RIASSUNTO I sub-bacini di Chilivani-Ardara e di Bonorva del Logudoro orientale sono stati parzialmente colmati da una sequenza di tre unità piroclastiche. L’Unità IC è costituita da depositi cineritici de- bolmente porfirici per Pl, Bt, Cpx, Kf e Qz. L’Unità PL è pomiceo-ci- neritica, porfirica per Pl, Bt ± Kf ± Qz. L’Unità CH è maggiormente porfirica per Pl, Qz, Bt ed ha un maggior contenuto in litici. Impor- tanti fasi di quiescenza vulcanica sono documentate da depositi epi- clastici che si rinvengono intercalati a diversi livelli stratigrafici. La sequenza è ricoperta da sedimenti marini del Burdigaliano sup.-Ser- ravalliano. I differenti caratteri petrografici e minerochimici suggeri- scono che le unità piroclastiche provengano da differenti serbatoi magmatici localizzati in posizione alto-crostale lungo faglie attive. In un progressivo contesto transtensionale-estensionale, tali serbatoi magmatici favoriscono per svuotamento l’instaurarsi di collassi cal- derici. I dati raccolti, indicano che le eruzioni abbiano colmato le parti più interne dei sub-bacini subsidenti del Logudoro orientale, favorendo il perdurare di condizioni continentali che cessano con l’attività vulcanica. La strutturazione e l’evoluzione dei sub-bacini descritti è caratterizzata dal progressivo sbloccamento e tilting dei blocchi tettonici, evidenziato dalla giacitura delle ignimbriti lapidee (WI), e dalla messa in posto di prodotti pomiceo-cineritici intercalati da epiclastiti e conglomerati. Nel quadro dell’evoluzione polifasica del Rift Sardo, questi sub-bacini appartengono alla seconda fase di rifting che prelude l’inizio della tettonica estensionale nel dominio nord tirrenico. TERMINI CHIAVE: Rift Sardo, sub-bacini, piroclastiti, epi- clastiti, stratigrafia vulcanica, petrografia. ABSTRACT The second stage of the Sardinian Rift: volcanism and evo- lution of Ardara-Chilivani and Bonorva sub-basins (Northern Sardinia). The Oligo-Miocene Sardinian Rift (27-15 Ma; CHERCHI & MON- TADERT, 1982; LECCA et alii, 1997), is a typical intra arc-massif basin (sensu DICKINSON, 1974) made up of several sub-basins filled by thick orogenic volcanic sequences and marine sediments. This arc is linked to the subduction of the Neotethys oceanic plate under the European plate and to the Africa-Europe convergence. The displace- ment of the arc or, in other words, the drift of the Sardinia-Corsica Block, is related to the orogenesis of the northern Apennine and of the Maghrebid-Sicilian chain. Despite the key-role played by the Sardinia Rift in the knowl- edge of the geodynamic evolution of the western Mediterranean basin, the stratigraphy of volcanic sequences is still incomplete because large portions of this rift still require detailed mapping. At the regional scale, the Sardinia Rift shows marked differ- ences with regard to volcanic and tectonic style. In northern Sar- dinia, the rift is arranged into several half-graben type sub-basins, related to tilted block and horst block systems. The basins are schematically attributable to the relative mobility of the main blocks with minor relative strike-slip to transtensional movements along NE-SW trending faults among the sub-blocks of eastern Sardinia. The Chilivani-Ardara and Bonorva sub-basins contain a poorly known thick pyroclastic sequence buried by epicontinental marine sediments. On the basis of field, petrographic and volcanological cri- teria, the volcanic pile is constituted, from bottom to the top, by early volcanics of broadly andesitic composition (M. Cuguttada Unit: MC), densely welded and rheomorphic ignimbrites (WI) followed by ash and pumice pyroclastic flow deposits named throughout this paper Pianu Ladu (PL), Pianu de Puma (IC) and Chilivani Unit (CH). Epiclastic deposits (EVL) are observed locally at