Cap de Creus massif

from Wikipedia, the free encyclopedia

The Cap-de-Creus-Massif is a 200-square-kilometer break in the basement of the eastern Pyrenees , which is mainly made up of metamorphic Paleozoic schists and ganitoids . It enables a continuous insight into a series of rocks around 2,000 meters thick, the degree of metamorphism of which ranges from very low-grade green slate to high-grade migmatites . The massif belongs to the axial zone of the Pyrenees and represents their most easterly located basement breakup.

Geographical location

The Cap de Creus with lighthouse. The steep, highly metamorphic schists are interspersed with pegmatite trains.

The Cap de Creus massif, named after the Cap de Creus , is the south-eastern continuation of the Albères massif , from which it is separated by the Valleta Fault . At this fault south of Llançà , the Cap de Creus massif was raised compared to the Albères massif, recognizable by the sudden transition from the lower grade chlorite zone of the Albères massif to the Biotite zone and Andalusite zone of the northern Cap de Creus massif.

The maximum length of the massif in the southeast direction is around 18 kilometers, its maximum width perpendicular to this is almost 13 kilometers. The southwest side of the massif disappears under the neogene sediments of the Empordà basin . The north-east and south-east sides are bounded by the Mediterranean Sea , into which it dips.

overview

Geological map of the Cap de Creus massif

Like other Gundgebirge massifs in the primary axial zone of the Pyrenees, the Cap-de-Creus-Massif consists of a Neoproterozoic-Paleozoic sediment series that metamorphosed during the Variscan orogeny and towards the end of the orogeny of two granitoids, the Rodes granodiorite and the Roses granodiorite , was intruded. Housed within the sedimentary series, there are igneous rocks of the Paleozoic , which are present now also metamorphosed. Tiny basaltic intrusion bodies such as the Puig Ferral near Cadaqués , which penetrated the massif in the Neogene, are a rarity .

Metasediments

The metasediments of the Cap-de-Creus peninsula are around 2,000 meters thick. They can be divided into a lower series and an upper series , which are separated from each other by a discordance . The lower series in turn consists of two series (from hanging to lying ):

The Cadaqués series is an alternation of metapsammites and metapelites, predominantly a monotonous sequence of predominant metamorphosed, ocher-gray greywacke with subordinate, lead-gray pelites and rare inclusions of other types of lithologies. Usually it is a few centimeters thick greywacke layers, which are separated by only very thin pelite layers. However, the thickness of the Grauwackenschüttungen can swell very strongly and reach 10 meters at Cala Culip and Cala Portaló, for example . Since the Grauwacken banks cannot withstand laterally, they cannot be mapped out. The metasediments of the Cadaqués series become darker and more pelitic towards the hanging wall, until they are finally available in the Montjoi series as black slate , in which smaller marble lenses are embedded.

The upper series is also known as the Norfeu series . The Norfeu series is a silica-carbonate sequence that occurs mainly in the southeast of the peninsula. It also girdles the eastern boundary of the two granodiorites in a north-west-south-east trending band .

The aforementioned external interferences in the Cadaqués series are predominantly plagioclase - amphibole- rich, unbearable layers in the millimeter and centimeter range, which are associated with the greywacke. They resemble amphibolite layers in the Ollo-de-Sapo gneisses in the Central Iberian Zone or insertions in the Villalba series . They are usually concordant and have a gradation relationship with the greywacke. Their lenticular secretion is mostly tectonic ( boudinage ), but can also be primarily sedimentary.

Banded and partly boudinated quartzite

The Cadaqués series features well-banked quartzite layers that are several centimeters to meters thick . The dark variety, the so-called Rabassers quartzite , consists of meter-thick, black and white striped quartzites, the predominantly dark parts of which are rich in organic matter. The pure white colored Culip quartzites are much more coarse-grained than the Rabassers quartzite. Transition types with black and white striped bands are also to be found.

The Montjoi series also contains external connections, for example a larger marble lens and metaporphyries in the hanging wall . The Sant Baldiri complex - carbon-rich black shale, lime-silicate stones, marbles, white quartzites, leucogneiss and amphibolites - acts as a further inclusion (possibly of tectonic origin) . The Sant Baldiri complex is surrounded by rust-colored slates. Together with the two quartzites already mentioned, it can be used as a prominent horizon for mapping purposes.

