The hercynian chain in Sardinia (Italy)

Abstract

A new geodynamic model for the Sardinian segment of the Hercynian chain is presented. The improvement of knowledge regarding several geological, metamorphic, magmatic and geochronological aspects of the Sardinian Palaeozoic basement, mainly achieved in the last few years, allows us to propose a more complete picture of its evolution.

The occurrence of remnants of an oceanic suture along a major tectonic lineament in northern Sardinia, as well as the products of Ordovician calc-alkaline magmatism, testifies to the presence, during the Lower Paleozoic, of an ancient (Precambrian- Cambrian) oceanic domain and its consumption along an Andean- type subduction zone. The following Carboniferous continental collision caused crustal stacking with Barrovian metamorphism and southward-migrating deformation from the suture zone toward the foreland.

Early Carboniferous Culm-type facies sediments, deposited in the outermost zone of the chain, imply that continental collision took place earlier in the internal zone, from Late Devonian or Early Carboniferous.

The collisional orogenic wedge experienced ductile extension during the Late Carhoniferous as a result of gravitational collapse of the thickened continental crust.

Extensional tectonism enhanced the uplift of the chain and some regions underwent tectonic denudation or LP/HT metamorphism and somewhere anatexis. The emplacement of calc-alkaline batholiths and the development of Late Carboniferous - Early Permian molasse basins occurred during extension that prolonged throughout the Permian.     

Baseline geochemical mapping of Sardinia (Italy)

A total of about 20,000 stream sediment samples from about 13,000 km² were collected in Sardinia and analysed for eleven elements (Ag, Co, Cr, Cu, F, Hg, Mn, Mo, Ni, Pb, Sb and Zn) by Ente Minerario Sardo (EMS), aimed at mineral exploration. We have reutilized 16,890 samples (covering an area of 7987 km²) of the above data to compile geochemical baseline maps for all single elements as well as maps showing the regional variability of factor scores resulting from R-mode factor analysis. This type of maps has a particular relevance to environmental issues, especially in a region such as Sardinia which has been mined since pre-Roman times.     

Large-scale Hercynian West-directed tectonics in southeastern Sardinia (Italy)

Abstract

Widespread Hercynian west-directed thrusting and west-facing folds, until now only reported in a limited area, are ubiquitous in the Sarrabus unit, southeastern Sardinia. Intense thrusting and imbrication in the higher tectonic levels of the Sarrabus unit potentially originated during west-directed thrusting of higher tectonic units, which have been eroded. The floor thrust of the Sarrabus unit, the Villasalto thrust, is a west- directed thrust that places the Sarrabus unit above the Gerrei and Meana Sardo units. East-west shortening in southeastern Sardinia followed north-south shortening in central Sardinia and internal deformation in the Castello Medusa-Riu Gruppa, Gerrei, Meana Sardo and Barbagia units. After restoration of movement along the Villasalto thrust, the Sarrabus unit is located east of the Gerrei and Meana Sardo unit. Therefore, it cannot be directly correlated with the Meana Sardo or the Barbagia units. © Elsevier, Paris     

Large-scale Hercynian west-directed tectonics in southeastern Sardinia (Italy)

Widespread Hercynian west-directed thrusting and west-facing folds, until now only reported in a limited area, are ubiquitous in the Sarrabus unit, southeastern Sardinia. Intense thrusting and imbrication in the higher tectonic levels of the Sarrabus unit potentially originated during west-directed thrusting of higher tectonic units, which have been eroded.The floor thrust of the Sarrabus unit, the Villasalto thrust, is a westdirected thrust that places the Sarrabus unit above the Gerrei and Meana Sardo units. East-west shortening in southeastern Sardinia followed north-south shortening in central Sardinia and internal deformation in the Castello Medusa-Riu Gruppa, Gerreì, Meana Sardo and Barbagia units. After restoration of movement along the Villasalto thrust, the Sarrabus unit is located east of the Gerrei and Meana Sardo unit. Therefore, it cannot be directly correlated with the Meana Sardo or the Barbagia units.     

The abandoned Pb-Zn mine of Ingurtosu, Sardinia (Italy)

The Ingurtosu Pb-Zn mine, situated in the southwestern part of Sardinia, has been in production from the beginning of the last century up to 1968. The orebody consists of galena/sphalerite/barite/siderite veins in Ordovician arenaceous and phyllitic sediments, near the contact with a Hercynian batholith.

