Guéret granite

The Guéret granite is a layered granite composed of many individual intrusions in the northwest of the French Massif Central . In terms of area, it is the largest granite deposit in the Limousin region and the second largest in the Massif Central after the Velay granite . The granite was formed during the Variscan Orogeny around 350 million years ago.

etymology

The granite is named after its type locality , the prefectural town of Guéret in the Creuse department , Nouvelle-Aquitaine region .

geography

Quarry in the Guéret granite on Maupuy

The Guéret granite, often referred to as the Guéret massif , Guéret-Allochthon or Magmatic Complex of Guéret ( French Complexe Magmatique de Guéret - CMG for short ), forms the main part of the Guéret Terran , which is delimited by significant crustal faults . The massif has a length of around 100 kilometers in an east-south-east direction. Its width is 20 kilometers in the west and grows to 40 kilometers in the east. Its area thus takes up about 3000 square kilometers.

The boundaries of the massif to the west are formed by the north-striking Bussière-Madeleine fault , which acts here as a deportation against the granite. The northern boundary is provided by the approximately east-trending, right -shifting Chambon-Voueize shear zone . Further to the east, the massif then extends in places up to the terrang border of the north-north-east- trending and left-shifting Sillon houiller . The southern delimitation begins in the west with the southeast steichenden and right- shifting Arrènes fault . The Saint-Michel-de-Veisse Fault , also shifting to the right, running in the same direction, then separates the Guéret granite from the Millevaches massif to the south . The north-trending, staggered and also right-handed Felletin fault limits the northeast edge of the Millevaches massif to the Guéret granite. Finally, the south-east trending, right -shifting La Courtine Fault forms the extreme south-east boundary of the terran until it too is cut off by the Sillon houiller.

The Creuse Fault - a south-east trending, right-shifting, staggered shear zone - divides the Guéret granite into a western and an eastern section. Burglary basins such as the Ahun Lavaveix coal basin of the Stefanium have emerged along the shear zone .

geology

General Introduction

The sea of ​​rocks of the Pierres Civières on Maupuy consists of Guéret granite

The tectonomic development of the Massif Central can be divided into four sections. After an initial, eovarisian subduction on the northern edge of Gondwana in the course of the Silurian Mountains (430 to 400 million years), high pressure conditions arose (2 gigapascals and 700 ° C), documented by metamorphosed eclogite remains of an oceanic crust. The following mesovarican continental collision in the period 400 to 340 million years led to ceiling stacking and was accompanied by a regional metamorphosis of the Barrow type, with anatectic melts occurring for the first time in the Limousin . In the area of ​​the Guéret granite, the anatexis occurred at the turn of the Frasnian / Famennian 375 to 372 million years ago and was complete. The Aubusson-Migmatite developed .

In general, the anatectic melt formations themselves can be divided into two types - the magmatites of the calcareous Limousin tonalite line from 370 million years and the peraluminous magmatites of the Guéret type from 359 million years . The latter appear as gigantic laccolites and are mainly made up of cordierite - and occasionally hornblende - containing biotite granites.

During the subsequent neovariszischen development was the Orogen in the period from 340 to 310 million years by numerous transpressional side shifts severed, leading to the formation of peraluminosen two mica granites ( Leukograniten ) of the Limousin type led. In the post-orogenic late stage between 320 and 280 million years, the orogen stretched and dome-like bulges of migmatite (for example the huge Velay dome in the east of the Massif Central) and infiltration basins were formed, which were filled with carbonaceous sediments. Recent magmatic developments in the Limousin were limited to very small, extremely specialized granite intrusions ( English Rare Metal Granites ), which were usually bound to faults.

Granitoids of the Guéret type

In addition to Guéret granite, the granitoids of the Guéret type include, among others, the 352 million year old Cieux-Vaulry granite , the 346 million year old Aureil granite and, as late stages, the 325 million year old Piégut-Pluviers-Granodiorite and the 324 Millions of years old Auriat granite .

The granites of the Guéret type generally appear as extensive, but only very thin, laccolithic plates at most 1 to 2 kilometers thick. Their magmas are syntectonic formations that had penetrated along nappe thrust orbits. Their foliation is mostly flat and concordant to the surrounding gneisses , which indicates a common formation towards the end of the thrusts. Stretch linears usually point in an east to east-southeast direction. The foliation becomes steeper in certain areas in which an increase in thickness can be observed at the same time. These are shear zones along which magma influx may also have taken place. The Guéret type is usually not mineralized. Any mineralization is mostly bound to small internal leuco granite domes.

