is spinel (Usp26.6-51.6Mt28.6-51.7Sp16.1-27.4). Latite and trachyte contain abundantly phenocrysts of plagioclase, sanidine, clinopyroxene and brown mica with minor quartz, ore and orthopyroxene. Apatite and zircon are accessory phases. Clinopyroxene is augite/diopside (Wo41.8-46.1En37.9-42.8Fs13.3-17.5). Brown mica is biotite/phlogopite (5.9 wt.% TiO2, 0.78 wt.% BaO ; Mg# = 58.7-79.2). Large plagioclase phenocrysts (An16.8-62.3Or1.4-11.8) may be normal and reverse zoned within the same grain. Feldspar phenocrysts have corroded plagioclase core and sanidine or anorthoclase mantle. Sanidine phenocrysts have composition Or44.5-57.6An2.0-3.8. Opaque mineral is Ti-magnetite or ilmenite. In the basaltic andesite phenocrysts are subheadral sieve-textured plagioclase with minor olivine and clinopyroxene. Olivine phenocrysts have pigeonite reaction rims. Dacites are represented by dry, welded ignimbrites. Detailed geochemical data on basaltic andesites and the ignimbrite are given elsewhere. Overall signatures of the rocks showed that they must have been derived from different geotectonic provenances, which include magmatic orogenic belts and intraplate volcanic provinces. In the adjacent regions these provenances are represented by the Apennine-Calabrian belt, Aeolian and Aegean volcanic arc, and Veneto Volcanic Province. The geodynamic models of the Eocene-Early Miocene collisional belt of the Adriatic plate and NW External Dinarides do not suggest favourable setting for the syn- or post-collisional magmatism. Magmatic rocks in this region are calc-alkaline basaltic andesite, andesites and dacites of proved or assumed Middle Triassic age. The volcanic rocks similar to the described volcanic fragments have not been found at the outcrops in the East Adriatic islands and hinterland. Thus, the fragments were envisaged as artefacts derived from the extra East Adriatic volcanic sources. The artefacts of dacitic ignimbrite make an exception in this context since they perfectly match the rocks outcropping near the village of Brušane (brus = whetstone) located in the mainland 25 km NNE from the island of Pag. The artefacts decipher no clear shape or form, these likely being obliterated by the natural abrasion. The majority of collected samples might have been used as the ballast stone and for stowing the material transported by ships. Other pieces may represent remnants of grinding tools and millstones. At this stage of research we paid more attention to the sourcing of the volcanic artefacts than to their usage. Although our data are incomplete, few conclusions on the artefact provenance may be inferred. Hawaiites and mugearites are apparently akin to the volcanic rocks from the Mt. Etna and are identical to the hawaiite and mugearite found in the Middle Bronze Age to the Late Iron Iron Age settlements from the border of Italy to Slovenia (Caput Adriae) and W Istria in Croatia. Our trachytes are comparable to the trachytes from the Caput Adriae used as grinding tools which are believed to have been excavated from the Monte Rosso quarry in the Euganean Hills volcanic complex near Padua. The source of latites and basaltic andesites is ambiguous. The latites from East Adriatic collection do not contain amphibole, which is the common mineral of Eugenean Hills latites. Basaltic andesites show typical arc-like geochemical signatures and were certainly not derived from the Veneto Volcanic Province (see in Milani et al., 1999). We search for their source(s) in the East Mediterranean. Our preliminary observations on provenance of the volcanic artefacts may be used to trace the route of ancient merchant ships through the Adriatic Sea. The artefacts, originating from Sicily and possibly from Aegean islands, arrived in the eastern Adriatic Sea after docking at some Greek colonies, as for instance, Apollonia in Albania, founded in the 7th and 6th centuries B.C., where the pieces of similar volcanic rocks were collected. Concerning the recovering sites of our volcanic artefacts, the longitudinal Adriatic routes seem to be liable. Naval characteristics of ships from that period were not favourable for an open sea sailing and the channels between the coastal line and inner islands were used as relatively safe routes. However, there is an evidence of trans-Adriatic sailing older than 6, 230 years B.C. Coves hidden from the hazardous tempestuous winds (i.e. sampling sites) were occasionally used as shelters. In the same time an opposite route was operating, as concluded from spreading of pieces of Eugenean trachytes, from the Caput Adriae to the northern Dalmatian islands, by the Veneti population. When the Greek and Veneti population groups met each other at the northern Dalmatian island, is a hard task to answer since the volcanic fragments are found out of stratigraphic context. The collaboration of archaeologists and geologists is expected to yield solutions to the problems.

volcanic artefacts; sourcing; ancient merchant routes; Adriatic Sea

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