Abundant tailings were collected in a pond from which they were discharged during the winter months into the sea via the Rio Naracauli stream. Today this area of outstanding scenic beauty, is degraded by tailings and waste rock scattered all along the stream.

Material derived from mining activities was studied for grain size, mineralogical and geochemical composition. The Rio Naracauli was sampled over a length of ten kilometers at regular intervals.

The results can be summarized as follows: (a) tailings contain a varying, but often quite high, percentage of Pb; lower percentages were found for Zn and Cd; (b) a sharp increase in dissolved Zn, Cd and Pb was observed in the Rio Naracauli where it leached the tailings; and (c) tailings and waste rocks should be recycled or revegetated.     

The structural evolution of the Asinara Island (NW Sardinia,Italy)

The metamorphic basement of the Asinara island represents a key area of the Sardinia Variscan segment, because it displays an almost complete cross-section through the inner part of the Sardinia Variscan belt, where different tectono-metamorphic complexes have been juxtaposed along narrow belts of high-strain concentration. Detailed field mapping coupled with preliminary studies on the structural and metamorphic features of this small island, allow to draw a better picture of the structural frame issued from the Variscan collision in the inner zone of the belt. Three deformation phases related to crustal thickening in a compressive and transpressive, partitioned tectonic regime, followed by a later phase of extensional deformation have been recognised. In spite of a general HT/LP metamorphic overprint, linked to the post-collisional deformation phases, a relic Barrovian zoneography is still detectable. The Barrovian assemblages are preto syn-kinematic with respect to the D2 deformation phase, and pre-date the third, contractional tectonic event.

The HT/LP assemblage indicates a static growth of weakly deformed by the last deformation events. The complex geometry of the fabric associated to the D2 and D3 deformation events suggests an heterogenous deformation history with a monoclinic geometry characterized by switching of the stretching lineation orientation and a contrasting sense of displacement, probably controlled by a northward partitioned pure shear.     

From thickening to extension in the Variscan belt — kinematic evidence from Sardinia (Italy)

The Variscan nappe stack of SE Sardinia originated as a result of several stages of nappe imbrication during the Lower Carboniferous phases of the Variscan orogeny. The crustal shortening caused regional SSW-and W-directed thrusting, greenschist facies metamorphism and open-to-isoclinal polyphase folding. The final stage of shortening produced large-scale antiforms and synforms.
Post-collisional deformation resulted in inversion of earlier thrusts as normal faults, development of low-angle normal faults, and refolding of earlier foliation and thrust planes by asymmetric folds with subhorizontal axial planes. Facing directions of these latest folds are directed horizontally outward from the hinge zones of main antiforms, suggesting that they cannot be regarded as parasitic folds of the latest thickening phase, but instead are the consequence of vertical shortening during gravitational collapse of dome-like km-scale antiforms, leading to denudation of antiformal culminations.     

Distribution of heavy metals in the Piscinas beach sediments (SW Sardinia, Italy)

We have studied sediments of the Piscinas beach (SW Sardinia, Italy), which is supplied by two streams that wash mine dumps of abandoned lead and zinc mines at Montevecchio and Ingurtosu, situated inland from the supply basin of the beach itself. A study of the texture, mineralogy and geochemistry of the sediments was conducted for the purpose of assessing the possible influence of the mine waste on the composition of the sediments, looking for any anomalous enrichments in heavy metals. Furthermore, to evaluate and quantify metal release into the sea, samples of Posidonia oceanica, a bioaccumulator marine plant, were also examined. The results indicate that the distribution of heavy metals in the foreshore sediments is particularly affected by the contribution of the streams, while in the shoreface the distribution is affected by the currents that disperse the sediments both out to sea and southwards. The metal contents of the Posidonia oceanica are correlated with the different stages of activity of the mines. Received: 28 January 1998 · Accepted: 22 April 1998     

Hydrogeochemical baseline studies prior to gold mining: A case study in Sardinia (Italy)

