GEOLOGICAL HISTORY OF THE MORVAN

GLOSSARY

A * B * C * D *E* F * G * H * I * K * L * M * N * O * P * R * S * T * U *V * W

A

Accretion: accretion prism : part of new sediments adds to the edge of the continentale crust.

Acidic: acidic rock, plutonic or volcanic rock having a silica content (SiO2) greater than 66 %.

Active continental margin: edge of continent, which located above a subduction plane or zone, is the site of an importante magmatic activity: plutonism and volcanism.

Amphibolite: mesozonal metamorphic rock composed of amphibole (hornblende) and plagioclase.

Anatexis: process of partial melting of the deep metamorphic rocks leaving to a granitic magma. Anatectite: rock produces by this process.

Andesite: volcanic rock, mesocratic, with a microlitic or porphyritic texture, composed of plagioclase (oligoclase/andesine), amphibole (green hornblende) pyroxene (augite) in a cryptofelsitic groundmass. In the paleovolcanic rocks, this groundmass contains secondary minerals : chlorite, epidote and opaque minerals.

Anticline: fold with an arch shape where geologic stratas dip in opposite direction from axis arch.

Autunian: Early Permian age ranging from 295 Ma to 270 Ma.

B

Back-arc: back-arc basin: basin with an oceanic floor open back to an active islanc arc.

Basalt: basic volcanic rock, melanocratic, with a microlitic or porphyritic texture, composed of calcic plagioclase (andesine/labrador), or pyroxene (augite) in a cryptofelsitic groundmass. In the paleovolcanites this groundmass contains secondary minerals (chlorite, epidote and opaque minerals).

Basement: group of crystalline or crystallophyllian rocks doing the background on which ly trangressi and/or unconformably overlie younger sedimentary series.

Basic: basic rock, plutonic or volcanic rock having a silica content (SiO2) comprised between 45% and 52 %.

Batholith: more or less well-shaped body of large dimensions of plutonic rocks, mainly of granitic kind.

Bimodal: concerns volcanic series only composed of acidic and basic rocks, intermediate rocks been lacking.

Breccia: rock composed of angular rock fragments with a size greater than 2 mm.

C

Calc-alkaline : magmatic series found in orogenic zones and characterized by presence of feldspar and amphibole or calcic augite (non-sodic).

Cambrian: Early Paleozoic period ranging from 540 Ma to 500 Ma.

Carboniferous: Late Paleozoic period ranging from 360 Ma to 295 Ma.

Catazone: metamorphic zone where T > 800°C and 3 kbs < P < 10 kbs and characterized by the granulite facies.

Cenozoic: geologic era ranging from 65 Ma to present times. Comprises Tertiary ande Quaternary.

Clastic: detritical rock composed of angular or more or less rounded rock or mineral fragments deriving from degradation of older rocks, mainly by erosion.

Cobble: rock fragment rounded under water action (sea, river), size ranging from 2 mm to 25 cm. Main components of conglomerates.

Cogenetic: rocks having same genetic conditions.

Comagmatic: rocks deriving from the same magma.

Conglomerate: consolidated clastic rock composed of rounded fragments (pebbles and cobbles) of various rocks having a size greater than 2 mm.

Continental crust: part of the Earth crust extending above the Moho and mainly composed of granitic rocks and constituting continents; thickness from 20 to 60 km.

Crystallophyllian: character of metamorphic granular rocks having a foliated or schistose fabrics outlined by micas rich layers.

Culm: particular facies of Dinantian characterized by the association of schists, sandstones, graywackes, rare limestones and acidic volcanic rocks (lava and tuff).

D

Dacite: volcanic rock, mesocratic, with a microlitic or porphyritic texture, composed of quartz, plagioclase (andesine), biotite, amphibole (green hornblende) in a vitric groundmass. In paleovolcanics this vitric groundmass is recrystalized with chlorite, epidote....

Deposit: bed, lens or mass of peculiar material; ex. coal deposit, clay deposit, fossiliferous deposit.

Devonian: Middle Paleozoic age ranging from 410 Ma to 360 Ma.

Dextral: clockwise displacement of the compartments along a fault.

Dike: geologic structure deriving from fracture filled by foreign material and younger than country rocks.

Dinantian: Early Carboniferous age ranging from 360 Ma to 325 Ma.

