Diapiric and Blister Structures in Welded Ignimbrites: the Example of the “Serra di Paringianu” Ignimbrite (SW Sardinia, Italy).

Diapiric structures (DS) have been observed in many different geological context e.g.: salt-sediment intrusions, plutonism and texturally stratified obsidian lavas [e.g. Fink, 1983]. In all these cases DS are generated by an unstable gravitational density stratification (Rayleigh-Taylor instability) which forced the intrusion and the buoyant rise of lower lighter material into the denser upper cover. These structures are uncommon in welded ignimbrite deposits where they have been described only in few cases [e.g. Leat and Schmincke, 1993]. In welded ignimbrites, the time scale of the cooling process below the glass transition temperature represents the upper limit for the duration of the process of plastic deformation. The knowledge of the density contrast between the DS and the host rocks, together with the style of deformation and the knowledge of the cooling time scale allow to constrain the rheological behavior of the system. The Serra di Paringianu rhyolitic ignimbrite (SEP) represents the youngest major ignimbrite of the Cenozoic volcanism of SW Sardinia. It widely crops out on San Pietro and Sant’Antioco islands, while scattered outcrops are present in the Sulcis mainland. At “La Punta”, north of San Pietro Island, SEP is constituted by a single cooling unit, subdivided into four eruptive units: 1) a lower unit (U1), comprising basal , dm thick, argillified fall and surge deposits, overlain by a 1.4 m thick, black ignimbrite (vitrophyre) and by a 10-12 m thick, densely welded ( = 2200 kg m-3), red ignimbrite with parataxitic texture; 2) an intermediate lower unit (U2), comprising a 10 m thick, lithic rich, white to pink to red, partially welded ignimbrite ( = 1600 kg m-3); 3) an intermediate upper unit (U3), represented by a 5- 6 m thick, red to pink, densely welded ignimbrite ( = 2200 kg m-3) with eutaxitic texture 4) an uppermost 4 m thick, grey-violet, partially welded ignimbrite unit (U4). At the same locality, tens meters-sized mushroom-shaped diapiric structures (DS) and metric to decametric lens-shaped voids, named blisters, have been observed within SEP. DS are constituted by the partially welded intermediate unit (U2) intruding the upper densely welded cover (U3). DS are generally connected with their source region and their roots, characterized by vertical re-orientation of flattened purple scoriae, are placed about 8-10 m below the roof, within the U2. In plane-view, stem region is generally elongated and characterized by sub-vertical dip of the foliation planes. Cup region presents horizontal cross-sectional shape, varying from circular to elliptical. Within the cup, foliation shows an approximately concentrical distribution, with dip increasing from the margins trough the center. Eutaxitic structures as well as flow banding, foliations and contact-transition between the different eruptive units are mainly sub-horizontal at “La Punta”. These structures deviate from their planar-horizontal character close to the contact with blister and inside and outside DS. Below the cup region of DS, the host-rock (U3) is folded, with axial plane gently dipping outward the diapir axis. In correspondence of the maximum diameter of the cup, at about 2 m out of the margins, the host rock shows undisturbed horizontal dip, which rapidly changes to vertical close to the margin. Close to the roof of the cup region, host-rock shows upward convex arrangement of foliation, grading into planar-horizontal at about 1 m above the cup. At least 30 blister are present within U3 in about 1 km2 wide area. Blisters have horizontal, circular to elliptical, cross-sectional shape and vertical, lens to cupola cross-sectional shape. Horizontal vs. vertical dimensions ratio ranges between 3 and 4. Foliation seems to envelope completely the blisters, reproducing their shape close to the contact with them (first 20-40 cm from the wall). Within the host-rock the vertical strike of foliation, close to the blister wall, changes progressively to horizontal at 1.3-2.5 m from the wall. In order to simulate the ascent of the DS in a reliable conditions and to estimate the cooling and compaction history of SEP, we performed several numerical simulations (using a trial and error approach) with the AshPac software [Riehle et al., 1995]. The simulated cooling history shows that U2, emplaced at 625 °C, heated up during the first year, due to the thermal equilibration with of the lower and overlaying hottest units, approaching its glass transition temperature (Tg, 650° C) 54 days after its emplacement. At the same time, a cooling front moved downwards the upper surface of the deposit and after 5 months, U3, emplaced at 750°C, cooled its upper portion below Tg. The density gradient at the interface between U2 and U3 increased during the firsts days, approaching a maximum at 42 days and then decreasing asymptotically. We estimated the viscosity of the melt through the model of Giordano et al. [2008]. Melt viscosity corrected for the crystal content ranges between 1012 and 109.8 Pa s at temperature between 650 and 760°C. The ascent velocity of a diapir with a Newtonian rheology ascending through a rock having Newtonian rheology can be calculated using the Stokes equation [Burov et al., 2003]. We simulated the ascent of a 10 m wide DS, with a stepwise process (1 m each step) assuming that temperature of DS and host rocks varied according to the variation in temperature and density reproduced by the simulated cooling-compaction history. The model of development of DS can be schematized in three steps: 1) During the first phase of post-emplacement cooling-compaction, density contrast between U2 and U3 increased, while the difference in the emplacement temperature favored the U2 heating up above Tg. 2) After about 50 days plume ascent is triggered by the formation of a Rayleigh-Taylor instability at the interface between U2 and U3, driven by the maximum density gradient. 3) The plume rose up with a velocity in the order of 10-7 m s-1 until it approached the ductile-fragile transition level of the host rock, where it slowed down and stopped starting a lateral intrusion for the continuous feeding from the source region. The degassing marginal parts of the plume head decreased its density, and possibly contributed to feed blisters formation. The blisters and the diapirs are strictly associated in a small area of about 1 km2. They occur inside the same unit deforming the foliation planes and so suggesting a contemporaneous development. We state that blisters represent degassing structures in which volatiles of SEP converge during compaction in many cases favored by the presence of the DS, which could represent preferential degassing channels. A detailed geochemical work (in prep.) could solve the uncertainty in interpreting blisters structures and their genetic relationships with DS.