RARE-ELEMENT (Li-RICH) GRANITIC AND PEGMATITIC PLUTONS: A PRIMARY OR SUPERIMPOSED SIGNATURE?

BERNARD CHAROY, FERNANDO NORONHA

Resumo


Bulk mineral compositions, phase relationships and primary versus metasomatic status of mineral assemblages in Li-rich pegmatites (or equivalents) are generally complex. In recent years, a considerable amount of interest has been paid to rare-met al granite-like entities characterized by an extreme enrichment in rare met als, together with volatiles as fluxing agents. A central question about the meaning of such evolved systems (intrusive, eruptive or pegmatitic equivalents) is whether their distinctive chemical signature is (1) magmatic or (2) due to subsolidus modification through addition from a superimposed or derivative fluid phase or a mixture of both. Another question is the proportions of the subsolidus fluids and whether it is a specific character or a convergence phenomenon. The study on evolved granitic and pegmatitic plutons from the Hercynian belt of France and Portugal indicate that some plutons reached their specialized status at the magmatic stage, and Li is hosted by the major primary phases. In other plutons, however, rare elements are of secondary, metasomatic origin. All studied plutons are volumetrically small, implying in small batches of magma, due to either a small melting fraction of crustal source of restricted composition, or a late withdraw of a residual liquid. The real significance of the high concentrations of the rare elements Li-rich pegmatites is undefined, because spatial and genetic relationships with larger magmatic plutons are often lacking.

Palavras-chave


Rare met al; Granites; Lithium; Magmatism; Metasomatism.

Texto completo:

PDF (English)

Apontamentos

  • Não há apontamentos.