RIASSUNTO

Il sito di Fizza Ona, oggetto di questa nota, è localizzato a ovest dell’abitato di Olbia e rappresenta una buona opportunità per effettuare osservazioni su morfostrutture localizzate in prossimità del contatto fra differenti intrusioni di composizione monzogranitica e leucogranitica. Si tratta di un orizzonte ricco di inclusi microgranitici dispersi nell’incassante granitico, costantemente circondati da una sottile corona verdastra di facies granofiriche. I fenomeni di sollevamento tettonico recente hanno favorito lo sviluppo di profonde valli e la ridefinizione della superficie erosionale post-ercinica nota come «Salti di Buddusò». Il reticolo idrografico è strettamente connesso con le strutture dell’area investigata. La parte superiore di questo plateau indica un forte denudamento dei blocchi che costituiscono il basamento sardo avvenuto alla fine dell’orogenesi ercinica (penepiano post-ercinico) e ridefinito successivamente durante le fasi tettoniche Oligo-Mioceniche.

TERMINICHIAVE: Granitoidi, geomorfositi, intrusioni gra-nitiche, Sardegna.

ABSTRACT

Geomorphosites in granitic environments: an example fromnorthern Gallura (north-eastern Sardinia, Italy).

In spite of the occurrence of spectacular forms in the granitic basement of Sardinia, geomorphosites in these environments are still poorly known. Morphosites of geological interest in northern Sardinia are mainly located along the coastline, favoured by marine activity processes forming spectacular landscapes. Recent geological surveys, focused on the evolution of the hydrographic network of north-eastern Sardinia, outline examples of morphosites of geological interest linked to the complex evolution of pluton emplacement. Schematically the major plutonic units of Sardinia batholith show mainly granodioritic and granitic composition and are broadly crosscut by late-satellite stocks mainly of fine-grained granites, as well as by a dike swarm of basaltic and aplogranitic rocks. The Fizza Ona outcrop, located near the small town of Padru west of Olbia, isa good example of morphosite in the inner zone developed at the edge of different plutonic units. It is a well exposed pillow-like horizon that does not exceed 5 m in thickness, observable in the field atleast for 200 m, underlined by the occurrence of large tafoniform. In detail, the pillow-like horizon is made up of sub-roundly to sub-rounded elements of granitic to micropegmatitic rocks up to 100 cm in diameter, dispersed into the fine-grained granitic host-rockmatrix; these elements are commonly edged by a continuous greenly coloured corona of aphyric rocks that does not exceed 5 cm in thickness. In the field, the observed pillow-like structure looks-like the mingling structures commonly observed for mafic and acidicmagma interactions. Under the microscope however, aphyric rocks may be classified as granophyres; graphic textures are commonly observed in the surrounding fine-grained rocks. In addition, similar granitic composition observed in samples of investigated area, indicate that pillow-like structures may be due to the occurrence of local filter-press mechanism under fast-cooling conditions (i.e. granophyric and graphic textures occurrence), during the emplacementin upper crustal levels of satellite bodies. Overall, the Fizza Ona out-crop represents a quite good and rare example of emplacement mechanism of granitic magmas of the Sardinian batholith. Collapse, dismembering and complex uplift of several parts forming the hercynian collisional chain, favoured the development of deep valley sand the incision of the erosional surface in the so called «Salti di Buddusò» plateau. The hydrographic network is strictly controlled by the structural features of exposed granitoids. The upper part of this plateau indicates a strong denudation process of the Sardinia block at the end of the hercynian orogenesis (the so-called post-hercynian penepiano), to be later redefined mainly during Oligo-Miocene time. Many preserved forms underlined the up mentioned denudation processes testified by the occurrence of inselbergs that characterize the whole Gallura landscapes as well as the north-eastern tilting of northern block of Sardinia basement referable to the Oligo-Miocenetectonic movements.

KEYWORDS: Granitoids, geomorphosites, granitic intrusions, Sardinia.