An epithermal gold deposit is planned to be exploited in the Osilo area (North Sardinia). A detailed hydrogeochemical study was carried out to evaluate the water quality prior to mining, and to assess the potential impact of mining on the aquatic system. Forty-eight surface and ground waters were sampled at different seasonal conditions. A large set of chemical components was determined, taking into account the composition of ore and host rocks, as well as those components, such as cyanide, planned to be used in the mineral processing. Waters show different salinity and element concentrations depending on the composition of rocks drained, the time of circulation, the interaction with mineralised bodies, and climatic conditions. Most waters show relatively low concentrations of toxic or potentially harmful elements.Waters interacting with the ore deposit are generally non acidic, likely due to the low sulphidation system, the limited extension of the ore veins, the presence of calcite in the hydrothermal alteration assemblage, and the weathering of silicate rocks, which appears to contribute in buffering the acidity produced by the oxidation of sulphide minerals, especially pyrite. The nonacid environment does not favour the mobility of elements associated with the mineralisation, such as Cd, Co, Ni, Pb, Zn, and As. In addition, aqueous transport of these elements seems limited by the precipitation of ferrihydrite, which is able to reduce or remove these elements, particularly As, from solution. However, considering that exploitation will expose large quantities of mineralised materials to weathering, an adequate disposal of waste dumps and tailings needs to be carefully planned to prevent the potential dispersion of contaminants.     

An analysis of debris-flow events in the Sardinia Island (Thyrrenian Sea, Italy)

On 6 December 2004, the Villagrande Strisaili area (middle-east Sardinia), was struck by debris flows; 330 mm of rainfall took place within 3 h with an hourly intensity of 120 mm, which is far more above than normal for the study area. In the urban center stony and driftwood deposits accounted for a total volume estimated as 10,000 m³. The event claimed huge amount of infrastructural loss and two human lives. According to the chronicle reports, the area experienced two debris-flow events in the last century. The present paper is the outcome of an intensive study of such debris-flow events including their physical processes and geomorphological effects through both field survey and laboratory analysis.     

Evaluating disturbance on mediterranean karst areas: the example of Sardinia (Italy)

Evaluating the human disturbance on karst areas is a difficult task because of the complexity of these peculiar and unique environments. The human impact on karstic geo-ecosystems is increasingly important and there is an increasing need for multidisciplinary tools to assess the environmental changes in karst areas. Many disciplines, such as biology, geomorphology, hydrology and social-economical sciences are to be considered to sufficiently evaluate the impact on these intrinsically vulnerable areas. This article gives an overview of the evolution of environmental impact on karst areas of the island Sardinia (Italy). For this particular case, the most important impacts in the past 50 years are derived from the following activities, in decreasing importance: (1) mining and quarrying; (2) deforestation, agriculture and grazing; (3) building (widespread urbanisation, isolated homes, etc.) and related infrastructures (roads, sewer systems, aqueducts, waste dumps, etc.); (4) tourism; (5) military activities. To evaluate the present environmental state of these areas the Disturbance Index for Karst environments [Van Beynen and Townsend (Environ Manage 36:101–116)] is applied in a slightly modified version. Instead of considering the indicators of environmental disturbances used in the original method, this slightly modified index evaluates the disturbances causing the deterioration of the environmental attributes. In the Sardinian case study, 27 disturbances have been evaluated, giving rise to the definition of a Disturbance Index ranging between 0 (Pristine) and 1 (highly disturbed). This Disturbance Index simplifies the original KDI method, appears to adequately measure disturbance on Mediterranean karst areas and could be applied with success to other similar regions.     

Middle Jurassic continental to marine transition in an extensional tectonics context: the Genna Selole Fm depositional system in the Tacchi area (central Sardinia,Italy)

In Eastern Sardinia during the early Middle Jurassic, Alpine Tethys opening triggered the rise of a temporary tectonic high. The high collapsed rapidly, was fragmented into separate blocks, and subsequently covered by continental, transitional, and finally shallow marine deposits forming a narrow depositional system comprising the Genna Selole Fm. Present‐day exposures in the southern part of the palaeo‐high allow the sedimentological evolution of the transgressive cover sequence to be ascertained. Initial terrestrial deposits comprise alluvial fan deposits located at the mouths of palaeovalleys. These pass into braid‐deltas and in the coastal areas located between adjacent valleys mouths, palustrine and coastal plain tidally‐influenced environments developed. These environments interfingered laterally and passed seaward into a transitional, siliciclastic to carbonate tidal environment. With the collapse of the tectonic high, the continental to transitional environments were transgressed with deposition of marine carbonates. A comparison with similar coeval deposits of the W‐Mediterranean domain has been undertaken. Copyright © 2015 John Wiley & Sons, Ltd.     