It is also assumed that the entire Guéret Terran is allochthonous and, while stretching, slid from the Upper Carboniferous towards east-south-east to south-east along the above-mentioned shear zones in the north and south-west. The kinematics required were provided by synkinematic leukogranites such as the Brame leukogranite , the Saint-Sylvestre leukogranite and the Saint-Goussaud leukogranite , which had penetrated the ceiling pile between 324 and 318 million years and raised the Guéret terran.

stratigraphy

A deep borehole at Créchat – Les Sibieux hit the 377 to 373 million year old, cordierite-bearing Aubusson-Migmatite (also known as Aubussonite ) of the lower gneiss cover under the plate-like Guéret granite at a depth of 500 meters , which is below 0.5 GPa ( corresponding to a depth of 15 kilometers) and 680 ° C. The migmatite / granite contact is of a tectonic nature and is marked by a breccia . The migmatite also appears in windows within the granite as well as on the southeast edge directly under the granite. Below the migmatite follows biotite-sillimanite-gneiss. The lying is formed by parautochthonous mica schists of the Chavanon series . It is assumed that the gneiss / mica schist contact zone represents a listric surface along which the granite and its gneiss bed slid in a south-easterly direction and at the same time was subject to an internal hanging-slope to south-east shear. The mica schists thus form a more easily deformable, ductile horizon that allowed the more rigid Guéret-Allochthon to slide off.

From the middle Viseum (around 335 million years) , the Guéret granite is covered discordantly by sediments of volcanic origin, which are counted among the tufs anthracitifères . At this point in time, the granite was already on the surface and was subject to erosion. A larger occurrence of the Tufs anthracitifères can be found north of Auzance . Later the already mentioned braziers followed in the Stefanium (e.g. Ahun-Lavaveix brazier, Bosmoreau brazier and Saint-Michel-de-Veisse brazier ).

Petrology

The Guéret granite is not a uniform rock formation, but is made up of several facies , some of which can be viewed as independent intrusions .

Ranchin (1971) was able to separate five petrological facies in the western section, which merge progressively and continuously into one another. Smaller diapir-like intrusions can also be seen in the generally flat rock, especially in the eastern section. Vauchelle (1988) then distinguished four main facies, namely the facies Villatange , the facies Paulhac (or Saint-Fiel ), the facies Grand-Bourg (or Peyrabout ) and the facies Aulon . These correspond to the predominant types of granite tonalite , granodiorite , monzogranite and monzoleucogranite.

In the meantime (2012) 19 smaller massifs can be mapped in the western section alone, which can be assigned to individualized magma pulses. These massifs differ from each other either petrologically and / or by tectonic conditions. A distinction is now made between, for example, the massifs of Villatange, Créchat, Paulhac, Grand-Bourg, Marsac, Saint-Priest-la-Plaine, Trois Cornes, Saint-Vaury, Montjourde, La Souterraine, Forges, Ribbes, Voudy, Lachaud, Aulon, Bénévent -l'Abbaye, Fursac, Noth and Salagnac. The magma pulses form flat, laccolithic secretions in the west, but can appear as dome-like bulges in the east, which penetrate previous shallow intrusions - examples of this are the Crocq granite and Fernoël granite .

In the Guéret granite there are numerous, mostly deformed inclusions made of crystalline slate, aluminum-rich residues, tonalites and dioritic MME (fine-grained mica-rich inclusions).

Rocks equivalent to Guéret granite also appear further west, such as the biotite granites of the Marche-Terran , the Oradour-Saint-Genest granite and others.

The type facies of the Guéret granite is the Chénérailles-Peyrabout-Monzogranite southwest of Guéret . It is composed of the following minerals :

• Alkali feldspar - 13 percent by volume
• Plagioclase - 37 percent by volume
• Quartz - 30 percent by volume
• Biotite - 17 percent by volume
• Muscovite - 2 percent by volume
• Cordierite
• Sillimanite

Apatite , ilmenite and zircon can also be used as accessories . The type facies appear in several varieties and can also be porphyry .

