Albères massif

from Wikipedia, the free encyclopedia

The Albères massif is a 600 square kilometer large break in the basement of the eastern Pyrenees , which is mainly made up of metamorphic Paleozoic schists , gneiss and ganitoids . It allows the continuous insight into up to eight kilometers powerful rock series whose metamorphosis degree of quite low-grade green slates to high grade granulites and associated anatectic rocks ranges.

Geographical location

The Albères massif seen from the north-west from the Força Réal . The view extends over the western edge of the Roussillon basin.

The Albères massif is a semi-dome with roughly rectangular dimensions, 25 to 30 kilometers in an east-west direction and 20 kilometers in a north-south direction. On its east side it dips into the Mediterranean . In the southeast it is separated from the Cap-de-Creus massif by the Valetta Fault . To the south it sinks under the neogene sediments of the Empordà basin . To the west it is cut off by the north-northwest-south-southeast trending La Jonquera fault . Its northern boundary is the Tech Fault , an east-northeast-west-southwest trending fault at which the neogene Roussillon basin collapsed.

construction

The Cap Cerbère (Pointe d'Oiseau) with slates of the chlorite zone

Like other Gundgebirg massifs in the primary axial zone of the Pyrenees, the Albères massif consists of a Neoproterozoic-Paleozoic sediment series that was metamorphosed during the Variscan orogeny and intruded by various granitoids towards the end of the orogeny .

Metasediments

Metasediments of the biotite zone on the Mediterranean coast near Banyuls-sur-Mer

The metasedimentary sequence is of Areniten and pelites dominates and dates back to the Neoproterozoic ( Ediacaran ) back, but is mostly Kambro-Ordovician age. Occasional layers of limestone (marbles) and volcanic rocks (metamorphosed rhyolite porphyries ) are included in the sediments . The sediment sequence can be divided into three parts by two sustaining horizons: a basal, 2000 meter thick layer of paragneiss , which is covered by a 500 meter thick granite corridor, which is now in the form of orthogneiss . Above that follow 1500 meters of the intermediate layer, which in turn is separated from the upper layer by a 500 meter thick black slate horizon. The upper layer is up to 3000 meters thick. In total, the sediment sequence thus comprises a maximum of 8,000 meters of rock. The lithology of the sediment package changes little and remains relatively monotonous.

metamorphosis

Steep metasediments (black slate) of the andalusite-cordierite zone at Fort Carré near Collioure

The pelites and arenites of the sediment succession suffered a high-temperature-low-pressure metamorphosis (HT-LP) during the Variscan orogeny processes in the Upper Carboniferous and Lower Permian and were transformed into metapelites and metaarenites. However, the metamorphosis did not take place uniformly and ranges from the low-metamorphic green slate facial chlorite zone to highly metamorphic anatectic migmatites . The maximum metamorphic conditions in the migmatite zone were generally pressures of 0.35 gigapascals and temperatures of 650 ° C, whereas the base of the sediment package registered up to 0.55 gigapascals and 750 ° C. The following metamorphic zones have formed in the Albères massif with an increasing degree of metamorphism:

  • Chlorite-Muscovite Zone
  • Biotite zone
  • Andalusite-cordierite zone
  • Sillimanite zone
  • Migmatite zone

The low-metamorphic chlorite-muscovite zone is present in the south-eastern sector of the massif (between Espolla and Port Bou ). The other zones then follow successively towards the northwest, where the heat pole of the migmatitic melting zone in the sector south of Laroque-des-Albères is finally reached. A comparable HT-LP zoning can also be found in other basement massifs of the Pyrenees, such as the Aston-Hospitalet-Massif , the Canigou-Massif , the Trois-Seigneurs-Massif , the Agly-Massif and the Cap-de-Creus-Massif .

Magmatism

At the end of the Variscan regional metamorphosis, the sediment package was intruded by igneous rocks , which can be assigned to four different types:

The peraluminous and the anatectic leukogranites can be combined to form a magma group due to their very similar chemistry, thus leaving three families of magmas - mafites, calcareous alkaline rocks and leucogranites.

