Limousin Tonalit Line

The Limousin tonalite line is a belt-like string of tonalitic , quartz dioritic and related rocks in the northwestern Massif of Central France . The rocks were formed between the Upper Devonian and Lower Carboniferous . They differ from the other granitoids of the Massif Central in their predominantly intermediate chemistry and their richness in hornblende . After the main phase of the Variscan regional metamorphosis, they are the first igneous secretions and geochemically resemble andesitic lavas with a calcareous affinity.

designation

The term Limousin-Tonalitlinie (abbreviated LTL ), French Ligne tonalitique du Limousin , was first introduced into the scientific literature in 1971 by J. Didier and J. Lameyre. Previously, the rocks were called the quartz diorite massifs of the Limousin .

Geography and geology

The northwest-southeast trending Limousin-Tonalit line extends over a distance of 220 kilometers from L'Isle-Jourdain in the Vienne department to Capdenac in the Lot department . It contains a total of 25 larger massifs of igneous origin, the rocks of which occur predominantly in the upper gneiss cover .

Their line-up begins on the northwestern edge of the Massif Central with the grouping around Saint-Barbant (with the Saint-Barbant quartz diorite , the Availles-Port-de-Salles granodiorite and the Oradour-Fanais quartz monzodiorite ), then the grouping follows in a southeastern direction east of Confolens (with the Lacouture quartz diorite , the La Gouyonnerie tonalite , the Saint-Quentin quartz diorite and the Saulgond tonalite ). Several smaller, scattered bodies can be found on the Vienne in the vicinity of Saint-Junien , such as the Saint-Brice-sur-Vienne-Tonalit , the Saillat-sur-Vienne-Tonalit (with smaller occurrences of associated quartz diorites and diorites ), the Saint Junien quartz diorite and the La Bregère quartz diorite . To the south of Limoges are the Aixette-Nexon quartz diorite , the Saint-Jean-de-Ligoure diorite and the small Lalet diorite . The Les Cars granite also contains smaller quartz diorite inclusions. Further south in Bas-Limousin is the Saint-Julien-le-Vendomois quartz diorite .

Further to the southeast there is a continuation into the Corrèze with tonalites near Tulle and north of Brive-la-Gaillarde as a frame around the Tulle anticline ( Ladignac massif , Lagraulière-Tonalit , Sainte-Féréole-Tonalit and Tulle-Gabbro with hornblendite ). Further south follow the Beaulieu tonalite near Beaulieu-sur-Dordogne and the two deposits near Saint-Céré ( Anglars quartz diorite and Lagineste tonalite ). Finally, the southernmost end point is a tonalite deposit south of Capdenac ( Capdenac-Tonalite ).

There are also deposits slightly off this main line in the lower gneiss cover . Examples are the Exideuil quartz diorite and the Mazières quartz diorite . Smaller tonalite and quartz diorite bodies are also included in the porphyry Chirac-Étagnac granite .

The tonalite belt is not limited to the Limousin and the western edge of the Massif Central, but is also extended to the northwest via the Charroux-Civray massif, hidden under Mesozoic sediments in the subsoil of the Aquitaine basin ( Seuil du Poitou ) into the Vendée department near Parthenay - Examples are the Le Tallud quartz diorite and the Moncoutant quartz diorite in the Haut Bocage . If these massifs of the Armorican Massif are added, the Tonalit line reaches a total length of 400 kilometers and thus represents one of the most important igneous lineaments of the Variscan orogen.

geomorphology

Grus formation and spherical weathering forms can occasionally be observed.

Petrology

Mazières elongated quartz diorite. Relatively little stressed quartz diorite above the coin. Underneath, a shear gap filled with calcite, still open in places. Including very heavily sheared quartz diorite.

The massifs of Tonalitlinie form predominantly Lakkolithkörper , often concordant to foliation run the metamorphic host rocks. Also Phakolithe are to be found which are either as an anti-form (for example, the occurrence of the tulle anticline), or organized as but Synform (Saint-Julien-le-Vendomois-quartz diorite).

Often oval amphibolite enclaves and rarer gneiss inclusions can be found inside the rock . The rock bodies can be injected from medium-grain, random or regulated granites ; pink pegmatite veins are also common.