Diorite: plutonic rock, mesocratic, composed of plagioclase and amphibole crystals.

Dolerite: fine-grained hypabyssal eruptive rock, having a basaltic composition.

E

Eclogite: catazonal metamorphic rock deriving from basalt, composed of green pyroxene (diopside altered in amphibole) and kelifitized garnet (amphibole and plagioclase alteration).

Epizone: metamorphism zone where 400°C < T < 500°C and 2 kb < P < 7 kb and characterized by green schist facies.

Epicontinental: concerns a sedimentation or a platform lying on a continental crust.

Epirogenic: which is related to epirogenese: large-scale movements, uplift or sinking of continental blocks.

Erosion: physical and/or chemical degradation process of the rocks under action of water, wind, freezing...

Eruptive: or igneous rock: rock deriving from magma consolidation; comprise plutonic rocks and volcanic rocks.

Extension: surface enlargement of the lithosphere under tectonic strengths. Contrary: compression.

Extrusion: eruption of viscous lava mass above the volcano vent.

F

Facies: group of petrographic, lithologic, and paleontologic characters which define a geologic unit.

Famennian: Late Devonian age ranging from 365 Ma to 360 Ma.

Fault: fracture with size ranging from a rock mass to a part of the Earth crust and characterized by relative displacement of the two compartments located on each side of the plane of fracture. Faults with vertical displacement: normal or reverse; faults with horizontal displacement: strike slip fault, dextral or sinistral.

Fold: structure shown by geologic layers deformed under tectonic strengths and giving succession of synclines and anticlines of various sizes (few meters to several kilometers).

Folding: deformation process of the geologic beds giving way to folds structures.

Foliation: structure of metamorphic granular rocks having layers rich in micas flakes.

Fossil: animal or plant relics remaining in geologic beds.

Flow: volcanic flow: outpour of low-viscosity lava, flowing away from the volcanic vent.

Fracturation: dislocation process, breaking of rocks giving way to faults.

Frasnian: Late Devonian age ranging from 375 Ma to 365 Ma.

G

Gabbro: plutonic, melanocratic, granular rock, composed of calcic plagioclase, pyroxene and peridot.

Geochemistry: Earth science section dealing to studies of abundance and repartition of chemical elements in rocks of the Earth crust.

Geochronology: Earth science section dealing to the absolute datations of rocks and minerals with isotopic methods.

Geodynamics: Earth science section dealing to studies of deformations of lithospheric plates: sea-floor spreading, continental drift, mountains building.

Geophysics: Earth science section dealing to the studies of physical characters of rocks and Earth globe constitution.

Geotectonic site: large-scale structure whose morphology and evolution are related to the lithosphere deformations. Among the main types of sites: mid-ocean rigde, active continental margin, island arc, intra-continental rift .

Gneiss: granular metamorphic, mesozonal rock composed of quartz, feldspars, micas, amphibole. Paragneiss: rock deriving from sediments; Orthogneiss: rock deriving from eruptive rocks

Graben: regional scale collapse trench, normal faults bounded.

Granite: granular plutonic, leucocratic rock composed of quartz, feldspatrs, micas.

Granoblastic: texture of metamorphic rock composed of minerals grains having the same size.

Granodiorite: granular plutonic, leucocratic rock composed of quartz, feldspatrs, micas, amphibole.

Granophyre: hypabyssal eruptive rock with a mineral and chemical composition of granite and characterized by a peculiar texture: micropegmatitic or micrographic a symplectic association of quartz and feldspars.

Granulite: granular metamorphic, catazonal rock, composed of less hydrous minerals: garnet, cordierite, hypersthene.

Graywacke: rock composed of angular grains of quartz, feldspars and various rocks enclosed in a weakly metamorphosed argillaceous matrix. Metagraywacke: metamorphosed graywacke.

H

Hercynian: initially dealing to folded and faulted structures NE-SW oriented of the German Harz; distinct from the NW-SE Armorican strikes.

Horst: uplifted regional size block and faults bounded.

Hypabyssal: dealing with eruptive rocks emplaced at shallow depths, often related to volcanic activity.

I

Igneous: or eruptive rock: rock deriving from magma consolidation; comprise plutonic rocks and volcanic rocks.

Ignimbrite: tuff whose acidic lava fragments are welded by consolidation of ash flows.