The magmatites occupy a total of almost 30% of the Albères massif. With the exception of the anatectic leukogranites, these late Variscan intrusive rocks have no direct genetic connection with the metamorphic rocks. They belong to different floors and have changed their neighboring rocks through contact metamorphosis. During the deformation phase D2, they intruded into the upper crust, comparable to other igneous complexes in the Pyrenees, e.g. B. the complex of Mont Louis - Andorra , the complex of Cauterets - Panticosa and the granodiorite of the Trois-Seigneurs massif.

geochemistry

In a diagram with the initial isotope ratios of 143 Nd / 144 Nd compared to 87 Sr / 86 Sr, the magmatites of the Albères massif show positions that are quite comparable to other representative magmatites of the Pyrenees. The mafites are located near the intersection of the two bulk earth straight lines (with Nd value at 0.51215, Sr value at 0.704) in the mantle array (and thus formed in the area of ​​the earth's mantle ), the anatectic magmatites are in the immediate vicinity The proximity of orthogneiss (Nd value at 0.5120, Sr values ​​at 0.720) and the calcareous granitoids occupy an intermediate position (Nd value at 0.51195, Sr values ​​at 0.710). The very constant isotope signature of the calcareous igneous rocks does not speak for magma mixing in the final stage of igneous development, but refers to either very homogeneous parent rocks or to crystallization of a very homogeneous magma in the lower crust under the conditions of a closed system, i.e. without any other magma or substance supply.

Petrogenesis

The mafites are likely to have arisen through fractional crystallization from hydrous calcareous basalt magmas of the earth's mantle. Smaller accumulations of hornblendites and cortlandites are likely to represent cumulates , whereas the diorites and tonalites are more developed melts. The Mafites are probably responsible for the initial heat and, to a lesser extent, also the mass supply in the orogen. Their different enrichment in light rare earths (LREE) indicates very differently developed mantle areas.

Partial melting of metasediments and orthogneiss in the migmatite zone produced numerous peraluminous leucogranite bodies, which then rose to higher coastal areas and established themselves there as sticks and tunnels of anatectic leucogranites. The El-Castellar leukogranite is likely to come from the deepest part of the migmatite zone and also refers to the extent of anatectic enamel formation.

The interpretation of the granodiorites is difficult because of their high CaO content and excludes a simple mixture of mafites and leukogranites. Rather, they are likely to have emerged from metatonalites, metaandesites or undeveloped metasediments through partial melting in the lower crust under granulite-facial conditions. The quartz monzodiorites represent cumulates in this scenario, whereas the La Jonquera granodiorite represents the most developed magma.

Age

In 1984, La Cocherie found an age of 282 ± 5 million years BP for granodiorites and quartz monzodiorites of the La Jonquera massif using the rubidium-strontium method , which corresponds to an age from the Lower Permian ( Artinsian ). More recent dating using the uranium-lead method on zirconia , however, has shown a much higher age at 305 million years BP (Upper Carboniferous, Kasimovium ) and thus question the reliability of the results obtained using the rubidium-strontium method. In 2011, Liesa and colleagues were also able to date the storage corridor and the metarhyolites in the metasediments using the uranium-lead method on zirconia. As a result, they found practically identical ages with 470 ± 3 million years BP for the camp passage and 465 to 472 million years BP for the rhyolite porphyries (transition from Lower to Middle Ordovician ). Since the rhyolites intruded as dikes, their age of 472 million years BP represents a minimum age for the metasediments, which are consequently older than the Middle Ordovician.

tectonics

The Puig Neulós, at 1256 meters, the highest point of the Albères massif

Characteristic of the Pyrenees are dome-like basement eruptions (massifs), which are girded by tightly folded areas. The fold axes generally strike east-southeast-west-northwest and the fold axis planes are inclined slightly to the southwest from the vertical.

In the Albères massif, two deformation phases D1 and D2 can be distinguished in the metasediments. The first deformation phase D1 is responsible for the formation of the regional foliation S1. The second deformation phase D2 is inconsistent. There have F2 wrinkle formed about dominate the D1 structures, in all orders of magnitude. The axial planes of the F2 folds dip steeply to the north or northeast. The F2 folds have usually caused creeping with the formation of a new foliation S2. The S2 foliation can be found in all granitoids, whereas the S1 foliation does not affect the granitoids and is therefore much older. Stretch linear associated with foliation S2 dip only slightly to the west or northwest.