The type rock is light, blue-gray in color, medium-grained and has a porphyry tendency. The idiomorphic to hypidiomorphic alkali feldspar phenocrystals are poikilitic (with biotite inclusions). Their grain size varies between 5 and 10 millimeters, but in porphyries it can reach up to 10 centimeters. These are only moderately perthitic microclines with Carlsbad twins. Annular growth zones can be seen. The idiomorphic plagioclase occurs as synneus groupings and shows discontinuous, oscillating zoning with An _35-40 in the core and An _10-20 at the edges. Single crystals are often surrounded by albitic outgrowths . Quartz appears idiomorphic with undissolved extinction and subgrain formation. Its recrystallization can progress to polygonization. The hypidiomorphic biotite forms small, torn or bent crystals that appear either singly or in pulled apart groups. Primary muscovite is very closely associated with biotite, whereas the very common secondary muscovite is an interstitial conversion product of plagioclase and cordierite. The hypidiomorphic or nodular cordierite is often poikilitic and mostly converted to pinnite (muscovite ± chlorite ) and muscovite. Occasionally fresh crystals have micro-cracks in the core area. There are rare relics of Sillimanit.

A special feature is the frequently observed occurrence of myrmekite , which grows like a bud in the contact area between alkali feldspar and plagioclase.

The other facies are basically just modifications of the type facies, which differ in the frequency of crystallization of the minerals plagioclase and biotite - an increase in the direction of tonalite and a decrease in the direction of leukomonic granite.

Chemism

Oxide wt.%	Tonalite Villatange	inclusion	Aubussonite	Granodiorite Saint-Fiel	Tonalit Paulhac	Monzogranite Peyrabout	Mean value of Monzogranit Peyrabout	Type facies Peyrabout Maupuy	Leukomonic granite
SiO 2	61.38	63.68	65.70	65.80	67.60	69.30	70.20	71.80	72.40
TiO 2	0.80	0.60	0.75	0.63	0.55	0.43	0.41	0.27	0.28
Al 2 O 3	17.73	16.19	16.70	15.90	16.40	15.20	15.20	15.40	15.10
Fe 2 O 3	5.74	0.84	1.76	0.75	3.50 dead	0.56	2.52 dead	2.20 dead	0.15
FeO	4.63	4.05	3.59	2.80		2.22			1.40
MnO	0.07	0.06	0.07	0.08	0.07	0.07	0.05	0.04	0.06
MgO	2.77	4.05	1.70	1.95	1.67	1.32	1.12	0.86	0.53
CaO	3.04	2.30	0.67	2.95	2.07	1.65	1.68	1.18	1.20
Na 2 O	3.07	2.96	1.91	3.50	2.81	3.30	3.38	3.41	3.90
K 2 O	4.12	2.59	4.49	4.95	4.24	4.42	4.44	4.60	4.55
P 2 O 5	0.25	0.18	0.09	0.04		0.05			0.02
dehydration	0.95	1.90	2.21	0.60	1.35	1.01	0.48	0.94	0.51
A '/ F	0.46	0.35	0.90	- 0.01		0.36			0.49
A / CNK	1.18	1.37	1.81	0.96	1.26	1.15	1.13	1.21	1.12

A continuous spectrum can be observed in the chemical compositions - from tonalite to leukomonzranite, the latter being the most differentiated. Therefore, conclusions can be drawn about a parent magma of the differentiate. The SiO ₂ values ​​range from 61 to 73 percent by weight and are thus intermediate to felsic (or acidic). They allow the following classification: Tonalite <64 percent by weight, Granodiorite 64 - 67.5 percent by weight, Monzogranite 67.5 - 70 percent by weight and Leukomonzogranite> 70 percent by weight.

Except for the metaluminous granodiorite from Saint-Fiel (or hypaluminos, since A '/ F <0), all rocks are peraluminos (with A / CNK ≥ 1 and A' / F> 0.33) and represent granites of the S type This is also expressed in the occurrence of normative corundum with more than one weight percent. The 353 ± 6 million year old Villatange tonalite , which is a mafic cumulative magma of the earth's mantle caused by dynamic filter pressure, is somewhat set off from the peraluminous differentiation series and is to be regarded as a precursor. It is characterized by a higher concentration of total iron, MgO, CaO and also P ₂ O ₅ . Inclusions and in particular aubussonites differ in their significantly increased aluminosity.