The Norfeu series at Cap Norfeu

Marbles also appear in the Norfeu series. The series begins with a basal marble layer , above which a sandstone layer and a conglomerate layer are set off. Marbles follow, which are dolomitic in the hanging wall .

Prevariscan igneous rocks

Two types of former magmatites occur in the metasediments : orthogneiss and metabasite. Orthogneiss includes the Port-de-la-Selva gneiss and several leucocratic lenses in the Sant Baldiri complex. The layered intruded Port-de-la-Selva gneiss has a granitic to quartz-monzonitic composition and a relict-like porphyry structure with occasional feldspar phenocrystals. The metabasites occurring in different levels of the metasediments and also in the Sant Baldiri complex were originally intrusions of gabbros or dolerites . Due to the Variscan metamorphosis, they are now available as green slate or amphibolite.

Variscan igneous rocks

Even during the Variscan orogeny, two larger synkinematic granodiorite stocks, elongated in the southeast direction, penetrated the metasediments along the southwest border of the Cap de Creus massif , the Rodes granodiorite in the northwest and the Roses granodiorite on the Gulf of Roses . Their heat content induced a contact metamorphosis in the surrounding phyllites with spotted schist and horn rock. The intrusions must have taken place before the deformation phase D 2, since newly formed porphyroblasts in the contact zone grow over the regional foliation S 1, but are in turn penetrated by creeps from the second deformation phase D 2.

The originally magmatic structure of the two granodiorites was then penetrated in the final magmatic stage by leucocratic aplites and pegmatites, which used expansion cracks as routes of ascent. These residual melts probably caused rheological stiffening.

After the magmatism ended, low-temperature, anastomosing shear zone networks arose in the granodiorites, which were followed by cataclasis in millimeter-thick bands at the very end of the cooling process .

Roses granodiorite in particular is very rich in inclusions and also contains metasediments.

Chemical composition

Main elements

Oxide
wt.%		 Phyllite
Muskovitzone		 Slate
sillimanite zone		 Gneiss
Port de la Selva		 Pegmatite Migmatite
melanosome		 Plagioclase
amphibolite		 Tonality Granodiorite Leuco granite
SiO 2 63.00 69.21 65.40 73.46 46.14 52.60 57.50 66.80 75.10
TiO 2 0.74 0.62 0.32 0.08 1.13 1.66 0.89 0.75 0.14
Al 2 O 3 16.40 13.22 16.90 14.78 22.73 14.67 18.60 15.80 13.80
Fe 2 O 3 6.03 (dead) 5.22 (dead) 2.43 (dead) 0.70 (dead) 11.81 (dead) 12.03 (dead) 9.87 (dead) 4.97 (dead) 1.06 (dead)
MnO 0.07 0.07 0.04 0.14 0.18 0.22 0.12
MgO 2.55 2.45 1.34 0.05 5.35 5.39 1.51 1.66 0.18
CaO 0.83 2.04 2.75 0.13 2.56 6.41 4.62 3.06 0.88
Na 2 O 2.63 3.22 5.37 4.82 3.66 4.19 3.40 3.12 3.27
K 2 O 3.67 3.33 3.16 3.72 4.37 0.46 2.26 2.81 5.09
P 2 O 5 0.24 0.18 0.10 0.08 0.26 0.40
H 2 O 2.25 0.55 0.68 0.46 1.33 1.91 0.66 0.60 0.34

The rock samples listed are mainly intermediate to acidic rocks of dioritic , tonalitic , granodioritic to granitic composition. They are quite high in potassium and also in sodium ; they are therefore to be classified as potassium-rich (high-K) alkaline rocks . The MgO, CaO and also the total iron values ​​are quite low overall. The leuco granites and the migmatites are peralkaline , the remaining igneous rocks belong to a calcareous cafemic rock association.

