Roses granodiorite
The Roses granodiorite is a Variscan granodiorite stick that penetrated into the metasediments of the Cap de Creus massif at the turn of the Carboniferous / Permian .
Geographical location
The 5 km long and a maximum of 2 km wide Roses granodiorite, named after its location Roses on the northern Costa Brava in Catalonia , is a small granodiorite body elongated in the southeast-northwest direction. It represents the easternmost intrusive body in the axial zone of the Pyrenees . Along its southwest side it borders the Mediterranean Sea (Bay of Roses) and dips in the northwest under the neogene sediments of the Empordà basin . On its northeast side, it has changed the metasediments of the Montjoi and Norfeu series that envelop it in contact metamorphosis and turned it into knot slate and horn rock.
geology
The pluton that penetrated synschistos shortly before the main phase of the Pyrenean deformation D 2 (and thus before the climax of the regional metamorphosis) , like the neighboring Rodes granodiorite located a little further north, shows a well-developed planar structure (foliation), but with very different intensity. The same deformation phase D 2 was also responsible for the development of the distinct foliation in the neighboring rocks . The foliation in granodiorite is concentrated in bands in the centimeter to hectometer range (up to a maximum of 500 meters). The granodiorite, which was originally covered with foil, is now mylonitized or gneissized .
Petrology
Roses granodiorite contains many quartz dioritic inclusions that can reach up to 50 percent by volume. In the undeformed state, the granodiorite has a more or less isotropic structure. The preferred orientation of the inclusions and the existing streaks should reflect a primarily magmatic flow direction. Occasionally xenolites and horn rock septa can be observed in the granodiorite, the foliation of which is clearly crossed by the granodiorite. Also Aplitgangscharen enforce the granodiorite. More leucocrate granitoids appear at the edge of the intrusion, which also send apophyses into the adjacent rock. Granodiorite dikes also run through the metasediments quite often. Furthermore, very late, dark lamprophyte ducts can be observed , which penetrate all structures in the granodiorite.
mineralogy
In a thin section, the undeformed Roses granodiorite contains the minerals quartz , plagioclase ( oligoclase to andesine ), alkali feldspar , biotite and hornblende . It can thus be characterized as biotite hornblende granodiorite. Frequently occurring accessories are allanite , epidote and clinozoisite . Oriented rutile needles are often included in biotite (Sagenite).
deformation
Granodiorite
At its northern northeast boundary, the granodiorite appears clearly and persistently foiled over a strike width of almost 1000 meters . This foliation S gm falls mainly at around 40 ° to the southwest. This foliation does not exist in the southern section of the Pluton, but the granodiorite, which is quite homogeneous here, is traversed by gneissified or mylonitized bands over a strike width of another 1000 meters. These bands run more or less parallel in the southeast-northwest direction, but are much steeper than the foliation to the southwest. Their incidence can, however, also fan out, so that even directions of incidence to the northeast can occasionally be observed. The predominantly left-shifting ligaments anastomize and then enclose diamond-shaped, sigmoidal areas (shear lenses, called lozenges in English ) with relatively undeformed granodiorite. In only very weakly deformed areas of the granodiorite, the mylonite zones can give up their parallelism and even conjugate shear zones with a right-shifting sense of movement are found. In places it can also be observed how a weak, flat-lying foliation bends into steep, discrete mylonite bands.
The mylonitized granodiorite has distinctive mineral lines that run parallel to the direction of deformation. They dip very regularly flat to the southeast and are thus practically perpendicular to the great circle of the foliation levels. The mylonitic foliation can in turn be folded and crenulated, with the associated fold axes also being arranged parallel to the mineral linear.
Kontakthof
The granodiorite has intruded its neighboring rocks at different levels, clearly penetrating the regional S 1 foliation of the metasediments. New nodular porphyroblasts arose in the phyllites of the contact zone due to intrusion . Subsequently, late folds and crenulations formed in the metasediments, which were placed around the porphyroblasts and are therefore clearly younger. The porphyroblasts could in turn act as instabilities from which disharmonious krenulations then emanated.
This late deformation event finally spread to the porphyroblasts themselves, which were sericited and folded. Muskovite and biotite in the horn rocks were buckled. Small veins emanating from the granodiorite were also folded in places, with the same symmetry as existing folds in the phyllites. The fold axis direction in the metasediments can also be found in mineral linear lines of quartz veins in the contact zone.
interpretation
Similar to other mylonite belts, all the structural elements mentioned in the Roses granodiorite are attributed to a progressive deformation, which is due to the gradual emergence of the intrusion. Simpson (1981) is of the opinion that after the initial stage of a homogeneous deformation, a weak but continuous foliation had developed under irrotational conditions. This was then subjected to the conditions of a simple shear (English simple shear ), which concentrated on narrow shear zones and there deformed the previously created foliation.
The mylonitization of granodiorite is accompanied by microstructural changes, characterized by drastic changes in grain size and grain refinement and, in some cases, mineralogical new formations such as chlorite , epidote , muscovite and albite . Chemical processes can also play a role, such as the removal of quartz in chlorite-albite-mylonites.
Age
The exact timing of the Roses granodiorite is still controversial. Age dating carried out so far has shown 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 and must therefore be older. 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 .
The ages of the mylonite zones, the lamprophyr intrusion and the general south-west tilt of the Pluton are also in the dark. For the latter, an alpine age is usually assumed. The retrograde shear zone formation and mylonitization in the Cap de Creus massif is assumed by Vissers and colleagues (2016) with the Middle Jurassic and even Tertiary . This casts serious doubts on the previously accepted model of a continuous, purely Variscan development of Roses granodiorite.
Individual evidence
- ↑ Carreras, J. and Losantos, M .: Geological setting of the Roses granodiorites (E-Pyrenees, Spain) . In: Acta Geològica Hispànica . 17, n ° 4, 1982, p. 211-217 .
- ^ Carreras, J .: Petrologia y análisis estructural de las rocas metamórficas en la zona del Cabo de Creus (Prov. De Gerona) (doctoral thesis) . Barcelona, S. 154 .
- ↑ Simpson, C .: Ductile shear zones: a mechanism of rock deformation in the ortho-gneisses of the Maggia Nappe, Ticino, Switzerland . In: Unpubl. Ph.D. Thesis . ETH Zurich, Switzerland, 1981.
- ↑ Cobbold, PR and Quinquis, H .: Development of sheath folds in shear regime . In: Journal of Structural Geology . tape 2 (112) , 1980, pp. 119-126 .
- ↑ 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 .
- ↑ 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 .
- ↑ Vissers, RLM et al: Middle Jurassic shear zones at Cap de Creus (Eastern Pyrenees, Spain): a record of pre-dift extension of the Piemonte-Ligurian Ocean? In: Journal of the Geological Society . 2016, doi : 10.1144 / jgs2016-014 .