Inclusion analysis has the highest MgO value and also quite high FeO and P ₂ O ₅ . It lies outside the differentiation series.

The aubussonite is an extremely peraluminous rock and is clearly separated from the Guéret igneous rocks. It is therefore very unlikely that it is the principal parent rock of the magmatites.

Trace elements

Trace elements ppm	Villatange tonalite	Paulhac Tonalite	Peyrabout monzogranite	inclusion
Ba	826	928	491	104
Ce	94.6	84.2	63.0	65.6
Co	12.4	11.2	5.7	11.4
Cr	62.1	51.5	29.5	97.7
Hf	6.5	6.1	4.3	5.1
Nb	14.7	14.7	10.0	11.7
Ni	22.6	19.6	15.8	37.6
La	44.5	42.5	31.5	32.9
Rb	206	180	218	203
Sm	7.0	5.9	5.2	5.5
Sr	331	380	208	189
Th	19.0	15.6	16.5	16.5
U	6.7	2.7	3.7	4.9
V	84	75	37	55
Y	17.8	22.2	15.1	14.4
Zr	237	222	145	173

The trace element concentrations are within the normal range of granodiorites and granites of the active continental margin (for example in the central volcanic zone of the Andes ). However, the trace elements cerium , chromium , lanthanum , nickel and rubidium show comparatively higher concentrations . The relatively higher content of chromium and nickel shows that the earth's mantle is more involved .

Isotope ratios

Downes and colleagues (1997) found an initial ⁸⁷ Sr / ⁸⁶ Sr for the different facies of the Guéret granite , ranging between 0.7089 and 0.7121. For the initial ε _Nd , they were able to determine values ​​from - 3.9 to - 7.5. Taken together, this is quite a wide range of variation and shows the heterogeneity of the magmas involved. Nevertheless, a significant sedimentary component emerges from metasediments of the lower crust. Similar values ​​can be found in the Banda island arc , which is heavily contaminated by sediments of continental origin.

Compared to other granitoids of the Massif Central, the initial lead isotope ratios are only slightly radiogenic.

geophysics

The Guéret massif has a pronounced positive gravity anomaly , despite its low density of 2620 to 2640 kg / m³. Model calculations carried out by Gébelin (2004) show that the granite can only be present as a very thin layer-like layer of several hundred meters - which was also confirmed by the deep drilling. The positive anomaly must therefore be explained by a significantly denser bluff below the Guéret massif.

tectonics

The main part of the magmatic intrusions of the Guéret granite had infiltrated a very extensive crustal area at a depth of 14 ± 2 kilometers from 350 million years ago. At this point in time, this was under a right-shifting, transpressive field of tension, the main narrowing axis of which was oriented N 150 to N 180 (north-south). The magmatic structure of the intrusions shows a more or less flat foliation with elongation lines in the northwest-southeast direction, ie the cooling magma was under an expansion in the southeast direction. Studies of magnetic anisotropy also provide very similar results.

Age

The following ages could be determined for the Aubussonites, which developed from the anatexis: 375 ± 2, 373 ± 5 and 371 ± 8 million years. The bulk of the Guéret granite was formed around 20 million years later. The following age determinations have been established so far: for the precursor tonalite from Villatange 353 ± 6, the monzogranite from Aulon 351 ± 6, the monzogranite of the Peyrabout type facies 348 ± 3, the granodiorite from Créchat 347 ± 3, the monzogranite from Saint-Fiel 346 ± 6 and the monzogranite of La Souterraine 345 ± 4 million years.

Emergence

The isotope ratios of Granite of Gueret-type suggest that their stock magma from a generated under partial melting of magma mixing had emerged. The parent rocks were both peraluminous metasediments and mafic igneous rocks.

Granulitic metasediments were partially melted in the lower crust. The resulting magma then mixed with mantle magmas of basaltic to andesitic composition from 360 million years ago, which had settled under the lower crust and infiltrated it. This formation model can explain the geochemical heterogeneities in the Guéret massif well, regardless of the individual petrographic facies.

