Capraia volcano

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The Capraia volcano is an extinct, polygenetic stratovolcano that built up the island of Capraia , which belongs to the Tuscan Archipelago , during the Tortonian in the northern Tyrrhenian Sea .

Geological framework

Punta del Trattoio. Clearly recognizable are the volcanic rocks falling to the south-west towards the sea and an angular discordance in the sequence of layers along the base

During the Burdigalium , the Corsican microcontinent carried out a counterclockwise rotation, which, although further removed from the European mainland, brought it closer to the northern spur of Apulia . In its eastern apron, due to the spatial constriction, Apulia was submerged in a south-westerly direction and the slowly emerging Apennine orogen was stacked on the ceiling . The crust expansion caused by the subduction in the back arc area of the Apennine arch now made it possible to penetrate accumulated magmas on fracture systems in the subsurface of Capraia. The magmas emerged from several extraction centers and lava domes , and by means of overlapping lava, ash and block flows as well as pyroclastic deposits, an asymmetrical stratovolcano was finally built - the Capraia volcano .

Stratigraphy and volcanic structure

The youngest lava flow of the Capraia volcano at the Punta del Ferraione

The volcanic deposits of the Capraia volcano exposed on the surface can be divided into two sequences, an older and a younger series. The older series was deposited around 7.6 to 7.55 million years BP and includes the following distinguishable groups:

  • Monte Castello
  • Monte Campanile
  • Monte Rosso
  • Punta del Trattoio

The younger series, consisting of the group of Monte Ruscitello, followed around 7.2 to 7.15 million years BP.

In the structure of the stratovolcano, two larger extraction centers and two dome structures can be seen. The two extraction centers, about 3 kilometers apart, face each other in an east-west direction, but both were seriously affected or completely destroyed by later flank tears. For example, nothing has survived from the older support center; it is now in the sea east of the Garitta del Bagno . The younger and much more important support center is located west of Monte Ruscitello at Punta de Recisello . It is responsible for the bulk of the volcanic deposits produced on Capraia. The more important of the two cathedral structures is the Monte Campanile on the east coast. The second, much smaller cathedral structure is about 2 kilometers further north at the Punta di Porto Vecchio, also on the east coast. Two smaller eruptive centers are in the debris of the Garitta del Bagno mining center. The last lava flow of the Capraia volcano came from one of these centers and flowed north-northeast past the fortress of San Giorgio to Punta del Ferraione .

Petrology

The high-K-calcareous ( English high-K calc-alkaline or abbreviated HKCA ) Capraia volcano is either part of the Corsican or the Tuscan magma province . Petrologically, the lavas that are extracted consist of potassium-rich andesites , dazites and rhyolites , but can sometimes also switch to the TAS fields latite and trachyte . The mineralogical structure of volcanic rocks is generally porphyritic and is dominated by plagioclase pheocrystals, which are joined by clinopyroxene , orthopyroxene and biotite . Occasionally, olivine and amphibole also appear. The base mass contains alkali feldspar , apatite , zircon and opaque minerals.

Chemical composition

The SiO 2 values for the main elements vary between 60 and 71 percent by weight, the rocks saturated with silicon thus have an intermediate to acidic composition, with the volcanites of the younger series being more acidic than those of the older series. The K 2 O content is between 2.6 and 4.6 percent by weight and the Na 2 O content between 3.4 and 3.8 percent by weight. The MgO values ​​fluctuate between 4.3 and 1.3 percent by weight and are increased in the intermediate volcanic rocks.

The trace element distribution in the volcanic rocks of the Capraia volcano shows an enrichment of incompatible elements and is characterized by a depletion of HFSE - compared to LILE elements. It is therefore quite comparable with the distribution in other volcanic rocks of the Tuscan Magma Province and also shows similarities to Sisco-Lamproit on Corsica . The initial ratio of the radioisotopes 87 Sr / 86 Sr (0.708720-0.710200) is significantly lower than that of the volcanic rocks of the Tuscan Magma Province and is closer to the value of the Corsican lamproite (0.712290). The initial ratio of 143 Nd / 144 Nd (0.512234–0.512280), on the other hand, is slightly above the Tuscan value, but is again similar to the Sisco value (0.512150).

