The genesis of LCT-type granitic pegmatites, as illustrated by lithium isotopes in micas

Abstract

Isotopic compositions in the Monts d'Ambazac Pegmatite Field (French Massif Central) exhibit a narrow range of mica δ7Li values, ranging from −3.6 to +3.4‰. The value obtained in biotite from the host Saint Sylvestre granite falls within this range (δ7Li=−1.5‰). Lithium concentrations are consistent with the degree of magmatic evolution of each pegmatite type: from 630ppm in Type II up to 13,500ppm in the more evolved Type VI pegmatite. Although the rare-element contents e.g., Li, Cs and Ta of the micas are consistent with pegmatite differentiation, δ7Li (‰) are firstly, independent of the degree of magmatic differentiation (independent of pegmatite type) and secondly, independent of the content of Li and other flux-elements such as Be and Cs. Muscovite sampled in pegmatite V from the Chabannes locality is the only pegmatite to exhibit a δ7Li variation from an intermediate unit (−1.7‰) to an internal pegmatitic unit (+3.4‰). The nature of this δ7Li variation suggests that there was extensive fractional crystallization during the pegmatite's consolidation. The independence of δ7Li (‰) evolution from the degree of magmatic evolution and the presence of distinct major rare-element bearing phases throughout the pegmatite field tend to confirm that the δ7Li (‰) values recorded in mica are inherited from crustal source rocks common to the granite and pegmatite-forming melts. We propose that the distinct pegmatite subtypes (beryl columbite vs lepidolite–petalite subtypes) observed throughout the Monts d'Ambazac Pegmatite Field reflect the diverse contributions of crustal protoliths. The lack of evidence of surrounding alteration combined with the absence of increasing Li-content within the host granite tends to confirm that the δ7Li values obtained within this pegmatite field are primary, and that no Li-diffusional process and/or mixing-driven Li-isotope fractionation has overprinted these isotopic compositions. In light of these results, the process of partial melting of protoliths enriched in rare-element bearing phases, e.g., mica and garnet, seems to be more responsible for Li-isotope fractionation than Li-diffusion or fractional crystallization at the temperature of pegmatite consolidation. Finally, we discuss the use of Li isotopic compositions to identify the most highly evolved pegmatitic systems.