The relatively dark rocks of the tonalite belt are characterized by their intermediate to basic chemical composition. Quartz diorites and tonalites predominate, diorites are relatively rare. Their petrological character is always calcareous .

mineralogy

The medium to coarse-grained structure (grain diameter 1 to 10 millimeters) shows a preferred orientation of the mineral grains, which is predominantly concordant with regional foliation and lineation (orthogneissification). The dark rock color is caused by black mica lamellae ( biotite ) and black to green, prismatic or fibrous amphiboles ( hornblende ). In addition, there are light minerals such as shiny to milky plagioclase and milky-glassy quartz . The plagioclase can take on a pink color due to potassium substitution in micro-granite inclusions and pegmatite veins. Honey-brown titanite , allanite and sulfide minerals such as pink pyrrhotite and golden-yellow chalcopyrite should be mentioned as accessories .

Tonalite

In addition to the occurrences mentioned at the beginning, the medium-grain tonalites often form lenticular layers in the much more common quartz diorites. Their original, hypidiomorphic grain structure is mostly overlaid by a regulated, granoblastic structure. This orthogneissification is often only very weakly developed, but it can progress to the point where the original structure is completely erased. The tonalites are by definition very rich in quartz and contain a lot of biotite and little or no hornblende. The quartz appears in layers, recrystallized and shows clear stretching when orthogneissification begins. Traces of potassium feldspar are very rare. Endogenous and exogenous transformations of the tonalites are sericization and sometimes saussuritization of the plagioclase as well as discoloration and / or chloritization of the biotite.

Quartz diorite

They are by far the most common type of rock. Their structure regulation is even more clear than with the tonalites. With them, too, the primarily hypidiomorphic grain structure is superimposed / replaced granoblastically or granonematoblastically. The plagioclase is a (sometimes oscillating) zoned andesine (An _40-50 ). The mostly xenomorphic quartz occurs in mono- or polycrystalline, layered accumulations, shows indolent extinction and recrystallization. It can also be present as inclusions in plagioclase and hornblende. The green hornblende (MgO 10.5%, FeO 16.7%) forms subautomorphic accumulations, when orthogneissification begins, blastic and poikiloblastic structures develop. The very pleochroic biotite appears in places as kink lamellae with very rare zirconium inclusions . The chloritization of the biotite can be very advanced with the formation of new epidote and prehnite ; Occasionally, almond-shaped potassium feldspar is formed, pushing the mica layers apart. Accessory minerals, mostly in association with the Fe-Mg minerals, are magnetite and / or ilmenite , sulfides, titanite, apatite , allanite and, more rarely, zircon.

Percentage composition (vol .-%):

• Quartz: 0-16%
• Plagioclase: 20-82%
• Microcline: 0-4%
• Hornblende: 6–31%
• Biotite: 8-25%
• Accessories: 4%

Diorite

The very rare diorites are homogeneous, fine-grained, meso- to melanocrate rocks that have very little or no quartz. A structure regulation can be seen very indistinctly. They usually appear sporadically as trains within the quartz diorite. Separate hornblendit layers with centimeter-sized, automorphic, green hornblinds can also be present within these dioritic lines . The mineral inventory of the diorite includes plagioclase, biotite, green hornblende, titanite, apatite as well as accessory oxides and sulfides. The plagioclase are xenomorphic to polygonal. Occasionally there are still pseudomorphoses from Hornblende to Pyroxen . The biotite is also chloritized or prehnitized in the diorites.

Percentage of mineral content (vol .-%):

• Hornblende: 47%
• Plagioclase: 34%
• Biotite: 15%
• Accessories: 4%

Chemical composition

Main elements

Below are several, partly averaged chemical analyzes of rocks in the tonalite belt:

Oxide wt.%	Availles-Port-de-Salles Granodiorite	Chirac-Etagnac Tonalite	Saint-Barbant quartz diorite	Saint-Brice-sur-Vienne Tonalit	Saulgond quartz diorite	Quartz diorite 6 analyzes	Anglars quartz diorite	La Guyonnerie Diorite	Diorite 3 analyzes	Saint-Quentin diorite	Lacouture diorite
SiO 2	69.00	67.24	60.07	55.99	55.18	54.29	52.94	50.98	49.89	49.80	49.08
TiO 2	0.32	0.40	0.82	1.06	1.06	1.29	1.25	0.75	1.47	1.96	1.30
Al 2 O 3	15.90	16.61	17.23	17.64	17.32	17.31	19.77	19.20	18.72	16.75	18.90
Fe 2 O 3	2.80 dead	1.07	6.57 dead	7.74 dead	8.55 dead	2.46	2.61	8.51 dead	2.70	11.08 dead	9.89 dead
FeO		2.84				6.55	5.82		7.16
MnO	0.06	0.09	0.13	0.13	0.15	0.11	0.07	0.15	0.18	0.22	0.16
MgO	0.91	1.74	2.93	3.63	4.54	4.01	3.09	6.29	4.28	6.00	4.64
CaO	2.60	4.33	5.82	7.14	7.65	7.16	7.22	9.31	8.20	9.00	8.01
Na 2 O	3.55	3.38	3.44	3.42	3.43	3.36	3.90	2.76	3.64	3.18	3.62
K 2 O	4.19	1.69	1.94	1.57	1.56	1.59	1.28	1.26	0.78	0.95	1.62
P 2 O 5	0.13	0.11	0.13	0.26	0.10	0.27	0.25	0.09	0.47	0.40	0.51
dehydration	1.00	0.63	0.82	1.00	1.06	1.07	0.90	1.02	1.02	0.90	1.02