metamorphosis

Migmatite of the sillimanite zone

Like the rest of the Pyrenees, the Cap-de-Creus massif was covered by a high-temperature-low-pressure metamorphosis (HT-LP) with very steep temperature gradients towards the end of the Upper Carboniferous . The metasediments passed through almost all metamorphic conditions, from low grade (green schist facies) to high grade (upper amphibolite facies) including anatexis . The degree of metamorphosis documented in the metasediments shows a clearly increasing gradient from the southwest to the northeast edge of the peninsula. The two granodiorite intrusions on the south-western edge of the massif, which imposed an additional contact metamorphosis on the low-grade slates to the northeast , recognizable by horn rocks and knot slates, are an exception . The contact aureole, which strikes roughly south-south-east, is around one kilometer wide. Folded phyllites of the chlorite zone (also chlorite-muscovite zone) then join to the northeast , characterized by the presence of chlorite and muscovite . The chlorite zone reaches a maximum width of 7 kilometers along the southeast coast, but gradually wedges to the northwest. North of Cadaqués, the chlorite zone is followed by the south-east trending biotite zone, which becomes a maximum of 2 kilometers wide (appearance of newly formed biotite in mica schists ) and from Cala s'Alqueria the 2 kilometer wide andalusite zone (also andalusite-cordierite zone), characterized by its first appearance of andalusite and cordierite .

Mylonitized pegmatite, Cala de Portixo

When the biotite zone is reached, mylonitic shear zones are encountered for the first time in the metasediments, which are becoming more and more common towards the northeast. The final sillimanite zone , characterized by the presence of sillimanite and divisible into a sillimanite-muskovite zone and a sillimanite-potash sparrow zone , follows the northeast coast in a 1.5 kilometer wide band from Cap de Creus to Cap Gros . It is characterized by scherzone networks, the presence of migmatites and bright pegmatite tunnels up to 10 meters thick , especially at Cap Gros and Tudela .

For the pT conditions prevailing during the metamorphosis, Druguet (1997) was able to determine a temperature range from 450 ° C for the chlorite zone to 670 ° C for the sillimanite zone. Maximum pressures of 0.47 GPa were registered in the sillimanite zone . In migmatites of the Punta dels Farallons , the temperatures had even risen to 700 ° C and the pressures had reached 0.74 GPa. This corresponds to a sinking depth of around 15 to 20 kilometers.

tectonics

Folded and boudinated pegmatite dike in a shear zone

The Variscan deformation in the Cap-de-Creus massif can be assigned to a general dextral (right-shifting) transpression and can be divided into three sections D 1, D 2 and D 3. During the first phase of the regional deformation, generally flat-lying layer-parallel was foliation S 0/1. The deformation phase D 2 was very intensive, as the progressive metamorphosis gradually reached its climax in the upper amphibolite facies. The heating of the sediments was so strong that along the northeast coast in the vicinity of smaller intrusions of tonalite and quartz diorite, the slate was melted and partially migmatized. At the height of the metamorphic development the anatectic pegmatites took place . During D 2, the old foliation S 1 was transposed into a new, steep foliation S 2, and at the same time close to isoclinal D 2 folds formed. The associated linear stretching lines L 2 drop flat to steep to the northwest. The deformation phase D 3 took place after reaching the thermal maximum under the retrograde metamorphic conditions of the green slate facies. It is responsible for the formation of the shear zones and mylonites that characterized all older structures. The deformation was now localized in a predominantly right-shifting network of shear zones. The structures accompanying the network are folds , pocket folds, shear bands and also shear fractures. The occurrence of shear fractures already indicates that the brittle upper crustal area has been reached and thus the gradual emergence of the Cap de Creus massif.

Structural structure

Structurally, the Cap de Creus massif can be divided into four areas (from southwest to northeast):

  • Southern belt of joke zones
  • Pleated belt
  • Transition zone
  • Northern joke zone belt

The southern shear zone belt comprises the two relatively shallow intruded granodiorite massifs including their shear zones and their contact zone area. The associated metasediments show a slight to moderate dip to the southwest in their foliation and fold axis planes. In the subsequent belt of folds , which extends over the chlorite and biotite zones, the foliation visibly flattens and lies more or less horizontally. The transition zone is identical to the Andalusite zone and is characterized by the gradual appearance of shear zones; from here the foliation now dips to the northeast until it assumes very steep dips to the northeast in the northern shear zone belt.

In general, the large-scale foliation structures therefore strike north-west-south-east, but can also bend in the east-west direction. The fold axes generally dip to the south-east in the south-west, but turn to east, north-east and even north in the north-east.

It is believed that the fold belt and the southern shear zone belt were obscured by an alpine block rot, as is also observed in other areas of the southern Pyrenees.

