Columbretes volcanic rocks
The Columbretes volcanic rocks are Pleistocene volcanic rocks from which the Columbretes Islands in the Gulf of Valencia are built, the superficially visible part of the much more extensive Columbretes volcanic field of the same name .
geography
The Columbretes Islands are located almost 100 kilometers northeast of Valencia in the middle of the southwestern Balearic Sea and the Gulf of Valencia . It is around 100 kilometers to Ibiza in the southeast, and around 150 kilometers to Mallorca in the east. The Iberian mainland in the west is 55 kilometers away with Castellón de la Plana . The largest island in the archipelago, Columbrete Grande , is a partially eroded volcano made up of basaltic rocks. The island archipelago can be divided into four archipelagos, which are likely arranged on north to northeast trending fault systems. Starting in the north, the group of Columbrete Grande, followed further southwest by the group around Islote de Ferrera , then further south the group of Islote de Lobo and finally the group around El Bergantín . All islands show traces of severe coastal erosion .
geology
Regional geological framework: The Valencia trough
The Balearic Sea with the Gulf of Valencia corresponds geologically to the Valencia Trough ( Spanish Surco de Valencia ), a rift valley that extends between the Iberian mainland and the Balearic Islands in a north-east-south-west direction. The Valancia trough can be divided into two areas: the north-western "Catalonia-Valencia area" (Spanish dominio Valenciano-catalán ) and the south-eastern "Betiden-Balearic area" ( dominio Bético-balear ). The former is limited to the northwest by the Catalan Coastal Mountains and the Iberian Mountains (or includes their coastal edges). It is characterized by extensional and in the coastal portion through Ostnordost- to North trending normal faults in Horst and grave structures dismantled. The Betiden-Balearic area is bordered to the southeast by the "Balearic spur" ( English Balearic Promontory ). It is pushed to the northwest on the Catalonia-Valencia area and consists internally of a stack of ceilings pushed over to the northwest , which is overprinted by a system of almost northwest-striking Listrian faults. entire paragraph after
Geophysically, the Valencia trough is characterized by a highly thinned continental crust , only 9 to 15 kilometers thick , which rests on an upper mantle with abnormally low P-wave speeds (7.2 to 7.9 kilometers / second). Short-wave magnetic anomalies of very high amplitude up to 500 nanotesla are attributed to underground volcanic structures. The high heat flow of 70 to 90 watts / square meter also indicates volcanism.
The tectonic development of the Valencia trough took place in two steps. The first section lasted from the late Oligocene ( Chattian ) to the middle Miocene ( Burdigalium ) (25 to 18 million years ago). He saw the establishment of the Valencia trough as well as the advancement of the betic ceilings, the stacking of which caused an additional lowering of the thinned northwest crust due to tectonic load. The second section lasts from the Serravallian to the present day (13.85 million years ago). In the Catalonia-Valencia area, the stretching tectonics subsided and retreated towards the mainland. The Betiden-Balearic area, on the other hand, was affected by an even greater stretch, the causes of which can be traced back to the opening of the Algeria basin to the south .
Columbretes volcanic rocks
The volcanic field of the Columbretes Islands sits on the outer edge of the Ebro shelf on a 60-meter-high, 12-kilometer and 4-kilometer-wide submarine bulge that culminates at a depth of 80 meters. Its core is likely to be built up from Paleozoic metamorphic rocks that form an eyrie on the left-shifting Western European rift system . For example, the metamorphic minerals sillimanite , andalusite , staurolite , garnet , etc. were found during explorations . Inclusions of metasediments, gabbros and nepheline syenites in phonolites from the southern archipelago confirm such an assumption. It is around 10 kilometers to the shelf edge further east in 160 meters of water. It falls into the Valencia trough at a little over 5 ° to a depth of 1200 meters. A huge mass flow up to 20 kilometers wide has loosened at its edge, covering almost the entire edge of the shelf and coming to a stop after 15 kilometers at a depth of 1,150 meters.
The actual Columbretes volcanic field is, however, much more extensive with an area of 90 × 40 kilometers.
The largely submerged island archipelago consists of a variety of volcanic structures such as B. needles, dunnage, chimneys and their fillings, remains of volcanic cones, but also lava flows. Some intrusive structures have raised and partially penetrated the sea floor. They are partially or completely covered by sediment drift, which was transported by the south-west flowing Catalonia current.
The Columbretes volcanic field was developed in two cycles. The first cycle was of a calcareous nature and lasted from the end of the Oligocene to the Middle Miocene . The second cycle produced alkaline volcanic rocks from the Middle Miocene to the Holocene . Rocks from both cycles can occur together and it is quite conceivable that younger lavas were created through the reactivation of older volcanic centers. Lopez-Ruiz and colleagues (2002) connect the calcareous volcanic rocks with a stretching regime that was initiated by the clockwise rotation of the Balearic spur and the opening of the Valencia trough. The expansion caused subithosphere material to billow up, the lithospheric mantle melted, and magmas rose along eyrie and trench faults. The authors see the trigger for the second alkaline cycle in the left-shifting shear movements along the Western European Rift system.