For the main elements, the SiO ₂ content in the rocks of the tonalite line varies between 49 and 69 percent by weight, so they are essentially intermediate rocks. The sum of the alkalis Na ₂ O + K ₂ O fluctuates mainly between 4 and 5 percent by weight - typical for sub-alkaline main series rocks. On the basis of the K ₂ O content of 0.78 to usually not more than 2 percent by weight, a calcareous character of the middle K series can be recognized. The mafic diorites (around 50 percent by weight SiO ₂ ) sometimes have very high concentrations of TiO ₂ , MnO, total iron, MgO, CaO and P ₂ O ₅ . The Na ₂ O contents are consistently high and indicate a possible spilitization of the rocks.

Trace elements

Trace elements ppm	Chirac-Etagnac Tonalite	Saint-Brice-sur-Vienne Tonalit	Saint-Junien diorite	Quartz diorite 6 analyzes	Diorite 3 analyzes
Ba	597	667	996	741	444
Co	6th	70	13	56	61
Cr	43	31	71	52	70
Hf	2.3		5.4	5.4
Li	50		69	69
Nb	4.8		11.6	11.6
Ni	12	15th	12	13	29
Rb	52	49	50	43	27
Sr	372	604	531	596	685
Pb	15th		10	10
Th	7.2		2.6	2.6
U	3.8		0.8	0.8
V	49		272	272
Y	7.2		29.2	29.2
Zn	69		139	139
Zr	88		268	268

The trace element analyzes are unfortunately somewhat incomplete. However, in comparison with island arc andesites, they reveal relatively high values ​​for the elements barium , strontium , vanadium and in some cases also for cobalt , niobium , zinc and zirconium . However, the analytical values ​​determined are generally very similar to the concentrations found in andesites of continental subduction zones. Nickel , uranium and thorium are somewhat depleted.

Isotope ratios

Shaw and colleagues (1993) state the following isotope ratios for the Limousin tonalite line :

• Compared to other granitoids of the Massif Central, relatively low ⁸⁷ Sr / ⁸⁶ Sr ratios, ranging between 0.7047 and 0.7059
• relatively high εNd between - 0.7 and + 1.8
• the initial lead ratios are quite homogeneous with ²⁰⁶ Pb / ²⁰⁴ Pb = 18.15–18.38, ²⁰⁷ Pb / ²⁰⁴ Pb = 15.57–15.62 and ²⁰⁸ Pb / ²⁰⁴ Pb = 38.06–38.26. Here the initial ²⁰⁷ Pb / ²⁰⁴ Pb values ​​are significantly lower than in other granitoids of the variscical.

Overall, the isotope ratios encountered indicate island arc magmatism or the segment of an active continental margin.

Regional overview

The individual tonal occurrences are lined up in a slightly curved, arched segment of a circle, which roughly follows the north-western edge of the Massif Central. The radius of curvature is roughly the same as that of the right-shifting, south-east- trending South Armorican Shear Zone (SASZ) - an important terrestrial boundary of the Armorican Massif , which ends about 30 kilometers further west in the Aquitaine Basin . Some tonalitic occurrences, such as the Saint-Barbant quartz diorite, the Lacouture quartz diorite and the Aixette-Nexon quartz diorite, are closely related to the Oradour-sur-Glane fault , another important, southeast-facing, dextral lateral shift to the northeast the SASZ. The Saulgond quartz diorite is captured internally by a SE-trending lateral shift and shifted to the right. This relatively short Scherbahn lies parallel to the Oradour-sur-Glane fault, which runs about 10 kilometers to the northeast.

The tonalite belt ends at Figeac , where it is cut off by the Sillon houiller , the most important NNE-trending fault zone in the Massif Central.

