Ax granite
The Ax granite is a small granite deposit from the Aston massif in the French Pyrenees . It froze at the end of the Variscan orogeny around 306 million years ago during the Upper Carboniferous ( Kasimovian ).
geography
The Ax-Granite, completely also Ax-les-Thermes-Granite or Ax-les-Thermes-Leukogranit , is only 3.5 kilometers long and 2.2 kilometers wide, with its longitudinal axis pointing in the north-northwest direction. Its surface is 7 square kilometers. It follows the course of the Ariège south of Ax-les-Thermes to the Gorges de Mérens . Its eastern edge is cut off south of the Oriège by a north-northwest trending fault . The outcrop area of the granite is largely masked by tertiary sediments that are related to the leveling area of the Pyrenees.
geology
The Ax granite is embedded in the orthogneiss of the Aston massif ( Riète gneiss ) and almost meets metasediments of the Paleozoic Era in the north . Until the North Pyrenean fault the Col du Chioula further north there are still some 5 kilometers. It therefore belongs to the axial zone of the Pyrenees. The Aston massif forms a 40 × 15 kilometer lenticular, dome-like bulge that is tectonically elongated in the east-west direction. It is enveloped by Cambodian-Ordovician metasediments, which have undergone a multifacial metamorphosis (very low-grade to high-grade). The orthogneiss occurs as a migmatite on the western and southern edges of the massif . In addition to the Ax granite, there are five other small granite deposits in the Aston massif, including a slightly larger one in the migmatite in the southwest. A smaller granodiorite appears on the southeast edge , which is encased by a quartz diorite . In the metasediments in the southwest of the massif near Pla de Soulcem there are numerous smaller pegmatites and muscovite granites, which are chemically very similar to the Ax granite. The Aston massif is joined to the west by the Bassiés granodiorite and to the east by the granitoids of the Quérigut-Milhas granite .
Petrology
The Ax-Granite is a peraluminoser two mica granite ( leucogranite ) and most intrusive Aston massif. As already mentioned, it is petrologically similar to the pegmatites and the muscovite granite of Pla de Soulcem , but has less mineralogical variability. The granite also occasionally forms a gneissy foliation .
Coarse-grained veins and alternating pegmatite layers with tourmaline, clinozoisite and biotite as well as associated aplites may also be present . Occasionally, metasediments are to be mentioned as foreign rock inclusions, and biotite-rich streaks also occur.
The rock is generally light, but can take on dark colors due to rich biotite and accessories. The chlorite is an indicator of retrograde conversions. Microtectonic deformation structures are very common.
In their typology of Pyrenees intrusives, Pouget and colleagues (1989) count the Ax granite among the plutons of homogeneous composition (but small volumes) in metamorphic areas, more precisely to the HA type with acidic chemistry. It melted at a depth of 12 to 15 kilometers during an expansion phase before the actual main deformation D 1. The pluton took place in a diapiric manner during the second regional deformation phase D 2 at a depth of about 7 to 10 kilometers. The contacts with the host rock are progressive.
The Pla-de-Soulcem-Granite and the Tramezaïgues-Granite are very similar to the Ax-Granite .
mineralogy
The mineralogy of Ax granite is composed as follows:
Accessory occur:
Chemical composition
The following table is intended to illustrate the effects of albitization on Ax granite:
| Oxide wt.% |
Ax granite | Albitized granite |
|---|---|---|
| SiO 2 | 73.45 | 62.55 |
| TiO 2 | 0.14 | 0.39 |
| Al 2 O 3 | 14.42 | 21.85 |
| Fe 2 O 3 | 0.89 | 0.12 |
| FeO | ||
| MnO | 0.02 | 0.01 |
| MgO | 0.22 | 0.11 |
| CaO | 0.85 | 3.96 |
| Na 2 O | 3.06 | 9.17 |
| K 2 O | 5.30 | 0.45 |
| P 2 O 5 | 0.30 | 0.53 |
Trace elements
Table with trace elements:
| Trace element ppm |
Ax granite | Albitized granite |
|---|---|---|
| Cr | 20.3 | 17.1 |
| Cs | 3.78 | 0.55 |
| Zr | 33.9 | 252 |
| Nb | 6.91 | 18.57 |
| Nd | 6,054 | 37.39 |
| Rb | 187 | 21.0 |
| Sr | 73.4 | 1043 |
| Ba | 242 | 41.2 |
| Th | 3.26 | 40.6 |
| Ta | 0.93 | 2.26 |
| Hf | 1.13 | 7.53 |
| La | 6.14 | 38.09 |
| Ce | 12.04 | 77.66 |
| Sm | 1.525 | 7.359 |
| Eu | 0.39 | 0.62 |
| Tb | 0.295 | 0.834 |
| Yb | 1.045 | 2.69 |
Metasomatosis
A special feature is the occurrence of myrmecite , which indicates metasomatic processes . The albitization of the granite in the former Petches quarry , which is associated with a ductile shear zone, indicates a sodium-calcium metasomatosis . The plagioclase is then present as An 13 to An 16 (albite) and epidote appears. The result of the albitization was an increase in the elements strontium , phosphorus , sodium , calcium and aluminum as well as a simultaneous loss of rubidium , lead , potassium , silicon , iron and magnesium . In conjunction with the albitization, there was also a very strong silicification with the formation of new titanite, which covered 70 to 80% of the parent rock.