The placement of the granites in the Tournaisium was accompanied by fractional crystallization . This resulted in the tonalitic cumulates of the Villatange facies (cumulates of plagioclase and aluminum-rich biotite), but also facies that are rich in SiO ₂ up to leukomonic granites of the Aulon facies , which are the only ones rich in cordierite. Nevertheless, magma contamination from the surrounding Aubussonites cannot be ruled out, although this is likely to have been rather low.

literature

• C. Cartannaz: Magmatismes et déformations polyphasés. Exemple of the Massifs de Guéret et de Millevaches (Massif central français). Origine des magmas et context de mise en place. Thesis. (PhD thesis) . Besançon 2006, p. 260 .
• C. Cartannaz and A. Cocherie: Notice explicative, Carte géol. France (1/50 000), feuille La Souterraine (641) . BRGM, Orléans 2012, p. 165 .
• M. Faure and J. Pons: Crustal thinning recorded by the shape of the Namurian-Westphalian leucogranites in the Variscan belt of the Northwest Massif Central, France . In: Geology . tape 19 , 1991, p. 730-733 .
• O. Jover: Les massifs granitiques de Guéret et du nord Millevaches (Nord du massif central Français). Analysis of structurale et modèle de mise en place (doctoral thesis). Univ. Nantes, 1986, p. 233 .
• P. Rolin, C. Cartannaz, P. Henry, M. Rossy, A. Cocherie, F. Salen, B. Delwaulle and B. Mauroux: Notice explicative, Carte géol. France (1/50 000), feuille Saint-Sulpice-les-Champs (666) . BRGM, Orléans 2006, p. 178 .
• L. Vauchelle: L'extrémité occidentale du massif de Guéret (Massif central français). Thèse, Université Clermont-Ferrand (doctoral thesis) . In: Ann. Sci. n ° 88, fasc. 12, 1988, pp. 397 .

