Stratigraphy of a Slightly Peralkaline, Rheomorphic Welded Ignimbrite:The Monte Ulmus Unit (SW Sardinia).

Volcanic deposits of the south-western sector of Sardinia are referred to an eruptive activity owing to a magmatic arc active between 32.4 and 13.8 Ma [Lecca et al.1997] as results of subduction of oceanic lithosphere NNW direct [Cioni et al. 2005]. Magmatic activity is linked with the opening of Mediterranean sea and 60° counter-clock-wise rotation of the Sardinia-Corse microplate that ends 19 Ma ago [Savelli et al. 2002]. The oligo-miocene volcanic activity can be subdivided into two phases: an Older phase (28.4-17.7 Ma) and an Younger phase (17.6 – 13.8 Ma) during that sixteen calc-alkaline and peralkaline, dacitic to rhyolitic ash flow tuff were emplaced [Mundula et al. 2009]. Monte Ulmus unit has yielded a K-Ar date of 15,5 ± 0,5 Ma [Morra et al. 1994]. It is a rheomorphic high-grade, low-aspect-ratio rhyolite to comenditic ignimbrite with thickness comprise between 8 to 98 m, spread in an area of 300 km2. About 41 stratigraphic sections, site in San Pietro, Sant’Antioco and Sulcis-Iglesiente areas , were studied and described . The Monte Ulmus unit drapes the Upper Commenditic Units (Ventrischio unit) in all its outcroups even if in the northern sector of San Pietro island, rest on some comenditic lava flow units ( Nasca and Vinagra units). The Monte Ulmus is overlains by the late Miocene events, testify by the Paringianu and Serra di Paringianu ignimbrite units and by the last comenditic lava flow event (Monte Tortorisio unit). The Monte Ulmus unit is divided into four main eruptive phases: a fall-out deposit followed by three pyroclastic flow events (named A, B1 and B2), separate by a sharp contact and as viewable by the vertical bulk rock densities trends; erosion surface absence and a columnar jointing that cross cutting the ignimbrites point out that this phases were emplaced during the same eruption or in a shortly time. The eruptive centre is hardly to locate but flow lineation trend (NW-SE in Sulcis area and NE-SW in San Pietro island), isophacs of ground fall-out and proximal facies present in the northern sector of San Pietro island may be locate the eruptive centre in the northern sector. The coarse, massive, totally argillified ash fall deposit have thickness comprise between 20cm and 2 cm. Where physical alteration is lesser (San Pietro), the average grain-size is comprise between fine to medium lapilli. Upon the fall deposit there is a basal vitrophire, 30 cm thick, generally strongly argillified although in some section is more preserved. In Sant’Antioco areas the basal vitrophire, have oblate eutaxitic white fiamme (10%) with average size of 3 cm and aspect ratio 2:1-3:1. Quartz and alkali feldspar crystal (lower than 5%) have 2 mm maximum size and rare rounded lithics clast with pressure shadows have maximum size of 3 cm. The core of sub-unit “A” is a 2 to 32m thick, reddish, fine ash, locally rheomorphic high-grade ignimbrite that croup out principally in Sant’Antioco island and it shows character of deposition in sub-aerial and shallow water environment. It contain two kinds of fiamme : a blackish(10-15 %) , obsidian-like, parataxitic fiamme with aspect ratio variable from 15:1 to 30:1 and a whitish ones (6-7%), highly compacted , sub-rounded to oblate shapes(aspect ratio variable from 2:1 to 5:1). Quartz and alkali feldspar crystal (1-2%) have maximum size of 3 mm and lithics (1-2 %) are sub-angular shaped and they are an older ignimbrites units fragments. The sub-unit, in section thicker than 10 meters, shows a gradual changes in welding e rheomorphic facies. A lower massive densely welded zone is overlain by a flow-banded densely welded core with rotational structure and by a highly rheomorphic zone that increase magnitude upward. Folds smaller than 0.5 m , prolate vesicles, sheet-joint and breccias are localized at the ground of rheomorphic zone, while, at top of sub-unit, where the ignimbrite is lava-like, there are folds bigger than 1meter, diapiric structures and 10meters radius blisters. Sub-unit B is a 3 to 52 m thick, locally reomorphic, dm-size components rich, poor porfiritic (Qtz+Sd), high grade ignimbrite composed by two layers (B1 and B2) divided by a sharp contact that croup out principally in San Pietro island and Sulcis area. This sub-unit is characterize by an high-concentration of chemically different juvenile pyroclasts randomly oriented, that are deposed in physical different status. The sub-unit B1, 2 to 24m thick, is a reddish, massive, fine grained welded tuff matrix with three typologies of juvenile clasts: a blackish high porphiritic (Sd) no vesiculated fiamme, a reddish low porfiritic fine grained fiamme and a whitish vesiculated fiamme. The blackish fiamme, around lithics clast, are warp while the white fiamme, where undergo no-coaxial stress, bend the matrix. Lithics fragments are principally old ignimbrite fragments with square to partially rounded border. This sub-unit shows rheomorphic facies in Sant’Antioco where covered the thicker crop out of sub-unit A and in San Pietro island where is filled into a valley. Sub-unit B2, 5 to 28 thick, is light-gray locally rheomorphic moderately welded tuff with two typologies of juvenile clast (blackish and whitish) . Fragments decrease in size (from dm-size to mm-size) and percentage upwards. Furthermore at ground, juvenile clasts have size comprise between 5 to 60 cm and they are randomly oriented with a rough out parataxitic structure. Juvenile fragment are blackish,glassy, high porfiritic (Sd) eutaxitic, no-vesiculated, dm-size fiamme and whitish eutaxitic vesiculated dm-size fiamme. Lithics fragments are rounded reddish high porfiritic older welded tuff with maximum size of 70cm in San Pietro. Into San Pietro island paleovalley, the ground portion of sub-unit B2 is highly rheomorphic whitish densely welded with >1m folds while at top it pass, sideways gradually and vertically with a sharp contact, to a blackish, component poor, vitric.