Age
Jäger and Zwart (1968) had granted the orthogneiss of the Aston massif a Lower-Ordovician age of 488 million years ( Tremadocium ). Denèle and colleagues (2009) meanwhile classify it as a little younger at 470 ± 6 million years ( Floium / Dapingium ).
Majoor (1987) was able to assign the Ax granite a cooling age of 301 ± 15 million years using the rubdidium-strontium method. A new date with the uranium-lead method (using LA-ICPMS ) by Denèle and colleagues (2014) provided a slightly older date of 306.2 ± 2.3 million years. With SIMS , Denèle (2007) determined an even older age of 321 ± 7 million years.
Faillourd and colleagues (2013) were able to determine the Na-Ca metasomatosis at 105.9 ± 4.2 million years ( Albium ).
Emergence
Like other Variscan intrusives with a high initial 87 Sr / 86 Sr ratio of 0.710 to 0.714, the Ax granite shows a very high value of 0.7142, which suggests crustal parent rocks. Zwart (1979) was of the opinion that the Ax granite had emerged from the orthogneiss of the Aston massif by anatexis . The orthogneiss, however, has an even higher 87 Sr / 86 Sr initial ratio of 0.7192 and also a different rubidium / strontium ratio (mean). The anatexis should therefore not be attributed to the orthogneiss alone, but also requires other melting components - other crustal rocks or an even deeper gneiss body.
literature
- Martignole, J .: Recherches pétrographiques et structurelles dans la région d'Ax-le-Thermes . In: Thèse de spécialité . Toulouse 1964, p. 180 .
Individual evidence
- ↑ Monod, B. et al .: Postorogenic planar paleosurfaces of the central Pyrenees: weathering and neotectonic records . In: Comptes Rendus Geoscience . tape 348 , 2016, p. 184-193 , doi : 10.1016 / j.crte.2015.09.005 .
- ↑ Pouget, among others: Typologie et mode de mise en place des roches magmatiques dans les Pyrénées hercyniennes . In: Geologische Rundschau . tape 78/2 , 1989, pp. 537-554 .
- ↑ a b Sylvain Fallourd include: In situ La-ICP-MS U-Pb dating of titanite Na-Ca metasomatism in orogenic belts: the North Pyrenean Example . In: International Journal of Earth Sciences . tape 103 , 2014, p. 667-682 , doi : 10.1007 / s00531-013-0978-1 .
- ↑ Denèle, Y. et al .: Middle Ordovician U-Pb age of the Aston and Hospitalet orthogneissic laccoliths: their role in the Variscan evolution of the Pyrenees . In: Bull. Soc. Geol. Fr. Band 180 , 2009, p. 209-216 .
- ↑ Majoor, FJM: A geochronological study of the axial zone of the Central Pyrenees, with emphasis on Variscan events and Alpine resetting . In: Verhandeling Laboratorium vour Isotope-Geologie . Amsterdam 1988, p. 117 .
- ↑ Denèle, Y., Laumonier, B., Paquette, JL, Olivier, P., Gleizes, G. and Barbey, P .: Timing of granite emplacement, crustal flow and gneiss dome formation in the Variscan segment of the Pyrenees . In: Geol. Soc. Lond. Spec. Publ. Volume 405 , 2014, p. 265-287 , doi : 10.1144 / SP405.5 .
- ↑ Denèle, Y .: Formation of dômes gneissiques hercyniens dans les Pyrénées: you exemple massif de l'Aston-Hospitalet (PhD) . 3 Université de Toulouse, 2007, p. 300 .