Emergence

Strongly tectonically stressed Mazières quartz diorite with dark amphibolite inclusions and armor stripes dipping backwards at the upper edge of the picture

The origin of the Limousin tonal line is still not completely clear. There are two explanations: The older one from 1971 assumes an andesitic arch of the island , the younger one sees the rocks as anatectic melting products of amphibolite-bearing paragneiss during the mediovariszischen regional metamorphosis in the Massif Central.

Island arc

According to this hypothesis, the tonalite belt represents the course of a former subduction zone , where Aquitania (Aquitanian microplate) plunged in a northeastern direction under the Central European continent. Analogous to today's subduction zones, the tonalite rocks represent the plutonic root area of ​​a Devonian-Sub-Carboniferous, Andesitic island arc , which was created above the subducting Aquitanian microplate.

Shaw and colleagues (1993) assume a depleted mantle magma that was modified by the subduction of oceanic crust and terrigenous sediments. These primitive quartz diorite magmas were then further contaminated in the lower crust by an AFC process (assimilation combined with fractional crystallization) - whereby acid magmatites or metasediments were assimilated.

Anatexis

The anatexis hypothesis does not contradict the subduction model, but focuses on immediate geological / petrological conditions (for example, anatectic processes are also required in the subduction model to generate the island arc magmas). As early as 1957, M. Chenevoy had recognized the preferred structural regulation of tonalite rocks. In addition, the gradual transition from the oriented tonalite rocks to the surrounding paragneiss can be observed in the terrain, with the gneisses themselves showing the first signs of melt formation. Strictly speaking, the gneisses are metatexites in the contact area with the tonalites . M. Chenevoy explained this fact with the anatectic, in situ formation of the tonalite rocks within the gneiss. He assumed that they had arisen through melting from mafic amphibolite lenses embedded in the gneiss and thus represent foliation-concordant migmatites . Nevertheless, in addition to these progressive transitions, paradoxically, there are also clearly intrusive contacts, especially at the edge of larger tonalite rock massifs. The depth of education provides an explanation for this apparent contradiction. The tonalite intrusives were formed at a much greater depth than the migmatites, were able to migrate out of the rock formation and then intrude into higher crustal areas, whereas the oriented rocks remained in their much shallower starting position in the original rock formation.

The work of Clemens and Vielzeuf (1987) also speaks in favor of the anatectic model. The authors show that rocks with a high content of water-containing Fe-Mg minerals such as biotite or hornblende can be melted relatively easily by dehydration under favorable pressure-temperature conditions. The plagioclase-carrying paragneiss of the upper gneiss cover (and also the lower gneiss cover) are therefore well suited for anatexis, as they are very rich in biotite and amphibole. In their study of similar rocks in the Artense nappes of the Auvergne , Mercier et al. (1992) concluded that anatexis had already set in in the parent rock before the regional foliation formation began and that this continued during and after the deformation had subsided. Melt products are rocks containing hornblende (± biotite) with a quartz dioritic to tonalitic composition.

Age

Radiometrically determined ages on rocks of the tonalite belt range from 379 to 350 million years BP ( Upper Devon to Lower Mississippium ), i.e. H. they emerged towards the end of the Mediovarisian epoch. The Le Tallud quartz diorite in the Vendée, for example, was dated 373 + 6 / −11 million years ago (U / Pb method on zircon). New determinations or recalculations by Bertrand and colleagues (2001) gave the tonalite of the Charroux-Civray massif 360 ± 3 and 356 ± 5 million years, for the Availles-Port-de-Salles granodiorite 355 ± 5 million years, for the Saint -Jean-de-Ligoure diorite 354 ± 7 million years and for the Isle Jourdain tonalite 351 ± 6 million years.

In addition, neighboring rocks such as the Oradour-sur-Glane diatexite and the Lanneau diatexite have an age of 375 ± 6 million years BP (rubidium-strontium method), the somewhat younger Cieux-Vaulry granite is 352 ± 12 million years ago BP (also Rb-Sr).

Summary

The formation of the rocks of the tonalite belt took around 25 million years. They are an example of the dichotomy of magmatic / intrusive and dynamometamorphic forms of formation. Which process was ultimately decisive cannot be clearly decided; both processes were probably involved. Due to the favorable chemism of the paragneiss, the long-lasting anatexis was facilitated. The fact that tectonic movements played an important role in the development of the rocks is underlined by the spatial association of the tonalite belt with significant, dextral lateral shifts. Some rock bodies were severely deformed, for example the Mazières quartz diorite, which was intensely sheared in places and in which open shear fissures occurred, which in turn were then folded again. Other tonalite bodies such as the Exideuil quartz diorite, on the other hand, appear as very massive, dark, relatively slightly deformed quartz diorites.

