Leuco granite
The leuco granite is an igneous rock and belongs to the granitoids . The rock is a leucocrate variety of granite with more than 95 percent by volume of light mineral components.
history
Originally, in 1900, the American geologist JE Spurr used the name alaskite for plutonic quartz-alkali feldspar rocks with little or no mafic minerals. Leukogranite was first scientifically described by Jean Lameyre in 1966, who used the term synonymously for two-mica granite . The description was based on leuco granites in the French Massif Central .
Rock description
Leuco granites are very light to white rocks ( ancient Greek λευκός leukós , German 'white' ), when weathered they take on slightly yellowish tones. The grain sizes are usually between 3 and 5 millimeters, but there are also fine-grained and porphyry to pegmatitic varieties. The structure is usually uniform, but tectonically stressed rocks form a foliation , which is noticeable in the regulation of the mica , especially the muscovite. The quartz aggregates contained can then also experience a flattening.
The color number is defined as less than 5 s.
Mineral inventory and composition
Leuco granites have the minerals typical of granites . They usually consist of more than 70 percent by weight SiO 2 .
Modal mineral inventory
The following can also be added:
The following appear as accessories :
Chemical composition
The average composition of the Saint-Mathieu leukogranite (14 analyzes) and of the Cognac-la-Forêt leukogranite (25 analyzes) - all from the northwestern Massif Central:
| Oxide weight percent |
Saint-Mathieu leuco granite | Cognac-la-Forêt leuco granite |
CIPW norm percent |
Saint-Mathieu leuco granite | Cognac-la-Forêt leuco granite |
|---|---|---|---|---|---|
| SiO 2 | 72.64 | 73.31 | Q | 29.72 | 32.47 |
| TiO 2 | 0.23 | 0.22 | Or | 30.33 | 30.44 |
| Al 2 O 3 | 15.19 | 14.88 | From | 29.73 | 27.32 |
| Fe 2 O 3 | 0.80 | 0.96 | On | 3.81 | 3.31 |
| FeO | 0.83 | 1.01 | C. | 2.45 | 2.77 |
| MnO | 0.04 | 0.04 | Hy | 2.60 | 1.71 |
| MgO | 0.70 | 0.36 | Mt | 1.41 | 1.73 |
| CaO | 0.84 | 0.74 | Il | 0.42 | 0.41 |
| Na 2 O | 3.52 | 3.23 | Ap | 0.14 | 0.14 |
| K 2 O | 5.13 | 5.15 | |||
| P 2 O 5 | 0.06 | 0.06 |
Leukogranites are oversaturated with SiO 2 , their average SiO 2 content is 73%, with a range of 70 to 75%. They are also corundum normative and peraluminous . According to the alphabet soup classification of the granitoids, they belong to the S-type .
Origin and Association
Leukogranites arise mainly from a very advanced anatexis of metasediments ( greywacke and metapelite) in the continental crustal area. They are mainly associated with the type of collision in ore formation processes ; In this context, they can be linked to larger, shallow thrusts or to regionally significant, steep faults .
Occurrence and locations
Leukogranites are found worldwide. A very important and huge leukogranite deposit is located in the Himalayas , which occurs along the Main Central Thrust and owes its genesis to the main Alpid thrust in the Tertiary . Well-known examples are the Manaslu granite and the Lhotse-Nuptse leuco granite . North of the Himalayas is another leuco granite dome, the Yalaxiangbo .
Numerous examples of leuco granites can also be found in the French Massif Central. They emerged here towards the end of the Variscan mountain formation in the Upper Carboniferous and are linked to overthrusts and / or lateral shifts.
Locations in Europe are the Black Forest in Germany , north-east Portugal , north-east Sardinia , the province of Salamanca in Spain , south-east Bulgaria and north-west Turkey ( Pontic Mountains ). An example of a leucogranite from the Proterozoic is the Hearney-leucogranite in the Black Hills , United States .
raw materials
Leukogranites can accumulate very strong lithophilic elements , such as uranium and thorium . The latter are mostly bound to pegmatites, micro-granite or lamprophyre ducts.
Individual evidence
- ↑ Classification of igneous rocks according to composition. American Mineralogist 25: 229-232
- ↑ Amos B. Aikman, T. Mark Harrison, Joerg Hermann: Age and thermal history of Eo- and Neohimalayan granitoids, eastern Himalaya. In: Journal of Asian Earth Sciences . Volume 51, 2012, pp. 85-97, doi : 10.1016 / j.jseaes.2012.01.011 .