The Velay dome (French The Velay dome (French Massif Central): Melt Massif Central): Melt

generation and granite generation and granite emplacement during emplacement during orogenic evolutionorogenic evolution

P. LedruP. Ledrua,*a,*, G.Courrioux, G.Courriouxaa, C. , C. DallainDallainbb, J.M Lardeaux, J.M Lardeauxcc, J.M , J.M

MontelMonteldd, O. Vanderhaegha, O. Vanderhaeghaee, G. , G. VitelVitelff

Meagan Webster 14528436

Geology G314

Introduction:Introduction:

Velay domeVelay dome Largest Granite- migmatite dome of the Largest Granite- migmatite dome of the

Variscan Belt located in the SE Massif Variscan Belt located in the SE Massif centralcentral

Formed during an orogenic collapse Formed during an orogenic collapse around 300 Maaround 300 Ma

Composed of peraluminous granites Composed of peraluminous granites characterized by nodular and prismatic characterized by nodular and prismatic cordierite and by enclaves of cordierite and by enclaves of gneissesgneisses and and granitesgranites of various sizes and natures of various sizes and natures

HOW it happened…HOW it happened…

The French Varsican belt resulted The French Varsican belt resulted from the collision between Laurussia from the collision between Laurussia and Gondwanaand Gondwana

North Gondwana continental margin North Gondwana continental margin represented by lower gneiss unit and represented by lower gneiss unit and sedimentary parautochlthonous sedimentary parautochlthonous sequence sequence

Southward thrusting and thickening Southward thrusting and thickening Velay was emplaced during this…Velay was emplaced during this…

3 3 mainmain structural zones are defined: structural zones are defined: The The host rockshost rocks intruded by the intruded by the

syntectonic granites precursor to of the syntectonic granites precursor to of the Velay domeVelay dome

The The gneiss-migmatite zonegneiss-migmatite zone, at the , at the periphery and roof of the Velay domeperiphery and roof of the Velay dome

The The cordierite bearing migmatite-cordierite bearing migmatite-granitegranite domain consisting of the most domain consisting of the most evolved pole of granitisation of the crust.evolved pole of granitisation of the crust.

Host rocks and periphery porphyric Host rocks and periphery porphyric granites of the Velay domegranites of the Velay dome

Host is primarily composed of rocks of Host is primarily composed of rocks of the lower Gneiss Unit: the lower Gneiss Unit:

(i) metasediments derived from pelites and (i) metasediments derived from pelites and argilitesargilites

(ii) augen orthogneiss originating from (ii) augen orthogneiss originating from peralumious peralumious porphyric granite (528+/- porphyric granite (528+/-9 Ma)9 Ma)

Mylonitic textures within the porphyric Mylonitic textures within the porphyric granites (emplaced 335 - 315Ma) are granites (emplaced 335 - 315Ma) are defined by the orientation of large K-defined by the orientation of large K-feldspar phenocrysts and biotitefeldspar phenocrysts and biotite

Indicating that there was regional Indicating that there was regional deformation and the plutonism were deformation and the plutonism were coeval.coeval.

The Velay migmatitesThe Velay migmatites Migmatites which occur in the core in the velay Migmatites which occur in the core in the velay

dome range from metatexites, who’s structure was dome range from metatexites, who’s structure was inherited from the parent gneiss, to diatexites and inherited from the parent gneiss, to diatexites and granitesgranites

Preserving K feldspar phenocrysts and Preserving K feldspar phenocrysts and diatexites with biotite-silliminite+/-cordieritediatexites with biotite-silliminite+/-cordierite

Migmatitic gneisses preserved within the dome Migmatitic gneisses preserved within the dome may be screens between intrusions of refractory may be screens between intrusions of refractory layerslayers

Contain cm size micaceous enclavesContain cm size micaceous enclaves From the base to top - Discontinuous layer of From the base to top - Discontinuous layer of

migmatitic paragneiss, migmatitic orthogneiss migmatitic paragneiss, migmatitic orthogneiss overlain by nonmigmatitic orthogneiss, and overlain by nonmigmatitic orthogneiss, and micaschist with amphibolite layers. This is due to :micaschist with amphibolite layers. This is due to :

Shearing and temperature plastic deformationShearing and temperature plastic deformation

Melting reactions in migmatites and Melting reactions in migmatites and gneiss hosts:gneiss hosts:

