THE ROLE OF CRUSTAL ANATEXIS AND MANTLE-DERIVED MAGMAS IN THE GENESIS OF SYNOROGENIC HERCYNIAN GRANITES OF THE LIVRADOIS AREA, FRENCH MASSIF CENTRAL

摘要

In the Livradois area of the French Massif Central, the Hercynian synorogenic porphyritic monzogranites and two-mica leucogranites intruded a migmatitic paragneiss sequence. Data on trace-element abundances and Rb/Sr and Sm/Nd isotopic values suggest a genetic link between the two-mica leucogranites and the migmatites. Numerical modeling of partial melting in the paragneiss can replicate the composition of the two-mica leucogranites in terms of trace elements if the accessory minerals zircon, monazite or xenotime remain in the residuum. The origin of the porphyritic monzogranite is more difficult to constrain; it belongs to a peculiar high-K, high-Mg suite that is rich both in compatible (e.g., Mg) and incompatible (e.g., K) elements. The porphyritic monzogranite is heterogeneous and contains microgranular mafic enclaves (MME) derived from a mafic magma. A model of mixing between a mafic magma with a composition similar to the MME, and a felsic magma similar to the two-mica leucogranite, accounts for the major- and trace-element characteristics and the Rb/Sr and Sm/Nd isotopic values of the porphyritic monzogranite. The MME are rich in incompatible elements, which implies an enriched source in the mantle. Considering the gological context of the Variscan belt in the French Massif Central, a possible origin for the enriched magmas is a subcontinental lithospheric mantle that was contaminated by crustal material during prior subduction, between 450 and 400 Ma. The results show that partial melting of a paragneiss generated the two-mica granite, and that the porphyritic monzogranite formed by mixing to various degrees of this melt with more mafic magmas generated by partial melting of an enriched mantle source. These magmas were formed and emplaced during the period 350 to 290 Ma when the orogen passed from the contractional (crustal thickening) age to orogenic gravitational collapse after detachment of its eclogitic root.