Transcript The Velay dome (French Massif Central): Melt generation and

The Velay dome (French Massif
Central): Melt generation and
granite emplacement during
orogenic evolution
P. Ledrua,*, G.Courriouxa, C. Dallainb, J.M
Lardeauxc, J.M Monteld, O. Vanderhaeghae,
G. Vitelf
Geology G314
Meagan Webster 14528436
Introduction:
Velay dome
 Largest
Granite- migmatite dome of the Variscan
Belt located in the SE Massif central
 Formed during an orogenic collapse around 300 Ma
 Composed of peraluminous granites characterized
by nodular and prismatic cordierite and by enclaves
of gneisses and granites of various sizes and
natures
HOW it happened…
The French Varsican belt resulted from the
collision between Laurussia and Gondwana
 North Gondwana continental margin
represented by lower gneiss unit and
sedimentary parautochlthonous sequence
 Southward thrusting and thickening
 Velay was emplaced during this…

3 main structural zones are defined:

The host rocks intruded by the syntectonic
granites precursor to of the Velay dome

The gneiss-migmatite zone, at the periphery and
roof of the Velay dome

The cordierite bearing migmatite-granite
domain consisting of the most evolved pole of
granitisation of the crust.
Host rocks and periphery porphyric granites
of the Velay dome

Host is primarily composed of rocks of the lower
Gneiss Unit:
(i) metasediments derived from pelites and argilites
(ii) augen orthogneiss originating from peralumious
porphyric granite (528+/-9 Ma)

Mylonitic textures within the porphyric granites
(emplaced 335 - 315Ma) are defined by the
orientation of large K-feldspar phenocrysts and
biotite
 Indicating
that there was regional deformation and
the plutonism were coeval.
The Velay migmatites

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Migmatites which occur in the core in the velay dome range
from metatexites, who’s structure was inherited from the
parent gneiss, to diatexites and granites
 Preserving K feldspar phenocrysts and diatexites with
biotite-silliminite+/-cordierite
Migmatitic gneisses preserved within the dome may be
screens between intrusions of refractory layers
Contain cm size micaceous enclaves
From the base to top - Discontinuous layer of migmatitic
paragneiss, migmatitic orthogneiss overlain by nonmigmatitic
orthogneiss, and micaschist with amphibolite layers. This is
due to :

Shearing and temperature plastic deformation
Melting reactions in migmatites and gneiss
hosts:

First melting stage (314+/- 5 Ma):
Developed under P_T conditions exceeding those for watersaturated quartz-feldspathic rocks (biotite stable) (700OC, 4 – 5 Kbar)
 Presence of corundum paragneiss enclaves
 Unlikely large of granite were produced and extracted at this stage.
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
Second melting stage(301+/-5 Ma):
Characterized by high-temperature metamorphism in the cordierite
stability field (biotite destabilized)(760-850OC, 4.4-6.0 Kbar)
 Considered to be synchronous with emplacement of the main
cordierite-bearing granites
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Migmatitic structural and textural evolution reflects the
progressive impact on the rheological behavior of partially
molten rocks
Biotite breakdown = 30 – 50 %melt production
The Velay Granites
 Define
a suite, which consists of 3 main granite
types distinguished according to age, structure,
homogenity
 Heterogeneous
banded biotite granite
 Main biotite – cordierite granite which has several
subtypes
 Late magmatic activity which includes homogeneous
granite with K- feldspar porphyrocrysts and common
prismatic cordierite; and shephanian leucogranite,
microgranite and aplite-pegmatite dykes and permian
rhyolites
Conclusion:

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Composite and heterogeneous nature of the dome
reflects the successive generation and emplacement of
the granite suite.
Foliation is well defined by the mineral and enclave
orientation, developed during magma crystallization
and final formation of the dome, delineating the shape
of the dome.
Velay migmatite-granite dome results from:

Partial melting of the thickened crust started at 340Ma, while
thrusting in the hinterland of the Variscan belt was still active
and ended with the collapse of the orogenic crust ~300Ma

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
Partial melting took place within a dominantly
metasedimentay crustal layer. Melting reactions evolved from
hydrous minerals and indicate that the melting started at the
end of the prograde metamorphasism and ended during the
decompression associated with exhumation of the migmatitegranite dome
A rise in the temperature during the evolution of the
Variscan orogenic crust is due to the thermal relaxation and
increased radioactive heat production following crustal
thickening.
Formation of the Velay dome, coeval with the activation of
the crustal-scale detachments, potentially corresponds to the
flow of a partially molten crustal layer in response to the
gravitational collapse
Other References:

http://www.oxfordjournals.org/our_journals/p
etroj/online/Volume_40/Issue_09/html/egc06
7_gml.html
Thank you
Questions …?