First melting stage (314+/- 5 Ma):First melting stage (314+/- 5 Ma): Developed under P_T conditions exceeding those for Developed under P_T conditions exceeding those for

water- saturated quartz-feldspathic rocks (biotite stable) water- saturated quartz-feldspathic rocks (biotite stable) (700(700OOC, 4 – 5 Kbar)C, 4 – 5 Kbar)

Presence of corundum paragneiss enclavesPresence of corundum paragneiss enclaves Unlikely large of granite were produced and extracted at Unlikely large of granite were produced and extracted at

this stage.this stage. Second melting stage(301+/-5 Ma):Second melting stage(301+/-5 Ma):

Characterized by high-temperature metamorphism in the Characterized by high-temperature metamorphism in the cordierite stability field (biotite destabilized)(760-850cordierite stability field (biotite destabilized)(760-850OOC, C, 4.4-6.0 Kbar)4.4-6.0 Kbar)

Considered to be synchronous with emplacement of the Considered to be synchronous with emplacement of the main cordierite-bearing granitesmain cordierite-bearing granites

Migmatitic structural and textural evolution Migmatitic structural and textural evolution reflects the progressive impact on the rheological reflects the progressive impact on the rheological behavior of partially molten rocks behavior of partially molten rocks

Biotite breakdown = 30 – 50 %melt production Biotite breakdown = 30 – 50 %melt production

The Velay GranitesThe Velay Granites Define a suite, which consists of 3 main Define a suite, which consists of 3 main

granite types distinguished according granite types distinguished according to age, structure, homogenityto age, structure, homogenity Heterogeneous banded biotite graniteHeterogeneous banded biotite granite Main biotite – cordierite granite which has Main biotite – cordierite granite which has

several subtypesseveral subtypes Late magmatic activity which includes Late magmatic activity which includes

homogeneous granite with K- feldspar homogeneous granite with K- feldspar porphyrocrysts and common prismatic porphyrocrysts and common prismatic cordierite; and shephanian leucogranite, cordierite; and shephanian leucogranite, microgranite and aplite-pegmatite dykes microgranite and aplite-pegmatite dykes and permian rhyolites and permian rhyolites

Conclusion:Conclusion: Composite and heterogeneous nature of the Composite and heterogeneous nature of the

dome reflects the successive generation and dome reflects the successive generation and emplacement of the granite suite. emplacement of the granite suite.

Foliation is well defined by the mineral and Foliation is well defined by the mineral and enclave orientation, developed during magma enclave orientation, developed during magma crystallization and final formation of the dome, crystallization and final formation of the dome, delineating the shape of the dome.delineating the shape of the dome.

Velay migmatite-granite dome results from:Velay migmatite-granite dome results from: Partial melting of the thickened crust started at Partial melting of the thickened crust started at

340Ma, while thrusting in the hinterland of the 340Ma, while thrusting in the hinterland of the Variscan belt was still active and ended with the Variscan belt was still active and ended with the collapse of the orogenic crust ~300Macollapse of the orogenic crust ~300Ma

Partial melting took place within a dominantly Partial melting took place within a dominantly metasedimentay crustal layer. Melting metasedimentay crustal layer. Melting reactions evolved from hydrous minerals and reactions evolved from hydrous minerals and indicate that the melting started at the end of indicate that the melting started at the end of the prograde metamorphasism and ended the prograde metamorphasism and ended during the decompression associated with during the decompression associated with exhumation of the migmatite-granite domeexhumation of the migmatite-granite dome

A rise in the temperature during the evolution A rise in the temperature during the evolution of the Variscan orogenic crust is due to the of the Variscan orogenic crust is due to the thermal relaxation and increased radioactive thermal relaxation and increased radioactive heat production following crustal thickening.heat production following crustal thickening.

Formation of the Velay dome, coeval with the Formation of the Velay dome, coeval with the activation of the crustal-scale detachments, activation of the crustal-scale detachments, potentially corresponds to the flow of a potentially corresponds to the flow of a partially molten crustal layer in response to partially molten crustal layer in response to the gravitational collapsethe gravitational collapse

Other References:Other References:

http://www.oxfordjournals.org/our_johttp://www.oxfordjournals.org/our_journals/petroj/online/Volume_40/Issuurnals/petroj/online/Volume_40/Issue_09/html/egc067_gml.htmle_09/html/egc067_gml.html

Thank youThank you Questions Questions …?…?