different strati- graphic levels (the so-called «lacustre», VARDABASSO & ATZENI, 1962; PECORINI, 1962). Marine deposits onlap the volcano-sedimen- tary sequence in the upper Burdigalian-Serravallian interval. Pub- lished K-Ar (LECCA et alii, 1997) and Ar-Ar (EDEL et alii, 2001) radio- metric data obtained for the pyroclastic flow deposits indicate a quite narrow time span in the range of 20-18 Ma. With regard to the regional stratigraphic picture proposed by LECCA et alii (1997), MC andesites belong to the A1 andesites; WI are the densely welded reomorphic ignimbrites, whereas the PL, IC and CH ash and pumice flow-Units constitute the AP2 group. In detail, pyroclastic units belonging to the latter group show the following: Pianu Ladu Unit (PL). The PL unit is a light grey-coloured ash and pumice pyroclastic flow deposit, outcropping with continuity at the northern edge of the Chilivani-Ardara sub-basin. It lies normally on WI ignimbrites or locally on reddish matrix-supported conglom- erates made up of clasts of Palaezoic basement. Content of pumice and xenoliths (accidental and cognate), ranges from 20 to 10% mov- ing from S. A. of Bisarcio towards M. Ladu, while size and degree of welding decrease. Pianu de Puma Unit (IC). The IC Unit constitutes discontinuous outcrops in the Bonorva basin as well as in the southern part of Chili- vani-Ardara. It lies with nonconformity on the Palaeozoic basement and/or on the WI. Locally (Pianu de Puma), it is preceded by greenish to yellow coloured epiclastic deposits at least 5 m thick, containing rare clasts of the Palaeozoic basement dispersed in an ashy matrix. In the field, it is a poorly porphyritic greyish coloured ashy pyroclastic flow deposit ranging in thickness from a few metres to 30 m the greater thickness being observed eastward of Pranu Mannu. Epiclastic deposits (EVL). They constitute discontinuous interbeds among the PL, IC and CH Units; characterized essentially by ash, crys- tals and pumice-fragments and containing conglomerate beds formed at the expense of Palaeozoic basement and WI ignimbrites. In the field they commonly show a greenish colour and typical cross and parallel bedding structures. Chilivani Unit (CH). This is an ash and pumice compound ign- imbrite made up of at least three different flow units well exposed in the Chilivani-Ardara sub-basin. It represents the volumetrically most important volcanic unit of the investigated area constituting wide plateaux dismembered by post-depositional faulting. The thickness of the unit CH ranges from 10 m (M. Salattu and M. Filigosu sec- tors), up to 100 m in M. Cordianu-M. S. Bernardo and in the west of Pranu Mannu (sub-basin of Bonorva). Macroscopically, it is easily recognizable because of its high porphyritic index. Petrographic characters suggest that the pyroclastic units recog- nized may be related to different magmatic reservoirs localized in the upper crust, along active fault zones. In a tectonic/volcanism feedback scenario, the progressive discharge of magmatic reservoirs gives rise to caldera-like structures favoured by a progressive exten- sional regime. Several lines of evidence indicate that the PL, IC and CH units mark the early supply in the studied basins, predating the marine Boll. Soc. Geol. It., 124 (2005), 3-20, 6 ff., 2 tabb. La seconda fase del Rift Sardo: vulcanismo ed evoluzione dei sub-bacini di Ardara-Chilivani e Bonorva (Sardegna settentrionale) ANTONIO SAU (*), LUCIANO LECCA (**), ROBERTO LONIS (*), FRANCESCO SECCHI (***) & MARIA LUISA FERCIA (*) (*) Progemisa S.p.A. Via Contivecchi 7, Cagliari. (**) Dipartimento Scienze della Terra, Università di Cagliari, Via Trentino 51; e-mail: leccal@unica.it. (***) Istituto di Scienze Geologiche e Mineralogiche, Università di Sassari, Corso Angioj 10.