1,720,981 research outputs found
Syn-eruptive crystallization during a subplinian event: the Greenish Pumice Eruption of Somma-Vesuvius (Italy)
No full textBubble growth during subplinian events: the Greenish Pumice Eruption of Somma-Vesuvius (Italy).
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I caratteri tessiturali delle ignimbriti saldate dell’Isola di San Pietro (Sardegna; Italia)
No full textWelded ignimbrites show a large variability of welding intensity and of post-depositional crystallization. According to existing literature, a fine characterization of welded ignimbritic units involves: 1) a classificatiory scheme which includes 5 up to 6 categories of welding intensity, 2) the definition of the style of post depositional crystallization. This classification can result time-consuming and out of the standards required by a general description of ignimbrite units in the case of regional geological survey of volcanic terrains.
In the new 1:50,000 geological mapping survey of the San Pietro Island (Sardinia, Italy) performed in the framework of the national CARG Project, a large number of samples of ignimbrite products has been collected. The geology of the area is characterized by Oligo-Miocene volcanic deposits, which testify to an intense explosive activity basically featured by the deposition of many variably welded ignimbritic units and, subordinately, by effusive activity, mainly characterized by lava coulees. All the products derive from the eruption of highly evolved magmas of both calc-alkaline and peralkaline affinity.
We present here a simplified scheme for microscopic facies classification of ignimbrite deposits, based on the aspect ratio of the fiammae, the strength of glass shards alignment, and the style of crystallization of the matrix, that can be used for the lithological characterization of ignimbritic units in other mapping projects
Diapiric and Blister Structures in Welded Ignimbrites: the Example of the “Serra di Paringianu” Ignimbrite (SW Sardinia, Italy).
No full textDiapiric 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
The Monte Ulmus Unit (SW Sardinia, Italy): stratigraphy of a rheomorphic welded ignimbrite.
No full textMonte Ulmus Ignimbrite (MUI) is a high-grade, rhyolitic, welded, locally rheomorphic, low-aspect-ratio ignimbrite. Three main pyroclastic flow units (in the sense of Smith, 1960), preceded by a basal fall-out deposit (F) have been distinguished, named A, B1 and B2. Each flow unit is constituted by different lithofacies strongly related to the morphology at emplacement. Unit A is a 2 to 32 m thick, reddish to grey, fine grained, high-grade, welded ignimbrite that crops out mainly in Sant’Antioco Island. A basal vitrophyre, 30 cm thick, represents the base of unit A, overlain by a massive glassy lithofacies. These two lithofacies develop independently by paleotopography. In paletopographic lows, the main part of the ignimbrite is characterized by the presence of a rheomorphic facies with thinly spaced sheet joints and decimetric to decametric rheomorphic folds. This rheomorphic portion of unit A develops a secondary vesiculation and a granophyric crystallization style. Units B1 and B2, 5 to 52 m thick, crop out mainly in San Pietro Island and Sulcis area. They are represented by a poorly porphyritic, Quartz- and Sanidine-bearing, high grade deposit in which four different lithofacies have been described. These units are characterized by a high concentration of different, randomly oriented, juvenile pyroclasts and present rheomorphic deformation in correspondence of topographic lows. Vertical variation of textural features along MUI clearly reflect into the observed variation of bulk rock density. On topographic highs, discontinuities in the textural and physical features along the ignimbrite sequence clearly trace the transition between different flow units. On topographic lows and on gently dipping paleovalley slopes, the rheomorphic processes smoothed the discontinuities between the different flow units, locally inducing secondary vesiculation which disturbs the vertical density profile. The occurrence of coarse lithic boulders in the northern sector of San Pietro Island, and the inferred dispersal of the fall deposits at the base of the ignimbrite suggest that the source was possibly located north-northeast of the San Pietro Island
Rheomorphic diapirs in densely welded ignimbrites: The Serra di Paringianu ignimbrite of Sardinia, Italy.
No full textRheomorphic structures in welded ignimbrites are commonly associated with deposition from hot pyroclastic
flows on inclined topography or by tractional shear on aggrading agglutinate by the pyroclastic density
current. We describe a type of rheomorphic deformation involving the formation of diapirs in ignimbrite following
the re-equilibration of internal gravitational instabilities within the deposit. We show that diapirs can
develop in horizontal welded ignimbrites that have an inverted density stratification. The interpretation of
these structures in terms of their dynamics and kinematics can help in defining the timing of the deformation
history of ignimbrites.
The rhyolitic Serra di Paringianu Ignimbrite is the uppermost, largest ignimbrite of Cenozoic volcanism in SW
Sardinia. A detailed study at La Punta, north of San Pietro island (Sardinia; Italy), schematically characterized
by a twin sequence of densely welded and partially welded ignimbrite flow units of the Serra di Paringianu
Ignimbrite, revealed the presence of mushroom and pillow-shaped diapiric structures, constituted by material
of an intermediate, partially welded, flow unit intruding into an upper, densely welded, unit. The diapirs
are generally connected with their source region and show both vertical and lateral variations in the textural
and physical features. The uppermost densely welded unit is deformed within a few meters of the diapiric
structures, with the foliation pattern of the surrounding ignimbrite enveloping the cup region of diapirs.
We used the existing numerical models of cooling–compaction of welded ignimbrites to constrain the time
scale for the development of specific rheomorphic structures. We present a model in which the diapirism involves
the buoyant rise of partially welded lithofacies into the densely welded cover in a time scale of a few
months. The diapir model takes into account the rheology of the host and intruding layers and explains the
textural and physical modifications both experienced by the partially welded lithofacies during rise and induced
in the host layers. The model shows that the time scale of the diapiric rise well agrees with the time
scale of the cooling process of the upper portion of the ignimbrite
Time-space relationships of deformative structures in welded ignimbrites : the diapiric structures of the “Serra di Paringianu” Ignimbrite (SW Sardinia, Italy)
No full textThe rheology of ignimbrites during welding and compaction has been matter of interest in recent years for the ability of welded ignimbrites to develop a wide variety of rheomorphic structures. The existence of numerical models able to reproduce the cooling-compaction history of welded ignimbrites allows to constrain the time scale for the development of specific rheomorphic structures and to test existing viscosity models. The Serra di Paringianu rhyolitic ignimbrite 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. A detailed study of La Punta section, north of San Pietro Island, schematically characterized by a twin sequence of densely welded and partially welded ignimbrite units, reveals the presence of mushroom-shaped diapiric structures constituted by the partially welded intermediate unit intruding the upper densely
welded cover
Paleotopographic study of a rheomorphic rhyolitic welded ignimbrite: Monte Ulmus (Sardinia - Italy).
No full textStratigraphy of a Slightly Peralkaline, Rheomorphic Welded Ignimbrite:The Monte Ulmus Unit (SW Sardinia).
No full textVolcanic 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
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