Comendite

The term Comendite derives from the area of Le Commende on San Pietro Island (Sardinia) in Italy, where the rock type is found. Originally described as a variety of porphyritic leucocratic alkali rhyolite containing phenocrysts of quartz, alkali feldspar, aegirine, arfvedsonite or riebeckite and minor biotite. It is now defined and distinguished from pantellerite as a variety of peralkaline rhyolite of TAS in which Al2O3 > 1.33 x total iron as FeO + 4.4.

San Pietro island

San Pietro Island, located at the southwestern tip of Sardinia, is part of the Sulcis complex and is completely formed by volcanic terrains. The geological peculiarity of this site is the presence of a peralkaline rhyolitic lavic and ignimbritic complex, interlayered in the products of a regionally extended calc-alkaline activity.

In 1895 S. Bertolio studied for the first time the peralkaline lava flows of Le Commende, in the northern part of the S. Pietro Island, giving them the name of Comenditi. This name entered the petrographical literature, and is presently used to indicate peralkaline rhyolites which contrast with pantelleritic compositions for a lower normative content of mafic minerals and for an iron content less than 4% by weight.

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image 1: Geological sketch of the area. Inferrd area is the provenance area for the main ignimbrites and fall deposits. From (Pioli; 2001).



The Cenozoic volcanism of western Sardinia forms a magmatic arc running along the western margin of Sardinia and southern Corse microplates. Volcanic activity occurred during Oligocene and Miocene (32.4-13.8 Ma,) as a result of subduction of oceanic litosphere in a NNW direction along the Apennines- Maghrebides subduction zone.

The activity preceded and partly accompanied the opening of the western Mediterrean sea through the formation of the Provençal Balearic and Algerian basins. This lead to a 60° counterclockwise rotation of Corsica-Sardinia blocks around a pole located at 42.7° N and 9.6° E. The magmatic products related to this activity range from calc-alkaline basalts to dacites with more subordinate rhyolites. The rocks show mineralogical and geochemical features closely comparable to those from other orogenic areas, showing systematic regional variations for several lithophile elements. Dostal et al. observed a progressive increase, from south to north and for a given SiO2 content, of the abundance of K, Li, Rb and Sr and of the K/Na, K/Ba and Rb/Sr ratios, associated to a decrease of the K/Rb ratio.

The Sulcis volcanic province (Image 1) is located in the southern portion of the magmatic arc, west of the main branch of the Sardinian rif, a Cenozoic N-S intra arc basin now occupying almost all of the western half of the island. Volcanic activity in the region was subaerial and can be grouped into two main phases:

Older Phase (28.4-17.7 Ma)
Dominated by the emplacement of calc-alkaline basaltic to intermediate lavas and subordinate pyroclastic products erupted from vents concentrated in the southern part of the basin (M. Narcao and southern S. Antioco island).

Younger phase (17.6-13.8 Ma)
Characterized by the emission of large-volume dacite to rhyolite ash flow tuffs with calc-alcaline to peralkaline affinity. The succession includes 11 main ignimbrite sheets separated by paleosols and minor, non-welded, pyroclastic deposits. The main tuffs are high-grade ignimbrites and are mostly distributed in the southern part of the sector.

No vents or caldera related to the younger phase are exposed in the basin, except for some effusive vents on the S. Pietro island. Field data (thickness variations, pumice and lithic maximum diameters, flow indicators) suggest a provenance for the main ignimbrites and fall deposits from a submerged sector located W of the island of S. Pietro (image 1).
Comenditic products are quite important; they are best exposed and thickest on the Island of S. Pietro. They are represented by three ignimbritic sheets and subordinate volumes of comenditic to pantelleritic lava flows and domes. No effusive products occur in the Sulcis mainland.

Volcanology of S.Pietro island



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Image 2: Geological skelch map of the S.Pietro island From (Cioni, 2005).



The S. Pietro Island can be subdivided into four different sectors, distinguished on the basis of their geological and morphological features:

• A southern sector, generally covered by the youngest ignimbrite sheet, characterized by a low, smooth relief and by a large depression;
• A northwestern sector, with the highest hills mainly formed by lava domes and coulees;
• A northeastern sector, formed by a small ignimbrite plateau cut by high cliffs along the coastline;
• A central sector, with a smooth relief, where the older volcanics of the S. Pietro Island crop out.

The volcanic succession cropping out on the San Pietro Island is represented by the units forming the upper half of that described in the Sulcis mainland. In particular, the older cropping units are the Nuraxi and Matzaccara ignimbrites, which mainly form the central sector of the island. The Matzaccara Ignimbrite forms the base of the comenditic sequence. This is particularly well exposed on San Pietro, with the occurrence of both effusive and pyroclastic products and where some vents can also be recognized.