The structures of the two deformation phases D1 and D2 can also be overprinted by mylonites of uncertain age. After the ductile Hercynian tectonics had been completed, the entire massif in the Paleogene was penetrated by thrusts (compression tectonics). In the Neogene, expansion tectonics then set in, which permeated Meso- and Cenozoic sediments with faults and, for example, is also responsible for the surrounding Neogene basins (Empordà and Roussillon).

The deformation phase D1 is assigned to a generally south-facing thrust tectonics. It took place in the course of the progressive metamorphosis and lasted until the climax of the metamorphosis. The deformation phase D2 was predominantly retrograde and was still compressive in a north-south direction, but also had a right-shifting shear component (dextral transpression). It had started shortly before the peak of metamorphosis, survived the maximum and only ended when the pressure-temperature conditions dropped.

See also

literature

  • Vilà, M., Pin, C., Enrique, P. and Liesa, M .: Telescoping of three distinct magmatic suites in an orogenic setting: Generation of Hercynian igneous rocks of the Albera Massif (Eastern Pyrenees) . In: Lithos . tape 83 , 2005, pp. 97-127 .

Individual evidence

  1. ^ Guitard, G .: Métamorphisme Hercynien . In: Barnolas, A. and Chiron, JC (eds.): Synthèse Géologique et Géophysique des Pyrénées . vol. 1. Editions BRGM-ITGE, 1995, p. 501-584 .
  2. ^ Laumonier, B .: Cambro-ordovicien . In: Barnolas, A. and Chiron, JC (eds.): Synthèse Géologique et Géophysique des Pyrénées . vol. 1st edition BRGM-ITGE, 1995, p. 157-209 .
  3. ↑ a b La Cocherie, A .: Interaction manteau-croûte: son rôle dans la génèse d'associations plutoniques calco-alcalines, contraintes géochimiques (élements en traces et isotopes du strontium et de l'oxygène) (doctoral thesis) . tape 90 . CNRS-BRGM, 1984, pp. 246 .
  4. ^ Wickham, SM and Oxburgh, ER: Low-pressure regional metamorphism in the Pyrenees and its implications for the thermal evolution of rifted continental crust . In: Philos. Trans. A . tape 321 (1557) , 1987, pp. 219-242 .
  5. ↑ Debon, F., Enrique, P. and Autran, A .: Magmatisme Hercynien . In: Barnolas, A. and Chiron, JC (eds.): Synthèse Géologique et Géophysique des Pyrénées . vol. 1st edition BRGM-ITGE, 1995, p. 361-499 .
  6. ↑ Roberts, MP, Pin, C., Clemens, JD and Paquette, JL: Petrogenesis of mafic to felsic plutonic rock associations: the calc-alkaline quérigut complex, French Pyrenees . In: Journal of Petrology . tape 41 , 2000, pp. 809-844 .
  7. ↑ Liesa, M., Carreras, J., Castiñeiras, P., Casas, JM, Navidad, M. and Vilà, M .: U-Pb ircon age of Ordovician magmatism in the Albera Massif (Eastern Pyrenees) . In: Geologica Acta . Vol. 9, N ° 1, 2011, p. 93-101 , doi : 10.1344 / 105.000001651 .
  8. ↑ Vilà, M .: Petrogénesi i estructura hercinianes del massís de l'Albera (Pirineus orientals) - doctoral thesis . Univ. de Barcelona 2003, p. 294 .
  9. ↑ Carreras, J .: Zooming on Northern Cap de Creus shear zones . In: Journal of Structural Geology . tape 23 , 2001, p. 1457-1486 .
  10. ↑ Gleizes, G., Leblanc, D. and Bouchez, JL: The main phase of the Hercynian orogeny in the Pyrenees is a dextral transpression . In: Holdsworth, RE, Strachan, RA and Dewey, JF (Eds.): Continental Transpressional and Transtensional Tectonics . vol. 135. Geol. Soc. London Spec. Publ., 1998, pp. 267-273 .