literature

• Bernard-Griffiths et al: The tonalite belt of Limousin (French Massif Central): U-Pb zircon ages and geotectonic implications . In: Bull. Soc. Géol. Fr. (8), I, n ° 4, 1985, p. 523-529 .
• Chenevoy, M. et al.: Feuille Nexon (712) . In: BRGM (ed.): Carte géologique de la France à 1/50 000 . Orléans 1990, ISBN 2-7159-1712-0 .
• Chèvremont, P. et al .: Feuille Rochechouart (687) . In: BRGM (ed.): Carte géologique de la France à 1/50 000 . Orleans.
• Floc'h, J.-P. inter alia: Feuille La Rochefoucauld (686) . In: BRGM (ed.): Carte géologique de la France à 1/50 000 . Orleans.
• Peiffer, MT: La ligne tonalitique du Limousin - sa contribution à la connaissance de la géologie régionale . In: Annales scientifiques du Limousin . tape 3 , 1987, pp. 3-15 .
• Shaw, A., Downes, H. and Thirlwall, MF: The quartz-diorites of Limousin: Elemental and isotopic evidence for the Devono-Carboniferous subduction in the French Massif Central. In: Chem. Geol. Volume 107 , 1993, pp. 1-18 .

Individual evidence

1. ↑ ^{a b} Didier, J. and Lameyre, J .: Les roches granitiques du Massif Central . Ed .: Plein-Air Service: Symposium J. Jung. Clermont-Ferrand 1971, p. 133-155 .
2. ↑ Cuney, M., Brouand, M. and Stussi, JM: Le complexe plutonique de Charroux-Civray (Vienne): témoin du magmatisme infra-carbonifère dans le segment occidental de la chaîne varisque européenne. In: Géologie de la France . n ° 1-2, 2001, p. 143-166 .
3. ↑ Cuney, M., Brouand, M., Dautel, D., Stussi, JM, Michard, A., Gros, Y., Poncet, D., Bouton, P., Colchen, M. and Vervialle, JP: Géochimie et géochronologie U / Pb des diorites quartziques du Tallud et de Moncoutant: new arguments for an extension of the “Ligne Tonalitique Limousine” in Vendée. In: CR Acad. Sci., Fr. Band 316 . Paris 1993, p. 1383-1390 .
4. ↑ Michel Cuney, Marc Brouand and Jean-Marc Stussi: Le magmatisme hercynien en Vendée. Corrélations avec le socle du Poitou et l'ouest du Massif central français. In: Géologie de la France . n ° 1-2, 2001, p. 117-142 .
5. ^ A. Ewart: The mineralogy and petrology of Tertiary-Recent orogenic volcanic rocks: with special reference to the andesitic-basaltic compositional range . In: RS Thorpe (Ed.): Andesites: orogenic andesites and related rocks . tape 26 . Wiley, Chichester 1982, pp. 87 .
6. ↑ ^{a b c} Shaw, A., Downes, H. and Thirlwall, MF: The quartz-diorites of Limousin: Elemental and isotopic evidence for the Devono-Carboniferous subduction in the French Massif Central. In: Chem. Geol. Volume 107 , 1993, pp. 1-18 .
7. ↑ Clemens, JD and Vielzeuf, D .: Constraints on melting and magma production in the crust . In: Earth Planet. Sci. Lett. tape 86 , 1987, pp. 287-306 .
8. ↑ Mercier, L. et al.: Évolutions tectono-métamorphiques des nappes de l'Artense (Massif central français): nouveaux marqueurs de la collision dans la chaine varisque . In: Bull. Soc. géol. Fr. 163, n ° 3, 1992, p. 293-308 .
9. ^ Bernard-Griffiths et al .: The tonalite belt of Limousin (French Massif Central): U-Pb zircon ages and geotectonic implications . In: Bull. Soc. Géol. Fr. (8), I, n ° 4, 1985, p. 523-529 .
10. ↑ Jean-Michel Bertrand et al .: Géochronologie U-Pb sur zircons de granitoïdes du Confolentais, du massif de Charroux-Civray (seuil du Poitou) et de Vendée . In: Géologie de la France . n ° 1-2, 2001, p. 167-189 .
11. ↑ Duthou, JL: Chronology Rb-Sr et géochimie of granitoïdes d'un segment de la chaîne varisque, Relations avec le métamorphisme: le North Limousin (Massif central français) (thèse d'Etat) . In: Annales sci. univ. Clermont II . n ° 63, fasc. 30, 1977, pp. 294 .