Intermediate: intermediate rock: plutonic or volcanic rock having a silica content (SiO2) comprised between 45 and 66 %.

Intrusion: emplacement process of a magma mass, concerns also the plutonic body.

Island arc : archipelago of volcanic islands built above a subduction zone or plane.

K

Keratophyre: intermediate or acidic (quartz-keratophyre) volcanic rock containing sodic plagioclase, similar to soda-dacite and soda-rhyolite. Spilites-keratophyres are encountered in orogenic zones with calc-alkaline suite rocks.

L

Lamprophyre: hypabyssal eruptive rock. Two types are encountered in Morvan region, minette with abundant biotite and vogesite with augite or hornblende.

Lapilli: from Italian word meaning small stone; lava fragment main component with crystals of volcanic tuffs.

Lava: molten matter more (acidic lava) or less (basic lava) viscous, pour out from volcanoes; concern also consolidated rocks : lava flow.

Leptynite: quartz rich metamorphic rock deriving from eruptive rock (rhyolite, microgranite) or volcano-sedimentary rock; granoblastic texture with quartz, microcline, albite, garnet.

Leucocratic: concern rocks where light colored minerals (quartz, feldspar) predominate on dark colored minerals (amphibole, pyroxene, peridot).

Leucogranite: light colored granite with predominant muscovite and subordinate biotite or amphibole.

Lithology: concern petrographic characters of sedimentary rocks.

Lithosphere: rigid upper part of the Earth crust, comprises oceanic crust and continental crust.

Lydienne: black and consolidated sedimentary rock composed of siliceous radiolarian tests.

M

Magma: partly or fully molten silicates mass of lithosphere deep parts (oceanic crust and continental crust); this gas rich magma intrude through the lithosphere, accumulates in magmatic chambers related or not to volcanic eruptions.

Magmatic series: group of eruptive rock with various compositions but deriving from a basaltic parental magma essentially by fractional cristallization.

Magmatic suite: eruptive rocks unit showing, for a bounded region and time span, common mineralogical and chemical characters that are strictly linked to the geotectonic site type.

Mantle: part of the Earth globe located between the lithosphere and core, 2 900 km thick and composed of fluid silicated magma.

Marble: calcareous rock composed of grains or crystals of calcite or dolomite, recrystallized limestone.

Melanocratic: concern rocks where dark colored minerals (biotite, amphibole, pyroxene, peridot) predominate on light colored minerals (quartz, feldspar).

Mesocratic: concern rocks where light colored minerals (quartz, feldspar) and dark collored minerals (amphibole, pyroxene, peridot) are in same proportions.

Mesozoic : (Secondary) geologic era ranging from 245 Ma to 65 Ma.

Mesozone: metamorphism zone where 500°C < T < 750°C and 2 kb < P < 9 kb and characterized by the amphibolite facies.

Metamorphism: physical and chemical process that modifies the mineralogical assemblage of rocks under pressure and temperature increase when buried at depth. Three zones of metamorphism: epizone, mesozone and catazone.

Microgranite: hypabyssal eruptive rock with a mineralogical and chemical granite composition.

Microgranulite: old denomination for Late Visean rhyolites and microgranites; equivalent to tufs anthracifères.

Microlitic: texture of volcanic or hypabyssal rocks characterized by abundant small and acicular feldpars crystals: the microlites; appear generally in the last stage of cooling and crystallization of the magma.

Migmatite: rock deriving from partial melting of gneiss and intimately mixed with this country rock; composed of quartz, feldspars and micas.

Moho: or Mohorovicic discontinuity is the boundary between the lithosphere (continental crust + oceanic crust) and the mantle.

Molasse: geologic formation composed of coarse clastic deposits deriving from the erosion and degradation of a mountain belt during the late building stages.

Mylonite: fine-grained rock, with a more or less ribbony texture, deriving from intense crushing of country rocks inside deep-sited faults or overthrusts.

N

Namurian: Late Carboniferous stage ranging from 325 Ma to 315 Ma.

Nappe: Large-scale tectonic structure (several dozen of km) in which a group of geologic units overthrusts with abnormal and sub-horizontal contact another group of geologic units.

O

Obduction: overthrusting of the oceanic crust on the continental crust during orogeny.

Oceanic crust: part of the Earth crust, the lithosphere, extending under oceans and located above the Moho; mainly composed of basic rocks (basalt then gabbro) and ultrabasic rocks (peridotite) and constituting sea floor; thickness from 5 to 8 km.