The main volcanic succession of S.Pietro:

Lower Lava Unit
This unit is represented by blocky lavas cropping out discontinuously along the cliffs of the northern coastline, where they form the base of the comenditic ignimbrites. Where their central portion is exposed, they show a strongly porphyritic texture, with Sanidine and quartz in a vesicular groundmass. The upper portion of the unit is a monogenic breccia formed by vitrophiric blocks, from centimetric to metric in size, suggesting a strong autobrecciation of the lava crust.

Upper Lava Unit (Nasca, Borrona, Tortoriso flows)
It comprises several comenditic and subordinate pantelleritic domes and coulees. They generally represent the base of the ignimbritic sequence. All these products are characterized by very abundant phenocrysts of quartz and sanidine in a spherulitic groundmass. Mafic minerals are scarce, essentially represented by Na-clinopyroxene, biotite and minor arfvedsonite and aenigmatite. Several coulees are associated to dome-like structures. Coulees are characterized by a typical folded surface morphology. Columnar jointing is very common.

Lower Comenditic Ignimbrite (LCI)
The LCI locally crops out mainly along the cliffs of the northern sector, where it directly covers the Lower Lava Unit. An air fall deposit is found immediately below the ignimbrite. The upper part of the unit can be subdivided in two members ( A and B), mainly basing on their welding and crystal content. Member A, the lower, has a lower crystal content (around 10% by volume of the total) and is densely welded. It has a basal vitrophyre 20 cm thick, with a clear, thinly spaced columnar jointing that becomes more and more spaced toward the top of the Member. Flattened fiammae are very common, and the matrix shows a clearly eutaxitic texture. Member B generally shows a higher crystal and lithic content. Juvenile material is represented by two types of fragments, mainly differing for their different flattening ratios, vesicularity and crystal abundance.

Upper Comenditic Ignimbrite (UCI)
This ash flow tuff crops out mainly in the nortwestern portion of the island, covering the main units of the Upper Lava Sequence. The basal portion of the UCI is a cross-stratified, non-welded to poorly welded, coarse ash deposit with a variable thickness, resting on a pumice fall deposit. Altered, highly vesicular pumice, porphyritic with quartz and sanidine, is present. Toward the base it is characterized by lithophysae, while its top is densely welded and shows a well-developed eutaxitic texture with abundant fiammae. Fiammae increase in size toward the topmost portion of the UCI, where they are up to 50 cm in size. Columnar jointing is well developed in this part of the unit.

Monte Ulmus Ignimbrite
This is the most widespread comenditic ignimbrite, cropping out also on the Sulcis mainland. At least three different facies in a strict vertical association can be distinguished.

• A thin, very typical, basal vitrophire, around 30 cm thick;
• A densely welded, fine-grained, crystal-poor strongly rheomorphic zone, with planar foliation. This facies is well developed in the outcrops of the Sulcis mainland, while it is restricted to few tens of centimeters on the San Pietro Island;
• An upper, densely welded zone characterized by very large (up to 150 cm), black fiammae, and white, rounded pumice fragments. Fiammae are porphyritic with sanidine.

Paringianu Unit
It is a complex unit, represented by an alternation of decimetric pumice fall layers interbedded with pyroclastic flow deposits. It probably groups the deposits of several plinian to phreatoplinian eruptions. Pumice fall layers are medium to fine grained. Accretionary lapilli-bearing beds are interlayered with the fine-grained beds. The lower pyroclastic flow units show a white color and are fine-grained and non-welded, with abundant loose crystals of plagioclase. The upper pyroclastic flow units are characterized by a progressive increase of welding and an increasingly reddish color upward. A marked columnar jointing characterizes this portion of the Paringianu Unit.

Serra di Paringianu Ignimbrite
It crops out both in the northern and in the southern sectors of the island. The ignimbrite is characterized by a basal vitrophyre, up to 50 cm thick. The main body of the ignimbrite shows different facies. A non-welded facies formed by a lithic-rich, whitish pumice flow unit with a finegrained matrix is locally interlayered in the densely welded tuff. The welded portion of the unit has a typical appearance, with a highly developed foliation given by strongly flattened fiammae. Fiammae are porphyritic with plagioclase and sanidine, the latter often bordering the large plagioclase phenocrysts. Foliation is locally disturbed by rheomorphic movements, forming folds and ramps in the upper portion of the unit. Ramps structures in the top, vitrophyric portion of the ignimbrite clearly evolve toward autobrecciation.