These two cycles were preceded by the rotation of the Corso-Sardinian block, which had taken place at around 30 ° counterclockwise and had led to rifts in the Golfe du Lion and the North Balearic Basin as well as the opening of the Ligurian Sea .
The construction of the volcanic cone of Columbrete Grande took place in a total of six episodes (A to F), with four discordances in the sequence of the pyroclastics . In the initial phase (Episodes A and B) there were very violent submarine eruptions and submarine tuffs in the form of surge deposits ( English wet surges ) were produced. When the volcano emerged, the tuffs were thrown out subaerically. These dark lapilli deposits are very rich in volcanic bombs (Episodes C and F, found on Columbrete Grande, Mancolibre, Mascarat and Senyoreta). Towards the end of the volcanic activity, there was again an interaction with the sea water and therefore phreatomagmatic eruptions in the form of wet and dry surges. The volcano of Columbrete Grande is likely to have protruded 300 to 325 meters from the sea before its erosion.
Petrology
The predominantly alkaline basalt rocks of the Columbretes Islands have a fairly homogeneous composition, they are mainly basanites with a very low SiO 2 content. The basanites from Columbrete Grande contain plagioclase , olivine , augite and the amphibole kaersutite as phenocrystals . Quartz and plagioclase act as metamorphic xenocrystals, some of which are corroded and show undiluted extinction. The basic mass is microcrystalline or glassy, secondary minerals are zeolites and carbonates . Sedimentary foreign rocks such as limestone , but also other basalts, can be found in the surge deposits .
But there are also higher differentials. According to Aparicio and colleagues (1994), the island groups south of Columbrete Grande consist of massive lava flows from phonolithic to tephritic-phonolithic composition, including greenish-gray and dark, glassy phonolites. Their phenocrystals are sanidine , nepheline , kaersutite and opaque minerals. As enclaves (diameter <10 centimeters) they contain sedimentary rocks, gabbros and nepheline syenites.
The sodium- accentuated intraplate volcanic rocks were deposited on the thinned crust of Hercynian age.
The following table is intended to illustrate the range in the compositions of the vulcanites:
| Oxide wt.% |
Basanite / nephelinite | Hawaiit | Bergantín phonolite |
CIPW norm percent |
Basanite / nephelinite | Hawaiit |
|---|---|---|---|---|---|---|
| SiO 2 | 39.60 | 46.94 | 55.80 | Q | 0.0 | 0.0 |
| TiO 2 | 2.99 | 2.33 | 0.78 | Or | 14.18 | 7.80 |
| Al 2 O 3 | 13.20 | 15.25 | 20.21 | From | 18.35 | 5.06 |
| Fe 2 O 3 | 5.33 | 3.97 | 3.96 | On | 17.62 | 16.08 |
| FeO | 4.81 | 6.10 | 0.39 | No | 12.06 | 9.62 |
| MnO | 0.15 | 0.15 | 0.13 | Tuesday | 26.23 | 19.75 |
| MgO | 9.98 | 7.25 | 1.02 | Oil | 8.89 | 9.30 |
| CaO | 11.45 | 9.23 | 3.49 | Mt | 7.33 | 5.76 |
| Na 2 O | 3.23 | 4.11 | 8.03 | Il | 5.68 | 4.43 |
| K 2 O | 1.32 | 2.40 | 4.69 | Hem | 0.28 | 0.0 |
| P 2 O 5 | 0.84 | 0.75 | 0.22 | Ap | 1.95 | 1.74 |
The volcanic rocks are undersaturated with SiO 2 (no normative quartz), but nepheline and olivine are normative. Their melting depths were 15 kilometers, their outlet temperatures between 1018 and 1021 ° C.
Compared to the volcanic rocks in Catalonia and in the region around Valencia, the Columbretes volcanic rocks are clearly depleted of incompatible elements . The compatible elements such as iron , magnesium , manganese , nickel and chromium are also less concentrated. However, phosphorus shows higher values - this can be explained by a less significant SiO 2 under-saturation of the magmas. It can therefore be assumed that the Columbretes volcanic rocks were secreted by a higher melting rate with an otherwise identical mantle reservoir. The relatively low LREE / HREE ratio also points to a higher melting rate . The normalized ratio (La / Yb) n is between 14.57 and 16.43, but in Catalonia it is between 16.65 and 21.12. The subsequent differentiation within the Columbretes volcanic rocks can partly be explained by fractional crystallization .
