The Velay dome (French Massif Central): Melt generation and granite emplacement during orogenic evolution

1. 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 Meagan Webster 14528436 Geology G314

2. 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

3. 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…

6. 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.

7. 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.

9. 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 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

10. Melting reactions in migmatites and gneiss hosts: • 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

11. 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

12. Conclusion: • 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

13. 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

14. Other References: • http://www.oxfordjournals.org/our_journals/petroj/online/Volume_40/Issue_09/html/egc067_gml.html Thank you Questions …?