Ophiolite: association of basic (basalt, dolerite, gabbro) and ultrabasic rocks (peridotite) deriving from obduction of oceanic crust.

Outcrop: occurence to the ground surface of bedrock (rocky or not) representative of the geologic background.

Ordovician: Early Paleozoic age ranging from 500 Ma to 435 Ma.

Orogen: mountains belt taken in the meaning of regional unit and time period of the geological history.

Orogeny: mountains building process.

Orthogneiss: gneiss deriving from eruptive rocks

Orthophyre: old name for trachyte.

Overthrust: tectonic structure in which one compartment overlaps the other. Emphasizing of a reverse fault giving way to an intermediate structure betwen this type of fault and a tectonic nappe.

P

Paleontology: Earth science section dealing to the studies of fauna and flora fossils.

Paleovolcanic: Paleovolcanics, paleovolcanic rocks : characters of old volcanic rocks having undergone varions degrees of alteration.

Paleozoic: (Primary) geologic era ranging from 540 Ma to 245 Ma.

Paragneiss: gneiss deriving from sedimentary rocks.

Pelite: rock composed of very small clay particles of silt size (0.05 to 0.005 mm). Metapelite: metamorphosed pelite.

Peridotite: deep eruptive rock composed of olivine and rare pyroxenes.

Permian: Late Paleozoic age ranging from 295 Ma to 245 Ma.

Petrography: Earth science section dealing to the studies of rocks, mainly descriptive.

Petrology: Earth science section dealing to the studies of rocks, mineralogical and chemical compositions, genesis.

Phyllade: bedded and foliated metamorphic and epizonal rock. Quartzo-phyllade: silica rich phyllade.

Pillow-lava: structure shown by lavas, mainly basaltic, flowed in water (lave en coussinets for French geologits).

Plate tectonics: displacement mechanism of lithosphere fragments (oceanic crust and continental crust) under the action of mantle convection currents. The lithosphere deformations induced by plate tectonics create various geotectonic sites.

Platform: continental platform: continental crust margin covered by sea and extending between shoreline and oceanic crust domain.

Plutonic: eruptive or igneous rock consolidated at depth.

Porphyre: old denomination for volcanic or hypabyssal porphyritic rock; ex. Green antic porphyry (andesite), red porphyry (microgranite) or granite porphyry.

Porphyritic: igneous rocks texture characterized by large euhedral phenocryts belonging to a first stage of crystallization (quartz, feldspars, feldspathoides, amphibole, pyroxene, peridot).

Precambrian: geologic era ranging from 3400 Ma to 540 Ma. Comprise Archean from 3400 Ma to 2500 Ma and Proterozoic from 2500 Ma to 540 Ma.

Protolith: primordial rock subsequently mineralogically modified (ex. by metamorphism).

Pseudoporphyritic: plutonic rocks texture, especially granite and characterized by large euhedral feldspars crystals.

Pyroclastic: general term for fragmented matter ejected by volcanoes: lava blocks, volcanic bombs, ashes (crystals, lapilli, volcanic glass).

Q

Quartzite: sandstone composed of quartz grains firmly cemented by silica.

R

Radiometric: radiometric dating: determination method of absolute age of minerals or rocks crystallisation or of subsequent events (ex. metamorphism). Method based on isotopic analyses of radioactive elements and their end members (K/Ar, Rb/Sr, U/Pb...)

Regression: progressive retreat of the sea on a continental shelf.

Retrograde: concerns metamorphic grade with temperature and pressure conditions lower than previously undergone by the metamporphic rocks. Contrary : prograde.

Rhyolite: volcanic leucocratic rock with a granitic chemical composition. Composed of quartz and feldspars phenocrysts, biotite flakes in a vitric groundmass recrystallized in paleovolcanic facies.

Rhyodacite: volcanic leucocratic rock with a chemical composition between rhyolite and dacite.

Rift: continental scale collapse trench, normal faults bounded.

S

Sandstone: clastic rock mainly composed of quartz and feldspars grains; cement can be siliceous, argillaceous or calcareous. Grains size ranges from 0.05 mm to 2 mm.

Saxonian: Early Permian age ranging from 270 Ma to 258 Ma.