Age

Age dating carried out so far resulted in an age of 299 million years BP for the migmatites and an age of 290.8 ± 2.9 million years BP for the Roses granodiorite. As a result, the peak of the deformation D 2 occurred in the early Unterperm ( Asselium and Sakmarium ). Laumonier and colleagues (2014), however, doubt the somewhat very young age of the Roses granodiorite determined by Druguet, since it is crossed by a pegmatite dated to 297 ± 3 million years BP. They also indicate that the bulk of the igneous intrusions in the Pyrenees occurred in the period 309 to 299 million years BP, ie in the Moscovian , Kasimovian and Gzhelian .

As far as the metasediments are concerned, Casas and colleagues (2014) found Neoproterozoic ages of the Ediacarian for the lower series . The Port de la Selva gneiss could be dated to 553 ± 4 million years BP using the U-Pb method on zirconia . Former acidic and basic tufa layers in the lower series resulted in ages between 577 ± 3 and 558 ± 3 million years BP.

The timing of the deformation D 3 and the retrograde shear zones is still controversial. New findings by Vissers and colleagues (2016) with Central Jurassic and even tertiary ages for the shear zones allow us to distance ourselves from the previously accepted model of a continuous Variscan development.

literature

  • Druguet, Elena: The structure of the Cap de Creus peninsula. Relationships with metamorphism and magmatism. (PhD thesis) . Universitat Autonoma de Barcelona, ​​1997.

Individual evidence

  1. ↑ Ábalos, B., inter alia: Variscan and Pre-Variscan Tectonics . Ed .: Gibbons, W. and Moreno, T., The Geology of Spain. Geological Society, London 2002, p. 155-183 .
  2. ↑ Navidad, M. and Carreras, J .: Pre-Hercynan magmatism in the Eastern Pyrenees (Cap de Creus and Albera Massifs) and its geodynamical setting . In: Geologie en Mijnbouw . tape 74 , 1995, pp. 65-77 .
  3. ↑ Druguet, E., Enrique, P. and Galán, G .: Tipología de los granitoides i las rocas asociadas del complejo migmatitico de la Punta dels Farallons (Cap de Creus, Pirineo Oriental) . In: Geogaceta . tape 18 , 1995, p. 199-202 .
  4. ↑ a b Druguet, E .: Development of high thermal gradients by coeval transpression and magmatism during the Variscan orogeny: insight from the Cap de Creus (Eastern Pyrenees) . In: Tectonophysics . tape 332 , 2001, pp. 275-293 .
  5. ↑ a b Druguet, E. et al .: Zircon geochronology of intrusive rocks from the Cap de Creus, Eastern Pyrenees . In: Geological Magazine . tape 151 , 2014, pp. 1095-1114 .
  6. ↑ Druguet, Elena: The structure of the Cap de Creus peninsula. Relationships with metamorphism and magmatism. (PhD thesis) . Universitat Autonoma de Barcelona, ​​1997.
  7. ↑ Fusseis, F. and Handy, MR: Micromechanism of shear zone propagation at the brittle-viscous transition . In: Journal of Structural Geology . tape 30 , 2008, p. 1242-1253 .
  8. ^ Carreras, J. and Casas, JM: On folding and shear zone development: a mesoscale structural study on the transition between two different tectonic styles . In: Tectonophysics . tape 135 , 1987, pp. 87-98 .
  9. ↑ Laumonier, Bernard et al .: Réconcilier les données stratigraphiques, radiométriques, plutoniques, volcaniques et structurales au Pennsylvania supérieur (Stéphanien - Autunien pp) dans l'Est des Pyrénées hercyniennes (France, Espagne) . In: Revue de Géologie pyrénéenne . tape 1, 2 , 2014, pp. 10 .
  10. ↑ Casas, JM et al .: The Late Neoproterozoic magmatism in the Ediacaran series of the Eastern Pyrenees: new ages and isotope geochemistry . In: International Journal of Earth Sciences . 2014, doi : 10.1007 / s00531-014-1127-1 .
  11. ↑ Vissers, RLM et al: Middle Jurassic shear zones at Cap de Creus (Eastern Pyrenees, Spain): a record of pre-drift extension of the Piemonte-Ligurian Ocean? In: Journal of the Geological Society . 2016, doi : 10.1144 / jgs2016-014 .