sabato 7 marzo 2015

Gli esotici neolitici dell'Oristanese

Riassunto del rif. 1. "Asce in pietra levigata sono ampiamente note nella Sardegna preistorica. Esse fanno la lora comparsa nelle prime comunità neolitiche (VI millennio a.C.) e perdurano fino all' età dei primi metalli. Questa ricerca sulla definizione tipologica delle asce in pietra levigata della Sardegna è basata su determinazioni archeometriche effettuate su un campionamento preliminare di manufatti provenienti da alcuni insediamenti all' aperto della Sardegna centro-occidentale.
Nei siti neolitici studiati il 95% dei litotipi delle asce in pietra levigata e costituito da «nephriti». Le «nephriti» possono essere suddivise in due gruppi a seconda che contengano o siano prive di epidoto. L' associazione mineralogica ecostituita da actinolite e tremolite, clorite, ±epidoto e ossido di ferro. Le altre litologie trovate sono: uno scisto a glaucofane, un metadiabase, una cornubianite ad andalusite e una fonolite.
Questi dati confermano una massiccia presenza di litotipi di origine esotica, che sono simili a quelli frequentemente impiegati nella produzione di manufatti continentali, soprattutto nei centri neolitici dell'Italia settentrionale. Sembrano invece mancare, o essere molto rare, asce in giadeitite* e in eclogite. II ritrovamento in Sardegna di esemplari non finiti, costituiti da litotipi di provenienza continentale, suggerisce una parziale lavorazione in loco del materiale grezzo.
La presenza di un' ascia in fonolite del Montiferro attesta anche una produzione locale dei manufatti in pietra levigata finora non documentata. " (1)


Fig. 1. Sin., gli insediamenti da cui provengono i manufatti neolitici studiati in (1). 1: Gribaia; 2: Conca Illonis; 3: Cuccuru is Arrius; 4: Su Pranu Mannu; 5: Interacquas; 6: San Ciriaco; 7: San Giovanni; 8: Serra sa Furca. Dx: una delle accettine litiche levigate da Conca Illonis (1). 

[..] All these artefacts come from archaeological surveys, generally carried out on long-term settlements dating at least as far back as the Middle Neolithic Age (4000-3250 B.C.). As regards the cultural and chronological timescale proposed for prehistoric Sardinia, this kind of tool fits between the Early Neolithic and the early Copper Age, and seems to disappear in the middle of the third millennium B.C. during the Monte Claro culture[..] (1)