Schist: metamorphic epizonal fine-grained and foliated rock.

Sea-floor spreading: surface and volume increase of the oceanic crust by new magma upwelling at the site of the mid-ocean ridge.

Sediment: sedimentary rock, deposit deriving from the accumulation solid fragments (ex. sandstones, clay, limestone) or chemical deposit (limestone, salt, gypsum...).

Serpentine: altered peridotite.

Sinistral: anti-clockwise displacement of the compartments along a fault.

Siltite: consolidated sedimentary rock composed of small clastic particles of silt size (0.05 to 0.005 mm)

Silurian: Middle Paleozoic age ranging from 435 Ma to 410 Ma.

Spilite: basic volcanic rock with primary minerals: calcic plagioclases and pyroxenes (augite) altered respectivelly to albite-oligoclase and actinote, chlorite and epidote. This alteration results from interaction with seawater during lava extrusion. The spilite-keratophyre association is often encountered in orogenic zones with rocks of the calc-alkaline suite.

Stephanian: Late Carboniferous age ranging from 305 Ma to 295 Ma.

Stock: plutonic rocks mass, more or less well shaped and of smaller size than a batholith.

Strata: bed, layer of homogeous kind of rock.

Stratification: characters of sedimentary rocks being a succession of horizontal or inclined stratas.

Stratigraphy: Earth science section dealing to studies of chronological succession of terranes deposited along geologic times, mainly based on fossils study.

Subduction: sinking of the oceanic crust in the upper mantle along an inclined plane: Benioff plane or subduction zone.

Subsidence: progressive sinking of a basin under tectonic strengths often accompanied with a more or less continuous sedimentary filling.

Sub-alkaline: one of the magmatic associations presented by the eruptive or igneous rocks and characterized by monzonitic granite or monzogranite (divided in magnesio-potassic monzogranite and in ferro-potassic monzogranite ). Other associations being the calc-alkcaline association (mainly adamellite), the alumino-potassic association characterized by muscovite and cordierite leucogranites, the alkaline association (sodic amphibole granite) and the tholeiitic association.

Syncline: fold structure with a trough shape where geologic stratas dip toward the trough axis.

T

Tectonics: Earth science section dealing to studies of the rocks deformation (folding and faulting) under the strengths generated by lithosphere displacements.

Tholeiite: basalt composed of calcic plagioclase, augite-pigeonite or hypersthene and few or without olivine. Tholeiitic basalts occur in mid-ocean ridges, island arcs and huge intra-continental basalts plateaus.

Tournaisian: Early Carboniferous ranging from 360 Ma to 350 Ma.

Trachyte : volcanic leucocratic rock, without quartz crystals and containing potassic feldspar.

Trachyandesite: volcanic rock with a composition comprised between trachyte and andesite, where plagioclase predominates on potassic feldspar.

Trangression: progressive spread of the sea on a continental shelf.

Tuff: consolidated rock deriving from volcanic ashes and composed of crystals, lapilli and glass shards.

Tuff-lava: described an outpouring (probably sub-aqueous) of volcanic rock composed of abundant fragments of pumiceous basic to intermediate lavas whose emplacement is similar to ignimbrite.

Tufs anthracifères: old denomination proper to the French Massif central and for Late Visean volcanic rocks also known as microgranulitic tuffs.

Turbidity current: sub-aquatic current induced by gravity flowage of water containing suspended sediments.

U

Ultrabasic: plutonic or volcanic rock having a silica content (SiO2) less than 45 %.

Unconformity: break in the succession of geologic stratas owing to the lack of sedimentation, the erosion or tectonic movements.

V

Variscan: concern the Western Europe orogenic belt and its geodynamic evolution during Paleozoic times.

Visean: Early Carboniferous age ranging from 350 Ma to 325 Ma.

Volcanic: concern all phenomenons related o the volcanoes activity.

Volcanism: volcanoes activity.

Volcanic breccia: volcanic rock composed of lava fragments. Several origins, for example: flow breccia containing already solidified fragments of lava (auto-breccia); extrusion breccia resulting of semi-solidified magma uplift in the volcano vent; slope breccia on the volcano flanks...

Volcano-sedimentary: concerns the association of volcanic rocks (lavas and tuffs) interbedded with sedimentary rocks.

W

Westphalian: Late Carboniferous age ranging from 315 Ma to 305 Ma.

 

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