Individual evidence

1. ↑ , M .: Berthier, F., Duthou, JL and Roques Data Transportation géochronologique Rb / Sr sur roches total du granite de Gueret (Massif Central). Age fini-dévonien de mise en place de l'un de ses faciès types. In: Bulletin BRGM . tape 2 , 1979, p. 60-71 .
2. ↑ M. Lespinasse, B. Mollier, J. Delair and Y. Bladier: Structuration tangentielle et chevauchements carbonifères dans les leucogranites du NW du Massif Central Français: L'exemple des failles de Bussières- Madeleine et d'Arrènes-Ouzilly . In: CR Acad. Sci. Ser. 2, 303, 1986, pp. 1575-1580 .
3. ^ B. Mollier and M. Lespinasse: Déformation magmatique et plastique en limite nord du granite de Saint-Sylvestre (Nord-Ouest du Massif Central francais): La faille d'Arrènes-Ouzilly . In: CR Acad. Sci. Ser. 2, 300, 1985, pp. 681-686 .
4. ↑ Christian Le Carlier de Veslud include: Relationships between granitoid and mineral deposits: three-dimensional modeling of the Variscan Limousin Province (NW French Massif Central) . In: Transactions of the Royal Society of Edinburgh: Earth Sciences . tape 91 , 2000, pp. 283-301 , doi : 10.1017 / S0263593300007446 .
5. ↑ J. Duthou: Les granitoïdes du Haut Limousin (Massif Central français) - chronology de leur mise en place - le thermométamorphisme carbonifère . In: Bulletin de la Société Géologique de France . tape (7), 20 , 1978, pp. 229-235 .
6. ↑ H. Gebauer et al .: U / Pb zircon and monazite dating of mafic-ultramafic complex and its country rocks. Example: Sauviat-sur-Vige, French Massif Central . In: Contributions to Mineralogy and Petrology . tape 76 , 1981, pp. 292-300 .
7. ↑ G. Guineberteau et al .: Structure magmatique et plastique des granites de la Marche occidentale: un couloir transformant hercynien dans le NW du Massif Central Français . In: Académie des Sciences, Comptes Rendus, Série II . tape 309 , 1989, pp. 1695-1702 .
8. ↑ Michel Faure: Late orogenic carboniferous extensions in the Variscan French Massif Central . In: Tectonics . tape 14 (1) . American Geophysical Union (AGU), 1995, p. 132-153 , doi : 10.1029 / 94TC02021 .
9. ↑ P. Hollinger, M. Cuney, M. Friedrich and L. Turpin: Age carbonifère de l'Unité de Brame du complexe granitique peralumineux de St-Sylvestre (NO du Massif Central) défini par les données isotopiques U-Pb sur zircon et monazite . In: CR Acad. Sci. Ser. 2, 303, 1986, pp. 1309-1314 .
10. ^ ^{A b} P. Rolin, C. Cartannaz, F. Salen, B. Delwaulle and N. Thalouarn: Carte géol. France (1/50 000), feuille Saint-Sulpice-les-Champs (666) . BRGM, Orléans 2006, p. 178 .
11. ↑ M. Chenevoy and J. Ravier: L'histoire des “gneiss d'Aubusson”, migmatites à cordiérite du Massif central français, d'après le chimisme de leurs grenats . In: Bulletin de la société géologique de France . 8 (V, n ° 2), 1989, p. 295-307 .
12. ↑ J. Lameyre et al .: Démonstration par sondage de la présence de Gneiss d'Aubusson sous les granites du batholite de Guéret (Massif central français) et de la nature tectonique du contact . In: CR Acad. Sci., Fr., sér. 2 . n ° 307, 1988, p. 2077-2083 .
13. ↑ M. Faure, P. Monié, C. Pin, H. Maluski and C. Leloix: Late Visean thermal event in the northern part of the French Massif Central: New 40Ar / 39Ar and RbSr isotopic constraints on the Hercynian syn-orogenic extension . In: Int. J. Earth Sci. tape 91 , 2002, p. 53-75 .
14. ↑ A.-M. Hottin et al .: Notice explicative, Carte géologique France (1/50 000), feuille Evaux-les-Bains (n ​​° 643) . BRGM, Orléans 1991, pp. 102 .
15. ↑ G. Ranchin: La géochimie de l'uranium et la différenciation granitique dans la province uranifère du Nord-Limousin . In: Sci. Terre, Mém., Fr. Band 19 , 1971, p. 394 .
16. ↑ C. Cartannaz and A. Cocherie: Notice explicative, Carte géol. France (1/50 000), feuille La Souterraine (641) . BRGM, Orléans 2012, p. 165 .
17. ↑ C. Cartannaz: Magmatismes et deformation polyphasés. Exemple of the Massifs de Guéret et de Millevaches (Massif central français). Origine des magmas et context de mise en place. Thesis. (PhD thesis) . Besançon 2006, p. 260 .
18. ↑ G. Sabourdy and P. Tempier: Caractère composite de la partie méridionale du massif granitique de Guéret (Massif central français) . In: CR Acad. Sci. t. 295 (series II), 1982, p. 1135-1138 .
19. ↑ ^{a b} J. Faure: Etude pétrographique de l'extrémité Nord Ouest du granite de Guéret et de ses enclaves. DES (diploma thesis) . Clermont-Ferrand 1963, p. 56 .
20. ↑ H. Downes, A. Shaw, BJ Williamson and MF Thirlwall: Sr, Nd and Pb isotopes of Hercynian granodiorites and monzogranites, Massif central, France . In: Chemical Geology . n ° 136, 1997, p. 99-122 .
21. ↑ A. Gébelin: Déformation et mise en place des granites (360-300 Ma) dans un segment de la Chaîne varisque (plateau de Millevaches), Massif central. Thèse . Université Montpellier II, 2004, p. 235 .
22. ↑ O. Laurent: Le sondage de Créchat-les Sibieux, apports à la connaissance géologique de l'Ouest du Massif central français, Thèse Nancy (PhD) . 1988, p. 335 .
23. ^ R. Freiberger, L. Hecht, M. Cuney and G. Morteani: Secondary Ca-Al silicates from Mid-European Hercynian granitoids: Implication for the cooling history of granitic plutons . In: Contrib. Mineral. Petrol. tape 141 , 2001, pp. 415-429 .
24. ↑ O. Jover: Les massifs granitiques de Guéret et du nord Millevaches (Nord du massif central Francais). Analysis of structurale et modèle de mise en place (doctoral thesis). Univ. Nantes, 1986, p. 233 .
25. ^ L. Turpin et al .: Meta-igneous origin of peraluminous granites in NW French Massif Central - implication for crustal history reconstitutions . In: Contributions to Mineralogy and Petrology . tape 104 , 1990, pp. 163-172 .