Comenditic lava flows

Several comenditic lava flows and coulees are present in the S.Pietro island, issued from different vents scattered in the northern sector of the island. They spread in different directions on a gently sloping, complex preexisting topography and have variably elongated shapes. The coulees show two different types of lineaments:

curved ridges and troughs with a convex shape in the sense of the flow, and straight fractures, generally orthogonal to the ridges and in a few cases nearly parallel to them. All these lineaments can be traced for several hundred meters and are clearly visible in the aerial photographs . They consist of alternating light- anddark-colored belts, due to the juxtaposition of bare rock and vegetated zones.



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Image 3: A) Aerial photograph of the comenditic lava flows of Le Commende area, San Pietro Island. B) Main geological lineaments drawn by the aerial photograph in A. The Arrow shows where the Becco Nasca coulee has been diverted by the preexisting Montagna di Ravenna coulee. From (Cioni, 2005).



Foliations:
Three penetrative foliations, with chronological superposition generally well defined in the field, are recognized at different scales. They are planar surfaces that can be widespread in the bulk lava flows or localized over restricted areas, depending on strain and progressive deformation experienced by the lava flows. Field data suggest that secondary foliations generally developed by mechanical rotation, in some cases followed by "transposition" processes, and were derived from preexisting layering that suffered compression.

Primary Foliations (S0). The older foliation recognized in the field is assumed to be directly related to the magma flow (flow banding). It is pervasive and involves the bulk of the lava flow, from the bottom to the top surface. This foliation is a primary magmatic feature and at macroscopic scale, it looks like a disjunctive layering, but differences between adjacent domains are often not very evident. In thin sections these planar anisotropies are marked by alternation of levels with different phenocryst content and matrix glass texture.

Secondary Foliations (S1 and S2). Two different foliations (S1) and (2l) developed during syn-emplacement deformation of the lava and overprinting the So foliation, have been recognized in some coulee. The S1 is a penetrative disjunctive cleavage, spaced few millimeters, that crosscut the So foliation. S1 is parallel to the axial plane of decameter-scale folds that deformed the S0. Along the border of the lava flows, another foliation, called S2 is well exposed. It is a disjunctive sub-vertical cleavage, decimeter to centimeter spaced. S2 shows a parallel-to-margin straight trend. In the area dose to the vent it overprints the So and the axial plane cleavage S1 that deformed the S0.



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Image 4: a) S1 folialion crosscutting the So foliation. From (Cioni, 2005).



Folds:
The lava coulees are affected by different fold systems recognizable from their shape, axial plane, and axis orientation, and also from their location. The ridges shown by the aerial photographs, similar in shape to the classical surface ropy structures of basaltic flows, appear to affect the main parts of the three lava flow.

F1 Folds: Ridges and troughs distinguished in the aerial photographsare antiforms and synforms of the older fold system, which formed during the forward movement of the lava flow by buckling of the S0 foliation under flow-parallel compression. Buckling occurred over the whole thickness of the coulee, which can be considered as a multilayered sequence partitioned by the S0 foliation. Where not deformed, in the near-vent sector, F1 folds are upright, cylindrical folds with sub-horizontal axes and vertical axial planes orthogonal to the flow direction.

F2 Folds: Ridges and troughs, which characterize the coulees, have a curvilinear arrangement, resulting from the refolding of F1 folds. Refolding occurred as a disharmonic multilayer bending involving the whole thickness of the coulee. F2 folds are typically symmetric upright, steeply plunging folds (vertical axial plane and axis steeply plunging toward the vent.

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Image 5: Sketch illustrating the formation of F1 and F2 folds and foliation transposition along margins (not to scale). F1, F1A.P = F1 folds and F1 axial plane; S0, S,1 S2, = (foliations); F2, F1A.P = F2 folds and F2 axial plane. From (Cioni, 2005).




