Meagan-The Velay dome _French Massif Central_ by liuqingyan


									    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
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
         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
        Indicating that there was regional deformation and
         the plutonism were coeval.
The Velay migmatites
     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
     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
   First melting stage (314+/- 5 Ma):
      Developed under P_T conditions exceeding those for water-
       saturated 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.

   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
   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,
      Heterogeneous   banded biotite granite
      Main biotite – cordierite granite which has several
      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
   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
   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 migmatite-
    granite 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:

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