Table with trace elements:
| Trace element ppm |
Basanite / nephelinite | Hawaiit |
|---|---|---|
| Cr | 177 | 207 |
| Ni | 135 | 128 |
| Zr | 103 | 105 |
| Nb | 3 | 7th |
| Y | 21st | 23 |
| Rb | 17th | 32 |
| Sr | 1123 | 839 |
| Ba | 967 | 838 |
| Th | 5.0 | 5.71 |
| Ta | 4th | |
| Hf | 5.8 | |
| La | 65 | 45.9 |
| Ce | 155 | 66.6 |
| Nd | 30th | |
| Eu | 2.1 | |
| Tb | 0.7 | |
| Yb | 2.08 |
Age
The Columbretes volcanic rocks were radiometrically dated to 0.3 to 1.0 million years using the potassium-argon method; they were thus formed during the Pleistocene.
Basaltic volcanic rocks from mainland Spain are similarly young, for example near Olot in northern Catalonia, Cofrentes and Picassent near Valencia, Campo de Calatrava near Ciudad Real and Cartagena in the province of Murcia . The volcanic complex of Agde in southern France and the volcanic rocks in northern Sardinia are also comparable .
Submarine, andesitic / Dacitic volcanic rocks in the immediate vicinity, which belong to the first cycle, could be dated to 21.9 to 20.8 and 24.4 to 19.4 million years. Their significantly higher age roughly corresponds to rhyolites on Mallorca, the age of which was determined to be 19.0 to 18.6 million years, or the calcareous igneous rocks of southeastern Spain.
Muñoz and colleagues (2005) are of the opinion that some of the submarine volcanic structures surrounding the Columbretes Islands are very young (<13,000 years) because they do not have any erosion structures and deform or pierce Holocene sediments.
literature
- Aparicio, A., Mitjavila, JM, Arañja, V. and Villa, M .: La edad del volcanismo de las islas Columbrete Grande y Alborán (Mediterráneo occidental) . In: Boletin Geologico y Minero . tape 102-4 , 1991, pp. 562-570 .
- Martí, J., Mitjavila, J., Roca, E. and Aparicio, A .: Cenozoic magmatism of the Valencia trough (western Mediterranean): relationship between structural evolution and volcanism . In: Tectonophysics . tape 203 , 1992, pp. 145-165 .
Individual evidence
- ↑ Fontboté, JM, Guimera, J., Roca, E., Sabat, F., Santanach, P. and Fermindez-Ortigosa, F .: The Cenozoic geodynamic evolution of the Valencia Trough (Western Mediterranean). In: Revista de la Sociedad Geológica de España. Volume 3, No. 3–4, 1990, pp. 249–259 ( PDF 850 kB)
- ^ Galdeano, A. and Rossignol, J.-C .: Contribution de l'aeromagnétisme à l'étude du Golfe de Valence (Méditerranée Occidentale) . In: Earth and Planetary Science Letters . tape 34 , 1977, pp. 85-99 .
- ↑ Cañada-Guerrero, F .: Contribución al estudio geolgóico de la plataforma continental submarina en el area de las Islas Columbretes . In: Boletin Geol. Minero . LXXXII-II, 1971, pp. 152-156 .
- ↑ a b c Aparicio, A., Arañja, V., Garcia, R. and Grachev, A .: The orogin of the Columbretes Islands basanitic and phonolitic magmas (western Mediterranean) . In: Mineralogical Magazine . Vol. 58A, 1994, pp. 21-22 .
- ^ Casas, D. et al .: Recent mass-movement processes on the Ebro continental slope (NW Mediterranean) . In: Marine and Petroleum Geology . tape 20 , 203, pp. 445-457 , doi : 10.1016 / S0264-8172 (03) 00078-3 .
- ↑ Maillard, A. and Mauffret, A .: Structure et volcanisme de la fosse de Valence (Méditerranée north-occidentale) . In: Bull. Soc. Géol. Ms. Band 164 (3) , 1993, pp. 365-383 .
- ↑ a b Muñoz, A. et al .: Sea floor morphology of the Ebro Shelf in the region of the Columbretes Islands, Western Mediterranean . In: Geomorphology . tape 72 , 2005, p. 1-18 , doi : 10.1016 / j.geomorph.2005.04.012 .
- ↑ Lopez-Ruiz, J., Cebría, J.-M. and Doblas, M .: Cenozoic volcanism: 1. The Iberian peninsula . In: Gibbons, M. and Moreno, T. (Eds.): The geology of Spain . The Geological Society, London 2002, pp. 417-438 .
- ↑ Hernández-Pacheco, F. and Asensio Amor, I .: Datos fisiográfico-sedimentológicos de la Columbrete Grande . In: Bol. Real Soc. Española Hist. Nat (Geol) . tape 64 , 1966, pp. 179-198 .
- ↑ Alonso Mantilla, WP : Variaciones petroquímicas en el volcanismo de las islas Columbretes (Castellón) . In: Publ. Cat. geol. Aplic. Obras Publ. Unív. Politec Valencia . ETS-ICCP, 1985, p. 59-101 .
- ↑ Martí, J., Mitjavila, J., Roca, E. and Aparicio, A .: Cenozoic magmatism of the Valencia trough (western Mediterranean): relationship between structural evolution and volcanism . In: Tectonophysics . tape 203 , 1992, pp. 145-165 .