Bibliography



• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press
• Shelley D (1992): Igneous and Metamorphic Rocks under the Microscope: Classification, textures, microstructures and mineral preferred orientation
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Eric A.K. (1985): Middlemost Magmas and Magmatic Rocks. Longman, London
• A. Morra, V. Secchi, F. A. (1994): Petrogenetic significance of peralkaline rocks from Cenozoic calc-alkaline volcanism from SW Sardinia, Italy. Chem. Geol. 118, 109-142.
• Cioni. R. Salaro. L. Pioli. L. (2001): The Cenozoic volcanism of San Pietro Island (Sardinia, Italy).
• Cioni. R. Gioncada. A. (2003): The Comenditic ignimbrites and lavas of San Pietro Island (Sardinia, Ital y): Inferences on degassing and evolution of peralkaline magmas from melt inclusions.
• Cioni. R. Funedda. A. (2005): Structural geology o[ crystal-rich, silicic lava flows: A case study from San Pietro Island (Sardinia, Italy). Geological Society or America, 396.
• Pioli. L.,Rosi. M. (2005): Rheomorphic structures in a high-grade ignimbrite: The Nuraxi tuff, Sulcis volcanic district (SW Sardinia, Italy). Journal of Volcanology and Geothermal Research.
• Morra, V. Secchi F.A. Assorgia, A. (1994): Petrogenetic significance of peralkaline rocks from Cenozoic calcalkaline volcanism from SW Sardinia, Italy. Chem. Geol. 118, 109–142.


Ignimbrite
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Glass fragment in a comenditic ignimbrite. PPL image, 10x (Field of view = 2mm)
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Glass fragment in a comenditic ignimbrite. PPL image, 10x (Field of view = 2mm)
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Glass fragment in a comenditic ignimbrite. PPL image, 10x (Field of view = 2mm)
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Glass fragment and dark fiammae in a comenditic ignimbrite. PPL image, 2x (Field of view = 7mm)
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Glass fragment and dark fiammae in a comenditic ignimbrite. PPL image, 2x (Field of view = 7mm)
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Glass fragment and dark fiammae in a comenditic ignimbrite. PPL image, 2x (Field of view = 7mm)


Comenditic Rhyolite
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Interstitial green Arfvedsonite in a comenditic Rhyolite. PPL image, 2x (Field of view = 7mm)
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Interstitial Arfvedsonite in a comenditic Rhyolite. XPL image, 2x (Field of view = 7mm)
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Interstitial green Arfvedsonite in a comenditic Rhyolite. PPL image, 10x (Field of view = 2mm)
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Interstitial green Arfvedsonite in a comenditic Rhyolite. PPL image, 2x (Field of view = 7mm)
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Interstitial green Arfvedsonite in a comenditic Rhyolite. PPL image, 2x (Field of view = 7mm)
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Interstitial green Arfvedsonite in a comenditic Rhyolite. PPL image, 10x (Field of view = 2mm)
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Sanidine and quartz in a comenditic Rhyolite. XPL image, 2x (Field of view = 7mm)
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Sanidine and quartz in a comenditic Rhyolite. XPL image, 2x (Field of view = 7mm)
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Interstitial green Arfvedsonite in a comenditic Rhyolite. PPL image, 10x (Field of view = 2mm)
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Sanidine and quartz in a comenditic Rhyolite. XPL image, 2x (Field of view = 7mm)
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Quartz in a comenditic Rhyolite. XPL image, 2x (Field of view = 7mm)
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Interstitial green Arfvedsonite in a comenditic Rhyolite. PPL image, 10x (Field of view = 2mm)
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Interstitial green Arfvedsonite in a comenditic Rhyolite. PPL image, 10x (Field of view = 2mm)
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Quartz crystals with interstitial green Arfvedsonite in a comenditic Rhyolite. XPL image, 10x (Field of view = 2mm)
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Interstitial green Arfvedsonite in a comenditic Rhyolite. PPL image, 10x (Field of view = 2mm)
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Comenditic Rhyolite with felsic (devitrified) groudmass. XPL image, 2x (Field of view = 7mm)
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Comenditic Rhyolite with felsic (devitrified) groudmass. XPL image, 2x (Field of view = 7mm)
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Comenditic Rhyolite with felsic (devitrified) groudmass. XPL image, 2x (Field of view = 7mm)
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Sanidine and quatz in a comenditic Rhyolite. XPL image, 2x (Field of view = 7mm)
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Sanidine and Arfvedsonite in a comenditic Rhyolite. PPL image, 2x (Field of view = 7mm)
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Sanidine and quatz in a comenditic Rhyolite. XPL image, 2x (Field of view = 7mm)
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Sanidine and quatz in a comenditic Rhyolite. XPL image, 2x (Field of view = 7mm)
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Comenditic Rhyolite with felsic (devitrified) groudmass. XPL image, 2x (Field of view = 7mm)
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Sanidine and Arfvedsonite in a comenditic Rhyolite. PPL image, 2x (Field of view = 7mm)
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Comenditic Rhyolite with felsic (devitrified) groudmass. XPL image, 2x (Field of view = 7mm)
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Comenditic Rhyolite with felsic (devitrified) groudmass. XPL image, 2x (Field of view = 7mm)