" DISCUSSION. Mineralogical and petrological characterisation of the examined Sardinian polished stone axes provides some useful information on their raw materials. However, conclusions are only approximate, since only a small number of samples was examined. In addition, the samples represent a group of 54 polished stone axes collected from the surface, a part which does not allow precise chronological or cultural attribution. The commonest lithologies are «nephrites», with very minor glaucophane schist, andalusite hornfels, and only one sample of volcanic rock. These data are partially consistent with the raw materials of Neolithic polished stone axes found in northern Italy and western Europe (D'Amico et al., 1991, 1992, 1995, 1996, 1997), where most artefacts are made from metamorphic rocks (i.e., jadeitite, eclogite, serpentinite, prasinite) and minor volcanics. «Nephrites» have also been reported, but as minor components (only 1.2% of all lithologies). In studied Neolithic sites in Sardinia, «nephrites» make up 95% of all lithologies, and jadeitite, eclogite and serpentinite axes have not yet been found. This may be due either to a sampling shortage or to an actual lack of this lithotype among Sardinian Neolithic axes. In fact, Lovisato (1886) described finding jadeitite, serpentinite, metagabbro and eclogite axes mainly in
northern Sardinia. Raw materials are unlikely to have come from a single geologic source. The considerable lithological variations, which include metapsammite, greenstone, glaucophane schist and phonolite, suggest that the raw materials came from various areas. Nevertheless, except for phonolite, the raw materials of the other axes come from an exotic source. This is easy to prove because, in Sardinia, high-pressure rocks such as glaucophane schist and «nephrites» are lacking,
whereas glaucophane schist, eclogite and serpentinite are common in the Alps and NE Corsica. The Alpine origin of the jadeitite, eclogite, and serpentine axes described by Lovisato (1886) is well ascertained, whereas the origin of the «nephrite», most probably multiple, is more problematic.
«Nephrite» has been found at several archeological sites in northern Italy, often associated with eclogite and jadeitite. In all the sites studied, a very small percentage of axes are made of «nephrite». Primary deposits of «nephrite» are known in Val de Faller, Peschiavo, Les Hauderes, Gottard (Swiss Alps), Isere Valley (France), Sestri Levante,  Monterosa (Italy), Mt. Hartz (Germany), south western Poland, and Pakile (Finland) (Gunia, 1999). Secondary deposits of «nephrite» have been reported from morainic deposits in Scandinavia and Germany (Gunia, 1999).
Glaucophane schists are common both in the Alps and in Alpine Corsica (Gibbons et al., 1986). The hypothesis of a Corsican source for both the Sardinian «nephrite» and glaucophane schist appears attractive, but still awaits confirmation.
There is petrographical evidence that local raw materials were used to produce polished stone axes in Sardinia. Phonolite, similar to that of sample GRI65 outcrops extensively in the Pliocene volcanic complex of Montiferro, central Sardinia. This complex consists of two main magmatic series of alkaline affinity (Beccaluva et al., 1977): one sequence of terms from analcite-basanite to tephritic phonolite; and one, more extensive, containing terms from alkaline to mildly-alkaline basalt, to phonolite (sometimes peralkaline). Some geochemical  features of peralkaline phonolite are relatively high Zr, Hf, Th, and Rb contents, and depletion in Y, Nb, Ta and intermediate REE. The latter defines a characteristic V-shaped REE pattern (fig. 5b) (Beccaluva et al., 1977). Table 1 lists the chemical analyses of phonolitic axe sample GRI65, which is very similar to peralkaline
phonolite 203F of Montiferro. This is confirmed by the REE pattern, which is characteristic of phonolite of the volcanic series from alkali-basalt to phonolite, and by the aegirine-augite composition of clinopyroxene.

One other point which deserves further exploration is the quality and working of raw materials. Most of the implements are finished tools such as axes, chisels and axe-amulets, but unfinished or rough-hewn instruments made by hammering have also been found. This suggests circulation of partially processed materials to be completed locally, as in other regions of Southern Italy (Leighton, 1992).

CONCLUDING REMARKS The 14 samples of polished stone axes from central Sardinia allow us to draw the following conclusions:
1. «Nephrite» axes make up 95% of sampled Sardinian Neolithic polished stone axes.
2. The origin of «nephrite» and glaucophane schist is undoubtedly exotic to Sardinia, since these kinds of rocks («nephrites» or highpressure rocks such as glaucophane schist) are not known on the island and their occurrence is precluded, for geological reasons.
3. The polished stone axe made of phonolite (GRI65) certainly comes from a local source, since its mineralogical and geochemical  features are very similar to the peralkaline
phonolite of the Montiferro area." (1)

*: All'Antiquarium Arborense, vetrina 2 sono tuttavia attestate: "Due accettine, di cui una in giadeite di provenienza alpina, in pietra levigata. Neolitico Tardo. Sinis di Cabras"

(1) BERTORINO G., FRANCESCHELLI M., LUGLIÈ C., MARCHI M., COLUMBU S. (2002)–Petrographic characterisation of polished stone axes from Neolithic Sardinia: archaeological implications. Archaeometry and Classical Heritage. Periodico di Mineralogia 71, special issue, 87-100.N. Jb. Miner. Mh., 8:337-351
(2) A. Belladonna, 9000-7000 a.C. e nel Mediterraneo già si navigava alla grande: parola di archeomammiferi, 17 SETTEMBRE 2014, monteprama.blogspot.it, ; in particolare si osservi che lo studio degli archeomammiferi "migranti" ha evidenziato che: "Per la Sardegna questi migranti che accompagnavano gli uomini arrivano con una prima ondata critica ca. 7700 anni fa (5700 a.C.): questo rafforza  ciò che raccontano archeologia e genetica sul cromosoma Y (4), confermando che la Sardegna non era certo isolata durante il Neolitico, ma che le genti da lì andavano e venivano via mare."

vd. anche: 
Le nuove rotte "dirette" dell'ossidiana sarda:VI-IV mill. a.C., 18 NOVEMBRE 2014

E qualcuno vorrebbe ancora chiamarlo Golfo dei Fenici?