Impact of past mining activity on the quality of groundwater in SW Sardinia (Italy)

Hydrogeochemical surveys were carried out in SW Sardinia (Italy) to investigate the impact of past mining activities on the quality of groundwater. The chemistry of waters from flooded galleries, adits and dumps has been compared with that of springs and wells in the same area at sites relatively far from any mine legacy. A feature, common to all waters, is the circumneutral pH, since the carbonate formations in the area neutralise the acidity produced by the oxidation of Fe-bearing sulphide minerals in the mine impacted water. However, groundwater interacting with mine workings is degraded in quality; it shows high dissolved SO₄, Zn, Cd and Pb contents. In some cases groundwater exceeds the limit established by the guidelines of the World Health Organization for Pb content in drinking water, so that groundwater is mixed before entering the local aqueducts. Results from this study suggest that more attention needs to be paid to the impact on the streams from contaminated water flowing out from some mine areas because during the dry season these streams are only fed by mine groundwater. We recommend focusing efforts to reduce the chemical contamination prior to discharge.     

Multi-stage metamorphic re-equilibration in eclogitic rocks from the Hercynian basement of NE Sardinia (Italy)

Summary Eclogitic rocks are hosted within gneisses and migmatites of the Hercynian basement of NE Sardinia. They are characterized by two compositional layers: garnet-pyroxene rich-layers and amphibole-plagioclase layers. The former contain structural, mineralogical and compositional relics of eclogite facies re-equilibration. Four stages of evolution have been identified: an eclogite stage, a granulite stage and a retrograde amphibolite to greenschist stage. A possible pre(?)-eclogite stage is documented by inclusions of euhedral tschermakitic amphibole + zoisite within the core of garnet. This early stage was followed by an increase in pressure under which the eclogite climax developed (T up to 700 °C, P =13-15 kbar), as documented by omphacite inclusions towards the rim of garnet.Characteristic mineral reactions after the eclogitic stage are: omphacite diopside + plagioclase (symplectite) and garnet orthopyroxene + plagioclase. These reactions testify the presence of a granulite stage during which the peak of metamorphism was reached (T up to 870'C, P 10 kbar). The orthopyroxene cummingtonite + quartz and garnet + diopside hornblende + plagioclase (kelyphite) transformations indicate extensive amphibolite retrogression (T = 550650 °C, P = 3-7 kbar). Finally, actinolite and chlorite developed (greenschist stage) at falling temperature and pressure (T = 300-400 °C, P < 2-3 kbar).

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Mehrstufige metamorphe Reequilibration eklogitischer Gesteine aus dem hercynischen Basement NE Sardiniens (Italien)

Zusammenfassung Eklogitische Gesteine kommen in Gneisen und Migmatiten des hereynischen Basements in NE Sardinien vor. Sie sind durch einen modalen Zweitagenbau von einerseits Granat-Pyroxen andererseits Amphibol-Plagioklas charakterisiert. In ersteren sind Relikte einer strukturellen, mineralogischen und mineralchemischen Reequilibration unter eklogitfaziellen Bedingungen enthalten. Vier Entwicklungstadien sind unterscheidbar: ein Eklogit-, ein Granulit-, sowie ein retrogrades Amphibolit- und Grünschieferstadium. Ein mögliches prä(?)-eklogitisches Stadium ist durch die Einschlüsse von tschermakitischem Amphibol + Zoisit in Granatkernen angedeutet. Auf dieses Frühstadium folgte, dokumentiert durch Omphaziteinschlüsse im Granatrandbereich, Druckzunahme und es wurden eklogitfazielle Bedingungen (T bis zu 700 °C, P = 13-15 kbar) erreicht.Charakteristische Mineralreaktionen nach der Eklogitbildung sind Omphazit Diopsid + Plagioklas (Symplektite) und Granat Orthoyroxen + Plagioklas. Diese Reaktionen belegen das Vorliegen eines Granulitstadiums, während dem der Metamorphosehöhepunkt (T bis 870°C, P 10 kbar) erreicht wurde. Die Umwandlungen von Orthopyroxen Cummingtonit + Quarz und Granat + Diopsid Hornblende + Plagioklas (Kelyphit) belegen eine intensive retrograde amphibolitfazielle Überprägung (T = 550-650°C, P = 3-7 kbar). Aktinolith und Chlorit (Grünschieferstadium) bildeten sich bei weiter fallenden Temperaturen und Drucken (T = 300-400 °C, P < 2-3 kbar).