Petrogenesis

The volcanic rocks of the Capraia volcano show similarities with lamproites, recognizable by their trace element pattern, the isotope ratios and the composition of their clinopyroxenes, whereas strangely, there is little in common with the calcareous and shoshonitic rocks of the Aeolian island arc . This confirms the hypothesis that the HKCA rocks from the Capraia volcano are actually related to lamproites and that a lamproic magma component played an important role in their formation. This would also explain the great variability of the potassium content in the Andesites of the Capraia volcano, which otherwise is difficult to understand using normal evolutionary processes.

Very similar lamproites occur (in addition to Corsica and Tuscany) in the western Alps and in the Betic Cordillera in Andalusia . It is assumed that the strong variability in the potassium content of the lamproic rocks is achieved through partial melting (with different melting rates) of lithospheric mantle rocks, which are interspersed with metasomatic phlogopitveins . In this scenario, lamproites represent almost pure vein melts, whereas in the case of the less potassium-rich calcareous and shoshonite magmas, in addition to the veins, mantle mother rock was also melted. In other words, calcareous alkaline rocks such as those on the Capraia volcano and Shoshonite represent lamproic magma seducts that have been diluted by peridotite magmas that are depleted in trace elements .

Dating

The Capraia volcano grew over a submarine base several hundred meters above the surrounding seabed, which is bounded by north-northeast-south-southwest trending fracture structures. This older base area is likely to go back to 8.3, possibly also to 9.5 million years BP. The stratovolcano lying above sea level is dated to 7.59 to 7.25 million years BP (also 7.50 to 6.90 million years BP). With the beginning of the Messinian, activities on the Capraia volcano ceased. After a pause in production of around 3 million years, the Zenobito volcano emerged in the Zancleum on the extreme southern tip of the island , which, however, clearly differs from its predecessor in petrological terms.

Individual evidence

  1. ^ A b Aldighieri, B., Groppelli, G., Norini, G. and Testa, B .: Capraia Island: Morphology and Geology of a Complex Volcanic Activity during the Miocene and Pliocene . In: Morini, D. and Bruni, P. (Eds.): The Regione Toscana project of Geological Mapping, case histories and data acquisition . 2004, p. 51-59 .
  2. ↑ Poli, G., Prosperini, N. and Conticelli, S .: Petrology and Geochemistry of Capraia and Zenobito volcanoes, Tuscan Archipelago - Italy: Complex origin of an High-K calc-alkalic volcano . 2006.
  3. ↑ a b Conticelli, S., D'Antonio, M., Pinarelli, L. and Civetta, L .: Source contamination and mantle heterogeneity in the genesis of Italian potassic and ultrapotassic volcanic Rocks: SrNd-Pb Isotope data from Roman Province and Southern Tuscany . In: Mineral. Petrol. tape 74 , 2002, pp. 189-222 .
  4. ↑ Chelazzi, L. et al .: A lamproitic component in the high-K calc-alkaline volcanic rocks of the Capraia Island, Tuscan Magmatic Province: evidence from clinopyroxene crystal chemical data . In: Periodico di Mineralogia . tape 75, 2-3 , 2006, pp. 75-94 .
  5. ↑ Peccerillo, A. and Martinotti, G .: The Western Mediterranean lamproitic magmatism : origin and geodynamic significance . In: Terra Nova . tape 18 , 2006, p. 109-117 .
  6. ↑ Peccerillo, A .: Pliocene-Quaternary volcanism in Italy. Petrology, Geochemistry, geodynamics . Springer, Heidelberg 2005, p. 365 .
  7. ↑ Conticelli, S., Carlson, RW, Widow, E. and Serri, G .: Chemical and isotopic composition (Os, Pb, Nd, and Sr) of Neogene to Quaternary Calcalkalic, shoshonitic and Ultrapotassic mafic rocks from the Italian Peninsula: inferences on the nature of their mantle sources . In: L. Beccaluva, G. Bianchini and M. Wilson, Cenozoic volcanism in the Mediterranean area (Eds.): Geological Society of America, Special Paper . 2006, p. 418 .
  8. ↑ Ferrara, G. and Tonarini, S .: Radiometric geochronology in Tuscany: results and problems . In: Rend. Soc. Ital. Mineral. Petrol. tape 40 , 1985, pp. 110-124 .
  9. ↑ Barberi, F., Ferrara, G., Franchi, F., Serri, G., Tonarini, S. and Treuil, M .: Geochemistry and geochronology of the Capraia Island volcanic complex (North Tyrrhenian Sea, Italy) . In: Terra Cognita . tape 6 , 1986, pp. 185 .