     

Zeolitization of Oligo-Miocene volcaniclastic rocks from Logudoro (northern Sardinia, Italy)

The present study reports the results of research on volcaniclastic products from Logudoro (northern Sardinia), a reconstructed stratigraphic succession some hundreds of meters thick, comprising two different ignimbritic units separated by an epiclastic layer (generally ˜10 m thick). Clinoptilolite is the most widely distributed authigenic phase in epiclastite and unwelded units, always associated with opal-CT and sometimes with smectite. An adularia-like feldspar, cristobalite and epigenetic quartz are typical phases of welded units. Within the zeolitized units (unwelded ignimbrite and epiclastic units) clinoptilolite and opal-CT constitute the cement deriving from the transformation of the precursor rhyolitic glass, in agreement with a single zeolitization process that developed after the deposition of the entire succession. Silica increases in zeolitized rocks with respect to the precursor material, leading to hypothesize that the secondary mineralization process was favored by mineralized hydrothermal fluids preferentially circulating through the fault system of the area. Quartz veins in welded subunit and K-feldspar (adularia-like) preferentially located near the faults are in agreement with this hypothesis. The interaction of these fluids with the glassy fraction favored its dissolution and the consequent crystallization of clinoptilolite. Furthermore, the pH increase and the silica supersaturation enhanced the contemporary or subsequent precipitation of opal-CT. Received: 5 June 2000 / Accepted: 14 August 2000     

Crystal chemistry of clinopyroxenes from Oligo-Miocene volcanics of Montresta (Sardinia, Italy)

Summary Augite and diopside crystals from the Oligo-Miocene high-Mg and high-Al basalts, basaltic andesites and andesites of Montresta (Sardinia, Italy) have been analysed by means of X-ray single crystal diffraction and electron microprobe. It has been found that crystallization or equilibration processes took place within the crust at pressures below 5 kbar. In particular, samples from high-Al basalts, basaltic andesites and andesites crystallised at pressures below one kbar while those from high-Mg basalts crystallised in a range from about 1.5 to 4.5 kbar. The intracrystalline temperatures range between about 550 °C for high-Mg basalts and 750 °C for andesites. The M2′ site is occupied in each of the studied crystals, which implies slow cooling rates.     

Anatomy of the middle ordovician sevier shale basin,eastern Tennessee

The Sevier Shale basin in eastern Tennessee comprises one of the thickest clastic sequences (nearly 2500 m) of Middle Ordovician age in North America. The lower one-half of the sequence is composed of Lenoir, Whitesburg, Blockhouse and Sevier Formations, in ascending order. The sequence ranges in age from Whiterockian to lower Wilderness in North American stages.The Middle Ordovician sequence exhibits tidal flat (Mosheim Member of Lenoir Fm.), subtidal (main body of Lenoir Fm.), slope (Whitesburg Fm.), anoxic basin (Blockhouse Fm), turbidite and contourite (Sevier Fm.) facies. The Sevier basin evolved in five stages: First, a widespread marine transgression initiated carbonate-shelf deposition in the study area. Second, a major tectonic downwarping event caused the stable shelf to break and subside rapidly at a rate of 60–65 cm 1000 yrs^?1, and areas of shelf facies became areas of slope and basin facies. Third, global transgressions maintained the deep anoxic conditions for nearly 10 Ma. Fourth, turbidites began to fill the basin from a westward-prograding submarine fan system. Fifth, contour currents reworked the turbidites and progressively ventilated the Sevier basin. The basin-filling process terminated with shallow-water/subaerial clastics at the end of Middle Ordovician.